Steel material for cold forging

Abstract

To provide a steel material capable of sufficiently securing surface layer hardness, crystal grain coarsening resistance and cold forgeability even if a refining process and a spheroidizing annealing process are omitted.SOLUTION: The steel for cold forging has a predetermined component composition, a carbon equivalent Ceq satisfying the following formula (1), a solid-solution N index Fx1 satisfying the following formula (2), and a predicted value DRe of deformation resistance during cold forging satisfying the following formula (3). Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 ≥ 0.44 Fx1 = [N] - [Ti] * 14.01 / 47.88 - [B] * 14.01 / 10.82 ≥ 0.0010 DRe = 2744 * Fx1 + 526 * Ceq + 398 ≤ 700 SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to cold forging steel materials. [Background technology]

[0002] Mechanical structural parts (gears, shafts, steering racks, etc.) for automobiles, construction machinery, etc. are manufactured by forming steel material into the part shape. After forming the mechanical structural parts, they undergo quenching and tempering (thermal refining process) and induction hardening processes to ensure sufficient strength (for example, surface hardness).

[0003] Patent Document 1 proposes a rolled steel material that can obtain high strength without carrying out a tempering treatment, in order to omit the tempering treatment that causes an increase in costs.

[0004] On the other hand, cold forging, which allows for the omission of machining such as cutting, is mainly used as a part forming method. Cold forging has problems such as high deformation resistance of steel materials and the tendency for steel materials to crack. Therefore, spheroidizing annealing is generally performed before cold forging to solve these problems. From the viewpoint of energy conservation, there is also a demand for shortening the time for this spheroidizing annealing. Patent Documents 2 to 4 propose steel materials for cold working in which the time for spheroidizing annealing can be shortened or even omitted. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2010-168624 [Patent Document 2] Patent Publication No. 2018-003106 [Patent Document 3] International Publication No. WO2015 / 189978 [Patent Document 4] Patent Publication No. 2001-042357 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, from the viewpoint of further reducing part manufacturing costs and protecting the environment, there has been a demand for omitting both the tempering process and the spheroidizing annealing process. Omitting the tempering process can reduce the surface hardness after induction hardening (hereinafter also simply referred to as "surface hardness") and the grain coarsening resistance, which affects part strength, while omitting the spheroidizing annealing process can reduce cold forgeability. In the prior art such as those disclosed in Patent Documents 1 to 4, omitting both processes while sufficiently ensuring various properties has not been considered.

[0007] The present disclosure has been made in view of the above circumstances, and one of its objectives is to provide a steel material that can sufficiently ensure surface hardness, grain coarsening resistance, and cold forgeability even when the thermal refining process and the spheroidizing annealing process are omitted. [Means for solving the problem]

[0008] Aspect 1 of the present invention is C: 0.30~0.40% by mass, Si:0.01~0.30% by mass, Mn: 0.50~1.00% by mass, P: 0.030% by mass or less (including 0% by mass), S: 0.030 mass% or less (including 0 mass%), Cr:0.01~0.30% by mass, Al: 0.020~0.100% by mass, Ti:0.0001~0.0040% by mass, B: 0.0010 to 0.0050 mass%; and N:0.0020~0.0100% by mass and the balance being Fe and inevitable impurities, The carbon equivalent Ceq satisfies the following formula (1), The solute N index Fx1 satisfies the following formula (2), A steel material for cold forging, in which the predicted value DRe of deformation resistance during cold forging satisfies the following formula (3). Ceq=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5≧0.44···(1) Fx1=[N]-[Ti]×14.01 / 47.88-[B]×14.01 / 10.82≧0.0010···(2) DRe=2744×Fx1+526×Ceq+398≦700···(3) In formulas (1) and (2), [C], [Si], [Mn], [Ni], [Cr], [N], [Ti], and [B] represent the contents of C, Si, Mn, Ni, Cr, N, Ti, and B, respectively, expressed in mass%.

[0009] Aspect 2 of the present invention is Cu: 0.001 to 0.100 mass%; and Ni: 0.001~0.100% by mass The steel material for cold forging according to aspect 1 further contains at least one selected from the group consisting of: [Effects of the Invention]

[0010] According to an embodiment of the present invention, it is possible to provide a steel material that can sufficiently ensure surface hardness, grain coarsening resistance, and cold forgeability even when the thermal refining process and spheroidizing annealing process are omitted.

[0011] The present inventors have conducted research from various angles to realize a steel material that can sufficiently ensure surface hardness after induction hardening (hereinafter also simply referred to as "surface hardness"), grain coarsening resistance, and cold forgeability (low deformation resistance and high drawing ability) even when the tempering process and spheroidizing annealing process are omitted. As a result, they have found that by adjusting the composition to a predetermined level and then appropriately controlling the carbon equivalent Ceq and the solute N index Fx1, it is possible to realize a steel material that can sufficiently ensure surface hardness, grain coarsening resistance, and cold forgeability even when the tempering process and spheroidizing annealing process are omitted. The following provides details of each requirement stipulated by the embodiment of the present invention.

[0012] <1. Steel composition> The cold forging steel material according to an embodiment of the present invention preferably contains C: 0.30 to 0.40 mass%, Si: 0.01 to 0.30 mass%, Mn: 0.50 to 1.00 mass%, P: 0.030 mass% or less (including 0 mass%), S: 0.030 mass% or less (including 0 mass%), Cr: 0.01 to 0.30 mass%, Al: 0.020 to 0.100 mass%, Ti: 0.0001 to 0.0040 mass%, B: 0.0010 to 0.0050 mass%, and N: 0.0020 to 0.0100 mass%, with the balance being Fe and unavoidable impurities. Each component will be described in detail below.

[0013] (C:0.30~0.40% by mass) C is an element effective in ensuring the hardness of the steel material, and too little C leads to insufficient strength in the final product. Therefore, the C content is set to 0.30% by mass or more, preferably 0.31% by mass or more, and more preferably 0.32% by mass or more. On the other hand, if the C content is excessive, the hardness of the steel material becomes excessively high and the cold forgeability (mainly the drawing characteristic) deteriorates. Therefore, the C content is set to 0.40% by mass or less, preferably 0.39% by mass or less, and more preferably 0.38% by mass or less.

[0014] (Si:0.01~0.30% by mass) Si is effective as a deoxidizer and also contributes to improving strength by ferrite solid solution strengthening. To effectively exert these effects, the Si content is set to 0.01 mass% or more, preferably 0.03 mass% or more, and more preferably 0.05 mass% or more. On the other hand, if the Si content is excessive, the hardness of the steel material becomes excessively high and cold forgeability deteriorates. Therefore, the Si content is set to 0.30 mass% or less, preferably 0.27 mass% or less, and more preferably 0.25 mass% or less.

[0015] (Mn:0.50~1.00% by mass) Mn is an element that improves the hardenability of steel, acts as a deoxidizer, and reduces the amount of oxide-based inclusions in steel, thereby improving internal quality. To effectively exert these effects, the Mn content is set to 0.50% by mass or more, preferably 0.53% by mass or more, and more preferably 0.55% by mass or more. On the other hand, if the Mn content is excessive, the hardness of the steel becomes excessively high and the cold forgeability deteriorates. Therefore, the Mn content is set to 1.00% by mass or less, preferably 0.95% by mass or less, and more preferably 0.90% by mass or less.

[0016] (P: 0.030% by mass or less (including 0% by mass)) P is an element that is inevitably contained in steel. P segregates at grain boundaries and reduces the impact properties of mechanical parts. Therefore, the P content is set to 0.030% by mass or less, preferably 0.027% by mass or less, and more preferably 0.025% by mass or less. On the other hand, the production cost increases as the purity is increased to eliminate P, so the P content is preferably 0.001% by mass or more, more preferably 0.003% by mass or more, and even more preferably 0.005% by mass or more. In this specification, "including 0% by mass" means that it includes embodiments in which it is not intentionally added, i.e., cases in which the content is below the level of unavoidable impurities (it does not exclude cases in which it is intentionally added).

[0017] (S: 0.030% by mass or less (including 0% by mass)) Like P, S is an element that is inevitably contained in steel. In steel, S combines with Mn to form MnS, which reduces cold forgeability. Therefore, the S content is set to 0.030% by mass or less, preferably 0.027% by mass or less, and more preferably 0.025% by mass or less. On the other hand, the production cost increases as the purity is increased to eliminate S, so the S content is preferably 0.001% by mass or more, more preferably 0.003% by mass or more, and even more preferably 0.005% by mass or more.

[0018] (Cr:0.01~0.30% by mass) Cr is an element that improves the hardenability of steel and is effective in ensuring a stable hardened layer depth and sufficient core hardness. Cr also combines with C to form carbides, reducing the amount of solute C and contributing to improved cold forgeability. To effectively exert this effect, the Cr content is set to 0.01% by mass or more, preferably 0.03% by mass or more, and more preferably 0.05% by mass or more. On the other hand, if the Cr content is excessive, the steel hardness becomes excessively high and the cold forgeability deteriorates. Therefore, the Cr content is set to 0.30% by mass or less, preferably 0.27% by mass or more, and more preferably 0.25% by mass or less.

[0019] (Al:0.020~0.100% by mass) Al acts as a deoxidizer, reducing the amount of oxide-based inclusions and improving the internal quality of steel. Furthermore, Al combines with N to form AlN, thereby suppressing grain coarsening. To effectively exert this effect, the Al content is set to 0.020% by mass or more, preferably 0.023% by mass or more, and more preferably 0.025% by mass or more. On the other hand, excessive Al content reduces cold forgeability. Therefore, the Al content is set to 0.100% by mass or less, preferably 0.090% by mass or less, and more preferably 0.080% by mass or less.

[0020] (Ti:0.0001~0.0040% by mass) Ti combines with C and N in the steel to form carbonitrides, improving grain coarsening resistance. To effectively exert this effect, the Ti content is set to 0.0001% by mass or more, preferably 0.0003% by mass or more, and more preferably 0.0005% by mass or more. On the other hand, Ti nitrides act as starting points for cracks during cold forging, so an excessive Ti content reduces cold forgeability (mainly drawing characteristics). Therefore, the Ti content is set to 0.0040% by mass or less, preferably 0.0037% by mass or less, and more preferably 0.0035% by mass or less.

[0021] (B:0.0010~0.0050% by mass) B is an element effective in improving hardenability. Furthermore, B combines with N to form BN, thereby reducing the amount of solute N and contributing to improved cold forgeability. To exert this effect, the B content is set to 0.0010% by mass or more, preferably 0.0011% by mass or more, and more preferably 0.0012% by mass or more. However, even if excessive B is added, the effect saturates, so the B content is set to 0.0050% by mass or less, preferably 0.0045% by mass or less, and more preferably 0.0040% by mass or less.

[0022] (N:0.0020~0.0100% by mass) N is an element inevitably contained in steel material. When present as solute N in steel material, it increases hardness and reduces ductility due to strain aging, thereby reducing cold forgeability. Therefore, the N content is set to 0.0100% by mass or less, preferably 0.0095% by mass or less, and more preferably 0.0090% by mass or less. On the other hand, N forms AlN, which is effective in improving grain coarsening resistance. Therefore, the N content is set to 0.0020% by mass or more, preferably 0.0025% by mass or more, and more preferably 0.0030% by mass or more.

[0023] The cold forging steel material according to the embodiment of the present invention preferably contains the above-described chemical composition, with the balance being Fe and unavoidable impurities. Elements introduced due to the conditions of raw materials, materials, manufacturing equipment, etc. are permitted as unavoidable impurities. Note that, for example, P and S, the lower their content, the better, and therefore they are unavoidable impurities, but some elements have composition ranges separately specified as above. Therefore, in this specification, the term "unavoidable impurities" constituting the balance excludes elements whose composition ranges are separately specified. Examples of such unavoidable impurities include Zn, Pb, and As. The total amount of unavoidable impurities may be, for example, 0.010% by mass or less.

[0024] The cold forging steel material according to this embodiment may further contain the following optional additive elements.

[0025] (at least one selected from the group consisting of Cu: 0.001 to 0.100 mass% and Ni: 0.001 to 0.100 mass%) Cu is an effective element for improving the hardenability of steel, so it may be added in an amount of, for example, 0.001% by mass or more. However, since excessive addition of Cu increases the cost of the steel, when Cu is added, the Cu amount is set to 0.100% by mass or less, preferably 0.090% by mass or less, and more preferably 0.080% by mass or less. Like Cu, Ni is also an effective element for improving the hardenability of steel, and so may be added in an amount of, for example, 0.001% by mass or more. However, since excessive addition of Ni increases the cost of the steel, when Ni is added, the Ni amount is set to 0.100% by mass or less, preferably 0.090% by mass or less, and more preferably 0.080% by mass or less.

[0026] <2. Carbon equivalent Ceq> The steel material for cold forging according to the embodiment of the present invention satisfies the following formula (1). Ceq=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5≧0.44···(1) In formula (1), [C], [Si], [Mn], [Ni], and [Cr] represent the contents of C, Si, Mn, Ni, and Cr, respectively, expressed in mass %.

[0027] The present inventors have found that the surface hardness after induction hardening can be sufficiently increased by setting the carbon equivalent Ceq to 0.44 mass% or more, as shown in formula (1). Ceq is preferably 0.45 mass% or more, and more preferably 0.46 mass% or more. This can further increase the surface hardness. There is no particular upper limit for Ceq, but it is preferably set to, for example, 0.56 mass% or less, which can further improve cold forgeability.

[0028] <3.Solute N index Fx1> The steel material for cold forging according to the embodiment of the present invention satisfies the following formula (2). Fx1=[N]-[Ti]×14.01 / 47.88-[B]×14.01 / 10.82≧0.0010···(2) In the formula (2), [N], [Ti] and [B] represent the contents of N, Ti and B, respectively, expressed in mass %.

[0029] In formula (2), "[Ti] × 14.01 / 47.88" can correspond to the amount of N contained in TiN, which is the nitride that can be formed most preferentially in this embodiment, and "[B] × 14.01 / 10.82" can correspond to the amount of N contained in BN, which is the nitride that can be formed most preferentially after TiN in this embodiment. The amount of solute N in a steel material can be roughly estimated from the solute N index Fx1.

[0030] The present inventors came up with the idea of ​​increasing the solute N index Fx1 sufficiently to precipitate a large amount of AlN (a nitride with a lower precipitation temperature than TiN and BN), which effectively suppresses the coarsening of crystal grains (austenite grains). The present inventors then discovered that sufficient resistance to crystal grain coarsening can be obtained by setting Fx1 to 0.0010% by mass or more, as shown in formula (2). Fx1 is preferably 0.0012% by mass or more, more preferably 0.0014% by mass or more, and even more preferably 0.0016% by mass or more. There is no particular upper limit to Fx1, but it is preferably set to, for example, 0.0050% by mass or less, which can further improve cold forgeability.

[0031] <4. Predicted value of deformation resistance during cold forging DRe> The steel material for cold forging according to the embodiment of the present invention satisfies the following formula (3). DRe=2744×Fx1+526×Ceq+398≦700···(3)

[0032] The deformation resistance during cold forging correlates with both the carbon equivalent Ceq and the solute N index Fx1. This can be explained as follows: The surface hardness after induction hardening, as mentioned above, may primarily reflect the hardness of the martensite phase in the surface layer that can form after induction hardening. Because the martensite phase has a high dislocation density, solute N is thought to have little influence on surface hardness, given that solute N can increase deformation resistance primarily by suppressing dislocation movement. On the other hand, the deformation resistance during cold forging may reflect the deformation resistance of the metal structure (mainly ferrite and pearlite) before induction hardening. Therefore, the solute N, which can suppress dislocation movement in the structure, is thought to have a strong influence on the deformation resistance during cold forging. From the examples described below, the inventors derived equation (3) using the measured value DRa of deformation resistance during cold forging as the objective variable and Ceq and Fx1 as explanatory variables. It was found that the measured value DRa of deformation resistance during cold forging can be accurately predicted using the predicted value DRe of deformation resistance during cold forging in equation (3), and that satisfying (3) allows the deformation resistance during cold forging to be sufficiently low. In equation (3), DRe is preferably 690 or less, and more preferably 675 or less.

[0033] <5. Manufacturing method of steel for cold forging> The cold forging steel material according to this embodiment is obtained by a manufacturing method including preparing a steel billet that satisfies the above-mentioned composition and formulas (1) to (3) and hot working (hot rolling, etc.) by a known method. Known conditions can also be used for the hot working conditions.

[0034] <6. Parts and their manufacturing methods> Conventional machine structural parts are obtained by subjecting cold forging steel to (1) spheroidizing annealing, (2) cold forging, (3) thermal refining, and (4) induction hardening. By using the cold forging steel material according to this embodiment, the part according to this embodiment can omit (1) the spheroidizing annealing step and (3) the thermal refining step. That is, the method for manufacturing a part according to this embodiment includes the steps of preparing the cold forging steel material according to this embodiment, cold forging the cold forging steel material, and induction hardening. The parts according to this embodiment can be used as machine structural parts for automobiles, construction machines, etc., and examples of machine structural parts include gears, shafts, steering racks, etc. [Example]

[0035] The present embodiment will be described in more detail below with reference to examples. The present embodiment is not limited to the following examples, and can be implemented with appropriate modifications within the scope of the above-mentioned and below-mentioned aims, and all such modifications are included in the technical scope of the present embodiment.

[0036] Ingots with the chemical compositions shown in Table 1 were obtained by the steel ingot method. These were heated at 1150°C to 1300°C for 60 minutes or more, hot rolled or hot forged, and then cooled to room temperature. After cooling, they were reheated to 800°C to 1000°C and hot rolled to produce steel materials for cold forging (steel bars with diameters of 55 to 65 mm) for Test Nos. 1 to 9.

[0037] [Table 1]

[0038] The cold forging steel materials of Test Nos. 1 to 9 described above were evaluated as follows.

[0039] <Surface hardness after induction hardening> Cylindrical test pieces with an outer diameter of 35 mm, an inner diameter of 25 mm, and a length of 40 mm were cut out from the cold forging steel materials of Test Nos. 2 to 9 so that the rolling direction and the longitudinal direction of the test piece were parallel. A coil suitable for the cylindrical test pieces was made, and high-frequency heat was applied from the outer diameter to perform induction hardening treatment (conditions: 100 kHz, 80 kW, heating time 2 seconds). The cold forging steel material of Test No. 1 was roughly machined to an outer diameter of 35.1 mm, an inner diameter of 24.9 mm, and a length of 40 mm so that the rolling direction and the longitudinal direction of the test piece were parallel, and then tempered under known conditions. The other conditions were the same as Test Nos. 2 to 9, except that it was further finished to an outer diameter of 35 mm, an inner diameter of 25 mm, and a length of 40 mm. The induction-hardened samples were embedded in resin so that the polished surface was in the longitudinal direction of the test piece, and the hardness was measured at three points 0.2 mm deep from the outer diameter using a Vickers hardness tester. The average value of the three points was used as the surface hardness of each steel material after induction hardening. A surface hardness of 530 HV or more was determined to be sufficient.

[0040] <Grain coarsening resistance> Each cold forging steel was heat treated by quenching (850°C, 30 minutes) and tempering (550°C, 1 hour). To enable observation at a position 1 / 4D (D is the steel bar diameter) from the surface of the heat-treated material, the heat-treated material was cut in the rolling direction, filled with resin, etched, and then the grain size was measured using an optical microscope. The grain size was determined in accordance with JIS G 0551 (2020). A grain size number of 5.0 or higher was considered to have sufficient resistance to grain coarsening.

[0041] The cold forgeability was evaluated by evaluating the deformation resistance and drawing characteristics as follows.

[0042] <Cold forgeability evaluation ~ deformation resistance ~> Solid cylindrical test pieces (φ12 mm, length 18 mm) were cut from the 1 / 4D position of each cold forging steel so that the rolling direction and the longitudinal direction of the test piece were parallel. The test pieces were compressed in a press tester until the reduction ratio reached 60%, and the deformation resistance DRa at 50% was measured. The reduction ratio is expressed as [{1-(L / L0)} × 100(%)] (where L is the longitudinal length of the cylindrical test piece before reduction (mm), and L0 is the longitudinal length of the cylindrical test piece after reduction (mm)).

[0043] <Cold forgeability evaluation ~Drawing characteristics~> A tensile test was carried out on each cold forging steel material in accordance with JIS Z 2241 (2022), and the reduction in area during the test was determined.

[0044] When the deformation resistance was 700 MPa or less and the reduction in area was 54% or more, the steel was determined to have sufficient cold forgeability.

[0045] The evaluation results are shown in Table 2.

[0046] [Table 2]

[0047] The following can be seen from Table 2. Test Nos. 4 to 7 are examples that satisfy all of the requirements of this embodiment, and even when the thermal refining process and spheroidizing annealing process were omitted, they had sufficient surface hardness, sufficient resistance to grain coarsening, and sufficient cold forgeability. On the other hand, Test Nos. 1 to 3 and 8 to 9 did not satisfy the requirements of this embodiment, and either the grain coarsening resistance property or the cold forgeability was insufficient.

[0048] Test No. 1 did not satisfy formula (3), and therefore the deformation resistance during cold forging exceeded 700 MPa.

[0049] In Test Nos. 2 and 3, the Ti content was less than 0.0001 mass % and formula (2) was not satisfied, so the grain size number was less than 5.0.

[0050] In Test No. 8, the Ti content exceeded 0.0040 mass %, and therefore the reduction in area was less than 54%.

[0051] In Test No. 9, the C content was more than 0.40% by mass and the Ti content was more than 0.0040% by mass, and therefore the reduction of area was less than 54%.

Claims

1. C: 0.30 to 0.40% by mass, Si: 0.01 to 0.30% by mass, Mn: 0.50 to 1.00% by mass, P: 0.030% by mass or less (including 0% by mass), S: 0.030% by mass or less (including 0% by mass), Cr: 0.01 to 0.30% by mass, Al: 0.020 to 0.100% by mass, Ti: 0.0001 to 0.0040% by mass, B: 0.0010 to 0.0050 mass%, and N: 0.0020 to 0.0100% by mass and the balance being Fe and inevitable impurities, The carbon equivalent Ceq satisfies the following formula (1): Solid solution N index Fx １ satisfies the following formula (2), A steel material for cold forging, in which a predicted value DRe of deformation resistance during cold forging satisfies the following formula (3). Ceq=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5≧0.44...(1) F8 １ =[N]-[Ti]×14.01 / 47.80-B]×14.01 / 10.82≧0.0010・・・(2) DRe=2744×Fx １ +526×Ceq+398≦700・・・(3) In the formulas (1) and (2), [C], [Si], [Mn], [Ni], [Cr], [N], [Ti], and [B] represent the contents of C, Si, Mn, Ni, Cr, N, Ti, and B, respectively, expressed in mass%.

2. Cu: 0.001 to 0.100 mass%, and Ni: 0.001 to 0.100% by mass The steel material for cold forging according to claim 1, further comprising at least one selected from the group consisting of:

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