Preparation method of 980MPa-grade hot-dip galvanizing enhanced formability complex-phase steel

Abstract

The invention relates to a preparation method of 980MPa-grade hot-dip galvanizing enhanced formability complex-phase steel, which comprises the following steps: annealing cold and hard strip steel with set chemical components, and then carrying out hot-dip galvanizing to obtain the 980MPa-grade hot-dip galvanizing enhanced formability complex-phase steel, wherein the annealing treatment comprises the following steps: heating the cold and hard strip steel, then soaking, and controlling the soaking temperature; and slowly cooling the uniformly-heated cold and hard strip steel, then rapidly cooling the strip steel at a first cooling speed, controlling the final temperature of rapid cooling, and carrying out heat preservation at the final temperature. The formability of the cold-rolled complex-phase steel with the enhanced formability is improved.

Description

Technical Field

[0001] The present application relates to the technical field of steel preparation, and in particular to a method for preparing 980MPa grade hot-dip galvanized enhanced formability complex phase steel. Background Art

[0002] Lightweight and high safety are important directions for the development of the automotive industry, and the use of more high-strength steel in automobile bodies can improve their safety and fuel efficiency. Compared with traditional high-strength steel, ferrite-martensite dual-phase steel has been widely used in automobile body structures due to its low yield ratio, high initial work hardening rate, and good strength-plasticity matching. However, low yield ratio dual-phase steel has a contradictory relationship between ductility and hole expansion. Although it has good strength-plasticity matching, it also has poor local formability in actual use, such as frequent cracking during bending and flanging. In particular, it has poor hole expansion performance and is difficult to resist early edge cracking during complex forming, which further hinders its promotion and application in the body. Due to its ferrite, bainite and martensite dual-phase structure, complex phase steel has a high yield ratio and good flanging and hole expansion performance. However, with the increase of part strength, the corresponding forming difficulty gradually increases, which makes the technical difficulty of cold stamping continue to increase. Cold-rolled complex phase steel still has frequent cracking when dealing with complex forming with relatively high drawing and local forming, which undoubtedly hinders the development of complex phase steel production and application.

[0003] Therefore, at this stage, how to develop a cold-rolled complex phase steel with enhanced formability to meet the common needs of flanging, hole expansion and drawing is a difficult problem that needs to be solved urgently. Summary of the invention

[0004] The present application provides a method for preparing 980MPa grade hot-dip galvanized enhanced formability complex phase steel to solve the existing technical problem of how to improve the formability of cold-rolled complex phase steel with enhanced formability.

[0005] In a first aspect, the present application provides a method for preparing 980MPa grade hot-dip galvanized enhanced formability complex phase steel, the method comprising:

[0006] The cold-hardened strip steel with set chemical composition is annealed and then hot-dip galvanized to obtain 980MPa grade hot-dip galvanized enhanced formability complex phase steel; wherein the annealing treatment includes:

[0007] The chilled steel strip is heated, then evenly heated, and the temperature of the evenly heated steel strip is controlled;

[0008] The cold-hardened steel strip after homogenization is slowly cooled, and then rapidly cooled under the condition of a first cooling rate, and the terminal temperature of the rapid cooling is controlled, and the steel strip is kept warm under the condition of the terminal temperature.

[0009] Optionally, the uniform heating temperature is 850°C to 950°C.

[0010] Optionally, the first cooling rate is 40°C / s to 55°C / s.

[0011] Optionally, the endpoint temperature of the rapid cooling is 340°C to 420°C.

[0012] Optionally, the insulation time is 80s to 140s.

[0013] Optionally, the cold-hardened steel strip comprises a steel substrate and a pre-plated metal layer attached to at least a portion of the surface of the steel substrate.

[0014] The thickness of the pre-plated metal layer is 200 mg / m 2 ~800 mg / m 2 .

[0015] Optionally, the set chemical composition includes: C, Si, Mn, Cr, Mo, Nb, Ti, Alt, P, S and Fe; wherein, in terms of mass fraction,

[0016] The C content is 0.10% to 0.30%, the Si content is 1.2% to 1.8%, the Mn content is 1.8% to 2.4%, the Cr content is 0.2% to 0.6%, the Mo content is 0.1% to 0.3%, the Nb content is 0.01% to 0.04%, the Ti content is 0.01% to 0.04%, the Alt content is 0.03% to 0.06%, the P content is ≤0.010%, and the S content is ≤0.006%.

[0017] Optionally, the 980MPa grade hot-dip galvanized enhanced formability complex phase steel meets the following mechanical performance indicators:

[0018] Tensile strength greater than 980MPa, yield strength ≥780MPa, elongation A80 ≥12%, hole expansion ≥30%; and

[0019] / or,

[0020] The microstructure of the 980MPa grade cold-rolled hot-dip galvanized enhanced formability complex phase steel includes: bainite, martensite, film-like retained austenite and ferrite; wherein, in terms of area fraction,

[0021] The content of bainite is 15% to 35%, the content of martensite is 10% to 25%, and the content of film-like retained austenite is 5% to 15%;

[0022] The average grain size of the ferrite is 2.1 μm to 4.2 μm, and the average grain size of the martensite is 1.2 μm to 3 μm;

[0023] The microstructure contains precipitates, and the diameter of the precipitates is 10 nm or less.

[0024] Optionally, the cooling rate of the slow cooling is 4°C / s to 12°C / s, and / or the terminal temperature of the slow cooling is 800°C to 850°C.

[0026] Optionally, the annealing treatment of the cold-hardened steel strip with the set chemical composition may also include:

[0027] The slab is heated in a furnace, then rolled, cooled and coiled to obtain a hot rolled coil;

[0028] The hot rolled coil is cold rolled to obtain the cold hardened steel strip; wherein,

[0029] The heating temperature in the furnace is 1250° C. to 1330° C., the final temperature of the rolling is 860° C. to 920° C., the coiling temperature is 600° C. to 700° C., and the reduction rate of the cold rolling is 30% to 60%.

[0030] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0031] The preparation method of the 980MPa grade hot-dip galvanized enhanced formability complex phase steel provided in the embodiment of the present application prepares a 980MPa grade hot-dip galvanized complex phase steel formability enhanced product through chemical composition design and annealing process optimization and regulation, and especially obtains excellent ductility and flanging and hole expansion performance, thereby improving the formability of the cold-rolled complex phase steel with enhanced formability. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0033] In order to more clearly illustrate the embodiments of the present application 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, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0034] Figure 1 A schematic flow chart of a method for preparing a 980MPa grade hot-dip galvanized enhanced formability complex phase steel provided in an embodiment of the present application;

[0035] Figure 2 This is a microstructure diagram of a 980MPa grade hot-dip galvanized enhanced formability complex phase steel provided in Example 1 of the present application. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0037] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0038] In the present application, in the absence of any contrary description, the directional words used, such as "upper" and "lower", are specifically the directions of the drawings in the accompanying drawings. In addition, in the description of the present specification, the terms "including", "comprising", etc. refer to "including but not limited to". In this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. In this article, "and / or" describes the association relationship of the associated objects, indicating that there may be three relationships, for example, A and / or B, which can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A, B can be singular or plural. In this article, "at least one" refers to one or more, and "plural" refers to two or more. "At least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of singular or plural items. For example, "at least one of a, b, or c", or "at least one of a, b and c", can both mean: a, b, c, ab (i.e. a and b), ac, bc or abc, where a, b, c can be single or plural, respectively.

[0039] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0040] Figure 1 A schematic diagram of a method for preparing a 980MPa grade hot-dip galvanized enhanced formability complex phase steel provided in an embodiment of the present application; see Figure 1 The present application provides a method for preparing 980MPa grade hot-dip galvanized enhanced formability complex phase steel, the method comprising:

[0041] S1. Annealing a cold-hardened steel strip having a set chemical composition, and then hot-dip galvanizing to obtain a 980MPa grade hot-dip galvanized enhanced formability complex phase steel; wherein the annealing treatment comprises:

[0042] The chilled steel strip is heated, then evenly heated, and the temperature of the evenly heated steel strip is controlled;

[0043] The cold-hardened steel strip after homogenization is slowly cooled, and then rapidly cooled under the condition of a first cooling rate, and the terminal temperature of the rapid cooling is controlled, and the steel strip is kept warm under the condition of the terminal temperature.

[0044] In some embodiments, the soaking temperature is 850°C to 950°C.

[0045] In some embodiments, the first cooling rate is 40° C. / s to 55° C. / s.

[0046] In some embodiments, the endpoint temperature of the rapid cooling is 340°C to 420°C.

[0047] In some embodiments, the insulation time is 80s to 140s.

[0048] In some embodiments, the cooling rate of the slow cooling is 4°C / s to 12°C / s, and / or the terminal temperature of the slow cooling is 800°C to 850°C.

[0049] In the embodiment of the present application, the above-mentioned soaking temperature is used, that is, the annealing temperature. After full austenitization, the fibrous structure after cold rolling completes recovery, recrystallization and is completely transformed into austenite. At the same time, the residual cementite is fully dissolved, and the composition and structure in the austenite are fully homogenized, thereby ensuring the ductility and strength of the steel plate. Exemplarily, the above-mentioned soaking temperature can be 850℃, 870℃, 890℃, 910℃, 930℃, 950℃, etc. In addition, the heating temperature can be 850℃~950℃, the heating speed can be 50℃ / s~96℃ / s, and the soaking time can be 2min~5min.

[0050] Then slowly cooling to a higher temperature ensures that the formation of new ferrite is suppressed during the initial cooling process, which not only prevents the evolution of structural inhomogeneity, but also avoids carbon-enriched austenite from increasing austenite stability. Exemplarily, the cooling rate of the above slow cooling can be 4°C / s, 6°C / s, 8°C / s, 10°C / s, 12°C / s, etc., and the terminal temperature of the slow cooling can be 800°C, 810°C, 820°C, 830°C, 840°C, 850°C, etc.

[0051] After slow cooling, the strip steel is rapidly cooled at a cooling rate of 40-55°C / s (first cooling rate) to 340-420°C (end temperature of rapid cooling) under 75% high hydrogen cooling conditions and kept warm for 80-140s. During this stage, bainite transformation is carried out and element distribution is carried out. The carbon content in bainite is fully transferred to austenite, so that the austenite structure is stable at room temperature, ensuring the ductility of the steel plate. Exemplarily, the cooling rate of the above rapid cooling can be 40°C / s, 42°C / s, 44°C / s, 46°C / s, 48°C / s, 50°C / s, 52°C / s, 54°C / s, 55°C / s, etc., and the end temperature of the above rapid cooling can be 340°C, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, 410°C, 420°C, etc.

[0052] Then, the strip is induction heated to a galvanizing temperature of 450-460°C and then galvanized. The dew point of the annealing heating section is -30--50°C, and the dew point of the galvanizing grate is (-40)--50°C. After galvanizing, the strip is air-cooled to 400-420°C, then naturally cooled to room temperature, and finally subjected to skin-finishing treatment on a four-roller skin-finishing mill to obtain a finished product, with a skin-finishing elongation of 0.2-0.4%.

[0053] In some embodiments, the chilled steel strip comprises a steel substrate and a pre-plated metal layer attached to at least a portion of the surface of the steel substrate.

[0054] The thickness of the pre-plated metal layer is 200 mg / m 2 ~800 mg / m 2 .

[0055] In an embodiment of the present application, the above-mentioned pre-plated metal layer is attached between the steel substrate and the zinc layer. The role of the above-mentioned pre-plated metal layer is to prevent other alloy elements of the steel substrate from being oxidized and enriched in the surface layer during the annealing process, affecting the wettability of the galvanizing and the adhesion of the coating, thereby obtaining excellent surface quality. The chemical composition of the above-mentioned pre-plated metal layer includes at least one of the following: iron, nickel, and copper. The thickness of the above-mentioned pre-plated metal layer is limited. During the annealing process, due to selective oxidation, Mn or Si oxides are easily formed on the surface of the steel plate. The nickel layer is sufficient to cover the oxides, so that the zinc liquid has sufficient wettability during the subsequent galvanizing process to avoid plating defects. At the same time, the pre-plated metal layer should not be too thick, which will cause unnecessary waste and increase manufacturing costs. Exemplarily, the thickness of the above-mentioned pre-plated metal layer can be 200mg / m 2 , 300mg / m 2 , 400mg / m 2 , 500mmg / m 2 , 600mg / m 2 , 700mg / m 2 , 800mg / m 2 wait.

[0056] In some embodiments, the set chemical composition includes: C, Si, Mn, Cr, Mo, Nb, Ti, Alt, P, S and Fe; wherein, in terms of mass fraction,

[0057] The C content is 0.10% to 0.30%, the Si content is 1.2% to 1.8%, the Mn content is 1.8% to 2.4%, the Cr content is 0.2% to 0.6%, the Mo content is 0.1% to 0.3%, the Nb content is 0.01% to 0.04%, the Ti content is 0.01% to 0.04%, the Alt content is 0.03% to 0.06%, the P content is ≤0.010%, and the S content is ≤0.006%.

[0058] In the embodiment of the present application, the C element is an effective element, which is used to stabilize the residual austenite and ensure the required target strength. In order to avoid excessive C content deteriorating the welding performance and excessive carbon causing casting defects, the carbon content should not exceed 0.3%. For example, the C content can be 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, etc.

[0059] Si is also an important solid solution strengthening element. At the same time, Si can effectively promote the enrichment of C elements into austenite, which helps to stabilize the residual austenite. At the same time, it can inhibit the precipitation of carbides in ferrite, purify the ferrite phase, and improve the elongation. In order to obtain the desired effect, the Si content should not be less than 1.2%, but when the Si content is greater than 1.8%, it will significantly deteriorate the surface of the hot-rolled plate and have a very deteriorating effect on the galvanizability during the galvanizing process. For example, the Si content can be 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, etc.

[0060] Mn is also an important element for solid solution strengthening and stabilizing austenite. It plays an important role in strengthening, can inhibit ferrite phase transformation, and is also an effective element for the formation and stabilization of residual austenite. When the manganese content is less than 1.8%, a large amount of ferrite phase transformation will occur, and it is difficult to obtain the target strength. However, if the Mn content is too high, it is easy to cause structural segregation, which is easy to cause forming cracks and deteriorate the comprehensive properties of steel. In addition, if the manganese content is too high, it will delay the bainite phase transformation and form a large amount of martensite, making it difficult to ensure the desired ductility. Exemplarily, the Mn content can be 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, etc.

[0061] The P element can inhibit the formation of carbides, so it is considered that containing a very small amount of the P element is beneficial. However, segregation at the grain boundary will lead to a decrease in grain boundary strength and thus deteriorate the mechanical properties of the material. It is also an element that significantly affects the contribution of carbon equivalent, so the P element content is controlled below 0.010%.

[0062] As a harmful element, S element is mainly used to prevent the combination with Mn to produce MnS, thereby deteriorating the material properties. It is also an element that significantly affects the carbon equivalent contribution. Therefore, the S element content is controlled below 0.006%.

[0063] The Cr element can improve the hardenability of austenite, thereby obtaining a sufficient amount of martensite to ensure strength. Adding a certain amount of Cr to obtain strength ensures that the alloy manufacturing cost is sufficiently low; but at the same time, the Cr element is a ferrite zone expansion element. Too much Cr element will cause the two-phase zone to shrink and hinder the bainite transformation. Therefore, the addition of Cr is limited to ensure the acquisition of bainite. Exemplarily, the content of Mn can be 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, etc.

[0064] Mo is an element that helps stabilize austenite. This element works in combination with elements such as C and Si to help stabilize austenite. Therefore, it is necessary to add more than 0.1% to ensure the desired effect. However, when it exceeds 0.3%, not only the alloy cost increases greatly, resulting in a reduction in the two-phase region, but also hinders the bainite transformation. Therefore, the addition of Mo is limited to ensure the acquisition of bainite. Exemplarily, the content of Mo can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc.

[0065] Nb / Ti is an effective element for precipitation strengthening and grain refinement. It combines with carbon and nitrogen in steel to form fine carbides and nitrides, which plays an important role in grain refinement. On the other hand, the above-mentioned fine carbides and nitrides strengthen the matrix structure when precipitated, and reduce the hardness difference between the phases, thereby improving the yield strength and hole expansion performance of the steel. When its content is less than 0.01%, it is difficult to achieve the above effect. When its content exceeds 0.04%, it not only increases the alloy cost, but also greatly reduces the ductility of the steel plate due to the formation of too many precipitates. Exemplarily, the contents of Nb and Ti can be 0.01%, 0.02%, 0.03%, 0.04%, etc.

[0066] Alt is a good deoxidizer in steel, so it needs to be kept above 0.03%. Al also acts similarly to Si and is a good austenite indirect stabilizing element. However, sufficient Si is added in the embodiment of the present application, so there is no need to add too much Al. Excessive addition of Al will cause nozzle blockage during casting, affecting continuous casting efficiency. For example, the contents of Nb and Ti can be 0.03%, 0.04%, 0.05%, 0.06%, etc.

[0067] In some embodiments, the 980MPa grade hot-dip galvanized enhanced formability complex phase steel meets the following mechanical performance indicators: tensile strength greater than 980MPa, yield strength ≥780MPa, elongation A80 ≥12%, and hole expansion ratio ≥30%; and

[0068] / or,

[0069] The microstructure of the 980MPa grade cold-rolled hot-dip galvanized enhanced formability complex phase steel includes: bainite, martensite, film-like retained austenite and ferrite; wherein, in terms of area fraction,

[0070] The content of bainite is 15% to 35%, the content of martensite is 10% to 25%, and the content of film-like retained austenite is 5% to 15%;

[0071] The average grain size of the ferrite is 2.1 μm to 4.2 μm, and the average grain size of the martensite is 1.2 μm to 3 μm;

[0072] The microstructure contains precipitates, and the diameter of the precipitates is 10 nm or less.

[0073] In the embodiment of the present application, the above-mentioned 980MPa grade hot-dip galvanized enhanced formability complex phase steel has excellent mechanical properties. Within the above-mentioned microstructure range, it is beneficial to ensure the strength and ductility of the steel plate steel, and it plays a role in alleviating the expansion of interface cracks during hole expansion and improving the hole expansion performance. Retained austenite is an indispensable structure to ensure its ductility. The content of the above-mentioned retained austenite is limited so that the retained austenite is fully stable at room temperature. Exemplary, Figure 2 This is a microstructure diagram of a 980MPa grade hot-dip galvanized enhanced formability complex phase steel provided in Example 1 of the present application; see Figure 2 , the structure includes: bainite, martensite, film-like residual austenite and ferrite. The ferrite grain size is controlled within a certain size range to achieve a fine size effect, thereby ensuring an excellent yield strength ratio; the average martensite grain size is limited, and its function is that the finely dispersed martensite is conducive to the uniform distribution and distribution of strain during forming, which is conducive to obtaining better hole expansion performance. The formation of martensite bands is avoided, and the formability is improved without forming cracks at the phase interface. The above-mentioned precipitates are mainly carbides, nitrides or carbonitrides of niobium or titanium. By uniformly distributing the above-mentioned precipitates in the matrix, the hardness of the soft phase ferrite is properly controlled, which is conducive to reducing the hardness difference between the two phases, thereby ensuring excellent hole expansion performance, relative to 1μm 2 The microstructure includes more than 100 precipitates. For example, the content of bainite can be 15%, 20%, 25%, 30%, 35%, etc., the content of martensite can be 10%, 15%, 20%, 25%, and the content of film-like retained austenite can be 5%, 10%, 15%, etc.; the average grain size of ferrite can be 2.1μm, 2.5, 3.0, 3.5, 4.0, 4.2μm, etc., and the average grain size of martensite can be 1.2μm, 1.5μm, 2.0μm, 2.5μm, 3μm, etc.; the diameter of the precipitate can be 10nm, 9nm, 8nm, 7nm, etc.

[0074] In some embodiments, the annealing of the chilled steel strip having the set chemical composition may further include:

[0075] S0, heating the slab in a furnace, and then rolling, cooling and coiling to obtain a hot-rolled coil;

[0076] The hot rolled coil is cold rolled to obtain a cold hardened steel strip; wherein:

[0077] The heating temperature in the furnace is 1250° C. to 1330° C., the final temperature of the rolling is 860° C. to 920° C., the coiling temperature is 600° C. to 700° C., and the reduction rate of the cold rolling is 30% to 60%.

[0078] In the embodiment of the present application, the molten steel is smelted in a converter and a slab is obtained by continuous casting. The end temperature of rolling is limited to ensure the uniformity of the microstructure. Within the above-mentioned range of the end temperature of rolling, the uniformity of the grains of the steel plate structure is ensured, and it is beneficial to energy saving and consumption reduction. The furnace heating temperature is limited to homogenize the alloy elements of the slab. The coiling temperature is limited to achieve more ideal organizational properties and surface quality. The cold rolling reduction rate is limited to obtain the required target cold rolling thickness, and at the same time, the recrystallization nucleation energy storage during annealing is obtained. Within the above-mentioned range of cold rolling reduction rates, sufficient recrystallization nucleation energy storage is obtained, and at the same time, the rolling load is not increased and the roller life is not reduced. Exemplarily, the temperature of the heating in the furnace can be 1250℃, 1260℃, 1270℃, 1280℃, 1290℃, 1300℃, 1310℃, 1320℃, 1330℃, etc., the end temperature of the rolling can be 860℃, 870℃, 880℃, 890℃, 900℃, 910℃, 920℃, etc., the coiling temperature can be 600℃, 620℃, 630℃, 640℃, 660℃, 680℃, 700℃, etc., and the reduction rate of the cold rolling can be 30%, 40%, 50%, 60%, etc.

[0079] In addition, the process parameters in the specific S0 step include: the furnace time is 210-280min, the furnace temperature is 1100-1220°C; the above rolling includes rough rolling and finish rolling, the rough rolling is performed for 3 passes, the rough rolling outlet temperature is 900-1020°C, the finish rolling is performed for 6 passes, and then the hot rolled coil is coiled at a rate of 10-20°C / s by laminar cooling to the coiling temperature, and naturally cooled to room temperature to obtain a hot rolled coil;

[0080] The above cold rolling is a single-stand reciprocating rolling of 5 passes to obtain a cold hardened strip of target thickness. Among them, the above furnace temperature is limited to comprehensively consider the guarantee of the finishing rolling entrance temperature and obtain the required comprehensive performance. The reason for the rough rolling of 3 passes and the finishing rolling of 6 passes is to reduce the rolling load of each pass. The cooling rate of laminar cooling is limited to make the cooling more uniform.

[0081] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.

[0082] The present application provides a method for preparing 980MPa grade hot-dip galvanized enhanced formability complex phase steel, the method comprising:

[0083] S00, heating the slab in a furnace, and then rolling, cooling and coiling to obtain a hot-rolled coil;

[0084] Cold rolling the hot rolled coil to obtain a cold hardened steel strip;

[0085] S11, annealing the cold-hardened steel strip with set chemical composition, and then hot-dip galvanizing to obtain 980MPa grade hot-dip galvanized enhanced formability complex phase steel; wherein the annealing treatment includes:

[0086] The chilled steel strip is heated, then evenly heated, and the temperature of the evenly heated steel strip is controlled;

[0087] The cold-hardened steel strip after homogenization is slowly cooled, and then rapidly cooled under the condition of a first cooling rate, and the terminal temperature of the rapid cooling is controlled, and the steel strip is kept warm under the condition of the terminal temperature.

[0088] For example, please refer to the chemical composition of 980MPa grade hot-dip galvanized enhanced formability complex phase steel shown in Table 1, the hot rolling process and product thickness of 980MPa grade hot-dip galvanized enhanced formability complex phase steel shown in Table 2, the annealing process of 980MPa grade hot-dip galvanized enhanced formability complex phase steel shown in Table 3, the mechanical properties of 980MPa grade hot-dip galvanized enhanced formability complex phase steel shown in Table 4, and the microstructure of 980MPa grade hot-dip galvanized enhanced formability complex phase steel shown in Table 5.

[0089] Table 1 Chemical composition (wt%) of the steel matrix of 980MPa grade cold-rolled hot-dip galvanized enhanced formability complex phase steel, the remainder is Fe and unavoidable impurities

[0090] Serial number C Si Mn P S Alt Cr Mo Nb Ti Example 1 0.18 1.5 1.8 0.008 0.002 0.04 0.42 0.22 0.021 0.027 Example 2 0.17 1.4 2.3 0.009 0.002 0.05 0.28 0.18 0.020 0.024 Example 3 0.18 1.6 2.2 0.008 0.003 0.05 0.38 0.20 0.018 0.028 Example 4 0.22 1.7 2.3 0.007 0.001 0.04 0.50 0.21 0.024 0.018 Example 5 0.21 1.6 2.3 0.006 0.002 0.06 0.24 0.22 0.024 0.020 Comparative Example 1 0.18 1.5 1.8 0.008 0.002 0.04 0.42 0.22 0.021 0.027

[0091] Table 2 Hot rolling process and product thickness of 980MPa grade hot-dip galvanized enhanced formability complex phase steel

[0092]

[0093] Table 3 Annealing process of 980MPa grade hot-dip galvanized enhanced formability complex phase steel

[0094]

[0095] The finished products of Examples 1 to 5 and Comparative Example 1 were sampled for mechanical property testing, and the results are shown in Table 4.

[0096] Table 4 Mechanical properties of 980MPa grade hot-dip galvanized enhanced formability complex phase steel

[0097] Serial number Rm / MPa Rp0.2 / MPa A80 / ％ Hole expansion rate / % Example 1 1022 785 12.5 38 Example 2 1018 834 13 31 Example 3 1032 815 12.5 35 Example 4 1022 828 14 38 Example 5 1048 822 14 36 Comparative Example 1 1050 840 9 22

[0098] Table 5 Microstructure of 980MPa grade hot-dip galvanized enhanced formability complex phase steel

[0099]

[0100]

[0101] In summary, the preparation method of a 980 MPa grade hot-dip galvanized enhanced formability complex phase steel provided in the embodiment of the present application, combined with Table 5, shows that the 980 MPa grade hot-dip galvanized enhanced formability complex phase steel has excellent mechanical properties; while the soaking temperature, the rapid cooling end temperature and the rapid cooling rate in the technical scheme of Comparative Example 1 are not within the scope of the embodiment of the present application, resulting in relatively low elongation and hole expansion rate of the final steel plate.

[0102] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A method for preparing 980MPa grade hot-dip galvanized enhanced formability complex phase steel, characterized in that: The method comprises: The cold-hardened strip steel with set chemical composition is annealed and then hot-dip galvanized to obtain 980MPa grade hot-dip galvanized enhanced formability complex phase steel; wherein the annealing treatment includes: The chilled steel strip is heated, then evenly heated, and the temperature of the evenly heated steel strip is controlled; The cold-hardened steel strip after homogenization is slowly cooled, and then rapidly cooled under the condition of a first cooling rate, and the terminal temperature of the rapid cooling is controlled, and the steel strip is kept warm under the condition of the terminal temperature.

2. The method according to claim 1, characterized in that The temperature of the soaking is 850°C to 950°C.

3. The method according to claim 1, characterized in that The first cooling rate is 40°C / s to 55°C / s.

4. The method according to claim 1, characterized in that: The terminal temperature of the rapid cooling is 340°C to 420°C.

5. The method according to claim 1, characterized in that The insulation time is 80s to 140s.

6. The method according to claim 1, characterized in that The chilled steel strip comprises a steel substrate and a pre-plated metal layer attached to at least a portion of the surface of the steel substrate. The thickness of the pre-plated metal layer is 200 mg / m 2 ~800 mg / m 2 .

7. The method according to claim 1, characterized in that The set chemical composition includes: C, Si, Mn, Cr, Mo, Nb, Ti, Alt, P, S and Fe; wherein, in terms of mass fraction, The C content is 0.10% to 0.30%, the Si content is 1.2% to 1.8%, the Mn content is 1.8% to 2.4%, the Cr content is 0.2% to 0.6%, the Mo content is 0.1% to 0.3%, the Nb content is 0.01% to 0.04%, the Ti content is 0.01% to 0.04%, the Alt content is 0.03% to 0.06%, the P content is ≤0.010%, and the S content is ≤0.006%.

8. The method according to claim 1, characterized in that The 980MPa grade hot-dip galvanized enhanced formability complex phase steel meets the following mechanical performance indicators: Tensile strength greater than 980MPa, yield strength ≥780MPa, elongation A80 ≥12%, hole expansion ratio ≥30%; and / or, The microstructure of the 980MPa grade cold-rolled hot-dip galvanized enhanced formability complex phase steel includes: bainite, martensite, film-like retained austenite and ferrite; wherein, in terms of area fraction, The content of bainite is 15% to 35%, the content of martensite is 10% to 25%, and the content of film-like retained austenite is 5% to 15%; The average grain size of the ferrite is 2.1 μm to 4.2 μm, and the average grain size of the martensite is 1.2 μm to 3 μm; The microstructure contains precipitates, and the diameter of the precipitates is 10 nm or less.

9. The method according to claim 1, characterized in that: The cooling rate of the slow cooling is 4°C / s to 12°C / s, and / or the terminal temperature of the slow cooling is 800°C to 850°C.

10. The method according to claim 1, characterized in that The annealing treatment of the cold hardened strip steel with the set chemical composition also includes: The slab is heated in a furnace, then rolled, cooled and coiled to obtain a hot rolled coil; The hot rolled coil is cold rolled to obtain the cold hardened steel strip; wherein, The heating temperature in the furnace is 1250° C. to 1330° C., the final temperature of the rolling is 860° C. to 920° C., the coiling temperature is 600° C. to 700° C., and the reduction rate of the cold rolling is 30% to 60%.

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