Low-cost and high-bending-performance steel for automobile piston rod and preparation method of low-cost and high-bending-performance steel

Abstract

The invention belongs to the technical field of metallurgy, and particularly discloses a low-cost and high-bending-performance steel for an automobile piston rod, which comprises the following chemical components in percentage by weight: 0.43 to 0.45 percent of C, 0.18 to 0.22 percent of Si, 0.63 to 0.67 percent of Mn, less than or equal to 0.05 percent of residual Cr element, 0.025 to 0.035 percent of Al, 0.0050 to 0.0080 percent of N and the balance of Fe. Meanwhile, the invention further discloses a novel preparation method of the steel for the automobile piston rod, the low-cost chemical component design is adopted, the alloy cost is reduced, through the specific rolling and online heat treatment technology, the strength and toughness of the piston rod machined by downstream users through continuous heat treatment are indirectly improved, the service life of the piston rod is prolonged, and the service life of the piston rod is prolonged. The straightness of the piston rod during heat treatment by a user is guaranteed, and the requirement that the piston rod cannot be broken after being bent by 25 mm at three points is met

Description

Technical Field

[0001] The present invention belongs to the field of metallurgical technology, and particularly relates to a steel for automotive piston rods with low cost and high press-bending performance and a preparation method thereof. Background Art

[0002] The piston rod is the core stress-bearing component of an automotive shock absorber. During the driving process of the vehicle, it has to bear alternating tensile, compressive, and lateral forces, and plays the role of connecting the upper and lower parts of the shock absorber and damping vibration. This puts high requirements on the hardness, toughness, and other indicators of the piston rod. The steel used for the piston rod is generally 45 steel or S45C with a diameter of 14 mm to 30 mm. The processing technology of the piston rod is as follows: steel wire drawing - blanking - grinding - induction hardening - low-temperature tempering - straightening - grinding - electroplating - dehydrogenation annealing - post-plating grinding. With the popularization of new energy vehicles and the increase in the weight of new energy vehicles, the shock absorber piston has higher requirements for strength and toughness. After processing, a three-point press-bending test must be carried out on the piston rod. The two ends of the piston rod are fixed, and it is required that the center is pressed down by 25 mm without fracture. According to the feedback from the piston rod processing industry, for the 45 or S45C steel provided by major domestic special steel mills, after being processed into piston rods and subjected to a static bending test, fractures often occur when the press-bending is 16 mm to 22 mm, and the fracture performance cannot meet the requirements (high-end vehicles require that the press-bending is 25 mm without fracture). The fracture source is located within the range from the surface to a position 10 mm away from the surface. Some are in the quenched layer, and some are outside the quenched layer. There is abnormal ferrite tissue in the quenched layer, and the hardness of the quenched layer is on the low side. The heat treatment process equipment has reached its limit state, and it is impossible to improve the strength and toughness of the piston rod by adjusting the heat treatment parameters. Increasing the chemical composition in the steel can improve the hardenability and strength of the steel. For this reason, a large number of verifications have been carried out, but the results are not ideal: the manganese element in the steel is increased from 0.65% to 0.77% (the upper limit of the national standard is 0.80%), and 0.18% of chromium is added. Although the alloy element cost increases and the strength and toughness of the piston rod after heat treatment can be improved, the qualified rate is still on the low side and cannot meet the normal supply requirements.

[0003] At present, for the 45 and S45C steels used to produce piston rods in China, in order to achieve higher strength and toughness, the carbon element is controlled at the upper-middle limit of the standard, and the main alloy elements are controlled at the upper limit of the standard. The alloy cost is relatively high, but due to certain problems in microstructure control, the improvement of the three-point press-bending test performance is not obvious.

[0004] Based on the above problems, it is necessary to find a brand-new process method to reduce the alloy cost, improve the strength and toughness of the piston rod after heat treatment by downstream users, ensure the straightness of the piston rod during heat treatment of users, and meet the requirement that the three-point press-bending is 25 mm without fracture. Summary of the Invention

[0005] The object of the present invention is to provide a steel for automotive piston rods with low cost and high hot bending performance. Providing its preparation method is the second object of the present invention.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] As the first aspect of the present invention, a steel for automotive piston rods with low cost and high hot bending performance is disclosed. In terms of weight percentage, its chemical composition is: C 0.43 - 0.45%, Si 0.18 - 0.22%, Mn 0.63 - 0.67%, residual element Cr ≤ 0.05%, Al 0.025 - 0.035%, N 0.0050 - 0.0080%, and the balance is Fe.

[0008] The steel forms three layers of structure from the outer layer to the core along its cross-section. The outer layer is tempered sorbite with a thickness of 0.8 mm - 1.0 mm (prepared for further heat treatment by the user. The thickness is determined according to the hardened layer of 0.8 - 1.0 mm after the user's heat treatment). The middle part is troostite. The troostite lamellar spacing is finer, and it has high strength and toughness. The core structure is pearlite + ferrite. Due to the optimized cooling rate of the core steel, the pearlite lamellar spacing changes from 0.50 - 0.70 μm in the normal process to 0.25 - 0.40 μm, and the ferrite grains change from large block morphology to fine-grained dispersion distribution, significantly improving the strength and toughness of the steel.

[0009] As the second aspect of the present invention, a preparation method for the above-mentioned steel for automotive piston rods with low cost and high hot bending performance is further disclosed, including the following steps: 1) top and bottom combined blowing converter steelmaking, 2) LF refining, 3) RH vacuum treatment, 4) continuous casting, 5) reheating of the steel billet, and 6) rolling process; in step 6), the rolling process adopts a segmented on-line heat treatment process. The specific operation is to first carry out hot rolling. The temperature after KOCKS finishing rolling is 920 ± 20 °C. In order to control the structure and reduce the residual stress, through a large number of industrial test verifications, the key process parameters are determined: after finishing rolling, water cooling is carried out in two sections using water tanks. The first section of water tank water cooling has low pressure and large flow: pressure 0.10 MPa, flow rate 520 m 3 / h, controlling the surface temperature of the steel after water passing to be reduced to 820 ± 30 °C. The structure is still austenite, but through rapid cooling, the grain growth rate is slowed down. The water pressure of the second section of the water tank is increased: pressure 0.25 MPa, flow rate 410 m 3 / h, control the surface temperature of the steel to drop to 250±30°C instantaneously after water cooling. Before the steel enters the cooling bed, utilize the maximum core heat of the steel to heat up to 550 - 650°C, and precisely control to obtain a grade 2 tempered sorbite structure within the outer layer of 0.8 - 1.0 mm (determined according to the required hardened layer thickness of 0.8 - 1.0 mm after heat treatment by the user). After entering the cooling bed, the steel is densely arranged without gaps between teeth, covered with a heat preservation cover for more than 20 minutes. After leaving the heat preservation cover at 250 - 300°C, it can be sheared and stored in the warehouse.

[0010] As a further preference of the present invention, the specific operation of the top and bottom combined blowing converter steelmaking in step 1) is as follows: control C in the molten steel in the furnace at the end of blowing to be 0.16 - 0.25%, P≤0.012%. After converter alloying, the chemical composition is controlled as: C 0.37 - 0.41%, Si 0.13 - 0.18%, Mn 0.58 - 0.62%, residual element Cr≤0.05%, Al 0.050 - 0.060%.

[0011] As a further preference of the present invention, in step 2) LF refining, control the final slag basicity to be 4.5 - 6.0, the white slag holding time to be 20 - 45 minutes, and the Al 2 O 3 in the refining slag is controlled to be 15 - 30%. The main chemical elements are finely adjusted to the target values: C 0.43 - 0.45%, Si 0.18 - 0.22%, Mn 0.63 - 0.67%, Al 0.035 - 0.045%, and residual element Cr does not exceed 0.05%.

[0012] As a further preference of the present invention, in step 3) RH vacuum treatment, control the vacuum degree ≤66.7 Pa, the vacuum holding time to be 15 - 30 minutes, control the circulation flow rate to be 1000 - 1500 NL / min, the circulation gas is nitrogen, the soft blowing time after vacuum is 15 - 25 minutes, control the nitrogen content in the molten steel to be 0.0050 - 0.0080%, and Al 0.025 - 0.035%.

[0013] As a further preference of the present invention, in step 4) continuous casting, control the tundish superheat to be 20 - 30°C, the crystallizer electromagnetic stirring parameters to be 300 A * 3.5 Hz, the final electromagnetic stirring to be 320 A * 4 Hz, and carry out full protection casting.

[0014] As a further preference of the present invention, in step 5) when heating the steel billet, control the soaking section temperature to be increased to 1200 - 1250°C, the heat preservation time to be 50 - 80 minutes, reserve heat for subsequent heat treatment, and in order to prevent grain coarsening, increase the aluminum content and nitrogen content.

[0015] Overall, the present invention reduces costs by down-regulating the main elements in steel. Each element is designed according to weight percentage as follows: C 0.43 - 0.45%, Si 0.18 - 0.22%, Mn 0.63 - 0.67% (standard requirement 0.60 - 0.80%), Cr is designed as a residual element and does not exceed 0.05%, Al 0.025 - 0.035%, N content 0.0050 - 0.0080%. By controlling aluminum and nitrogen, the coarsening of austenite grains during heating is prevented. The heating furnace adopts high-temperature heating to provide heat for subsequent on-line heat treatment. During rolling, an on-line heat treatment process of segmented cooling is adopted. The steel on the cooling bed is closely arranged and the moving speed is reduced. The cooling bed is covered with a protective cover, and the waste heat of the steel is utilized for heat preservation treatment to control the surface structure and internal structure state, reduce the residual stress of the steel, and prevent the piston rod from bending due to residual stress, ultimately achieving the expected goal.

[0016] Specifically, compared with the prior art, the present invention has the following technical advantages:

[0017] 1) The present invention adopts a chemical composition design with lower costs. Currently, when producing 45 steel for piston rods in China, in order to improve the mechanical properties of the steel, the design mainly focuses on improving the chemical composition, resulting in a relatively high consumption of alloys and an unsatisfactory improvement effect. Based on the heat treatment principle, the microscopic structure and mechanical properties of different tissues, the present invention uses reverse thinking, adopts a low-cost main element design strategy, and takes changing the microscopic structure of the steel (compensating for the adverse effects caused by the reduction of alloying elements) as the main improvement goal. An on-line heat treatment process is developed and used as a pretreatment means for high-frequency quenching of users to optimize the product structure. On the premise of meeting the performance requirements, the target chemical composition of the required low-cost main elements is designed as follows: C 0.43 - 0.45%, Si 0.18 - 0.22%, Mn 0.63 - 0.67%, Cr is designed as a residual element and does not exceed 0.05% (agreement requirement ≤ 0.20%), reducing the alloy cost. During the RH treatment process of 45 steel in the present invention, the vacuum circulating gas is innovatively changed from expensive argon to inexpensive nitrogen, thereby controlling the nitrogen content in the steel between 0.0050 - 0.0080% and reducing the cost of circulating argon. The present invention finds that the C element is reduced from 0.47% to 0.44%; the Mn element is reduced from 0.75% to 0.65%; the Cr element is reduced from 0.15% to below 0.05%; the RH vacuum treatment is changed from conventional argon to nitrogen, and the cost per ton of steel can be reduced by 25 yuan;

[0018] 2) The preparation process of the present invention can effectively prevent grain coarsening during high-temperature heating: In the general process of 45 steel, it is required to reduce the nitrogen content in the steel, and the aluminum content range is controlled at 0.010 - 0.020%. In the present invention, the nitrogen content in the steel is controlled between 0.0050 and 0.0080%, and at the same time, the aluminum content in the steel is controlled between 0.025 and 0.035%, which can effectively prevent the coarsening of austenite grains during heating;

[0019] 3) The present invention adopts a specific on-line heat treatment process. At present, there are mainly two purposes for the controlled rolling and controlled cooling process in China. One is to perform low-temperature rolling at 750 - 800 °C during finish rolling to achieve the purpose of grain refinement. The other is to perform rapid cooling after rolling to control the surface temperature to rapidly cool between 650 - 750 °C, aiming to inhibit the precipitation of harmful structures such as network carbide, which is mainly used in bearing steel and high-carbon steel. In the present invention, a high-temperature finish rolling + post-rolling heat treatment process is adopted, which is essentially different from the ordinary controlled cooling process. According to the requirement that the induction hardening layer of the piston rod after heat treatment by the user must reach a thickness of 0.8 - 1.0 mm, the finish rolling temperature is designed to be 920 ± 20 °C (storing heat at high temperature to prepare for subsequent temperature rise and control of the final microstructure). After rolling, surface forced cooling is carried out through the cooling water in the water tank, accurately controlling the cooling speed and intensity to make the instantaneous temperature within the range of 0.8 mm - 1.0 mm thickness of the steel surface reach below 250 °C. After surface forced cooling quenching, using the high-temperature heat of the steel core to transfer outward, the surface temperature rises, and the temperature range is 500 - 600 °C, completing the on-line high-temperature tempering of the extremely cold structure within 0.8 mm - 1.0 mm of the steel surface. After being transferred to the cooling bed, they are densely arranged and quickly covered with a heat preservation cover, and at the same time, the steel transfer speed of the cooling bed is reduced to ensure that the time in the cover is more than 20 minutes. Due to the high temperature of the steel core, the morphology of the lamellar cementite with high hardness and poor toughness in the structure changes, and the toughness of the steel core structure is effectively improved. Finally, three layers of tissue are formed from the surface to the core of the steel cross-section. The outer layer is tempered sorbite with a thickness of about 0.8 mm - 1.0 mm (preparing the tissue for further heat treatment by the user), the middle part is troostite, the strength and toughness of troostite are high, and the core is pearlite + ferrite, and the pearlite and ferrite grains in the core are more refined (see Figure 1) The lamellar cementite morphology in troostite and pearlite is optimized, and the strength and toughness of the overall steel are greatly improved (see Table 1). The tempered sorbite structure with a thickness of 0.8 - 1.0 mm on the surface lays a good organizational foundation for the next surface induction hardening. After induction hardening + low-temperature tempering by downstream users, tempered martensite with a thickness of 0.8 mm - 1.0 mm is formed on the surface, with fine and uniform grains, no residual abnormal ferrite, and good metallographic structure. The structure in the middle and the core remains the same as the optimized hot-rolled material structure. The overall strength and toughness of the piston rod are greatly improved. The static press-bending test performance is improved from breaking when pressed down by 16 mm - 22 mm originally to still not breaking when pressed down by 38 mm - 45 mm, far exceeding the requirement of not breaking when pressed down by 25 mm specified in the technical standard. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is the cross-sectional tissue state (metallurgical microscope + electron microscope) of 45 steel with a diameter of φ24 mm under the process of the present invention, Figure 2 is the cross-sectional tissue state (metallurgical microscope + electron microscope) of 45 steel with a diameter of φ24 mm under the normal process. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention will be further described below in conjunction with the detailed description of the invention.

[0022] Example 1

[0023] For the low-cost, high press-bending performance steel for automotive piston rods in this example, by weight percentage, its chemical composition is: C 0.44%, Si 0.20%, Mn 0.65%, Cr residual element ≤0.05%, Al 0.030%, N 0.0070%, and the balance is Fe.

[0024] Its preparation method includes the following steps:

[0025] 1) Top and bottom combined blown converter steelmaking, and the specific operation is: controlling C 0.20% and P ≤0.012% in the molten steel in the converter at the end of blowing. After converter alloying, the chemical composition is controlled as: C 0.39%, Si 0.15%, Mn 0.60%, Cr residual element ≤0.05%, Al 0.055%;

[0026] 2) Controlling the final slag basicity at 5.0 during LF refining, maintaining the white slag for 30 minutes, and controlling Al 2 O 3 in the refining slag at 20%, and finely adjusting the main chemical elements to the target values: C 0.44%, Si 0.20%, Mn 0.65%, Al 0.040%, Cr residual element, not exceeding 0.05%;

[0027] 3) RH vacuum treatment, controlling the vacuum degree ≤ 66.7 Pa, the vacuum holding time is 20 min, controlling the circulation flow rate at 1200 NL / min, the circulating gas is nitrogen, the soft blowing time after vacuum is 20 min, controlling the nitrogen content in the molten steel at 0.0070%, and Al at 0.030%;

[0028] 4) In continuous casting, controlling the tundish superheat at 25 °C, the mold electromagnetic stirring parameters are 300 A * 3.5 Hz, the final electromagnetic stirring is 320 A * 4 Hz, and full protection casting;

[0029] 5) When heating the billet, controlling the soaking section temperature to be raised to 1220 °C and the heat preservation time is 60 min;

[0030] 6) Rolling process; the rolling process adopts a segmented on-line heat treatment process. The specific operation is to first carry out hot rolling. The temperature after KOCKS finishing rolling is 920 ± 20 °C. In order to control the microstructure and reduce the residual stress, through a large number of industrial test verifications, the key process parameters are determined: after finishing rolling, water cooling through the water tank is carried out in two sections. The first section of the water tank water cooling has low pressure and large flow rate: the pressure is 0.10 MPa, and the flow rate is 520 m 3 / h, controlling the surface temperature of the steel after passing through the water to be reduced to 820 ± 30 °C. The microstructure is still austenite, but through rapid cooling, the grain growth rate is slowed down. The second section of the water tank increases the water pressure: the pressure is 0.25 MPa, and the flow rate is 410 m 3 / h, controlling the surface temperature of the steel after passing through the water to be instantly reduced to 250 ± 30 °C. Before the steel goes onto the cooling bed, using the highest heat in the steel core to heat up to 600 °C, a grade 2 tempered sorbite microstructure can be obtained within 0.9 mm of the surface layer. After going onto the cooling bed, the steel is densely arranged without gaps between teeth, covered with a heat preservation cover for more than 20 min. After coming out of the heat preservation cover at 280 °C, it can be sheared and stored in the warehouse.

[0031] Example 2

[0032] For the low-cost, high-pressure bending performance steel for automotive piston rods in this example, in terms of weight percentage, its chemical composition is: C 0.43%, Si 0.18%, Mn 0.63%, Cr residual element ≤ 0.05%, Al 0.025%, N 0.0050%, and the balance is Fe.

[0033] Its preparation method includes the following steps:

[0034] 1) Top-bottom combined blowing converter steelmaking. The specific operation is: controlling C at 0.17% and P ≤ 0.012% in the molten steel in the converter at the end of blowing. After converter alloying, the chemical composition is controlled as: C 0.37%, Si 0.13%, Mn 0.58%, Cr residual element ≤ 0.05%, Al 0.050%;

[0035] 2) During LF refining, control the final slag basicity at 4.5, keep the white slag for 20 min, and control the Al 2 O 3 in the refining slag at 15%. Fine-tune the main chemical elements to the target values: C 0.43%, Si 0.18%, Mn 0.63%, Al 0.035%, and the residual element Cr not exceeding 0.05%. 2 O 3

[0036] 3) During RH vacuum treatment, control the vacuum degree ≤ 66.7 Pa, keep the vacuum for 15 min, control the circulation flow rate at 1000 NL / min, the circulation gas is nitrogen, the soft blowing time after vacuum is 15 min, and control the nitrogen content in the molten steel at 0.0050% and Al at 0.025%.

[0037] 4) During continuous casting, control the tundish superheat at 20 °C, the mold electromagnetic stirring parameters at 300 A * 3.5 Hz, the final electromagnetic stirring at 320 A * 4 Hz, and carry out full protection casting.

[0038] 5) When heating the steel billet, control the soaking section temperature to be increased to 1200 °C and the holding time to be 50 min to reserve heat for subsequent heat treatment.

[0039] 6) Rolling process; the rolling process adopts a segmented on-line heat treatment process. The specific operation is to first carry out hot rolling, the temperature after KOCKS finishing rolling is 920 ± 20 °C. In order to control the microstructure and reduce the residual stress, through a large number of industrial test verifications, the key process parameters are determined: after finishing rolling, use the water tank to carry out water cooling in two sections. The first section of the water tank water cooling has low pressure and large flow rate: the pressure is 0.10 MPa, the flow rate is 520 m 3 / h, control the surface temperature of the steel after water passing to be reduced to 820 ± 30 °C, the microstructure is still austenite, but through rapid cooling, the grain growth rate is slowed down. The second section of the water tank increases the water pressure: the pressure is 0.25 MPa, the flow rate is 410 m 3 / h, control the surface temperature of the steel after water passing to be instantly reduced to 250 ± 30 °C. Before the steel enters the cooling bed, use the highest core heat of the steel to heat up to 550 °C, and a grade 2 tempered sorbite microstructure can be obtained within 0.8 mm of the surface layer. After entering the cooling bed, the steel is densely arranged without gaps between teeth, covered with a heat preservation cover and kept warm for more than 20 min. After leaving the heat preservation cover at 250 °C, it can be sheared and stored in the warehouse. 3 / h, control the surface temperature of the steel after water passing to be reduced to 820 ± 30 °C, the microstructure is still austenite, but through rapid cooling, the grain growth rate is slowed down. The second section of the water tank increases the water pressure: the pressure is 0.25 MPa, the flow rate is 410 m 3 / h, control the surface temperature of the steel after water passing to be instantly reduced to 250 ± 30 °C. Before the steel enters the cooling bed, use the highest core heat of the steel to heat up to 550 °C, and a grade 2 tempered sorbite microstructure can be obtained within 0.8 mm of the surface layer. After entering the cooling bed, the steel is densely arranged without gaps between teeth, covered with a heat preservation cover and kept warm for more than 20 min. After leaving the heat preservation cover at 250 °C, it can be sheared and stored in the warehouse.

[0040] Example 3

[0041] The low-cost, high-pressure bending performance steel for automotive piston rods in this example, by weight percentage, has the following chemical composition: C 0.45%, Si 0.22%, Mn 0.67%, Cr residual element ≤ 0.05%, Al 0.035%, N 0.0080%, and the balance is Fe.

[0042] ​Its preparation method includes the following steps:

[0043] 1) Top and bottom combined blowing converter steelmaking. The specific operation is as follows: Control the C content in the molten steel in the converter at the end of blowing to 0.25%, P≤0.012%. After converter alloying, the chemical composition is controlled as: C 0.41%, Si 0.18%, Mn 0.62%, residual Cr element≤0.05%, Al 0.060%;

[0044] 2) Control the final slag basicity at 6.0 during LF refining, keep the white slag for 45 min, and control the Al2O3 in the refining slag at 30%. Fine-tune the main chemical elements to the target values: C 0.45%, Si 0.22%, Mn 0.67%, Al 0.045%, residual Cr element, not exceeding 0.05%; 2 O 3 3) RH vacuum treatment. Control the vacuum degree≤66.7 Pa, the vacuum holding time at 30 min, control the circulation flow rate at 1500 NL / min, change the circulation gas from argon to nitrogen, the soft blowing time after vacuum at 25 min, and control the nitrogen content in the molten steel at 0.0080%, Al 0.035%;

[0045] 4) During continuous casting, control the tundish superheat at 30℃, the mold electromagnetic stirring parameters at 300 A*3.5 Hz, the final electromagnetic stirring at 320 A*4 Hz, and conduct full protection casting;

[0046] 5) When heating the steel billet, control the soaking section temperature to be increased to 1250℃ and the holding time at 80 min;

[0047] 6) Rolling process; The rolling process adopts a segmented on-line heat treatment process. The specific operation is to first carry out hot rolling. The temperature after KOCKS finishing rolling is 920±20℃. In order to control the microstructure and reduce the residual stress, through a large number of industrial test verifications, the key process parameters are determined: After finishing rolling, use water tanks in two sections for water cooling through the water. In the first section, the water tank water cooling has low pressure and large flow rate: the pressure is 0.10 MPa, the flow rate is 520 m

[0048] / h, control the surface temperature of the steel after passing through the water to be reduced to 820±30℃. The microstructure is still austenite, but through rapid cooling, the grain growth rate is slowed down. In the second section, increase the water pressure of the water tank: the pressure is 0.25 MPa, the flow rate is 410 m 3 / h, control the surface temperature of the steel after passing through the water to be instantly reduced to 250±30℃. Before the steel enters the cooling bed, use the maximum heat in the steel core to heat up to 650℃. A grade 2 tempered sorbite microstructure can be obtained within 1.0 mm of the surface layer. After entering the cooling bed, the steel is densely arranged without gaps between teeth, cover the heat preservation cover and keep it warm for more than 20 min. After leaving the heat preservation cover at 300℃, it can be sheared and stored in the warehouse. 3 Comparative Example 1

[0049] ​

[0050] Comparative Example 1 was prepared using normal chemical compositions and normal rolling processes. The specific preparation process is as follows:

[0051] In terms of weight percentage, the chemical composition of this comparative example is: C 0.47%, Si 0.25%, Mn 0.77%, Cr 0.18%, Al 0.015%, N is the residual content of 0.0038%, and the balance is Fe.

[0052] Its preparation method includes the following steps:

[0053] 1) Top and bottom combined blowing converter steelmaking. The specific operation is as follows: Control C at 0.15% and P ≤ 0.012% in the molten steel in the converter at the end of blowing. After converter alloying, the chemical composition is controlled as: C 0.44%, Si 0.22%, Mn 0.75%, Cr residual element 0.15%, Al 0.030%.

[0054] 2) Control the final slag basicity at 5.0 in LF refining, keep the white slag for 40 minutes, and control Al in the refining slag 2 O 3 at 23%. Fine-tune the main chemical elements to the target values: C 0.47%, Si 0.25%, Mn 0.77%, Al 0.025%, Cr 0.18%.

[0055] 3) RH vacuum treatment. Control the vacuum degree ≤ 66.7 Pa, keep the vacuum for 30 minutes, control the circulation flow rate at 1500 NL / min, the circulation gas is argon, the soft blowing time after vacuum is 25 minutes, the nitrogen in the molten steel is the residual content of 0.0038%, and Al is 0.015%.

[0056] 4) In continuous casting, control the tundish superheat at 30°C, the mold electromagnetic stirring parameters at 300 A * 3.5 Hz, the final electromagnetic stirring at 320 A * 4 Hz, and carry out full protection casting;

[0057] 5) When heating the steel billet, control the soaking section temperature at 1180°C and the holding time at 60 minutes;

[0058] 6) Rolling process; Adopt normal temperature rolling. The temperature after KOCKS finishing rolling is 840 ± 20°C. After rolling, there is no strong cooling. It is directly sent to the cooling bed. On the cooling bed, the steel is arranged with teeth spaced apart, the heat preservation cover is open, and it is sheared and stored in the warehouse after leaving the cooling bed.

[0059] Comparative Example 2

[0060] The chemical composition of the main elements in this Comparative Example 2 was designed according to the low-cost target in the present invention, but normal heating and process rolling were adopted. The specific preparation process is as follows:

[0061] By weight percentage, the chemical composition of this comparative example is: C 0.44%, Si 0.20%, Mn 0.65%, Cr residual element ≤ 0.05%, Al 0.015%, N is the residual content 0.0040%, and the balance is Fe.

[0062] Its preparation method includes the following steps:

[0063] 1) Top and bottom combined blowing converter steelmaking, and the specific operation is: controlling C 0.15% and P ≤ 0.012% in the molten steel in the converter at the end of blowing. After converter alloying, the chemical composition is controlled as: C 0.40%, Si 0.16%, Mn 0.62%, Cr residual element ≤ 0.05%, Al 0.035%;

[0064] 2) Control the final slag basicity at 5.2 in LF refining, the white slag holding time at 43 min, and the Al in the refining slag 2 O 3 is controlled at 25%, and the main chemical elements are fine-tuned to the target values: C 0.44%, Si 0.20%, Mn 0.65%, Al 0.025%, Cr 0.04% (residual element);

[0065] 3) RH vacuum treatment, controlling the vacuum degree ≤ 66.7 Pa, the vacuum holding time at 30 min, controlling the circulation flow rate at 1500 NL / min, the circulation gas is argon, the soft blowing time after vacuum at 25 min, the nitrogen in the molten steel is the residual content 0.0040%, and Al 0.015%;

[0066] 4) In continuous casting, the tundish superheat is controlled at 28 °C, the mold electromagnetic stirring parameters are 300 A * 3.5 Hz, the final electromagnetic stirring is 320 A * 4 Hz, and full protection casting;

[0067] 5) When heating the steel billet, control the soaking section temperature at 1180 °C and the holding time at 60 min;

[0068] 6) Rolling process; Normal temperature rolling is adopted, the temperature after KOCKS finishing rolling is 840 ± 20 °C, no forced cooling is carried out after rolling, directly sent to the cooling bed, the steel on the cooling bed is arranged with teeth separated, the heat preservation cover is open, and it is sheared and stored in the warehouse after leaving the cooling bed.

[0069] Performance experiment

[0070] Using the process of the present invention (Example 1), normal chemical composition, normal rolling process (Comparative Example 1), and low-cost design of main chemical elements, normal rolling process (Comparative Example 2) to prepare φ24 mm 45 steel respectively. Three mechanical testing specimens are prepared for each process for mechanical property testing. The results are shown in Table 1. After the steel of the three processes is heat-treated by the user into piston rod blanks, three piston rod blanks are taken for three-point bending testing respectively. The results are shown in Table 2.

[0071] Table 1 Comparison of Mechanical Properties of Steel for Three Processes

[0072]

[0073]

[0074] As can be seen from Comparative Example 1 and Comparative Example 2 in Table 1, for the same normal rolling process, when the main chemical elements are designed with low cost, the average yield strength is 13 MPa lower, the average tensile strength is 18 MPa lower, and the average U-notch impact energy is 9 J lower. For the 45 steel produced by the process of the present invention, although the main chemical elements are also designed with low cost, the rolling and specific on-line heat treatment processes of the present invention precisely control the steel structure from the outside to the inside. Compared with Comparative Example 2 with the same low-cost design of main chemical elements, the average yield strength is increased from 393 MPa to 595 MPa, the average tensile strength is increased from 652 MPa to 825 MPa, and the normal-temperature U-notch impact energy is increased from 54 J to 79 J. The rolling and cooling process of the present invention plays a key role.

[0075] Table 2 Comparison of Three-Point Bending Test Results of φ24mm 45 Steel for Normal Rolling Process and the Process of the Present Invention

[0076]

[0077] As can be seen from Comparative Example 1 and Comparative Example 2 in Table 2, for the same normal rolling process, when the main chemical elements are designed with low cost, the performance of the static bending test decreases from an average bending of 19.7 mm to fracture to an average bending of 17.7 mm to fracture. Compared with Comparative Example 2, although the main elements of the process of the present invention are also designed with low cost, after the rolling and on-line heat treatment processes of the present invention and then the surface heat treatment by the user, the piston rod is improved from breaking at an average pressing down of 17.7 mm to still not breaking at an average pressing down of 41.3 mm, far exceeding the index of not breaking at a pressing down of 25 mm required by the technical standard. This shows that the overall strength and toughness of the finished piston rod processed from the 45 steel produced by the process of the present invention are greatly improved.

[0078] In addition, for the steel produced by the process of the present invention, the large residual stress generated when the steel is extremely cold is eliminated, and the straightness of a single piston rod is guaranteed during the subsequent processing by the user, and it has been supplied to the downstream market in batches.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A low-cost, high-pressure bending performance steel material for automobile piston rods, characterized in that: The chemical composition thereof is, in weight percentage, C 0.43-0.45%, Si 0.18-0.22%, Mn 0.63-0.67%, residual Cr element ≤0.05%, Al 0.025-0.035%, N 0.0050-0.0080%, and the balance is Fe.

2. The low-cost, high-pressure bending performance automobile piston rod steel according to claim 1 is characterized in that: The steel material forms a three-layer structure from the surface to the core along its cross section, wherein the outer layer is tempered troostite with a thickness of 0.8 to 1.0 mm, the middle part is troostite, and the core structure is pearlite + ferrite, and the core pearlite is more refined, and the pearlite lamellar spacing is 0.25 to 0.40 μm.

3. The method for preparing the low-cost, high-pressure bending performance steel for automobile piston rod according to any one of claims 1-2, characterized in that: The following steps are involved: 1) top and bottom combined blowing converter steelmaking, 2) LF refining, 3) RH vacuum treatment, 4) continuous casting, 5) billet heating and 6) rolling process; wherein, the rolling process of step 6) adopts a segmented online heat treatment process, and the specific operation is to first adopt high temperature rolling, the temperature after KOCKS final rolling is 920±20℃, and then use water tanks for water cooling in two stages after final rolling. The first stage water tank water cooling has low pressure and large flow: pressure 0.10MPa, flow 520m 3 / h, control the surface temperature of the steel after water penetration to 820±30℃, and increase the water pressure in the second water tank: pressure 0.25MPa, flow rate 410m 3 / h, control the surface temperature of the steel to drop to 250±30℃ instantly after water penetration, and use the heat in the core of the steel to raise the temperature to 550-650℃ before the steel goes on the cooling bed, accurately control the outer layer within 0.8-1.0mm (determined according to the user's hardened layer after heat treatment must reach 0.8-1.0mm thickness) to obtain a second-level tempered bainite structure. After going on the cooling bed, the steel is densely arranged without teeth, covered with a heat preservation cover for more than 20 minutes, and after leaving the heat preservation cover at 250-300℃, it can be sheared and stored.

4. The method for preparing low-cost, high-pressure bending performance automobile piston rod steel according to claim 3 is characterized in that: In step 1), the specific operation of top and bottom combined blowing converter steelmaking is: by controlling C 0.16-0.25% and P ≤ 0.012% in the molten steel in the furnace at the end of blowing, the chemical composition after converter alloying is controlled to be: C 0.37-0.41%, Si 0.13-0.18%, Mn 0.58-0.62%, Cr residual element ≤ 0.05%, Al 0.050-0.060%.

5. The method for preparing the low-cost, high-pressure bending performance steel for automobile piston rod according to claim 3 is characterized in that: In step 2), the final slag basicity is controlled at 4.5-6.0 during LF refining, the white slag retention time is 20-45 min, the Al2O3 in the refined slag is controlled at 15-30%, and the main chemical elements are fine-tuned to the target values: C 0.43-0.45%, Si 0.18-0.22%, Mn 0.63-0.67%, Al 0.035-0.045%, and the residual Cr element does not exceed 0.05%.

6. The method for preparing the low-cost, high-pressure bending performance steel for automobile piston rod according to claim 3 is characterized in that: In step 3), during RH vacuum treatment, the vacuum degree is controlled to be ≤66.7Pa, the vacuum holding time is 15 to 30min, the circulation flow rate is controlled to be 1000 to 1500NL / min, the circulation gas is nitrogen, the soft blowing time after vacuum is 15 to 25min, and the nitrogen content in the molten steel is controlled to be 0.0050 to 0.0080%, and Al to be 0.025 to 0.035%.

7. The method for preparing a low-cost, high-pressure bending performance steel material for automobile piston rods according to claim 3, characterized in that: Step 4) During continuous casting, the superheat is controlled at 20-30°C, the electromagnetic stirring parameters of the crystallizer are 300A*3.5Hz, the terminal electromagnetic stirring is 320A*4Hz, and full protection casting is performed.

8. The method for preparing a low-cost, high-pressure bending performance steel material for automobile piston rods according to claim 3, characterized in that: Step 5) When heating the steel billet, the temperature of the soaking section is controlled to be raised to 1200-1250° C., and the holding time is 50-80 minutes.

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