Low-yield-ratio cold-formed steel and manufacturing method thereof

Abstract

The invention discloses cold working steel with a low yield ratio. The cold working steel further comprises the following chemical elements in percentage by mass besides Fe: 0.35 to 0.42 percent of C, less than 0.15 percent of Si, 0.85 to 1.35 percent of Mn, 0.1 to 0.3 percent of Cr, less than 0.2 percent of Ni, 0.001 to 0.02 percent of S, 0.015 to 0.05 percent of Al, 0.004 to 0.0260 percent of N, 0.01 to 0.05 percent of Ti, 0.001 to 0.005 percent of B, less than 0.85 percent of 12 (Si / 28 + Mn / 55) / C and less than 1.5 percent of 1 < 14 (Ti / 48 + Al / 27) / N. In addition, the invention further discloses a manufacturing method of the cold working steel. The cold working steel has excellent plasticity and cold working characteristics, can effectively improve the production efficiency and reduce the energy consumption, can be used for cold forging of shaft parts and the like, and has good popularization prospects and application values.

Description

Technical Field

[0001] The present invention relates to a metal material and a manufacturing method thereof, in particular to a steel for cold-formed shaft parts with a low yield ratio and a manufacturing method thereof. Background Art

[0002] With the accelerating development of green and low-carbon manufacturing, some parts processing is shifting from hot working to cold working. Over 50% of parts in Japan and Europe are cold forged. To ensure ultimate service strength, automotive parts typically incorporate carbon and alloying elements. This addition of alloying elements increases the yield strength and cost of the steel matrix, thereby increasing the material's yield-to-strength ratio. This negatively impacts the material's cold working plasticity and can increase mold wear during cold working.

[0003] At present, the yield strength ratio of most cold-processed materials is not less than 0.7. A high yield strength ratio means that the number of dislocations that can be multiplied during plastic deformation is limited, that is, the plasticity of the material is limited. At the same time, high yield strength will also increase the mold loss of the material.

[0004] Based on this, it is expected to obtain a steel with excellent plasticity and cold working properties, and at the same time use B to improve the hardenability of the steel, so as to reduce the alloy cost of the steel and reduce the yield strength ratio of the cold-worked steel, thereby reducing mold loss. Summary of the Invention

[0005] One of the purposes of the present invention is to provide a cold-worked steel which not only has excellent plasticity but also has a low alloy content, has a wide range of applicability, and has good promotion prospects and application value.

[0006] In order to achieve the above object, the present invention provides a cold-worked steel containing the following chemical elements in the following mass percentages: C: 0.35~0.42%, Si: <0.15%, Mn: 0.85~1.35%, Cr: 0.1~0.3%, Ni<0.2%, S: 0.001~0.02%, Al: 0.015~0.05%, N: 0 .004~0.0160%, Ti: 0.01-0.05%, B: 0.001~0.005%, 12(Si / 28+Mn / 55) / C<0.85, 1<14(Ti / 48+Al / 27) / N<1.5.

[0007] The balance is Fe and other inevitable impurity elements.

[0008] In the cold-worked steel of the present invention, the design principles of the chemical elements are as follows: C: An appropriate amount of C ensures the steel possesses adequate strength after being processed into parts, which benefits the service life of the parts. However, increasing the C content increases the steel's hardness, leading to excessive cold-worked material strength, increased mold wear during the cold forging process, and higher downstream processing costs. The C content in the cold-forming steel described herein is controlled to be between 0.35% and 0.42% by weight.

[0009] Si: As a deoxidizer, Si effectively reduces the oxygen content in molten steel. However, excessive Si content can reduce the plasticity of the steel. Therefore, the mass percentage of Si in the cold-worked steel described herein is controlled to be below 0.15%.

[0010] Mn: As a solid solution strengthening element, Mn can increase the yield strength of steel, which has certain benefits for its service life. However, excessive Mn content increases the yield strength of the steel, which can increase mold wear during subsequent cold working. The mass percentage of Mn in the cold-working steel described herein is controlled between 0.85% and 1.35%.

[0011] S: Adding an appropriate amount of S to the steel forms MnS with Mn, improving cutting performance and preventing tool sticking during subsequent finishing. Therefore, the mass percentage of S in the cold-worked steel described herein is controlled between 0.001% and 0.020%.

[0012] Al: Al effectively deoxidizes steel during the steelmaking process and combines with nitrogen to form AlN precipitation, which refines austenite grains and improves material plasticity. However, excessive Al content can lead to the formation of large Al2O3 oxides, which can form large Class B inclusions. These coarse aluminum oxide inclusions can degrade the fatigue properties of the steel and cause tool chipping during machining. Therefore, the mass percentage of Al in the cold-worked steel described herein is controlled between 0.015% and 0.05%.

[0013] Cr: Cr is a strong carbide-forming element. It readily combines with carbon in steel to form precipitation phases, which promotes carbide spheroidization, effectively reducing the proportion of lamellar carbides and improving the steel's plasticity. If the Cr content in steel is too high, some Cr will dissolve in the matrix. Furthermore, Cr can significantly affect the hardenability of gear steel. Therefore, the Cr content is controlled within a range of 0.01-0.2% by weight.

[0014] B: The control of alloying elements in the present invention may lead to insufficient hardenability of the material, and thus insufficient strength and toughness after the final heat treatment of the part, affecting the service performance of the part. B is added to compensate for the reduced hardenability caused by insufficient alloying elements. Therefore, the mass percentage of B is controlled to be 0.001-0.005%.

[0015] Ti: Ti will preferentially combine with N in steel, reducing the excessive combination of B and N in the steel, ensuring sufficient free B in the steel, and further ensuring the hardenability improvement effect of B. Therefore, the mass percentage of Ti is controlled at 0.01-0.05%.

[0016] Accordingly, although nitrogen can form AlN or TiN in steel, which can refine austenite grains, increasing the nitrogen content in steel leads to increased concentration at defects and the formation of coarse nitride precipitation particles, which affects the fatigue life of the steel. Therefore, the mass percentage of nitrogen should be controlled within the range of 0.004% to 0.0160%.

[0017] Furthermore, the cold-worked steel of the present invention further contains at least one of the following elements: 0<Ca≤0.005%.

[0018] In the above technical solution, the above-mentioned Ca can further improve the performance of the cold-worked steel of the present invention. The design principle of its chemical elements is as follows: Ca: Adding an appropriate amount of Ca to steel can improve its castability. However, excessive Ca content in steel should not be too high, as it can produce large DS inclusions. Therefore, the mass percentage of Ca should be controlled to 0 < Ca ≤ 0.005%.

[0019] Furthermore, in the cold-worked steel of the present invention, among other inevitable impurities, P≤0.020%, O≤0.0030%.

[0020] In the above technical solution, in the cold-worked steel of the present invention, P, N and O are all inevitable impurity elements in the steel.

[0021] Phosphorus (P) combines with iron to form a hard and brittle Fe3P phase, which causes cold brittleness during cold working. This deteriorates the steel's plasticity and causes intergranular fracture when subjected to impact loads, severely impacting the cold working of parts. Therefore, to prevent increased brittleness, the mass percentage of phosphorus (P) in the cold-worked steel described herein is controlled to ≤ 0.020%.

[0022] The impurity element O can form Al2O3, TiO, etc. with Al and Ti elements in the steel. Therefore, in order to ensure the uniformity of the steel structure, in the cold-worked steel described in the present invention, the mass percentage of O is controlled to be O≤0.0030%.

[0023] Furthermore, in the cold-worked steel of the present invention, its microstructure is ferrite+spheroidal carbide.

[0024] Furthermore, in the cold-worked steel of the present invention, the spheroidization ratio of the microstructure reaches more than 90% (rated according to ASTM F2282-2023).

[0025] Accordingly, another object of the present invention is to provide a method for manufacturing low-yield ratio cold-formed steel with excellent plasticity and cold working properties.

[0026] In order to achieve the above object, the present invention provides the above-mentioned method for manufacturing cold-worked steel, comprising the steps of: (1) Smelting and casting; casting into a cross-sectional area exceeding 120,000 mm 2 Large square bloom, or directly cast into 140-220mm square billet.

[0027] (2) Heating and intermediate billet rolling: The billet is hot-charged and sent into the heating furnace. The billet heating temperature is 1100±30℃ and kept warm for 3-5 hours. The rolling strain rate is not less than 2s -1 , rolling the ingot into an intermediate square billet of 140mm~220mm.

[0028] (3) Rolling round steel: heat the intermediate billet to 1070-1130℃, keep it warm for 4 hours, and adopt large deformation continuous rolling. The deformation of each rolling pass is not less than 15%. The finishing rolling temperature is controlled at T1+80~T1+140℃, where T1=940-480C+56Si-20Mn+136Ti+200Al+3300B. The round steel size is 10-60mm.

[0029] (4) Post-rolling cooling control: After rolling, the round steel is pile-cooled to below 300℃, and then placed in a continuous roller hearth annealing furnace for annealing. The specific annealing process parameters are as follows: the round steel is placed in a roller hearth furnace, the first section temperature is set to 600-700℃, and kept warm for 1-2 hours, the second section is heated to T2 (±5℃), where T2 = 742-27C-12Mn+18Si+24Cr-25Ni-180Al-7Ti-900B, and kept warm for 4-5 hours, the third section is cooled to T2-20℃ (±5℃) and kept warm for 1.5 hours, the fourth section is cooled to T2-50℃ (±5℃) and kept warm for 7.5 hours, the fifth section is cooled to 630-650℃ and kept warm for 2 hours, and then taken out of the furnace and air-cooled.

[0030] The theoretical basis and innovations involved in the composition design and manufacturing method of the cold-worked steel described in the present invention are as follows: (1) Cold-processed steel grades usually need to undergo heat treatment to ensure the final strength of the parts, and usually require the addition of sufficient alloys. The present invention utilizes the improvement effect of B on hardenability, greatly reducing the use of alloys such as Ni, Cr, and Mo, reducing the solid solution strengthening effect of the alloy, and can effectively reduce the yield strength of the matrix and reduce mold loss.

[0031] (2) The forms of B in steel are divided into solid solution state and combined state. The combined state B has little effect on improving hardenability. The present invention controls 1<14(Ti / 48+Al / 27) / N<1.5, so that Ti and Al can fully consume N in the steel, effectively reducing the formation of BN compounds, so that the solid solution state B of the steel can fully play its role in improving hardenability, and realize the effectiveness of B.

[0032] (3) Si and Mn are core elements for solid solution strengthening, which easily lead to an increase in the yield strength ratio of the material. In the present invention, by controlling 12(Si / 28+Mn / 55) / C < 0.85, the coupling effect of solid solution strengthening of Si and Mn and precipitation strengthening of carbides is achieved, thereby controlling the yield strength ratio of the material.

[0033] By controlling the process conditions, especially the heat treatment process parameters, and controlling the forged or rolled bars and then adopting an annealing process, the cold-worked steel produced by the manufacturing method of the present invention obtains a matrix structure of ferrite + spheroidal carbides, with a large amount of ferrite on the matrix, which effectively ensures that the cold-worked steel of the present invention has good plasticity, eliminates the internal stress of the steel, and has good structural uniformity.

[0034] The cold-worked steel and the manufacturing method thereof of the present invention have the following advantages and beneficial effects compared with the prior art: In summary, the present invention utilizes a rational chemical composition design, fully utilizing the effects of various alloying elements on phase transformation and microstructure, and combining it with a specific controlled rolling and controlled cooling process to form a uniform ferrite + spheroidal carbide matrix structure. The spheroidization structure is rated according to the ASTM F2282-2023 standard, with a spheroidization rate exceeding 90%. Furthermore, the present invention effectively controls the contents of P, N, and O, ensuring that the resulting cold-worked steel has suitable strength, excellent ductility, and elongation.

[0035] The cold-worked steel of the present invention has good plasticity and cross-sectional reduction at low temperatures and excellent cold forging performance. The yield strength of the cold-worked steel does not exceed 350 MPa, the tensile strength does not exceed 550 MPa, the elongation is ≥25%, and the cross-sectional reduction is ≥60%. This steel has excellent plasticity and cold working properties.

[0036] In addition, it should be noted that the cold-processed steel described in the present invention has a reasonable chemical composition and process design, a wide process window, and can be mass-produced commercially on a rod and wire production line, with good promotion prospects and application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a microstructure photograph of the cold-worked steel of Example 4 under an optical microscope.

[0038] Figure 2 This is a microstructure photograph of the cold-worked steel of Comparative Example 3 under an optical microscope. DETAILED DESCRIPTION

[0039] The cold-worked steel and the manufacturing method thereof described in the present invention will be further explained and illustrated below in conjunction with specific embodiments and the accompanying drawings. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0040] The cold-worked steels of Examples 1-6 were all prepared by the following steps: (1) Smelt and cast in an electric furnace or converter according to the chemical composition shown in Table 1.

[0041] (2) Rolling round steel: heat the steel billet to 1100-1150℃, keep it warm for 4 hours, and use large deformation continuous rolling with a rolling strain rate of not less than 2s -1 The finishing rolling temperature is controlled at T1+50~T1+100℃, where T1=940-480C+56Si-20Mn+136Ti+200Al+3300B, and the round steel size is 10-60mm.

[0042] (4) Post-rolling heat treatment: After rolling, the round steel is pile-cooled to below 200℃, and then placed in a continuous roller hearth annealing furnace for annealing. The specific annealing process parameters are as follows: the round steel is placed in the roller hearth furnace, the first section temperature is set to 600-700℃, and kept warm for 1-2 hours, the second section is heated to 1 / 2 (T1+T2), where T2=762-27C-12Mn+18Si+24Cr-25Ni-180Al-7Ti-900B, and kept warm for 4-5 hours, the third section is cooled to T2-10℃ (±5℃) and kept warm for 2 hours, the fourth section is cooled to T2-30℃ (±5℃) and kept warm for 7.5 hours, the fifth section is cooled to 600-630℃ and kept warm for 2 hours, and then taken out of the furnace and air-cooled.

[0043] Table 1. (wt%, the balance is Fe and other unavoidable impurities except P, N and O) (1) Smelt and cast in an electric furnace or converter according to the chemical composition shown in Table 1.

[0044] (2) Rolling round steel: heat the steel billet to 1100-1150℃, keep it warm for 4 hours, and use large deformation continuous rolling with a rolling strain rate of not less than 2s -1 The finishing rolling temperature is controlled at T1+50~T1+100℃, where T1=940-480C+56Si-20Mn+136Ti+200Al+3300B, and the round steel size is 10-60mm.

[0045] Table 2 lists the specific process parameters of the cold-worked steels of Examples 1-6.

[0046]

[0047] Post-rolling cooling control: After rolling, the round steel is pile-cooled to below 300℃, and then placed in a continuous roller hearth annealing furnace for annealing. The specific annealing process parameters are as follows: the round steel is placed in the roller hearth furnace, the temperature of the first section is set to 600-700℃, and kept warm for 1-2 hours, the second section is heated to 1 / 2 (T1+T2) (±5℃), where T2=742-27C-12Mn+18Si+24Cr-25Ni-180Al-7Ti-900B, and kept warm for 4-5 hours, the third section is cooled to T2-10℃ (±5℃) and kept warm for 1.5 hours, the fourth section is cooled to T2-30℃ (±5℃) and kept warm for 7.5 hours, the fifth section is cooled to 630-660℃ and kept warm for 2 hours, and then air-cooled after being taken out of the furnace.

[0048] The cold-worked steels of Examples 1-6 were subjected to various performance tests, and the test results are listed in Table 4. Table 4 lists the mechanical property test results of the cold-worked steels of Examples 1-6, as well as the results of the spheroidized structure rating according to ASTM F2282-2023.

[0049] As can be seen from Table 4, the cold-worked steels of Examples 1-6 of the present invention exhibit excellent mechanical properties. The yield strength of each example is between 300-350 MPa, the tensile strength is between 480-550 MPa, the elongation is ≥25%, and the reduction of area is ≥60%. The production of this steel can effectively reduce energy consumption. Furthermore, the cold-worked steels of each example exhibit excellent mechanical properties, including good plasticity and reduction of area, comparable in strength and plasticity to conventional round steel after spheroidizing annealing, and exhibit excellent cold-working characteristics.

[0050] Figure 1 This is a microstructure photograph of the cold-worked steel of Example 4 under an optical microscope.

[0051] Figure 2 This is a microstructure photograph of the comparative example of the cold-worked steel of Example 4.

[0052] Combine Figure 1 and Figure 2 It can be seen that in the cold-worked steel of Example 4 of the present invention, the microstructure of the cold-worked steel is spheroidal carbides and ferrite.

[0053] From the above, it can be seen that the present invention fully utilizes the influence of various alloy elements on phase transformation and microstructure through reasonable chemical composition design, and then cooperates with a specific heat treatment process to form a uniform ferrite + spheroidal carbide matrix structure. At the same time, the present invention effectively controls the content of P, N and O, and effectively ensures that the steel has suitable strength and excellent plasticity and elongation. The cold-worked steel of the present invention has good plasticity and cross-sectional shrinkage at low temperatures, and has excellent cold forging performance. The yield strength of the cold-worked steel does not exceed 350MPa, the tensile strength is not more than 550MPa, the elongation is ≥25%, and the cross-sectional shrinkage is ≥60%.

[0054] It should be noted that the cold-processed steel described in the present invention has reasonable chemical composition and process design, a wide process window, and can be mass-produced commercially on bar or plate production lines, with good promotion prospects and application value.

[0055] In addition, the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0056] It should also be noted that the above examples are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above examples, and many similar variations are possible. Any variations that can be directly derived or conceived by those skilled in the art from the present disclosure are intended to fall within the scope of protection of the present invention.

Claims

1. A low yield ratio cold-worked steel, characterized in that: Contains the following chemical elements in percentage by mass: C: 0.35~0.42%, Si: <0.15%, Mn: 0.85~1.35%, Cr: 0.1~0.3%, Ni <0.2%, S: 0.001~0.02%, Al: 0.015~0.05%, N: 0.004~0.0160%, Ti: 0.01-0.05%, B: 0.001~0.005%, the balance is Fe and other inevitable impurity elements; among them, 12 (Si / 28+Mn / 55) / C <0.85, 1 <14 (Ti / 48+Al / 27) / N <1.5; the yield ratio of this cold-worked steel does not exceed 0.

65.

2. The cold-worked steel according to claim 1, wherein Among other inevitable impurities, P≤0.020%, O≤0.0030%, 0<Ca≤0.004%.

3. The cold-worked steel according to claim 1 or 2, wherein: Its microstructure is ferrite + spheroidal carbide. The spheroidized structure is rated according to the ASTM F2282 standard, and the spheroidization rate reaches more than 90%.

4. The cold-worked steel according to claim 1 or 2, characterized in that: Its performance meets at least one of the following items: the yield strength of the cold-worked steel does not exceed 350MPa, the tensile strength does not exceed 550MPa, the elongation is ≥27%, and the cross-sectional shrinkage is ≥60%.

5. A method for manufacturing cold-worked steel according to any one of claims 1 to 4, characterized in that: The following steps are involved: (1) smelting and casting the chemical components using an electric furnace or a converter; (2) Rolling round steel: heat the steel billet to 1100-1150℃, keep it warm for 4 hours, adopt large deformation continuous rolling, the rolling strain rate is not less than 2s-1, and the finishing temperature is controlled at T1+50~T1+100℃, where T1=940-480C+56Si-20Mn+136Ti+200Al+3300B, and the round steel size is 10-60mm; (3) Post-rolling cooling control: After rolling, the round steel is pile-cooled to below 300℃, and then placed in a continuous roller hearth annealing furnace for annealing. The specific annealing process parameters are as follows: the round steel is placed in a roller hearth furnace, the first section temperature is set to 600-700℃, and kept warm for 1-2 hours, the second section is heated to 1 / 2 (T1+T2) (±5℃), where T2=742-27C-12Mn+18Si+24Cr-25Ni-180Al-7Ti-900B, and kept warm for 4-5 hours, the third section is cooled to T2-10℃ (±5℃) and kept warm for 1.5 hours, the fourth section is cooled to T2-30℃ (±5℃) and kept warm for 7.5 hours, the fifth section is cooled to 630-660℃ and kept warm for 2 hours, and then taken out of the furnace and air-cooled.

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