Production method of 270MPa-grade hot-based galvanized deep-drawing steel strip

Abstract

The invention provides a production method of a 270 MPa grade hot-based galvanized deep drawing steel strip. According to the production method provided by the invention, the chemical composition is controlled, the conditions of soaking temperature, finish rolling temperature, coiling temperature and the like are controlled in the hot rolling process, and the conditions of annealing temperature, galvanizing temperature, finishing, withdrawal and straightening and the like are controlled in the hot galvanizing process, so that the obtained hot-based galvanized 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 problems that in the acid pickling-hot galvanizing technology, in the annealing process, deformation-free structure recovery, recrystallization, grain growth and other processes are prone to occurring, finished product crystal mixing and performance deterioration are effectively solved, the microstructure of the product is 100% ferrite, and the ferrite grain size is 8.0-10.5 grade. In addition, the pickling direct hot galvanizing technology adopted in the invention omits the conventional cold rolling step, and reduces the process cost.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a production method of a 270MPa grade hot-dip galvanized deep-drawn steel strip. Background Art

[0002] my country's automobile industry has entered a period of rapid development of passenger cars. Ti micro-alloyed interstitial-free hot-dip galvanized deep-drawing steel sheets developed on the basis of ultra-low carbon steel have very good stamping forming properties and corrosion resistance. They are usually used to make complex internal parts of automobiles. Due to their good mechanical properties, process properties and high corrosion resistance, they have become the preferred material for passenger car steel and have been widely used.

[0003] Recently, major domestic and foreign automobile manufacturers have proposed the use of high-strength coated plates with a thickness of >2.5mm for automobiles; the limit thickness of the acid rolling-hot dip galvanizing process of conventional domestic production lines is usually ≤2.5mm, and it is impossible to develop this product. However, the coated plates developed by the hot-based coated plate production technology, i.e., hot rolling + pickling + hot dip galvanizing process technology, have the advantages of thick specifications, thick coatings, and low costs due to the reduction of cold rolling processes, and have good market promotion and application prospects.

[0004] According to the design requirements of new models, major domestic automobile manufacturers have put forward the demand for the use of thick hot-dip galvanized steel sheets with excellent formability and yield strength of 270MPa. The main technical indicators are: p0.2 185~255MPa, Rm≥270MPa, A 50mm ≥42%, n 90 ≥0.17. Thick hot-dip galvanized steel sheets with excellent formability at 270MPa are used to process the internal structural parts of passenger cars. The main technical requirements of steel sheets are: sufficient strength and plasticity; good formability; low anisotropy of mechanical properties; good corrosion resistance. The mechanical properties of thick hot-dip galvanized steel sheets with excellent formability at 270MPa meet the requirements of Table 1.

[0005] Table 1: Mechanical properties

[0006]

[0007] The hot-dip galvanizing production process is quite different from the traditional production process, especially the hot-rolled pickled raw materials are significantly different from the cold-rolled raw materials used in the traditional pickling-hot-dip galvanizing process in terms of deformation storage energy and surface roughness. The deformation storage energy generated by the cold-rolled strip after large plastic deformation has a significant impact on the control of the recrystallization annealing structure in the continuous hot-dip galvanizing production process, which is directly related to the mechanical properties and uniformity of the finished product. Due to the lack of cold processing deformation storage energy, the annealing process of the hot-rolled pickled raw materials using the direct galvanizing process after pickling has no thermal history processes such as deformation structure recovery, recrystallization, and grain growth, and the structure and texture state after conventional process annealing cannot be obtained. Under normal conditions, the finished products obtained by ordinary processes have low plasticity and poor forming performance. Therefore, how to develop a full-process production process for hot-rolled galvanized deep-drawing steel sheets that replaces the cold rolling link to obtain the mechanical properties of hot-dip galvanized products for deep drawing that are equivalent to those of cold-rolled-annealed products has become an urgent problem to be solved. Summary of the invention

[0008] In view of this, the present invention provides a 270MPa grade hot-dip galvanized deep-drawing steel strip and a production method thereof. The production method of the present invention can effectively improve the mechanical properties of the hot-dip galvanized deep-drawing steel strip, has excellent deep-drawing performance, and good performance anisotropy, overcomes the problem of mixed crystals in the finished product and deterioration of performance in the process of no deformation structure recovery, recrystallization, grain growth, etc. in the pickling-hot-dip galvanizing technology during the annealing process, and improves the plasticity index.

[0009] The present invention provides a method for producing a 270MPa grade hot-dip galvanized deep-drawn steel strip, comprising the following steps:

[0010] A) Molten steel:

[0011] The molten iron from the blast furnace and the smelting charge 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 deoxidation and alloying treatment to obtain molten steel;

[0016] In terms of mass percentage, the molten steel obtained in step C) comprises:

[0017] C: ≤0.0035%;

[0018] Si: ≤0.020%;

[0019] Mn: 0.10%~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] Continuously casting the molten steel obtained in step C) to obtain a slab;

[0028] E) Hot rolling:

[0029] hot rolling the slab obtained in step D);

[0030] The hot rolling process includes: reheating and soaking treatment, rough rolling, finishing rolling, cooling and coiling;

[0031] in,

[0032] The reheating soaking temperature of the hot rolling is 1190-1240°C and the time is 30-45 minutes;

[0033] The temperature of the rough rolling is 1140-1200°C;

[0034] The final rolling temperature of the finishing rolling is 900-940°C;

[0035] The cooling is laminar cooling; wherein the length of the laminar cooling is 10 to 20 m, and the cooling rate is 10 to 20 ° C / min;

[0036] The coiling temperature is 660-690°C;

[0037] F) Pickling:

[0038] Pickling the steel strip obtained in step E);

[0039] G) Hot dip galvanizing:

[0040] Preheating, recrystallization annealing, cooling and hot-dip galvanizing the steel strip after pickling in step F) to obtain a 270MPa grade hot-base galvanized deep-drawn steel strip;

[0041] in,

[0042] The temperature of the recrystallization annealing is 860-890°C;

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

[0044] Preferably, in step G), the preheating temperature is 550-700°C;

[0045] The recrystallization annealing time is 4 to 8 minutes.

[0046] Preferably, in step G), the cooling comprises sequentially performing slow cooling and rapid cooling;

[0047] in,

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

[0049] The rapid cooling is cooling to 460-470° C. at a rate of 15-30° C. / min.

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

[0051] The cooling rate after plating is 35-50°C / min, and the target is to cool to 260°C.

[0052] Preferably, in step G), after cooling after plating, the following steps are further performed: online skin-finishing, tensioning and straightening, online inspection, and coiling;

[0053] Among them, the elongation of skin straightening is preferably 1.00% to 1.40%, and the elongation of tension leveling is preferably 0% to 0.6%.

[0054] Preferably, in step F), the pickling tensile elongation is 0.6% to 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 RH treatment time is 20 to 35 minutes, and the exit temperature is 1590 to 1610°C.

[0057] Preferably, in step B), the LF refining time is 10 to 25 minutes, and the outlet temperature is 1630 to 1650°C.

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

[0059] The production method provided by the present invention controls the chemical composition, the hot rolling process controls the conditions such as the soaking temperature, the final rolling temperature, and the coiling temperature, and the hot-dip galvanizing process controls the conditions such as the annealing temperature, the galvanizing temperature, the skin pass and the straightening, so that the obtained hot-dip galvanized steel strip has excellent mechanical properties (Rp0.2 is 185-255MPa, Rm≥270MPa, A50mm≥42%, n90≥0.17); it effectively overcomes the problem that the pickling-hot-dip galvanizing technology is prone to mixed crystals and performance deterioration in the process of no deformation structure recovery, recrystallization, and grain growth during the annealing process. The microstructure of the product is 100% ferrite, and the ferrite grain size is 8.0-10.5. The refined hot-rolled microstructure, TiC and Ti (C, N) particles are used to hinder the non-uniform growth of annealed grains, refine the ferrite grain size of the finished product, and improve the plasticity index of the finished product. Moreover, the pickling direct hot-dip galvanizing process technology adopted by the present invention omits the conventional cold rolling step, reduces the process cost, and the finished product has good physical performance, effectively adapts to the use needs of downstream users, and has good promotion value and prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0061] Figure 1 This is a microstructure diagram of the product obtained in Example 1;

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

[0063] Figure 3 This is 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 those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0065] In this article, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

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

[0067] Herein, when it comes to numerical ranges, unless otherwise specified, the numerical ranges are deemed to be continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0068] In this article, when referring to the unit of a data range, if there is a unit only after the right endpoint, it means that the units of the left and right endpoints are the same. For example, 1660-1675℃ means that the units of the left endpoint "1660" and the right endpoint "1675" are both ℃.

[0069] The present invention provides a method for producing a 270MPa grade hot-dip galvanized deep-drawn steel strip, comprising the following steps:

[0070] A) Molten steel:

[0071] The molten iron from the blast furnace and the smelting charge are smelted in a converter to obtain molten steel; then, the molten steel is pre-deoxidized;

[0072] B) LF refining:

[0073] The molten steel obtained in step A) is subjected to LF refining treatment;

[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] In terms of mass percentage, the molten steel obtained in step C) comprises:

[0077] C: ≤0.0035%;

[0078] Si: ≤0.020%;

[0079] Mn: 0.10%~0.20%;

[0080] Ti: 0.045%~0.065%;

[0081] N: 0.002%~0.005%;

[0082] P: 0%~0.010%;

[0083] S: 0%~0.010%;

[0084] Als: 0.020%~0.050%;

[0085] Fe and inevitable impurity elements: balance;

[0086] D) Continuous Casting:

[0087] Continuously casting the molten steel obtained in step C) to obtain a slab;

[0088] E) Hot rolling:

[0089] hot rolling the slab obtained in step D);

[0090] The hot rolling process includes: reheating and soaking treatment, rough rolling, finishing rolling, cooling and coiling;

[0091] in,

[0092] The reheating soaking temperature of the hot rolling is 1190-1240°C and the time is 30-45 minutes;

[0093] The temperature of the rough rolling is 1140-1200°C; the temperature of the finish rolling is 900-940°C;

[0094] The cooling is laminar cooling; wherein the length of the laminar cooling is 10 to 20 m, and the cooling rate is 10 to 20 ° C / min;

[0095] The coiling temperature is 660-690°C;

[0096] F) Pickling:

[0097] Pickling the steel strip obtained in step E);

[0098] G) Hot dip galvanizing:

[0099] Preheating, recrystallization annealing, cooling and hot-dip galvanizing the steel strip after pickling in step F) to obtain a 270MPa grade hot-base galvanized deep-drawn steel strip;

[0100] in,

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

[0102] The temperature of the hot-dip galvanizing is 455-465°C.

[0103] The 270MPa grade hot-dip galvanized steel sheet involved in the present invention adopts an ultra-low carbon steel + Ti microalloying scheme, with an annealing temperature of 860-890°C, good strength-plasticity matching, and good stamping performance. The cold rolling process is omitted and the problem of mixed crystals and deteriorated performance of the finished product in the process of no deformation structure recovery, recrystallization, and grain growth in the pickling-hot-dip galvanizing technology during the annealing process is effectively overcome. This scheme uses refined hot-rolled microstructures and carbides precipitated at grain boundaries / intra-grains to hinder the non-uniform growth of annealed grains, refine the ferrite grain size after annealing, and improve the plasticity index of the finished product. The cold rolling process is omitted in the manufacturing process of the 270MPa grade hot-dip galvanized steel sheet of the present invention, and the mechanical properties are still comparable to the stamping properties of traditional acid-rolled-hot-dip galvanized products. It also has certain corrosion resistance, can effectively meet the needs of the automobile manufacturing industry, and has good promotion prospects.

[0104] [About Step A]:

[0105] A) Molten steel smelting: Molten iron from a blast furnace and smelting charge are smelted in a converter to obtain molten steel; then, the molten steel is pre-deoxidized.

[0106] In the present invention, the equipment for smelting molten steel is a top-blown converter, and the molten iron after vanadium extraction from the blast furnace molten iron is used as the raw material, and the smelting charge, i.e., the steelmaking auxiliary material, is added for smelting. There is no special restriction on the type of the auxiliary material, and it can be any auxiliary material commonly used in steelmaking in this field, and the ingredients can be prepared according to the chemical composition of the target molten steel. The smelting temperature is preferably 1660-1675°C (i.e., the converter tapping temperature), specifically 1660°C, 1665°C, 1670°C, 1675°C. After smelting, molten steel is obtained.

[0107] After obtaining the molten steel, the molten steel is tapped into a steel tank. The molten steel is pre-deoxidized during the tapping process. Specifically, when 1 / 3 of the steel is tapped, an aluminum-iron alloy is added to the steel tank for pre-deoxidation, and the steel tank is bottom-blown with argon. Among them, tapping 1 / 3 of the steel means that 1 / 3 of the molten steel (the amount is measured by mass) is poured out when the converter taps, that is, the ratio of the amount of molten steel poured out / the total amount of molten steel = 1 / 3. Among them, the argon pressure of the bottom blowing argon is preferably 200-400Pa, specifically 200Pa, 250Pa, 300Pa, 350Pa, 400Pa. The time of the bottom blowing argon is preferably 4-10min, specifically 4min, 5min, 6min, 7min, 8min, 9min, 10min, and more preferably 4min.

[0108] [About step B]:

[0109] B) LF refining: The molten steel obtained in step A) is subjected to LF refining treatment.

[0110] In the present invention, during LF refining, argon is introduced into the bottom of the molten steel tank, and the pressure of the argon is preferably 200-400Pa, specifically 200Pa, 250Pa, 300Pa, 350Pa, 400Pa. The time for passing argon is preferably 4-6min, specifically 4min, 5min, 6min. The argon flow rate is based on the condition that the molten steel does not overturn, which can avoid secondary oxidation of the molten steel and too fast temperature drop, so that the inclusions in the steel can fully float, and further improve the cleanliness of the steel. The time for LF refining is preferably 10-25min, specifically 10min, 15min, 20min, 25min. The outlet temperature of LF refining is preferably 1630-1650℃, specifically 1630℃, 1635℃, 1640℃, 1645℃, 1650℃.

[0111] [About Step C]:

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

[0113] In the present invention, the RH treatment time is preferably 20 to 35 minutes, specifically 20 minutes, 25 minutes, 30 minutes, and 35 minutes. In the treatment, aluminum-iron alloy, metal manganese, titanium-iron alloy, etc. are added for alloying. The specific means are not particularly limited. The corresponding auxiliary materials can be added according to the chemical composition of the target product in accordance with the conventional alloying method in the field. The outlet temperature of the RH treatment is preferably 1590 to 1610°C, specifically 1590°C, 1595°C, 1600°C, 1605°C, and 1610°C.

[0114] In the present invention, in order to ensure the uniformity of chemical composition of steel, after RH treatment of deoxidation and alloying, the molten steel in the RH vacuum chamber circulates for 5 to 8 minutes, and the vacuum degree is ≤2.0mba. Then, the molten steel is also treated with bottom argon blowing; the pressure of the bottom argon blowing is 200 to 400Pa, specifically 200Pa, 250Pa, 300Pa, 350Pa, 400Pa. The time of the bottom argon blowing is preferably 4 to 6 minutes, specifically 4 minutes, 5 minutes, 6 minutes. After the above treatment, molten steel is obtained.

[0115] After 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%~0.005%;

[0121] P: 0%~0.010%;

[0122] S: 0%~0.010%;

[0123] Als: 0.020%~0.050%;

[0124] Fe and inevitable impurity elements: balance

[0125] In one embodiment of the present invention, the composition of molten steel is as follows: 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 impurities. In another embodiment of the present invention, the composition of molten steel is as follows: 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 is Fe and inevitable impurities. Among them, Als is a well-known concept in the art, i.e., acid-soluble aluminum.

[0126] [About Step D]:

[0127] D) Continuous casting: The molten steel obtained in step C) is continuously cast to obtain a slab.

[0128] Continuous casting (continuous pouring) Continuous casting specifically involves pouring refined molten steel into a pre-baked tundish and casting it into a slab through a fully protected slab continuous casting machine. After pouring, it is preferably cooled, for example, naturally cooled at room temperature.

[0129] [About Step E]:

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

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

[0132] In the present invention, the hot rolling treatment includes reheating and soaking treatment. After the slab is obtained by continuous casting, the slab is sent to a heating furnace for heating, which is a reheating and soaking treatment. The temperature of reheating and soaking refers to the rolling temperature of the steel billet out of the furnace. Soaking at this temperature can fully dissolve the microalloying elements, eliminate the chemical element segregation caused by the dendrite bias of the casting, and avoid the liquid precipitation carbide reducing the effect of the microalloying elements in the steel. In the present invention, the temperature of the reheating and soaking treatment is 1190-1240°C, specifically 1190°C, 1200°C, 1208°C, 1210°C, 1220°C, 1230°C, 1240°C. The time of the reheating and soaking treatment is 30-45min, specifically 30min, 35min, 40min, 45min.

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

[0134] In the present invention, after the rough rolling, finishing rolling is performed. The starting rolling temperature of the finishing rolling is 1010-1040°C, specifically 1010°C, 1015°C, 1020°C, 1025°C, 1030°C, 1035°C, 1040°C. The final rolling temperature of the finishing rolling is 900-940°C, specifically 900°C, 905°C, 910°C, 915°C, 920°C, 925°C, 930°C, 935°C, 940°C. The above temperature control during the hot rolling process can make the steel in a completely austenitic structure in the finishing rolling stage and avoid the austenitic structure being too coarse.

[0135] In the present invention, cooling is performed after the above-mentioned finish rolling. The cooling is laminar cooling. The length of the laminar cooling is 10 to 20 m, specifically 10 m, 15 m, and 20 m. The cooling rates of the upper and lower headers of the laminar cooling are preferably 75% and 50%, respectively. The above-mentioned cooling rate is expressed as a proportion of the amount of water passed. In actual production, the cooling rate is adjusted by switching the cooling header nozzle. The percentage expression method is well known in the industry. For example, the full-open mode is 100%, that is, 100% of the water is directly sprayed on the surface of the strip. Through the above control, the cooling rate is 10 to 20 ° C / min, specifically 10 ° C / min, 15 ° C / min, and 20 ° C / min. After cooling, the hot-rolled thin strip steel adjusts the internal structure of the steel and is then coiled into a coil. In order to meet the requirements of good strength and plasticity of hot-dip galvanized steel sheet products, the present invention controls the ferrite grain size in the microstructure of the strip after hot rolling to about 10.5 levels, and the precipitation of AlN, TiC and Ti(C,N) inhibits the non-uniform growth of ferrite in the high temperature zone, thereby refining the hot-rolled base plate structure.

[0136] In the present invention, the material is cooled to 660-690°C by the cooling process and then coiled. The temperature may be 660°C, 665°C, 670°C, 675°C, 680°C, 685°C, or 690°C.

[0137] [About Step F]:

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

[0139] The present invention has no special restrictions on the pickling process, and it can be carried out according to the conventional process procedures; usually, the thin steel strips rolled by hot rolling are welded at the head of the pickling mill to form a continuous steel strip, and then straightened, straightened and dephosphorized, pickled, alkali washed, and dried. In the present invention, the pickling straightening elongation is preferably 0.6% to 1.0%, and can be specifically 0.6%, 0.7%, 0.8%, 0.9%, and 1.0%. After pickling, the surface quality of the steel plate 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-drawn steel strip.

[0142] In the present invention, after pickling in step F), the steel strip is sent to a hot-dip galvanizing unit for hot-dip galvanizing. Specifically, the pickled thin steel strip is welded at the entrance of the hot-dip galvanizing unit to form a continuous steel strip, and after continuous surface cleaning and drying, it enters the continuous hot-dip galvanizing unit for a series of treatments, including preheating, recrystallization annealing, cooling and hot-dip galvanizing.

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

[0144] In the present invention, after preheating, recrystallization annealing is performed. The temperature of the recrystallization annealing is 860-890°C, specifically 860°C, 865°C, 870°C, 875°C, 880°C, 885°C, 890°C. The time of the recrystallization annealing is preferably 4-8 minutes, specifically 4 minutes, 5 minutes, 6 minutes, 7 minutes, 8 minutes.

[0145] In the present invention, after recrystallization annealing, cooling is performed. In the present invention, the cooling includes slow cooling and rapid cooling in sequence. Among them, the slow cooling rate is preferably 5-9°C / min, specifically 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min; the target of the slow cooling is preferably cooled to 570-580°C, specifically 570°C, 571°C, 572°C, 573°C, 574°C, 575°C, 576°C, 577°C, 578°C, 579°C, 580°C. The rapid cooling rate is preferably 15-30°C / min, specifically 15°C / min, 20°C / min, 25°C / min, 30°C / min. The target of the rapid cooling is preferably to cool to 460-470°C, specifically 460°C, 461°C, 462°C, 463°C, 464°C, 465°C, 466°C, 467°C, 468°C, 469°C, 470°C.

[0146] In the present invention, after cooling, hot-dip galvanizing is performed. The temperature of the hot-dip galvanizing is preferably 455-465°C, specifically 455°C, 456°C, 457°C, 458°C, 459°C, 460°C, 461°C, 462°C, 463°C, 464°C, 465°C.

[0147] In the present invention, after hot-dip galvanizing, post-galvanizing cooling is preferably performed. The post-galvanizing cooling rate is preferably 35-50°C / min, specifically 35°C / min, 40°C / min, 45°C / min, 50°C / min. The target of the post-galvanizing cooling is preferably cooled to 260°C. After the post-galvanizing cooling to 260°C, final cooling is performed; the final cooling is preferably cooled to ≤90°C. After cooling to the above temperature, the subsequent skin-passing unit is entered.

[0148] In the present invention, after the above cooling, it is preferred to further carry out: online skin-passing, tensioning, online inspection, and coiling. Among them, the elongation of skin-passing is preferably 1.00% to 1.40%, and can be specifically 1.00%, 1.10%, 1.20%, 1.30%, and 1.40%. The elongation of tensioning is preferably 0% to 0.6%, and can be specifically 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, and 0.6%. After the above treatment, a 270MPa grade hot-dip galvanized deep-drawn steel strip product is obtained.

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

[0150] The 270MPa grade hot-dip galvanized deep-drawn steel strip prepared by the present invention 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 C dissolving in α-Fe and has a body-centered cubic crystal structure. Ferrite can be measured by a method known to those skilled in the art, such as the GB / T 6394 metallographic method.

[0151] The 270MPa grade hot-dip galvanized steel sheet involved in the present invention adopts an ultra-low carbon steel + Ti microalloying scheme (C combines with Ti to form TiC, and N combines with Ti to form TiN). The production process adopts key technical measures such as off-furnace refining, controlled rolling and controlled cooling, recrystallization annealing, hot-dip galvanizing, and light finishing processes. The resulting product has stable mechanical properties, good strength and plasticity matching, excellent deep drawing performance, good performance anisotropy, and good corrosion resistance of the coating. The finished product inspection results show that the 270MPa grade hot-dip galvanized steel sheet produced by this method has good mechanical properties and process performance. Because the steel sheet involved in the present invention has stable mechanical properties, small anisotropy of the steel sheet, excellent deep drawing performance, and good corrosion resistance of the coating, it can effectively adapt to different stamping processes of users (bending, stamping, flanging, etc.). The hot-dip galvanized steel sheet produced by the present invention has excellent comprehensive performance, high cost performance, and good prospects for promotion and use.

[0152] The production method provided by the present invention controls the chemical composition, the hot rolling process controls the conditions such as the soaking temperature, the final rolling temperature, and the coiling temperature, and the hot-dip galvanizing process controls the annealing temperature, the galvanizing temperature, the skin pass and the straightening conditions, so that the obtained hot-dip galvanized steel strip has excellent mechanical properties (Rp0.2 is 185-255MPa, Rm≥270MPa, A50mm≥42%, n90≥0.17); it effectively overcomes the problem that the pickling-hot-dip galvanizing technology is prone to mixed crystals and performance deterioration in the process of no deformation structure recovery, recrystallization, and grain growth during the annealing process. The refined hot-rolled microstructure, TiC and Ti (C, N) particles are used to hinder the non-uniform growth of annealed grains, refine the ferrite grain size of the finished product, and improve the plasticity index of the finished product. Moreover, the pickling direct hot-dip galvanizing process technology adopted by the present invention omits the conventional cold rolling step, reduces the process cost, and the physical performance of the finished product is good, which effectively meets the use needs of downstream users, and has good promotion value and promotion prospects.

[0153] The chemical composition detection methods in the present invention are respectively spark source atomic emission spectrometry analysis methods for carbon steel and medium and low alloy steel, and the national standard is GB / T4336; Determination of low carbon content in non-alloy steel Part 2: Infrared absorption method after combustion in an induction furnace (preheated), and the national standard is GB / T 20126-2006.

[0154] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0155] Example 1

[0156] A) Molten steel:

[0157] The smelting equipment is a top-blown converter. The semi-steel after vanadium extraction from the blast furnace molten iron is used as raw material. Steelmaking auxiliary materials are added to melt to 1670℃ and then the steel is tapped into the molten steel tank. When 1 / 3 of the steel is tapped, 200Kg of aluminum-iron alloy is added for pre-deoxidation and alloying. Then, argon is bottom-blown into the molten steel tank on a small platform outside the furnace. The argon pressure is 300Pa and the time is 4 minutes.

[0158] B) LF refining:

[0159] Argon gas at a certain pressure (350Pa) is introduced into the bottom of the molten steel tank for 4 minutes. The argon gas flow rate is based on the condition that the molten steel does not overturn. The LF refining time is 20min and the outlet temperature is 1635℃.

[0160] C) RH treatment:

[0161] The molten steel tank was transported to the RH refining furnace for final deoxidation, alloying treatment, and composition fine-tuning. The RH treatment time was 30 minutes, and the outlet temperature was 1595°C. The composition of the molten steel was: 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 was Fe and unavoidable impurity elements.

[0162] D) Continuous Casting:

[0163] The molten steel tank is transported to the casting position, and the bottom of the molten steel tank slides the Al stopper rod, and the molten steel automatically flows into the tundish, passes through the Al stopper rod, and is drained to the crystallizer for continuous casting. The whole process uses protective slag for protective casting, and after casting, it is cooled to obtain hot-rolled plate billets.

[0164] E) Hot rolling:

[0165] The continuous casting slab is sent to a heating furnace for reheating and soaking treatment, the soaking temperature is 1208℃, and the soaking time is 40min; then, rough rolling is carried out, the rough rolling temperature is 1157℃; thereafter, finish rolling is carried out, the start rolling temperature of finish rolling is 1025℃, and the final rolling temperature is 930℃; the rolling passes have 6 rough rolling steps and 7 finishing rolling steps.

[0166] After the above-mentioned finish rolling, laminar cooling is performed, and the cooling rates of the upper and lower headers are 75% and 50% respectively, and the cooling rate is 15°C / min. The steel is cooled to a coiling temperature of 670°C for coiling.

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

[0168] F) Pickling:

[0169] The steel strip obtained in step E) is pickled online, wherein the pickling tensile leveling elongation is 0.9%.

[0170] G) Hot dip galvanizing:

[0171] The steel strip and sheet steel strip after pickling in step F) are welded to form a continuous steel strip at the entrance of the hot-dip galvanizing unit, and after continuous surface cleaning and drying, enter the continuous hot-dip galvanizing unit for the following series of treatments:

[0172] Preheating: temperature 600℃;

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

[0174] Cooling: first slow cooling and then fast cooling; slow cooling: cooling to 575℃ at 6℃ / min; fast cooling: cooling to 465℃ at 20℃ / min;

[0175] Hot dip galvanizing: temperature 457℃;

[0176] Cooling after plating: Cool to 260℃ at 40℃ / min; then, finally cool to 90℃;

[0177] Post-processing: online skin-finishing, tension-leveling, online inspection, and roll separation; among which, the skin-finishing elongation is 1.20%, and the tension-leveling elongation is 0.1%.

[0178] test:

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

[0180] Example 2

[0181] Implement according to Example 1, except that:

[0182] In step C), the composition of the molten steel is: 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 is Fe and unavoidable impurity elements.

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

[0184] In step F), the pickling and straightening elongation is 1.0%.

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

[0186] test:

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

[0188] Comparative Example 1

[0189] Implement according to Example 1, except that:

[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%, and the rest is Fe and unavoidable impurity elements.

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

[0192] In step F), the pickling and straightening elongation is 1.4%.

[0193] In step G), the recrystallization annealing temperature is 760° C., the hot-dip galvanizing temperature is 455° C., the final cooling temperature is 93° C., the skin-pass elongation is 0.8%, and the tension-leveling elongation is 0.3%.

[0194] test:

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

[0196] Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core ideas, including the best mode, and also enable any technician in the field to practice the present invention, including making and using any device or system, and implementing any combined method. It should be pointed out that for ordinary technicians in this technical field, without departing from the principle of the present invention, the present invention can also be improved and modified, and these improvements and modifications also fall within the scope of protection of the claims of the present invention. The scope of patent protection of the present invention is defined by the claims and may include other embodiments that can be thought of by those skilled in the art. If these other embodiments have structural elements similar to the literal expression of the claims, or if they include equivalent structural elements that are not substantially different from the literal expression of the claims, then these other embodiments should also be included in the scope of the claims.

Claims

1. A method for producing a 270MPa grade hot-dip galvanized deep-drawn steel strip, characterized in that: The following steps are involved: A) Molten steel: The blast furnace molten iron and smelting charge are smelted in a converter to obtain molten steel; then, the molten steel is pre-deoxidized; B) LF refining: The molten steel obtained in step A) is subjected to LF refining treatment; 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; In terms of mass percentage, the molten steel obtained in step C) comprises: 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: Continuously casting the molten steel obtained in step C) to obtain a slab; E) Hot rolling: hot rolling the slab obtained in step D); The hot rolling process includes: reheating and soaking treatment, rough rolling, finishing rolling, cooling and coiling; in, The reheating soaking temperature of the hot rolling is 1190-1240°C and the time is 30-45 minutes; The temperature of the rough rolling is 1140-1200°C; The final rolling temperature of the finishing rolling is 900-940°C; The cooling is laminar cooling; wherein the length of the laminar cooling is 10 to 20 m, and the cooling rate is 10 to 20 ° C / min; The coiling temperature is 660-690°C; F) Pickling: Pickling the steel strip obtained in step E); G) Hot dip galvanizing: Preheating, recrystallization annealing, cooling and hot-dip galvanizing the steel strip after pickling in step F) to obtain a 270MPa grade hot-base galvanized deep-drawn steel strip; in, The temperature of the recrystallization annealing is 860-890°C; The temperature of the hot-dip galvanizing is 455-465°C.

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

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

4. The production method according to claim 1, characterized in that In step G), after the hot-dip galvanizing, post-galvanizing cooling is also performed; The cooling rate after plating is 35-50°C / min, and the target is to cool to 260°C.

5. The production method according to claim 4, characterized in that: In step G), after cooling after plating, the following steps are also performed: online skin-finishing, tensioning, online inspection, and coiling; Among them, the elongation of skin straightening is preferably 1.00% to 1.40%, and the elongation of tension leveling is preferably 0% to 0.6%.

6. The production method according to claim 1, characterized in that: In step F), the pickling tensile elongation is 0.6% to 1.0%.

7. The production method according to claim 1, characterized in that: In step E), the cooling rates of the upper and lower headers of the laminar cooling are preferably 75% and 50% respectively.

8. The production method according to claim 1, characterized in that: In step C), the RH treatment time is 20 to 35 minutes, and the exit temperature is 1590 to 1610°C.

9. The production method according to claim 1, characterized in that: In step B), the LF refining time is 10 to 25 minutes, and the outlet temperature is 1630 to 1650°C.

10. The production method according to claim 1, characterized in that: In step A), the smelting temperature is 1660-1675°C.

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