Steel for 700Mpa-grade hot-rolled wheel and preparation method of steel

Abstract

The invention discloses 700Mpa-grade hot-rolled wheel steel and a preparation method thereof, and belongs to the technical field of metallurgy. The steel for the wheel comprises the following chemical components in percentage by mass: less than or equal to 0.1% of C, 0.05-0.10% of Si, less than or equal to 1.65% of Mn, less than 0.015% of P, less than 0.005% of S, less than 0.060% of Nb, less than 0.020% of Ti, less than 0.03% of Cr, less than 0.045% of A1s, less than 0.005% of N, less than 0.003% of O and the balance of iron and inevitable impurities. The production method comprises the procedures of molten iron pretreatment, converter smelting, LF refining, RH refining, slab continuous casting, heating, rolling, laminar cooling and coiling. Under the condition that expensive Mo element is not added, through reasonable component design and a controlled rolling and controlled cooling process, the yield strength of the wheel steel is larger than or equal to 500 MPa, the tensile strength is larger than or equal to 680 MPa, the ductility is larger than 18%, and the wheel steel has good welding performance and machining performance.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a 700 MPa grade hot-rolled wheel steel and a preparation method thereof. Background Art

[0002] Automobile wheels are crucial components, enduring various forces and moments during driving, performing functions such as load-carrying, steering, driving, and braking, directly impacting the safety and reliability of the vehicle. With the development of heavy-duty transportation, the trend in steel wheel manufacturing is to minimize weight while ensuring safety and reliability. Consequently, stringent requirements are placed on the material's welding properties, fatigue resistance, formability, surface quality, and steel cleanliness and uniformity.

[0003] National patent CN113957359 A, applied for on October 28, 2021, discloses a high-strength steel for automobile wheels and a preparation method thereof. This method adopts a chemical composition with a high Si content, which is prone to surface iron scale control problems, and the Nb and Ti alloy additions are both high, resulting in a high alloy cost.

[0004] National patent CN115058644 A, applied for on April 26, 2022, discloses a 700MPa grade hot-rolled wheel steel and its manufacturing method. The method adopts a chemical composition Ti content of 0.45-0.55%. Excessive Ti content is more likely to cause fluctuations in performance control, and large particles of TiN are not conducive to wheel fatigue performance.

[0005] National patent CN115125432 A, applied for on June 13, 2022, discloses a method for manufacturing 650MPa-grade high-strength, lightweight steel for automotive spokes. This method uses a chemical composition with an Si content of 0.40-0.70%, and there is also a risk of difficult-to-control surface iron scale. Summary of the Invention

[0006] The present invention provides a 700 MPa-grade hot-rolled wheel steel and a preparation method thereof. Through reasonable chemical composition design and controlled rolling and controlled cooling process, a moderately cost-effective high-strength wheel steel with a tensile strength of 700 MPa can be obtained. The high-strength wheel steel has both good welding performance and processing performance, and the fatigue performance of the wheels produced by the steel is excellent.

[0007] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions: A 700 MPa grade hot-rolled wheel steel, having the following chemical composition and mass percentages: C≤0.1%, Si: 0.05-0.10%, Mn≤1.65%, P<0.015%, S<0.005%, Nb<0.060%, Ti<0.020%, Cr<0.03%, Als<0.045%, N<0.005%, O<0.003%, and the balance being Fe and unavoidable impurities.

[0008] Furthermore, the microstructure of the wheel steel is ferrite+pearlite+bainite, the grain size is ≥14 levels, and the banded structure is ≤1 level.

[0009] Furthermore, the wheel steel has a yield strength of ≥500 MPa, a tensile strength of ≥680 MPa, and an elongation of >18%.

[0010] The design principle of each chemical element of the present invention is as follows: a low-carbon, low-silicon, medium-manganese component system is adopted, low C is used to reduce segregation, reduce carbon equivalent, and improve weldability; Mn improves the hardenability of the material, plays a role in solid solution strengthening, and improves the strength of the material; Cr can increase the hardenability of steel, thereby improving the hardness of steel, and at the same time can increase the stability of supercooled austenite, so that austenite undergoes phase transformation at a lower temperature and a larger supercooling degree, thereby refining the structure; Nb element increases the temperature of the non-recrystallization zone in the finishing rolling stage, increases the deformation of the non-recrystallization zone of austenite, thereby refining the grains; Si has the effect of deoxidation and strengthening, but Si exceeds a certain value. , which will affect the surface quality and weldability of the steel. Therefore, the present invention adopts a low Si design, and the upper limit of the value range is set to 0.1%; trace Ti can reduce the hardness of the weld and improve the ductility and plastic deformation ability of the weld, but when the Ti content is higher than a certain value, micron-level Ti (CN) will be generated in the steel, which will seriously affect the forming performance and fatigue life of the material. Therefore, the upper limit of the Ti value range of the present invention is set to 0.020%; P seriously damages the plasticity and toughness of the steel plate; S combines with Mn and other elements in steel to form plastic inclusions MnS, which is detrimental to the plasticity and toughness of the steel. Therefore, the harmful elements S and P are strictly controlled.

[0011] The production method of the above-mentioned 700 MPa grade hot-rolled wheel steel includes the steps of molten iron pretreatment, converter smelting, LF refining, RH refining, slab continuous casting, heating, rolling, laminar cooling, and coiling; In the rolling process, the rough rolling is rolling in the austenite recrystallization zone; the finishing rolling is rolling in the austenite non-recrystallization zone, the finishing rolling inlet temperature is 980-1010°C, the finishing rolling adopts the speed increase rolling, and the final rolling temperature is controlled at 840±20°C; The laminar cooling process adopts a two-stage cooling mode, with a front-stage cooling rate of ≥70°C / s and a final cooling temperature of 440-480°C.

[0012] Furthermore, in the hot metal pretreatment process, the S content in the hot metal after desulfurization is controlled to be ≤ 0.005 wt%.

[0013] Furthermore, in the converter smelting process, slag detection during tapping and a slide gate for slag blocking are adopted, the slag volume is controlled at 1 - 3 kg / t, and 3 - 5 kg / t of post-furnace modifying agent is added to the top slag.

[0014] Furthermore, in the LF refining process, FeO in the top slag is < 1 wt%, and the pure degassing time in the RH refining process is ≥ 6 min.

[0015] Furthermore, in the continuous casting process, soft reduction of ≥ 4.5 mm is applied, a peritectic steel powder is used, the superheat in the tundish is controlled at 15 - 25 °C, and weak cooling mode is adopted for secondary cooling water.

[0016] Furthermore, in the slab heating process, the heating time is 2 - 3 h, the residence time in the soaking section is ≥ 45 min, and the tapping temperature is 1220 - 1260 °C.

[0017] Furthermore, in the laminar flow cooling process, the intermediate point temperature is 690 - 710 °C, the air cooling time is 5 - 8 s, and the cooling rate in the latter section is 15 - 30 °C / s.

[0018] The beneficial effects of adopting the above technical solutions are as follows: (1) Through reasonable composition design, the present invention can ensure that the properties of the material meet the standard requirements without adding expensive metal elements such as Mo; (2) The present invention controls the Si content to be ≤ 0.10%, ensuring good welding performance of the steel for wheels. At the same time, it can improve defects such as scale and pitting caused by high Si, improve the surface quality of the hot rolled coil and the appearance quality of parts, thereby enhancing the processing efficiency and yield rate of the wheel manufacturing process.

[0019] (3) Cr can also inhibit the pearlite transformation to generate a certain amount of bainite, improving the comprehensive mechanical properties of the material. In the present invention, Cr is added for microalloying. As a strengthening element, Cr can replace Mn, increasing the strength of the steel while reducing segregation and improving the comprehensive mechanical properties of the material; (4) Large particle TiN has a regular shape with sharp corners, seriously affecting the formability and fatigue performance of the material. To avoid the generation of large particle TiN caused by excessive Ti content, the present invention controls the Ti content < 0.020, which can not only refine the grains, increase the strength, but also improve the welding and fatigue performance of the material.

[0020] (5) Without adding a large amount of alloying elements, the present invention controls the finish rolling temperature at 980 - 1010 °C, ensuring rolling stability while adopting low-temperature rolling to obtain a finer grain structure with a grain size of up to 14 grades, effectively enhancing the strength of the material.

[0021] (6) By adopting the front-end high cooling rate cooling mode with a cooling rate ≥ 70 °C / s in the front stage and a finishing cooling temperature of 440 - 480 °C and a lower coiling temperature, the present invention improves the microscopic banded structure of the sheet, controls the banded structure to below grade 1, which is beneficial to the welding and forming properties of the material. DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the metallographic structure morphology diagram of the hot-rolled wheel steel for Example 1; Figure 2 It is the metallographic structure morphology diagram of the hot-rolled wheel steel for Example 2; Figure 3 It is the metallographic structure morphology diagram of the hot-rolled wheel steel for Example 3; Figure 4 It is the metallographic structure morphology diagram of the hot-rolled wheel steel for Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be further described in detail below in conjunction with the embodiments and the drawings.

[0024] The following equipment is adopted in the embodiments: LF refining furnace and RH vacuum degassing furnace are used for steelmaking, 230 mm thick continuous casting billets are used for hot-rolling steel billets, regenerative reheating furnace is used for the heating process, two roughing mills are used for rough rolling, descaling is carried out at the roughing inlet, seven finishing mills are used for finishing rolling. According to the rolling direction, the seven finishing mills are numbered as F1, F2, F3, F4, F5, F6, F7 in sequence. Descaling is carried out at the finishing inlet. The laminar cooling process with ultra-fast cooling is used for the post-rolling cooling process, and an underground coiler is used for the coiling process. Example 1

[0025] First, pre-desulfurization of hot metal is carried out, and the S content is controlled to be 0.002 wt%; converter smelting is carried out, alloying of Si, Mn, Cr, and Nb is carried out, and the steel is tapped with a slide gate slag stopper, the slag volume is 1.0 kg / t, and 3 kg / t of ladle top slag modifier is added; in the LF refining process, desulfurization is carried out with white slag, and the top slag FeO is controlled to be < 1 wt%; in the RH refining process, the vacuum degassing time is 8 min, and the inclusions are modified by Ca treatment after Ti alloying; in the continuous casting process, soft reduction of ≥ 4.5 mm is used, a peritectic steel casting powder is used, the superheat of the tundish is controlled at 18 - 25 °C, and weak cooling mode is used for secondary cooling water. The mass percentages of the chemical components of the continuous casting billet are: C: 0.09%, Si: 0.05%, Mn: 1.6%, P: 0.010%, S: 0.004%, Nb: 0.058%, Ti: 0.017%, Cr: 0.015%, Al: 0.038%, N: 0.0046%, O: 0.0025%, and the rest is Fe and unavoidable impurities.

[0026] Put the above-mentioned billets into a heating furnace for heating for 2 hours, with a residence time of 46 minutes in the soaking zone, and then take them out of the furnace after heating to 1240°C. After heating, the slab is descaled by high-pressure water and then rough-rolled, with full-pass descaling used in rough rolling; the entry temperature for finish rolling is 982°C, and it is rolled into a thickness of 5.0 mm through 7 finish-rolling mills, with a finishing temperature of 822°C; the laminar cooling mode is two-stage cooling, with a cooling rate of 80°C / s in the first stage, a midpoint temperature of 709°C, an air-cooling time of 6 s, a cooling rate of 30°C / s in the second stage, and a coiling temperature of 442°C. Example 2

[0027] First, pre-desulfurize the hot metal to control the S content to 0.005 wt%; smelt in a converter, carry out alloying of Si, Mn, Cr, and Nb, tap steel with a slide gate slag stopper, with a slag entrainment amount of 2.6 kg / t, and add 5 kg / t of ladle top slag modifier; in the LF refining process, desulfurize with white slag and control the top slag FeO < 1 wt%; in the RH refining process, the vacuum degassing time is 6 min, and Ca treatment is carried out to modify inclusions after Ti alloying; in the continuous casting process, apply soft reduction of ≥4.5 mm, use a peritectic steel mold powder, control the superheat in the tundish at 17 - 23°C, and adopt a weak cooling mode for secondary cooling. The mass percentages of the chemical components of the continuous casting billet are: C: 0.08%, Si: 0.10%, Mn: 1.65%, P: 0.012%, S: 0.003%, Nb: 0.050%, Ti: 0.015%, Cr: 0.022%, Al: 0.044%, N: 0.0048%, O: 0.0026%, and the rest are Fe and inevitable impurities.

[0028] Put the above-mentioned billets into a heating furnace for heating for 2.5 hours, with a residence time of 49 minutes in the soaking zone, and then take them out of the furnace after heating to 1260°C. After heating, the slab is descaled by high-pressure water and then rough-rolled, with full-pass descaling used in rough rolling; the entry temperature for finish rolling is 1004°C, and it is rolled into a thickness of 4.8 mm through 7 finish-rolling mills, with a finishing temperature of 857°C; the laminar cooling mode is two-stage cooling, with a cooling rate of 74°C / s in the first stage, a midpoint temperature of 700°C, an air-cooling time of 7 s, a cooling rate of 15°C / s in the second stage, and a coiling temperature of 478°C. Example 3

[0029] First, desulfurize the hot metal preliminarily to control the S content at 0.002 wt%; smelt in a converter, alloyize with Si, Mn, Cr, and Nb, tap steel with a slide gate to slag, with the slag carry-over amount being 2.5 kg / t, and add 4.3 kg / t of ladle top slag modifier; in the LF refining process, desulfurize by making white slag and control the top slag FeO < 1 wt%; in the RH refining process, the vacuum degassing time is 9 min, and modify inclusions by Ca treatment after Ti alloying; in the continuous casting process, apply soft reduction of ≥4.5 mm, use peritectic steel protective slag, control the superheat of the tundish at 16 - 24 °C, and adopt a weak cooling mode for secondary cooling. The mass percentages of the chemical components of the continuous casting billet are: C: 0.10%, Si: 0.08%, Mn: 1.62%, P: 0.009%, S: 0.002%, Nb: 0.053%, Ti: 0.018%, Cr: 0.028%, Al: 0.042%, N: 0.0027%, O: 0.0014%, and the rest are Fe and unavoidable impurities.

[0030] Put the above-cast billet into a heating furnace for heating for 3 hours, with a soaking time of 45 minutes in the soaking section, and take it out of the furnace after heating to 1260 °C. After heating, the slab is descaled by high-pressure water and then rough-rolled, with full-pass descaling used in rough rolling; the entry temperature for finish rolling is 1010 °C, and it is rolled into a thickness of 4 mm through 7 finish rolling mills, with a final rolling temperature of 845 °C; the laminar cooling mode is two-stage cooling, with a front-stage cooling rate of 70 °C / s, an intermediate point temperature of 690 °C, an air-cooling time of 8 s, a rear-stage cooling rate of 25 °C / s, and a coiling temperature of 462 °C. Example 4

[0031] First, desulfurize the hot metal preliminarily to control the S content at 0.003 wt%; smelt in a converter, alloyize with Si, Mn, Cr, and Nb, tap steel with a slide gate to slag, with the slag carry-over amount being 3 kg / t, and add 3.5 kg / t of ladle top slag modifier; in the LF refining process, desulfurize by making white slag and control the top slag FeO < 1 wt%; in the RH refining process, the vacuum degassing time is 9 min, and modify inclusions by Ca treatment after Ti alloying; in the continuous casting process, apply soft reduction of ≥4.5 mm, use peritectic steel protective slag, control the superheat of the tundish at 15 - 22 °C, and adopt a weak cooling mode for secondary cooling. The mass percentages of the chemical components of the continuous casting billet are: C: 0.09%, Si: 0.09%, Mn: 1.65%, P: 0.009%, S: 0.003%, Nb: 0.057%, Ti: 0.014%, Cr: 0.020%, Al: 0.040%, N: 0.0033%, O: 0.002%, and the rest are Fe and unavoidable impurities.

[0032] Put the above billets into a heating furnace for heating for 3 hours, with a residence time of 47 minutes in the soaking section, and then take them out of the furnace after heating to 1220°C. After heating, the slab is descaled by high-pressure water and then rough-rolled, with descaling in all passes during rough rolling; the entry temperature for finish rolling is 981°C, and it is rolled into a thickness of 5 mm through 7 finish-rolling mills, with a finishing temperature of 840°C; the laminar cooling mode is two-stage cooling, with a cooling rate of 72°C / s in the first stage, an intermediate point temperature of 694°C, an air-cooling time of 5 s, a cooling rate of 23°C / s in the second stage, and a coiling temperature of 480°C.

[0033] Samples of the hot-rolled plates obtained in Examples 1-4 were taken for testing and observation, and their mechanical properties are shown in Table 1. The microstructural morphology diagrams under the metallurgical microscope are respectively as Figures 1-4 shown. It can be seen from the figures that the microstructures of the hot-rolled wheel steels in each example are all ferrite + pearlite + bainite, and the metallographic test results are shown in Table 1.

[0034] Table 1. Mechanical properties and metallographic test results of the hot-rolled wheel steels in each example .

Claims

1. A steel for hot-rolled wheels of 700 Mpa grade, characterized in that, The chemical composition and mass percentage of the steel for wheels are as follows: C ≤ 0.1%, Si: 0.05 - 0.10%, Mn ≤ 1.65%, P < 0.015%, S < 0.005%, Nb < 0.060%, Ti < 0.020%, Cr < 0.03%, A1s < 0.045%, N < 0.005%, O < 0.003%, and the balance is Fe and unavoidable impurities.

2. The hot-rolled steel for 700Mpa-class hot-rolled wheels according to claim 1, characterized in that, The microstructure of the steel for wheels is ferrite + pearlite + bainite, the grain size ≥ 14 grades, and the banded structure ≤ 1 grade.

3. The hot-rolled steel for 700Mpa-class hot-rolled wheels according to claim 2, characterized in that, The yield strength of the steel for wheels ≥ 500 MPa, the tensile strength ≥ 680 MPa, and the elongation > 18%.

4. The preparation method of the 700Mpa grade hot-rolled steel for wheels according to any one of claims 1-3, characterized in that, It includes the processes of hot metal pretreatment, converter smelting, LF refining, RH refining, slab continuous casting, heating, rolling, laminar flow cooling, and coiling. In the rolling process, the rough rolling is carried out in the austenite recrystallization zone; the finish rolling is carried out in the austenite non-recrystallization zone, the finish rolling inlet temperature is 980 - 1010 °C, the finish rolling adopts speed-up rolling, and the finish rolling temperature is controlled at 840 ± 20 °C. In the laminar flow cooling process, a two-stage cooling mode is adopted, the cooling rate in the first stage ≥ 70 °C / s, and the final cooling temperature is 440 - 480 °C.

5. The preparation method of the hot-rolled steel wheel for 700Mpa level according to claim 4, characterized in that In the hot metal pretreatment process, the S content in the hot metal after desulfurization is controlled ≤ 0.005 wt%.

6. The preparation method of the hot-rolled steel wheel for 700Mpa level according to claim 5, characterized in that, In the converter smelting process, slag detection during tapping and a slide gate slag stopper are used, the slag volume is controlled at 1 - 3 kg / t, and 3 - 5 kg / t of post-furnace modifier is added to the top slag.

7. The preparation method of the 700Mpa grade hot-rolled steel for wheels according to claim 6, characterized in that, In the LF refining process, FeO in the top slag < 1 wt%, and in the RH refining process, the pure degassing time ≥ 6 min.

8. The preparation method of the hot-rolled steel for 700Mpa-class hot-rolled wheels according to claim 7, characterized in that, In the continuous casting process, soft reduction of ≥ 4.5 mm is applied, a peritectic steel powder is used, the superheat in the tundish is controlled at 15 - 25 °C, and weak cooling mode is adopted for secondary cooling water.

9. The preparation method of the 700Mpa grade hot-rolled steel for wheels according to claim 8, characterized in that, In the slab heating process, the heating time is 2 - 3 h, the residence time in the soaking section ≥ 45 min, and the tapping temperature is 1220 - 1260 °C.

10. The preparation method of the 700Mpa grade hot-rolled steel for wheels according to any one of claims 1-9, characterized in that, In the laminar flow cooling process, the intermediate point temperature is 690 - 710 °C, the air cooling time is 5 - 8 seconds, and the cooling rate in the second stage is 15 - 30 °C / s.

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