Hot rolling method for high-strength wheel steel for rim

Abstract

The present invention discloses a hot continuous rolling method for high-strength wheel steel for rims, comprising heating, rough rolling, finish rolling, and cooling and coiling steps. The rough rolling step involves rolling the heated ingot into an intermediate billet, with the reduction in each pass below the Tnr temperature accounting for no less than 80% of the total rough rolling reduction. The finishing rolling step involves controlling the strip threading speed of the finishing stand to ≥5.5 m / s. The cooling and coiling step involves laminar cooling of the finished strip before coiling, with the relaxation time T of the strip head before entering laminar cooling being ≥3.5 s. This method utilizes a strict controlled rolling and cooling process and controlled relaxation time at the finishing exit, utilizing the principle of controlling the ferrite phase ratio by the deformation and relaxation time of the austenite non-recrystallized zone. This method can produce high-strength steel for rims with uniform transverse properties. It can effectively increase and control the ferrite phase ratio, thereby improving welding and processing performance, and reduces production control difficulty.

Description

Technical Field

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

[0002] There is increasing pressure to reduce vehicle weight, fuel consumption, and emissions. Research shows that for every 10% decrease in vehicle mass, fuel consumption decreases by 8% and emissions by 4%. Steel wheels, rotating components of commercial vehicles, play a significant role in reducing energy consumption and emissions. Steel wheels consist of two parts: the rim and the spokes. The rim's processing is complex, involving flattening, rounding, flash butt welding, slagging, trimming, flaring, roll forming, expansion, airtightness testing, and puncturing. The hot-rolled steel plates used to make the wheels are required to exhibit excellent strength, toughness, ductility, weldability, post-weld formability, and fatigue resistance.

[0003] Major commercial vehicle manufacturers are currently pushing for lightweight wheels. The thickness of hot-rolled steel plates used in rims is being continuously reduced, from the initial 7.0mm for the 380MPa grade to approximately 4.2mm for the 750MPa grade. Consequently, requirements for the material's weldability, formability, and lateral uniformity are becoming increasingly stringent. Currently, when using hot-rolled wheel steel with a tensile strength of 750MPa, wheel manufacturers are experiencing widespread defects during the rim forming process, such as cracking in the weld and heat-affected zone, and cracking in the original hot-rolled plate edge. This results in a rim scrap rate exceeding 10%, which is often less cost-effective than using thicker steel plates with lower strength grades. This has dampened wheel manufacturers' enthusiasm for using 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;

[0006] The rough rolling process is as follows: the heated ingot is rolled into an intermediate billet, and during the rolling process, the reduction amount of each pass below the Tnr temperature accounts for no less than 80% of the total reduction amount of the rough rolling;

[0007] The finishing rolling process: controlling the strip threading speed of the finishing rolling last stand rolling mill to be ≥5.5m / s;

[0008] The cooling and coiling process: after finishing rolling, the strip is cooled by laminar flow and then coiled, and the relaxation time T before the head of the strip enters the laminar flow cooling is greater than or equal to 3.5s.

[0009] Furthermore, in the cooling coiling process, n cooling valves are closed at the front end of the laminar cooling process, and the number of cooling valves closed, n, is calculated using the following formula (1);

[0010] n=(T×V F -D) / d (1);

[0011] Where: n is an integer; T is the relaxation time before the strip head enters laminar cooling, s; V F The speed of the last finishing mill strip threading is set, 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.

[0012] Furthermore, 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.

[0013] Furthermore, in the finishing rolling process, cooling water is added between the stands of the finishing rolling mill at least three times.

[0014] The beneficial effects of adopting the above technical solution are:

[0015] The present invention uses a strict controlled rolling and cooling process and controls the relaxation time at the finish rolling exit, utilizing the principle of controlling the ferrite phase ratio by the deformation and relaxation time of the austenite non-recrystallization zone, to obtain high-strength steel for rims with uniform transverse performance. The present invention can quickly, accurately, and simply control the relaxation time of the strip at the finish rolling exit, mainly applying the principle of precisely controlling the transformation of phase structure in steel, and can effectively improve and control the ferrite phase ratio, thereby improving welding performance and processing performance, with low production control difficulty. The product obtained by the present invention has a yield strength of 650-750 MPa, a tensile strength of 750-850 MPa, an elongation of ≥19%, a transverse strength difference of ≤40 MPa for the strip, a difference between the hardness of the base material after welding and the peak hardness of the heat-affected zone of the weld ≤50 HV, a rim processing scrap rate of less than 2%, and satisfactory fatigue performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0017] 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

[0018] The hot continuous rolling method of high-strength wheel steel for rims has 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%, Nb 0.04% to 0.06%, V 0.035% to 0.045%, Ti 0.07% to 0.09%, Al 0.02% to 0.05%, N ≤ 0.0045%, and the remainder is Fe and unavoidable impurities.

[0019] The hot rolling method includes heating, rough rolling, finishing rolling and cooling and coiling processes, and the processes of each process are as follows:

[0020] (1) Heating: The billet is heated, and the tapping temperature after heating is 1230-1260℃.

[0021] (2) Rough rolling: The heated ingot is rolled into an intermediate billet with a thickness of 32 to 34 mm. The billet is rolled using two rough rolling mills, R1 and R2. The inlet temperature of the R2 mill is lower than the non-recrystallization critical temperature (Tnr temperature). The R2 reduction accounts for ≥80% of the total rough rolling reduction. The R1 mill rolls in one pass and the R2 mill rolls in five passes.

[0022] (3) Finishing rolling: The intermediate billet is rolled into strip steel, the starting rolling temperature of the finishing rolling is 1030-1050℃, and the final rolling temperature is 850-890℃; the cooling water between the stands of the finishing rolling mill is put into operation for at least 3 times to reduce the temperature and increase the speed, and 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 reducing the lateral temperature difference of the strip steel.

[0023] (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 before the strip head enters the laminar cooling is controlled to be ≥3.5s. The laminar cooling front-end cooling valve 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 n headers before the laminar cooling are set as fault valves and no water is sprayed. Water spraying starts from the n+1th header. The following formula (1) is used to calculate the number of cooling valves closed n;

[0024] n=(T×V F -D) / d (1);

[0025] 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 of the last finishing mill strip threading is set, 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.

[0026] (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 650-750 MPa, the tensile strength is 750-850 MPa, the elongation is ≥19%, the transverse strength difference of the strip is ≤40 MPa, the difference between the hardness of the base material after welding and the peak hardness of the heat-affected zone of the weld is ≤50 HV, the scrap rate of the rim processing is less than 2%, and the fatigue performance meets the requirements.

[0027] Example 1-5: The hot continuous rolling method of the high-strength wheel steel for rim is specifically described as follows.

[0028] The following equipment is used: regenerative heating furnace, two roughing mills, seven finishing mills, laminar cooling equipment, and downcoiler.

[0029] 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 is Fe and unavoidable impurities.

[0030] (1) Heating process: Billet heating and steel tapping temperature: 1230-1260℃.

[0031] (2) Rough rolling process: After heating, the ingot is rolled to 197 mm in one pass on a two-roll reversible rough rolling mill. Tnr is calculated as 1158 °C according to the classic formula. The inlet temperature of the rolled piece R2 is 1110-1140 °C. The rolled piece is rolled into an intermediate billet with a thickness of 32-34 mm in five passes on a four-roll reversible rough rolling mill R2. The R2 reduction accounts for 83% of the total reduction in rough rolling.

[0032] (3) Finishing rolling process: the starting temperature of finishing rolling is 1030-1050℃, the final rolling temperature is 850-890℃, and 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 the laminar cooling is obtained according to the above formula 1, and the n headers in front of the laminar cooling are set as fault valves through automatic control.

[0033] (4) Cooling and coiling process: The cooling process uses a front-end centralized cooling mode. Before the strip enters the laminar cooling area, the first n headers set as fault valves 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 by the coiler. The number of fault valves set at the front end of each embodiment is shown in Table 1.

[0034] Table 1: Number of fault valves set when relaxation time is met in each embodiment

[0035]

[0036] 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.

[0037] (5) Product performance: The performance of the hot-rolled steel strips in the width direction obtained in each embodiment is shown in Table 2; the 1000X metallographic structure diagram of the hot-rolled steel strip 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, can be seen from the figure that its microstructure is ferrite + pearlite + bainite, and the ferrite phase ratio exceeds 39%.

[0038] Table 2: Width-direction properties of finished hot-rolled steel strips in various embodiments

[0039]

[0040] 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 steel belt width, 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 steel belt width, the "drive side 1 / 8" is a position where the distance from the edge of the drive side steel belt is 1 / 8 of the steel belt width, the "drive side 1 / 4" is a position where the distance from the edge of the drive side steel belt is 1 / 4 of the steel belt width, and the "middle 1 / 2" is a position where the distance from the edges of the drive side and operating side steel belts is 1 / 2 of the steel belt width.

Claims

1. A hot rolling method for high-strength wheel steel for rims, characterized by: 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 billet, and the reduction in each pass below the Tnr temperature during the rolling process accounts for no less than 80% of the total reduction in the rough rolling process; the Tnr temperature is the critical temperature for non-recrystallization; 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 finishing 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; 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 is the strip threading speed of the last finishing mill, 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.

2. The hot continuous rolling method of high-strength wheel steel for rim according to claim 1, characterized in that: The rough rolling process adopts 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.

3. The hot continuous rolling method of high-strength wheel steel for rim according to claim 1 or 2, 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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