A kind of easily weldable hot-dip galvanized TRIP steel sheet with 600MPa grade and high surface quality and its preparation method

Abstract

The invention discloses an easily weldable hot-dip galvanized TRIP steel sheet with a 600 MPa grade and a preparation method thereof, aiming to solve the technical problems in the prior art such as poor welding performance of hot-dip galvanized TRIP steel sheets caused by high carbon equivalent and poor surface quality caused by high contents of Si and Mn elements; by adding a certain content of Al to replace part of Si on the basis of C-Mn steel, a low-carbon equivalent and low-Pcm alloy system is designed, improving the welding performance of high-strength TRIP steel sheets; at the same time, the low contents of Si and Mn reduce the surface oxides of the TRIP steel. With a reasonable pre-oxidation-reduction process and control of the pretreatment cleaning section, the surface quality has no defects and is good. The surface quality of the strip steel reaches grade C. The good welding performance and high surface quality enable the 600 MPa grade hot-dip galvanized TRIP steel sheet to expand its application in the automotive field and have a broad market prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of automotive steel, and particularly relates to a 600MPa grade hot-dip galvanized TRIP steel sheet with easy weldability and high surface quality and a preparation method thereof. Background Art

[0002] With the increasing requirements for environment, safety and greenness, automotive steel is moving towards lightweight. In particular, the development and application of the new generation of automotive TRIP steel has further improved the mechanical properties, formability and collision energy absorption of automotive steel. However, with the increase in the strength of TRIP steel, more alloying elements often need to be added. Especially, a large amount of alloying elements such as Si and Mn that reduce weldability are added to obtain the TRIP effect. Therefore, the weldability of high-strength TRIP steel is an important factor restricting the application of TRIP steel.

[0003] The application proportion of hot-dip galvanized steel sheets in the field of automotive steel is increasing. Its hot-dip galvanized sheets have good corrosion resistance and low cost, and are important materials for automotive body panels and structural parts. The surface quality of hot-dip galvanized steel sheets is an important factor affecting their application. The surface quality of hot-dip galvanized strip steel is greatly affected by the pretreatment and the substrate composition. Alloying elements and surface attachments have a great impact on the surface quality after plating.

[0004] While hot-dip galvanized TRIP steel is widely used in the field of automotive steel due to its good mechanical properties, excellent formability, good corrosion resistance and low cost, there are two reasons restricting its wider application: one is the surface quality defects caused by the precipitation of alloying elements such as Si and Mn on the iron-based surface, and the surface quality defects caused by attachments such as iron powder and emulsion during the pretreatment process of the substrate surface; the other is that the high content of elements such as C, Si and Mn affects the Weld Crack Sensitivity Index (Pcm) and Carbon Equivalent Value (CEV), resulting in a decline in its weldability and restricting its wide application. Due to its high carbon equivalent, the weldability of TRIP steel is poor. In hot-dip galvanized TRIP steel, the excessive content of alloying elements Si and Mn leads to external oxidation on the strip surface, generating Si and Mn oxides on the strip surface, resulting in poor wettability of the strip surface and causing galvanized surface defects. In addition, due to the presence of oxides on the strip surface, the surface roughness increases, and impurities such as residual alkali liquor, emulsion and iron powder remain on the surface, further deteriorating the surface quality and resulting in defects such as black spots on the surface of hot-dip galvanized strip steel. Summary of the Invention

[0005] In order to overcome the defects existing in the above-mentioned prior art, the object of the present invention is to provide an easily weldable hot-dip galvanized TRIP steel sheet with high surface quality at 600 MPa level and its preparation method, aiming to solve the problems of poor welding performance caused by high carbon equivalent and poor surface quality caused by high contents of Si and Mn elements in the existing hot-dip galvanized TRIP steel sheet at 600 MPa level. In view of the problems occurring in the existing hot-dip galvanized TRIP steel at 600 MPa level, a low-carbon equivalent alloy system, a reasonable pre-oxidation process and a reasonable pre-cleaning process are designed to achieve a hot-dip galvanized TRIP steel at 600 MPa level with excellent welding performance and good surface quality.

[0006] In order to achieve the above object of the invention, the present invention provides an easily weldable hot-dip galvanized TRIP steel sheet with high surface quality at 600 MPa level. The chemical composition of the hot-dip galvanized TRIP steel sheet at 600 MPa level in mass percentage is as follows: C: 0.07 - 0.12%, Si: 0.1 - 0.35%, Mn: 0.85 - 1.35%, P: 0.01 - 0.035%, S: ≤0.02%, Al: 1.0 - 1.5%, Nb: 0.003 - 0.15%, Ti: 0.003 - 0.15%, V: 0.003 - 0.1%, B: 0.0001% - 0.005%, Cu ≤0.10%, where Nb + Ti + V ≤0.3%. The welding crack sensitivity index of the steel sheet: Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Cr / 20 + Mo / 15 + Ni / 60 + V / 10 + 5B ≤0.24; the carbon equivalent: CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ≤0.37. The carbon equivalent CEV of this patent is less than or equal to 0.37, and the welding crack sensitivity index Pcm is less than or equal to 0.24. Both of these two welding performance indexes are within the optimal values of CEV ≤0.4 and Pcm ≤0.25, and the welding performance is excellent.

[0007] The functions of each element in the steel:

[0008] C: An interstitial solid solution element in the steel and an austenite stabilizing element. If the content is too low, the C enrichment degree in austenite is insufficient to form retained austenite. If the content is too high, the welding performance is poor, and at the same time, the hardenability increases, and martensite is easily formed during the rapid cooling process with a large cooling rate and a low temperature. The carbon content C in the present invention is 0.07 - 0.12%.

[0009] Si: Plays a role of solid solution strengthening in the steel, increases the strength of ferrite, inhibits the formation of carbides, delays the bainite transformation, increases the carbon content in austenite, and increases the austenite stability. However, too high Si will cause oxides to form on the surface, reduce the wettability of the strip surface, and the surface quality will decline. The silicon content Si in the present invention is 0.1 - 0.35%.

[0010] Mn: Increases the stability of austenite, lowers the Ms point of martensitic transformation below room temperature. The Mn content in this invention is Mn: 0.85 - 1.35%.

[0011] P: Under the low Si content in this patent, the addition of P can increase the retained austenite content. P: 0.01 - 0.035%.

[0012] Al: Has an effect similar to Si, partially replaces Si, but Al does not affect the surface quality of the strip steel. The Al content in this patent is Al: 1.0 - 1.5%.

[0013] Nb: Can refine grains, inhibit the precipitation of cementite, inhibit pearlite transformation, and increase the carbon content in retained austenite. Nb in this invention: 0.003 - 0.15%.

[0014] Ti: Plays a role in precipitation strengthening. The precipitated phase helps to inhibit the growth of austenite grains and is helpful for improving the welding performance of the strip steel. Ti: 0.003 - 0.15%.

[0015] V: Plays a role in precipitation strengthening and refines ferrite grains. V in this invention: 0.003 - 0.1%.

[0016] Cu: Improves strength through grain refinement and precipitation strengthening, hinders the formation of carbides, and improves the stability of retained austenite. Cu ≤ 0.10%

[0017] A preparation method for an easily weldable hot-dip galvanized TRIP steel sheet with high surface quality at the 600 MPa level, the method comprising: a smelting process, a hot rolling process, a cold rolling and pickling process, a hot-dip galvanizing annealing process, and a skin pass process.

[0018] In the above technical solution, further, in the hot rolling process: the hot rolling heating temperature is 1150 - 1280 °C, the hot rolling holding time is 0.5 - 2 h, the starting rolling temperature is 1050 - 1150 °C, the final rolling temperature is 850 - 950 °C, and the coiling temperature is 550 - 650 °C. A coiling temperature of 550 - 650 °C can inhibit the precipitation of alloying elements Nb, Ti, and V, and at the same time avoid the bainite transformation temperature. The hot rolling structure is ferrite + pearlite.

[0019] Adopting low-temperature coiling in the hot rolling process can accelerate pearlite phase transformation, and at the same time is beneficial to the precipitation of the second phase. The precipitation of the second phase can effectively hinder dynamic recrystallization, ensure the uniformity of the hot rolling structure, thereby reducing the influence of recrystallization on the mechanical property fluctuations between the head and the middle of the hot rolled coil, and making the through-coil performance of the hot rolled coil stable.

[0020] Further, in the cold rolling and pickling process: after hot rolling and pickling, it undergoes multiple passes of rolling, and the total cold rolling reduction rate is 55 - 85%.

[0021] Furthermore, the galvanizing annealing process: the annealing temperature is 780 - 860°C, the strip is kept at 780 - 860°C in the annealing furnace for 40 - 140 s, the slow cooling temperature is 660 - 740°C, the rapid cooling temperature is 350 - 430°C. Subsequently, the strip is heated to 455 - 460°C through induction heating, and then the strip enters the zinc pot at 455 - 460°C, stays in the pot for 1 - 8 s, and is air-cooled to room temperature at a cooling rate of 5 - 35°C / s after leaving the zinc pot.

[0022] The zinc liquid composition of the zinc pot: the Al content is 0.18 - 0.22%, and Fe ≤ 0.016%. Precise control of the aluminum content in the zinc pot can ensure the stability of the zinc layer composition, and the low Fe content can ensure the uniformity of the zinc layer inhibition layer and the stability of the surface quality.

[0023] Two-phase zone annealing is adopted for annealing, and different annealing temperatures are used for different thicknesses to precisely control the two-phase zone structure, so that the two-phase zone structure is 40 - 55% ferrite and 45 - 60% austenite.

[0024] The rapid cooling temperature is 350 - 430°C. The two-phase zone structure is rapidly cooled to 350 - 430°C after slow cooling. The rapid cooling rate is 30 - 170°C / s. A large cooling rate is adopted to ensure avoiding the pearlite transformation and the nose temperature, and the rapid cooling temperature is above the martensite transformation temperature Ms. Using this large cooling rate for low-temperature cooling can promote the solution of more alloying elements, and also ensure the enrichment of C in austenite. Subsequently, it is induction-heated to 455 - 460°C. One is to meet the strip temperature requirement for entering the zinc pot, and the other is that the strip enters the bainite isothermal transformation, causing some carbon-rich austenite phases to transform into bainite and the C in some austenite to further enrich. After leaving the zinc pot, it is air-cooled to room temperature, and the room temperature structure is 45 - 65% ferrite, 15 - 40% bainite, and residual austenite ≥ 5%. Different annealing processes are adopted for different thickness specifications as shown in Table 1 below.

[0025] Table 1 Different annealing processes are adopted for different thickness specifications

[0026]

[0027] In the above technical solution, furthermore, the pre-treatment process before the cleaning section: the strip is alkali-washed with NaOH alkali solution with a concentration of more than 10%, and then washed with strong water flow. The high alkali solution concentration ensures the removal of emulsion, iron powder, foreign matter, oil stain, and rolling fluid on the strip surface. At the same time, the alkali solution neutralizes some Mn and Cr oxides, reducing the surface oxides of the strip, and the reflectivity of the strip ≥ 95%.

[0028] Obtaining strip steel with high surface quality mainly involves two processing technologies: one is the pre-treatment process before the cleaning section and the post-treatment process after skin pass rolling formulated to ensure that the strip steel has a clean surface, no foreign matter, no iron powder, no oil stain, no emulsion, and no rolling fluid, and the other is the pre-oxidation-reduction process formulated for the coating defects caused by the oxides formed by alloying elements on the surface of transformation-induced plasticity steel of 600 Mpa grade.

[0029] Post-treatment after skin pass rolling: The roughness Ra of the skin pass roll is ≤1.0, RPc is ≤100, the skin pass elongation is ≥0.6%, and the skin pass rolling force is ≥2500 kN.

[0030] Pre-oxidation-reduction process: The hydrogen content in the pre-oxidation section is 10 - 15%, the oxygen content in the pre-oxidation section is 1.0 - 1.5%, the dew point in the pre-oxidation section is -30 - -40°C, the hydrogen content in the rapid cooling section is 15 - 20%, and the dew point in the rapid cooling section is -35 - -45°C.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] In the present invention, a certain content of Al is added on the basis of C-Mn steel to replace part of Si, and low carbon equivalent and low Pcm are designed, so that its welding performance is excellent. At the same time, the low Si and Mn contents reduce the surface oxides of TRIP steel. Combined with a reasonable pre-oxidation-reduction process and the control of the pre-treatment cleaning section, the surface quality is defect-free, the surface quality is good, the surface quality of the strip steel reaches grade C, and the good welding performance and high surface quality make the 600 MPa grade hot-dip galvanized TRIP steel sheet have a wide market application prospect. Description of the Drawings

[0033] Figure 1 It is the hot-rolling microstructure diagram of the 600 MPa grade hot-dip galvanized TRIP steel sheet of the embodiment;

[0034] Figure 2 It is the schematic diagram of the galvanizing annealing process of the 600 MPa grade hot-dip galvanized TRIP steel sheet of the embodiment;

[0035] Figure 3 It is the finished product microstructure diagram of the 600 MPa grade hot-dip galvanized TRIP steel sheet prepared in the embodiment;

[0036] Figure 4 It is the SEM morphology of the zinc layer on the surface of the finished product of the 600 MPa grade hot-dip galvanized TRIP steel sheet of the embodiment. Detailed Embodiments

[0037] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited in any way. To avoid repetition, the raw materials in the following embodiments are commercially available without special instructions, and the quality grades are all industrial grades; the methods used are all conventional methods without special instructions. The steel plate composition in the embodiments meets the requirements: C: 0.07 - 0.12%, Si: 0.1 - 0.35%, Mn: 0.85 - 1.35%, P: 0.01 - 0.035%, S: ≤0.02%, Al: 1.0 - 1.5%, Nb: 0.003 - 0.15%, Ti: 0.003 - 0.15%, V: 0.003 - 0.1%, B: 0.0001% - 0.005%, Cu ≤0.10%, where Nb + Ti + V ≤0.3%, and the balance is Fe and other inevitable impurities. The chemical compositions of the molten steel of the steel billets in Examples 1 - 7 are shown in Table 2.

[0038] Table 2 Actual smelting chemical composition table of the examples (wt%)

[0039] Example C Si Mn P S Al Nb Ti V B Cu 1 0.08 0.13 0.9 0.015 0.008 1.22 0.13 0.005 0.004 0.003 0.02 2 0.09 0.31 1.15 0.026 0.008 1.39 0.007 0.14 0.009 0.002 0.06 3 0.09 0.24 1.07 0.023 0.004 1.27 0.09 0.1 0.007 0.002 0.04 4 0.11 0.11 0.89 0.016 0.004 1.48 0.005 0.006 0.1 0.004 0.09 5 0.08 0.33 1.32 0.031 0.006 1.04 0.1 0.11 0.008 0.001 0.02 6 0.08 0.11 0.88 0.012 0.006 1.02 0.005 0.006 0.004 0.0006 0.01 7 0.12 0.33 1.34 0.032 0.005 1.49 0.145 0.144 0.009 0.005 0.09

[0040] A preparation method of a 600MPa - grade hot - dip galvanized TRIP steel plate with easy weldability and high surface quality, the method comprising: a smelting process, a hot - rolling process, a cold - rolling and pickling process, a hot - dip galvanizing annealing process, and a skin - pass process.

[0041] Hot - rolling process: The hot - rolling heating temperature is 1150 - 1280°C, the hot - rolling holding time is 0.5 - 2h, the starting rolling temperature is 1050 - 1150°C, the final rolling temperature is 850 - 950°C, and the coiling temperature is 550 - 650°C. The hot - rolled structure is ferrite + pearlite.

[0042] Table 3 Control parameters of the hot - rolling process for each example

[0043]

[0044] Cold - rolling and pickling process: After multiple passes of rolling, the total cold - rolling reduction rate is 55 - 85%.

[0045] Table 4 Control parameters of the cold - rolling and pickling process for each example

[0046] Example Cold rolling reduction rate / % Cold rolled sheet thickness / mm Example Cold rolling reduction rate / % Cold rolled sheet thickness / mm 1-1 82 0.45 5-1 66 0.85 1-2 55 2.25 5-2 83 0.935 2-1 56 1.1 6-1 83 0.425 2-2 84 0.8 6-2 66 1.53 3-1 74 0.65 7-1 77 0.575 3-2 78 1.1 7-2 60 2.4 4-1 68 0.8 4-2 55 2.475

[0047] Hot - dip galvanizing process: The annealing temperature is 780 - 860°C, the strip is held at 780 - 860°C in the annealing furnace for 40 - 140s, the slow - cooling temperature is 660 - 740°C, the rapid - cooling temperature is 350 - 430°C, then the strip is heated to 455 - 460°C by induction heating, and then the strip enters the zinc pot at 455 - 460°C, stays in the pot for 1 - 8s, and is air - cooled to room temperature at a cooling rate of 5 - 35°C / s after exiting the zinc pot.

[0048] Annealing is carried out in the two-phase region. Different annealing temperatures are adopted for different thicknesses to precisely control the two-phase region structure, so that the two-phase region structure is 40 - 55% ferrite and 45 - 60% austenite.

[0049] The rapid cooling temperature is 350 - 430°C. The two-phase region structure is slowly cooled and then rapidly cooled to 350 - 430°C. The rapid cooling rate is 30 - 170°C / s. A large cooling rate is adopted to ensure avoiding pearlite transformation and the nose temperature, and the rapid cooling temperature is above the martensite transformation temperature Ms. This large cooling rate and low-temperature cooling promote the solution of more alloying elements, and also ensure the enrichment of C in austenite. Subsequently, it is induction heated to 455 - 460°C. One is to meet the strip entry temperature requirement of the galvanizing pot, and the other is that the strip enters the bainite isothermal transformation, so that part of the carbon-rich austenite phase transforms into bainite, and C in part of the austenite is further enriched. After exiting the galvanizing pot, it is air-cooled to room temperature. The room temperature structure is 45 - 65% ferrite, 15 - 40% bainite, and residual austenite ≥ 5%.

[0050] Table 5 Control parameters of the hot-dip galvanizing annealing process for each example

[0051]

[0052] Pre-treatment process in the cleaning section: The strip is pickled with 10 - 15% concentration of NaOH alkaline solution and then cleaned with strong water flow. The high alkaline solution concentration ensures the removal of emulsified liquid, iron powder, foreign matter, oil stain, and rolling fluid on the strip surface. At the same time, the alkaline solution neutralizes part of the Mn and Cr oxides, reducing the surface oxides of the strip, and the strip reflectivity ≥ 95%.

[0053] Post-treatment of skin pass: The roughness Ra of the skin pass roll ≤ 1.0, RPc ≤ 100, the skin pass elongation ≥ 0.6%, and the skin pass rolling force ≥ 2500 kN.

[0054] Pre-oxidation - reduction process: The hydrogen content in the pre-oxidation section is 10 - 15%, the oxygen content in the pre-oxidation section is 1.0 - 1.5%, the dew point in the pre-oxidation section is -30 - -40°C, the hydrogen content in the rapid cooling section is 15 - 20%, and the dew point in the rapid cooling section is -35 - -45°C.

[0055] Composition of the zinc bath in the galvanizing pot: The Al content is 0.18 - 0.22%, and Fe ≤ 0.016%. Precise control of the aluminum content in the galvanizing pot can ensure the stability of the zinc layer composition, and the low Fe content can ensure a uniform inhibition layer of the zinc layer and stable surface quality. The SEM of the finished product surface is as Figure 4 shown, the coating has no defects and good uniformity.

[0056] Table 6 Control parameters of the pre-treatment, post-treatment of skin pass (and pre-oxidation) processes in the cleaning section for each example

[0057]

[0058] The mechanical properties of the easily weldable hot-dip galvanized TRIP steel sheet with high surface quality and 600 MPa grade prepared in the examples are shown in Table 7.

[0059] Table 7 Mechanical properties of the 600 MPa grade hot-dip galvanized TRIP steel sheet prepared in each example

[0060] Example Yield strength / MPa Tensile strength / MPa Elongation / % n value 1-1 432 617 37 0.2 1-2 427 627 36.5 0.2 2-1 438 629 36.5 0.2 2-2 435 626 37 0.21 3-1 434 637 39 0.21 3-2 430 644 38 0.2 4-1 444 665 35 0.2 4-2 454 678 35 0.2 5-1 431 643 35.5 0.2 5-2 439 634 35.5 0.2 6-1 393 605 40 0.21 6-2 384 601 40 0.2 7-1 466 674 34.5 0.18 7-2 478 689 35 0.19

[0061] For any person skilled in the art, without departing from the scope of the technical solution of the present invention, many possible changes and modifications can be made to the technical solution of the present invention by using the technical content disclosed above, or it can be modified into an equivalent embodiment with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A hot-dip galvanized TRIP steel sheet with 600 MPa grade that is easy to weld and has high surface quality, characterized in that, The chemical composition of the 600 MPa grade hot-dip galvanized TRIP steel sheet is by mass percentage: C: 0.07% - 0.09%, Si: 0.10% - 0.35%, Mn: 0.85% - 1.15%, P: 0.01% - 0.035%, S ≤ 0.008%, Al: 1.02% - 1.50%, Nb: 0.003% - 0.09%, Ti: 0.11% - 0.15%, V: 0.003% - 0.1%, B: 0.0001% - 0.005%, Cu ≤ 0.10%, where Nb + Ti + V ≤ 0.3%; The welding crack sensitivity index Pcm of the hot-dip galvanized TRIP steel sheet is Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Cr / 20 + Mo / 15 + Ni / 60 + V / 10 + 5B ≤ 0.24; the carbon equivalent CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ≤ 0.37; The preparation method of the easily weldable high surface quality 600 MPa grade hot-dip galvanized TRIP steel sheet includes the following processes: smelting process, hot rolling process, cold rolling and pickling process, hot-dip galvanizing annealing process and skin pass process; In the hot rolling process, the hot rolling heating temperature is 1150°C - 1280°C, the hot rolling holding time is 0.5 - 2 h, the starting rolling temperature is 1050°C - 1150°C, the finishing rolling temperature is 850°C - 950°C, and the coiling temperature is 550°C - 650°C; In the cold rolling and pickling process, after hot rolling and pickling, it is rolled in multiple passes, and the total cold rolling reduction rate is 55% - 85%; In the galvanizing annealing process, the annealing temperature is 780°C - 860°C, the holding time is 40 - 140 s, the slow cooling temperature is 660°C - 740°C, the rapid cooling temperature is 350°C - 430°C. Then the strip is induction heated to 455°C - 460°C, and then enters the zinc pot. The time in the pot is 1 - 8 s. After exiting the zinc pot, it is air-cooled to room temperature at a cooling rate of 5 - 35°C / s. The composition of the zinc bath in the zinc pot: the Al content is 0.18% - 0.22%, Fe ≤ 0.016%; The annealing adopts two-phase region annealing, and different annealing temperatures are used for different thicknesses to precisely control the two-phase region structure, so that the two-phase region structure is 40% - 55% ferrite and 45% - 60% austenite; In the pre-treatment process before the cleaning section, the strip is alkali-washed with NaOH lye with a concentration of more than 10%, and then cleaned with strong water flow. The reflectivity of the strip is ≥ 95%; Skin pass post-treatment: the roughness Ra of the skin pass roll is ≤ 1.0, RPc ≤ 100, the skin pass elongation is ≥ 0.6%, and the skin pass rolling force is ≥ 2500 kN; Pre-oxidation-reduction process: the hydrogen content in the pre-oxidation section is 10% - 15%, the oxygen content in the pre-oxidation section is 1.0% - 1.5%, the dew point in the pre-oxidation section is -30°C - -40°C, the hydrogen content in the rapid cooling section is 15% - 20%, and the dew point in the rapid cooling section is -35°C - -45°C.

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