A rolling method of a 1.2mm ultra-thin gauge hot-rolled automobile structural steel strip

Abstract

The application discloses a rolling method for 1.2mm extremely thin gauge hot-rolled automobile structure steel strip in the field of hot-rolling steel production, and the method comprises the following steps: controlling the thickness and length of a steel casting blank; adopting a hot feeding and hot charging mode for heating; performing rough rolling and finish rolling; performing laminar flow cooling on the steel strip after the finish rolling; and finally performing coiling. The application adopts a slab with a proper length, which is beneficial to reducing the temperature difference between the intermediate blank and the head and tail of the steel coil, the hot feeding and hot charging mode is adopted for heating, the core temperature of the slab can be improved, the rolling is stable, the rough rolling is performed for four passes, the temperature drop at the entrance of the finish rolling is further reduced, and through reasonable control of the rolling speed, the plate crown, the loop amount, the bending force and the roll shifting amount and other processes during the finish rolling, the 1.2mm thick hot-rolled steel strip has the advantages of good rolling stability and excellent comprehensive performance, defects such as rolling breakage, wave shape and rolling waste at the tail of the steel strip in the prior art are avoided, and the yield of the extremely thin gauge hot-rolled steel strip can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of hot rolling steel production, in particular to a rolling method of 1.2mm ultra-thin gauge hot-rolled automobile structure steel strip. BACKGROUND

[0002] At present, the conventional thickness of automobile structure steel is 2.0-10.0mm, the tensile strength is 400-700MPa, the elongation is ≥40%, and the yield ratio is ≤0.80. With the gradual development of the automobile industry towards light weight and low cost, the demand for ultra-thin gauge hot-rolled steel gradually increases. Thinning the thickness of the steel plate can reduce the weight of the vehicle and reduce carbon emissions, and can also replace cold-rolled plates to further reduce carbon emissions and save production costs.

[0003] Some production methods of ultra-thin gauge hot-rolled steel are disclosed in the prior art, such as:

[0004] CN104550256A discloses a TMCP thin gauge automobile structure steel plate shape control method: heating the billet to 1100-1200℃, controlling the temperature difference of the upper, middle and lower layers of the billet to be 0-30℃; setting the rolling process parameters unchanged, using the rolling schedule set by the two-stage model for rolling; after finish rolling, the steel plate is cooled by ACC, the ACC roller speed is set to 1.0-1.25m / s, and the final cooling temperature is controlled in the range of 400-500℃; after cooling, the steel plate is straightened online, and the two-stage model is used for automatic straightening, and the red temperature of the straightened steel plate is controlled to be 500-550℃.

[0005] CN112496085B discloses a production method for improving the edge wave defect of 1500MPa grade hot-rolled ultra automobile structure steel. According to the wave type and wave height of the edge wave, the rough rolling and finishing rolling parameters are set as follows: when the wave height is 5-10mm / m, the lower roll position of the rough straightening and finishing straightening work roll is set to 4-8mm; when the wave height is 10.1-15mm / m, the lower roll position of the rough straightening and finishing straightening work roll is set to 9-12mm. When the wave type is single edge wave on the operation side, the work roll inclination of the rough straightening and finishing straightening is set to 0.5-1.2mm; when the wave type is single edge wave on the drive side, the work roll inclination of the rough straightening and finishing straightening is set to -0.5--0.2mm; when the wave type is double edge wave, the work roll inclination of the rough straightening and finishing straightening is set to 0.1-0.4mm. The present application can control the unevenness of the steel plate to be below 5mm / m.

[0006] CN113522988B discloses a DQ process thin gauge ultra-car structure steel plate shape control method, including: rolling, cooling, full-process plate shape control of flattening transverse cutting. The invention emphasizes the micro-mid-waves compensation strategy during rolling. After the steel coil is rolled, a segmented cooling mode is adopted to mainly reduce the problem of poor plate shape caused by excessive cooling rate after rolling. The steel plate is flattened immediately within 48 hours after rolling to avoid excessive work hardening of the steel plate and ensure the flattening effect. The unevenness of the steel plate produced in this aspect is between 5-8mm / m, and the best can reach 2mm / m.

[0007] As can be seen from the above, the industrial production of extremely thin gauge hot-rolled automobile structure steel is extremely difficult, and generally uses continuous casting and rolling process. However, the forming performance of automobile structure steel produced by continuous casting and rolling process is poor, and correction is often needed when using conventional hot rolling method, which has low production efficiency and low steel strip yield. SUMMARY

[0008] To overcome the problems of large production difficulty, low production efficiency and low yield of existing extremely thin gauge hot-rolled steel strip, the technical problem to be solved by the present application is to provide a rolling method of 1.2mm extremely thin gauge hot-rolled automobile structure steel strip using a conventional hot rolling mill.

[0009] The technical scheme adopted by the present application to solve the technical problem is:

[0010] A rolling method of 1.2mm extremely thin gauge hot-rolled automobile structure steel strip, wherein the chemical composition of the steel casting blank includes, by weight percentage: C:≤0.10%, Si:0.05~0.25%, Mn:0.30~1.00%, P:≤0.015%, S≤0.005%, Ti:0.015%~0.045%, N≤0.005%, the rest being Fe and unavoidable impurity elements, and the rolling process includes the following steps:

[0011] Step one, control the thickness of the steel casting blank to be 200-250mm, and the length of the casting blank to be 8.5-9.5m;

[0012] Step two, the steel casting blank is loaded into the slab heating furnace in a hot charging mode, and the slab discharge temperature is 1220-1280℃;

[0013] Step three, the steel casting blank is roughed, two groups of rolling mills are used, the first group of rolling mills is rolled for one pass, and the second group of rolling mills is rolled for three passes, the inlet temperature of rough rolling is 1160-1200℃, the outlet temperature of rough rolling is 1120-1160℃, and the outlet speed of rough rolling is 2-5m / s;

[0014] Step four, the intermediate blank obtained in step three is finish-rolled, the finish-rolling inlet temperature is 1060-1120℃, the finish-rolling outlet temperature is 860-900℃, the finish-rolling outlet speed is 10.0-11.5m / s, and the thickness of the steel strip after finish-rolling is controlled to be 1.2mm; the number of groups of cooling water opened between the stands during finish-rolling is not more than 1 group, and the number of groups of lubricating rolling opened is not less than 3 groups; the finish-rolling unit includes 7 finish-rolling rollers, which are F1-F7 respectively, and 6 loop rollers, which are L1-L6 respectively, and the finish-rolling rollers and the loop rollers are controlled according to the following requirements:

[0015] The finish-rolling roller crown is F1: 30-40μm, F2: 15-20μm, F3: 10-15μm, F4: 5-10μm, F5: 4-8μm, F6: 4-6μm, and F7: 4-6μm;

[0016] The finish-rolling roller gap is F1: 15mm, F2: 7mm, F3: 3mm, and F4-F7: 2mm;

[0017] The finish-rolling roller speed is F1: 0.8-1.1m / s, F2: 1.6-2.2m / s, F3: 2.5-4.0m / s, F4: 4.5-6.2m / s, F5: 6.6-8.6m / s, F6: 8.4-10.1m / s, and F7: 10.0-11.5m / s;

[0018] The loop roller tension is L1: 700-800N / mm2, L2: 800-900N / mm2, L3: 900-1000N / mm2, L4: 1100-1200N / mm2, L5: 1300-1400N / mm2, and L6: 1600-1800N / mm2;

[0019] The finish-rolling roller bending force is F1: 200N / mm2, F2-F4: 180N / mm2, F5-F6: 160N / mm2, and F7: 120N / mm2;

[0020] The finish-rolling roller roll shifting amount is F1: -50--150mm, F2: -20--70mm, F3: -10--60mm, F4: -5--40mm, F5: -100--150mm, F6: -60--130mm, and F7: -30--100mm;

[0021] Step five, the steel strip after finish-rolling is laminar flow cooled, the laminar flow cooling rate is 10-30℃ / s, the lower header and the upper header cooling flow rate ratio is 1.2-1.4, the laminar flow cooling model adopts sparse cooling, and the steel strip is coiled after being cooled to 600-650℃.

[0022] Further, in step three, the thickness of the intermediate blank after rough rolling is controlled to be 30-35mm.

[0023] Further, a heat preservation cover and a hot coil box are arranged on the transfer roller way between the third step and the fourth step, and the intermediate blank enters the finishing rolling mill group after passing through the heat preservation cover and the hot coil box in sequence.

[0024] The present application has the advantages that: the present application adopts the slab with appropriate length, which is beneficial to reduce the temperature difference of the intermediate blank and the head and tail of the coil, and the hot feeding and hot charging mode is adopted for heating, which can improve the core temperature of the slab, and is easy to roll stably, the 4-pass rough rolling further reduces the temperature drop at the entrance of the finishing rolling, and through reasonable control of the rolling speed, the plate crown, the loop amount, the bending force and the roll shifting amount and other processes during the finishing rolling, the 1.2mm thick hot continuous rolling steel strip has the advantages of good rolling stability and excellent comprehensive performance, and the defects such as rolling breakage, wave shape and rolling waste at the tail of the steel strip in the prior art are avoided, the yield of the extremely thin specification hot continuous rolling steel strip can be improved, and the yield can be more than 95%. DETAILED DESCRIPTION

[0025] The present application will be further described below in combination with the embodiments.

[0026] The present application is a rolling method of a 1.2mm extremely thin specification hot rolling automobile structure steel strip, wherein the chemical composition of the rolling steel ingot includes, in terms of weight percentage: C: ≤0.10%, Si: 0.05-0.25%, Mn: 0.30-1.00%, P: ≤0.015%, S ≤0.005%, Ti: 0.015%-0.045%, N ≤0.005%, and the rest is Fe and inevitable impurity elements, and the rolling process includes the following steps:

[0027] Step one, the thickness of the steel ingot is controlled to be 200-250mm, and the length of the ingot is 8.5m-9.5m;

[0028] Step two, the steel ingot is loaded into the slab heating furnace in a hot feeding and hot charging mode, and the slab discharge temperature is 1220-1280℃;

[0029] Step three, the steel ingot is rough rolled, two groups of rolling mills are adopted, the first group of rolling mills is rolled for 1 pass, and the second group of rolling mills is rolled for 3 passes, the inlet temperature of the rough rolling is 1160-1200℃, the outlet temperature of the rough rolling is 1120-1160℃, and the outlet speed of the rough rolling is 2-5m / s;

[0030] Step four, the intermediate blank obtained in step three is subjected to finish rolling, the finish rolling inlet temperature is 1060-1120℃, the finish rolling outlet temperature is 860-900℃, the finish rolling outlet speed is 10.0-11.5m / s, and the thickness of the steel strip after finish rolling is controlled to be 1.2mm; the number of groups of cooling water opened between the stands during finish rolling is not more than 1 group, and the number of groups of lubricating rolling opened is not less than 3 groups; the finish rolling mill set includes 7 finish rolling rollers, which are F1-F7 respectively, and 6 loop rollers, which are L1-L6 respectively, and the finish rolling rollers and the loop rollers are controlled according to the following requirements:

[0031] The finish rolling roller crown is F1: 30-40μm, F2: 15-20μm, F3: 10-15μm, F4: 5-10μm, F5: 4-8μm, F6: 4-6μm, and F7: 4-6μm;

[0032] The finish rolling roller gap is F1: 15mm, F2: 7mm, F3: 3mm, and F4-F7: 2mm;

[0033] The finish rolling roller speed is F1: 0.8-1.1m / s, F2: 1.6-2.2m / s, F3: 2.5-4.0m / s, F4: 4.5-6.2m / s, F5: 6.6-8.6m / s, F6: 8.4-10.1m / s, and F7: 10.0-11.5m / s;

[0034] The loop roller tension is L1: 700-800N / mm2, L2: 800-900N / mm2, L3: 900-1000N / mm2, L4: 1100-1200N / mm2, L5: 1300-1400N / mm2, and L6: 1600-1800N / mm2;

[0035] The finish rolling roller bending force is F1: 200N / mm2, F2-F4: 180N / mm2, F5-F6: 160N / mm2, and F7: 120N / mm2;

[0036] The finish rolling roller roll shifting amount is F1: -50--150mm, F2: -20--70mm, F3: -10--60mm, F4: -5--40mm, F5: -100--150mm, F6: -60--130mm, and F7: -30--100mm;

[0037] Step five, the steel strip after finish rolling is subjected to laminar cooling, the laminar cooling rate is 10-30℃ / s, the lower header and the upper header cooling flow rate ratio is 1.2-1.4, the laminar cooling model adopts sparse cooling, and the steel strip is coiled after being cooled to 600-650℃.

[0038] In the step three, in order to improve the finishing efficiency and the steel strip forming quality, the thickness of the intermediate blank after rough rolling is preferably controlled within 30-35mm.

[0039] In order to avoid the intermediate blank from cooling too fast during the process from the rough rolling mill to the finishing mill, and to reduce the finishing inlet temperature, a heat preservation cover and a hot coil box can be arranged on the transmission roller way between the step three and the step four, and the intermediate blank passes through the heat preservation cover and the hot coil box in sequence and then enters the finishing mill. The heat preservation cover and the hot coil box can make the temperature of the intermediate blank uniform and stable, and at the same time, can help to reduce the burning loss and the generation of secondary oxide scale, and improve the surface quality of the steel.

[0040] The reasons for limiting the main alloying elements in the steel according to the present application are described below.

[0041] C is an important strengthening element in the steel, and can improve the strength of the steel through solid solution strengthening and precipitation strengthening. However, the C content should not be too high, otherwise Ti and other elements are easy to form Ti-containing alloy cementite during subsequent phase transformation. The cementite phase not only has a large size, but also is easy to enrich at the grain boundary, which affects the forming performance of the material. Therefore, the C content is controlled at a lower level of 0.05-0.10%.

[0042] Si is an important interstitial solid solution strengthening element in the steel, but should not be too high, otherwise it is easy to form red scale which is not easy to remove.

[0043] Mn plays a role in solid solution strengthening and improving toughness in the steel. However, when the Mn content is too high, it is easy to form composition segregation at the center of the thickness, reduce the uniformity of the structure, and also increase the hot deformation resistance and the rolling load.

[0044] P, S and N are impurity elements in the steel, which are easy to form inclusions and reduce the toughness and plasticity of the steel. When the S and N content is too high, it is easy to form sulfide and nitride inclusions with Ti in the steel, among which the liquid-precipitated TiN has a cubic shape and is easy to form a crack source when it exists in a large amount in the steel, which has a great influence on the toughness and plasticity. Therefore, the S content is limited to ≤0.005%, and the N content is limited to ≤0.005%.

[0045] P is also one of the impurity elements in the steel. However, adding an appropriate amount of P can improve the corrosion resistance. However, it should not be added too high, otherwise the steel will become brittle.

[0046] Ti can combine with C elements in the steel to form nanoscale TiC precipitates, which can have a strong precipitation strengthening effect. At the same time, Ti can inhibit the coarsening of the original austenite structure during slab reheating, which is beneficial to the grain refinement. However, the Ti content should not be too high, so as to avoid the formation of large-size and high-density liquid-precipitated TiN inclusions.

[0047] The reasons for limiting the production process are described below in combination with the requirements of the steel strip forming control according to the present application.

[0048] First, in the heating system. Since the slab heating furnace uses nozzle heating, the main heat transfer mode is thermal radiation, and the heat is transferred from the surface to the center of the slab. Therefore, when the slab is cold, the core temperature is low, and the heating speed is slow. If the heating system is not appropriate, the core temperature will be low, which will lead to insufficient deformation transmission to the core of the slab during subsequent rough rolling, increase the rolling load and increase the rolling difficulty, and also lead to coarse core organization. When hot charging is used, that is, the billet is directly heated in the heating furnace after being heated and transported in a high temperature state. Since the slab is naturally cooled in the air after being discharged from continuous casting, the core temperature is higher than the surface, and the core temperature is high after the slab is heated, which is easy to roll stably. Therefore, the present application requires that the billet obtained after continuous casting is hot charged into the slab heating furnace, and the furnace temperature is controlled at a high level of 1220-1280℃.

[0049] Second, in the rough rolling process. The conventional rough rolling adopts a "3+3" mode, that is, rough rolling R1 and R2 are rolled for 3 passes respectively. The present application adopts a "1+3" mode, which reduces two rolling passes, reduces the temperature reduction, and the temperature and deformation system design is considered. The austenite recrystallization zone is rolled, and the austenite organization is refined through recrystallization to improve the strength and plasticity of the material. Therefore, the rough rolling temperature is controlled at a high level, the rolling deformation is controlled at a large level, and the intermediate billet thickness is controlled at a low level. Secondly, the rough shape control is considered, and a higher rough rolling temperature is adopted to reduce the rolling load. Therefore, the present application requires that the rough rolling "1+3" process has an inlet temperature of 1160-1200℃, a rough rolling outlet temperature of 1120-1160℃, a rough rolling outlet speed of 2-5m / s, and a rough rolling intermediate billet thickness of 30-35mm.

[0050] Third, an insulation cover and a hot coil box are provided on the roller bed between the rough rolling mill and the finishing rolling mill. On the one hand, it is to improve the finishing rolling temperature and reduce the rolling load, and on the other hand, it is to remove the surface iron oxide scale by coiling the high-temperature intermediate billet, so that the head and tail temperatures of the steel plate are uniform, and the stable rolling of the thin-gauge steel plate is realized.

[0051] Fourth, in the finishing rolling process. First, the main role of finishing rolling is to provide a large number of nucleation sites for subsequent phase transformation through large compression ratio rolling, to promote the formation of fine and uniform structure for subsequent phase transformation, and to improve the strength and toughness of the material through fine-grain strengthening. The present application adopts a higher finishing rolling temperature, i.e. the finishing rolling inlet temperature is 1060-1120℃, and the finishing rolling outlet temperature is 860-900℃, mainly to ensure that the early stage of finishing rolling (F1-F4) is still in the austenite recrystallization zone for rolling, and to form uniform and fine equiaxed grains through recrystallization. The late stage of finishing rolling (F5-F7) is in the austenite non-recrystallization zone for rolling, and to provide sufficient nucleation work and a large number of nucleation sites for subsequent phase transformation through the flattening of the austenite structure. In addition, the present application adopts a higher finishing rolling outlet speed, which is 10-11.5m / s, mainly to make the steel plate quickly complete the finishing rolling, reduce the temperature difference between the head and tail caused by the natural temperature drop of the steel plate, and thus reduce the performance difference between the head and tail of the steel plate.

[0052] Secondly, the setting of the finishing rolling process also considers the problem of shape control of thin gauge steel plate. The steel plate described in the present application is thin in thickness, and the finishing rolling compression ratio is large, so the shape control is difficult. Therefore, the present application requires: first, the number of open groups of inter-stand cooling water is required to be not more than 1 group, in order to reduce the temperature drop of the steel plate during finishing rolling; second, the number of open groups of lubricating rolling is required to be not less than 3 groups, in order to reduce the friction force on the surface of the steel plate through oil-water lubrication, and to reduce the finishing rolling load; third, the finishing rolling process is reasonably controlled, such as rolling speed, plate crown, loop amount, bending force and roll shifting amount, etc., so that the hot continuous rolling steel strip has the characteristics of good rolling stability and excellent comprehensive performance.

[0053] The finishing rolling roll crown is set to a reasonable value, which is beneficial to improve the plate shape accuracy of the whole strip and avoid the generation of strip wave defects; the intermediate billet and finished product thickness of thin gauge strip and the temperature drop during rolling are considered comprehensively, and through the design of the finishing rolling roll gap, reasonable rolling load distribution is realized under the premise of ensuring the finishing rolling deformation; too high loop roll tension is easy to cause the strip to be narrowed, and too low loop roll tension is easy to cause the extremely thin gauge strip to not be stretched enough between two stands, which makes the strip stability between stands poor and causes the shape to change, in addition, there is also a risk of causing the strip to be rolled to pieces; the finishing rolling roll bending force and roll shifting amount are set mainly to adapt to the intermediate billet crown after rough rolling, and to realize the control of the strip crown, flatness and shape, etc., and to avoid the generation of strip wave defects, etc.

[0054] The finishing rolling outlet speed is mainly affected by the temperature drop of the strip during finishing rolling, and a certain rolling speed is required to ensure that the temperature drop of the strip during finishing rolling can meet the requirements of the finishing rolling inlet temperature and outlet temperature. The thinner the thickness, the greater the temperature drop; in addition, like automobile structural steel and high-strength weathering steel, due to the addition of Ti, Cr, Cu and other alloy elements, the strip temperature drop rate increases, so the rolling speed needs to be further improved.

[0055] Fifth, in the aspect of laminar cooling. First, in order to avoid the formation of coarse ferrite or cementite organization, the laminar cooling rate is required to be higher, 10-30℃ / s, to refine the grain and improve the strength and plasticity of the material. Second, the coiling temperature is controlled at 600-650℃, and appropriately increasing the coiling temperature is beneficial to improve the elongation of the thin gauge steel plate. Third, due to the influence of gravity, the upward spraying rate of the cooling water of the lower header to the surface of the steel plate is low, therefore, in order to ensure that the cooling rates of the upper and lower surfaces are close, the cooling water flow of the lower header should be appropriately higher than that of the upper header. In combination with the industrial production experience, the flow ratio of the lower header to the upper header is controlled at 1.2-1.4 in the present application. Fourth, due to the phase change of the steel plate during the laminar cooling, internal stress is easily generated during the phase change, and the internal stress will increase when the cooling rate is higher, therefore, under the premise of meeting the performance requirements of the steel plate, the cooling rate of the steel plate is appropriately reduced, i.e. the cooling water mode is sparser, and every 2 groups of headers are opened and every group of headers is closed.

[0056] The specific embodiments of the present application are further described below in combination with examples, and the present application is not limited in the scope of the described examples.

[0057] Example 1

[0058] A steel billet with a thickness of 200mm and a billet length of 9.0m is prepared by adopting the process flow of converter-LF electric heating-RH vacuum refining-continuous casting, and the mass percentage of the steel billet is as follows: 0.05% C, 0.05% Si, 0.30% Mn, 0.015% P, 0.003% S, 0.015% Ti, 0.0043% N, and the rest is Fe and inevitable impurities. The steel billet is prepared into a steel plate with a thickness of 1.2mm by adopting the process flow of heating-coarse rolling-hot coiling box-fine rolling-laminar cooling-coiling. The specific process is as follows: the steel billet is sent into the billet heating furnace by hot sending and hot loading, and the discharge temperature is 1261℃. The coarse rolling inlet temperature is 1192℃, the coarse rolling outlet temperature is 1145℃, the coarse rolling outlet speed is 3.3m / s, and the intermediate billet thickness obtained after coarse rolling is 31mm. The intermediate billet after coarse rolling is transmitted by a roller way, and is sequentially subjected to heat preservation in a heat preservation cover and coiling after hot coiling box coiling, and is then sent into a fine rolling unit. The fine rolling inlet temperature is 1085℃, the fine rolling outlet temperature is 871℃, the fine rolling outlet speed is 11.5m / s, 1 group of inter-stand cooling water is opened, and 3 groups of lubrication rolling are opened. The steel plate after fine rolling is subjected to laminar cooling, the laminar cooling rate is 20℃ / s, the laminar cooling model is 2 groups of pipes 1 group, the water flow ratio of the lower header to the upper header is 1.33, and the coiling temperature is 650℃.

[0059] The steel plate in Example 1 has a thickness of 1.2mm, a tensile strength of 425MPa, an elongation of 48%, and a yield strength ratio of 0.75.

[0060] Example 2

[0061] A slab with thickness 200mm, slab length 9.4m, and mass percentage of 0.07% C, 0.25% Si, 0.98% Mn, 0.11% P, 0.004% S, 0.045% Ti, 0.0032% N, and the rest of Fe and inevitable impurities is prepared by the process of converter-LF electric heating-RH vacuum refining-continuous casting. A steel plate with thickness 1.2mm is prepared by the process of heating-coarse rolling-hot coil box-fine rolling-laminar cooling-coiling. The specific process is as follows: the slab is sent into the slab heating furnace by hot charging, and the discharge temperature is 1278℃. The coarse rolling inlet temperature is 1195℃, the coarse rolling outlet temperature is 1164℃, the coarse rolling outlet speed is 4.0m / s, and the intermediate slab thickness after coarse rolling is 35mm. The intermediate slab after coarse rolling is transmitted by roller way, and is successively kept warm by heat preservation cover, and is coiled by hot coil box, and is sent into the fine rolling unit. The fine rolling inlet temperature is 1105℃, the fine rolling outlet temperature is 891℃, the fine rolling outlet speed is 11.0m / s, the interstand cooling water is not opened, 5 groups of lubrication rolling are opened, the steel plate after fine rolling is subjected to laminar cooling, the laminar cooling rate is 30℃ / s, the laminar cooling model is 2 groups of pipes opened 1 group, the water flow rate ratio of lower header to upper header is 1.27, and the coiling temperature is 635℃.

[0062] The steel plate in Example 2 has thickness 1.2mm, tensile strength 695MPa, elongation 45%, and yield ratio 0.78.

[0063] Example 3

[0064] A slab with thickness 200mm, slab length 8.5m, and mass percentage of 0.07% C, 0.18% Si, 0.75% Mn, 0.013% P, 0.002% S, 0.036% Ti, 0.0034% N, and the rest of Fe and inevitable impurities is prepared by the process of converter-LF electric heating-RH vacuum refining-continuous casting. A steel plate with thickness 1.2mm is prepared by the process of heating-coarse rolling-hot coil box-fine rolling-laminar cooling-coiling. The specific process is as follows: the slab is sent into the slab heating furnace by hot charging, and the discharge temperature is 1261℃. The coarse rolling inlet temperature is 1192℃, the coarse rolling outlet temperature is 1163℃, the coarse rolling outlet speed is 3.2m / s, and the intermediate slab thickness after coarse rolling is 30mm. The intermediate slab after coarse rolling is transmitted by roller way, and is successively kept warm by heat preservation cover, and is coiled by hot coil box, and is sent into the fine rolling unit. The fine rolling inlet temperature is 1100℃, the fine rolling outlet temperature is 892℃, the fine rolling outlet speed is 10.5m / s, the interstand cooling water is not opened, 5 groups of lubrication rolling are opened, the steel plate after fine rolling is subjected to laminar cooling, the laminar cooling rate is 30℃ / s, the laminar cooling model is 2 groups of pipes opened 1 group, the water flow rate ratio of lower header to upper header is 1.32, and the coiling temperature is 615℃.

[0065] The steel of Example 3 has a thickness of 1.2 mm, a tensile strength of 595 MPa, an elongation of 46%, and a yield ratio of 0.77.

[0066] Comparative Example 1

[0067] A steel slab with a thickness of 200 mm and a length of 11 m was prepared by a converter-LF electric heating-RH vacuum refining-continuous casting process, and had the following mass percentages: 0.07% C, 0.18% Si, 0.75% Mn, 0.013% P, 0.002% S, 0.036% Ti, 0.0034% N, and the balance of Fe and inevitable impurities. The steel slab was prepared into a 1.2 mm thick steel plate by a heating-coarse rolling-hot coil box-finish rolling-laminar cooling-coiling process. The specific process was as follows: the slab was fed into the slab heating furnace at a cold slab, and the discharge temperature was 1231℃. The coarse rolling inlet temperature was 1163℃, the coarse rolling outlet temperature was 1135℃, the coarse rolling outlet speed was 3.2 m / s, and the intermediate slab thickness after coarse rolling was 30 mm. The intermediate slab after coarse rolling was transmitted by a roller way and fed into the finish rolling unit. The finish rolling used a conventional rolling process, the finish rolling inlet temperature was 1100℃, the finish rolling outlet temperature was 858℃, the finish rolling outlet speed was 10.5 m / s, the inter-stand cooling water was not turned on, 5 groups of lubrication rolling were turned on, the steel plate after finish rolling was subjected to laminar cooling, and the coiling temperature was 595℃.

[0068] The steel of Comparative Example 1 has a thickness of 1.2 mm, a poor plate shape, a tensile strength of 625 MPa, an elongation of 38%, and a yield ratio of 0.81.

[0069] Therefore, by using the production process of the present application, the rolling difficulty and the material forming performance problem of the ultra-thin gauge automobile structure steel can be solved by using a conventional hot continuous rolling unit, a new idea is provided for the preparation of the ultra-thin gauge automobile structure steel, and good operability and generalizability are possessed.

Claims

1. A rolling method for 1.2mm ultra-thin hot-rolled steel strip for automotive structures, characterized in that: The chemical composition of the steel billet for rolling, by weight percentage, includes: C: ≤0.10%, Si: 0.05~0.25%, Mn: 0.30~1.00%, P: ≤0.015%, S≤0.005%, Ti: 0.015%~0.045%, N≤0.005%, with the remainder being Fe and unavoidable impurity elements. The rolling process includes the following steps: Step 1: Control the thickness of the steel billet to be 200~250mm and the length of the billet to be 8.5m~9.5m; Step 2: The steel billet is loaded into the slab heating furnace using a hot charging method, and the slab exit temperature is 1220~1280℃. Step 3: Rough rolling of the steel billet is carried out using two sets of rolling mills. The first set of rolling mills performs one pass rolling, and the second set of rolling mills performs three passes rolling. The inlet temperature of the rough rolling mill is 1160~1200℃, the outlet temperature of the rough rolling mill is 1120~1160℃, the outlet speed of the rough rolling mill is 2~5m / s, and the thickness of the intermediate billet after rough rolling is controlled to be 30~35mm. Step 4: The intermediate billet obtained in Step 3 is finished rolled. A heat insulation cover and a hot coil box are installed on the transfer roller table between Step 3 and Step 4. The intermediate billet passes through the heat insulation cover and the hot coil box sequentially before entering the finishing mill. The finishing mill inlet temperature is 1060~1120℃, the finishing mill outlet temperature is 860~900℃, and the finishing mill outlet speed is 10.0~11.5m / s. The thickness of the finished strip is controlled to be 1.2mm. During the finishing rolling process, the number of cooling water groups between stands opened does not exceed one group, and the number of lubrication rolling groups opened is not less than three groups. The finishing mill includes 7 sets of finishing rolls, F1~F7, and 6 sets of looper rolls, L1~L6. The finishing rolls and looper rolls are controlled according to the following requirements: Finishing roll crown: F1: 30~40μm, F2: 15~20μm, F3: 10~15μm, F4: 5~10μm, F5: 4~8μm, F6: 4~6μm, F7: 4~6μm; Finishing roll gap: F1: 15mm, F2: 7mm, F3: 3mm, F4~F7: 2mm; Finishing roll speeds: F1: 0.8~1.1m / s, F2: 1.6~2.2m / s, F3: 2.5~4.0m / s, F4: 4.5~6.2m / s, F5: 6.6~8.6m / s, F6: 8.4~10.1m / s, F7: 10.0~11.5m / s; Looping roller tension: L1: 700~800 N / mm 2 L2: 800~900 N / mm 2 L3: 900~1000 N / mm 2 L4: 1100~1200 N / mm 2 L5: 1300~1400 N / mm 2 L6: 1600~1800 N / mm 2 ; Bending force of finishing roll: F1: 200 N / mm 2 F2~F4: 180N / mm 2 F5~F6: 160N / mm 2 F7: 120 N / mm 2 ; Finishing roll shifting amount: F1: -50~-150mm, F2: -20~-70mm, F3: -10~-60mm, F4: -5~-40mm, F5: -100~-150mm, F6: -60~-130mm, F7: -30~-100mm; Step 5: Perform laminar flow cooling on the finished steel strip. The laminar flow cooling rate is 10~30℃ / s, and the cooling flow ratio between the lower manifold and the upper manifold is 1.2~1.

4. The laminar flow cooling model adopts sparse cooling. After the steel strip is cooled to 600~650℃, it is coiled.