Steel for low-residual-stress steering rack and manufacturing method thereof

Abstract

The invention relates to steel for a low-residual-stress steering rack and a manufacturing method of the steel. The steel comprises the following chemical components in percentage by mass: 0.45-0.48% of C, 0.25-0.35% of Si, 0.80-1.00% of Mn, 0.16-0.25% of Cr, less than or equal to 0.25% of Ni, less than or equal to 0.015% of P, 0.04-0.09% of S, less than or equal to 0.10% of Cu, less than or equal to 10ppm of [O] and the balance of Fe and inevitable impurities. The manufacturing method comprises the steps of converter smelting, LF refining, RH vacuum degassing, continuous casting, rolling, hardening and tempering, straightening, flaw detection and packaging. Wherein the heat treatment of the steel billet comprises two steps of quenching and tempering, the quenching temperature is 880-980 DEG C, the heat preservation time is 60-120 minutes, and water cooling is carried out; and the tempering temperature is 600-650 DEG C, the heat preservation time is 120-180 min, and water cooling is conducted. The quenching and tempering processes are optimized, the heat treatment sensitivity of the steel is reduced, and the mechanical property of the steel is guaranteed; the straightening process is optimized, and the residual stress of the steel is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and specifically relates to a steel for a low residual stress steering rack and a manufacturing method thereof. Background Art

[0002] At present, the physical quality level and stability of steering rack steel produced by domestic enterprises cannot fully meet the requirements of customers, and the physical quality has not been fully recognized by well-known foreign companies. Therefore, China urgently needs to develop high-quality and high-stability steering rack steel, and at the same time make the steering rack steel produced in my country very competitive in the international market.

[0003] In order to ensure safety and stability, the steering rack must have excellent properties such as high precision, high strength, and high wear resistance. It requires low non-metallic inclusions, harmful elements, and gas content in steel, and high comprehensive mechanical properties and quality stability. High-quality automotive steering gears require that the deformation of each tooth on the same rack after quenching must be less than 2 microns to ensure a constant transmission ratio during rack transmission, thereby improving rack transmission efficiency. High residual stress can cause deformation of multiple steering rack teeth.

[0004] At present, in order to improve the quality stability and purity of the steering rack steel for cars, the process technologies being developed at home and abroad mainly include: smelting with high-quality raw and auxiliary materials, precise control technology of molten steel composition and temperature when tapping the primary furnace, selection and addition process optimization of deoxidizers, research and control technology of refined slag, optimization of argon blowing and stirring process and vacuum treatment process, full protection casting technology, etc. The content of non-metallic inclusions, harmful elements and gases is low, and the comprehensive mechanical properties and quality stability requirements are high. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a low residual stress steering rack steel and a manufacturing method thereof in view of the above-mentioned prior art, so as to meet the requirements of low residual stress, hardness and mechanical properties of automobile rack steel.

[0006] The technical solution adopted by the present invention to solve the above problems is: a steel for a low residual stress steering rack, wherein the chemical composition and the mass percentage of the steel are: C 0.45-0.48%, Si 0.25-0.35%, Mn 0.80-1.00%, Cr 0.16-0.25%, Ni≤0.25, P≤0.015%, S 0.04-0.09%, Cu≤0.10%, [O]≤10ppm, and the remainder is Fe and unavoidable impurities.

[0007] The present invention uses blast furnace molten iron as the main raw material for steelmaking, controls the contents of five harmful elements and residual elements such as P, S, Cu, Ni, etc. in the steel; adopts LF refining outside the furnace and RH vacuum degassing to greatly remove non-metallic inclusions and gas content in the steel, so that the purity of the steel is high and the intrinsic quality of the steel is good; through molten steel refining, billet continuous casting, tundish induction heating, continuous casting light reduction and other technologies, the chemical composition of the steel is accurately controlled within a narrow range, and the uniformity of the molten steel composition is good; adopts continuous rolling mill rolling, finished product shot blasting, magnetic leakage, ultrasonic flaw detection technology, so that the steel has high dimensional accuracy and good surface quality, thereby effectively ensuring that the product is used and adopts reasonable straightening technology to achieve low residual stress control of the steel.

[0008] The chemical element composition of the rack steel of the present invention is determined based on the following:

[0009] Determination of C content: Taking into account the plasticity, toughness, machinability and weldability of the steel, the C content of the present invention is determined to be in the range of 0.45-0.48%.

[0010] Determination of Si content: Si here strengthens ferrite, can improve the matrix strength of the material, and contribute to the tensile strength of the material. The Si content range of the present invention is determined to be 0.25-0.35%.

[0011] Determination of Mn content: Mn has a good deoxidation effect in the steelmaking process, and because manganese can combine with sulfur to form MnS with a high melting point (1600°C), the harmful effect of sulfur is eliminated to a certain extent. The range of Mn content in the present invention is determined to be 0.80-1.00%.

[0012] Determination of Cr content: Cr is a medium carbide-forming element. Cr dissolved in austenite during heating strongly improves hardenability. Part of the Cr in steel replaces iron to form alloy cementite, improving stability; part of it dissolves in ferrite, producing solid solution strengthening, improving the strength and hardness of ferrite. The range of Cr content in the present invention is determined to be 0.16-0.25%.

[0013] Determination of O content: A large number of tests have shown that the reduction of oxygen content is significantly beneficial to the improvement of steel fatigue life. The range of O content in the present invention is determined to be [O] ≤ 10 ppm.

[0014] Determination of P and S content: P in steel causes serious segregation during solidification. P dissolves in ferrite to cause grain distortion and coarsening, and increases cold brittleness. The range of P content in the present invention is determined to be ≤0.015%. S makes steel hot brittle, reduces the ductility and toughness of steel, but adding an appropriate amount of S to steel can improve machinability. The range of S content in the present invention is determined to be 0.040-0.090%.

[0015] Another object of the present application is to provide a method for manufacturing steel for steering racks with low residual stress, the process of which is: converter smelting---LF refining---RH vacuum degassing---continuous casting---rolling---tempering---straightening---defect detection---packaging.

[0016] The main production processes are as follows:

[0017] (1) High-purity steelmaking control technology: Use converter hot-charging hot metal production technology to reduce harmful elements; use self-produced scrap steel and clean scrap steel to effectively control harmful impurity elements in steel (such as lead, tin, arsenic, antimony, bismuth and other low-melting-point metals); select special refining slag and tundish protection slag to fully absorb inclusions during the refining process; perform deep dephosphorization during hot metal pretreatment to make the phosphorus content reach the standard of premium grade steel; use RH or VD vacuum degassing, the maximum vacuum degree in the vacuum furnace is ≤1mbar, and the vacuum circulation treatment time of molten steel is maintained at ≥15min to ensure that harmful gases in the steel are effectively removed and the oxygen content of the steel is below 10ppm, so that the steel meets the high purity requirements and greatly improves the fatigue life and reliability of the rack.

[0018] (2) High-purity steel continuous casting control technology: The continuous casting adopts a large section of 390mm×510mm, uses a specific protective slag, and uses argon protection throughout the casting process to prevent secondary pollution oxidation of the molten steel and the formation of inclusions; electromagnetic stirring technology is used to improve the uniformity of the molten steel and promote the floating of inclusions. The ladle slag detection technology is used to control the ladle slag flowing into the continuous casting tundish to avoid contaminating the molten steel and improve the purity of the molten steel.

[0019] (3) Rolling: High temperature heating of 1150-1240℃ is adopted, and the heating time is ≥3.5h. The rolling is started at 1050~1100℃. The high reduction rolling is adopted to improve the uniformity and density of the steel structure. The round steel with the final specification of φ20~φ30 is rolled through continuous rolling. The high-precision KOCKS rolling mill is used for finishing rolling. Through the precise hole design, the dimensional tolerance of the finished product is controlled within the high-precision dimension of ±0.1.

[0020] (4) Quenching and tempering process: The quenching and tempering of the steel billet include two steps: quenching and tempering. The induction quenching temperature is 850℃~950℃, water cooling; the tempering temperature is 600℃~650℃, the holding time is 120~180min, water cooling. Further preference: the induction quenching temperature is 890℃, water cooling; the tempering temperature is 620℃, the holding time is 180min, water cooling. In order to ensure the performance requirements of the user's steering rack, there are clear requirements for the hardness after quenching and the hardness after tempering, and requirements are also made for the uniformity of hardness. Regulations for equipment affecting hardness and uniformity: The water volume is 8-12m 3 / h, the distance between the water ring and the induction coil is 32mm, the diameter of the water pipe is 30mm; suitable quenching process parameters: 860±10℃ quenching, 620℃±5℃ tempering.

[0021] (5) Low residual stress control: Due to the user's steering gear processing, if the stress of the steel is large, it will cause deformation of the tooth spacing during processing, so there are strict residual stress requirements for the rack steel. In order to meet the user's residual stress requirements, the straightening machine process parameters are strictly controlled to achieve low residual stress requirements and explore the best process parameters: the straightening machine pressure is controlled at 0.3-0.5mm, the straightening roller angle is controlled at 12-15°, the current change does not exceed 2A, and a standardized process document for this variety (quenching and tempering - straightening - flaw detection) is formed.

[0022] Compared with the prior art, the advantages of the present invention are:

[0023] The present invention designs the heat treatment process for low stress rack steel, optimizes the temperature range of quenching and tempering, makes the hardness uniformity of the steel better, and ensures the mechanical properties of the steel. By controlling the pressure reduction and current of the straightening machine, the steel has the characteristics of low residual stress. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a heat treatment process curve diagram in the manufacturing method of the present invention.

[0025] Figure 2 This is the changing trend of residual stress deformation after straightening. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0027] The chemical composition (wt%) of the new low stress steering rack steel of each embodiment of the present invention is shown in Table 1

[0028] Table 1 Ingredients

[0029] C Si Mn P S Cr Ni Cu O Example 1 0.47 0.28 0.92 0.009 0.32 0.16 0.01 0.01 0.0009 Example 2 0,46 0.30 0.88 0.010 0.32 0.16 0.01 0.01 0.0010 Example 3 0.46 0.28 0.87 0.008 0.35 0.18 0.02 0.01 0.0008

[0030] The present embodiment relates to a novel low-stress steering rack steel and a manufacturing method thereof, wherein the chemical composition is (by weight): C 0.45-0.48%, Si 0.25-0.35%, Mn 0.80-1.00%, Cr 0.16-0.25%, Ni≤0.25, P≤0.015%, S 0.04-0.09%, Cu≤0.10%, [O]≤10ppm. The balance is Fe and unavoidable impurities.

[0031] The manufacturing process is: converter smelting---LF refining---RH vacuum degassing---continuous casting---rolling---quenching and tempering---straightening---defect detection---packaging.

[0032] The conditions of the quenching and tempering process in this embodiment are: quenching temperature is 890°C, holding time is 120min, water cooling, tempering temperature is 620°C, holding time is 180min, water cooling, and the heat treatment process parameters refer to Figure 1 .

[0033] The obtained steering rack steel was optimized to adjust the pressure of the straightening machine and control the current at ±1.5A, and the residual stress remained stable. Figure 2 .

[0034] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A steel for a steering rack with low residual stress, characterized in that The chemical composition of the steel is C0.45~0.48%, Si0.25~0.35%, Mn 0.80~1.00%, Cr0.16~0.25%, Ni ≤0.25, P ≤0.015%, S0.04~0.09%, Cu ≤0.10%, [O] ≤10ppm, and the balance is Fe and unavoidable impurities.

2. The method for manufacturing a low residual stress steering rack steel according to claim 1, characterized in that: The method comprises the following steps: 1) Adopt converter hot-charging hot metal production technology to reduce harmful elements; use scrap steel to effectively control harmful impurity elements in steel; select refining slag and tundish protection slag to fully absorb inclusions during the refining process; perform deep dephosphorization on hot metal pretreatment to make the phosphorus content reach the standard of premium grade steel; use RH or VD vacuum degassing to ensure that harmful gases in steel are effectively removed so that the steel meets high purity requirements; 2) Continuous casting uses large-section 390mm×510mm continuous casting billets, uses protective slag, and uses argon protection throughout the casting process to prevent secondary pollution oxidation of molten steel and the formation of inclusions; electromagnetic stirring technology is used to improve the uniformity of molten steel and promote the floating of inclusions; large ladle slag detection technology is used to control the large ladle slag flowing into the continuous casting tundish to avoid contaminating the molten steel and improve the purity of the molten steel; 3) Rolling: adopt high temperature heating of 1150-1240℃, heating time ≥3.5h, rolling at 1050~1100℃, and high reduction rolling to improve the uniformity and density of steel structure. The final round steel with specifications of φ20~φ30 is rolled through continuous rolling. The high-precision KOCKS rolling mill is used for finishing rolling. Through the hole design, the finished product size tolerance is controlled within the high-precision size of ±0.1; 4) Quenching and tempering include two steps: quenching and tempering. The induction quenching temperature is 850℃~950℃, water cooling; the tempering temperature is 600℃~650℃, the holding time is 120~180min, and water cooling; 5) The tempered steel is straightened, flaw detected and packaged.

3. The method for manufacturing a steel for a steering rack with low residual stress according to claim 2, characterized in that: In step 1), the maximum vacuum degree in the vacuum furnace is ≤1mbar, the molten steel vacuum circulation treatment time is maintained ≥15min, and the oxygen content of the steel after vacuum degassing reaches below 10ppm.

4. The method for manufacturing a steel for a steering rack with low residual stress according to claim 2, characterized in that: In step (1), the final carbon content of the primary steel is controlled at 0.06-0.12%, the final phosphorus content is controlled at ≤0.20%, and the steel tapping temperature is between 1650-1700°C; vacuum degassing is performed in an RH or VD furnace, and a sufficient vacuum degree is achieved in the furnace, the vacuum degree is 1.0-1.45 mbar, and the high vacuum time is ensured to be ≥35 min, while maintaining sufficient circulation processing time, and the circulation time is ≥40 min.

5. The method for producing carburized bearing steel for large-size oil drilling rig turntable bearings according to claim 3, characterized in that: In step 4), the induction quenching temperature is 890°C, water cooling; the tempering temperature is 620°C, the holding time is 180 minutes, and water cooling.

6. The method for producing carburized bearing steel for large-size oil drilling rig turntable bearings according to claim 3, characterized in that: During the tempering process of step 4), the water volume of water cooling is 8-12m³ / h, the distance between the water ring and the induction coil is 32mm, and the diameter of the water pipe is 30mm.

7. The method for producing carburized bearing steel for large-size oil drilling rig turntable bearings according to claim 3, characterized in that: The straightening parameters in step 5) are controlled as follows: the straightening machine pressure is controlled at 0.3-0.5 mm, the straightening roller angle is controlled at 12-15°, and the current change does not exceed 2A.

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