Hot-rolled high-strength steel and method for shape control and optimization thereof

Abstract

The application discloses a kind of hot-rolled high-strength steel and its plate shape control and optimization method, belong to the production method field of hot-rolled high-strength steel.The tensile strength grade of hot-rolled high-strength steel is 500-700MPa, and the thickness is 2.0-12.0mm;Plate shape control and optimization method is obtained by slab heating, rough rolling, finish rolling, laminar cooling, coiling production process to get steel coil, send steel coil into bell type annealing furnace to carry out stress relief annealing treatment to obtain hot-rolled high-strength steel with good plate shape, annealing process is: from room temperature to 450-550 DEG C, heating time 3.5-4.5h;Annealing temperature 450-550 DEG C, annealing time 19.5-20.5h;Fire stop in bell type furnace furnace cooling, cooling to 180-220 DEG C, take out air cooling to room temperature.The method of the application can effectively solve the problem of uneven internal stress of existing hot-rolled steel plate, which leads to the problem of poor plate shape after the release of internal stress after subsequent steel plate uncoiling or dividing.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of production method of hot-rolled high-strength steel, and relates to a method for improving the shape of hot-rolled high-strength steel plate by using a bell-type annealing furnace, in particular to a hot-rolled high-strength steel and a method for controlling and optimizing the shape of the hot-rolled high-strength steel. BACKGROUND

[0002] With the popularization and application of micro-alloying elements such as Nb, V and Ti in hot-rolled high-strength steel, the strength grade of hot-rolled steel plate is greatly improved. As a result, the microstructure of high-strength steel is significantly refined, and the second phase precipitation is significantly increased. Under the influence of conditions such as large fluctuation in hot-rolling process and non-uniform temperature, the grain size and the number of second phase precipitations of high-strength steel are uneven in the length, width and thickness directions of the steel plate, which also leads to uneven internal stress of the steel plate. After the internal stress of the steel plate is released after uncoiling or dividing, the problem of poor plate shape occurs, which affects the final use of the user. Therefore, solving the problem of plate shape is of great significance for improving the quality of hot-rolled high-strength steel products and promoting the application.

[0003] After searching, CN202311268132.8 discloses a method for eliminating the edge wave of hot-rolled free-rolling strip steel, which adopts a "bamboo joint" rolling mode of "narrow-wide-narrow-wide-narrow-wide", specifically including: 1) the products with an out-of-furnace temperature in the range of 1210-1240℃ are subjected to free rolling; 2) when the length of the rolled strip steel reaches 40-45 kilometers, reverse wide free rolling is started, and the longest rolling cycle of the entire finishing work roll is 110 kilometers; 3) the maximum width of the transition material in the reverse wide transition is less than 300mm different from the width of the previous product before rolling; 4) the hot-rolled strip steel with a thickness of 1.2-25.4mm is divided into more than 5 thickness levels, and the thickness of the first transition material in free rolling is not less than two thickness levels of the main rolled material. In solving the problem of the shape of the reverse wide product, the advantage of the rolling mill that can change the crown of the strip steel is fully utilized to control the hot-rolled plate shape by the crown, solve the problem of plate shape, realize the alternating rolling of wide material and narrow material in the cycle, and improve the production capacity.

[0004] CN202210835311.4 discloses a method for controlling the shape of hot-rolled thin plates, characterized by the following steps: 1) reheating under the condition that the surface temperature of the rough-rolled blank is above 500℃, heating time ≥ 150min, heating temperature 1280-1320℃; 2) rough rolling + finish rolling, the surface temperature of the blank at the end of rough rolling ≥ 1100℃, the surface temperature of the blank at the end of finish rolling 880-930℃. The rolling crown continuously and uniformly decreases from 60IU to 20IU along the rolling direction of the blank; 3) controlling the cooling process after finish rolling, first water spray cooling, then slow cooling in air. The upper and lower water quantities during the cooling process are both 25% of the rated total water quantity, the water pressure is 1.2-1.6bar, the cooling speed is 25-35℃ / s, the strip steel out-of-water temperature after water spray cooling is controlled between 640-700℃, and the slow cooling time in air is controlled between 18-25s; 4) after cooling, the plate is coiled when the surface temperature is ≥ 600℃. After coiling, the coil enters the insulation wall for further cooling, the temperature when entering the insulation wall is ≥ 500℃, and the cooling time is ≥ 48h, and the coil is removed from the insulation wall when the temperature is below 100℃.

[0005] From the above, the prior art mainly controls the shape of hot-rolled high-strength steel from the aspects of rolling process and laminar cooling process, and there is no method for improving the shape of high-strength steel through a bell-type annealing furnace. SUMMARY

[0006] The technical problem to be solved by the present application is that the uneven internal stress of the existing hot-rolled steel plate leads to the problem of poor plate shape after the release of internal stress after subsequent uncoiling or uncoiling of the steel plate.

[0007] The technical solution adopted by the present application to solve the technical problem is:

[0008] In a first aspect, the present application provides a method for controlling and optimizing the shape of hot-rolled high-strength steel: obtaining a steel coil through the production process flow of slab heating, rough rolling, finish rolling, laminar cooling and coiling, and sending the steel coil into a bell-type annealing furnace for stress relief annealing treatment to obtain hot-rolled high-strength steel with good plate shape; wherein the annealing process comprises the following steps:

[0009] 1) heating section: heating from room temperature to 450-550℃, heating time 3.5-4.5h;

[0010] 2) annealing section: annealing temperature 450-550℃, annealing time 19.5-20.5h;

[0011] 3) cooling section: stopping fire in the bell-type furnace, cooling to 180-220℃, then taking out the steel coil from the bell-type furnace, and air cooling to room temperature.

[0012] In step 1) above, N2 is introduced from the beginning of the heating section, the N2 flow rate is 14.5m3 / h~15.5m 3 / h.

[0013] In the step 2), the annealing section is continuously supplied with N2, and the flow rate of N2 is 29.5m 3 / h~30.5m 3 / h.

[0014] The slab heating is performed in any one of the following modes:

[0015] The slab is sent into the slab heating furnace in a hot charging mode, and the charging temperature is 500-700 DEG C.

[0016] After the cast slab is stacked and slowly cooled to room temperature, the slab is sent into the slab heating furnace in a cold charging mode, wherein the cast slab is stacked with hot slabs with a temperature higher than 500 DEG C on both sides, and the slab should not be placed on the uppermost and lowermost layers of the stack.

[0017] The rough rolling is performed by two single-stand reversing rough rolling mills, the camber value at the outlets of the two rough rolling mills is monitored in real time, the control system is fed back to control the rolling mill inclination value in real time, and the rough rolling camber is controlled in the range of -20mm-20mm.

[0018] The finish rolling is performed by 6-7 stands of irreversible finish rolling mills, the front two stands are in a positive shifting mode, the shifting amount is 50mm-100mm, the middle 2-3 stands are in a positive shifting mode, the shifting amount is 0mm-50mm, the last two stands are in a negative shifting mode, the shifting amount is -50mm-0mm, the convexity at the outlets of the finish rolling mills is controlled in the range of 20um-50um, and the wedge is controlled in the range of not higher than 30um.

[0019] The laminar cooling is performed in a sparse cooling mode, the header opening mode is open 1 and close 1, and the cooling rate is 10 DEG C / s-30 DEG C / s; after being cooled to 590 DEG C-610 DEG C, the steel coil is coiled, and after being cooled to room temperature, the steel coil is sent into a bell-type annealing furnace for stress relief annealing treatment.

[0020] In the second aspect, the application provides a hot-rolled high-strength steel prepared by the method for controlling and optimizing the shape of the hot-rolled high-strength steel plate.

[0021] The tensile strength grade of the hot-rolled high-strength steel is 500MPa-700MPa (tensile strength is 500MPa-800MPa), and the thickness specification is 2.0mm-12.0mm.

[0022] The application has the beneficial effects that the stress relief annealing is performed by using the bell-type annealing furnace, a new idea for improving the shape is provided, compared with the traditional slow cooling pit stress relief annealing or the conventional heat treatment furnace stress relief annealing, the bell-type annealing furnace is protected by N2, the plate shape can be improved, and thick iron oxide skin can be avoided during heating. Attached Figure Description

[0023] Figure 1 The sheet shape after uncoiling and slitting the steel in Example 1;

[0024] Figure 2 This refers to the sheet shape of the steel after uncoiling and slitting in Example 2;

[0025] Figure 3 The sheet shape after uncoiling and slitting of steel in Comparative Example 1;

[0026] Figure 4 The image shows the sheet shape of steel after uncoiling and slitting, as shown in Comparative Example 2. Detailed Implementation

[0027] The technical solution of the present invention can be implemented in the following manner.

[0028] This invention provides a method for controlling and optimizing the shape of hot-rolled high-strength steel plates. First, the reasons why hot-rolled high-strength steel is prone to shape defects are briefly explained.

[0029] With advancements in microalloying technology, microalloying elements such as Nb and Ti are widely used in hot-rolled high-strength steel. Domestic and international steel companies commonly employ Nb and Ti microalloying in industrial trials of hot-rolled high-strength steel. Nb primarily promotes grain refinement in steel. During finishing rolling, it readily forms NbC precipitates. These precipitates, along with Nb dissolved in austenite, can pin austenite grain boundaries, further promoting grain refinement. Ti readily forms second-phase precipitation during various hot-rolling processes, exhibiting deformation-induced precipitation, interphase precipitation, and supersaturated precipitation during rolling, laminar cooling, and coiling stages, respectively. Furthermore, these precipitates are highly temperature-sensitive. Unstable hot-rolling processes, uneven cooling, and uneven temperatures can lead to variations in grain size and the number of precipitates at different locations on the steel plate, resulting in uneven internal stress distribution and ultimately, plate shape defects.

[0030] Secondly, the reasons for the main limitations of the production process parameters in the method for shape control and optimization of hot-rolled high-strength steel plates described in this invention will be explained.

[0031] Firstly, regarding the heating regime: During the billet cooling process, the surface temperature drops rapidly while the core temperature drops more slowly. Therefore, normally air-cooled billets will generate thermal stress in the thickness direction. Stacking billets for slow cooling helps reduce the temperature difference between the surface and core, minimizing grain size differences and reducing internal stress. Hot charging also effectively reduces differences in grain size, temperature drop, and internal stress. Furthermore, compared to cold charging, hot charging results in a higher core temperature, which facilitates the penetration of rolling deformation into the core, improving the uniformity of the steel plate structure and reducing uneven internal stress in the thickness direction.

[0032] Second, in the rolling process. The shape of the steel plate has a genetic effect from the rough rolling mill to the finishing rolling mill, so the shape of the rough rolling steel plate should be controlled first. The present application detects the camber condition at the outlet of the rough rolling mill and feeds back to the rough rolling control system to adjust the rolling mill inclination value, and then controls the rough rolling camber in the range of -20mm~20mm. Secondly, the control of crown and wedge in the finishing rolling process is crucial to the improvement of the plate shape. The present application provides the roll shifting control requirements of each rack in the finishing rolling. Among them, the first two racks adopt positive shifting to reduce the crown of the steel plate, but the roll shifting amount cannot be too high to avoid the formation of middle wave in the steel plate. The present application limits the roll shifting amount of the first two racks to 50mm~100mm; the middle 2~3 racks continue to adopt positive shifting to reduce the crown of the steel plate. Since the thickness of the steel plate has been reduced after rolling by the first two racks, the roll shifting amount is reduced compared with the first two racks. The present application provides that the roll shifting amount of the middle 2~3 racks is 0mm~50mm; the last two racks adopt negative shifting, and the roll shifting amount is -50mm~0mm. One is to avoid the low crown leading to excessive reduction of the center of the steel plate and thus forming a middle wave, and the other is to increase the reduction of the edge of the steel plate to control the plate shape at the outlet of the finishing rolling mill to be "micro edge wave". Therefore, the present application limits the crown control at the outlet of the finishing rolling mill to the range of 20μm~50μm, and the wedge control to the range of not higher than 30μm.

[0033] Third, in the laminar cooling process. There is non-uniformity in the cooling of the steel plate along the width direction during laminar cooling. The heat transfer occurs along the upper and lower surfaces of the middle part of the steel plate along the width direction, while the heat dissipation also occurs along the side surface of the edge part of the steel plate along the width direction. The temperature of the edge part of the steel plate drops faster, resulting in finer microstructure and preferential austenite-ferrite phase transformation in the edge part of the steel plate. Since the linear expansion coefficient of austenite is higher than that of ferrite, the edge part of the steel plate shrinks, generating tensile stress. This tensile stress is combined with the "micro edge wave" formed by the finishing rolling mill, which is beneficial to offset the internal stress of the steel plate and improve the plate shape. On the other hand, the cooling rate of the laminar cooling water has a great influence on the plate shape of the steel plate. When a faster cooling rate is used, it is easy to cause the difference in microstructure between the surface and the center of the steel plate along the thickness direction, resulting in finer surface microstructure and coarser center microstructure, which also causes residual stress. At the same time, after the cooling rate increases, a large amount of cooling water accumulates on the surface of the steel plate, which cannot be discharged in time, and may also cause the laminar cooling water on the surface of the steel plate to fail to circulate immediately, affecting the cooling efficiency. Therefore, the present application requires sparse cooling for laminar cooling, and the header opening mode is open 1 and close 1, and a lower cooling rate of 10℃ / s~30℃ / s is required.

[0034] Fourth, the hood annealing process, since the main purpose of annealing process is to reduce the residual stress of the steel plate, therefore, the annealing temperature should not be too high, should be lower than the austenite recrystallization temperature interval, ferrite pearlite transformation interval, but the temperature should not be too low, otherwise the stress relief effect is limited. Therefore, the annealing temperature is limited to 450-550℃. At the same time, in order to reduce the thermal stress in the heating and cooling process, the heating rate and cooling rate are set at a lower level, the heating time is set to 3.5-4.5h in the heating stage, and the steel coil is cooled to 180-220℃ in the annealing furnace in the cooling stage.

[0035] The technical solutions and effects of the present application are further described below through actual examples.

[0036] Example

[0037] Example 1: The 610L steel with a thickness of 10mm was prepared by slab heating-rough rolling-finish rolling-laminar cooling-coiling-hood annealing, and the tensile strength was 663MPa. The specific preparation steps are as follows.

[0038] The cast blank was sent into the heating furnace in a hot charging manner, the inlet temperature was 603℃, and the outlet temperature was 1210℃; the rough rolling was carried out in 6 passes, and the middle blank camber value after rough rolling was 15mm; the finish rolling was carried out in 7 racks (2+3+2), and the roll shifting amount of each rack was 97mm, 55mm, 35mm, 27mm, 12mm, -10mm and -32mm, respectively, the finish rolling outlet convexity was 39μm, and the wedge shape was 18μm; the laminar cooling was carried out in an open-1-close-1 sparse cooling mode, and the steel was cooled to 608℃ for coiling at a cooling rate of 18℃ / s, and cooled to room temperature. The steel coil was sent into the hood annealing furnace for stress relief annealing, the steel coil was heated to 488℃, the heating time was 4.1h, and N2protective gas with a flow rate of 14.9m 3 / h was introduced during heating; the temperature was kept at 488℃ for 19.8h, and the N2flow rate introduced during the holding period was 30.3m 3 / h, and after the annealing was completed, the steel coil was placed in the hood furnace for furnace cooling, and cooled to 211℃, then the steel coil was taken out of the hood furnace and air cooled to room temperature.

[0039] The plate shape after the 610L steel prepared by the process of example 1 was unwound and divided into strips, as shown in the attached Figure 1 It can be seen that the example 1 steel did not show side bending and warping after unwinding and dividing into strips, and the plate shape was good.

[0040] Example 2: The 700L steel with a thickness of 8mm was prepared by slab heating-rough rolling-finish rolling-laminar cooling-coiling-hood annealing, and the tensile strength was 724MPa. The specific preparation steps are as follows.

[0041] The cast slab is sent into a heating furnace for heating in a way of cast slab stacking and slow cooling after cold charging, and the discharge temperature is 1231℃; the rough rolling is performed in 6 passes, and the intermediate slab camber value after rough rolling is 17mm; the finish rolling is performed in 7 stands (2+3+2), the roll shifting amounts of each stand are 89mm, 57mm, 32mm, 24mm, 15mm, -13mm and -28mm respectively, the exit crown of finish rolling is 30μm, and the wedge is 21μm; the laminar cooling is performed in a sparse cooling mode of opening 1 and closing 1, the steel is cooled to 595℃ for coiling at a cooling rate of 17℃ / s, and is naturally cooled to room temperature. The steel coil is sent into a batch annealing furnace for stress relief annealing, the steel coil is heated to 493℃, the heating time is 4.2h, and N2protective gas with a flow rate of 15.1m 3 / h is introduced during heating; the temperature is kept at 493℃ for 20.5h, N2with a flow rate of 30.0m 3 / h is introduced during keeping temperature, and after the annealing is finished, the steel coil is placed in the batch furnace for furnace cooling, and is taken out from the batch furnace after being cooled to 195℃, and is naturally cooled to room temperature.

[0042] The 700L steel prepared by the process described in Example 2 is uncoiled and divided into strips, and the plate shape is shown in FIG. 2. Figure 2 It can be seen that after being uncoiled and divided into strips, the steel of Example 2 does not show side bending and warping, and the plate shape is good.

[0043] The 700L steel with a thickness of 7mm is prepared by using slab heating-rough rolling-finish rolling-laminar cooling-coiling-batch annealing according to Comparative Example 1, and the tensile strength is 738MPa, and the specific preparation steps are as follows.

[0044] The cast slab is sent into a heating furnace for heating in a way of cast slab stacking and slow cooling after cold charging, and the discharge temperature is 1222℃; the rough rolling is performed in 6 passes, and the intermediate slab camber value after rough rolling is 20mm; the finish rolling is performed in 7 stands (2+3+2), the roll shifting amounts of each stand are 92mm, 61mm, 43mm, 31mm, 20mm, -5mm and -21mm respectively, the exit crown of finish rolling is 29μm, and the wedge is 15μm; the laminar cooling is performed in a mode of front concentrated cooling (i.e. the front five groups of headers are sequentially opened without interval until the coiling temperature reaches the requirement), and the steel is cooled to 608℃ for coiling at a cooling rate of 35℃ / s, and is naturally cooled to room temperature. Then the steel coil is sent into a batch annealing furnace for stress relief annealing, the steel coil is heated to 208℃, the heating time is 2.3h, and N2protective gas with a flow rate of 14.9m 3 / h is introduced during heating; the temperature is kept at 208℃ for 19.8h, N2with a flow rate of 30.3m 3 / h is introduced during keeping temperature, and after the annealing is finished, the steel coil is placed in the batch furnace for furnace cooling, and is taken out from the batch furnace after being cooled to 211℃, and is naturally cooled to room temperature.

[0045] The 700L steel prepared by the process of Comparative Example 1 had the plate shape after uncoiling and slitting as shown in the following table Figure 3 It can be seen that the steel of Comparative Example 1 had serious side bending after uncoiling and slitting, which was manifested as the strip on both sides bending outwardly, indicating that the edge of the steel in the width direction was in compressive stress, which was mainly due to the fact that the laminar cooling adopted the front concentrated cooling mode, resulting in a large cooling rate of the edge of the steel plate in the width direction, and the effects of grain refinement and thermal stress superimposed. In addition, the temperature of the cover annealing was relatively low, only 208℃, and the stress relief effect was not large, finally resulting in poor plate shape of the steel plate.

[0046] The 510L steel with a thickness of 12mm was prepared by slab heating-rough rolling-finish rolling-laminar cooling-coiling-cover annealing according to Comparative Example 2, and the tensile strength was 576MPa.

[0047] The cast blank was sent into the heating furnace in a hot charging manner, the entry temperature was 544℃, and the exit temperature was 1202℃; the rough rolling adopted 6 passes, and the camber value of the intermediate blank after rough rolling was 18mm; the finish rolling adopted 7 stands (2+3+2) rolling, the roll shifting amount of each stand was 134mm, 99mm, 57mm, 45mm, 32mm, 20mm and 12mm respectively, the convexity at the finish rolling exit was 33μm, and the wedge shape was 16μm; the laminar cooling adopted the sparse cooling mode of opening 1 and closing 1, and the steel was cooled to 615℃ for coiling at a cooling rate of 14℃ / s, and the coil was hot charged and slowly cooled to room temperature, and the temperature of other coils for hot charging and slow cooling was 485℃.

[0048] The 510L steel prepared by the process of Comparative Example 2 had the plate shape after uncoiling and slitting as shown in the following table Figure 4 It can be seen that the steel of Comparative Example 2 had side bending after uncoiling and slitting, which was mainly due to the fact that the roll shifting amount of the steel plate in the finish rolling process was large, and all the roll shifting amounts were positive, resulting in the plate shape of the steel plate after rolling being in the form of middle wave, and after the coiling of the steel coil, the hot charging and slow cooling were adopted to eliminate the internal stress, but the slow cooling temperature was relatively low, and the effect of reducing the internal stress of the steel plate was not significant, finally resulting in the side bending of the steel plate after slitting.

Claims

1. A method for shape control and optimization of hot rolled high strength steel sheets, characterized in that: The steel coil is obtained through a production process flow of slab heating, rough rolling, finish rolling, laminar cooling and coiling, and the steel coil is sent into a bell type annealing furnace for stress relief annealing treatment to obtain a hot-rolled high-strength steel with good flatness. The slab heating is sent into a slab heating furnace in a hot charging manner, and the charging temperature is 500-700 DEG C. The finish rolling is carried out by using 6-7 irreversible finish rolling mill groups, the first two mill groups adopt positive shifting, the shifting amount is 50-100 mm, the middle 2-3 mill groups adopt positive shifting, the shifting amount is 0-50 mm, and the last two mill groups adopt negative shifting, the shifting amount is -50-0 mm; the exit crown of the finish rolling mill is controlled in the range of 20-50 mu m, and the wedge is controlled in the range of not higher than 30 mu m. The laminar cooling is carried out in a sparse cooling manner, the header opening mode is open 1 and close 1, and the cooling rate is 10-30 DEG C / s; after cooling to 590-610 DEG C, the steel coil is coiled, and after cooling to room temperature, the steel coil is sent into the bell type annealing furnace for stress relief annealing treatment. The annealing process comprises the following steps: 1) heating section: heating from room temperature to 450-550 DEG C, and the heating time is 3.5-4.5 h; 2) annealing section: the annealing temperature is 450-550 DEG C, and the annealing time is 19.5-20.5 h; 3) cooling section: stopping fire in the bell type furnace, and when the temperature is cooled to 180-220 DEG C, the steel coil is taken out from the bell type furnace and air-cooled to room temperature.

2. The method of hot strip high strength steel shape control and optimization of claim 1, wherein: In step 1), the heating section is purged with N2 at a flow rate of 14.5 m 3 / h from the start of heating until the end of the heating period. 3 / h.

3. The method of hot strip high strength steel shape control and optimization of claim 1, wherein: In step 2), the annealing section lasts for 29.5 min with N2 flow rate of 30.5 m 3 / h~30.5m 3 / h.

4. The method of shape control and optimization of hot rolled high strength steel strip as claimed in claim 1, wherein: The slab heating is sent into the slab heating furnace in a cold charging manner after the cast slab is stacked and slowly cooled to room temperature, and the hot slab with a temperature higher than 500 DEG C must be stacked on both sides, and the slab should not be placed on the uppermost and lowermost layers of the stack.

5. The method of shape control and optimization of hot rolled high strength steel strip as claimed in claim 1, wherein: The rough rolling is carried out by using two single mill reversible rough rolling mills, the camber value at the outlets of the two rough rolling mills is monitored in real time, the control system is fed back in real time to regulate the mill inclination value, and the rough rolling camber is controlled in the range of -20-20 mm.

6. A hot-rolled high strength steel, characterized in that: The hot-rolled high-strength steel is obtained by the method for flatness control and optimization of the hot-rolled high-strength steel according to any one of claims 1-5.

7. The hot rolled high strength steel of claim 6, wherein: The tensile strength is 500-700 MPa, and the thickness specification is 2.0-12.0 mm.

Images