1200 MPa Grade Hot-Rolled Steel Plate for QP Heat-Treated Wheels and Production Method

Abstract

The present invention relates to a 1200 MPa grade hot-rolled steel plate for QP heat-treated wheels and a production method thereof. The chemical components in the steel plate are as follows by weight percentage: C 0.2% - 0.28%, Si 0.75% - 1.25%, Mn 1.6% - 2.2%, Al 0.01% - 0.07%, Cr ≤ 0.8%, P ≤ 0.008%, S ≤ 0.005%, Ti 0.05% - 0.1%, Nb 0.02% - 0.06%, and the balance is Fe and unavoidable impurity elements. After QP heat treatment, the overall strength of the wheel steel of the present invention can reach a yield strength of ≥ 800 MPa, a tensile strength Rm of ≥ 1200 MPa, and an elongation A50 of ≥ 12%.

Description

Technical Field

[0001] The present invention relates to the technical field of hot-rolled steel, and particularly relates to a 1200 MPa grade hot-rolled steel plate for QP heat-treated wheels and a production method thereof. Background Art

[0002] With the increasingly strict requirements of the automotive industry for energy conservation, consumption reduction, and environmental protection. Advanced high-strength steels will be more widely used in the automotive manufacturing industry to achieve its lightweight goal.

[0003] In heavy truck manufacturing, each heavy-duty truck has up to 24 wheels. Using steel plates with a higher strength level for the wheels will reduce the vehicle body weight, save energy, and improve the transportation efficiency, showing a good demonstration application effect in terms of lightweight.

[0004] The manufacturing process of automotive wheels adopts the spinning process, which requires the wheel steel to have good cold formability, elongation > 20%. In addition, it also has good flange elongation, high fatigue strength, and good welding performance. Currently, higher-grade steel types cannot achieve the required strength-toughness matching. Due to the contradiction between strength and toughness, when the strength exceeds 600 MPa, the spinning forming and application of the wheels are very difficult. Therefore, high-strength steels have not been widely promoted and applied in wheel manufacturing. Summary of the Invention

[0005] The purpose of the present invention is to provide a 1200 MPa grade hot-rolled steel plate for QP heat-treated wheels and a production method thereof. After QP heat treatment, the steel of the present invention has a tensile strength ≥ 1200 MPa, better plasticity, and elongation A50 ≥ 12%. And by the QP heat treatment method, retained austenite structure is introduced into the formed wheels, improving the fatigue resistance of the wheel parts. In addition, the present invention adopts a bell-type annealing process to reduce the strength of the base material before spinning forming to a yield strength ≤ 580 MPa, a tensile strength ≤ 780 MPa, and an elongation A50 ≥ 22%, thus greatly reducing various problems in the spinning forming process, including difficult uncoiling, uncoiling breakage, brittle cracking during shearing and blanking, excessive edge stress, damage to the shearing processing blade, spinning forming deviation, etc. It has very good feasibility and economy.

[0006] To achieve the above object, the present invention is realized by adopting the following technical solutions:

[0007] QP heat-treated 1200MPa grade hot-rolled steel plate for wheels, the chemical composition of the steel plate is as follows by weight: C 0.2%~0.28%, Si 0.75%~1.25%, Mn 1.6%~2.2%, Al 0.01%~0.07%, Cr≤0.8%, P≤0.008%, S≤0.005%, Ti 0.05%~0.1%, Nb 0.02%~0.06%, and the rest are Fe and unavoidable impurity elements.

[0008] In addition, the design principles of each alloy element and corresponding content of this steel grade are as follows:

[0009] C: Carbon is the most economical and effective solid solution strengthening element in steel. It is one of the main factors affecting yield strength and tensile strength. It is also an indispensable alloying element for improving hardenability, forming martensite and stabilizing austenite. Adding an appropriate amount of carbon content to steel will not only not reduce plasticity, but also because carbon diffuses into austenite during the partitioning process, it improves the stability of austenite and causes TRIP effect during deformation, which greatly improves its plasticity. Therefore, its content cannot be too low, but too high a content will cause the weldability and formability of steel to deteriorate. Therefore, the present invention controls the carbon content to 0.2-0.28%.

[0010] Si: Adding silicon to steel can improve the purity and deoxidation of steel, play a role in solid solution strengthening in steel, and can also play a role in inhibiting the precipitation of carbides. In the present invention, it is mainly used to stabilize the residual austenite. Si is distributed in the ferrite, which increases the chemical position of carbon in the ferrite, promotes the diffusion of carbon in the ferrite into the austenite, increases the carbon concentration in the residual austenite, forms carbon-rich residual austenite, and enhances the TRIP effect. However, if the silicon content is too high, red iron oxide scale will appear on the surface of the steel plate after rolling, which will deteriorate the surface quality. It will damage the welding performance and coating performance of the steel grade. Wheel steel has higher requirements on the surface of the steel plate, so the present invention controls the silicon content to 0.75% to 1.25%.

[0011] Mn: Manganese is the most effective element for improving strength and toughness, and is also one of the important alloying elements used in the present invention. Mn is a typical austenite stabilizing element, which can significantly delay the transformation of pearlite and bainite, reduce the critical cooling rate of martensite formation, and thus significantly improve the hardenability of steel. In addition, Mn combines with S in steel to form MnS to prevent hot brittleness of steel. However, a high Mn content will delay the precipitation of ferrite while delaying the transformation of pearlite, and will cause segregation in the center of the steel, deteriorating its fatigue properties, which is extremely unfavorable for wheel steel. Therefore, the present invention controls the Mn content within the range of 1.6% to 2.2%.

[0012] S: Sulfur is an impurity element in steel. If the sulfur content is too high, it will increase the hot brittleness tendency of the steel. Sulfur in steel often exists in the form of manganese sulfide. Such sulfide inclusions are very unfavorable to the impact toughness of the steel and cause anisotropy of properties. Therefore, the sulfur content in the steel should be controlled as low as possible. Thus, the sulfur content in the steel is controlled below 0.005%.

[0013] P: Phosphorus is an impurity element in steel. If the phosphorus content is too high, the eutectic structure of Fe2P will precipitate during the solidification of the steel billet and cause cold brittleness. Therefore, the lower the content of phosphorus element, the better. In actual production, it is generally controlled below 0.008%.

[0014] Ti: Titanium is one of the important alloying elements used in the present invention. Titanium can play a role in refining the grain size and improving the strength and toughness. In addition, the addition of Ti element is beneficial to improving the welding performance. If the content of Ti element is too high, it will increase the number of its coarse carbide and nitride inclusions, thus affecting the comprehensive mechanical properties. Therefore, the titanium content should be controlled at 0.050 - 0.10%.

[0015] Niobium: Niobium can significantly increase the recrystallization temperature of the steel and achieve grain refinement. During the hot rolling process, the carbide of niobium can hinder the recrystallization of deformed austenite. The addition of this element can further refine the microstructure of ferrite + pearlite, improving the strength and toughness of the steel. The present invention controls the content of niobium at 0.02% - 0.06%.

[0016] Cr: Chromium is one of the important alloying elements used in the present invention. Since the commercial vehicle wheels are quenched using thick - gauge steel plates, adding a certain amount of Cr element can effectively improve the hardenability during the heat treatment process, enabling the entire quenched wheel parts to be hardened. If the Cr content is too high, it will form additional Cr - containing oxides during the welding process, which is not conducive to the plasticity of the welded joint. Therefore, the content of Cr is controlled at ≤0.80%.

[0017] The thickness of the hot - rolled steel plate is 4.0 - 12.0 mm. This is because it can be used for both the rim and the spoke. Generally, the spoke of a commercial vehicle is thicker, the rim is generally 4.0 - 6.0 mm, and the spoke is 4 - 12 mm.

[0018] The metallographic structure of the hot - rolled steel plate is a ferrite + pearlite structure with fine grain size; the mechanical properties of the hot - rolled steel plate are: the tensile strength of the hot - rolled steel plate ≤780 MPa, the yield strength ≤580 MPa, and the elongation ≥22%.

[0019] The production method of the 1200 MPa - grade hot - rolled steel plate for QP heat - treated wheels includes smelting, refining, continuous casting, heating, rough rolling, finish rolling, laminar cooling, coiling, and bell - type furnace treatment processes. The specific method includes:

[0020] A slab with a thickness of 70 - 200 mm is charged into a heating furnace in a hot state or after being cooled to room temperature, heated at a temperature of 1150 - 1250 °C, with a holding time of 1.5 - 2.5 h, and then rolled in multiple passes on a rolling mill. The thickness of the intermediate billet is 35 - 45 mm, the hot rolling starting temperature is 1060 - 1120 °C, the hot rolling finishing temperature is 840 - 950 °C, the total reduction ratio in finishing rolling is ≥ 85%, and it is cooled to the coiling temperature after rolling. The coiling temperature is 650 °C - 750 °C to obtain a hot-rolled steel plate. When the hot rolling finishing temperature is lower than 840 °C, it is easy to form a mixed state of coarse and fine grains, resulting in a decline in the processing performance of the steel plate. When the finishing rolling temperature is higher than 950 °C, the obtained grains are too coarse, and the mechanical properties of the steel plate decline. High-temperature coiling is beneficial to the forming performance of the steel plate after hot rolling, so the coiling temperature is selected to be 650 °C - 750 °C. Since the technical solution of the present invention uses components of C combined with Si and Mn, the strength of the steel plate is relatively high after hot rolling, and it will be more difficult to carry out the next forming process and spin forming. Therefore, it is necessary to perform batch annealing treatment on the steel coil before cold rolling. When the temperature is too low, the yield strength cannot be sufficiently reduced. When the batch annealing temperature is too high, it will cause the grains to become coarse and reduce the final mechanical properties of the steel plate. Therefore, the steel coil after hot rolling is subjected to batch annealing, where the annealing temperature is 600 - 700 °C and the holding time is 6 - 12 h;

[0021] The heat treatment method of QP heat-treated wheels. The QP heat-treated wheels are formed by spin forming using a 1200 MPa grade hot-rolled steel plate. The QP heat treatment process used includes:

[0022] Heat the formed wheel to the two-phase region between 760 - 830 °C at a rate of 20 - 50 °C / s, with a holding time ≤ 250 s, and then quickly cool it to between 180 °C - 220 °C, which is the Ms - Mf point, in a heating furnace equipped with a blowing cooling device, with a cooling rate ≥ 30 °C / s. At this time, a primary martensite + austenite structure is formed. Subsequently, heat the wheel to 380 - 420 °C above the Ms point at a speed of 20 - 50 °C / s and maintain a partitioning of 120 s - 200 s, and then air cool it to room temperature. During this process, a primary martensite + secondary martensite + retained austenite structure is formed, making the tensile strength exceed 1200 MPa while maintaining an elongation rate of ≥ 10%. In addition, due to the addition of a certain amount of Cr element, the hardenability of the steel plate is increased, and a martensite + retained austenite structure can be formed throughout the wheel part. Therefore, the steel used for this wheel has good forming performance and fatigue performance, and has good surface quality, meeting the requirements for steel used in wheels.

[0023] The tensile strength of the wheel steel plate obtained after QP heat treatment is ≥1200 MPa, the yield strength is ≥800 MPa, and the elongation is ≥10%. Therefore, this hot-rolled steel plate not only has good cold formability and is suitable for wheel spinning. At the same time, after QP heat treatment, due to the retained austenite in the microstructure, it also has good fatigue resistance. Further, the wheel made by QP heat treatment is placed on a wheel test bench for fatigue life testing, and its fatigue life is greater than 1 million times.

[0024] The present invention proposes the composition and processing method of a thick-specification hot-rolled steel plate for QP heat-treated wheels, and proposes a bell-type furnace annealing process measure for the performance required by wheel spinning cold forming to ensure good formability of the steel plate in cold processing. After QP heat treatment, the tensile strength of the wheel steel will reach ≥1200 MPa, and the wheel parts have good fatigue performance.

[0025] The present invention adopts a bell-type furnace annealing heat treatment process after wheel forming to solve the problem of poor toughness before forming, and improves the strength of the wheel steel through QP heat treatment after forming, and introduces retained austenite into the steel through the quenching and partitioning process to improve the fatigue performance of the wheel steel.

[0026] Compared with the existing technology, the beneficial effects of the present invention are:

[0027] Compared with traditional wheel steels, the mechanical properties of the steel of the present invention are significantly improved in terms of strength and ductility, and due to the introduction of retained austenite, the fatigue strength of the wheels after heat treatment is good. The present invention adopts a bell-type annealing process to reduce the base material strength before cold forming processing to a yield strength ≤580 MPa, a tensile strength ≤780 MPa, and the elongation A50 is increased to ≥22%, thus greatly reducing a series of problems such as difficult uncoiling, uncoiling breakage, brittle cracking of shearing and blanking, excessive edge stress, and damage to the shearing processing tool blades caused by too high strength during the shape cutting and machining processes before cold forming, and has very good feasibility and economy.

[0028] After QP heat treatment, the overall strength of the wheel steel can reach a yield strength ≥800 MPa, a tensile strength Rm≥1200 MPa, and an elongation A50≥12%. Brief Description of the Drawings

[0029] Figure 1 is the microstructural photograph of the 1200 MPa grade QP steel in Example 1 of the present invention Detailed Embodiments

[0030] The following further illustrates the specific embodiments of the present invention in conjunction with the embodiments:

[0031] An embodiment of the present invention provides an application of the above-mentioned 1200 MPa grade hot-rolled steel plate for heat-treated wheels and its production method in the field of automobile manufacturing.

[0032] The following further elaborates on this solution through specific embodiments.

[0033] Table 1 shows the chemical compositions of the steel in Examples 1-5. The molten steel that meets the chemical compositions in Table 1 after smelting is subjected to vacuum degassing treatment and then continuously cast into billets, and then the billets are continuously rolled and coiled on a hot rolling mill.

[0034] Table 1 Chemical composition of examples Mass percentage / %

[0035] Example C Si Mn P S Ti Nb Cr Als 1 0.20 1.20 2.25 0.011 0.005 0.10 0.04 0.80 0.07 2 0.25 0.75 2.00 0.012 0.004 0.09 0.06 0.75 0.55 3 0.28 0.95 1.60 0.011 0.005 0.08 0.05 0.65 0.01 4 0.22 0.90 1.95 0.012 0.004 0.07 0.04 0.70 0.02 5 0.28 1.05 1.85 0.009 0.005 0.06 0.02 0.70 0.05

[0036] Table 2 shows the hot rolling manufacturing process parameters of the steel in Examples 1-5.

[0037] Table 2

[0038]

[0039] Table 3 shows the mechanical properties of the hot-rolled steel plates in Examples 1-5.

[0040] Table 3

[0041] Example Steel plate thickness / mm <![CDATA[Rp 0.2 / MPa]]> Rm / MPa A50 / ％ 1 4.0 428 616 26 2 4.5 485 680 24 3 6.0 503 730 25 4 8.5 540 748 26 5 11.0 580 760 23

[0042] Table 4 shows the process system of QP heat treatment in Examples 1-5

[0043] Table 4

[0044]

[0045] Table 5 shows the mechanical properties of the finally obtained wheel steel in Examples 1-5.

[0046] Table 5

[0047]

Claims

1. The hot-rolled steel sheet with a grade of 1200 MPa for QP heat-treated wheels is characterized in that, The chemical components in the steel plate are by weight percentage: C 0.25% - 0.28%, Si 0.75% - 1.25%, Mn 1.85% - 2.2%, Al 0.01% - 0.07%, Cr 0.65% - 0.8%, P ≤ 0.008%, S ≤ 0.005%, Ti 0.05% - 0.09%, Nb 0.02% - 0.06%, and the rest are Fe and inevitable impurity elements; A production method of a 1200MPa grade hot-rolled steel plate for QP heat-treated wheels, specifically including: Loading a slab with a thickness of 70 - 200mm into a heating furnace, heating at a temperature of 1150 - 1250°C, with a holding time of 1.5 - 2.5h, then performing multi-pass rolling on a rolling mill, the thickness of the intermediate billet is 35 - 45mm, the hot-rolling starting rolling temperature is 1060 - 1120°C, the hot-rolling finishing rolling temperature is 920 - 950°C, the total finishing rolling reduction rate ≥ 85%, cooling to the coiling temperature after rolling, the coiling temperature is 680°C - 750°C, to obtain a hot-rolled steel plate; after hot-rolling, the steel coil is annealed in a bell-type furnace, where the annealing temperature is 600 - 700°C and the holding time is 6 - 12h; The metallographic structure of the hot-rolled steel plate is ferrite + pearlite structure; the mechanical properties of the hot-rolled steel plate are: the tensile strength of the hot-rolled steel plate ≤ 780MPa, the yield strength ≤ 580MPa, and the elongation ≥ 22%.

2. The hot-rolled steel sheet with a yield strength of 1200 MPa for QP heat-treated wheels according to claim 1, wherein The thickness of the hot-rolled steel plate is 4.0 - 12.0mm.

3. The production method of the 1200 MPa grade hot-rolled steel sheet for QP heat-treated wheels according to claim 1, characterized in that, The specific method includes: Loading a slab with a thickness of 70 - 200mm into a heating furnace, heating at a temperature of 1150 - 1250°C, with a holding time of 1.5 - 2.5h, then performing multi-pass rolling on a rolling mill, the thickness of the intermediate billet is 35 - 45mm, the hot-rolling starting rolling temperature is 1060 - 1120°C, the hot-rolling finishing rolling temperature is 920 - 950°C, the total finishing rolling reduction rate ≥ 85%, cooling to the coiling temperature after rolling, the coiling temperature is 680°C - 750°C, to obtain a hot-rolled steel plate; after hot-rolling, the steel coil is annealed in a bell-type furnace, where the annealing temperature is 600 - 700°C and the holding time is 6 - 12h.

4. Heat treatment method for QP heat-treated wheels, characterized in that, The QP heat-treated wheel described above is spin-formed using the 1200MPa grade hot-rolled steel plate as claimed in claim 1, and the QP heat treatment process used includes: Heating the formed wheel to the two-phase region between 760 - 830°C at a rate of 20 - 50°C / s, with a holding time ≤ 250s, then quickly cooling to between 180°C - 220°C in a heating furnace with a blowing cooling device, the cooling rate ≥ 30°C / s, subsequently heating the wheel to 380°C - 420°C again at a speed of 20 - 50°C / s, and maintaining a partitioning of 120s - 200s, and then air-cooling to room temperature.

5. The heat treatment method of the QP heat-treated wheel according to claim 4, characterized in that, The tensile strength of the wheel steel obtained after QP heat treatment ≥ 1200MPa, the yield strength ≥ 800MPa, the elongation ≥ 12%, and its fatigue life is greater than 1 million times.

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