A production method for 270MPa grade hot-dip galvanized deep-drawing steel strip

Abstract

This invention provides a method for producing 270MPa grade hot-dip galvanized deep-drawing steel strip. The method controls the chemical composition, the hot rolling process (including soaking temperature, final rolling temperature, and coiling temperature), and the hot-dip galvanizing process (including annealing temperature, galvanizing temperature, finishing, and tension leveling). This results in hot-dip galvanized steel strip with excellent mechanical properties (Rp0.2 = 185–255 MPa, Rm ≥ 270 MPa, A50mm ≥ 42%, n90 ≥ 0.17). It effectively overcomes the problems of mixed crystals and performance deterioration in the pickling-hot-dip galvanizing technology during annealing due to lack of deformation recovery, recrystallization, and grain growth. The product's microstructure is 100% ferrite with a ferrite grain size of 8.0–10.5. Furthermore, the pickling-direct hot-dip galvanizing process used in this invention omits the conventional cold rolling step, reducing process costs.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metallurgy, and particularly relates to a production method of 270MPa-grade hot-based galvanizing deep-drawing steel strip. BACKGROUND

[0002] The automobile industry in China has entered a high-speed development period of passenger cars. Ti micro-alloyed interstitial-atom-free hot-dip galvanized deep-drawing steel plate developed on the basis of ultra-low carbon steel has very good stamping forming performance and corrosion resistance, and is usually used to make complex parts inside the automobile. Due to good mechanical properties and process performance and high corrosion resistance, it has become the preferred material for passenger car steel and has been widely used.

[0003] Recently, major domestic and foreign automobile manufacturers have proposed the use requirement of >2.5mm thick high-strength plated sheet for automobiles; the limit thickness of the conventional production line pickling- hot-dip galvanizing process is usually ≤2.5mm, which cannot develop the product. The plated sheet developed by the hot-based plating plate production technology, i.e. hot rolling + pickling + hot-dip galvanizing process technology, has the advantages of thick gauge, thick plating layer and low cost due to the reduction of the cold rolling process, and has good market application prospect.

[0004] The main automobile manufacturers in China have proposed the use requirement of thick-gauge hot-dip galvanized steel sheet with excellent forming performance of yield strength 270MPa according to the design requirements of new vehicle models. The main technical indexes are: R p0.2 185-255MPa, Rm≥270MPa, A 50mm ≥42%, n 90 ≥0.17. The 270MPa-grade thick-gauge hot-dip galvanized steel sheet with excellent forming performance is used for processing internal structural parts of passenger cars. The main technical requirements of the steel sheet are: sufficient strength and plasticity; good forming performance; low mechanical property anisotropy; and good corrosion resistance. The mechanical properties of the 270MPa-grade thick-gauge hot-dip galvanized steel sheet with excellent forming performance meet the requirements of Table 1.

[0005] Table 1: Mechanical properties

[0006]

[0007] The hot-based galvanizing production process is quite different from the traditional production process, especially the hot-rolled pickling raw material has obvious difference with the cold-rolled raw material used in the traditional pickling-rolling-hot galvanizing process in terms of deformation stored energy and surface roughness. The deformation stored energy of the cold-rolled strip steel after large plastic deformation has significant influence on the recrystallization annealing organization control in the continuous hot galvanizing production process, and is directly related to the mechanical property level and uniformity of the finished product. The hot-rolled pickling raw material lacks cold working deformation stored energy, and the annealing process of the pickling-rolling-hot galvanizing process has no thermal history process such as deformation organization recovery, recrystallization and grain growth, so the organization and texture state after the annealing process of the conventional process cannot be obtained. Under normal conditions, the finished product obtained by the conventional process has low plasticity and poor forming performance. Therefore, how to develop a hot-based galvanizing deep drawing steel plate full-process production process to replace the cold rolling process to obtain a hot galvanizing product with mechanical property level comparable to the cold-rolled-annealed product has become a problem to be solved. SUMMARY

[0008] Therefore, the present application provides a 270MPa hot-based galvanizing deep drawing steel strip and a production method thereof. The production method of the present application can effectively improve the mechanical properties of the hot-based galvanizing deep drawing steel strip, has excellent deep drawing performance and good performance anisotropy, overcomes the problem that the pickling-hot galvanizing technology is prone to mixed crystal and performance deterioration in the annealing process without deformation organization recovery, recrystallization and grain growth, and improves the plasticity index.

[0009] The present application provides a production method of a 270MPa hot-based galvanizing deep drawing steel strip, comprising the following steps:

[0010] A) smelting molten steel:

[0011] The blast furnace molten iron and smelting furnace materials are smelted in a converter to obtain molten steel; then, the molten steel is pre-deoxidized;

[0012] B) LF refining:

[0013] The molten steel obtained in step A) is subjected to LF refining treatment;

[0014] C) RH treatment:

[0015] The molten steel obtained in step B) is sent to an RH refining furnace for final deoxidization and alloying treatment to obtain molten steel;

[0016] The molten steel obtained in step C) comprises, by mass percentage:

[0017] C: ≤0.0035%;

[0018] Si: ≤0.020%;

[0019] Mn: 0.10% to 0.20%;

[0020] Ti: 0.045% ~ 0.065%;

[0021] N: 0.002% ~ 0.005%;

[0022] P: 0% ~ 0.010%;

[0023] S: 0% ~ 0.010%;

[0024] Als: 0.020% ~ 0.050%;

[0025] Fe and inevitable impurity elements: balance;

[0026] D) continuous casting:

[0027] the molten steel obtained in step C) is subjected to continuous casting to obtain a slab;

[0028] E) hot rolling:

[0029] the slab obtained in step D) is subjected to hot rolling treatment;

[0030] the hot rolling treatment comprises reheating soaking treatment, rough rolling, finish rolling, cooling and coiling;

[0031] wherein,

[0032] the reheating soaking temperature of the hot rolling is 1190 ~ 1240℃, and the time is 30 ~ 45min;

[0033] the temperature of the rough rolling is 1140 ~ 1200℃;

[0034] the finish rolling temperature is 900 ~ 940℃;

[0035] the cooling is laminar cooling; wherein, the length of the laminar cooling is 10 ~ 20m, and the cooling speed is 10 ~ 20℃ / min;

[0036] the coiling temperature is 660 ~ 690℃;

[0037] F) pickling:

[0038] the steel strip obtained in step E) is subjected to pickling;

[0039] G) hot galvanizing:

[0040] the steel strip after pickling in step F) is subjected to preheating, recrystallization annealing, cooling and hot galvanizing to obtain a 270MPa grade hot-based galvanized deep drawing steel strip;

[0041] wherein,

[0042] the recrystallization annealing temperature is 860 ~ 890℃;

[0043] The temperature of the hot-dip galvanizing is 455-465℃.

[0044] Preferably, in step G), the temperature of the preheating is 550-700℃.

[0045] The time of the recrystallization annealing is 4-8min.

[0046] Preferably, in step G), the cooling comprises slow cooling and fast cooling in sequence.

[0047] wherein,

[0048] The slow cooling is cooling to 570-580℃ at a rate of 5-9℃ / min.

[0049] The fast cooling is cooling to 460-470℃ at a rate of 15-30℃ / min.

[0050] Preferably, in step G), after the hot-dip galvanizing, post-cooling is further performed.

[0051] The speed of the post-cooling is 35-50℃ / min, and the target is cooling to 260℃.

[0052] Preferably, in step G), after the post-cooling, on-line smoothing, tension leveling, on-line inspection, and roll division are further performed.

[0053] wherein, the elongation of the smoothing is preferably 1.00%-1.40%, and the elongation of the tension leveling is preferably 0%-0.6%.

[0054] Preferably, in step F), the tension leveling elongation of the pickling is 0.6%-1.0%.

[0055] Preferably, in step E), the cooling rates of the upper and lower headers of the laminar cooling are preferably 75% and 50% respectively.

[0056] Preferably, in step C), the time of the RH treatment is 20-35min, and the out-station temperature is 1590-1610℃.

[0057] Preferably, in step B), the time of the LF refining is 10-25min, and the out-station temperature is 1630-1650℃.

[0058] Preferably, in step A), the temperature of the smelting is 1660-1675℃.

[0059] The production method provided by the application controls the chemical composition, the heat soaking temperature, the finish rolling temperature, the coiling temperature and other conditions in the hot rolling process, and controls the annealing temperature, the galvanizing temperature, the finishing and the tension leveling and other conditions in the hot galvanizing process, so that the obtained hot base galvanizing steel strip has excellent mechanical properties (Rp0.2 is 185-255 MPa, Rm≥270 MPa, A50mm≥42%, n90≥0.17); the problem that the finished product mixed crystal and the performance deterioration easily occur in the annealing process of the pickling-hot galvanizing technology are effectively overcome, the product microstructure is 100% ferrite, and the ferrite grain size is 8.0-10.5 levels. The annealing grain non-uniform growth is hindered by the refined hot rolling state microstructure, the TiC and Ti(C,N) particles, the finished product ferrite grain size is refined, and the finished product plasticity index is improved. Moreover, the pickling direct hot galvanizing process technology adopted by the application omits the conventional cold rolling step, reduces the process cost, the finished product has good physical performance, effectively adapts to the use demand of the downstream user, and has good popularization value and popularization prospect. BRIEF DESCRIPTION OF DRAWINGS

[0060] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only belong to the embodiments of the application, and the drawings provided by the application can also be obtained by the ordinary skilled in the art without any creative effort.

[0061] Figure 1 The microstructure diagram of the product obtained in Example 1;

[0062] Figure 2 The microstructure diagram of the product obtained in Example 2;

[0063] Figure 3 The microstructure diagram of the product obtained in Comparative Example 1. DETAILED DESCRIPTION

[0064] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0065] In the technical features described in an open manner herein, both the closed technical solution consisting of the listed features and the open technical solution containing the listed features are included.

[0066] The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0067] In this text, when referring to a numerical interval, unless otherwise specified, the numerical interval is considered to be continuous and includes the minimum and maximum values of the range and every value between such minimum and maximum values. Further, when the range refers to integers, every integer between the minimum and maximum values of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein are to be understood as including any and all sub-ranges subsumed therein.

[0068] In this text, when referring to a numerical interval, unless otherwise specified, the numerical interval is considered to be continuous and includes the minimum and maximum values of the range and every value between such minimum and maximum values. Further, when the range refers to integers, every integer between the minimum and maximum values of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein are to be understood as including any and all sub-ranges subsumed therein.

[0069] The present application provides a production method of a 270MPa grade hot-dip galvanizing deep drawing steel strip, comprising the following steps:

[0070] A) Smelting molten steel:

[0071] Smelting the blast furnace molten iron and smelting furnace materials in a converter to obtain molten steel; then, pre-deoxidizing the molten steel;

[0072] B) LF refining:

[0073] LF refining treatment is performed on the molten steel obtained in step A);

[0074] C) RH treatment:

[0075] The molten steel obtained in step B) is sent to an RH refining furnace for final deoxidation and alloying treatment to obtain molten steel;

[0076] The molten steel obtained in step C) comprises, in mass percentage:

[0077] C: ≤0.0035%;

[0078] Si: ≤0.020%;

[0079] Mn: 0.10% to 0.20%;

[0080] Ti: 0.045% to 0.065%;

[0081] N: 0.002% to 0.005%;

[0082] P: 0% to 0.010%;

[0083] S: 0% to 0.010%;

[0084] Als: 0.020% to 0.050%;

[0085] Fe and inevitable impurity elements: balance;

[0086] D) continuous casting:

[0087] The molten steel obtained in step C) is subjected to continuous casting to obtain a slab;

[0088] E) hot rolling:

[0089] The slab obtained in step D) is subjected to hot rolling treatment;

[0090] The hot rolling treatment comprises reheating soaking treatment, rough rolling, finish rolling, cooling and coiling;

[0091] wherein,

[0092] The reheating soaking temperature of the hot rolling is 1190-1240℃, and the time is 30-45min;

[0093] The temperature of the rough rolling is 1140-1200℃, and the temperature of the finish rolling is 900-940℃;

[0094] The cooling is laminar cooling; wherein the length of the laminar cooling is 10-20m, and the cooling speed is 10-20℃ / min;

[0095] The temperature of the coiling is 660-690℃;

[0096] F) pickling:

[0097] The steel strip obtained in step E) is subjected to pickling;

[0098] G) hot galvanizing:

[0099] The steel strip after pickling in step F) is subjected to preheating, recrystallization annealing, cooling and hot galvanizing to obtain a 270MPa grade hot-based galvanizing deep drawing steel strip;

[0100] wherein,

[0101] The temperature of the recrystallization annealing is 860-890℃;

[0102] The temperature of the hot galvanizing is 455-465℃.

[0103] The 270MPa hot-based galvanized steel plate relates to an ultra-low carbon steel + Ti micro-alloying scheme, an annealing temperature of 860-890 DEG C, good strength-plasticity matching and good stamping performance. The cold rolling process is omitted, and the problem of mixed crystal and performance deterioration in the annealing process of pickling-hot galvanizing technology is effectively overcome. The scheme adopts the refined hot rolling microstructure, the carbide precipitated at the grain boundary / grain to hinder the non-uniform growth of annealing grains, and the refined ferrite grain size after annealing, so that the plasticity index of the finished product is improved. The 270MPa hot-based galvanized steel plate of the application omits the cold rolling process in the manufacturing process, and the mechanical properties are equivalent to the stamping performance of the traditional pickling-hot galvanizing product. The product also has certain corrosion resistance, can effectively meet the needs of the automobile manufacturing industry, and has good popularization prospect.

[0104] [About step A]:

[0105] A) smelting molten steel: smelting the blast furnace molten iron and smelting furnace materials in a converter to obtain molten steel; then, pre-deoxidizing the molten steel.

[0106] In the application, the equipment for smelting molten steel is a top-blown converter, the blast furnace molten iron after vanadium extraction is used as raw material, and smelting furnace materials, i.e. steelmaking auxiliary materials, are added for smelting. The types of the auxiliary materials are not limited, and any commonly used auxiliary material in the art can be used, and the auxiliary materials can be dosed according to the chemical composition of the target molten steel. The smelting temperature is preferably 1660-1675 DEG C (i.e. the converter tapping temperature), and can be 1660 DEG C, 1665 DEG C, 1670 DEG C or 1675 DEG C. After smelting, molten steel is obtained.

[0107] After obtaining the molten steel, the molten steel is tapped into a molten steel ladle. The molten steel is pre-deoxidized during tapping. Specifically, at 1 / 3 of the tapping, aluminum-iron alloy is added to the molten steel ladle for pre-deoxidization, and the molten steel ladle is subjected to bottom argon blowing. Among them, 1 / 3 of the tapping refers to 1 / 3 of the amount of molten steel (the amount is measured by mass) tapped during the tapping of the converter, i.e. the ratio of the amount of molten steel tapped to the total amount of molten steel = 1 / 3. The argon pressure of the bottom argon blowing is preferably 200-400 Pa, and can be 200 Pa, 250 Pa, 300 Pa, 350 Pa or 400 Pa. The time of the bottom argon blowing is preferably 4-10 min, and can be 4 min, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min, and more preferably 4 min.

[0108] [About step B]:

[0109] B) LF refining: LF refining treatment is performed on the molten steel obtained in step A).

[0110] In the present application, in the LF refining, argon is introduced into the bottom of the molten steel ladle, the pressure of the argon is preferably 200-400 Pa, and specifically can be 200 Pa, 250 Pa, 300 Pa, 350 Pa or 400 Pa. The time for argon introduction is preferably 4-6 min, and specifically can be 4 min, 5 min or 6 min. The argon flow rate is under the condition that the molten steel is not stirred too much, so that secondary oxidation and too rapid temperature drop of the molten steel can be avoided, the inclusions in the steel are fully floated, and the cleanliness of the steel is further improved. The time for LF refining is preferably 10-25 min, and specifically can be 10 min, 15 min, 20 min or 25 min. The outlet temperature of the LF refining is preferably 1630-1650℃, and specifically can be 1630℃, 1635℃, 1640℃, 1645℃ or 1650℃.

[0111] [About step C]:

[0112] C) RH treatment: the molten steel obtained in step B) is sent to an RH refining furnace for final deoxidation and alloying treatment to obtain molten steel.

[0113] In the present application, the time for the RH treatment is preferably 20-35 min, and specifically can be 20 min, 25 min, 30 min or 35 min. In the treatment, aluminum-iron alloy, metallic manganese, titanium-iron alloy and the like are added for alloying, and the specific means are not particularly limited, and the corresponding auxiliary materials can be added according to the target product chemical composition according to the conventional alloying method in the art. The outlet temperature of the RH treatment is preferably 1590-1610℃, and specifically can be 1590℃, 1595℃, 1600℃, 1605℃ or 1610℃.

[0114] In the present application, in order to ensure the uniformity of the chemical composition of the steel, after the deoxidation and alloying of the RH treatment, the molten steel in the RH vacuum chamber is circulated for 5-8 min, and the vacuum degree is ≤2.0 mba. Then, the molten steel is subjected to bottom argon blowing treatment; the pressure of the bottom argon blowing is 200-400 Pa, and specifically can be 200 Pa, 250 Pa, 300 Pa, 350 Pa or 400 Pa. The time for the bottom argon blowing is preferably 4-6 min, and specifically can be 4 min, 5 min or 6 min. After the above treatment, the molten steel is obtained.

[0115] After the RH treatment, the composition of the obtained molten steel is as follows:

[0116] C: ≤0.0035%;

[0117] Si: ≤0.020%;

[0118] Mn: 0.10%-0.20%;

[0119] Ti: 0.045%-0.065%;

[0120] N: 0.002% to 0.005%;

[0121] P: 0% to 0.010%;

[0122] S: 0% to 0.010%;

[0123] Als: 0.020% to 0.050%;

[0124] Fe and inevitable impurity elements: balance.

[0125] In one embodiment of the present application, the molten steel has the following composition: C: 0.0028%, Si: 0.01%, Mn: 0.15%, Ti: 0.056%, N: 0.0034%, P: 0.008%, S: 0.003%, Als: 0.041%, and the balance being Fe and inevitable impurities. In another embodiment of the present application, the molten steel has the following composition: C: 0.0030%, Si: 0.01%, Mn: 0.14%, Ti: 0.058%, N: 0.0030%, P: 0.010%, S: 0.005%, Als: 0.028%, and the balance being Fe and inevitable impurities. Here, Als is a concept well known in the art, i.e., acid-soluble aluminum.

[0126] [Regarding Step D]:

[0127] D) Continuous casting: the molten steel obtained in Step C) is subjected to continuous casting to obtain a slab.

[0128] The continuous casting (continuous casting) is specifically casting the molten steel after refining into a pre-baked tundish, and casting into a slab by a full-flow-protected slab continuous casting machine. After casting, cooling is preferably also performed, for example, natural cooling at room temperature.

[0129] [Regarding Step E]:

[0130] E) Hot rolling: the slab obtained in Step D) is subjected to hot rolling.

[0131] The hot rolling step is to roll the cast slab after heating. The purpose of rolling is to make the continuous casting slab reach the required thickness. The rolling pass of the hot rolled steel strip of the present application has 6 rough rolling steps and 7 finishing rolling steps.

[0132] In the present application, the hot rolling treatment includes reheating soaking treatment. After the slab is obtained by continuous casting, the slab is sent into a heating furnace for heating, namely reheating soaking treatment. The temperature of the reheating soaking refers to the slab out-of-furnace rolling temperature, at which the soaking is performed, so as to fully solid-solution micro-alloying elements, eliminate chemical element segregation caused by dendrite bias of the casting slab, and meanwhile avoid liquid precipitation carbide to reduce the effect of micro-alloying elements in the steel. In the present application, the temperature of the reheating soaking treatment is 1190-1240℃, and specifically can be 1190℃, 1200℃, 1208℃, 1210℃, 1220℃, 1230℃, 1240℃. The time of the reheating soaking treatment is 30-45min, and specifically can be 30min, 35min, 40min, 45min.

[0133] In the present application, after the above-mentioned reheating soaking treatment, rough rolling is performed. The temperature of the rough rolling is 1140-1200℃, and specifically can be 1140℃, 1145℃, 1150℃, 1155℃, 1160℃, 1165℃, 1170℃, 1175℃, 1180℃, 1185℃, 1190℃, 1195℃, 1200℃.

[0134] In the present application, after the above-mentioned rough rolling, finish rolling is performed. The rough rolling temperature is 1010-1040℃, and specifically can be 1010℃, 1015℃, 1020℃, 1025℃, 1030℃, 1035℃, 1040℃. The finish rolling temperature is 900-940℃, and specifically can be 900℃, 905℃, 910℃, 915℃, 920℃, 925℃, 930℃, 935℃, 940℃. The above-mentioned temperature control in the hot rolling process can make the steel in the finish rolling stage be in complete austenite organization, and avoid the austenite organization being too coarse.

[0135] In the present application, after the finish rolling, cooling is performed. The cooling is laminar flow cooling. The length of the laminar flow cooling is 10-20 m, and can be specifically 10 m, 15 m or 20 m. The cooling rates of the upper and lower headers of the laminar flow cooling are preferably 75% and 50%, respectively. The cooling rate is expressed by the proportion of the water flow, and in actual production, the cooling rate is adjusted by opening and closing the cooling header nozzles. The percentage expression method is well known in the industry, for example, the full opening mode is 100%, that is, the water flow is 100% directly sprayed on the surface of the strip. Through the above control, the cooling rate is 10-20 ℃ / min, and can be specifically 10 ℃ / min, 15 ℃ / min or 20 ℃ / min. After the thin plate strip rolled by hot rolling is cooled, the internal organization state of the steel is adjusted, and then the thin plate strip is coiled. In order to meet the good strength and plasticity of the finished hot-dip galvanized steel plate, the present application controls the ferrite grain size in the microstructure of the hot-rolled strip to be about 10.5 grade, and the precipitation of AlN, TiC and Ti(C, N) inhibits the non-uniform growth of ferrite in the high temperature zone, and the hot-rolled substrate organization is refined.

[0136] In the present application, the material is cooled to 660-690 ℃ by the above cooling for coiling. The temperature can be specifically 660 ℃, 665 ℃, 670 ℃, 675 ℃, 680 ℃, 685 ℃ or 690 ℃.

[0137] [About step F]:

[0138] F) Pickling: the steel strip obtained in step E) is pickled.

[0139] The pickling process of the present application is not particularly limited, and can be performed according to the conventional process procedure; generally, the thin plate strip rolled by hot rolling is welded at the head of the pickling mill group to form a continuous steel strip, and then is subjected to straightening, tension leveling and phosphorus breaking, pickling, alkali washing and drying. In the present application, the tension leveling elongation of the pickling is preferably 0.6%-1.0%, and can be specifically 0.6%, 0.7%, 0.8%, 0.9% or 1.0%. After the steel plate is pickled, the surface quality is improved.

[0140] [About step G]:

[0141] G) Hot-dip galvanizing: the steel strip after pickling in step F) is preheated, recrystallized annealed, cooled and hot-dip galvanized to obtain a 270 MPa grade hot-dip galvanized deep drawing steel strip.

[0142] In the present application, after the pickling in step F), the steel strip is sent to the hot-dip galvanizing unit for the hot-dip galvanizing process. Specifically, the thin plate strip after pickling is welded at the inlet of the hot-dip galvanizing unit to form a continuous steel strip, and then is subjected to a series of treatments including preheating, recrystallization annealing, cooling and hot-dip galvanizing after being continuously cleaned and dried.

[0143] In the present application, the preheating temperature is preferably 550-700 DEG C, and can be 550 DEG C, 560 DEG C, 570 DEG C, 580 DEG C, 590 DEG C, 600 DEG C, 610 DEG C, 620 DEG C, 630 DEG C, 640 DEG C, 650 DEG C, 660 DEG C, 670 DEG C, 680 DEG C, 690 DEG C, 700 DEG C.

[0144] In the present application, after preheating, recrystallization annealing is performed. The recrystallization annealing temperature is 860-890 DEG C, and can be 860 DEG C, 865 DEG C, 870 DEG C, 875 DEG C, 880 DEG C, 885 DEG C, 890 DEG C. The recrystallization annealing time is preferably 4-8 min, and can be 4 min, 5 min, 6 min, 7 min, 8 min.

[0145] In the present application, after recrystallization annealing, cooling is performed. In the present application, the cooling includes sequentially performing slow cooling and fast cooling. The slow cooling speed is preferably 5-9 DEG C / min, and can be 5 DEG C / min, 6 DEG C / min, 7 DEG C / min, 8 DEG C / min, 9 DEG C / min; the slow cooling target is preferably cooling to 570-580 DEG C, and can be 570 DEG C, 571 DEG C, 572 DEG C, 573 DEG C, 574 DEG C, 575 DEG C, 576 DEG C, 577 DEG C, 578 DEG C, 579 DEG C, 580 DEG C. The fast cooling speed is preferably 15-30 DEG C / min, and can be 15 DEG C / min, 20 DEG C / min, 25 DEG C / min, 30 DEG C / min. The fast cooling target is preferably cooling to 460-470 DEG C, and can be 460 DEG C, 461 DEG C, 462 DEG C, 463 DEG C, 464 DEG C, 465 DEG C, 466 DEG C, 467 DEG C, 468 DEG C, 469 DEG C, 470 DEG C.

[0146] In the present application, after cooling, hot galvanizing is performed. The hot galvanizing temperature is preferably 455-465 DEG C, and can be 455 DEG C, 456 DEG C, 457 DEG C, 458 DEG C, 459 DEG C, 460 DEG C, 461 DEG C, 462 DEG C, 463 DEG C, 464 DEG C, 465 DEG C.

[0147] In the present application, after hot galvanizing, post-cooling is preferably performed. The post-cooling speed is preferably 35-50 DEG C / min, and can be 35 DEG C / min, 40 DEG C / min, 45 DEG C / min, 50 DEG C / min. The post-cooling target is preferably cooling to 260 DEG C. After the above post-cooling to 260 DEG C, final cooling is performed; the final cooling is preferably cooling to ≤90 DEG C, and after cooling to the above temperature, a subsequent finishing unit is entered.

[0148] In the present application, after the above cooling, preferably, the following is further carried out: on-line sizing, tension leveling, on-line inspection, and volume division. The elongation of the sizing is preferably 1.00% to 1.40%, and specifically can be 1.00%, 1.10%, 1.20%, 1.30%, or 1.40%. The elongation of the tension leveling is preferably 0% to 0.6%, and specifically can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or 0.6%. After the above treatment, a 270MPa hot-dip galvanized deep drawing steel strip product is obtained.

[0149] The present application also provides a 270MPa hot-dip galvanized deep drawing steel strip product prepared by the preparation method described in the above technical solution.

[0150] The 270MPa hot-dip galvanized deep drawing steel strip prepared by the present application has a microstructure of 100% ferrite, and a ferrite grain size of 8.0 to 10.5. Ferrite refers to an interstitial solid solution formed by dissolving C in alpha-Fe, and has a body-centered cubic crystal structure. The ferrite can be measured by a method known to those skilled in the art, such as GB / T 6394 metallographic method.

[0151] The 270MPa hot-dip galvanized steel plate according to the present application adopts an ultra-low carbon steel + Ti micro-alloying scheme (C combines with Ti to form TiC, and N combines with Ti to form TiN), and adopts key technical measures such as external refining, controlled rolling and controlled cooling, recrystallization annealing, hot-dip galvanizing, and sizing process in the production process. The obtained product has stable mechanical properties, good matching of strength and plasticity, excellent deep drawing performance, good performance anisotropy, and good corrosion resistance of the plated layer. The finished product inspection results show that the 270MPa hot-dip galvanized steel plate produced by the method has good mechanical properties and process performance. Since the steel plate according to the present application has stable mechanical properties, small anisotropy, excellent deep drawing performance, and good corrosion resistance of the plated layer, it can effectively adapt to different stamping processes (bending, stamping, flanging, etc.) of users. The hot-dip galvanized steel plate produced by the present application has excellent comprehensive performance, high cost performance, and good prospects for popularization and use.

[0152] The production method provided by the application controls the chemical composition, the soaking temperature, the finishing temperature, the coiling temperature and other conditions in the hot rolling process, and controls the annealing temperature, the galvanizing temperature, the finishing and the tension leveling and other conditions in the hot galvanizing process, so that the obtained hot base galvanizing steel strip has excellent mechanical properties (Rp0.2 is 185-255 MPa, Rm is greater than or equal to 270 MPa, A50mm is greater than or equal to 42%, and n90 is greater than or equal to 0.17); the problem that mixed crystals and performance deterioration of the finished product are prone to occur in the annealing process of the pickling-hot galvanizing technology is effectively overcome. The annealing grain non-uniform growth is hindered by the refined hot rolling state microstructure, TiC and Ti(C,N) particles, the finished product ferrite grain size is refined, and the finished product plasticity index is improved. Moreover, the pickling direct hot galvanizing process technology adopted by the application omits the conventional cold rolling step, reduces the process cost, the finished product has good physical performance, effectively meets the use requirements of downstream users, and has good popularization value and popularization prospect.

[0153] The detection method of the chemical composition in the application is a spark source atomic emission spectrometry analysis method for carbon steel and medium and low alloy steel, and the national standard is GB / T4336; the determination of low carbon content of non-alloy steel part 2: infrared absorption method after combustion in an induction furnace (after preheating), and the national standard is GB / T 20126-2006.

[0154] In order to further understand the application, the preferred embodiments of the application are described below in combination with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the application, and are not a limitation on the claims of the application.

[0155] Example 1

[0156] A) Smelting molten steel:

[0157] The smelting equipment is a top-blown converter, and the semi-steel after vanadium extraction of the blast furnace molten iron is used as the raw material, the steelmaking auxiliary materials are added for smelting to 1670 DEG C, and the molten steel is discharged to the molten steel ladle, 200 Kg of aluminum-iron alloy is added at 1 / 3 of the molten steel discharge for pre-deoxidization and alloying, and then the molten steel ladle is subjected to bottom argon blowing treatment on the small platform outside the furnace, the argon pressure is 300 Pa, and the time is 4 minutes.

[0158] B) LF refining:

[0159] The argon gas with a certain pressure (350 Pa) is introduced into the bottom of the molten steel ladle for 4 minutes, the argon flow is not too large, the LF refining time is 20 min, and the outlet temperature is 1635 DEG C.

[0160] C) RH treatment:

[0161] The molten steel ladle is transported to the RH refining furnace for final deoxidation, alloying treatment, and composition fine adjustment. The RH treatment time is 30 min, and the outlet temperature is 1595°C. The composition of the molten steel is: C: 0.0028%, Si: 0.01%, Mn: 0.15%, Ti: 0.056%, N: 0.0034%, P: 0.008%, S: 0.003%, Als: 0.041%, and the rest is Fe and inevitable impurity elements.

[0162] D) Continuous casting:

[0163] The molten steel ladle is transported to the casting position, the bottom of the molten steel ladle is slid, the Al quality stopper is automatically flowed into the tundish, and the continuous casting is carried out through the Al quality stopper and the flow guide to the crystallizer. The whole process adopts protective slag for protective casting, and the hot-rolled plate steel billet is obtained after cooling.

[0164] E) Hot rolling:

[0165] The continuous casting plate billet is sent into the heating furnace for reheating and soaking treatment, the soaking temperature is 1208°C, and the soaking time is 40 min; then, rough rolling is carried out, the rough rolling temperature is 1157°C; then, finish rolling is carried out, the finish rolling opening temperature is 1025°C, and the finish rolling temperature is 930°C; the rolling pass rough rolling step is 6, and the finish rolling step is 7.

[0166] After the above finish rolling, laminar cooling is carried out, the upper and lower header cooling rates are 75% and 50% respectively, and the cooling rate is 15°C / min. Cooling to the coiling temperature of 670°C for coiling.

[0167] The thickness of the steel strip obtained after the hot rolling process is 3.10 mm.

[0168] F) Pickling:

[0169] The steel strip obtained in step E) is subjected to online pickling, wherein the pickling straightening elongation is 0.9%.

[0170] G) Hot galvanizing:

[0171] The steel strip after pickling in step F) is welded into a continuous steel strip at the inlet of the hot galvanizing unit, and after surface continuous cleaning and drying, it is subjected to the following series of treatments in the continuous hot galvanizing unit:

[0172] Preheating: temperature 600°C;

[0173] Recrystallization annealing: temperature 885°C, time 6 min;

[0174] Cooling: slow cooling first and then fast cooling; slow cooling: cooling to 575°C at a rate of 6°C / min; fast cooling: cooling to 465°C at a rate of 20°C / min;

[0175] Hot galvanizing: temperature 457°C;

[0176] Post-cooling: cooling to 260°C at 40°C / min; then, final cooling to 90°C;

[0177] Post-processing: online skin pass, tension leveling, online inspection, and roll division; wherein the skin pass elongation is 1.20%, and the tension leveling elongation is 0.1%.

[0178] Test:

[0179] The obtained plate roll product was subjected to mechanical property testing, and the yield strength Rp0.2, tensile strength Rm, elongation A 50mm , and work hardening index n 10-20 at room temperature were detected, respectively. The tensile property was tested according to GB / T228 metal material room temperature tensile test method. The results showed that the product yield strength (Rp0.2) was 192 MPa, the tensile strength (Rm) was 304 MPa, the elongation (A 50mm ) was 46%, and n 90 was 0.21, which met the technical requirements of 270 MPa grade hot-dip galvanized steel plate. Figure 1 The finished product microstructure of Example 1 was shown, wherein the ferrite grain size was 10.0 grade.

[0180] Example 2

[0181] According to Example 1, except that:

[0182] In step C), the composition of the molten steel was: C: 0.0030%, Si: 0.01%, Mn: 0.14%, Ti: 0.058%, N: 0.0030%, P: 0.010%, S: 0.005%, Als: 0.028%, and the rest was Fe and inevitable impurity elements.

[0183] In step E), the conditions were as follows: soaking temperature was 1204°C, finish rolling temperature was 910°C, and coiling temperature was 685°C.

[0184] In step F), the pickling tension leveling elongation was 1.0%.

[0185] In step G), the recrystallization annealing temperature was 870°C, and the hot-dip galvanizing temperature was 460°C; the skin pass elongation was 1.10%, and the tension leveling elongation was 0.2%.

[0186] Test:

[0187] The obtained plate roll product was subjected to mechanical property testing according to the test method of Example 1, and the results showed that the product yield strength (Rp0.2) was 205 MPa, the tensile strength (Rm) was 310 MPa, the elongation (A 50mm ) was 45%, and n 90The value is 0.20, which meets the technical requirements for 270MPa grade hot-dip galvanized steel sheets. Figure 2 The microstructure of the finished product of Example 2 is shown, wherein the ferrite grain size is 10.0 grade.

[0188] Comparative Example 1

[0189] Implemented according to Example 1, with the following differences:

[0190] In step C), the composition of the molten steel is: C: 0.0028%, Si: 0.08%, Mn: 0.25%, Ti: 0.031%, N: 0.0035%, P: 0.010%, S: 0.004%, Als: 0.040%, with the remainder being Fe and unavoidable impurity elements.

[0191] In step E), the conditions are as follows: the soaking temperature is 1225℃, the finishing rolling temperature is 925℃, and the coiling temperature is 720℃. The hot-rolled thickness is 3.0 mm.

[0192] In step F), the elongation rate of acid washing and stretching is 1.4%.

[0193] In step G), the recrystallization annealing temperature is 760℃, the hot-dip galvanizing temperature is 455℃, the final cooling temperature is 93℃, the finishing elongation is 0.8%, and the tensile elongation is 0.3%.

[0194] test:

[0195] The mechanical properties of the obtained sheet coil product were tested according to the test method of Example 1. The results showed that the product elongation (A) was... 50mm The percentage was 36%, which does not meet the technical requirements for 270MPa grade hot-dip galvanized steel sheets. Figure 3 The microstructure of the finished product of Comparative Example 1 is shown, in which the ferrite grain size is 11.0 grade.

[0196] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of these embodiments are merely to aid in understanding the method and core ideas of the present invention, including the best mode, and to enable any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that those skilled in the art can make various improvements and modifications to the present invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims. The scope of protection of this patent is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to those expressed in the claims, or if they include equivalent structural elements that are not substantially different from those expressed in the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A method of producing a 270 MPa grade hot dip galvanised deep drawing steel strip, characterised in that, The method comprises the following steps: A) smelting molten steel: Smelting the blast furnace molten iron and smelting furnace materials in a converter to obtain molten steel; then, pre-deoxidizing the molten steel; B) LF refining: Performing LF refining treatment on the molten steel obtained in step A); C) RH treatment: Sending the molten steel obtained in step B) to an RH refining furnace for final deoxidation and alloying treatment to obtain molten steel; The molten steel obtained in step C) comprises, by mass percentage: C：≤0.0035%； Si: ≤0.020%; Mn: 0.10%-0.20%; Ti: 0.045%-0.065%; N：0.002%~0.005%； P：0%~0.010%； S：0%~0.010%； Als: 0.020%-0.050%; Fe and inevitable impurity elements: balance; D) continuous casting: Performing continuous casting on the molten steel obtained in step C) to obtain a slab; E) hot rolling: Performing hot rolling treatment on the slab obtained in step D); The hot rolling treatment comprises reheating soaking treatment, rough rolling, finish rolling, cooling, and coiling; wherein, The reheating soaking temperature of the hot rolling is 1190-1240°C, and the time is 30-45 min; The temperature of the rough rolling is 1140-1200°C; The finish rolling temperature is 900-940°C; The cooling is laminar cooling; wherein, the length of the laminar cooling is 10-20 m, and the cooling speed is 10-20°C / min; The coiling temperature is 660-690°C; F) pickling: Pickling the steel strip obtained in step E); G) hot galvanizing: Preheating, recrystallization annealing, cooling, and hot galvanizing the steel strip after pickling in step F) to obtain a 270 MPa grade hot-based galvanized deep drawing steel strip; wherein, The recrystallization annealing temperature is 860-890°C; The hot galvanizing temperature is 455-465°C.

2. The production method according to claim 1, characterized by, In step G), the preheating temperature is 550-700°C; The recrystallization annealing time is 4-8 min.

3. The production method according to claim 1, characterized by, In step G), the cooling comprises sequentially performing slow cooling and fast cooling; wherein, The slow cooling is cooling at a rate of 5-9°C / min to 570-580°C; The fast cooling is cooling at a rate of 15-30°C / min to 460-470°C.

4. The production method according to claim 1, characterized by, In step G), after the hot galvanizing, post-galvanizing cooling is further performed; The post-galvanizing cooling speed is 35-50°C / min, and the target is to cool to 260°C.

5. The production method according to claim 4, characterized by, In step G), after the post-galvanizing cooling, online finishing, tension leveling, online inspection, and coiling are further performed; wherein, the elongation of the finishing is 1.00%-1.40%, and the elongation of the tension leveling is 0.1%-0.6%.

6. The production method according to claim 1, characterized by, In step F), the tension leveling elongation of the pickling is 0.6%-1.0%.

7. The production method according to claim 1, characterized by, In step C), the RH treatment time is 20-35 min, and the outbound temperature is 1590-1610°C.

8. The production method according to claim 1, characterized by, In step B), the LF refining time is 10-25 min, and the outbound temperature is 1630-1650°C.

9. The production method according to claim 1, characterized by, In step A), the smelting temperature is 1660-1675°C.

10. The production method according to claim 1, characterized by, ​

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