Method for reducing strength difference in width direction of titanium microalloying hot-rolled high-strength steel

Abstract

The invention discloses a method for reducing the strength difference of titanium microalloying hot-rolled high-strength steel in the width direction, and belongs to the technical field of metallurgy. The titanium microalloyed hot-rolled high-strength steel is prepared according to the production technological processes of slab heating, rough rolling, finish rolling, laminar cooling and coiling. The tensile strength fluctuation range of the titanium microalloying hot-rolled high-strength steel in different positions in the plate width direction is not higher than 20 MPa. The method specifically comprises the following steps: a, fully heating a plate blank when the plate blank is heated, and ensuring that the temperature of each part of the plate blank is uniformly distributed; b, preserving heat of the plate blank before rough rolling, and ensuring complete recrystallization of austenite during rough rolling; c, carrying out heat preservation on the plate blank before finish rolling, and precisely controlling the temperature during finish rolling; d, laminar cooling to prevent water accumulation on the surface of the steel plate; and e, controlling the fluctuation range of the coiling temperature in the width direction to be not higher than 20 DEG C during coiling. The method can effectively solve the problems that the mechanical property fluctuation in the width direction of the titanium microalloyed steel is large, and the stability is poor.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, relates to a production method of hot-rolled microalloyed high-strength steel, and specifically relates to a method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel. Background Art

[0002] With the development of high-purity steel smelting technology, microalloyed steel has been increasingly favored by domestic and foreign steel companies and has been widely used in the research and development of hot-rolled high-strength steel. Among them, Ti microalloyed steel has been vigorously promoted in hot-rolled high-strength steel, especially high-strength automotive steel products, due to its lower alloy cost, better precipitation strengthening and fine grain strengthening effects. At the same time, due to the strict requirements of the automobile manufacturing industry on the safety performance and product stability of materials, it has also been fed back to upstream steel companies, and higher requirements have been put forward for the product quality and performance stability of high-strength automotive steel, especially titanium microalloyed high-strength steel. Ti microalloyed steel has the following disadvantages of large fluctuations in mechanical properties, mainly because Ti is easy to precipitate in each hot rolling process, such as deformation-induced precipitation, interphase precipitation, ferrite supersaturation precipitation, etc. in the rolling, laminar cooling, and coiling stages. The sizes of the above-mentioned precipitates vary greatly, so the precipitation behavior of the Ti element is greatly affected by the stability of the hot rolling process. Therefore, it is necessary to study the production process stability of Ti microalloyed steel, improve the performance uniformity of Ti microalloyed steel in the width direction, and reduce the uneven performance that leads to poor user experience, resulting in problems such as rebound and cracking.

[0003] After searching, CN114226453B discloses a low carbon steel and a manufacturing method for improving the uniformity of ferrite rolled low carbon steel. The chemical composition is calculated by weight percentage: C: ≤0.070%; Si: ≤0.050%; Mn: 0.08-0.30%; P: ≤0.025%; S: ≤0.015%; Al: 0.020-0.060%; the rest is Fe and inevitable impurities. The production process is: control the temperature of the billet out of the furnace at 1000-1050℃, and the edge temperature drop is less than 50℃; the cooling between the F1 racks and the F2 are used for secondary descaling water system, and the cooling water, descaling water and roll gap lubrication between the other racks are closed, and the temperature fluctuation in the width direction of the F4 rack entrance is controlled to be less than 20℃; the laminar cooling method is the rear-stage laminar sparse cooling, and the side spraying method adopts symmetrical side spraying, and the side spraying is water spraying or air spraying;.

[0004] CN118516521A discloses a production method for improving the stability of the performance of 800MPa-grade complex phase steel in the width direction and the produced 800MPa-grade complex phase steel. In the production of the present invention, the width setting machine is put into use to increase the deformation of the edge and promote the refinement of the grain structure of the edge; the temperature unevenness in the width direction during the finishing rolling is reduced by optimizing the water use strategy between the frames; the laminar cooling front-end fast cooling technology is adopted to quickly cool the strip to the coiling target temperature, reduce the retention time of the strip in the water environment, and reduce the temperature drop of the edge during the period; the laminar cooling edge shielding technology is used to accurately control the laminar cooling edge shielding amount, so that the edges of plate and strip products with different widths are cooled by the natural flow of cooling water in the middle area, and the edge temperature of the plate and strip is increased, thereby reducing the difference in the structure and performance of the edge and the middle part of the strip laminar cooling and hot coil cooling; the tensile strength extreme difference in the width direction of the product is ≤35MPa, and the low strength point is ≥815MPa.

[0005] From the above, it can be seen that the above invention patents mainly involve the hot rolling process control of conventional steel types such as carbon steel and complex phase steel, while Ti microalloyed steel is more sensitive to hot rolling process and temperature. In order to reduce the performance fluctuation range of Ti microalloyed steel, a more targeted and stricter hot rolling process design is needed. Summary of the invention

[0006] The technical problem to be solved by the present invention is that Ti microalloyed steel has high sensitivity to hot rolling process and temperature, large fluctuation in mechanical properties and poor stability.

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

[0008] Method for reducing the strength difference of titanium microalloyed hot-rolled high-strength steel in the width direction: titanium microalloyed hot-rolled high-strength steel is prepared according to the production process of slab heating, rough rolling, finish rolling, laminar cooling and coiling;

[0009] The tensile strength fluctuation range of the titanium microalloyed hot-rolled high-strength steel at different positions along the plate width direction is not higher than 20 MPa;

[0010] The method specifically comprises the following steps:

[0011] a. When heating the slab, the slab should be fully heated to ensure that the temperature of each part of the slab is evenly distributed;

[0012] b. Keep the slab warm before rough rolling to ensure complete recrystallization of austenite during rough rolling;

[0013] c. Keep the slab warm before finishing rolling and precisely control the temperature during finishing rolling;

[0014] d. Laminar cooling prevents water accumulation on the surface of the steel plate;

[0015] e. When coiling, control the coiling temperature so that the fluctuation range along the width direction is not higher than 20℃.

[0016] In the above step a, hot delivery and hot charging are adopted during the furnace loading and the charging temperature is controlled to be ≥500°C, or the slabs are stacked and slowly cooled before being cold loaded into the furnace.

[0017] In the above step a, the furnace temperature is controlled to be 1200° C. to 1250° C., the furnace time is 180 min to 300 min, and the soaking time is 40 min to 60 min.

[0018] In the above step b, a heat preservation cover is put into use on the roller table from the outlet of the heating furnace to the entrance of the roughing mill.

[0019] In the above step b, a fixed width press or a rough rolling mill vertical roller is used to perform lateral pressing of the slab, the lateral pressing amount is controlled to be 0 mm to 120 mm, and the rough rolling single pass reduction amount is ≥18%.

[0020] In the above step c, a heat preservation cover is put into use on the roller table from the outlet of the roughing mill to the entrance of the finishing mill.

[0021] In the above step c, the temperature difference of the finishing rolling work rolls along the width direction is not higher than 30°C.

[0022] In the above step c, the rolling mileage of the finishing rolling work rolls is not less than 10 km.

[0023] In the above step c, the finishing rolling exit speed is 5m / s to 8m / s.

[0024] In the above step c, the final rolling temperature fluctuates within a range of no more than 20° C. in the width direction.

[0025] In the above step d, the laminar cooling process adopts the front-stage sparse cooling method, and the header opening method is 1 on 1 or 1 on 2.

[0026] In the above step d, the side water spray and top air spray are turned on to sweep away the surface water, and the side water spray sweeping surface covers the entire width of the steel plate.

[0027] Furthermore, the purge medium is compressed air, and the pressure of the compressed air is 0.58MPa to 0.6MPa.

[0028] The beneficial effect of the present invention is that the present invention systematically proposes a method for reducing the performance fluctuation of Ti microalloyed steel in the width direction from the whole process of hot rolling. The method of reducing the performance fluctuation of Ti microalloyed steel described in the present invention is suitable for conventional hot rolling production lines, has strong process adaptability and promotion, and can be extended to other steel companies and the production of other steel products. The invention promotes the product research and development and quality stability control of Ti microalloyed hot-rolled high-strength steel, can promote the application of high-strength and lightweight steel in the fields of automobile manufacturing, engineering machinery, etc., and promotes carbon reduction and emission reduction in my country's infrastructure industry and manufacturing industry. DETAILED DESCRIPTION

[0029] The technical solution of the present invention can be specifically implemented in the following manner.

[0030] The reasons for the production process limitations of the titanium microalloyed hot-rolled high-strength steel of the present invention are explained below to clarify the influence of the production process on the performance stability of the titanium microalloyed steel.

[0031] First, in terms of the slab heating process. The nozzles of the heat storage heating furnaces equipped in general hot rolling mills are arranged near the narrow surface of the slab end, that is, the heat diffuses along the narrow surface of the slab end to the middle. Therefore, in order to improve the temperature uniformity of the steel billet when it comes out of the furnace and reduce the temperature difference and organizational and performance differences of the steel plate along the width direction, improvements can be made in the following aspects: 1) Use hot delivery and hot loading, the furnace loading temperature is ≥500°C, or the billet stack is slowly cooled and then cold loaded into the furnace. This is because hot delivery and hot loading or slow cooling of the billet stack is conducive to reducing the cooling rate of the billet during cooling, avoiding excessive cooling rate and causing excessive differences in microstructure grain size at different positions of the billet. 2) Use a higher slab out-of-furnace temperature and a longer heating and heat preservation time. The present invention limits the out-of-furnace temperature, furnace time, and soaking time to 1200°C to 1250°C, 180min to 300min, and 40min to 60min, respectively. This is because as the slab heating temperature and heating time increase, the billet is heated more fully, the heat diffuses more evenly, and the temperature distribution at different positions of the billet is more uniform.

[0032] Second, in terms of the rough rolling process. First, along the width direction of the steel billet, the cooling rate at the edge is higher than that in the middle. This is because the edge of the steel billet is located at the corner and has a larger contact area with the air. Therefore, the present invention requires that a heat preservation cover be used on the rollers from the heating of the slab to the rough rolling, and between the rough rolling and the finishing rolling, so as to reduce the temperature difference at different positions of the steel billet. Secondly, in addition to the temperature, the rolling reduction also has a great influence on the organization and performance fluctuations of the steel plate. The present invention limits the lateral pressure during the rough rolling of the steel billet to 0mm to 120mm. This is because when the lateral pressure of the steel billet is too high, it is easy to cause a bulge to form near the angle of the steel plate. Under the same rough rolling reduction, the reduction at the normal position where no bulge is formed may be low, resulting in incomplete recrystallization of austenite, inducing mixed crystal structure, and resulting in uneven performance of the steel plate. At the same time, the present invention limits the reduction in a single rough rolling pass to ≥18%. This is because when the reduction in a rough rolling pass is too low, the deformation of the core of the steel billet is insufficient, and the energy provided by the deformation cannot induce complete austenite recrystallization, resulting in incomplete austenite recrystallization, which leads to uneven steel plate structure and uneven performance.

[0033] Third, in terms of finishing rolling process. Since the finishing rolling work rolls are in a state of alternating heating and cooling during the rolling process and between the rolling gaps, it is easy for the finishing rolling work rolls to have uneven temperatures along the axial direction, that is, in the width direction. Therefore, the present invention requires controlling the rolling rhythm to ensure that the temperature difference of the working rolls along the width direction is not higher than 30°C. At the same time, the working roll rolling mileage is limited to no less than 10km because when the rolling mileage is low, the working roll temperature is low, which can easily lead to a decrease in temperature of the steel plate when it contacts the working roll during the rolling process. In addition, the present invention also requires the use of a faster finishing exit speed of 5m / s to 8m / s to avoid the tail of the steel coil being outside the rolling mill for too long, which in turn leads to a large temperature drop at the tail of the steel coil.

[0034] Fourth, in terms of laminar cooling process. The present invention requires the use of a front-stage sparse cooling method, and specifies the open water mode in detail, that is, 1 open 1 or 1 open 2. This is because when centralized rapid cooling is adopted, the amount of laminar cooling water is large, which can easily cause the steel plate to be soaked in the laminar cooling water, that is, serious water accumulation on the surface of the steel plate, which not only easily causes the steel plate to turn red and induces coarse structure in the core of the steel plate, but also easily causes the surface water to flow away from the width edge of the steel plate, causing the temperature of the edge of the steel plate to be lower than that of the middle. At the same time, in order to further prevent water accumulation on the surface of the steel plate, the present invention limits the opening angle of the side water spray, requires the side water spray sweeping surface to cover the entire plate surface, and requires the use of a larger flow of top air spray before coiling to further sweep the surface water.

[0035] Fifth, in terms of coiling process. The present invention puts forward specific requirements for typical hot rolling temperature control, that is, the fluctuation range of the final rolling temperature and the coiling temperature along the plate width direction is not higher than 20°C. This is because the final rolling temperature and the coiling temperature represent the temperature before and after laminar cooling, respectively. The laminar cooling process between the two is the key process for the steel to undergo γ→α phase transformation and form interphase precipitation TiC. When the final rolling temperature fluctuates greatly, it may cause the position where the final rolling temperature is relatively low to enter the two-phase region for rolling, forming a mixed crystal structure, affecting the uniformity of the steel structure, and the position where the final rolling temperature is relatively high. At the same time, when the final rolling temperature is different, the amount of deformation-induced precipitation of TiC during the rolling process is also different. When the final rolling temperature is higher, the deformation-induced precipitation formed during the rolling process and the interphase precipitation formed during the laminar cooling process are more. When the coiling temperature is low, the grain size of the finished steel structure is finer. At the same time, when the coiling temperature is close to 600°C, that is, the supersaturated precipitation nose temperature of Ti, the supersaturated precipitation of ferrite is the most. When the coiling temperature is high or low, the supersaturated precipitation phase of ferrite will decrease. It can be seen from the above that when the final rolling temperature and the coiling temperature fluctuate, the microstructure grain size and titanium-containing precipitation phase of the steel will change, resulting in the change of the fine grain strengthening and precipitation strengthening increment of the steel. Therefore, the present invention requires that the fluctuation range of the final rolling temperature and the coiling temperature in the width direction be controlled within 20°C.

[0036] The technical solution and effects of the present invention are further illustrated below through practical examples.

[0037] Example

[0038] Example 1

[0039] The titanium microalloyed steel ingot was prepared by converter-refining-continuous casting process, and was sent to the slab heating furnace by hot delivery and hot charging. The slab charging temperature was 612℃, the furnace discharge temperature was 1215℃, the furnace time was 260min, and the soaking time was 52min. The slab was sent to the rough rolling mill through the roller table with insulation cover, and the side pressure was 80mm on the rough rolling vertical roller. The rough rolling was carried out in 6 passes, and the deformation of each pass was 19%, 20%, 24%, 26%, 29%, and 30%. The intermediate slab obtained by rough rolling was sent to the finishing mill through the roller table with insulation cover. The rolling mileage of the finishing working roll was 15km, the temperature difference of the working roll along the width direction was 23℃, and the finishing exit speed was 6.5m / s. The laminar cooling adopted a sparse cooling mode of 1 on 1, and the side water spray and top air spray were turned on to purge the surface water. The compressed air pressure used for air spray was 0.6MPa. The extreme difference in finishing temperature along the width direction of the plate is 12°C, including 870°C in the middle of the plate width and 858°C at the edge of the plate width. The extreme difference in coiling temperature along the width direction of the plate is 11°C, including 600°C in the middle of the plate width and 589°C at the edge of the plate width.

[0040] The titanium microalloyed steel prepared by the above process has tensile strengths of 676MPa, 688MPa and 690MPa at the edge, 1 / 4 and middle of the plate width, respectively, and the tensile strength fluctuation range along the plate width is 14MPa.

[0041] Example 2

[0042] The titanium microalloyed steel ingot was prepared by converter-refining-continuous casting process. After stacking and slow cooling to room temperature, it was sent to the slab heating furnace in a cold loading manner. The slab temperature out of the furnace was 1238℃, the furnace time was 223min, and the soaking time was 46min. The slab was sent to the rough rolling mill through the roller table with a heat preservation cover, and the side pressure was 60mm on the rough rolling vertical roller. The rough rolling was carried out in 5 passes, and the deformation of each pass was 21%, 23%, 28%, 30%, and 32%. The intermediate billet obtained by the rough rolling was sent to the finishing mill through the roller table with a heat preservation cover. The rolling mileage of the finishing working roll was 22km, the temperature difference of the working roll along the width direction was 18℃, and the finishing exit speed was 5.7m / s. The laminar cooling adopted a sparse cooling mode of 1-2 intervals, and the side water spray and top air spray were turned on to purge the surface water. The compressed air pressure used for the air spray was 0.59MPa. The extreme difference in finishing temperature along the width direction of the plate is 15°C, including 880°C in the middle of the plate width and 865°C at the edge of the plate width. The extreme difference in coiling temperature along the width direction of the plate is 13°C, including 607°C in the middle of the plate width and 594°C at the edge of the plate width.

[0043] The titanium microalloyed steel prepared by the above process has tensile strengths of 822MPa, 830MPa and 839MPa at the edge, 1 / 4 and middle of the plate width, respectively, and the tensile strength fluctuation range along the plate width is 17MPa.

[0044] Comparative Example 1

[0045] The ingot of titanium microalloyed steel was prepared by converter-refining-continuous casting process, and was sent to the slab heating furnace by hot delivery and hot charging. The slab charging temperature was 603℃, the furnace discharge temperature was 1209℃, the furnace time was 213min, and the soaking time was 41min. The slab was side pressed by a fixed width press with a side pressure of 160mm. The rough rolling was carried out in 6 passes with deformation of 18%, 20%, 25%, 27%, 28%, and 30%. The rolling mileage of the finishing rolling working roll was 13km, the temperature difference of the working roll along the width direction was 45℃, and the finishing rolling exit speed was 6.4m / s. The laminar cooling adopted a sparse cooling mode of 1 on 1, and the side water spray and top air spray were turned on to purge the surface water. The compressed air pressure used for the air spray was 0.58MPa. The extreme difference in finishing temperature along the width direction of the plate is 30°C, including 886°C in the middle of the plate width and 856°C at the edge of the plate width. The extreme difference in coiling temperature along the width direction of the plate is 32°C, including 612°C in the middle of the plate width and 580°C at the edge of the plate width.

[0046] The titanium microalloyed steel prepared by the above process has tensile strengths of 639MPa, 663MPa and 682MPa at the edge, 1 / 4 and middle of the plate width, respectively, and the tensile strength fluctuation range along the plate width is 43MPa.

[0047] The tensile strength fluctuation range of the comparative steel exceeds 20MPa, which is mainly due to the excessive side pressure of the slab, resulting in uneven rough rolling reduction along the width direction. At the same time, the temperature control of the working rolls is poor, and the temperature difference along the width direction is 45°C, resulting in large differences in the final rolling temperature and coiling temperature of the steel plate along the width direction, which in turn causes large fluctuations in the tensile strength of the steel plate along the width direction.

[0048] Comparative Example 2

[0049] The ingot of titanium microalloyed steel was prepared by converter-refining-continuous casting process. After stacking and slow cooling to room temperature, it was sent to the slab heating furnace by cold loading. The slab temperature out of the furnace was 1240℃, the time in the furnace was 256min, and the soaking time was 52min. The slab was sent to the rough rolling mill through the roller table with insulation cover, and the side pressure was 50mm on the rough rolling vertical roller. The rough rolling was carried out in 5 passes, and the deformation of each pass was 20%, 24%, 29%, 31%, and 31%. The intermediate billet obtained by rough rolling was sent to the finishing mill through the roller table with insulation cover. The rolling mileage of the finishing working roll was 22km, the temperature difference of the working roll along the width direction was 18℃, and the finishing exit speed was 5.7m / s. The laminar cooling adopts the front-stage centralized opening mode, and the opened side spray water cannot completely blow away the surface water. The extreme difference in finishing temperature along the width direction of the plate is 39°C, including 890°C in the middle of the plate width and 851°C at the edge of the plate width. The extreme difference in coiling temperature along the width direction of the plate is 36°C, including 621°C in the middle of the plate width and 585°C at the edge of the plate width.

[0050] The titanium microalloyed steel prepared by the above process has tensile strengths of 783MPa, 815MPa and 828MPa at the edge, 1 / 4 and middle of the plate width, respectively, and the tensile strength fluctuation range along the plate width is 45MPa.

[0051] The tensile strength fluctuation range of the comparative steel exceeds 20MPa, mainly because the laminar cooling water volume is too large and the side water spray fails to completely sweep the surface water, resulting in a large temperature difference between the middle and edge of the steel plate. The extreme differences of the final rolling temperature and coiling temperature are 39°C and 36°C respectively, which leads to large fluctuations in the tensile strength of the steel plate along the width direction.

Claims

1. A method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel, characterized in that: Titanium microalloyed hot-rolled high-strength steel is prepared according to the production process of slab heating, rough rolling, finish rolling, laminar cooling and coiling, wherein the fluctuation range of the tensile strength of the titanium microalloyed hot-rolled high-strength steel at different positions along the width direction of the plate is not higher than 20 MPa, and the method specifically comprises the following steps: a. When heating the slab, the slab should be fully heated to ensure that the temperature of each part of the slab is evenly distributed; b. Keep the slab warm before rough rolling to ensure complete recrystallization of austenite during rough rolling; c. Keep the slab warm before finishing rolling and precisely control the temperature during finishing rolling; d. Laminar cooling prevents water accumulation on the surface of the steel plate; e. When coiling, control the coiling temperature so that the fluctuation range along the width direction is not higher than 20℃.

2. The method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel according to claim 1, characterized in that: In step a, hot delivery and hot charging are adopted during charging and the charging temperature is controlled to be ≥500°C, or the slabs are stacked and slowly cooled before being cold charged into the furnace.

3. The method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel according to claim 1, characterized in that: In step a, the furnace temperature is controlled to be 1200° C. to 1250° C., the furnace time is 180 min to 300 min, and the soaking time is 40 min to 60 min.

4. The method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel according to claim 1, characterized in that: In step b, a heat-insulating cover is put into use on the roller table from the outlet of the heating furnace to the entrance of the roughing mill.

5. The method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel according to claim 1, characterized in that: In step b, a fixed width press or a vertical roller of a rough rolling mill is used to perform side pressing of the slab, and the side pressing amount is controlled to be 0 mm to 120 mm, and the reduction amount of a single rough rolling pass is ≥18%.

6. The method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel according to claim 1, characterized in that: In step c, a heat-insulating cover is put into use on the roller table from the outlet of the roughing mill to the entrance of the finishing mill.

7. The method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel according to claim 1, characterized in that: In step c, at least one of the following is met: The temperature difference of the finishing rolling work roll along the width direction is not higher than 30℃; The rolling mileage of the finishing work roll shall not be less than 10km; Finishing rolling exit speed 5m / s~8m / s; The final rolling temperature fluctuation range along the width direction is not higher than 20℃.

8. The method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel according to claim 1, characterized in that: In step d, the laminar cooling process adopts the front-stage sparse cooling method, and the header opening method is 1 open 1 or 1 open 2.

9. The method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel according to claim 1, characterized in that: In step d, the side water spray and top air spray are turned on to sweep away the surface water, and the side water spray sweeping surface covers the entire width of the steel plate.

10. The method for reducing the strength difference in the width direction of titanium microalloyed hot-rolled high-strength steel according to claim 9, characterized in that: The purge medium is compressed air, and the compressed air pressure is 0.58MPa~0.6MPa.