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    • 专利摘要:
    Spring steel according to the present modality has a chemical composition consisting of, in mass%, C: 0.4 to 0.7%, Si: 1.1 to 3.0%, Mn: 0.3 to 1.5 %, P: equal to or less than 0.03%, S: equal to or less than 0.05%, Al: 0.01 to 0.05%, rare earth metal: 0.0001 to 0.002%, N: equal to or less than 0.015%, O: equal to or less than 0.0030%, Ti: 0.02 to 0.1%, and as optional elements, Ca, Cr, Mo, W, V, Nb, Ni, Cu, and B, with the balance being Fe and impurities. In spring steel, the number of oxide inclusions having an equivalent circular diameter equal to or greater than 5 ¿m is equal to or less than 0.2 / mm2, the oxide inclusions being each one between an oxide based on Al, a complex oxide containing REM, O and Al, and a complex oxisulfide containing REM, O, S, and Al. In addition, the maximum value between the equivalent circular diameters of the oxide inclusions is equal to or less than 40 m.
    公开号:BR112016023912B1
    申请号:R112016023912-1
    申请日:2015-04-22
    公开日:2021-02-23
    发明作者:Masayuki Hashimura；Junya Yamamoto；Kazumi Mizukami；Naotsugu Yoshida；Masafumi Miyazaki；Kenichiro Miyamoto
    申请人:Nippon Steel Corporation；
    IPC主号:
    专利说明:

    Technical field
    [0001] The present invention relates to a spring steel and a method for producing it. Background
    [0002] Spring steels are used in automobiles or machines in general. When a spring steel is used for a car's suspension springs, for example, the spring steel must have up to fatigue resistance. Recently, there has been a need for cars with reduced weight and greater power output for improved fuel economy. Consequently, spring steels that are used for engines or suspensions need to have even greater fatigue strength.
    [0003] Steel products may contain oxide inclusions typified by alumina. Crude oxide inclusions decrease fatigue resistance.
    [0004] Alumina is formed when the deep steel is deoxidized in the refining stage. Pans or the like often contain refractory alumina materials. For this reason, alumina can be formed in molten steel not only in the case of deoxidation by Al, but also when deoxidation is carried out with an element other than Al (for example, Si or Mn). Alumina in molten steel tends to agglomerate and form agglomerates. In other words, alumina tends to be rough.
    [0005] Techniques for refining oxide inclusions typified by alumina are described in Japanese Patent Application Publication No. 05-311225 (Patent Literature 1), Japanese Patent Application Publication No. 2009-263704 (Patent Literature 2), Japanese Patent Application Publication No. 09-263820 (Patent Literature 3), and Japanese Patent Application Publication No. 11-279695 (Patent Literature 4).
    [0006] Patent Literature 1 describes the following: an Mg alloy is added to the molten steel. As a result, alumina is reduced and spinel (MgO-Al2O3) or MgO is formed instead. Consequently, the stiffening of the alumina due to the agglomeration of the alumina is inhibited.
    [0007] However, the production method of Patent Literature 1 raises the possibility of clogging the nozzles in a continuous casting machine. In this case, gross inclusions are more likely to be captured by molten steel. This results in reduced fatigue strength in steel.
    [0008] Patent Literature 2 describes the following: the chemical composition of SiO2-Al2O3-CaO oxides in a longitudinal cross section of the steel rod is controlled to be SiO2: 30 to 60%, Al2O3: 1 to 30%, and CaO: 10 to 50% so that the melting point of the oxides is no more than 1400 ° C. In addition, 0.1 to 10% of B2O3 is included in the oxides. As a result, the oxide inclusions are finely dispersed.
    [0009] However, although B2O3 is effective for the above oxides, it sometimes cannot sufficiently inhibit alumina clumping. In such cases, resistance to fatigue decreases.
    [0010] Patent Literature 3 describes the following: in the production method of steel calmed to aluminum, an alloy made from two or more elements selected from the group consisting of Ca, Mg and rare earth metals (REM) and Al is added to the cast steel for deoxidation.
    [0011] However, in some cases, the addition of the above alloys to a spring steel does not refine the oxide inclusions. In such cases, the fatigue strength of the spring steel decreases.
    [0012] Patent Literature 4 describes the following: the steel rebar includes 0.010% or less of REM (0.003% in the example) so that the inclusions can be spheroidized.
    [0013] However, in some cases, the addition of the REM content above to a spring steel does not refine the oxide inclusions. In such cases, the fatigue strength of the spring steel decreases.
    [0014] In addition, suspension springs have the role of absorbing vibrations from the vehicle's chassis caused by irregularities on the surface of the road on which it is traveling. Consequently, suspension springs must have not only fatigue resistance, but also high toughness.
    [0015] Methods for producing a spring include hot forming and cold forming. In cold forming, winding is performed by cold operation to produce springs. Consequently, spring steels must have high ductility for cold operation. List of citations Patent Literature
    [0016] Patent Literature 1: Japanese Patent Application Publication No. 05-311225
    [0017] Patent Literature 2: Japanese Patent Application Publication No. 2009-263704
    [0018] Patent Literature 3: Japanese Patent Application Publication No. 09-263820
    [0019] Patent Literature 4: Japanese Patent Application Publication No. 11-279695 Summary of the invention
    [0020] An objective of the present invention is to provide a spring steel that presents excellent resistance to fatigue, toughness and ductility.
    [0021] A spring steel according to the present modality has a chemical composition consisting of, in% mass, C: 0.4 to 0.7%, Si: 1.1 to 3.0%, Mn: 0.3 to 1.5%, P: equal to or less than 0.03%, S: equal to or less than 0.05%, Al: 0.01 to 0.05%, rare earth metal: 0.0001 to 0.002%, N: equal to or less than 0.015%, O: equal to or less than 0.0030%, Ti: 0.02 to 0.1%, Ca: 0 to 0.0030%, Cr: 0 to 2.0% , Mo: 0 to 1.0%, W: 0 to 1.0%, V: 0 to 0.70%, Nb: 0 and less than 0.050%, Ni: 0 to 3.5%, Cu: 0 to 0.5%, and B: 0 to 0.0050%, with the balance being Fe and impurities. In spring steel, the number of oxide inclusions having an equivalent circular diameter of 5 μ m or less is equal to or less than 0.2 / mm2, the oxide inclusions each being one of Al-based oxides , a complex oxide containing REM, O and Al, and an oxysulfide complex containing REM, O, S and Al. In addition, the maximum value between the equivalent circular diameters of the oxide inclusions is equal to or less than 40 μ m.
    [0022] Spring steel according to the present modality has excellent resistance to fatigue, toughness and ductility. Brief Description of Drawings
    [0023] [FIG. 1] - FIG. 1 is an SEM image of a complex oxysulfite containing Al, O (oxygen), REM (Ce in this modality), and S in a steel spring of the present modality.
    [0024] [FIG. 2] - FIG. 2 is a cross-sectional view of a semi-finished product to illustrate a method for measuring the cooling rate of the semi-finished product in a casting step.
    [0025] [FIG. 3A] - FIG. 3A is a side view of an ultrasonic fatigue test specimen.
    [0026] [FIG. 3B] - FIG. 3B is a schematic diagram illustrating the location for cutting a test specimen that serves as material for the ultrasonic fatigue test specimen illustrated in FIG. 3A. Settings description
    [0027] A spring steel according to the present modality has a chemical composition consisting of, in% by mass, C: 0.4 to 0.7%, Si: 1.1 to 3.0%, Mn: 0.3 1.5%, P: equal to or less than 0.03%, S: equal to or less than 0.05%, Al: 0.01 to 0.05%, rare earth metal: 0.0001 to 0.002 %, N: equal to or less than 0.015%, O: equal to or less than 0.0030%, Ti: 0.02 to 0.1%, Ca: 0 to 0.0030%, Cr: 0 to 2, 0%, Mo: 0 to 1.0%, W: 0 to 1.0%, V: 0 to 0.70%, Nb: 0 to less than 0.050%, Ni: 0 to 3.5%, Cu: 0 to 0.5%, and B: 0 to 0.0050%, with the balance being Fe and impurities. In spring steel, the number of oxide inclusions having a circular diameter equivalent to or greater than 5 μ m is equal to or less than 0.2 / mm2, the oxide inclusions each being one among Al-based oxides , a complex oxide containing REM, O and Al, and a complex oxysulfide containing REM, O, S, and Al. In addition, the maximum value between equivalent circular diameters of the oxide inclusions is equal to or less than 40 μ m.
    [0028] In steel for springs according to the present modality, the oxide inclusions, each of which is one between Al-based oxide, a complex oxide (inclusion containing REM and containing Al and O), and a complex oxide (inclusion containing REM, and containing Al, O and S), are dispersed finely. As a result, spring steel has high resistance to fatigue. In addition, the spring steel of the present modality includes Ti and, therefore, has high toughness. As a result, the spring steel according to the present modality has excellent ductility.
    [0029] The chemical composition of the spring steel above may include Ca: 0.0001 to 0.0030%. The chemical composition of the spring steel above may include one or more elements selected from the group consisting of Cr: 0.05 to 2.0%, Mo: 0.05 to 1.0%, W: 0.05 to 1.0 %, V: 0.05 to 0.70%, Nb: 0.002 to less than 0.050%, Ni: 0.1 to 3.5%, Cu: 0.1 to 0.5%, and B: 0.0003 to 0.0050%.
    [0030] A method for producing the spring steel of the present modality includes the steps of refining the molten steel having the above chemical composition; produce a semi-finished product using cast steel refined by a continuous casting process, and hot work the semi-finished product. The step of refining the steel includes: a step of deoxidizing the molten steel using Al during refining in the pan, and a step of deoxidizing the molten steel using REM for at least 5 minutes after deoxidizing Al. The step of producing a semi product -finished includes: a step of stirring the molten steel inside a mold to circulate the molten steel in a horizontal direction at a flow rate of 0.1 m / min or faster, and a step of cooling the semi-finished product that it is being cast at a cooling rate of 1 to 100 ° C / min.
    [0031] In the refining step, deoxidation with Al and deoxidation with REM are performed in that order during refining in the pan with deoxidation with REM being performed for at least 5 minutes. Then, in the continuous casting step, the circulation is carried out at the flow rate mentioned above and the cooling is carried out at the cooling rate mentioned above. With this production method, it is possible to produce a spring steel that satisfies the number of crude oxide inclusions and the maximum value between equivalent circular diameters of the crude oxide inclusions mentioned above.
    [0032] The steel spring of the present modality will be described in detail below. In the contents of the elements, "%" means "% by mass". [Chemical composition]
    [0033] The chemical composition of spring steel according to the present modality includes the following elements:
    [0034] C: 0.4 to 0.7%
    [0035] Carbon (C) increases the strength of the steel. If the C content is too low, this advantageous effect cannot be produced. On the other hand, if the C content is too high, pro-eutectoid cementites will form excessively in the cooling process after hot rolling. In this case, the working capacity for drawing steel decreases. Consequently, the C content ranges from 0.4 to 0.7%. The lower limit of the C content is preferably greater than 0.4%, more preferably 0.45%, and even more preferably 0.5%. The upper limit of the C content is preferably less than 0.7%, more preferably 0.65%, and even more preferably 0.6%.
    [0036] Si: 1.1 to 3.0%
    [0037] Silicon (Si) increases the hardening capacity of the steel and increases the fatigue resistance of the steel. In addition, Si increases the resistance to bending. If the Si content is too low, these advantageous effects cannot be produced. On the other hand, if the Si content is too high, the ductility of the ferrite in the pearlite will decrease. In addition, if the Si content is very high, decarbonization will be promoted in the rolling, rapid cooling and quenching processes, resulting in a decrease in the strength of the steel. Consequently, the Si content ranges from 1.1 to 3.0%. The lower limit of the Si content is preferably greater than 1.1%, more preferably 1.2%, and even more preferably 1.3%. The upper limit of the Si content is preferably less than 3.0, more preferably 2.5% m, and even more preferably 2.0%.
    [0038] Mn: 0.3 to 1.5%
    [0039] Manganese (Mn) deoxides the steel. In addition, Mn increases the strength of the steel. If the Mn content is too low, these beneficial effects cannot be produced. On the other hand, if the Mn content is too high, segregation will occur. In the segregation portion, micro-tensile will form. The micromartensite will be a factor that will cause failures in the lamination process. In addition, micromartensite decreases the work capacity for steel drawing. Consequently, the Mn content ranges from 0.3 to 1.5%. The lower limit of the Mn content is preferably greater than 0.3%, more preferably 0.4%, and even more preferably 0.5%. The upper limit of the Mn content is preferably less than 1.5%, more preferably 1.4%, and even more preferably 1.2%.
    [0040] P: equal to or less than 0.03%
    [0041] Phosphorus (P) is an impurity. P secretes at the grain edges, which results in a decrease in the fatigue strength of the steel. Consequently, the P content is preferably as low as possible. The P content is equal to or less than 0.03%. The upper limit of the P content is preferably less than 0.03%, and more preferably 0.02%.
    [0042] S: equal to or less than 0.05%
    [0043] Sulfur (S) is an impurity. S forms crude MnS, which results in decreased fatigue strength of the steel. Consequently, the S content is preferably as low as possible. The S content is equal to or less than 0.05%. The upper limit of the S content is preferably less than 0.05%, more preferably 0.03%, and even more preferably 0.01%.
    [0044] Al: 0.01 to 0.05%
    [0045] Aluminum (Al) deoxides the steel. In addition, Al adjusts the steel grains. If the Al content is too low, these beneficial effects cannot be produced. On the other hand, if the Al content is too high, the advantageous effects above will reach saturation. In addition, if the Al content is too high, large amounts of alumina will remain in the steel. Consequently, the Al content ranges from 0.01 to 0.05%. The lower limit of the Al content is preferably greater than 0.01%. The upper limit of the Al content is preferably less than 0.05%, and more preferably 0.035%. The content of Al as referred to in that specification means the so-called total Al content.
    [0046] REM: 0.0001 to 0.002%
    [0047] Rare earth metal (REM) desulphurizes and deoxides steel. In addition, REM binds to Al-based oxides to refine the oxide inclusions. This is described below.
    [0048] In this specification, the oxide inclusions are one or more of the Al-based oxides typified by alumina, complex oxides, and complex oxysulfides. Al-based oxide, complex oxide and complex oxy-sulphide are defined as follows.
    [0049] Al-based oxide includes at least 30% O (oxygen) and at least 5% Al. Al-based oxide can also include at least one or more deoxidizing elements such as Mn, Si, Ca , in G. The REM content in the Al-based oxide is less than 1%.
    [0050] Complex oxide includes at least 30% O (oxygen), at least 5% Al, and at least 1% REM. The complex oxide can also include at least one or more deoxidizing elements such as Mn, Si, Ca, and Mg.
    [0051] Complex oxysulfide includes at least 30% O (oxygen), at least 5% Al, at least 1% REM, and S. The complex oxysulfide can also include at least one or more deoxidizing elements such as Mn , Si, Ca, and Mg.
    [0052] REM reacts with oxides based on Al in steel to form complex oxides. Complex oxides can also react with S to form complex oxysulfides. Thus, REM transforms oxides based on Al into complex oxides or complex oxysulfides. This inhibits the agglomeration of Al-based oxides in molten steel to form agglomerates, thus making it possible to disperse fine oxide inclusions in the steel.
    [0053] FIG. 1 is an SEM image illustrating an example of a complex oxysulfide in the spring steel of the present embodiment. The equivalent circular diameter of the complex oxysulfide The equivalent circular diameter of the complex oxysulfide in FIG. 1 is less than 5 μ m. The chemical composition of the complex oxysulfide in FIG. 1 includes 64.4% O (oxygen), 18.4% Al, 5.5% Mn, 4.6% S and 3.8% Ce (REM).
    [0054] Complex oxides and complex oxisulfides, which are represented by FIG. 1, have equivalent circular diameters of about 1 to 5 μ m and, therefore, are thin. In addition, neither complex oxides nor complex oxisulfides are extended to become crude or to form agglomerates. Thus, neither complex oxides nor complex oxisulfides are capable of acting as points of onset of fatigue fractures. As a result, the fatigue strength of spring steel is increased.
    [0055] The spring steel of the present embodiment preferably includes at least complex oxisulfides of all oxide inclusions. In this case, S is immobilized on the complex oxisulfides. As a result, MnS precipitation is inhibited and TiS precipitation at the grain edges is also inhibited. Consequently, the ductility of spring steel increases.
    [0056] If the REM content is too low, these beneficial effects cannot be produced. On the other hand, if the REM content is too high, the inclusions containing REM can clog the nozzles in the continuous casting. Even in the case that the inclusions containing REM do not clog the nozzles, the raw inclusions containing REM are included in the steel, which results in a decrease in the fatigue resistance of the steel. Consequently, the REM content ranges from 0.0001 to 0.002%. The lower limit of the REM content is preferably greater than 0.0001 ’%, more preferably 0.0002%, and even more preferably greater than 0.0003%. The upper limit of the REM content is preferably less than 0.002%, more preferably 0.0015%, even more preferably 0.0010%, and even more preferably 0.0005%.
    [0057] REM as referred to in this specification is a generic term for lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, scandium (Sc) with atomic number 21, and yttrium (Y) with number atomic 39.
    [0058] N: equal to or less than 0.015%
    [0059] Nitrogen (N) is an impurity. N forms nitrides, which results in decreased fatigue resistance of steel. In addition, N causes stress aging, which results in a decrease in the ductility and toughness of the steel. Consequently, the N content is preferably as low as possible. The N content is equal to or less than 0.015%. The upper limit of the N content is preferably less than 0.015%, more preferably 0.010%, even more preferably 0.008%, and even more preferably 0.006%.
    [0060] O: equal to or less than 0.0030%
    [0061] Oxygen (O) is an impurity. O forms Al-based oxides, complex oxides and complex oxysulfides. If the O content is too high, large quantities of crude Al-based oxides will form, which will shorten the fatigue life of the steel. Consequently, the O content is equal to or less than 0.0030%. The upper limit of the O content is preferably less than 0.0030%, more preferably 0.0020%, and even more preferably 0.0015%. The O content as referred to in this specification is the so-called total amount of oxygen (T. O).
    [0062] Ti: 0.02 to 0.1% Titanium (Ti) forms Ti carbides and fine Ti carbonitrides in the austenite temperature range above A3. During heating for quick cooling, Ti carbides and Ti carbonitrides exert a pinning effect on austenite grains to refine the grains and make them uniform. Thus, Ti increases the toughness of the steel.
    [0063] In general, when Ti is included, Ti carbides and Ti carbonates are formed and TiS also precipitates at the grain edges. TiS decreases steel ductility similarly to MnS.
    [0064] However, as described above, in the spring steel of the present modality, S binds to REM to form complex oxysulfides. As a result, S does not secrete at the grain edges and, therefore, neither TiS nor MnS are likely to form. Thus, in the present modality, the Ti contained increases the toughness and also provides high ductility. If the Ti content is too low, these beneficial effects cannot be produced.
    [0065] On the other hand, if the Ti content is too high, crude TiN will form. TiN tends to be a point of initiation of fractures and also to be a place of capture of hydrogen. As a result, the fatigue strength of the steel will decrease. Consequently, the Ti content ranges from 0.02 to 0.1%. The lower limit of the Ti content is preferably greater than 0.02%, and more preferably 0.04%. The upper limit of the Tié content is preferably less than 0.1%, more preferably 0.08%, and even more preferably 0.06%.
    [0066] The balance of the chemical composition of steel for springs according to the present modality is Fe and impurities. Impurities here refer to impurities that find their way into steel from ore and scrap as raw materials or from the production environment, for example, when a steel product is produced industrially and that are left within a range that does not affect adversely affect the advantageous effects of spring steel of the present modality.
    [0067] The chemical composition of spring steel according to the present modality may also include Ca instead of part of Fe.
    [0068] Ca: 0 to 0.0030% Calcium (Ca) is an optional element and may not be included. When Ca is included, Ca desulfurizes the steel. On the other hand, if the Ca content is too high, crude oxides Al-Ca-O will form. In addition, if the Ca content is too high, complex oxisulfides will absorb Ca. Complex oxisulfides that have absorbed Ca tend to become crude. Such crude oxides tend to be fracture initiation points for steels. Consequently, the Ca content ranges from 0 to 0.0030%. The lower limit of the Ca content is preferably not less than 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the Ca content is preferably less than 0.0030%, more preferably 0.0020%, and even more preferably 0.0015%.
    [0069] The chemical composition of spring steel according to the present modality may also include, instead of part of the Fe, one or more elements selected from the group consisting of Cr, Mo, W, V, Nb, Ni, Cu, and B All of these elements increase the strength of the steel.
    [0070] Cr: 0 to 2.0% Chromium (Cr) is an optional element and may not be included. When included, Cr increases the strength of the steel. In addition, Cr increases the hardening capacity of the steel and increases the fatigue strength of the steel. In addition, Cr increases the resistance to softening in tempering. On the other hand, if the Cr content is too high, the hardness of the steel increases excessively, which results in a decrease in ductility. Consequently, the Cr content ranges from 0 to 2.0%. The lower limit of the Cr content is preferably 0.05%. When the resistance to softening in tempering is to be increased, the lower limit of the Cr content is preferably 0.5%, and more preferably 0.7%. The upper limit of the Cr content is preferably less than 2.0%. When the spring steel product is to be produced by cold winding, the upper limit of the Cr content is more preferably 1.5%.
    [0071] Mo: 0 to 1.0% Molybdenum (Mo) is an optional element and may not be included. When included, Mo increases the hardening capacity and increases the strength of the steel. In addition, Mo increases the resistance to softening when tempering the steel. In addition, Mo forms the carbides to refine the grains. Mo carbides precipitate at temperatures lower than vanadium carbides. Thus, Mo is effective for refining the grains of steels for high-strength springs, which are tempered at low temperatures.
    [0072] On the other hand, if the Mo content is too high, an overcooled structure tends to form in the cooling process after hot rolling. Overcooled structures can be the cause of aging fractures or fractures during work. Consequently, the Mo stick content is 0 to 1.0%. The lower limit of the Mo content is preferably 0.05%, and more preferably 0.10%. The upper limit of the Mo content is preferably less than 1.0%, more preferably 0.75%, and even more preferably 0.50.
    [0073] W: 0 to 1.0% Tungsten (W) is an optional element and may not be included. When included, W increases the hardening capacity of the steel and increases the strength of the steel similarly to Mo. In addition, W increases the resistance to softening when tempering the steel. On the other hand, if the W content is too high, an overcooled structure will form as with Mo. Consequently, the W content ranges from 0 to 1.0%. When a high resistance to softening in the temper must be obtained, the lower limit of the W content is preferably 0.05%, and more preferably 0.1%. The upper limit of the W content is preferably less than 1.0%, more preferably 0.75%, and even more preferably 0.50%.
    [0074] V: 0 to 0.70% Vanadium (V) is an optional element and may not be included. When included, V forms fine nitrides, carbides and carbonitrides. These precipitates increase the resistance to softening in the temper of the steel and the resistance of the steel. In addition, these precipitates refine the grains. On the other hand, if the V content is too high, V nitrides, V carbides, and V carbon nitrides will not dissolve sufficiently when heated for rapid cooling. V nitrides, carbides, and undissolved V carbonitrides become crude and remain in the steel, resulting in a decrease in the ductility and fatigue strength of the steel. In addition, if the V content is too high, an overcooled structure will form. Consequently, the V content ranges from 0 to 0.70%. The lower limit of the V content is preferably 0.05%, more preferably 0.06%, and even more preferably 0.08%. The upper limit of the V content is preferably less than 0.70%, more preferably 0.50%, even more preferably 0.30%, and most preferably the upper limit is 0.25%.
    [0075] Nb: 0 less than 0.050% Niobium (Nb) is an optional element. And it may not be included, similarly to V, niobium forms nitrides, carbides and carbonites, which increases the strength of the steel and the resistance to softening in the tempering of the steel and refines the grains. On the other hand, if the Nb content is too high, the ductility of the steel will decrease. Consequently, the Nb content ranges from 0 to less than 0.050%. The lower limit of the Nb content is preferably 0.002%, more preferably 0.005%, and more preferably 0.008%. When springs are produced by cold winding, the upper limit of the Nb content is preferably less than 0.030%, and more preferably less than 0.020%.
    [0076] Ni: 0 to 3.5% Nickel (Ni) is an optional element and may not be included. When included, Ni increases the strength and hardening capacity of steel similarly to Mo. In addition, when Cu is included, NI forms an alloying phase with Cu to inhibit the decrease in the hot-working capacity of the steel. On the other hand, if the Ni content is too high, the amount of austenite retained will increase excessively, which results in a decrease in the strength of the steel after rapid cooling. In addition, the retained austenite will turn into martensite in use to cause expansion. As a result, the dimensional accuracy of the product decreases. Consequently, the Ni content ranges from 0 to 3.5%. The lower limit of Ni content is preferably 0.1%, more preferably 0.2%, and even more preferably 0.3%. The upper limit of the Ni content is preferably less than 3.5%, more preferably 2.5%, and even more preferably 1.0%. When Cu is included, the Ni content is preferably not less than the Cu content.
    [0077] Cu: 0 to 0.5% Copper (Cu) is an optional element and may not be included. When included, Cu increases the hardening capacity of the steel and increases the strength of the steel. In addition, Cu increases the corrosion resistance of the steel and inhibits the decarburization of the steel. On the other hand, if the Cu content is too high, the work capacity decreases. In this case, flaws in production processes such as casting, rolling and forging tend to occur. Consequently, the Cu content ranges from 0 to 0.5%. The lower limit of the Cu content is preferably 0.1%, and more preferably 0.2%. The upper limit of the Cu content is preferably less than 0.5%, more preferably 0.4%, and even more preferably 0.3%.
    [0078] B: 0 to 0.0050% Boron (B) is an optional element and may not be included. When included, B increases the hardening capacity of the steel and increases the strength of the steel.
    [0079] In addition, B is kept in a solid solution in the steel to segregate at the grain edges, the solute B inhibits segregation at the grain edges of fragility elements at the grain edges such as P, N, and S. Thus, B reinforces the grain edges. In the spring steel of the present modality, the segregation of S at the edges of the grains is significantly inhibited when B is included together with Ti and REM. As a result, the fatigue strength and toughness of the steel are increased.
    [0080] On the other hand, if the B content is too high, an overcooled structure such as martensite or bainite will form. Consequently, the B content ranges from 0 to 0.0050%. The lower limit of the B content is preferably not less than 0.0003%, more preferably 0.0005%, and even more preferably 0.0008%. The upper limit of the B content is preferably less than 0.0050%, more preferably 0.0030%, and even more preferably 0.0020%. [Microstructure] [TN number of crude oxide inclusions]
    [0081] In spring steel that has the chemical composition described above, the TN number of oxide inclusions that have an equivalent circular diameter equal to or greater than 5 μ m is equal to or less than 0.2 / mm2, the inclusions of oxides, each of which is one of Al-based oxides, a complex oxide, and a complex oxysulfide.
    [0082] The equivalent circular diameter refers to the diameter of a circle determined to have the same area as the area of each of the oxide inclusions (Al-based oxides, complex oxides, and complex oxisulfides). Hereinafter, oxide inclusions having an equivalent circular diameter equal to or greater than 5 μ m are referred to as "crude oxide inclusions". The TN number of crude oxide inclusions can be determined as follows.
    [0083] A steel for rod-shaped or linear springs is cut along its axial direction. The cross section is mirror polished. The Selective Potentiostatic Etching method by Electrolytic Dissolution (SPEED method) is performed on the mirrored cross section. In the etched cross section, five fields are freely selected which are rectangular regions with a width of 2 mm in the radial direction and 5 mm in length in the axial direction, with a location with R / 2 depth from the spring steel surface (R is the radius of the spring steel) being the center.
    [0084] Using a scanning electron microscope (SEM) equipped with an X-ray dispersive energy microanalyzer (EDX), the fields are each observed at 2000x magnification and images of the fields are acquired. Inclusions are identified in the fields. Using EDX, the chemical composition (Al content, O content, REM content, S content, etc. in the inclusion) of each of the inclusions identified is analyzed. Based on the results of the analysis, oxide inclusions (Al-based oxides, complex oxides, and complex oxysulfides) are identified among the inclusions.
    [0085] The equivalent circular diameters of the identified oxide inclusions (Al-based oxides, complex oxides, and complex oxysulfides) are determined by image processing to identify the inclusions of oxides having an equivalent circular diameter equal to or greater than 5 μ m (crude oxide inclusions).
    [0086] The total number of crude oxide inclusions in the five fields is determined and the TN number (number / mm2) of crude oxide inclusions is determined by the following formula. TN = Total number of crude oxide inclusions in the five fields / total area of the five fields
    [0087] In the spring steel of the present modality, the TN number of crude oxide inclusions is not greater than 0.2 / mm2. The appropriate amount of REM contained under suitable production conditions transforms oxides based on Al into complex oxides or fine complex oxysulfides. This results in reaching a low TN number. Consequently, a high resistance to fatigue is obtained. [Maximum Dmax value between equivalent circular diameters of oxide inclusions]
    [0088] In addition, for spring steel of the present modality, the maximum value Dmax between equivalent circular diameters of the oxide inclusions is equal to or less than 40 μ m.
    [0089] The maximum Dmax value is determined as follows. When measuring the TN number described above, the equivalent circular diameters of the oxide inclusions in the five fields are determined. The maximum value between the equivalent circular diameters determined is designated as the maximum value Dmax between the equivalent circular diameters of the oxide inclusions.
    [0090] In the spring steel of this modality, the maximum value Dmax is no more than 40 μ m. The appropriate amount of REM contained here transforms oxides based on Al into complex oxides or fine complex oxysulfides to thereby reach the low maximum Dmax value. Consequently, a high resistance to fatigue is obtained. [Production method]
    [0091] An exemplary method for producing the above spring steel is described. The method for producing the spring steel of the present modality includes: a step of refining the molten steel (refining process); a step of producing a semi-finished product using cast steel refined by a continuous casting process (casting process); a step of hot working the semi-finished product to produce the spring steel (hot working process). [Refining process]
    [0092] In the refining process, molten steel is refined. Initially, the molten steel is subjected to refining in the pan. Any known refining in the pan can be used as refining in the pan. Examples of refining in the pan include a vacuum degassing process using RH (Ruhrstahl-Heraeus).
    [0093] While refining the pan is running, Al is introduced into the molten steel to deoxidize the molten steel with Al. Preferably. The content of O (total amount of oxygen) in steel melted after deoxidation with Al is not greater than 0.0030%.
    [0094] After deoxidation with Al, REM is introduced into the molten steel to perform deoxidation with REM for at least 5 minutes.
    [0095] After deoxidation with REM, the refining of the pan including the vacuum degassing process can also be performed. With the refining step described above, a molten steel is produced having the chemical composition described above.
    [0096] In the refining process described above, REM deoxidation is performed after deoxidation with Al for at least 5 minutes. This results in the transformation of Al-based oxides into complex oxides or complex oxysulfides and their refining. Consequently, the hardening (agglomeration) of Al-based oxides as in the conventional technique is inhibited.
    [0097] If REM deoxidation lasts for less than 5 minutes, the transformation of Al-based oxides into complex oxides or complex oxisulfides will be insufficient. Consequently, the TN number will exceed 0.2 / mm2 and or the maximum Dmax value between the equivalent circular diameters of the oxide inclusions will exceed 40 μ m.
    [0098] In addition, if deoxidation is carried out with an element other than Al before deoxidation with REM, the transformation of Al-based oxides into complex oxides or complex oxisulfides will be insufficient. Consequently, the TN number will exceed 0.2 / mm2 and / or the maximum Dmax value between the equivalent circular diameters of the oxide inclusions will exceed 40 μ m.
    [0099] For REM deoxidation, for example, a metal misch (mixture of REMs) can be used. In this case, a fragment-type metal misch may be added to the molten steel. In the last stage of refining, a Ca-Si alloy, a CaO-CaF2 flux, or another substance can be added to the molten steel to perform desulfurization. [Casting process]
    [00100] Using refined molten steel in the pan, a semi-finished product is produced by a continuous casting process.
    [00101] Even after refining the analysis, REM and Al-based oxides react with each other in molten steel to form complex oxidesulfides and oxides. Therefore, by stirring the molten steel inside the mold, the reaction between REM and Al-based oxides can be facilitated.
    [00102] Consequently, in the casting process, the molten steel inside the mold is agitated and circulated in the horizontal direction at a flow rate of 0.1 m / min or faster. This promotes the reaction between REM and Al-based oxides to form complex oxides and complex oxysulfides. As a result, the TN number of crude oxide inclusions is not greater than 0.2 / mm2 and the maximum Dmax value of the oxide inclusions is not greater than 40 μ m. On the other hand, if the flow rate is less than 0.1 m / min, the reaction between REM and Al-based oxides is less likely to be promoted. Consequently, the TN number will exceed 0.2 / mm2 and / or the maximum Dmax will exceed 40 μ m. The stirring of the molten steel is carried out by electromagnetic stirring, for example.
    [00103] In addition, the RC cooling rate of the semi-finished product being cast will affect the hardening of the oxide inclusions. In this mode, the RC cooling rate varies from 1 to 100 ° C / min. The cooling rate refers to the cooling rate from liquidus temperature to solidus temperature in a location at a depth T / 4 (T is the thickness of the semi-finished product) from the top or bottom surface of the semi-finished product. If the cooling rate is too low, the hardening of the oxide inclusions is more likely to occur. Thus, if the RC cooling rate is less than 1 ° C / min, the TN number of crude oxide inclusions will exceed 0.2 / mm2 and / or the Dmax value between the maximum equivalent circular diameters of the oxide inclusions will exceed 40 μ m.
    [00104] On the other hand, if the RC cooling rate is greater than 100 ° C / min, inclusions of crude oxide will be captured in the steel before they float during the casting. Consequently, the TN number of crude oxide inclusions will exceed 0.2 / mm2 and / or the maximum Dmax value between equivalent circular diameters of the oxide inclusions will exceed 40 μ m.
    [00105] When the RC cooling rate varies from 1 to 100 ° C / min, the TN number of crude oxide inclusions is not greater than 0.2 / mm2 and the maximum Dmax value between equivalent circular diameters of the oxide inclusions is not greater than 40 μ m.
    [00106] The cooling rate can be determined as follows. FIG. 2 illustrates a cross-section (cross-section perpendicular to the axial direction of the semi-finished product) of the semi-finished ingot product. With reference to FIG. 2, in the cross-section of the semi-finished product, any point P that is T / 4 in depth is selected from the upper and lower surfaces of the semi-finished product at the time of casting. T is the thickness (mm) of the semi-finished product. In the solidified structure at point P, the spacing of the secondary dendrite arm À (μ m) in the direction of the thickness T is measured. Specifically, the spacing of the secondary dendrite arm in the direction of the thickness T is measured in 10 places and the average of the measurements is designated as À spacing.
    [00107] The given À spacing is replaced in Formula (1) to determine the RC cooling rate (° C / min). RC = (À / 770) - (1 / 0.41) (1)
    [00108] The lower limit of the RC cooling rate is preferably 5 ° C / min. The upper limit of the RC cooling rate is preferably less than 60 ° C / min and more preferably less than 30 ° C / min. Under the production conditions described above, the semi-finished product is produced. [Hot work process]
    [00109] The semi-finished product produced is subjected to hot work to produce a rebar. For example, the semi-finished product is laminated to bars to produce a bar. The bar is subjected to hot rolling to produce a rebar. Using the production method described above, rebar is produced.
    [00110] When springs are produced using rebar, a hot forming process or a cold forming process can be used. The hot forming process can be implemented as follows, for example. The rebar is subjected to wire drawing to obtain a spring steel wire. The spring steel wire is heated to the above A3 temperature. The spring steel wire (austenite structure) is wound around a mandrel to be formed into a coil (spring). The shaped spring is subjected to rapid cooling and tempering to adjust the resistance of the spring. The rapid cooling temperature ranges from 850 to 950 ° C, for example, with oil cooling being performed. The tempering temperature ranges from 420 to 500 ° C, for example. Using the steps described above, springs are produced.
    [00111] The cold forming process is implemented as follows. The rebar is subjected to wire drawing to obtain the steel wire for springs. The steel spring wire is subjected to rapid cooling and tempering to produce a steel wire with adjusted resistance. The blast chilling temperature ranges from 850 to 950 ° C, for example, and the quench temperature ranges from 420 to 500 ° C, for example. Cold roll forming is carried out using a cold winding machine to produce springs.
    [00112] Spring steel according to the present modality has excellent resistance to fatigue as well as excellent toughness and ductility. Thus, even when a cold forming process is employed to form springs, the plastic deformation of the spring steel is readily performed without breaks during forming. EXAMPLES
    [00113] The refining in the pan was carried out to produce molten steels having the chemical compositions shown in Tables 1 and 2. [Table 1]


    [Table 2]


    [Table 3]



    [00114] Specifically, in the refining in the pan, the molten steels were circulated for 10 minutes using an RH equipment. After refining the pan, deoxidation was performed. The "Order of addition" column in Table 3 shows the deoxidizers used and the order of addition. "Al ^ REM" indicates that after deoxidation was performed by adding Al, deoxidation was also performed by adding REM. "Al" indicates that only deoxidation with Al was performed without performing deoxidation with another deoxidizer (for example, REM). "REM ^ Al" indicates that REM deoxidation was performed and then Al deoxidation was performed. "Al ^ REM ^ Ca" indicates that Al deoxidation was performed and then REM deoxidation was performed, and finally it was performed deoxidation with Ca. Al metal was used for deoxidation with Al, a metal misch was used for deoxidation with REM, and a Ca-Si alloy and CaO flux: CaF2 = 50:50 (mass ratio) were used for deoxidation with Ca. The circulation time in Table 3 is the circulation time after the final deoxidizer has been added, that is, the deoxidation time with the added final deoxidizer. When the final deoxidizer added is REM, the deoxidation time with REM is indicated.
    [00115] The cast steel from Tests 1 to 47 shown in Tables 1 and 2 were subjected to refining under the conditions shown in Table 3. Specifically, in tests 1 to 33 and 35 to 47, refining in the pan was initially performed cast steel. On the other hand, for the cast steel of test n ° 34, refining in the pan was not carried out. In the column "Refining in the pan" in Table 3, "C" indicates that refining in the pan was performed on the cast steel of the corresponding number test and "NC" indicates that the refining in the pan was not performed. Refining in the pan was carried out under the same conditions for all test numbers.
    [00116] In the cases in which REM deoxidation was performed, the circulation times (deoxidation times) after adding REM were as shown in Table 3. By the steps described above, the test steels in 1 to 47 were produced .
    [00117] Using the cast steel produced, blocks (semi-finished products) having a cross section of 300 mm x 300 mm were produced by a continuous casting process. At that time, the molten steels inside the mold were agitated by electromagnetic agitation. The speeds (m / min) of the stirring flows of the molten steels within the mold in the horizontal direction during stirring were as shown in Table 3. Using one of the blocks produced from each test number, the RC cooling rate (° C / min) of the blocks of each test number was determined in the manner described above. The determined RC cooling rates are shown in Table 3.
    [00118] The blocks were heated up to 1200 to 1250 ° C. The heated blocks were subjected to the rolling of bars to produce bars having a cross section of 160 mm x 160 mm. The bars were heated to 1100 ° C or more. After heating, rebars (steel for springs) having a diameter of 15 mm were produced. [Evaluation test] [Preparation of the ultrasonic fatigue test specimens]
    [00119] For each test number, the ultrasonic fatigue test specimen illustrated in FIG. 3A was prepared as follows. The numerical values in FIG. 3A indicate dimensions (in mm) at the respective locations. "p3" indicates that the diameter is 3 mm.
    [00120] FIG. 3B is a view of a cross section (cross section perpendicular to the axis of the rebar) of the rebar 10 having a diameter of 15 mm. The broken line in FIG. 3B indicates the location where the raw test specimen 11 (a test specimen 1 mm larger than the shape illustrated in FIG. 3A) for the ultrasonic fatigue test specimen is cut. The longitudinal direction of the raw test specimen 11 was the longitudinal direction of the rebar 10. The raw test specimen 11 was cut at the cut location illustrated in FIG. 3B so that the loading portion of the ultrasonic fatigue test specimen does not include segregation at the center line of the rebar.
    [00121] The raw test specimens cut from the rebar of the respective test numbers were subjected to rapid cooling and tempering to adjust the Vickers (HV) hardness of the raw test specimens to 500 to 540. For all test numbers, the blast chilling temperature was 900 ° C and its retention time was 20 minutes. For test numbers in which the C content is greater than 0.50%, the tempering temperature was 430 ° C and the retention time was 20 minutes. For test numbers in which the C content was less than 0.50%, the tempering temperature was 410 ° C and its retention time was 20 minutes.
    [00122] After being heat treated as described above, crude test specimens were given substantially the same properties as coiled springs. Thus, these raw test specimens were used to evaluate the performance of the spring.
    [00123] After the heat treatment, the test specimens were subjected to a finishing process to prepare a plurality of ultrasonic fatigue test specimens having the dimensions illustrated in FIG. 3A for each test number.
    [00124] [Measurement of the TN number of crude oxide inclusions and maximum value Dmax.]
    [00125] The prepared ultrasonic fatigue test specimens were each cut along the axial direction to form a cross section containing the central axis. The cross section of each ultrasonic fatigue test specimen was mirror-polished. The Selective Potentiostatic Etching by Electrolytic Dissolution method (SPEED method) was performed on the polished cross section. In the cross section submitted to the SPEED method, 5 fields in the 10 mm diameter portion were freely selected. Each field was rectangular having a width of 2 mm in the radial direction and a length of 5 mm in the axial direction, with the center being located at an R / 2 depth from the surface of the ultrasonic fatigue test specimen (R is the radius , 5 mm in this example).
    [00126] Each field was observed using a scanning electron microscope (SEM) equipped with an X-ray dispersive energy microanalyser (EDX). Observation was performed at 1000x magnification. Inclusions were identified in the fields. Then, the chemical compositions of the inclusions identified were analyzed using EDX to identify Al-based oxides, complex oxides containing REM, and complex oxysulfides containing REM. In addition, the equivalent circular diameter of each of the identified inclusions was determined by image analysis. Based on the results of the analysis of the chemical compositions of the inclusions and the equivalent circular diameters of the inclusions, the TN numbers of the crude oxide inclusions and the maximum Dmax values of the oxide inclusions were determined. [Ultrasonic fatigue test]
    [00127] An ultrasonic fatigue test was conducted using the prepared ultrasonic fatigue test specimens. The test system used was an ultrasonic fatigue test system, USF-2000, produced by SHIMADZU CORPORATION. The frequency was adjusted to 20 kHz and the test stress was adjusted to 850 MPa to 1000 MPa. Six test specimens were used for each test number to perform the ultrasonic fatigue test. The maximum load at which a resonance greater than 107 cycles is possible is referred to as the fatigue strength (MPa) of the test number. [Vickers hardness test]
    [00128] A Vickers hardness test according to the JIS Z 2244 standard was conducted using the prepared ultrasonic fatigue test specimens. The test force was adjusted to 10 kgf = 98.07 N. The hardness was measured at three freely selected points in the 10 mm diameter portion on each ultrasonic fatigue test specimen and the mean value of the measurements was designated as the hardness Vickers (HV) of the test number. [Charpy impact test]
    [00129] Crude test specimens having a square cross section of 11 mm x 11 mm were prepared from the rebar of the respective test numbers. The raw test specimens were subjected to rapid cooling and tempering under the same conditions as the ultrasonic fatigue test specimens. Subsequently, they were subjected to a finishing process to prepare JIS No. 4 test specimens. In the finishing process, a U-shaped notch was formed. The depth of the U-shaped notch was 2 mm. A Charpy impact test in accordance with JIS Z 2242 was conducted using the prepared test specimens. The test temperature was room temperature (25 ° C). [Traction test]
    [00130] From the rebars of all test numbers, crude test specimens 1 mm larger than the shape of a round bar test specimen having a flat portion of 6 mm in diameter (corresponding to the test specimen n ° 14A) were prepared specified in JIS Z 2201). The raw test specimens were subjected to rapid cooling and tempering under the same conditions as those of the ultrasonic fatigue test specimens. Subsequently, they were subjected to a finishing process to prepare round bar test specimens. According to JIS 2241, a tensile test was conducted at room temperature (25 ° C) to determine elongation at fracture (%) and reduction in area (%). [Test results]
    [00131] The test results are shown in Table 4. Table 4



    [00132] In Table 4, in the "Results of the casting" column, "S" means that the casting was performed without causing the nozzles to clog. "F" means that the nozzle was clogged during casting. The "Main Inclusions" column lists the oxide inclusions that had an area fraction of not less than 5% in the five fields in the observation with an SEM. "REM-Al-O-S" refers to complex oxisulfides. "Al-O" refers to oxides based on Al. "MnS" refers to MnS. In tests Nos. 1 to 32 and 34 to 47, complex oxides having an area fraction of less than 5% were also present in the steels. .
    [00133] In relation to Table 4, in tests 1 to 32, the chemical compositions were adequate. In addition, in all of them, the TN number of crude oxide inclusions was not greater than 0.2 / mm2 and the maximum Dmax value between equivalent circular diameters of the oxide inclusions was not greater than 40 μ m. As a result, the fatigue strengths of tests 1 through 32 were all high, on the order of 950 MPa or more.
    [00134] In addition, the chemical compositions of tests in 5 to 10 included B. As a result, they had high Charpy impact values and showed excellent toughness compared to tests in 1 to 4 and 11 to 32.
    [00135] On the other hand, in test n ° 33, the chemical composition does not include REM. As a result, neither complex oxides nor complex oxisulfides were formed, and the TN number of crude oxide inclusions exceeded 0.2 / mm2 and also the maximum Dmax value of the oxide inclusions exceeded 40 μ m. Consequently, the fatigue strength was low at less than 950 MPa. In addition, in test No. 33, the chemical composition did not include Ti. As a result, the Charpy impact value was less than 40 x 104 J / m2 and the toughness was low. In addition, the elongation at the fracture was less than 9.5% and the reduction in the area was less than 50%.
    [00136] In test n ° 34, the O content was very high. As a result, the TN number was very high and the Dmax value was very large. Consequently, the fatigue strength was low at less than 950 MPa.
    [00137] In test 35, the chemical composition was adequate. However, circulation time during deoxidation with REM was very short. As a result, the maximum Dmax value has exceeded 40 μ m. Consequently, the fatigue strength was low at less than 950 MPa.
    [00138] In test n ° 36, the chemical composition was adequate. However, the electromagnetic agitation inside the mold was insufficient and the flow velocity inside the mold was less than 0.1 m / min. As a result, the TN number was very high. Consequently, the fatigue strength was low at less than 950 MPa.
    [00139] In test n ° 37, the REM content was excessively high. As a result, nozzle clogging occurred during continuous casting and, therefore, a semi-finished product cannot be produced.
    [00140] In test 38, the REM content was very high. As a result, crude oxide inclusions in the steel have increased, resulting in the excessively high TN number. Consequently, the fatigue strength was low at less than 950 MPa.
    [00141] In test n ° 39, the REM content was very low. As a result, neither complex oxides nor complex oxisulfides formed and therefore Al-based oxides became crude, resulting in an excessively high TN number. Consequently, the fatigue strength was low at less than 950 MPa. In addition, the very low REM content resulted in a low fracture elongation of less than 9.5% and a low reduction in the area of less than 50%. It is considered that a very low REM content caused the formation of TiS at the grain edges resulting in decreased ductility.
    [00142] In the tests in the 40 and 41, the Ti content was very high. Consequently, the fatigue strength was low at less than 950 MPa. Crude TiN is considered to have formed and this has resulted in decreased fatigue resistance.
    [00143] In test n ° 42, the chemical composition was adequate but the rate of RC cooling during continuous casting was very fast. As a result, the TN number was very high and the maximum Dmax was very large. Consequently, the fatigue strength was low at less than 950 MPa.
    [00144] In test n ° 43, the chemical composition was adequate but the RC cooling rate was very slow. As a result, the TN number was very high and the maximum Dmax was very large. Consequently, the fatigue strength was low at less than 950 MPa.
    [00145] In tests 44 to 46, none of the chemical compositions included REM. As a result, the TN number was very high and the maximum Dmax was very large. Consequently, the fatigue strength was low at less than 950 MPa.
    [00146] In addition, in test no. 45, the Ti content in the chemical composition was very low. As a result, the Charpy impact value was approximately 40 x 104 J / m2 and the toughness was low. In addition, the elongation at the fracture was less than 9.5% and the reduction in the area was less than 50%.
    [00147] In test n ° 47, the Ti content in the chemical composition was very low. As a result, the Charpy impact value was less than 40 x 104 J / m2 and the toughness was low. In addition, the elongation at the fracture was less than 9.5% and the reduction in the area was less than 50%. In the previous specification, an embodiment of the present invention has been described. However, it should be understood that the above modality is merely an illustrative example by which the present invention is implemented. Thus, the present invention is not limited to the above modality, and modifications of the above modality can be made properly without departing from the spirit and scope of the invention.
    权利要求:
    Claims (4)
    [0001]
    1. Spring steel, characterized by the fact that it has a chemical composition consisting of, in mass%: C: 0.4 to 0.7%, Si: 1.1 to 3.0%, Mn: 0.3 1.5%, P: equal to or less than 0.03%, S: equal to or less than 0.05%, Al: 0.01 to 0.05%, Rare metal: 0.0001 to 0.002 %, N: equal to or less than 0.015%, O: equal to or less than 0.0030%, Ti: 0.02 to 0.1%, Ca: 0 to 0.0030%, Cr: 0 to 2, 0%, Mo: 0 to 1.0%, W: 0 to 1.0%, V: 0 to 0.70%, Nb: 0 to less than 0.050%, Ni: 0 to 3.5%, Cu: 0 to 0.5%, and 8: 0 to 0.0050%, with the balance being Fe and impurities, with the number of oxide inclusions having an equivalent circular diameter equal to or greater than 5 μm equal to or less than 0.2 / mm2, the oxide inclusions each being one between Al-based oxide, complex oxide containing REM, O and Al, and complex oxysulfate containing REM, O, S and Al, and the value being maximum between equivalent circular diameters of the oxide inclusions is equal to or less than 40 μm.
    [0002]
    2. Spring steel, according to claim 1, characterized by the fact that the chemical composition includes Ca: 0.0001 to 0.0030%.
    [0003]
    3. Spring steel according to claim 1 or 2, characterized by the fact that the chemical composition includes one or more elements selected from the group consisting of: Cr: 0.05 to 2.0%, Mo: 0, 05 to 1.0%, W: 0.05 to 1.0%, V: 0.05 to 0.70%, Nb: 0.002 to less than 0.050%, Ni: 0.1 to 3.5%, Cu : 0.1 to 0.5%, and B: 0.0003 to 0.0050%.
    [0004]
    4. Method for the production of a spring steel, characterized by the fact that it comprises the steps of: refining the molten steel that presents the chemical composition, as defined in any one of claims 1 to 3; produce a semi-finished product from cast steel refined by a continuous casting process; and hot working the semi-finished product, the stage of refining the molten steel includes the steps of: performing the refining in the molten steel pan; deoxidize the molten steel using Al subsequent to the refining of the pan; and deoxidizing the molten steel using REM for at least 5 minutes after deoxidation with Al; and the step of producing the semi-finished product includes the steps of: stirring the molten steel within the mold to circulate the molten steel in a horizontal direction at a flow rate of 0.1 m / min or faster; and cool the semi-finished product being cast at a cooling rate of 1 to 100 ° C / min, the cooling rate being defined by the cooling rate from liquidus temperature to solidus temperature in a location at a depth T / 4; and where T is the thickness of the semi-finished product from the upper or lower surface of the semi-finished product.
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    同族专利:
    公开号 | 公开日
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    BR112016023912A2|2017-08-15|
    KR101830023B1|2018-02-19|
    CN106232849B|2018-01-30|
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    法律状态:
    2019-08-20| B06U| Preliminary requirement: requests with searches performed by other patent offices: suspension of the patent application procedure|
    2019-09-03| B25D| Requested change of name of applicant approved|Owner name: NIPPON STEEL CORPORATION (JP) |
    2020-12-08| B09A| Decision: intention to grant|
    2021-02-23| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 22/04/2015, OBSERVADAS AS CONDICOES LEGAIS. |
    优先权:
    申请号 | 申请日 | 专利标题
    JP2014-089420|2014-04-23|
    JP2014089420|2014-04-23|
    PCT/JP2015/002202|WO2015162928A1|2014-04-23|2015-04-22|Spring steel and method for producing same|
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