steel material

Abstract

The present invention relates to a steel material comprising, in mass%, 0.30%≤C≤0.45%, 0.10%≤Si≤1.00%, 0.60%≤Mn≤1.20%, 0.20%≤Cr≤0.70%, 0.30%≤V≤0.47%, Ti≤0.015%, P≤0.100%, and S≤0.080%, the balance being Fe and unavoidable impurities, and having a P0 value defined by P0=P0'×V / P1, satisfying P0≥0.30, where P0'=Mn+0.49Cu+0.89Ni+0.40Cr-0.30Si, and P1=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.17Ni+0.2Cr+V, in the formula, each element symbol represents the content of each element in mass%.

Description

Technical Field

[0001] This invention relates to a type of steel, and more specifically, to a type of steel that can be used to manufacture automotive parts without heat treatments such as quenching and tempering. Background Technology

[0002] The strength of steel can be increased by heat treatment such as quenching and tempering. However, from the viewpoint of simplifying manufacturing processes, non-heat-treated steels are widely used in automotive engine parts such as crankshafts and connecting rods. These steels are designed to achieve high strength even in the hot-forged state without heat treatment. For example, the composition of this type of non-heat-treated steel is disclosed in Patent Document 1.

[0003] Patent Document 1: WO 2019 / 203348A1 Summary of the Invention

[0004] In recent years, the application of non-heat-treated steel in high-power automotive engines has also been studied. With the increase in engine power output, there is a need to improve the strength of non-heat-treated steel. A widely used technique for improving the strength of non-heat-treated steel is the addition of v (V). Patent Document 1 also discloses that "precipitation strengthening of steel due to fine v carbides has been achieved by adding a large amount of v," and that "V, one of the elements that generates alloy carbides, has a large solid solution content in the steel, and the precipitation strengthening obtained by heating (approximately 1250°C) before hot forging is also large." On the other hand, Patent Document 1 discloses that "the solid solution content of v in steel is limited, so it is difficult to further improve the strength simply by increasing the v content." Therefore, in Patent Document 1, the steel contains both v and titanium (Ti) to further improve the strength. Patent Document 1 describes that Ti carbides precipitate first, and the Ti carbides act as nuclei for v carbides, resulting in finer and larger-volume precipitation of v carbides compared to the case where only v carbides precipitate. However, compared to V carbides, Ti carbides have a higher solution temperature and exhibit coarse precipitation in the austenite phase. Therefore, increasing the amount of Ti added is unlikely to further improve the strength of non-heat-treated steel.

[0005] The purpose of this invention is to provide steel with high strength without heat treatment.

[0006] To achieve the above objectives, the steel according to the present invention comprises, by weight percent:

[0007] 0.30% ≤ C ≤ 0.45%,

[0008] 0.10% ≤ Si ≤ 1.00%,

[0009] 0.60% ≤ Mn ≤ 1.20%,

[0010] 0.20% ≤ Cr ≤ 0.70%,

[0011] 0.30% ≤ V ≤ 0.47%,

[0012] Ti≤0.015%,

[0013] P≤0.100%, and

[0014] S≤0.080%,

[0015] The balance is Fe and unavoidable impurities, and

[0016] It has a P0 value defined by the following equation (1), satisfying P0≥0.30:

[0017] P0 = P0' × V / P1 (1)

[0018] Here,

[0019] P0' = Mn + 0.49Cu + 0.89Ni + 0.40Cr - 0.30Si (2)

[0020] P1=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.17Ni+0.2Cr+V(3)

[0021] In equations (1) to (3), the symbols for each element represent the content of each element in terms of mass % (%).

[0022] Steel can be used without heat treatment.

[0023] By weight percent, the steel may also contain at least one of the following:

[0024] 0% < Cu ≤ 0.50%,

[0025] 0% < Ni ≤ 0.50%,

[0026] 0% < Nb ≤ 0.010%,

[0027] 0% < Ti ≤ 0.015%,

[0028] 0% < Pb ≤ 0.30%,

[0029] 0% < Bi ≤ 0.20%,

[0030] 0% < Ca ≤ 0.0100%,

[0031] 0% < Zr ≤ 0.010%,

[0032] 0% < Mg ≤ 0.010%, and

[0033] 0% < Te ≤ 0.010%.

[0034] By mass percentage, the steel meets the following requirements:

[0035] Mo≤0.10%,

[0036] Al≤0.050%, and

[0037] N≤0.030%.

[0038] The value of P1 defined by equation (3) can satisfy 1.04≤P1≤1.15.

[0039] The steel may have a P2 value defined by equation (4) and a P3 value defined by equation (5), satisfying P2 / P3≥1.4:

[0040] P2=417-242C+30Si-25Mn-17Cu-22Ni-14Cr-35Mo(4)

[0041] P3=10^(1.35-0.54C+0.02Si+0.77Mn+0.47Cu+0.42Ni+0.52Cr+4.84Mo)(5)

[0042] In equations (4) and (5), the symbols for each element represent the content of each element in terms of mass % (%).

[0043] In the cross-section of steel in the hot-forged state, (V+Ti)≥30% and the area is 1μm. 2 The amount of the above precipitates can be per 1 mm 2 Less than 10.

[0044] Steel in the hot-forged state can have the following properties:

[0045] Yield point above 900MPa, and

[0046] Yield ratio of 0.80 or higher.

[0047] The steel according to the invention has a composition in which the contents of C, Si, Mn, Cr, and V are within predetermined ranges, the contents of Ti, P, and S are limited to below predetermined upper limits, and the P0 value obtained based on the content of the constituent elements satisfies P0 ≥ 0.30, thereby giving the steel high strength without heat treatment. In particular, the contents of Mn and Cr are set within such ranges that a high strength improvement effect is obtained through the fine precipitation of V-based carbides, while preventing the formation of bainite that reduces the yield ratio of the steel, thus giving the steel high strength without the addition of large amounts of V and Ti.

[0048] Here, by using steel without heat treatment, the heat treatment process is omitted, thus simplifying the manufacturing process of various components using steel. As described above, the steel according to the present invention has a predetermined composition, which enables the steel to have high strength even without heat treatment.

[0049] Furthermore, when the steel contains at least one element selected from Cu, Ni, Nb, Ti, Pb, Bi, Ca, Zr, Mg, and Te in predetermined amounts, it is possible to obtain the effect of increasing strength through Cu, Ni, Nb, and Ti and improving machinability through Pb, Bi, Ca, Zr, Mg, and Te.

[0050] In steel, by limiting the contents of Mo, Al, and N to below a predetermined upper limit, the formation of bainite due to excessive Mo content and the reduction of fatigue strength due to excessive Al and N content are prevented.

[0051] When the P1 value satisfies 1.04≤P1≤1.15, high hardness and strength improvement can be achieved in steel, while maintaining high machinability.

[0052] Furthermore, when the P2 and P3 values ​​satisfy P2 / P3≥1.4, the formation of bainite, which causes a decrease in yield ratio, is prevented in the steel, and a high strength increase effect is obtained.

[0053] In the cross-section of hot-forged steel, the number of precipitates with a predetermined composition and area is 1 mm. 2 In cases with fewer than 10, such non-heat-treated steel can be obtained, which prevents strength reduction due to the formation of coarse precipitates.

[0054] Furthermore, when the steel has a yield point of 900 MPa or higher and a yield ratio of 0.80 or higher in the hot-forged state, it has sufficiently high strength as a non-heat-treated steel for application in automotive engine parts, etc. Attached Figure Description

[0055] Figure 1A and Figure 1B Experimental results demonstrating the effect of adding Mn on improving the strength of steel are shown. The relationship between V content and yield point for two Mn contents is presented below. Figure 1A Furthermore, the relationship between the V content and the yield ratio is shown in... Figure 1B .

[0056] Figure 2A Experimental results showing the relationship between P0 value and yield ratio were presented, and Figure 2BExperimental results showing the relationship between P1 value and yield point are presented. Detailed Implementation

[0057] The steel according to the embodiments of the present invention will be described in detail below.

[0058] The steel according to an embodiment of the present invention comprises the following elements, with the balance being Fe and unavoidable impurities. The types, contents, and reasons for limitation of the constituent elements are as follows. The content is expressed in mass percent. Unless otherwise specified, the following properties are values ​​evaluated at room temperature (approximately 25°C). The steel according to an embodiment of the present invention can be used after heat treatment such as quenching and tempering. However, because it satisfies the following composition, the steel according to an embodiment of the present invention has high strength even in the untreated state, and the steel according to an embodiment of the present invention is preferably used in the untreated state, that is, in the hot-forged state without heat treatment.

[0059] [Content of each component element]

[0060] 0.30% ≤ C ≤ 0.45%

[0061] C contributes to the improvement of steel strength by forming carbides with V and Cr. From the viewpoint of fully obtaining the effect of improving strength, C is set to satisfy 0.30% ≤ C, and preferably 0.33% ≤ C.

[0062] On the other hand, excessive carbon content increases the formation of hard pearlite, leading to a decrease in the yield ratio and a deterioration in machinability of the steel. It also results in the formation of coarse precipitates containing carbides. From the viewpoint of preventing these phenomena, the carbon content is set to be C ≤ 0.45%, and preferably C ≤ 0.38%.

[0063] 0.10% ≤ Si ≤ 1.00%

[0064] Si has the effect of increasing the strength of steel and improving its machinability. From the viewpoint of fully utilizing these effects, Si is set to satisfy 0.10% ≤ Si, and preferably 0.40% ≤ Si.

[0065] On the other hand, excessive Si content leads to an excessive increase in hardness, resulting in a shortened die life for hot forging. To avoid this, Si should be set to ≤1.00%, and preferably ≤0.85%.

[0066] 0.60% ≤ Mn ≤ 1.20%

[0067] Mn has the effect of promoting the fine precipitation of V carbides in steel. The fine precipitation of the precipitates improves the strength of the steel. From the viewpoint of fully obtaining these effects, Mn is set to satisfy 0.60% ≤ Mn, and preferably 0.75% ≤ Mn.

[0068] On the other hand, Mn promotes the formation of bainite. With a large amount of bainite forming, the strength of the steel decreases due to the reduced yield ratio. From the viewpoint of avoiding these phenomena, Mn is set to satisfy Mn ≤ 1.20%, and preferably Mn ≤ 1.00%.

[0069] 0.20% ≤ Cr ≤ 0.70%

[0070] Similar to Mn, Cr also improves the strength of steel by promoting the fine precipitation of V carbides. In steel, from the viewpoint of obtaining sufficiently high strength, Cr is set to satisfy 0.20% ≤ Cr, and preferably 0.30% ≤ Cr.

[0071] On the other hand, similar to Mn, the formation of bainite reduces the yield ratio of the steel, which in turn causes a decrease in strength due to Cr. From the viewpoint of avoiding these phenomena, Cr is set to ≤ 0.70%, and preferably ≤ 0.50%.

[0072] 0.30% ≤ V ≤ 0.47%

[0073] V has the effect of increasing the strength of steel through the precipitation of carbides. From the viewpoint of obtaining sufficiently high strength, V is set to satisfy 0.30% ≤ V, and preferably 0.33% ≤ V.

[0074] On the other hand, when the V content is too high, the effect of improving the strength of the steel becomes saturated, and due to the formation of bainite, it is difficult to ensure sufficiently high strength. From the viewpoint of avoiding these phenomena, V is set to satisfy V ≤ 0.47%, preferably V ≤ 0.45%, and more preferably V ≤ 0.40%.

[0075] The steel according to an embodiment of the present invention comprises the predetermined amounts of C, Si, Mn, Cr, and V, with the balance being Fe and unavoidable impurities. Here, the steel according to an embodiment of the present invention may contain Ti, P, and S as unavoidable impurities, and the content of these impurities is limited to the following ranges.

[0076] Ti≤0.015%

[0077] Ti can form coarse carbides and carbonitrides. These coarse precipitates of carbides and carbonitrides reduce the strength of steel. Especially when containing a large amount of Ti, coarse precipitation is prone to occur in the austenite phase. Therefore, in the steel according to embodiments of the present invention, by limiting Ti to satisfying Ti ≤ 0.015%, and preferably Ti ≤ 0.010%, the reduction in strength due to the formation of coarse precipitates is prevented.

[0078] P≤0.100%, and S≤0.080%

[0079] P and S are elements that cause embrittlement due to grain boundary segregation in steel. From the viewpoint of avoiding the effects of grain boundary segregation, the content of P is limited to P ≤ 0.100%, and preferably P ≤ 0.080%, and the content of S is limited to S ≤ 0.080%, and preferably S ≤ 0.065%.

[0080] Furthermore, the steel according to embodiments of the present invention may contain Mo, Al and N as unavoidable impurities other than Ti, P and S, and the content of these elements is preferably limited to the following ranges.

[0081] Mo≤0.10%

[0082] Mo originates from the raw materials and therefore inevitably mixes into the steel, promoting the formation of bainite. The formation of bainite leads to a decrease in the strength of the steel. Therefore, from the viewpoint of preventing bainite formation, Mo is preferably limited to Mo ≤ 0.10%, and more preferably Mo ≤ 0.05%.

[0083] Al≤0.050%

[0084] Al forms coarse Al₂O₃ inclusions in steel. These Al₂O₃ inclusions reduce the fatigue strength of the steel. From the viewpoint of preventing the formation of Al₂O₃ inclusions, the Al content is preferably limited to Al ≤ 0.050%, and more preferably Al ≤ 0.030%.

[0085] N≤0.030%

[0086] Nitrogen (N) forms coarse nitride inclusions in steel. These nitride inclusions reduce the fatigue strength of the steel. From the viewpoint of preventing the formation of nitride inclusions, N is preferably limited to N ≤ 0.030%, and more preferably N ≤ 0.015%.

[0087] Besides Ti, P, S, Mo, Al, and N, examples of unavoidable impurities that may be included in steel according to embodiments of the present invention include Co ≤ 0.03%, As ≤ 0.010%, Sn ≤ 0.010%, Sb ≤ 0.010%, etc. Furthermore, the total amount of unavoidable impurities is preferably limited to 3.0% or less.

[0088] In addition to the essential elements mentioned above, the steel according to embodiments of the present invention may optionally contain one, two, or more elements selected from the following elements. The content of each element, the reasons for the limitation, etc., are as follows.

[0089] 0% < Cu ≤ 0.50%, and 0% < Ni ≤ 0.50%

[0090] Cu and Ni have the effect of causing fine precipitation of V carbides in steel. This fine precipitation improves the strength of the steel. Even small amounts of Cu and / or Ni are added, showing a high improvement in steel strength due to promoting fine precipitation; therefore, there is no particular limit to the lower limit of Cu and Ni content. From the viewpoint of obtaining particularly high effects, Cu and Ni can be set to satisfy 0.06% ≤ Cu and / or 0.03% ≤ Ni. Incidentally, Cu less than 0.06% and Ni less than 0.03% can be considered unavoidable impurities.

[0091] On the other hand, Cu and Ni promote the formation of bainite. With the formation of a large amount of bainite, the strength of the steel decreases due to the reduced yield ratio. Furthermore, since Cu and Ni are expensive elements, the cost of the steel increases with the addition of large amounts of Cu and Ni. From the viewpoint of avoiding these phenomena, Cu is set to satisfy Cu ≤ 0.50%, and preferably Cu ≤ 0.25%, and Ni is set to satisfy Ni ≤ 0.50%, and preferably Ni ≤ 0.25%.

[0092] 0% < Nb ≤ 0.010%, and 0% < Ti ≤ 0.015%.

[0093] Nb and Ti contribute to improving the strength of steel by forming carbides and carbonitrides. Since even small amounts of Nb and Ti exhibit significant strength-improving effects, there are no particular restrictions on the lower limits of their content. From the viewpoint of achieving particularly high effects, Nb and Ti can be set to satisfy 0.001% ≤ Nb and / or 0.001% ≤ Ti.

[0094] However, when coarse precipitates are formed as carbides or carbonitrides of Nb and / or Ti, the strength of the steel decreases. From the viewpoint of avoiding strength reduction, Nb is set to satisfy Nb ≤ 0.010%, and preferably Nb ≤ 0.007%, and Ti is set to satisfy Ti ≤ 0.015%, and preferably Ti ≤ 0.010%. Incidentally, Ti is also an unavoidable impurity as described above, and the content of Ti as an unavoidable impurity is limited to satisfy Ti ≤ 0.015%. However, the content of Ti as an unavoidable impurity varies depending on the raw materials used to manufacture the steel. When the content of Ti as an unavoidable impurity is low, Ti can be added to utilize the strength-improving effect of Ti, as long as Ti is within the range of Ti ≤ 0.015%.

[0095] 0% < Pb ≤ 0.30%, 0% < Bi ≤ 0.20%, 0% < Ca ≤ 0.0100%, 0% < Zr ≤ 0.010%, 0% < Mg ≤ 0.010%, and 0% < Te ≤ 0.010%.

[0096] Any one of Pb, Bi, Ca, Zr, Mg, and Te has the effect of improving the machinability of steel. Since even small amounts of any of these elements exhibit a high improvement in machinability, there is no particular limitation on the lower limit of their content. From the viewpoint of obtaining particularly high machinability, the content of Pb and / or Bi can be set to 0.02% or more, more preferably 0.05% or more; the content of Ca can be set to 0.0005% or more, more preferably 0.0010% or more; and the content of Zr, Mg, and / or Te can be set to 0.001% or more.

[0097] On the other hand, when steel contains large amounts of Pb, Bi, Ca, Zr, Mg, or Te, hot workability and fatigue strength may decrease. Therefore, from the viewpoint of ensuring high hot workability, the upper limits of the content of each element are determined as described above. To further improve this effect, the content of Pb is preferably set to 0.20% or less, the content of Bi is preferably set to 0.15% or less, and the content of Ca is preferably set to 0.0050% or less.

[0098] [The relationship between the content of constituent elements]

[0099] Next, the relationship between the contents of the constituent elements will be described. In the following mathematical formulas defining the relationship between the contents of the constituent elements, each element symbol represents the content of that element, expressed in mass percent. If the steel does not contain any elements other than the essential elements, the content of these elements in the formulas will be considered zero.

[0100] In the steel according to an embodiment of the present invention, the P0 value obtained based on the following formula (1) satisfies P0≥0.30.

[0101] P0 = P0' × V / P1 (1)

[0102] Here, the values ​​of P0' and P1 in equation (1) are defined by equations (2) and (3) respectively.

[0103] P0' = Mn + 0.49Cu + 0.89Ni + 0.40Cr - 0.30Si (2)

[0104] P1=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.17Ni+0.2Cr+V(3)

[0105] The Mn, Cu, Ni, and Cr contained in the limiting formula (2) of P0' promote the fine precipitation of V precipitates by lowering the ferrite phase transformation temperature. On the other hand, Si prevents the fine precipitation of V precipitates. Formula (2) represents the sum of the contents of each element taking into account the contribution, and P0' serves as an indicator of the degree to which the fine precipitation of V precipitates is promoted. The larger the value of P0', the greater the effect of promoting the fine precipitation of V precipitates. Furthermore, the molecule in formula (1) is obtained by multiplying the value of P0' by the content of V, and the larger the value of the molecule, the more it promotes the precipitation of fine V carbides, thereby exhibiting a greater effect on improving the strength of the steel.

[0106] On the other hand, the constituent elements included in the limiting formula (3) of P1 improve the hardness and tensile strength of the steel. In the limiting formula (1) of P0, P1 is the denominator, and when the value of P0' increases relative to the value of P1 and the value of P0 increases, the yield ratio is improved relative to the tensile strength of the steel due to the increase in yield point caused by promoting the fine precipitation of V precipitates.

[0107] As described above, as the P0 value defined by equation (1) increases, the effect of improving strength due to the fine precipitation of V carbides is enhanced. In particular, the effect of improving the yield ratio becomes higher. In the steel according to the embodiment of the present invention, since P0 ≥ 0.30, a high yield ratio of 0.80 or higher can be obtained. The P0 value is preferably P0 ≥ 0.35, and more preferably P0 ≥ 0.40. The higher the effect of improving the yield ratio, the more preferred. Therefore, no specific upper limit is determined for the P0 value.

[0108] As long as the overall P0 value satisfies P0≥0.30, P0' and P1 can individually take any value. However, it is preferable that P1 satisfies 1.04≤P1≤1.15. As mentioned above, each element contained in the limiting formula (3) of P1 has the effect of improving the strength (tensile strength) and hardness of the steel. Therefore, when 1.04≤P1, a high effect of improving the strength of the steel is obtained, especially a high effect of improving the yield point, and a high yield point such as 900MPa or above is easily achieved. A P1 value of 1.05≤P1 is more preferable.

[0109] On the other hand, the elements contained in the limiting formula (3) of P1 reduce the machinability of the steel. Therefore, when P1 ≤ 1.15, high machinability of the steel can be ensured. A more preferred value for P1 is P1 ≤ 1.10.

[0110] Furthermore, in the steel according to the embodiments of the present invention, it is preferable that the P2 value obtained based on the following formula (4) and the P3 value obtained based on the following formula (5) satisfy P2 / P3≥1.4.

[0111] P2=417-242C+30Si-25Mn-17Cu-22Ni-14Cr-35Mo(4)

[0112] P3=10^(1.35-0.54C+0.02Si+0.77Mn+0.47Cu+0.42Ni+0.52Cr+4.84Mo)(5)

[0113] The P2 calculated based on equation (4) is essentially equivalent to the temperature range from the ferrite transformation point to 500°C. The larger the P2 value, the easier it is for the ferrite-pearlite transformation to be completed, and the formation of bainite is prevented. On the other hand, the P3 calculated based on equation (5) is essentially equivalent to the cooling time (critical cooling time) of up to 500°C required to complete the ferrite transformation. The critical cooling rate is obtained by dividing the temperature range represented by P2 by the critical cooling time represented by P3.

[0114] Therefore, a high level of bainite formation prevention is achieved when P2 / P3 ≥ 1.4. P2 / P3 is more preferably ≥ 1.7, and even more preferably ≥ 1.9. The larger the P2 / P3 value, the more stable the ferrite-pearlite metallographic structure; therefore, no specific upper limit is set for P2 / P3.

[0115] [Properties of Steel]

[0116] The steel according to embodiments of the present invention has the above-described composition, resulting in high strength even in the non-heat-treated state. In particular, by controlling the content of elements such as Mn and Cr, which have properties that promote the formation of bainite (a metallographic structure that reduces steel strength) and the fine precipitation of V precipitates, and by controlling the balance between the content of these elements and other elements, a high degree of improvement in steel strength is achieved. Therefore, the steel according to embodiments of the present invention is suitable for use as a ferritic-pearlitic non-heat-treated steel for manufacturing automotive engine parts, etc.

[0117] According to embodiments of the present invention, the steel in its non-heat-treated state after hot forging tends to have a yield point of 900 MPa or more and a yield ratio of 0.80 or more, which corresponds to having the above-mentioned composition, particularly containing predetermined amounts of Mn, Cr, and V. More preferably, the steel can have a yield point of 920 MPa or more and a yield ratio of 0.81 or more. Steel with a high yield point and a high yield ratio indicates that the steel has high strength. The increase in yield point and yield ratio due to the addition of Mn is also shown in the embodiments described below. Here, the yield point and yield ratio can be evaluated by tensile testing according to JIS Z 2241:2011 with a yield strength of 0.2% and a yield strength ratio of 0.2% (the ratio of 0.2% yield strength to tensile strength), respectively. In the evaluation, hot forging can be performed, for example, at a heating temperature of 1,250°C and a reduction of area of ​​70%. Since higher strength of the steel is preferred, there is no specific upper limit to the strength of the steel.

[0118] As described above, in the steel according to embodiments of the present invention, high strength is achieved through the fine precipitation of V precipitates, and the size of the precipitates also reflects the strength. The less coarse precipitates containing V are formed, the higher the strength of the steel. For example, in the cross-section of the steel in the non-heat-treated state after hot forging, (V+Ti)≥30% and the area is 1μm. 2 The preferred amount of precipitates (with a square root area of ​​1 μm or more) is per 1 mm 2 The number of forgings is 10 or less, more preferably 8 or less, and even more preferably 5 or less. The hot forging can be evaluated, for example, at a heating temperature of 1,250°C and a reduction of area of ​​70%.

[0119] While this does not preclude the use of the steel according to embodiments of the present invention after heat treatment, as mentioned above, the steel possesses high strength even in its non-heat-treated state. Therefore, from the viewpoint of simplifying the manufacturing process of the product, it is preferable to use this steel as a non-heat-treated steel without heat treatment after hot forging. There are no particular limitations on the conditions for hot forging, but it is preferable to exemplify a mode in which the forging heating temperature is 1,100°C to 1,260°C and the reduction of area is 50% to 95%.

[0120] Example

[0121] The present invention will now be described in more detail with reference to embodiments.

[0122] [1] Strength improved by adding Mn

[0123] First, the effect of adding Mn on improving strength was confirmed, and its mechanism was examined.

[0124] [Sample Preparation]

[0125] Non-heat-treatable steel samples containing 0.4% or 0.9% Mn and 0.1% to 0.4% V were prepared. The constituent elements other than Mn and V and their contents were: C: 0.35%, Si: 0.50%, P: 0.05%, Cu: 0.12%, Ni: 0.07%, and Cr: 0.30%, with the balance being Fe and unavoidable impurities. For each non-heat-treatable steel sample preparation, steel with a predetermined composition was melted in a vacuum induction furnace and then cast into ingots. The obtained ingots were subjected to rough forging and final forging to control mechanical properties, and then evaluated. As a final forging to control mechanical properties, round bars were forged from 40D to 22D at a heating temperature of 1,250°C (reduction of area: 70%). The forging end temperature was set above 1,050°C, and the cooling rate was set to an average of 1.4°C / s to 1.6°C / s between 600°C and 800°C.

[0126] [Testing Method]

[0127] Test pieces (No. JIS 14A) were prepared from the obtained non-heat-treated steel, and tensile tests were performed in air at room temperature according to JIS Z2241:2011 to evaluate the yield point (0.2% yield strength) and tensile strength. Furthermore, the ratio of yield point to tensile strength was calculated as the yield ratio (0.2% yield strength ratio).

[0128] [Test Results]

[0129] Figure 1A and Figure 1BThe results of yield point and yield ratio measurements are shown separately. In the figure, the horizontal axis represents the V content, and the vertical axis represents the yield point or yield ratio value. Furthermore, a Mn content of 0.4% is indicated by a circle, and a Mn content of 0.9% is indicated by a square. Figure 1A Regardless of the Mn content, the yield point increases linearly with increasing V content. According to... Figure 1B The yield ratio also increases with increasing V content. These results indicate that the strength of non-heat-treated steel is improved by adding V, and further increasing the amount of V added. This is because the addition of V induces the precipitation of fine V carbides and precipitation strengthening.

[0130] Next, we compared the cases with an Mn content of 0.4% and 0.9%. With an Mn content of 0.9%, the yield point and yield ratio were higher across the entire range of V content. This indicates that increasing the Mn content improves the strength of the non-heat-treated steel. This is believed to be because adding Mn lowers the phase transformation temperature of ferrite, thereby promoting the fine precipitation of V precipitates such as V carbides.

[0131] Furthermore, when comparing the effects of increased Mn content on different V contents, the improvement range of yield point and yield ratio increased as the Mn content increased from 0.4% to 0.9%. For example, the yield point improvement was 55 MPa when the V content was 0.1%, and increased to 92 MPa when the V content was 0.4%. Additionally, the yield ratio improvement was 0.013 when the V content was 0.1%, and increased to 0.027 when the V content was 0.4%. That is, it can be considered that increasing the Mn content amplifies the strength improvement effect caused by increasing the V content. In particular, the effect of increasing the yield ratio is enhanced.

[0132] In this way, by adding V to non-heat-treated steel, the strength of the steel is improved due to the formation of fine precipitates. Furthermore, by adding Mn, the fine precipitation of V precipitates is promoted, thereby further improving the strength. It has been confirmed that not only M achieves this effect, but Cr, Ni, and Cu also achieve similar effects.

[0133] [2] Properties of non-heat-treated steel with various compositions

[0134] Next, non-heat-treated steels with various compositions were prepared, and their properties were evaluated.

[0135] [Sample Preparation]

[0136] Non-heat-treated steels according to the respective examples and comparative examples were prepared, having the compositional composition (unit: mass %) shown in Tables 1 to 3 below (balance being Fe and unavoidable impurities). The preparation method and hot forging conditions of the non-heat-treated steels were the same as those in the tests described above [1].

[0137] [Testing Method]

[0138] (1) Evaluation of metallographic structure and precipitates in steel

[0139] The cross-section of the hot-forged sample was observed using a scanning electron microscope (SEM). In this case, the region corresponding to half the radius from the center of the sample cross-section was observed. The type of metallographic structure formed in this region was determined. In the various embodiments and comparative examples, the observed metallographic structure was ferrite-pearlite (F+P) or ferrite-pearlite plus bainite (F+P+B). Furthermore, for samples with an F+P structure, the microstructure was evaluated per 1 mm using automated SEM analysis. 2 The content of (V+Ti) ≥ 30% and the area is 1 μm 2 The above refers to the quantity of precipitates (coarse precipitates).

[0140] (2) Evaluation of strength

[0141] The yield point and yield ratio of each sample were evaluated in the same manner as the test described above [1].

[0142] [Test Results]

[0143] Tables 1, 2, and 3 below show the composition of the non-heat-treated steels according to Examples 1 to 43 and Comparative Examples 1 to 14. Furthermore, Tables 4, 5, and 6 show the values ​​of P0 to P3 and P2 / P3 calculated based on the composition from equations (1) to (5) above, and the evaluation results of each test. In addition, for these samples where the content of each element meets the content range specified in the embodiments of the present invention above, i.e., for each Example and Comparative Examples 1 to 5, 10, 13, and 14, the relationship between the P0 value and the yield ratio is shown below. Figure 2A The relationship between P1 value and yield point is shown in... Figure 2B . Figure 2A and Figure 2B An approximate straight line is also shown.

[0144]

[0145]

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[0147]

[0148]

[0149]

[0150] according to Figure 2A A strong correlation was observed between the P0 value and the yield ratio, and it was found that as the P0 value increases, the yield ratio increases, resulting in higher material strength. As mentioned above, the P0 value can be used as a suitable indicator of the strength of non-heat-treated steel. It was found that when the P0 value is set above 0.30, a yield ratio of approximately 0.80 or higher can be obtained.

[0151] In addition, according to Figure 2B A strong correlation was observed between the P1 value and the yield point, and it was found that as the P1 value increases, the yield point increases, resulting in higher material strength. As mentioned above, the P1 value can also be used as a suitable indicator of the strength of non-heat-treated steel. It was found that when the P1 value is set to 1.04 or higher, a yield point of approximately 900 MPa or higher can be obtained.

[0152] According to Tables 1 to 4, in each embodiment where the content of each component element meets the content range specified in the above-described embodiments of the present invention and satisfies P0 ≥ 0.30, a ferrite-pearlite structure is obtained, and the number of coarse precipitates is limited to 10 per mm. 2 Furthermore, the yield point obtained was above 900 MPa, and the yield strength ratio was above 0.80, thus confirming that the non-heat-treated steel has high strength. In the examples, samples with higher contents of Mn, Cu, Cr, and Ni, as well as with higher P0 and P1 values, tended to have higher yield points, higher yield strength ratios, and higher material strength.

[0153] On the other hand, in each comparative example, the content of any component element does not meet the content range specified in the embodiments of the present invention described above, and / or does not meet P0 ≥ 0.30. In Comparative Examples 1 to 5, 10, 13 and 14, the content of each component element meets the predetermined range, but P0 < 0.30. Therefore, except for Comparative Examples 2 and 4, the yield point is less than 900 MPa and the yield ratio is less than 0.80.

[0154] In Comparative Example 6, the Ti content was greater than 0.015%, and the P0 value was less than 0.30. Therefore, per 1 mm 2In Comparative Example 7, the number of coarse precipitates exceeded 10, and the yield ratio did not reach 0.80. In Comparative Example 8, the C content exceeded 0.45%. Therefore, the yield ratio did not reach 0.80. In Comparative Example 9, the V content did not reach 0.30%, so the yield point obtained was less than 900 MPa, and the yield ratio was less than 0.80. It can be considered that the effect of improving strength through V precipitates was not sufficiently obtained. In Comparative Example 9, the Mn content was less than 0.60%, and the yield ratio was less than 0.80. On the other hand, in Comparative Example 11, with a Cr content greater than 0.70%, and Comparative Example 12, with a Mn content greater than 1.20%, the metallographic structure contained bainite, and the yield ratio was significantly less than 0.80. In Comparative Example 12, the yield point was also significantly lower than 900 MPa. Mn and Cr have the effect of improving strength by promoting the fine precipitation of V precipitates, but at the same time, they promote the formation of bainite, resulting in a decrease in strength. The results show that high strength cannot be obtained in non-heat-treated steel due to excessive addition of Mn and Cr.

[0155] The embodiments and examples of the present invention have been described above. The present invention is not particularly limited to these embodiments and examples, and various modifications can be made.

[0156] This application is based on Japanese Patent Application No. 2021-199356 filed on December 8, 2021 and Japanese Patent Application No. 2022-182428 filed on November 15, 2022, the contents of which are incorporated herein by reference.

Claims

1. A type of steel, It comprises, by weight %: 0.30%≤C≤0.45%， 0.50%≤Si≤1.00%, 0.60%≤Mn≤1.20%, 0.20%≤Cr≤0.70%， 0.33%≤V≤0.47%， Ti≤0.009%, P≤0.100%， S≤0.080%， 0.06%≤Cu≤0.50%, 0% < Ni ≤ 0.50%, 0% < Mo ≤ 0.10%, 0% < Al ≤ 0.050%, and 0%＜N≤0.030%， The balance is Fe and unavoidable impurities, and It has a P0 value defined by the following equation (1), satisfying P0≥0.35: P0 = P0' × V / P1 (1) Here, P0' = Mn + 0.49Cu + 0.89Ni + 0.40Cr - 0.30Si (2) P1 = C + 0.07Si + 0.16Mn + 0.61P + 0.19Cu + 0.17Ni + 0.2Cr + V (3) Having a P2 value defined by equation (4) and a P3 value defined by equation (5), satisfying P2 / P3≥1.4: P2 = 417 - 242C + 30Si - 25Mn - 17Cu - 22Ni - 14Cr - 35Mo (4) P3 = 10^(1.35 - 0.54C + 0.02Si + 0.77Mn + 0.47Cu + 0.42Ni + 0.52Cr +4.84Mo) (5) In equations (1) to (5), the symbols for each element represent the content of each element in terms of mass % (%). The value of P1, as defined by equation (3), satisfies 1.04 ≤ P1 ≤ 1.

15. In the cross-section of the steel in the hot-forged state, (V + Ti) ≥ 30% and the area is 1 μm. 2 The above amount of precipitates is per 1 mm 2 Less than 10.

2. The steel according to claim 1, wherein, The steel is used without heat treatment.

3. The steel according to claim 1 or 2, In terms of mass percentage, it also includes at least one of the following: 0% < Nb ≤ 0.010%, 0% < Ti ≤ 0.015%, 0% < Pb ≤ 0.30%, 0% < Bi ≤ 0.20%, 0% < Ca ≤ 0.0100%, 0%＜Zr≤0.010%， 0% < Mg ≤ 0.010%, and 0% < Te ≤ 0.010%.

4. The steel according to claim 1 or 2, which, in the hot-forged state, has: Yield point above 900 MPa, and Yield ratio of 0.80 or higher.

5. The steel according to claim 3, which has the following characteristics in the hot-forged state: Yield point above 900 MPa, and Yield ratio of 0.80 or higher.

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