Steel with crack resistance

Abstract

To provide a steel material that can further reduce deformation resistance and crack occurrence probability when compressive stress is applied during cold working, and to provide a low-cost steel material that has suppressed crack occurrence, low deformation resistance, and excellent workability, thereby eliminating the need for spheroidizing annealing treatment before cold forging. [Solution] A steel material for cold forging made of machine structural steel, characterized in that the ratio of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm), ΔP / ΔL, is 30 MPa / mm or less when the compression working rate by cold working is in the range of 10 to 60%.

Description

Technical Field

[0001] This application relates to steel materials with excellent workability, such as steel bars made of machine structural steel suitable for cold forging, etc.

Background Art

[0002] Conventionally, as steel with excellent workability, in mass%, C: 0.1 to 0.35%, Si: 0.03 to 1.0%, Mn: 0.2 to 20%, S: 0.1% or less (including 0%), Nb: 0.025 to 0.20%, Ti: 0.025 to 0.12%, N: 0.020% or less (including 0%), Al: 0.13% or less (including 0%), which satisfies, and the balance is substantially made of steel mainly composed of Fe, and carbides and / or carbonitrides satisfying the following formula ((Ti) / (Nb)≧0.05) are present at 2.0×10 7 pieces / mm 2 or more, and the average value of the Vickers hardness in the cross section is 180 or less, and the maximum value of the standard deviation of the Vickers hardness is 5 or less, and a skin annealing steel with excellent grain growth resistance characteristics and cold workability that can omit softening annealing has been proposed. In the formula, (Ti) and (Nb) represent the respective contents (mass%) of Ti and Nb in the carbide and / or carbonitride.

[0003] Also, in mass%, C: 0.05 to 0.35%, Si: 0.3% or less, Mn: 0.15 to 1.8%, P: 0.015% or less (including 0%), S: 0.02% or less (not including 0%), Cr: 0.01 to 0.5%, sol.Al: 0.01 to 0.06%, N: 0.0005 to 0.006%, B: 0.0003 to 0.0015%, Ti: 0.003 to 0.030% are satisfied, N, B, and Ti satisfy the formula (-0.0060≦[N]-1.3×[B]-0.29×[Ti]≦-0.0020), and the balance is steel composed of iron and inevitable impurities. This steel has a mixed structure of ferrite and pearlite. When the diameter of this steel is D, the grain size number of the ferrite present in the range from the center of the steel to the D / 8 position is 6 to 12, and a wire-shaped steel or bar-shaped steel that can omit spheroidizing annealing has been proposed.

[0004] Furthermore, a wire has been proposed that allows for the omission of softening heat treatment, characterized by having a microstructure in which, by mass%, C: 0.2~0.45%, Si: 0.02~0.4%, Mn: 0.3~1.5%, Cr: 0.3~1.5%, Al: 0.02~0.05%, Mo: 0.01~0.5%, N: 0.01% or less, with the remainder being Fe and other unavoidable impurities, and having a microstructure in which, by area%, the protoprecipitation ferrite fraction is 40% or more of the equilibrium phase, the regenerated ferrite and bainite fraction is 40% or more, and the martensite fraction is 20% or less, and the average size of pearlite colonies in the region from 2 / 5 to 3 / 5 of the diameter from the surface is 5 μm or less (see Patent Document 3). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2006-307271 [Patent Document 2] Japanese Patent Publication No. 2009-242916 [Patent Document 3] Japanese Patent Publication No. 2022-512514 [Overview of the project] [Problems that the invention aims to solve]

[0006] The proposal in Patent Document 1 focuses on achieving both grain coarsening prevention and cold workability, but it lacks accuracy in evaluating crack initiation and microstructure uniformity, making it insufficient as an evaluation of workability.

[0007] While the proposal in Patent Document 2 focuses on minimizing hardness variation, it does not accurately evaluate crack initiation or structural uniformity, and therefore falls short as an evaluation of machinability.

[0008] Patent Document 3 optimizes the average size of pearlite colonies and the microstructure before spheroidizing annealing, but it does not allow for the omission of spheroidizing annealing, nor does it consider crack resistance.

[0009] Therefore, the present invention aims to provide steel materials, such as steel bars made from structural steel, that can further reduce deformation resistance and crack occurrence probability when compressive stress is applied during cold working, for example, steel materials made from chromium steel (SCr), chromium-molybdenum steel (SCM), and hardenability-enhancing alloy steel with added Nb. Furthermore, it aims to provide a low-cost steel material that has suppressed crack occurrence, low deformation resistance, and excellent workability, thereby eliminating the need for spheroidizing annealing treatment before cold forging. [Means for solving the problem]

[0010] The first means for solving the problems of the present invention is a steel material for cold forging made of machine structural steel, characterized in that the ratio ΔP / ΔL, which is the ratio of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm), is 30 MPa / mm or less when the compression working rate by cold working is in the range of 10 to 60%.

[0011] The second method is a steel bar made of machine structural steel as described in the first method, wherein the ratio A4 / A2 of the ferrite + pearlite fraction A4 (area ratio) at position D / 4 (middle circumference) to the ferrite + pearlite fraction A2 (area ratio) at position D / 2 (center) is 0.80 to 1.10.

[0012] The third method is a steel bar according to the second method, characterized in that the average ferrite grain size is 10 to 40 μm.

[0013] The fourth method is a method for manufacturing steel materials as described in the first method, characterized in that a steel billet made of machine structural steel is rolled in a rolling mill to form an intermediate product, and the intermediate product having a surface temperature of 1000°C or less is introduced into a finishing rolling mill to perform finishing rolling.

[0014] The fifth method is a method for manufacturing steel bars as described in the second or third method, characterized in that a steel billet made of machine structural steel is rolled in a rolling mill to form an intermediate product, and the intermediate product having a surface temperature of 1000°C or less is introduced into a finishing rolling mill to perform finishing rolling.

[0015] The surface temperature described in the fourth and fifth means is preferably 850 to 1000°C, more preferably 850 to 980°C, and even more preferably 850 to 960°C.

[0016] In other words, this is a method for manufacturing steel materials such as steel bars suitable for forging, characterized by rolling a steel billet made of machine structural steel in a rolling mill to form an intermediate product, and then introducing the intermediate product, which has a surface temperature of 850 to 1000°C, into a finishing rolling mill to perform finishing rolling, wherein when the compression is performed in a cold working state with a compressibility of 10 to 60%, the ratio of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm), ΔP / ΔL, is 30 MPa / mm or less.

[0017] Furthermore, the present invention relates to a method for manufacturing steel materials such as steel bars suitable for cold forging, characterized by rolling a steel billet made of machine structural steel in a rolling mill to form an intermediate product, and then introducing the intermediate product with a surface temperature of 850 to 1000°C into a finishing rolling mill for finishing rolling. The method is characterized by the fact that when the compressibility of the cold work is compressed in the range of 10 to 60%, the ratio of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm), which is ΔP / ΔL, is 30 MPa / mm or less, and the ratio A4 / A2 of the ferrite + pearlite fraction A4 (area ratio) at the D / 4 position (middle circumference) to the ferrite + pearlite fraction A2 (area ratio) at the D / 2 position (center) is 0.80 to 1.10.

[0018] Also, a steel piece made of steel for mechanical structures is rolled by a rolling mill to form an intermediate product, and the intermediate product with a surface temperature of 850 to 1000 °C is introduced into a finishing rolling mill for finishing rolling. It is characterized in that when compression processing is performed in the range of 10 to 60% reduction ratio in cold working, ΔP / ΔL, which is the ratio of the change amount ΔP (MPa) of the compression stress to the compression deformation amount ΔL (mm), is 30 MPa / mm or less, and the ratio A4 / A2 of the ferrite + pearlite fraction A4 (area ratio) at the D / 4 position (mid-peripheral part) to the ferrite + pearlite fraction A2 (area ratio) at the D / 2 position (center part) is 0.80 to 1.10, and it is a manufacturing method of a steel material suitable for forging with an average ferrite grain size of 10 to 40 μm.

Effects of the Invention

[0019] According to the present invention, it is possible to provide a steel material suitable for cold forging processing with lower deformation resistance and crack generation probability when applying compression stress. Since the steel material such as bar steel of the present invention has excellent cold workability, cold working that omits spheroidizing annealing treatment or normalizing treatment can be omitted. Therefore, for example, if the steel material of the present invention is used for drive system parts such as automobiles manufactured by cold working, it is possible to reduce the manufacturing cost. Further, if a steel material such as bar steel made of steel for mechanical structures is manufactured by the manufacturing method of the present invention, since it becomes a steel material having excellent cold workability, cold working that omits spheroidizing annealing treatment or normalizing treatment can be omitted, so it is also suitable for cold forging processing.

Modes for Carrying Out the Invention

[0020] Prior to the description of the embodiments of the present invention, the reasons for defining the ratio of ΔP / Δ in the steel material such as bar steel of the present invention, the reasons for defining the ratio of the area ratio of A4 / A2, and the reasons for defining the average ferrite grain size will be described.

[0021] ΔP / ΔL is 30 MPa / mm or less ΔP / ΔL is the ratio of the change amount ΔP (MPa) of the compression stress to the compression deformation amount ΔL (mm) in the range where the reduction ratio at the time of compression processing in cold working is 10 to 60%. When cold working is performed using a steel material with ΔP / ΔL exceeding 30 MPa / mm, the probability of crack generation during cold working increases, and the load on the mold also increases. Therefore, in the present invention, ΔP / ΔL is set to 30 MPa / mm or less.

[0022] A4 / A2 is 0.80 to 1.10 The ratio of A4 / A2 is an index for evaluating the probability of crack generation due to the inhomogeneity of the structure. A4 / A2 is the ratio of the ferrite + pearlite fraction A4 (area ratio) at the D / 4 position (mid-peripheral part) to the ferrite + pearlite fraction A2 (area ratio) at the D / 2 position (center part). When A4 / A2 is less than 0.80, the probability of crack generation due to the inhomogeneity of the structure increases. Therefore, A4 / A2 is set to 0.80 or more, and preferably 0.90 or more. When A4 / A2 exceeds 1.10, the probability of crack generation due to the inhomogeneity of the structure also increases. Therefore, A4 / A2 is set to 1.10 or less. When the cross-sectional diameter of the bar steel is D, the depth of D / 4 from the surface is defined as the mid-peripheral part, and the depth of D / 2 from the surface is defined as the center part.

[0023] The average ferrite grain size is 10 to 40 μm When the average ferrite grain size is 10 μm or less, the deformation resistance decreases, but the deformation ability decreases, so the workability is not sufficient. On the other hand, when the average ferrite grain size exceeds 40 μm, the deformation resistance becomes too large, and the probability of crack generation increases. Therefore, it is preferable that the average ferrite grain size is 10 to 40 μm.

[0024] Performing finish rolling on an intermediate product with a surface temperature of 1000°C or less In the production of steel materials such as bar steel in the present invention, it is preferable to perform finish rolling of the intermediate product with a surface temperature of 1000°C or less using a finish rolling mill. In other words, the surface temperature of the intermediate product immediately before being introduced into the finishing rolling mill is preferably 1000°C or lower, more preferably 980°C or lower, and even more preferably 960°C or lower. If it exceeds 1000°C, it becomes difficult to obtain steel bars with a ΔP / ΔL of 30 MPa / mm or less, and the probability of cracking increases. Furthermore, if the surface temperature of the intermediate product immediately before being introduced into the finishing rolling mill is below 850°C, the ferrite grains become excessively fine, reducing the deformability. Therefore, it is preferable that the surface temperature of the intermediate product be 850°C or higher. In other words, it is preferable to finish-roll intermediate products with a surface temperature of 850 to 1000°C.

[0025] The steel material for which the ΔP / ΔL of the present invention is 30 MPa / mm or less is structural steel for machinery. For example, structural alloy steel for machinery as described in JIS (Japanese Industrial Standards) G4053:2023 can be used. By manufacturing using these structural steels according to a predetermined procedure, steel material satisfying ΔP / ΔL: 30 MPa / mm or less can be obtained. In the present invention, SCr420, SCM420, and the like are particularly suitable as structural steel for machinery. As long as the characteristics of structural steel are satisfied, it is not limited to steels conforming to JIS standards, and alloy steels with improved hardenability, as described later, can also be used.

[0026] For example, SCr steel is a type of steel whose composition, by mass%, is as follows: C: 0.12-0.48%, Si: 0.15-0.35%, Mn: 0.55-0.95%, P: 0.030% or less, S: 0.030% or less, Ni: 0.25% or less, Cr: 0.85-1.25%, Cu: 0.30% or less, with the remainder being Fe and unavoidable impurities.

[0027] Furthermore, SCM steel is a steel that, by mass%, contains C: 0.12-0.49%, Si: 0.15-0.35%, Mn: 0.30-1.00%, P: 0.030% or less, S: 0.030% or less, Ni: 0.25% or less, Cr: 0.85-1.50%, and Mo: 0.15-0.45%, with the remainder being Fe and unavoidable impurities.

[0028] Furthermore, the hardenability-enhancing alloy steel is a steel containing C: 0.14-0.25%, Si: 0.25-0.60%, Mn: 0.20-1.55%, P: 0.020% or less (0% is also acceptable), S: 0.020% or less (0% is also acceptable), Ni: 0.20% or less (0% is also acceptable), Cr: 1.30-3.00%, Nb: 0.070% or less (0% is also acceptable), with the remainder being Fe and unavoidable impurities.

[0029] For embodiments and comparisons of the present invention, JIS SCr420, SCM420, and steel of steel type A, consisting of the component compositions listed in Table 1 with the remainder being Fe and unavoidable impurities, were melted in a 100 kg vacuum induction melting furnace, yielding two steel billets for each steel type. Steel type A is a structural alloy steel designed with improved hardenability in mind.

[0030] [Table 1]

[0031] The embodiments will be described in detail below, using a rod-shaped steel material (steel bar) as an example. First, the steel billets of the steel types listed in Table 1 were subjected to continuous rolling using a roughing mill, an intermediate rolling mill, and a finishing rolling mill to obtain intermediate products. At that time, the surface temperature of the intermediate products immediately before being introduced into the finishing rolling mill was 1000°C for one steel billet of each steel type (labeled (a) in the examples in Table 2) and 960°C for the other steel billet (labeled (b) in the examples in Table 2). Next, the obtained intermediate products of these steel types were air-cooled to obtain steel bars with a circular cross-section and a diameter D of 30 mm.

[0032] As a comparative example, two steel billets were prepared for each steel grade, similar to the example. The surface temperature of the intermediate products immediately before being introduced into the finishing row rolling mill was set to 1150°C for one billet (the steel grade labeled (a) in the comparative example in Table 2) and 1200°C for the other billet (the steel grade labeled (b) in the comparative example in Table 2). Finish rolling was then performed, and steel bars were obtained using the same procedure as in the example.

[0033] <Evaluation Criteria> To evaluate the properties of each steel bar, (1) the microstructure was observed, (2) the deformation resistance was measured, and (3) a limit upsizing test was conducted.

[0034] (1) Microorganisms To observe the microstructure, a steel bar was cut through the center of the rolled material, parallel to the rolling direction, and then polished and nital-etched. Afterward, the surface was observed at positions D / 4 and D / 2 using an optical microscope. The results are shown in Table 2.

[0035] The microstructure of each steel after rolling was identified and determined by mirror polishing the steel after normalization, etching it with Nital solution, and then observing it with an optical microscope. For each identified microstructure, the proportion of each microstructure at positions D / 4 and D / 2 was determined by counting the number of pixels in the captured microscope images using image processing software.

[0036] From tissue observation using an optical microscope, the ferrite + pearlite fraction in D / 4 was defined as the area ratio of A4, and the ferrite + pearlite fraction in D / 2 was defined as the area ratio of A2, and A4 / A2 was calculated. The results are shown in Table 2.

[0037] (2) Deformed Resistor The obtained rolled steel bar was processed into a cylindrical test specimen (14 mm in diameter x 21 mm in length) from its center, and statically compressed with the end face fully constrained. The stress change ΔP (MPa) required for compressive deformation from 10% to 60% and the deformation amount ΔL (mm) were measured, and the maximum ΔP / ΔL was calculated. The results are shown in Table 2.

[0038] (3) Limit set-up test Cylindrical test specimens (14 mm in diameter x 21 mm in length) were prepared from the center of the rolled material. For the limit upsetting test, five test specimens were cold-compressed in the longitudinal direction, and the compression ratio at which cracking began to occur was confirmed using a magnifying glass. The average of the obtained compression ratios was evaluated as the "limit compressibility". In this invention, the target value for the critical compressibility during cold compression was set at 65% or higher. Samples that exhibited cracking were marked with an "x" and were deemed unsuitable, while samples that did not exhibit cracking were marked with a "○" and were deemed good. The results are shown in Table 2.

[0039] [Table 2]

[0040] In the embodiments of the present invention, a steel billet made of structural steel for machine use is a finish-rolled intermediate product with a surface temperature of 960 to 1000°C, has a ΔP / ΔL of 30 MPa / mm or less, an A4 / A2 ratio of 1 to 1.09, a ferrite grain size of 24 to 33 μm, is free from cracking, and achieves a limit compressibility of 65% or more.

[0041] In the comparative examples, intermediate products with surface temperatures of 1150-1200°C were subjected to finish rolling, and in all cases, the ΔP / ΔL ratio exceeded 30 MPa / mm, and the ferrite grain size was also large. Furthermore, in some cases, the A4 / A2 ratio exceeded 1.10. As a result, cracking occurred in all of the comparative examples, and the critical compressibility of 65% or more was not achieved.

Claims

1. A steel material for cold forging made of machine structural steel, characterized in that, when the compression working rate by cold working is in the range of 10 to 60%, the ratio of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm), ΔP / ΔL, is 30 MPa / mm or less.

2. The cold-working steel material made of machine structural steel according to claim 1 is a steel bar, and furthermore, the ferrite + pearlite fraction A at position D / 4 (middle circumference) 4 (Area ratio) and ferrite + pearlite fraction A at position D / 2 (center) 2 (Area ratio) Ratio A 4 / A 2 A steel bar characterized by having a coefficient of 0.80 to 1.

10.

3. The steel material according to claim 1, characterized in that the average ferrite particle size is 10 to 40 μm.

4. The steel bar according to claim 2, characterized in that the average ferrite particle size is 10 to 40 μm.

5. A method for manufacturing steel materials such as steel bars suitable for forging, characterized by rolling a steel billet made of machine structural steel in a rolling mill to form an intermediate product, and then introducing the intermediate product, which has a surface temperature of 850 to 1000°C, into a finishing rolling mill for finishing rolling, wherein when the material is compressed in a cold working state with a compressibility of 10 to 60%, the ratio of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm), ΔP / ΔL, is 30 MPa / mm or less.

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