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JP-2026076301-A - Method for manufacturing steel parts and steel parts

JP2026076301AJP 2026076301 AJP2026076301 AJP 2026076301AJP-2026076301-A

Abstract

[Problem] A method for manufacturing steel parts with improved resistance to hydrogen embrittlement. [Solution] By weight, the composition includes 0.05%≦C≦0.15%, 0.01%≦Si≦1%, 1.2%≦Mn≦2%, 0.1%≦Cr≦2%, 0.001≦Al≦0.1%, 0.003%≦N≦0.01%, 0≦S≦0.015%, 0≦P≦0.015%, 0%≦Ni≦1%, 0%≦B≦0.01%, 0%≦Mo≦1.0%, 0%≦Ti≦0.04%, 0%≦Nb≦0.1%, 0≦V≦0.5%, with the remainder being iron and non-metallic. A method for manufacturing steel parts made of impurities, comprising annealing a semi-finished product at an annealing temperature strictly lower than the steel's Ac1 temperature, cooling it to room temperature, cold forming the semi-finished product to obtain a cold-formed product, subjecting the cold-formed product to a heat treatment including heating it to a heat treatment temperature of the steel's complete austenitization temperature Ac3 or higher, quenching it to room temperature, and optionally reheating the product at a holding temperature of 180°C to 400°C for 15 minutes to 2 hours. [Selection Diagram] None

Inventors

  • フロテ，マリオン
  • レシアク，ベルナール

Assignees

  • アルセロールミタル

Dates

Publication Date
20260511
Application Date
20260209

Claims (16)

  1. A method for manufacturing steel parts, wherein the method is as follows: - A process for providing a semi-finished product made of steel, wherein the steel is by weight, 0.05% ≤ C ≤ 0.15% 0.01% ≤ Si ≤ 1% 1.2% ≤ Mn ≤ 2% 0.1% ≤ Cr ≤ 2% 0.001 ≤ Al ≤ 0.1% 0.003% ≤ N ≤ 0.01% 0 ≤ S ≤ 0.015% 0 ≤ P ≤ 0.015% 0% ≤ Ni ≤ 1% (optional choice) 0% ≤ B ≤ 0.01% 0% ≤ Mo ≤ 1.0% 0% ≤ Ti ≤ 0.04% 0% ≤ Nb ≤ 0.1% 0 ≤ V ≤ 0.5% The supply process includes, with the remainder consisting of iron and unavoidable impurities. - A step of annealing this semi-finished product at an annealing temperature that is strictly lower than the Ac1 temperature of the steel, - The process of cooling it to room temperature, - A cold forming process to turn the aforementioned semi-finished product into a cold-formed product. - A step of subjecting the cold-formed product to heat treatment, wherein the heat treatment consists of the following steps: - A step of heating the cold-formed product to a heat treatment temperature of Ac3 or higher, the complete austenitization temperature of the steel. - The process of hardening to room temperature, - Optionally, a heat treatment step including a step of reheating the product at a holding temperature of 180°C to 400°C for 15 minutes to 2 hours. Methods that include...
  2. The method according to claim 1, wherein during the heating step of the heat treatment, the cold-formed product is heated to a heat treatment temperature at least 50°C higher than the complete austenitization temperature Ac3 of the steel.
  3. The method according to claim 1 or claim 2, wherein the annealing temperature is Ac1 minus 20°C or higher.
  4. The method according to any one of claims 1 to 3, wherein the semi-finished product is a wire having a diameter of 5 mm to 25 mm.
  5. The method according to any one of claims 1 to 4, further comprising a step of preparing the surface of the semi-finished product, which includes a step of cleaning the surface of the semi-finished product and a step of forming a lubricating film on the surface, prior to the cold forming step.
  6. The method according to claim 5, wherein the step of forming a lubricating film on the surface of the semi-finished product includes the steps of phosphate treatment and washing with soap.
  7. The method according to any one of claims 1 to 6, wherein the carbon content of the steel is within 0.08 to 0.14% by weight.
  8. The method according to any one of claims 1 to 7, wherein the manganese content of the steel is 1.3 to 1.9% by weight.
  9. The method according to any one of claims 1 to 8, wherein the chromium content of the steel is within 0.2 to 1.6% by weight.
  10. The method according to any one of claims 1 to 9, wherein the cold forming step is a cold heading step.
  11. The method according to any one of claims 1 to 10, wherein the product is maintained at the holding temperature by immersing it in a molten salt bath during the holding step.
  12. A steel part made of an alloy, wherein the alloy is by weight, 0.05% ≤ C ≤ 0.15% 0.01% ≤ Si ≤ 1% 1.2% ≤ Mn ≤ 2% 0.1% ≤ Cr ≤ 2% 0.001 ≤ Al ≤ 0.1% 0.003% ≤ N ≤ 0.01% 0 ≤ S ≤ 0.015% 0 ≤ P ≤ 0.015% 0% ≤ Ni ≤ 1% (optional choice) 0% ≤ B ≤ 0.01% 0% ≤ Mo ≤ 1.0% 0% ≤ Ti ≤ 0.04% 0% ≤ Nb ≤ 0.1% 0 ≤ V ≤ 0.5% It contains, and the remainder consists of iron and unavoidable impurities. A steel component having a microstructure containing at least 80 area percent of bainite and a cumulative presence of 1 to 25 area percent of martensite and retained austenite, and having a tensile strength of 1100 MPa or more.
  13. The steel component according to claim 12, wherein the martensite of the steel has rod-shaped iron carbide, and the length of the rod is 50 to 200 nm.
  14. The steel part according to any one of claims 12 or 13, wherein the steel part has a hardness of 360 HV to 405 HV.
  15. A steel component according to any one of claims 12 to 14, having a hydrogen embrittlement index of less than 0.09.
  16. A steel component according to any one of claims 12 to 15, having a cross-sectional reduction ratio exceeding 58%.

Description

This invention relates to a method for manufacturing assembly parts such as screws and bolts, commonly used in vehicle chassis or wheel hub components, in the automotive industry, by cold forming, particularly through cold heading. As is well known, the automotive industry is constantly striving to reduce vehicle weight. This can be achieved by modifying its safety device assemblies. Weight reduction increasingly necessitates a reduction in the size of these components. However, these components remain subject to the same mechanical stresses and therefore must possess increasingly high mechanical properties, particularly tensile strength. WO2016/158470 is an age-hardening steel with excellent machinability before aging treatment and superior fatigue properties, toughness, and low-cycle fatigue properties after aging treatment. Specifically, it contains predetermined amounts of C, Si, Mn, S, Cr, Al, V, Nb, Ca, and REM, with P, Ti, and N content limited to predetermined levels or less, and has residual Fe and impurities, with a bainite structure area ratio of 70% or more. However, the steel of WO2016/158470 lacks hydrogen embrittlement. WO2011/124851 is a machine steel component in high-performance steel, with a composition of the following weight percent: 0.05% ≤ C ≤ 0.25%, 1.2% ≤ Mn ≤ 2%, 1% ≤ Cr ≤ 2.5% (where the content of C, Mn, and Cr is (830 - 270 C% - 90 Mn% - 70 Cr%) ≤ 560), 0 < Si ≤ 1.55, 0 < Ni ≤ 1%, 0 < Mo ≤ 0.5%, 0 < Cu ≤ 1%, 0 < V ≤ 0.3%, 0 < Al ≤ 0.1%. The steel contains 0 < B ≤ 0.005%, 0 < Ti ≤ 0.03%, 0 < Nb ≤ 0.06%, 0 < S ≤ 0.1%, 0 < Ca ≤ 0.006%, 0 < Te ≤ 0.03%, 0 < Se ≤ 0.05%, 0 < Bi ≤ 0.05%, and 0 < Pb ≤ 0.1%, with the remainder of the steel component being iron and impurities arising from processing. The steel's microstructure is bainite, and it contains a total of 20% or less of martensite and/or protocrystalline ferrite and/or pearlite. However, the steel of WO2011/124851 does not exhibit hydrogen embrittlement and a cross-sectional reduction rate of 58% or more. However, it is desirable to further improve the resistance of the components to hydrogen embrittlement. International Publication No. 2016/158470International Publication No. 2011/124851 Carbon is present in the steel of this invention at a concentration of 0.05% to 0.15%. Carbon imparts strength to the steel through solid solution strengthening, and because carbon is gamma-genetic, it delays ferrite formation. Carbon is an element that influences the formation of cementite-free lath-like bainite. A minimum of 0.05% carbon is required to achieve a tensile strength of 1100 MPa, but if carbon is present above 0.15%, it reduces the ductility and machinability of the final product due to cementite formation. The carbon content is advantageously in the range of 0.08% to 0.14%, more preferably 0.09% to 0.14%, to obtain both high strength and high ductility simultaneously. Silicon is present in the steel of the present invention at a concentration of 0.01% to 1%. Silicon imparts strength to the steel of the present invention through solid solution strengthening. In particular, at the above content, silicon has the effect of hardening the bainite microstructure through solid solution hardening. Silicon reduces the formation of cementite nuclei because it inhibits the controlled precipitation and diffusion growth of carbides by forming a Si-enriched layer around the precipitation nuclei. Therefore, cementite-free lath-like bainite is obtained. Silicon also acts as a deoxidizing agent. A minimum of 0.01% silicon is required to impart strength to the steel of the present invention. At amounts exceeding 1%, the activity of carbon in the austenite increases, promoting transformation to proterite ferrite, resulting in decreased strength, slower bainite formation during continuous cooling, and potentially excessive residual austenite at the end of cooling. The preferred limit for silicon is 0.01 to 0.9%, more preferably 0.01 to 0.5%. Manganese is added to this steel at a concentration of 1.2% to 2%. Manganese imparts hardenability to the steel. It allows for a reduction in the critical cooling rate, enabling bainite transformation through continuous cooling without prior transformation. Manganese lowers the bainite onset temperature of the steel, thus resulting in refinement of the bainite structure, forming lath bainite, and thus improving the mechanical properties of the part. A minimum content of 1.2% by weight is necessary to obtain the desired bainite microstructure. However, exceeding 2% may cause retained austenite to transform into MA islands or fresh martensite, and since these phases are detrimental to this property, manganese adversely affects the steel of this invention. Furthermore, manganese forms sulfides such as MnS. These sulfides can improve machinability if their shape and distribution are well controlled. Otherwise, the sulfides can have a very detrimental effect on elongation. The preferred limit for manganese is 1.3% to 1