A process for improving the longitudinal shear crookedness of qste650tm girders

Abstract

This invention discloses a process for improving the camber curve of QSTE650TM beam steel during slitting, including slab preparation, rolling and cooling, straightening, and slitting steps. The slab preparation step involves shortening the slab length to 8.5–9 m. The rolling and cooling step involves rough rolling and finish rolling of the slab, followed by cooling of the finished strip. During cooling, the strip is first air-cooled to 880–900°C, then laminar flow cooling is performed at a rate of 20–25°C / s, with a final cooling temperature of 550–580°C. The straightening step includes rough straightening and finish straightening. During rough straightening, the pressure of the rough straightener is 18–20 MPa. During finish straightening, the straightening insertion is controlled at 30%–45% of the strip thickness, and the pressure of the finish straightener is 8–10 MPa. The slitting step involves controlling the slitting machine blade gap at 0.15–0.2 mm and the coiling tension at 120–150 N / mm. This method systematically reduces the degree of camber after longitudinal shearing of the QSTE650TM main beam steel, keeping it within 3mm.

Description

Technical Field

[0001] This invention relates to the field of steel rolling processing technology, and in particular to a process for improving the longitudinal shearing sickle bend of QSTE650TM beam steel. Background Technology

[0002] In the production of automotive frame steel, high-strength steels such as QSTE650TM often exhibit camber after longitudinal shearing, such as... Figure 1 As shown, the bending deformation can reach 20mm, severely affecting the accuracy of subsequent bending, stamping, and other processing, as well as assembly quality. In existing technologies, the main causes of sickle bending include:

[0003] Uneven residual stress: The temperature difference between the head and tail of the slab during rolling and cooling, as well as the temperature difference between the edge and the middle during the rolling process, leads to the accumulation of thermal stress, and residual local stress in the straightening process.

[0004] Equipment and process defects: Uneven blade gap and improper tension control in the slitting machine can cause imbalance in shearing force.

[0005] Traditional processes only optimize the rolling or straightening stages without forming a multi-dimensional collaborative control scheme, making it difficult to effectively solve the sickle bend problem. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a process that effectively improves the longitudinal shearing sickle bend of the QSTE650TM main beam steel.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes the following steps: slab preparation, rolling and cooling, straightening treatment and slitting.

[0008] The slab preparation step involves shortening the slab length to 8.5–9 m.

[0009] The rolling and cooling steps are as follows: the slab is first roughed and finished, and the finished strip is cooled; during the cooling process, it is first air-cooled to 880-900℃, and then laminar flow cooling is performed at a rate of 20-25℃ / s and a final cooling temperature of 550-580℃.

[0010] The straightening process includes a rough straightening process and a fine straightening process. During the rough straightening process, the pressure of the rough straightening machine is 18-20 MPa. During the fine straightening process, the fine straightening insertion amount is controlled at 30%-45% of the strip thickness, and the pressure of the fine straightening machine is 8-10 MPa.

[0011] The slitting process involves controlling the blade gap of the slitting machine to be 0.15–0.2 mm and the winding tension to be 120–150 N / mm.

[0012] Furthermore, in the slab preparation step, the carbon content in the slab is controlled to be 0.12%±0.02% and the manganese content to be 1.5%±0.1%.

[0013] Furthermore, in the rolling and cooling steps, the edge heaters heat the material to 1050±20℃ during the finishing rolling process.

[0014] Furthermore, in the straightening process, the straightening speed is 15-20 m / min.

[0015] Furthermore, in the straightening process, the reduction ratio of each roller in the fine straightening process is as follows: 25% for the first roller, 35% to 40% for the second roller, 20% for the third roller, 10% to 15% for the fourth roller, and 5% for the fifth roller.

[0016] Furthermore, in the straightening process, the inlet and outlet tilt angle for fine straightening is 1.5°±0.2°.

[0017] The beneficial effects of adopting the above technical solution are as follows:

[0018] 1. Quality Improvement: The camber after slitting is reduced from 20mm to below 3mm, dimensional tolerances meet standards (width ±1mm, length 0 / +5mm), and mechanical properties meet ReL 650~730MPa, Rm 700~800MPa, and A% 16~23%;

[0019] 2. Efficiency Improvement: The scrap rate decreased from 8% to below 1.5%, production efficiency increased by 12%, and annual cost savings were approximately 3 million yuan;

[0020] 3. Technological breakthrough: Breaking through the technological barriers of foreign high-end beam steel production, the process parameters can be extended to other high-strength steel grades (such as QSTE700TM).

[0021] 4. This invention systematically reduces the degree of camber after longitudinal shearing of QSTE650TM beam steel by adjusting slab parameters, temperature field, straightening process and equipment precision, keeping it within 3mm to meet the high precision requirements of automobile manufacturing. Attached Figure Description

[0022] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0023] Figure 1 This is a schematic diagram of the sickle bend of a slab obtained by the traditional method;

[0024] Figure 2 This is a schematic diagram of the sickle bend of the slab obtained by the present invention;

[0025] Figure 3 This is a schematic diagram of the laminar flow cooling mode adjustment described in this invention;

[0026] Figure 4 This is a schematic diagram of the upper roller insertion amount of the straightening machine described in this invention. Detailed Implementation

[0027] The process for improving the longitudinal shearing sickle bend of the QSTE650TM main beam steel includes slab preparation, rolling and cooling, straightening, and longitudinal shearing steps, as described below:

[0028] (1) Slab preparation steps: The slab length is shortened to 8.5-9m; by shortening the slab length, the temperature drop at the tail end during rolling is reduced, and the temperature drop amplitude is reduced by about 30%, thus suppressing the uneven stress in the length direction; the carbon content in the slab is controlled at 0.12%±0.02% (wt) and the manganese content is controlled at 1.5%±0.1% (wt) to eliminate defects such as looseness and segregation under the liquid core of the slab and ensure the uniformity of the structure.

[0029] (2) Rolling and Cooling Steps: The slab is first roughed and then finished rolled. During the finishing rolling process, the edge heaters are heated to 1050±20℃. Vertical roll cooling water baffles are added on both sides of the vertical rolls of the finishing mill to prevent cooling water from splashing onto the edges. The finished strip is then cooled; during the cooling process, the traditional laminar flow cooling is changed from the first half cooling to the front 4 sets of water shut off + the second half cooling, such as... Figure 3 As shown, the rolled strip is first naturally cooled to 880-900℃, i.e., air-cooled to 880-900℃, and then subjected to laminar flow cooling. The temperature drop during the air cooling process is preferably 10-15℃, and the air cooling time is preferably 5-10 seconds. The air cooling process can give the deformed material a short recovery time, which promotes the dynamic softening of non-recrystallized austenite. In the laminar flow cooling process, a temperature gauge is added after the first four groups of laminar flow cooling to detect the temperature after air cooling. The laminar flow cooling rate is controlled at 20-25℃ / s, and the final cooling temperature is 550-580℃.

[0030] The shortcomings of traditional front-section 1 / 2 cooling: In the traditional front-section 1 / 2 laminar flow cooling process, the strip immediately enters the front-section cooling zone after rolling and undergoes high-intensity water cooling. At this time, the temperature field of the strip exhibits significant non-uniformity. This will cause: ① Temperature gradient imbalance: At the end of rolling, the temperature difference between the edge and the middle of the strip has reached 50-80℃. The rapid cooling in the front section further aggravates the temperature drop rate at the edge, reaching 40-50℃ / s, causing the edge microstructure to complete the ferrite phase transformation before the middle, forming a microstructure transformation time difference of "early phase transformation at the edge - late phase transformation in the middle"; ② Thermal stress accumulation: Rapid cooling causes the edge to shrink rapidly and generate compressive stress, while the middle, due to delayed cooling, forms tensile stress. The stress difference Δσ along the width direction of the strip is ≥150MPa, forming a moment M≈20-30N·m that causes the strip to bend.

[0031] This process employs a cooling mode of shutting off four sets of front-end water nozzles plus half-cooling. The technical advantage is that after shutting off the four sets of laminar flow cooling nozzles, the strip steel receives a natural cooling buffer time of 10–15°C, and its thermodynamic behavior is as follows:

[0032] ① Temperature field homogenization: Through radiation and air convection, the temperature difference between the edge and the middle is reduced to 20-30℃, and heat conduction reduces the overall temperature gradient of the strip by 40%-50%;

[0033] ② Dynamic recrystallization activation: In the temperature range of 880 to 900℃, the unrecrystallized austenite achieves dynamic softening through dislocation slip and subgrain merging, with a residual thermal stress relaxation rate of 30% to 40%.

[0034] ③ Improved phase transformation synchronization: The laminar cooling initiation temperature is uniformly controlled at 880~900℃, the phase transformation initiation time difference between the edge and the middle is ≤0.5s, and the ferrite grain size deviation is reduced from ±5μm to ±2μm;

[0035] ④ Stress equilibrium: When the laminar cooling rate is 20-25℃ / s, the bainite transformation is controlled at 5%-8%, avoiding the formation of brittle martensite phase, and the stress difference along the thickness direction is Δσ≤50MPa;

[0036] ⑤ Residual stress dissipation: The strip is first air-cooled to the laminar cooling initiation temperature of 880-900℃, and then laminar cooling is performed, thereby optimizing the pearlite lamellar spacing to achieve a pearlite lamellar spacing of 0.5-0.8μm. The structural stress generated by the phase transformation cancels out the thermal stress, and the final residual stress density is reduced from 120-150MPa to 70-80MPa.

[0037] The effect of the cooling mode used in this process on the microstructure is shown in Table 1;

[0038] Table 1: Effects of Cooling Mode on Microstructure

[0039]

[0040] (3) Straightening process: including coarse straightening and fine straightening, with a straightening speed of 15-20 m / min. During the coarse straightening process, the pressure of the coarse straightening machine is 18-20 MPa to eliminate macroscopic curvature. During the fine straightening process, the pressure of the fine straightening machine is 8-10 MPa to finely adjust the residual curvature; Figure 4As shown, the insertion amount of the fine straightening machine is controlled at 30% to 45% of the strip thickness, with an error of ≤0.5mm. For example, if the strip thickness is 4.0mm, the insertion amount of the fine straightening machine is set to 1.2 to 1.8mm. The inlet and outlet tilt angles during the fine straightening process are 1.5° ± 0.2°, and the difference in tilt between the drive side and the operating side is ≤0.1°. The insertion amount of the fine straightening machine is 1.2 to 1.8mm, and the distribution ratio of the reduction amount of each roller is: 25% for the first roller, 35% to 40% for the second roller, 20% for the third roller, 10% to 15% for the fourth roller, and 5% for the fifth roller. The straightness of the strip after straightening is ≤1mm / m.

[0041] (4) Slitting process: Use a feeler gauge to measure one point every 100mm along the width of the blade, and control the gap value to be 0.15~0.2mm with an error of ≤0.03mm; the concentricity deviation of the blade shaft is ≤0.05mm, the parallelism error of the blade holder is ≤0.1mm / m, and parts with excessive wear are replaced in time; during the slitting process, the winding tension is controlled at 120~150N / mm.

[0042] (5) After adopting the above process, the obtained QSTE650TM beam steel eliminates the longitudinal shear camber defect, controlling the camber within 3mm, such as Figure 2 As shown.

[0043] Example 1: This example focuses on 4.0mm thick QSTE650TM beam steel. By optimizing the slab preparation, rolling cooling, straightening, and slitting processes, the slitting camber defect is effectively improved. The specific steps are as follows:

[0044] (1) Slab preparation: The chemical composition is precisely controlled during the steelmaking process: carbon content 0.11%, manganese content 1.48%, and sulfur and phosphorus content ≤0.015%; a liquid core pressing device is used in the continuous casting stage, and the continuous casting slab is tested by ultrasonic flaw detection to ensure that there are no defects such as porosity and segregation inside, and the uniformity of the structure meets the standard. Continuous casting slabs are selected, and the slab length is controlled at 8.8m.

[0045] (2) Rolling and cooling: The slab is first rough rolled to the thickness of the intermediate slab, and then enters the finishing mill. Before finishing, the edge heater is started to heat the edge temperature of the strip to 1045℃. Cold water baffles are installed on both sides of the vertical roll of the finishing mill to prevent cooling water from splashing onto the edge of the strip and to avoid abnormal drop in edge temperature.

[0046] After finishing rolling, the strip first enters the air cooling section, where the temperature reaches 890℃. During air cooling, the temperature difference between the edge and center of the strip is reduced to 20-30℃ through radiation and air convection. Then, laminar flow cooling is started, using a "front-end 4 sets of water shut-off + 1 / 2 cooling" mode. Temperature gauges are installed after the 4 sets of cooling nozzles before laminar flow cooling to monitor the temperature after air cooling in real time. The cooling rate is controlled at 22℃ / s, and the final cooling temperature is controlled at 565℃.

[0047] Through this process, the microstructure of the strip is uniformly distributed ferrite + pearlite across its entire cross-section; the overall dislocation density is 1.0 × 10⁻⁶. 12 / cm 2 The deviation is ≤10%, avoiding the problem of "early phase transformation at the edge and late phase transformation in the middle" in traditional processes.

[0048] (3) Straightening treatment: The two-step method of "coarse straightening + fine straightening" is adopted, and the straightening speed is controlled at 18m / min. The pressure of the coarse straightening machine is set to 19MPa. The macroscopic bending of the strip is eliminated by the high pressure, and the plate shape is initially improved.

[0049] The pressure of the precision straightener is set to 9MPa; based on the strip thickness of 4.0mm, the insertion depth of the precision straightener is set to 1.3mm, with an insertion depth error of ≤0.5mm; the inlet and outlet tilt angle of the precision straightener is set to 1.5°, with a tilt difference between the drive side and the operating side of ≤0.1°, and the straightness of the strip after straightening is ≤1mm / m.

[0050] (4) Slitting: Use a feeler gauge to measure one point every 100mm along the blade width direction. Adjust the blade gap to 0.18mm with a gap error ≤0.03mm. Check the concentricity of the blade shaft to ensure a deviation ≤0.05mm. Calibrate the parallelism of the blade holder to ensure an error ≤0.1mm / m. Replace any worn blades and bearing components. During slitting, control the winding tension at 135N / mm to avoid uneven tension causing bending of the slitting strip. After slitting, inspect the strip. The measured camber is 2.3mm, which meets the quality requirement of ≤2.5mm camber after slitting of the main beam steel.

[0051] Example 2: This example is for QSTE650TM beam steel with a thickness of 5.0mm. The specific steps are as follows:

[0052] (1) Slab preparation: The slab length is controlled at 8.6m, and the temperature drop at the tail end is reduced by about 30%. Slab chemical composition control: carbon content 0.13%, manganese content 1.52%, sulfur and phosphorus content ≤0.015%; the continuous casting slab is subjected to ultrasonic testing, and there are no internal defects and the structure is uniform.

[0053] (2) Rolling and Cooling: The temperature of the edge heater before finishing rolling is set at 1055℃, and the vertical roll cooling water baffle works normally to avoid splashing of cooling water at the edge. After finishing rolling, the strip is air-cooled to 885℃, the laminar flow cooling rate is controlled at 24℃ / s, and the final cooling temperature is controlled at 570℃. The resulting strip has a uniform ferrite + pearlite microstructure across its entire cross section, with a dislocation density deviation of ≤10%, and balanced thermal stress and microstructure stress.

[0054] (3) Straightening treatment: The straightening speed is selected as 16m / min; a. The pressure of the coarse straightener is set to 19.5MPa to eliminate macroscopic curvature. The pressure of the fine straightener is set to 9.5MPa; based on the strip thickness of 5.0mm, the insertion amount of the fine straightener is set to 1.8mm, and the insertion amount error is ≤0.5mm; the inlet and outlet tilt angle is set to 1.4°, the tilt difference between the drive side and the operating side is 0.08°, and the straightness of the strip after straightening is ≤1mm / m.

[0055] (4) Slitting: The blade gap was adjusted to 0.19mm, with a gap error of ≤0.03mm; the concentricity deviation of the blade shaft was 0.04mm, and the parallelism error of the blade holder was 0.08mm / m. The equipment condition met the standards. The coiling tension was controlled at 140N / mm to ensure a smooth coiling process. The measured value of the sickle bend of the strip after slitting was 1.3mm, which met the quality standards for slitting of beam steel and was significantly improved compared with the traditional process (sickle bend 4.0~5.0mm).

Claims

1. A process for improving the longitudinal shearing sickle bend of the QSTE650TM main beam steel, characterized in that: This includes slab preparation, rolling and cooling, straightening, and slitting steps; The slab preparation step involves shortening the slab length to 8.5–9 m. The rolling and cooling steps are as follows: the slab is first roughed and finished, and the finished strip is cooled; during the cooling process, it is first air-cooled to 880-900℃, and then laminar flow cooling is performed at a rate of 20-25℃ / s and a final cooling temperature of 550-580℃. The straightening process includes a rough straightening process and a fine straightening process. During the rough straightening process, the pressure of the rough straightening machine is 18-20 MPa. During the fine straightening process, the fine straightening insertion amount is controlled at 30%-45% of the strip thickness, and the pressure of the fine straightening machine is 8-10 MPa. The slitting process involves controlling the blade gap of the slitting machine to be 0.15–0.2 mm and the winding tension to be 120–150 N / mm.

2. The process for improving the longitudinal shearing sickle bend of the QSTE650TM main beam steel according to claim 1, characterized in that: The slab preparation steps control the carbon content in the slab to be 0.12%±0.02% and the manganese content to be 1.5%±0.1%.

3. The process for improving the longitudinal shearing sickle bend of the QSTE650TM main beam steel according to claim 1, characterized in that: During the rolling and cooling process, the edge heaters heat the material to 1050±20℃ during the finishing rolling process.

4. The process for improving the longitudinal shearing sickle bend of the QSTE650TM main beam steel according to claim 1, characterized in that: The straightening process is carried out at a speed of 15-20 m / min.

5. The process for improving the longitudinal shearing sickle bend of the QSTE650TM main beam steel according to claim 1, characterized in that: In the straightening process, the reduction ratio of each roller is as follows: first roller 25%, second roller 35% to 40%, third roller 20%, fourth roller 10% to 15%, and fifth roller 5%.

6. A process for improving the longitudinal shearing sickle bend of QSTE650TM main beam steel according to any one of claims 1-5, characterized in that: In the straightening process, the inlet and outlet tilt angle for fine straightening is 1.5°±0.2°.

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