Hydrogen-resistant cold-rolled, hot-dip galvanized ultra-high-strength duplex steel and method for manufacturing the same

Abstract

The present invention provides a hydrogen-induced crack-resistant cold-rolled hot-dip galvanized ultra-high strength dual-phase steel including a substrate and a zinc plating layer plated on the substrate. The substrate contains Fe and inevitable impurity elements, and the substrate also contains the following chemical elements in the following mass percentages: C: 0.1 to 0.18%, Mn: 2.2 to 3.0%, Si: 0.2 to 0.6%, Al: 0.03 to 0.3%, Nb: 0.01 to 0.1%, Ti: 0.01 to 0.1%, Mo: 0.04 to 0.2%, B: 0.0005 to 0.003%. The matrix of the microstructure of the substrate is ferrite + martensite, and the microstructure also contains carbide precipitation phases with a size of less than 100 nm. These carbide particles and the matrix are in an aggregated state or a semi-aggregated state. In addition, the present invention further discloses a manufacturing method of the above steel material including the following steps: (1) smelting and casting, (2) hot rolling, (3) pickling, (4) cold rolling, (5) recrystallization annealing of hot-dip galvanizing, (6) cooling after galvanizing.

Description

Technical Field

[0001] The present invention relates to steel materials and a method for manufacturing the same, and particularly relates to ultra-high strength dual-phase steel and a method for manufacturing the same.

Background Art

[0002] In recent years, with the rapid development of the automotive industry, the steel materials used in the white body of automobiles have also been rapidly changing, and the proportion of ultra-high strength steel materials used in the automotive field has been increasing very significantly.

[0003] Compared with conventional high-strength steel, when the strength of ultra-high strength steel reaches 980 MPa or more, the risk of hydrogen-induced delayed cracking significantly increases, and the safety of the vehicle body is greatly impaired. In particular, when the tensile strength reaches a level exceeding 1180 MPa, the critical hydrogen content for hydrogen-induced cracking becomes very low, and in order to meet the requirements of corrosion resistance, the surface of the steel plate is usually plated with a zinc layer. Due to the presence of the zinc layer, the overflow of diffusible hydrogen is suppressed, and when the plated zinc becomes extremely high, the risk of hydrogen-induced cracking increases. Therefore, the development of ultra-high strength steel that can avoid the risk of hydrogen-induced cracking while meeting the user's requirements for corrosion resistance has become the focus of future research and development, but there are few related reports and patents of this kind.

[0004] For example, in a Chinese patent document with a publication number of CN1990894A, a publication date of July 4, 2007, and a title of "Ultra-high strength thin steel plate with excellent hydrogen embrittlement resistance", an ultra-high strength steel and a manufacturing method with excellent hydrogen embrittlement resistance are disclosed, and its typical composition is (0.1~0.3)C-(1~3.5)Mn-(1~3)Si. When the tensile strength of the material reaches the 1180 MPa level, it has high resistance to delayed cracking. However, in this technical solution, a higher C and Si design is adopted, which is disadvantageous for the spot welding performance of extremely high zinc plating. At the same time, the annealing temperature used when manufacturing the steel material is higher than the A3 temperature, and higher unit conditions are required for the process path, which does not conform to the low-carbon design concept.

[0005] For example, a Chinese patent document with publication number CN102449180A, publication date May 9, 2012, and title "High-strength steel sheet with excellent hydrogen embrittlement resistance" discloses a high-strength steel sheet with excellent hydrogen embrittlement resistance. Its typical composition is (0.15~0.25)C-(1.5~3)Mn-(1~2.5)Si, and high formability is obtained when the tensile strength of the material reaches the 1180 MPa level after the quenching-tempering process. However, this technical solution employs a design with higher C and Si, which is disadvantageous for the spot weldability and manufacturability of galvanized steel. At the same time, when manufacturing the steel material, the required quenching-tempering process path and high annealing temperature require high unit conditions and equipment, which does not contribute to product expansion.

[0006] Therefore, in order to meet the needs of the market and users, there is an urgent need to develop a new hydrogen crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel using the conventional duplex (DP) process, while ensuring manufacturability, production cost, and excellent weldability of cold-rolled hot-dip galvanized duplex steel. [Overview of the project] [Problems that the invention aims to solve]

[0007] One of the objectives of the present invention is to provide a hydrogen crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel, which, through rational component matching and process design, can achieve excellent manufacturability, formability, weldability, and hydrogen cracking resistance while ensuring ultra-high strength, with a yield strength of ≥800 MPa, tensile strength of ≥1180 MPa, and elongation A 50 With a hydrogen content of ≥6% and diffusible hydrogen content of ≤0.2 ppm, it has very good application prospects. [Means for solving the problem]

[0008] To achieve the above objective, the present invention provides a hydrogen-resistant, cold-rolled, hot-dip galvanized, ultra-high-strength duplex steel comprising a substrate and a zinc plating layer plated on the substrate, wherein the substrate contains Fe and unavoidable impurity elements, and the substrate also contains the following chemical elements in the following mass percentages: C: 0.1~0.18%, Mn: 2.2~3.0%, Si: 0.2~0.6%, Al: 0.03~0.3%, Nb: 0.01~0.1%, Ti: 0.01~0.1%, Mo: 0.04~0.2%, B: 0.0005~0.003%.

[0009] The aforementioned substrate has a ferrite + martensite matrix in its microstructure, and this microstructure also includes carbide precipitate phases with a size of less than 100 nm. In this disclosure, the size of the carbide precipitate phase refers to its particle size.

[0010] In the present invention, the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel is simply referred to as cold-rolled hot-dip galvanized duplex steel.

[0011] Furthermore, in the hydrogen-induced crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the mass percentages of each chemical element in the substrate are as follows: C: 0.1~0.18%, Mn: 2.2~3.0%, Si: 0.2~0.6%, Al: 0.03~0.3%, Nb: 0.01~0.1%, Ti: 0.01~0.1%, Mo: 0.04~0.2%, B: 0.0005~0.003%, with the remainder being Fe and unavoidable impurities.

[0012] In one or more embodiments, the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel has a mass percentage content of element C that is within the range of 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, or any two of the aforementioned values.

[0013] In one or more embodiments, the mass percentage content of Mn element in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel is within the range of 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3.0%, or any two of the aforementioned values.

[0014] In one or more embodiments, the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel has a mass percentage content of Si element that is within the range of 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, or any two of the aforementioned values.

[0015] In one or more embodiments, the mass percentage content of the element Al in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel is within the range of 0.03%, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, or any two of the aforementioned values.

[0016] In one or more embodiments, the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel has a mass percentage content of Nb element that is within the range of 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, or any two of the aforementioned values.

[0017] In one or more embodiments, the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel has a mass percentage content of Ti element that is within the range of 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, or any two of the aforementioned values.

[0018] In one or more embodiments, the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel has a mass percentage content of Mo which is within the range of 0.04%, 0.06%, 0.08%, 0.1%, 0.12%, 0.14%, 0.16%, 0.18%, 0.2%, or any two of the aforementioned values.

[0019] In one or more embodiments, the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel has a mass percentage content of element B that is within the range of 0.0005%, 0.001%, 0.0015%, 0.002%, 0.0025%, 0.003%, or any two of the aforementioned values.

[0020] In one or more embodiments, the size of the carbide precipitate phase is within the range of 5 nm, 10 nm, 15 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any two of the aforementioned values.

[0021] In this invention, the hydrogen-resistant cold-rolled, hot-dip galvanized ultra-high-strength duplex steel can be further enhanced by the introduction of a precipitation strengthening phase, i.e., a nanoscale carbide precipitate phase, into the ferrite and martensite structure of conventional duplex steels through rational composition matching and process design. These carbide precipitate phases not only improve the strength of the steel but also function as powerful hydrogen traps that fix diffusible hydrogen, which is beneficial for improving delayed cracking of the steel.

[0022] In the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the design principles for each chemical element are as follows.

[0023] C: In the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, carbon (C) is an important component element of the hot-dip galvanized duplex steel and contributes to the strong plasticity of the galvanized sheet. If the carbon content in the steel is too low, the austenite content formed during annealing in the critical region decreases, reducing austenite stability and martensitic hardening, making it difficult to ensure the strong plasticity of the steel. On the other hand, the carbon content in the steel should not be too high. If the carbon content in the steel is too high, the plasticity and weldability of the duplex steel will decrease. Therefore, considering the effect of the carbon content on the properties of the steel, in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the mass percentage content of carbon is controlled to 0.1 to 0.18%.

[0024] Mn: In the hydrogen-induced cracking-resistant cold-rolled hot-dip galvanized ultra-high strength dual-phase steel according to the present invention, the Mn element improves the stability of austenite, shifts the C curve to the right, and reduces the critical cooling rate of martensite. The content of the Mn element in the steel must not be too low. If the content of Mn is too low, the hardenability of the steel decreases, and the strengthening effect also weakens. At the same time, it is not desirable to add excessive Mn to the steel. If the content of the Mn element in the steel is too high, it will affect the weldability of the substrate and the quality of the surface zinc plating. At the same time, the grain boundaries become weak, and the risk of hydrogen-induced cracking of the material increases. Therefore, considering the influence of the content of the Mn element on the properties of the steel, in the hydrogen-induced cracking-resistant cold-rolled hot-dip galvanized ultra-high strength dual-phase steel according to the present invention, the mass percentage content of the Mn element is controlled to be 2.2 - 3.0%.

[0025] Si: In the hydrogen-induced cracking-resistant cold-rolled hot-dip galvanized ultra-high strength dual-phase steel according to the present invention, Si is a ferrite solid solution strengthening element and can strongly improve the strength of the steel sheet. At the same time, the Si element can also promote the enrichment of C atoms from ferrite to austenite, purify ferrite, suppress the precipitation of cementite and ε-carbide, and improve the stability of austenite. However, the content of the Si element in the steel must not be too high. If the content of the Si element is too high, it will directly affect the plating property and spot weldability of the substrate, so attention is required. Therefore, in order to exert the beneficial effects of the Si element, it is necessary to strictly control the content of the Si element in the steel. In the hydrogen-induced cracking-resistant cold-rolled hot-dip galvanized ultra-high strength dual-phase steel according to the present invention, the mass percentage content of the Si element is controlled to be 0.2 - 0.6%.

[0026] Al: In the hydrogen-induced cracking resistant cold-rolled hot-dip galvanized ultra-high strength dual-phase steel according to the present invention, the Al element functions similarly to the Si element, can effectively suppress the precipitation of carbides, and can promote the diffusion of carbon elements into austenite. However, the Al element cannot suppress the precipitation of ε-carbide. In the present invention, the Al element can increase the stacking fault energy of austenite and effectively suppress the weakening of grain boundaries caused by stress concentration and hydrogen enrichment. At the same time, Al can form a dense alumina protective layer on the surface to suppress the intrusion of hydrogen, and improve the hydrogen embrittlement resistance of the material. Furthermore, the Al added to the steel can also pin the grain boundaries by forming AlN and refine the crystal grains.

[0027] The content of the Al element in the steel should not be too high. If the content of Al is too high, problems such as nozzle clogging during continuous casting and a significant increase in Ac3 are likely to occur, so attention is required. Therefore, in order to exert the beneficial effects of the Al element, it is necessary to strictly control the content of the Al element in the steel. In the present invention, the mass percentage content of the Al element is controlled to be 0.03 - 0.3%.

[0028] Nb: In the hydrogen-induced cracking resistant cold-rolled hot-dip galvanized ultra-high strength dual-phase steel according to the present invention, the Nb element strongly suppresses dynamic recrystallization, combines with C and N to form Nb(C,N), effectively suppresses the coarsening of crystal grains during heat treatment, refines the crystal grains, and can strengthen the boundary strength. Furthermore, the Nb precipitates, like the Ti element, have the effect of increasing the grain boundary area and reducing the diffusible hydrogen content per unit area. At the same time, it is also beneficial to reduce the risk of delayed cracking of the steel plate by fixing diffusible hydrogen. However, the content of the Nb element in the steel should not be too high. Adding excessive Nb will deteriorate the hot working properties of the steel and the toughness of the steel plate. Based on this, in order to exert the beneficial effects of the Nb element, the mass percentage of the Nb element in the hydrogen-induced cracking resistant cold-rolled hot-dip galvanized ultra-high strength dual-phase steel according to the present invention is controlled to be 0.01 - 0.1%.

[0029] Ti: In the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, Ti combines with C and N to form Ti(C,N), TiN, and TiC. This refines the microstructure in the cast state, prevents grain coarsening during heat treatment, strengthens grain boundaries, and simultaneously refines the grains, increasing the grain boundary area, which is advantageous in reducing the diffusible hydrogen content per unit area. Furthermore, Ti precipitates fix diffusible hydrogen atoms, which is advantageous in avoiding localized accumulation of diffusible hydrogen, and this is beneficial in improving delayed cracking of the steel. However, the Ti content in the steel should not be too high. Adding excessive Ti increases costs and increases the content and size of the precipitates, reducing the ductility of the steel sheet, so care must be taken. Therefore, in order to exert the beneficial effects of the Ti element, it is necessary to control the mass percentage of the Ti element to 0.01~0.1% in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention.

[0030] Mo: In the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the element Mo can shift the C curve of pearlite and bainite to the right, thereby improving the hardenability of the steel. At the same time, the element Mo can also significantly improve the strength of the steel without affecting the quality of the surface zinc plating. Furthermore, the grain boundary strengthening effect of the element Mo further improves the resistance of the steel sheet to hydrogen-induced cracking, and the finely dispersed precipitate phase of Mo reduces the diffusible hydrogen content and prevents the accumulation of diffusible hydrogen. Therefore, in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, considering that the element Mo is expensive, the mass percentage of the element Mo is controlled to 0.04-0.2%.

[0031] B: In the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, element B can effectively inhibit the recrystallization of the steel during the hot-rolling process, which is beneficial for the refinement of the microstructure caused by cumulative deformation. However, the content of element B in the steel should not be too high, as adding too much B will generate BC, which will reduce the ductility of the steel, so care must be taken. Therefore, in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the mass percentage content of element B is controlled to 0.0005 to 0.003%.

[0032] Furthermore, in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the chemical elements of the substrate may optionally or preferably contain at least one of the following elements in the following mass percentages.

[0033] V:0.005~0.2%, Cr:0.01~0.8%, Cu:0.003~0.5%.

[0034] Furthermore, in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the mass percentages of each chemical element in the substrate are: The composition is C:0.1~0.18%, Mn:2.2~3.0%, Si:0.2~0.6%, Al:0.03~0.3%, Nb:0.01~0.1%, Ti:0.01~0.1%, Mo:0.04~0.2%, B:0.0005~0.003%, and optionally or preferably at least one of V:0.005~0.2%, Cr:0.01~0.8%, and Cu:0.003~0.5%, with the remainder being Fe and unavoidable impurities.

[0035] In one or more embodiments, if V is present, the mass percentage content of element V in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel is within the range of 0.005%, 0.01%, 0.02%, 0.05%, 0.07%, 0.1%, 0.2%, or any two of the aforementioned values.

[0036] In one or more embodiments, if Cr is present, the mass percentage content of the Cr element in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel is within the range of 0.01%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, or any two of the aforementioned values.

[0037] In one or more embodiments, when Cu is included, the mass percentage content of Cu in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel is within the range of 0.003%, 0.005%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or any two of the aforementioned values.

[0038] To further optimize the performance of the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, in some preferred embodiments, elements V, Cr, and Cu may be further added to the steel.

[0039] V: In this invention, V mainly exists in the form of VC in hot-dip galvanized duplex steel, pinning grain boundaries to refine crystal grains and improving the strength and toughness of the steel through dispersion precipitation strengthening in ferrite. At the same time, the delayed cracking improvement mechanism of the carbides of element V is the same as that of Nb and Ti. However, it should be noted that further addition of V increases the alloy cost of the steel, so in this invention, the mass percentage content of element V is limited to 0.005 to 0.2%.

[0040] Cr: In this invention, the element Cr can refine the grain structure and suppress grain coarsening during hot working. Furthermore, Cr is an element that forms ferrite, promotes the diffusion of C into austenite, improves the stability of austenite, and reduces the critical cooling rate during annealing. At the same time, the addition of Cr causes carbides in the steel to disperse and precipitate finely, forming an effective hydrogen trap and reducing the risk of delayed cracking. However, the Cr content in the steel should not be too high, as too much Cr content will impair the ductility and surface plating properties of the steel, so care must be taken. Therefore, in this invention, the mass percentage content of the element Cr is controlled to 0.01 to 0.8%.

[0041] Cu: In this invention, Cu significantly improves the stability of austenite, shifts the C-curve of bainite to the right, improves hardenability, and thereby reduces the critical cooling rate of martensite. At the same time, the Cu element can strengthen the matrix by precipitating nano-precipitation phases, and furthermore, Cu also functions as a hydrogen trap, effectively pinning diffusible hydrogen and reducing the risk of delayed cracking of the material. However, the Cu element content in the steel should not be too high, as too much Cu can easily lead to copper embrittlement during hot working, so care must be taken. Therefore, in this invention, the mass percentage content of the Cu element is controlled to 0.003 to 0.5%.

[0042] Furthermore, in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the mass percentage content of Si, Al, Mo, and Cr in the substrate satisfies Si + Al + Cr + Mo ≤ 1.0%.

[0043] In one or more embodiments, the hydrogen-induced crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention satisfies the condition that the mass percentage content of Si, Al, Mo, and Cr in the substrate is within the range of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0% of Si+Al+Cr+Mo or any two of the aforementioned values.

[0044] In the above technical solution of the present invention, the present invention may further control the elemental ratio in the steel so that the mass percentage content of Si, Al, Mo, and Cr satisfies Si+Al+Cr+Mo≦1.0%, while controlling the mass percentage content of a single chemical element. By setting Si+Al+Cr+Mo≦1.0%, it is possible to ensure sufficient hydrogen trapping while ensuring good manufacturability of the steel sheet, thereby achieving both manufacturability and delayed crack resistance.

[0045] Furthermore, in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the unavoidable impurities in the substrate are P ≤ 0.02%, S ≤ 0.01%, and N ≤ 0.008%.

[0046] In the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, elements P, S, and N are all impurity elements in the substrate. To obtain a steel sheet with better performance and quality, as far as technical conditions allow, it is necessary to reduce the content of impurity elements in the steel sheet as much as possible.

[0047] In this invention, the weakening effect of impurity elements on grain boundaries increases susceptibility to hydrogen embrittlement. However, if the content of P and S in the steel is too high, grain boundary segregation and grain boundary embrittlement will increase, and it will not contribute to the hydrogen embrittlement resistance of the material. Therefore, it is necessary to strictly control the mass percentage content of impurity elements P, S, and N in the steel so that P ≤ 0.02%, S ≤ 0.01%, and N ≤ 0.008%, and the higher the purity of the steel, the better the effect.

[0048] In one or more embodiments, in the hydrogen-induced crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the mass percentage content of element P is 0.001 to 0.02%, the mass percentage content of element S is 0.001 to 0.01%, and the content of element N is 0.001 to 0.008%.

[0049] Of course, in some preferred embodiments, it is even more preferable to control P ≤ 0.01%, S ≤ 0.006%, and N ≤ 0.005% to obtain better implementation effects.

[0050] Furthermore, in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the carbide precipitate phase and the matrix are in an aggregated or semi-aggregated state.

[0051] Furthermore, in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the volume phase ratio of ferrite in the microstructure of the substrate is 10 to 40%, for example, 15%, 20%, 25%, 30%, or 35%.

[0052] Furthermore, in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, the thickness of the zinc plating layer on one side of the substrate is 5 to 200 μm, for example, 10 μm, 25 μm, 35 μm, 50 μm, 75 μm, 100 μm, 120 μm, 130 μm, and 150 μm.

[0053] Furthermore, the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention has a yield strength of ≥800 MPa, a tensile strength of ≥1180 MPa, and elongation A 50 It is characterized by having a hydrogen content of ≥6% and a diffusible hydrogen content of ≤0.2 ppm.

[0054] In one or more embodiments, the yield strength of the hydrogen-induced crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention is within the range of 800 MPa, 850 MPa, 900 MPa, 950 MPa, 1000 MPa, or any two of the aforementioned values.

[0055] In one or more embodiments, the tensile strength of the hydrogen-induced crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention is within the range of 1180 MPa, 1190 MPa, 1200 MPa, 1220 MPa, 1240 MPa, 1260 MPa, 1270 MPa, or any two of the aforementioned values.

[0056] In one or more embodiments, the elongation A of the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention 50 This is within the range of 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or any two of the aforementioned values.

[0057] In one or more embodiments, the diffusible hydrogen content of the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention is within the range of 0.01 ppm, 0.02 ppm, 0.04 ppm, 0.06 ppm, 0.08 ppm, 0.1 ppm, 0.2 ppm, or any two of the aforementioned values.

[0058] Therefore, another object of the present invention is to provide a method for producing the above-mentioned hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel, which optimizes the design of the annealing process and can effectively produce the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention, thereby ensuring the hydrogen-resistant cracking properties of the steel material.

[0059] To achieve the above objective, the present invention provides a method for producing hydrogen-resistant, cold-rolled, hot-dip galvanized ultra-high-strength duplex steel, comprising the following steps: (1) Smelting and casting; (2) Hot rolling; (3) pickling; (4) Cold rolling; (5) Hot-dip galvanizing recrystallization annealing: (a) Heat the steel plate to a soaking temperature T1 at a heating rate V1 of 1-20°C / s, and hold the temperature for 30-240 seconds, where T1 > 760°C; (b) The soaked steel plate is cooled to an intermediate temperature T2 at a cooling rate V2 of 2-20°C / s, where T2 = 600-780°C; (c) Cool the steel plate to a zinc plating temperature T3 at a cooling rate V3 of 5-60°C / s, and hold it at that temperature for 20-300 seconds, where V3 > V2 and the zinc plating temperature is 400-500°C; (d) Send the steel plates to the zinc pot for galvanizing; (6) Cooling after zinc plating: After zinc plating is complete, rapidly cool to below 200°C at a rate of 10°C / s or more, hold in the temperature range of 100°C to 300°C for 15 to 100 seconds, and then cool to room temperature at a cooling rate of 5°C / s or more.

[0060] In the present invention, the production of hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel according to the present invention requires a series of processes including smelting and casting (such as continuous casting), hot rolling, pickling, cold rolling, hot-dip galvanizing recrystallization annealing process, and cooling after galvanizing. By adopting the above-described component design, rolling process, and hot-dip galvanizing recrystallization annealing process, the present invention can effectively produce hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel. The microstructure matrix of the produced steel sheet is ferrite + martensite, and also includes carbide precipitate phases with a size of less than 100 nm, and these carbide particles and the matrix are in an aggregated or semi-aggregated state.

[0061] This invention improves the delayed crack resistance of high-strength steel by controlling the relative content of ferrite and martensite, comprehensively utilizing fine-grain strengthening, precipitation strengthening, and other methods to obtain good plasticity, while also employing various control techniques such as increasing grain boundary strength, reducing diffusible hydrogen content, and avoiding localized accumulation of diffusible hydrogen.

[0062] In the manufacturing method of the present invention described above, the inventors have optimized the hot-dip galvanizing recrystallization annealing process in step (5) so that the steel sheet can be flexibly tempered after galvanizing. This process not only promotes the overflow of diffusible hydrogen, but also causes ε-carbide precipitation and increases hydrogen trapping, significantly reducing the diffusion coefficient of hydrogen in the martensite and improving the delayed crack resistance of the steel sheet.

[0063] Furthermore, the present invention utilizes a short-time aging treatment (i.e., a process of holding the steel plate at a temperature range of 100-300°C for 15-100 seconds during cooling after zinc plating) to promote hydrogen overflow, significantly reducing the diffusible hydrogen content in the steel plate, thereby enabling the steel material to have ultra-high strength as well as excellent resistance to hydrogen embrittlement.

[0064] It should be noted that the parameters of the hot-dip galvanizing annealing process are closely related to the compositional design of the steel, determining the relative content of soft ferrite and hard martensite in the galvanized sheet and affecting the optimal matching of ferroplasticity in the galvanized steel sheet.

[0065] In the present invention, a cold-rolled steel sheet is heat-treated by a continuous annealing method, and the hot-dip galvanizing recrystallization annealing process is as shown in steps (a) to (d) above.

[0066] In step (a), the cold-rolled steel sheet is heated to a soaking temperature T1 at a heating rate V1, and then held for a time t1. Specifically, V1 is selected to be 1-20°C / s, for example 2°C / s, 5°C / s, 7°C / s, 8°C / s, 10°C / s, 12°C / s, 15°C / s, 17°C / s, or 18°C; T1 is selected to be 760°C, for example 770°C, 780°C, 790°C, 800°C, 810°C, 820°C, or 830°C; and the holding time t1 is controlled to be 30-240 s, for example 40 s, 60 s, 80 s, 100 s, 120 s, 160 s, 180 s, 200 s, or 220 s. In this process, if the soaking temperature T1 is lower than 760°C and the soaking time t1 is shorter than 30 s, the matrix structure of the cold-rolled galvanized sheet corresponding to the design composition of the present invention will mostly be a banded structure, and sufficient austenite or steel sheet matrix carbides will not be obtained, making it impossible to completely dissolve and form austenite particles. In particular, the presence of a banded structure adversely affects the usability of the steel material, such as bending and hole enlargement.

[0067] In steps (b) and (c), the steel sheet after soaking treatment is slowly cooled to an intermediate temperature T2 at a cooling rate V2, then immediately rapidly cooled to the zinc plating temperature T3 at a cooling rate V3, and then held for a time t3.

[0068] Here, the selection of the slow cooling rate V2 and the intermediate temperature T2 mainly considers the following: promoting the transformation of some austenite into ferrite, ensuring that the austenite at temperature T2 has some degree of hardenability, mitigating decomposition at the subsequent zinc plating temperature T3, ensuring a certain amount of martensite and ferrite in the final structure, and allowing the steel sheet to acquire a certain degree of ferroplasticity.

[0069] If the intermediate temperature T2 is too high, a large amount of austenite will be present, which will have reduced stability and be more likely to transform into bainite at the subsequent zinc plating temperature T3, thereby affecting the strength and elongation of the steel. If the intermediate temperature T2 is too low, too much ferrite will be formed, resulting in less austenite, which will reduce the amount of martensite that is ultimately formed and may lead to insufficient strength. Therefore, based on the chemical elemental composition designed in this invention, V2 is selected as 2~20℃ / s, for example, 4℃ / s, 6℃ / s, 8℃ / s, 10℃ / s, 12℃ / s, 14℃ / s, 16℃ / s, 18℃ / s; and T2 is selected as 600~780℃, for example, 620℃, 640℃, 660℃, 700℃, 720℃, 740℃, 760℃.

[0070] Therefore, in the composition design of the present invention, V3 = 5~60°C / s, for example, 10°C / s, 15°C / s, 20°C / s, 25°C / s, 30°C / s, 35°C / s, 40°C / s, 45°C / s, 50°C / s, 55°C / s, and V3 > V2 are selected. In order to select a rapid cooling rate V3, it is necessary to minimize austenite decomposition in the steel sheet matrix during the cooling process. If the zinc plating temperature T3 exceeds 500°C, austenite decomposes and a microstructure containing pearlite or carbides is generated, thereby consuming the austenite content and its carbon content, and reducing the strength of the steel sheet. If the zinc plating temperature T3 is below 400°C, the bainite ferrite content increases, and the ferroplasticity of the zinc-plated sheet decreases.

[0071] From the standpoint of effectiveness and economics, in order to ensure the ferrite-to-martensite ratio for the final galvanized steel sheet to obtain the best trepidation, the temperature T3 is specifically limited to 450-500°C, for example, 460°C, 470°C, 480°C, and 490°C, and the stabilization time (i.e., the holding time at the galvanizing temperature T3) t3 is selected to be 20-300 s, for example, 30 s, 50 s, 70 s, 90 s, 100 s, 110 s, 120 s, 150 s, and 200 s.

[0072] Furthermore, in the manufacturing method of the present invention, in step (2), the slab is heated to 1180-1280°C (for example, 1200°C, 1220°C, 1240°C, 1260°C), the holding time is set to 0.5-4 hours (for example, 1 hour, 2 hours, 3 hours), the final rolling temperature is controlled to 850°C or higher (for example, 870°C, 880°C, 890°C, 900°C, 920°C, 930°C, 950°C), and the winding temperature is controlled to 700°C or lower (for example, 550°C, 570°C, 580°C, 600°C, 620°C, 640°C, 660°C, 680°C).

[0073] Furthermore, in the manufacturing method of the present invention, in step (4), the amount of cold rolling deformation is controlled to 30-70%, for example, 35%, 40%, 45%, 50%, 55%, 60%, or 65%.

[0074] Furthermore, in the manufacturing method of the present invention, in step (6), after the zinc plating is completed, the zinc-plated steel sheet is rapidly cooled to 200°C or less (for example, 150°C, 160°C, 170°C, 180°C, 190°C) at a cooling rate V4 of 10°C / s or more (for example, 20°C / s, 30°C / s, 40°C / s, 50°C / s), and then to 100-300°C (for example, 150°C, 180°C, 200°C, 22°C) The material is held at a holding temperature T4 (0°C, 250°C, 280°C) for a holding time t4 of 15 to 100 seconds (e.g., 20s, 30s, 40s, 50s, 60s, 70s, 80s, 90s), and then cooled to room temperature at a cooling rate V5 of 5°C / s or more (e.g., 10°C / s, 20°C / s, 30°C / s, 40°C / s, 50°C / s, 60°C / s, 70°C / s, 80°C / s).

[0075] With current existing technology, galvanized ultra-high-strength duplex steel with a tensile strength exceeding 1180 MPa is highly susceptible to delayed cracking, posing a significant safety risk. Whether or not 1180 MPa-class ultra-high-strength steel will be applied to vehicle bodies in the future hinges on improving the delayed cracking performance of the steel plates.

[0076] For hot-dip galvanized ultra-high-strength steel at the 1180 MPa level, the present invention employs a rational application of alloying elements and combines it with a rational manufacturing process to obtain a hot-dip galvanized ultra-high-strength duplex steel that possesses excellent manufacturability, formability, weldability, and resistance to hydrogen-induced cracking.

[0077] Compared with existing technologies, the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel and its manufacturing method according to the present invention have the following advantages and beneficial effects.

[0078] In this invention, the inventors, when designing the substrate for a zinc-plated sheet, employ various control techniques such as optimizing alloying elements, controlling grain refinement, and distributing finely dispersed precipitate phases to enhance grain boundary strength, reduce diffusible hydrogen content, and avoid localized accumulation of diffusible hydrogen, thereby effectively improving the delayed crack resistance of ultra-high-strength steel. This hydrogen-resistant cold-rolled, hot-dip galvanized ultra-high-strength duplex steel can be further enhanced by introducing precipitate strengthening phases, i.e., nanoscale carbide precipitate phases, into the ferrite and martensitic structure of conventional duplex steel through rational composition matching and process design. These carbide precipitate phases not only improve the strength of the steel but also function as powerful hydrogen traps that fix diffusible hydrogen, which is beneficial for improving delayed cracking.

[0079] Furthermore, the present invention utilizes a short aging treatment to promote hydrogen overflow, significantly reducing the diffusible hydrogen content in the steel sheet. As a result, the steel possesses ultra-high strength and excellent resistance to hydrogen embrittlement.

[0080] (1) The present invention achieves properties that combine the formability and weldability of duplex steel by optimizing the chemical composition and manufacturing process design, controlling the content of C and Si in the steel sheet, and ensuring a constant ferrite ratio in the microstructure.

[0081] (2) The present invention can promote the dispersion and precipitation of nanoscale carbide particles in steel, and these carbide particles can not only improve the strength of the steel but also function as a powerful hydrogen trap for fixing diffusible hydrogen, which is beneficial for improving the delayed cracking performance of the steel.

[0082] (3) In the present invention, the hot-dip galvanizing recrystallization annealing process has been optimized so that the steel sheet can be flexibly tempered after galvanizing. This process not only promotes the overflow of diffusible hydrogen, but also causes ε-carbide to precipitate and increases hydrogen trapping, which significantly reduces the diffusion coefficient of hydrogen in the martensite and consequently improves the delayed crack resistance of the steel sheet.

[0083] From the above, it can be seen that the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength steel provided by the present invention can achieve extremely excellent plasticity by controlling the relative content of ferrite and martensite and comprehensively utilizing microstructure control means such as fine-grain strengthening and precipitation strengthening.

[0084] At the same time, this steel not only possesses excellent hydrogen-induced cracking resistance but also has a certain degree of manufacturability and formability. The production of this hydrogen-induced cracking-resistant cold-rolled hot-dip galvanized ultra-high-strength steel can be carried out using existing, conventional hot-dip galvanized high-strength steel sheet production lines without significant adjustments. It can be used in the manufacture of vehicle parts such as automotive structural components and collision prevention components, and has a good outlook for widespread adoption and application. [Brief explanation of the drawing]

[0085] [Figure 1]Figure 1 schematically shows, in one embodiment, the control process during the continuous hot-dip galvanizing recrystallization annealing process and cooling after galvanizing, in the method for producing hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength steel of the present invention. [Figure 2] Figure 2 is a photograph of the microstructure of the hydrogen-resistant, cold-rolled, hot-dip galvanized ultra-high-strength duplex steel from Example 1. [Modes for carrying out the invention]

[0086] The following interpretations and explanations will be made based on the drawings and specific embodiments of the specification, but these interpretations and explanations will not unduly limit the technical proposal of the present invention.

[0087] Examples 1-7 The hydrogen-resistant, cold-rolled, hot-dip galvanized ultra-high-strength duplex steels of Examples 1-7 of the present invention were all manufactured by the following process.

[0088] (1) The chemical elements were smelted and cast according to the mass percentage composition of the chemical elements shown in Table 1. (2) Hot rolling: The obtained slab was hot-rolled, heated to 1180-1280°C, the holding time was controlled to 0.5-4 hours, the final rolling temperature was controlled to 850°C or higher, and the hot-rolled sheet was wound at a winding temperature of 700°C or lower.

[0089] (3) Pickling: The hot-rolled sheets were pickled after hot rolling. (4) Cold rolling: The hot-rolled sheet after pickling was subjected to cold rolling deformation, and the amount of cold rolling deformation was controlled to 30-70%.

[0090] (5) Hot-dip galvanizing recrystallization annealing: (a) The steel plate was heated to a soaking temperature T1 at a heating rate V1 of 1 to 20°C / s, and then kept warm for 30 to 240 seconds, where T1 > 760°C; (b) The soaked steel plate was cooled to an intermediate temperature T2 at a cooling rate V2 of 2 to 20°C / s, where T2 = 600 to 780°C; (c) The steel plate was cooled to the zinc plating temperature T3 at a cooling rate V3 of 5-60°C / s, and then kept warm for 20-300 seconds, where V3 > V2 and the zinc plating temperature was set to 400-500°C; (d) The steel plates were sent to a zinc pot for galvanizing.

[0091] (6) Cooling after zinc plating: After zinc plating was completed, the material was rapidly cooled to below 200°C at a cooling rate of 10°C / s or more, held in the temperature range of 100°C to 300°C for 15 to 100 seconds, and then cooled to room temperature at a cooling rate of 5°C / s or more to obtain the corresponding hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength steel.

[0092] In the present invention, the chemical elemental composition and related process design of the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel in Examples 1-7 of the present invention all satisfy the requirements of the design specifications of the present invention.

[0093] Table 1 lists the mass percentages of each chemical element in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel of Examples 1-7.

[0094] [Table 1]

[0095] Tables 2-1 and 2-2 list the specific process parameters used in the above manufacturing process steps for the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel of Examples 1-7.

[0096] [Table 2-1]

[0097] [Table 2-2]

[0098] In the above manufacturing process, before zinc-plating the manufactured substrates, the inventor took samples of the substrates manufactured in each example to analyze the microstructure of the substrates. Using a metallurgical microscope and a transmission electron microscope (TEM), the inventor observed and analyzed the microstructure of the substrate samples manufactured in Examples 1-7, and the relevant observation and analysis results are shown in Table 3 below.

[0099] Table 3 lists the microstructural observation and analysis results for hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel substrates in Examples 1-7.

[0100] [Table 3]

[0101] As shown in Table 3 above, in the present invention, the matrix of the microstructure of the substrates produced in Examples 1-7 is ferrite + martensite, and the microstructure also includes carbide precipitate phases with a size of 15-70 nm. Here, the volume phase ratio of ferrite in the microstructure of the substrates in Examples 1-7 is 20-40%. In the substrates of Examples 1-7 produced according to the present invention, it was observed that the carbide precipitate phase and the matrix were in an aggregated or semi-aggregated state.

[0102] Therefore, after completing the observation and analysis of the microstructure described above, in order to further explain the excellent properties of the hydrogen crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel of Examples 1-7 manufactured according to the present invention, the inventors further took samples of the finished hydrogen crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel manufactured in Examples 1-7, and performed various performance tests on the samples of the hydrogen crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel of Examples 1-7, and the relevant test results are shown in Table 4.

[0103] The relevant performance test methods are as follows: (1) Tensile test: In accordance with GB228.1-2021, a plate tensile test specimen with a gauge length of 50 mm was used to measure and determine the yield strength, tensile strength, and elongation of the steel material in each example in the laboratory.

[0104] (2) Hydrogen-induced cracking test: The diffusible hydrogen content of the steel material in each example was measured and determined in accordance with ISO 3690-2012. The higher the diffusible hydrogen content, the lower the resistance to hydrogen-induced cracking of the steel material and the higher the risk of delayed cracking. The lower the diffusible hydrogen content, the higher the resistance to hydrogen-induced cracking of the steel material and the lower the risk of delayed cracking.

[0105] Table 4 lists the performance test results of the hydrogen-induced crack-resistant cold-rolled, hot-dip galvanized ultra-high-strength duplex steels of Examples 1-7.

[0106] [Table 4]

[0107] Referring to Table 4, it can be seen that the hydrogen-induced crack-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steels obtained in Examples 1-7 according to the present invention all possess excellent mechanical properties, with a yield strength of 840-940 MPa, a tensile strength of 1190-1260 MPa, and an elongation of 8-11%.

[0108] Furthermore, the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-tensile duplex steels of Examples 1-7 designed according to the present invention also exhibit excellent hydrogen-induced cracking resistance, with a diffusible H content of 0.07-0.15 ppm, and have good prospects for widespread adoption and application.

[0109] Figure 1 schematically shows, in one embodiment, the control process during the continuous hot-dip galvanizing recrystallization annealing process and cooling after galvanizing, in the method for producing hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength steel of the present invention.

[0110] As shown in Figure 1, in the present invention, when performing a continuous hot-dip galvanizing recrystallization annealing process, first the cold-rolled sheet is heated to a soaking temperature T1 at a heating rate V1 and then held for a time t1. After soaking, the steel sheet is first slowly cooled to an intermediate temperature T2 at a cooling rate V2, then rapidly cooled to the galvanizing temperature T3 at a cooling rate V3, and then held for a time t3. After the above steps are completed, the steel sheet is sent to a zinc pot for galvanizing.

[0111] Accordingly, after the zinc plating is complete, the steel sheet needs to be cooled. The zinc-plated steel sheet is rapidly cooled to a holding temperature T4 at a cooling rate V4, held at this holding temperature for a time t4, and finally cooled at a cooling rate V5.

[0112] Figure 2 is a photograph of the microstructure of the hydrogen-resistant, cold-rolled, hot-dip galvanized ultra-high-strength duplex steel from Example 1.

[0113] As shown in Figure 2, in this embodiment, the volume fraction of light-colored ferrite in the hydrogen-resistant cold-rolled hot-dip galvanized ultra-high-strength duplex steel of Example 1 is approximately 30%, and at the same time, the micro-alloying effect of the micro-alloy elements makes the microstructure particles finer.

[0114] Furthermore, the combination methods of each technical feature in this application are not limited to the combination methods described in the claims of this application or the combination methods specifically described in the embodiments. All technical features described in this application can be freely combined or combined in any way, as long as they do not contradict each other.

[0115] It should be noted that the embodiments listed above are only specific embodiments of the present invention. Of course, the present invention is not limited to the embodiments above, and similar modifications or variations made based on the embodiments, which can be directly derived or easily conceived by those skilled in the art from the content disclosed herein, fall within the scope of protection of the present invention.

Claims

1. A cold-rolled, hot-dip galvanized duplex steel comprising a substrate and a zinc plating layer plated on the substrate, The substrate contains the following chemical elements in the following mass percentages: C: 0.1–0.18%, Mn: 2.2–3.0%, Si: 0.2–0.6%, Al: 0.03–0.3%, Nb: 0.01–0.1%, Ti: 0.01–0.1%, Mo: 0.04–0.2%, B: 0.0005–0.003%; optionally, at least one of V: ​​0.005–0.2%, Cr: 0.01–0.8%, Cu: 0.003–0.5%; the remainder being Fe and unavoidable impurity elements; The matrix of the microstructure of the substrate is ferrite + martensite, and the microstructure also includes a carbide precipitate phase, the size of which is less than 100 nm. The cold-rolled, hot-dip galvanized duplex steel is characterized by having a yield strength of ≥ 800 MPa, a tensile strength of ≥ 1180 MPa, an elongation A ≥ 6%, and a diffusible hydrogen content of ≤ 0.2 ppm.

2. The cold-rolled hot-dip galvanized duplex steel according to claim 1, characterized in that the mass percentage content of Si, Al, Mo, and Cr in the substrate satisfies Si + Al + Cr + Mo ≤ 1.0%.

3. The cold-rolled hot-dip galvanized duplex steel according to claim 1, characterized in that, among the unavoidable impurities in the substrate, the mass percentage content of P, S, and N satisfies P ≤ 0.02%, S ≤ 0.01%, and N ≤ 0.008%.

4. The cold-rolled hot-dip galvanized duplex steel according to claim 1, characterized in that the carbide precipitate phase and the matrix are in an aggregated or semi-aggregated state.

5. The cold-rolled hot-dip galvanized duplex steel according to claim 1, characterized in that the volume phase ratio of ferrite in the microstructure of the substrate is 10 to 40%.

6. The cold-rolled hot-dip galvanized duplex steel according to claim 1, characterized in that the thickness of the zinc plating layer on one side of the substrate is 5 to 200 μm.

7. A method for manufacturing cold-rolled hot-dip galvanized duplex steel according to any one of claims 1 to 6, comprising the following steps: (1) Smelting and casting; (2) Hot rolling; (3) Pickling; (4) Cold rolling; (5) Hot-dip galvanizing recrystallization annealing: (a) Heat the steel plate to a soaking temperature T1 at a heating rate V1 of 1 to 20°C / s, and hold the temperature for 30 to 240 seconds, where T1 > 760°C; (b) The soaked steel plate is cooled to an intermediate temperature T2 at a cooling rate V2 of 2 to 20°C / s, where T2 = 600 to 780°C; (c) The steel plate is cooled to a zinc plating temperature T3 at a cooling rate V3 of 5 to 60°C / s, and then kept warm for 20 to 300 seconds, where V3 > V2 and the zinc plating temperature is 400 to 500°C; (d) Send the steel plate to the zinc pot for galvanizing; (6) Cooling after zinc plating: After zinc plating is complete, rapidly cool to below 200°C at a cooling rate of 10°C / s or more, hold in the temperature range of 100°C to 300°C for 15 to 100 seconds, and then cool to room temperature at a cooling rate of 5°C / s or more. A method for producing cold-rolled hot-dip galvanized duplex steel, characterized by including the following:

8. The manufacturing method for the 7, characterized in that, in step (2) above, the slab is heated to 1180 to 1280°C, the holding time is 0.5 to 4 hours, the final rolling temperature is controlled to 850°C or higher, and the winding temperature is controlled to 700°C or lower.

9. The manufacturing method according to claim 7, characterized in that in step (4) above, the amount of cold rolling deformation is controlled to 30 to 70%.

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