Hot continuous rolling method of high-strength wheel steel for rims

Abstract

The invention discloses a hot continuous rolling method of high-strength wheel steel for rims. The hot continuous rolling method comprises the working procedures of heating, rough rolling, finish rolling and cooling and coiling. In the rough rolling process, the heated casting blank is rolled into an intermediate blank, and the pass reduction under the Tnr temperature in the rolling process accounts for not less than 80% of the total rough rolling reduction; in the finish rolling process, the strip threading speed of a finish rolling last frame rolling mill is controlled to be larger than or equal to 5.5 m / s; in the cooling and coiling process, the finish-rolled strip steel is coiled after being subjected to laminar cooling, and the relaxation time T of the head of the strip steel before laminar cooling is larger than or equal to 3.5 s. According to the method, through a strict controlled rolling and controlled cooling process and a mode of controlling the relaxation time of a finish rolling outlet, the high-strength steel with uniform transverse performance for the rim can be obtained by utilizing the principle that the ferrite phase proportion is controlled by the deformation and relaxation time of an austenite non-recrystallization region; and the ferrite phase proportion can be effectively improved and controlled, so that the welding performance and the machining performance are improved, and the production control difficulty is small.

Description

Technical Field

[0001] The invention relates to the technical field of iron and steel metallurgy, in particular to a hot continuous rolling method of high-strength wheel steel for rims. Background Art

[0002] The pressure to reduce vehicle weight, fuel consumption and emissions is increasing. According to research, for every 10% decrease in vehicle mass, fuel consumption decreases by 8% and emissions decrease by 4%. Steel wheels, as rotating parts of commercial vehicles, play a significant role in reducing consumption and emissions. Steel wheels consist of two parts: the rim and the spoke. The processing procedures of the rim are complicated. The production process includes flattening, rounding, flash welding, slag planing, trimming, flaring, rolling forming, expansion, air tightness testing, punching and filling of air holes, etc. The hot-rolled raw steel plates used to make wheels are required to have good strength, toughness, plasticity, weldability, post-weld formability and good fatigue performance.

[0003] At present, major commercial vehicle manufacturers are pushing for lightweight wheels. The hot-rolled steel plates used for rims are continuously thinned from the initial 7.0mm at the 380Pa level to about 4.2mm at the 750MPa level. Correspondingly, the requirements for the welding forming performance and lateral uniformity of the materials are getting higher and higher. At present, when wheel manufacturers use hot-rolled wheel steel with a tensile strength of 750MPa, there are generally defects such as thinning and cracking of welds and heat-affected zones, and thinning and cracking of the original edge of hot-rolled plates during the rim processing and forming process, resulting in a rim processing scrap rate of more than 10%. Compared with the use of thick-gauge steel plates with lower strength levels to make rims, it has no advantage in cost control, which affects the enthusiasm of wheel manufacturers to use high-strength wheel steel to promote lightweighting. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a hot continuous rolling method for high-strength wheel steel for rims with excellent product performance.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: comprising heating, rough rolling, finish rolling and cooling and coiling processes; The rough rolling process: rolling the heated ingot into an intermediate ingot, wherein the reduction amount of each pass below the Tnr temperature during the rolling process accounts for no less than 80% of the total reduction amount of the rough rolling; The finishing rolling process: controlling the strip threading speed of the finishing rolling last stand rolling mill to be ≥5.5m / s; The cooling and coiling process: after the finish rolling, the strip is coiled after laminar cooling, and the relaxation time T before the head of the strip enters the laminar cooling is ≥3.5s.

[0006] Furthermore, in the cooling coiling process, n cooling valves are closed at the front end of the laminar cooling, and the number of cooling valves closed n is calculated using the following formula (1); n=(T×V F-D) / d (1); Where: n is an integer; T is the relaxation time before the strip head enters laminar cooling, s; V F The speed set for the last finishing mill strip threading is m / s; D is the distance between the finishing mill outlet and the first laminar cooling header, m; d is the distance between every two headers at the front end of the laminar cooling, m.

[0007] Furthermore, in the rough rolling process, two rolling mills, R1 and R2, are used for rolling, the inlet temperature of the R2 rolling mill is lower than the Tnr temperature, and the R2 reduction accounts for ≥80% of the total rough rolling reduction.

[0008] Furthermore, in the finishing rolling process, cooling water is added between the stands of the finishing rolling mill at least three times.

[0009] The beneficial effects of adopting the above technical solution are: The present invention uses a strict controlled rolling and controlled cooling process and a method of controlling the relaxation time at the finish rolling exit, and utilizes the deformation and relaxation time of the austenite non-recrystallization zone to control the ferrite phase ratio principle, so as to obtain high-strength steel for rims with uniform transverse performance. The present invention can quickly, accurately and simply control the relaxation time at the finish rolling exit of the strip steel, and mainly applies the principle of accurately controlling the transformation of the phase structure in the steel, which can effectively improve and control the ferrite phase ratio, thereby improving the welding performance and processing performance, and the production control difficulty is small. The yield strength of the product obtained by the present invention is 650-750 MPa, the tensile strength is 750-850 MPa, the elongation is ≥19%, the transverse strength difference of the strip steel is ≤40MPa, the difference between the hardness of the base material after welding and the peak hardness of the heat affected zone of the weld is ≤50HV, the scrap rate of the rim processing is less than 2%, and the fatigue performance meets the requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0011] Figure 1 This is a 1000X metallographic structure diagram of the hot-rolled steel strip product obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0012] The hot rolling method of high-strength wheel steel for rims comprises the following components and their mass percentages: C 0.065% to 0.080%, Si≤0.2%, Mn 1.65% to 1.80%, P≤0.015%, S≤0.005%, Nb0.04% to 0.06%, V 0.035% to 0.045%, Ti 0.07% to 0.09%, Als 0.02% to 0.05%, N≤0.0045%, and the rest are Fe and unavoidable impurities.

[0013] The hot rolling method includes heating, rough rolling, finishing rolling and cooling coiling processes, and the processes of each process are as follows: (1) Heating: The ingot is heated, and the tapping temperature after heating is 1230-1260°C.

[0014] (2) Rough rolling: The heated ingot is rolled into an intermediate billet, and the thickness of the intermediate billet is set to 32-34 mm; two rough rolling mills, R1 and R2, are used for rolling, the inlet temperature of the R2 rolling mill is lower than the non-recrystallization critical temperature (Tnr temperature), and the R2 reduction accounts for ≥80% of the total rough rolling reduction; the R1 rolling mill is rolled in one pass, and the R2 rolling mill is rolled in five passes.

[0015] (3) Finishing rolling: The intermediate billet is rolled into strip steel. The starting rolling temperature of the finishing rolling is 1030-1050°C and the final rolling temperature is 850-890°C. The cooling water between the stands of the finishing rolling mill is put into use for at least 3 times to reduce the temperature and increase the speed. The initial flow rate is set at 25%-35% to meet the strip threading speed of the last stand of the finishing rolling mill ≥5.5m / s, so as to achieve the purpose of rapid rolling and reduce the lateral temperature difference of the strip steel.

[0016] (4) Cooling and coiling: After finishing rolling, the strip is coiled after laminar cooling. The final cooling temperature of laminar cooling is 580-620℃. The relaxation time T of the strip head before entering the laminar cooling is controlled to be ≥3.5s. The front cooling valve of the laminar cooling is closed to ensure that the strip has a certain relaxation time after leaving the finishing mill. Specifically, the laminar cooling uses the front-end centralized cooling mode. When the strip enters the laminar cooling area, the first n headers of the laminar cooling are set as fault valves and no water is sprayed. Water is sprayed starting from the n+1th header. The number of cooling valves closed n is calculated using the following formula (1); n=(T×V F -D) / d (1); Where: n is an integer and decimal places are discarded; T is the relaxation time before the strip head enters laminar cooling, s; V F The speed set for the last finishing mill strip threading is m / s; D is the distance between the finishing mill outlet and the first laminar cooling header, m; d is the distance between every two headers at the front end of the laminar cooling, m.

[0017] (5) The microstructure of the high-strength wheel steel strip obtained by this method is ferrite + pearlite + bainite, the ferrite phase ratio exceeds 39%, the yield strength is 650-750MPa, the tensile strength is 750-850MPa, the elongation is ≥19%, the transverse strength difference of the strip is ≤40MPa, the difference between the hardness of the base material after welding and the peak hardness of the heat affected zone of the weld is ≤50HV, the scrap rate of the rim processing is less than 2%, and the fatigue performance meets the requirements.

[0018] Embodiment 1-5: The hot continuous rolling method of the high-strength wheel steel for rim is specifically described as follows.

[0019] The following equipment is used: regenerative heating furnace, two roughing mills, seven finishing mills, laminar cooling equipment, and down coiler.

[0020] Ingot: thickness 230 mm; mass percentage content of each component of the ingot: C 0.075%, Si 0.15%, Mn 1.70%, P 0.010%, S 0.003%, Nb 0.05%, V 0.040%, Ti 0.08%, Als 0.025%, N 0.0034%, and the rest are Fe and unavoidable impurities.

[0021] (1) Heating process: Ingot heating and steel tapping temperature: 1230-1260℃.

[0022] (2) Rough rolling process: After heating, the ingot is rolled on a two-roller reversible mill for one pass to reduce the thickness to 197 mm. Tnr is calculated according to the classic formula to be 1158 °C. The inlet temperature of the rolled piece R2 is 1110-1140 °C. The rolled piece is rolled on a four-roller reversible mill for five passes to be an intermediate billet with a thickness of 32-34 mm. The R2 reduction accounts for 83% of the total reduction in rough rolling.

[0023] (3) Finishing rolling process: The starting temperature of finishing rolling is 1030-1050℃, and the final rolling temperature is 850-890℃. After the rolled piece reaches the finishing rolling entrance temperature detection point, the secondary system of the rolling mill obtains the strip threading speed parameter V of the last stand finishing mill. F At the same time, the number n of fault valves set in the front section of laminar cooling is obtained according to the above formula 1, and the front n headers of laminar cooling are set as fault valves through automatic control.

[0024] (4) Cooling and coiling process: The cooling process uses the front-end centralized cooling mode. Before the strip enters the laminar cooling area, the first n headers set as fault valves for laminar cooling are not put into use. Laminar cooling starts with water spraying from the n+1th header. The final cooling temperature is 580-620°C, and the hot-rolled steel coil is obtained through the coiler. The number of fault valves set at the front end of each embodiment is shown in Table 1; Table 1: Number of fault valves set when the relaxation time is satisfied in each embodiment

[0025] The calculation process of the number n of fault valves set at the front end in Example 1 is used as an example to illustrate: n=(T×V F -D) / d=(3.5×5.5-8.75) / 0.5=21.

[0026] (5) Product performance: The performance of the hot-rolled steel strip products in the width direction obtained in each embodiment is shown in Table 2; the 1000X metallographic structure diagram of the hot-rolled steel strip product obtained in Example 1, in which (a) is the operating side 1 / 8, (b) is the operating side 1 / 4, (c) is the middle 1 / 2, (d) is the transmission side 1 / 4, and (e) is the transmission side 1 / 8. It can be seen from the figure that its microstructure is ferrite + pearlite + bainite, and the ferrite phase ratio exceeds 39%.

[0027] Table 2: Width direction properties of hot-rolled steel strip finished products in various embodiments

[0028] In Table 2, the "operating side 1 / 8" is a position where the distance from the edge of the operating side steel belt is 1 / 8 of the width of the steel belt, the "operating side 1 / 4" is a position where the distance from the edge of the operating side steel belt is 1 / 4 of the width of the steel belt, the "transmission side 1 / 8" is a position where the distance from the edge of the transmission side steel belt is 1 / 8 of the width of the steel belt, the "transmission side 1 / 4" is a position where the distance from the edge of the transmission side steel belt is 1 / 4 of the width of the steel belt, and the "middle 1 / 2" is a position where the distance from the edges of the transmission side and operating side steel belts is 1 / 2 of the width of the steel belt.

Claims

1. A hot rolling method for high-strength wheel steel for rims, characterized in that: Including heating, rough rolling, finishing rolling and cooling coiling process; The rough rolling process is as follows: the heated ingot is rolled into an intermediate ingot, and the reduction amount of each pass below the Tnr temperature during the rolling process accounts for no less than 80% of the total reduction amount of the rough rolling; The finishing rolling process: controlling the strip threading speed of the finishing rolling last stand rolling mill to be ≥5.5m / s; The cooling and coiling process: after the finish rolling, the strip is coiled after laminar cooling, and the relaxation time T before the head of the strip enters the laminar cooling is ≥3.5s.

2. The hot continuous rolling method of high-strength wheel steel for rim according to claim 1, characterized in that: In the cooling coiling process, n cooling valves are closed at the front end of laminar cooling, and the number of cooling valves closed n is calculated using the following formula (1); n=(T×V F -D) / d (1); Where: n is an integer; T is the relaxation time before the strip head enters laminar cooling, s; V F The speed set for the last finishing mill strip threading is m / s; D is the distance between the finishing mill outlet and the first laminar cooling header, m; d is the distance between every two headers at the front end of the laminar cooling, m.

3. The hot continuous rolling method of high-strength wheel steel for rim according to claim 1, characterized in that: The rough rolling process uses two rolling mills, R1 and R2, the inlet temperature of the R2 rolling mill is lower than the Tnr temperature, and the R2 reduction accounts for ≥80% of the total rough rolling reduction.

4. The hot continuous rolling method of high-strength wheel steel for rim according to claim 1, 2 or 3, characterized in that: In the finishing rolling process, cooling water is added between the stands of the finishing rolling mill at least three times.

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