Zinc-based coating hot forming steel suitable for direct hot forming production and production method

Abstract

The invention relates to zinc-based coating hot forming steel suitable for direct hot forming production and a production method, and belongs to the technical field of coating. The hot-formed steel substrate with the zinc-based coating comprises the following chemical components in percentage by mass: 0.19 to 0.22 percent of C, 1.92 to 2.50 percent of Mn, 0.20 to 0.28 percent of Si, 0.0030 to 0.0050 percent of B, 0.020 to 0.030 percent of Ti, less than or equal to 0.005 percent of S, less than or equal to 0.022 percent of P, 0.030 to 0.050 percent of Als, 0.15 to 0.30 percent of Cr, less than or equal to 0.0050 percent of N and the balance of Fe and inevitable impurities. The zinc-based coating hot-formed steel production method comprises a hot-dip galvanizing process and a hot stamping forming process, is suitable for a hot-formed steel product produced by a direct hot forming process, can avoid the problem of liquid metal cracks, is stable in product quality, low in production cost and high in production efficiency, and obviously improves the market competitiveness of the product.

Description

Technical Field

[0001] The invention belongs to the technical field of coating and plating, and in particular relates to zinc-based coated hot-formed steel suitable for direct hot forming production and a production method. Background Art

[0002] Hot-formed steel has the characteristics of high strength and can be used to produce parts with complex shapes. Compared with ordinary high-strength steel, it can reduce the weight of parts, so hot-formed steel is widely used.

[0003] Zinc-based coated hot-formed steel is a type of hot-formed steel that combines the advantages of low cost of uncoated products and high corrosion resistance of aluminum-silicon products. It is lower in cost than aluminum-silicon products and has become a new variety that major OEMs are competing to try out.

[0004] The production of zinc-based coated hot-formed steel is prone to liquid metal brittle cracks, which restricts its promotion and application. Hot forming processes include direct hot forming processes and indirect hot forming processes. Since the direct hot forming process is more likely to produce liquid metal brittle cracks, most of the current production is produced using the indirect hot forming process. The indirect hot forming process is to first cold-form the steel plate to 90%, and then heat it to complete austenitization for hot stamping. Since cold forming is required first, the production process is increased, the production cost is increased, and the production efficiency is affected. At the same time, it is necessary to add cold forming stamping dies, which also increases the die cost and maintenance cost.

[0005] Therefore, how to use direct hot forming process to produce zinc-based coated hot-formed steel to avoid the problem of liquid metal brittle cracks, ensure product quality and improve production efficiency has become a problem that needs to be solved.

[0006] Chinese patent CN 107127238 discloses a hot stamping forming method for zinc-coated steel sheets or strips, which requires annealing the steel coil at 500-800℃ for 6-24h before taking it out of the furnace. The annealing time in the annealing furnace is long and the production efficiency is low. This production method does not have a preheating process, and directly heats to a temperature above Ac3 for austenitization. Due to the high heating temperature, when the heating temperature is higher than the melting point of the zinc layer, the surface zinc layer will melt and Zn will volatilize. After the subsequent hot forming process, the surface quality will deteriorate after the zinc layer melts and solidifies, and the corrosion resistance of the product will deteriorate.

[0007] Chinese patent CN 115029632 discloses a highly corrosion-resistant galvanized hot-formed hardened steel and its components and a preparation method. The method requires cold stamping pre-forming and is a typical product produced using an indirect hot forming method, with low production efficiency and high production cost.

[0008] Chinese patent CN 115125439 discloses a zinc-based coated 1800Mpa grade hot stamping steel and a preparation method thereof. This method also produces products by using an indirect hot forming method, with low production efficiency and high production cost. Summary of the invention

[0009] The present invention provides a method for producing zinc-based coated hot-formed steel suitable for direct hot-forming production, which can avoid the problem of brittle cracks of liquid metal, save production costs and improve production efficiency.

[0010] In order to solve the above technical problems, the technical solution adopted by the present invention is: a zinc-based coated hot-formed steel suitable for direct hot forming production, the chemical composition and mass percentage of the substrate of the zinc-based coated hot-formed steel are: C: 0.19-0.22%, Mn: 1.92-2.50%, Si: 0.20-0.28%, B: 0.0030-0.0050%, Ti: 0.020-0.030%, S≤0.005%, P≤0.022%, Als: 0.030-0.050%, Cr: 0.15-0.30%, N≤0.0050%, and the rest are Fe and unavoidable impurities.

[0011] The yield strength of the zinc-based coated hot-formed steel of the present invention is ≥1100MPa, the tensile strength is ≥1600MPa, and the elongation A80 is ≥5%.

[0012] The specification of the zinc-based coated hot-formed steel of the present invention is 1.4-1.8 mm.

[0013] Another object of the present invention is to provide a method for producing zinc-based coated hot-formed steel suitable for direct hot forming production, the production method comprising a hot-dip galvanizing process and a hot stamping forming process, and the specific steps are as follows:

[0014] 1) Hot dip galvanizing process:

[0015] The production steps of galvanized steel strip for hot stamping steel are as follows:

[0016] (1) Smelting and casting according to the composition requirements to form a casting billet;

[0017] (2) Hot rolling process: the ingot is heated and then rolled, with the final rolling temperature being 860-900°C and the coiling temperature being 620-650°C;

[0018] (3) Cold rolling process: cold rolling the steel coil to 1.4-1.8 mm, with a cold rolling reduction rate of 45-60%;

[0019] (4) Continuous hot-dip galvanizing process: soaking zone temperature 770-790°C, strip entering zinc pot temperature 460-480°C, zinc pot temperature 457-463°C;

[0020] (5) Skin finishing process: skin finishing machine elongation 0.7~1.0%;

[0021] 2) Hot stamping process:

[0022] Using galvanized steel strip, the production steps of zinc-based coated hot-formed steel include blanking, preheating, heating, precooling, and hot stamping forming processes;

[0023] (1) Blanking process: blanking the zinc-coated steel sheet and cutting the steel sheet according to the shape and size of the finished parts;

[0024] (2) Preheating process: Place the fallen sheet into an annealing furnace and heat it at a preheating temperature of 500-700°C for 260-380 seconds;

[0025] (3) Heating process: reheat to 890-910°C and keep warm for 120-240s to make the steel plate austenitized evenly;

[0026] (4) Pre-cooling process: After the steel plate comes out of the heating furnace, the heated steel plate is quickly transferred to the forming mold, and then pre-cooled to 620-690°C at a cooling rate of ≥35°C / s;

[0027] (5) Hot stamping forming process: forming quenching is performed on the die, the forming temperature range is 550-650°C, and the cooling rate is 40-60°C / s.

[0028] The (2) preheating process of the present invention is: firstly heating to 400-430°C, with a heating rate V1≤18°C / s, and after the temperature reaches 430°C, heating to a preheating temperature of 500-700°C at a heating rate V2≤8°C / s.

[0029] The (5) hot stamping forming process of the present invention has a holding time of 5 to 19 seconds and a holding pressure of 400 to 1200 tons during hot stamping.

[0030] The zinc-based coated hot-formed steel of the present invention has a yield strength of ≥1100 MPa, a tensile strength of ≥1600 MPa, and an elongation A80 of ≥5%.

[0031] The zinc-based coated hot-formed steel of the present invention has no cracks when observing the metallographic structure cross section.

[0032] The beneficial effects of the above technical solution are: 1. The present invention optimizes the galvanizing process and develops a production method for zinc-based coated hot-formed steel suitable for direct hot forming production, which can avoid the problem of liquid metal brittle cracks and ensure stable product quality; 2. Compared with the traditional indirect hot forming method, the present invention can save production costs, improve production efficiency, and significantly enhance the market competitiveness of products. 3. The hot stamping products produced by the present invention have good strength and toughness. The yield strength of the hot stamping products is ≥1100MPa, the tensile strength is ≥1600MPa, and the elongation A80 is ≥5%. DETAILED DESCRIPTION

[0033] The present invention will be further described in detail below in conjunction with specific embodiments.

[0034] Examples 1-8

[0035] A method for manufacturing zinc-based coated hot-formed steel for direct hot forming production includes a hot-dip galvanizing process and a hot stamping forming process, and the specific steps are as follows:

[0036] 1) Hot dip galvanizing process:

[0037] The production steps of galvanized steel strip for hot stamping steel are as follows:

[0038] (1) Smelting and casting according to the composition requirements to form a casting billet;

[0039] (2) Hot rolling process: the ingot is heated and then rolled, with the final rolling temperature being 860-900°C and the coiling temperature being 620-650°C;

[0040] (3) Cold rolling process: cold rolling the steel coil to 1.4-1.8 mm, with a cold rolling reduction rate of 45-60%;

[0041] (4) Continuous hot-dip galvanizing process: soaking zone temperature 770-790°C, strip entering zinc pot temperature 460-480°C, zinc pot temperature 457-463°C;

[0042] (5) Skin-passing process: the skin-passing mill elongation is 0.7-1.0%; the process control parameters of hot rolling, cold rolling, continuous hot-dip galvanizing and skin-passing processes of each embodiment are shown in Table 1.

[0043] 2) Hot stamping process:

[0044] Using galvanized steel strip, the production steps of zinc-based coated hot-formed steel include blanking, preheating, heating, precooling, and hot stamping forming processes;

[0045] (1) Blanking process: blanking the zinc-coated steel sheet and cutting the steel sheet according to the shape and size of the finished parts;

[0046] (2) Preheating process: Place the fallen sheet into the annealing furnace and heat it to 400-430°C, with a heating rate V1≤18°C / s. After the temperature reaches 430°C, the heating rate V2≤8°C / s. The preheating temperature is 500-700°C and the temperature is kept for 260-380s.

[0047] (3) Heating process: reheat to 890-910°C and keep warm for 120-240s to make the steel plate austenitized evenly;

[0048] (4) Pre-cooling process: After the steel plate comes out of the heating furnace, the heated steel plate is quickly transferred to the forming mold, and then pre-cooled to 620-690°C at a cooling rate of ≥35°C / s;

[0049] (5) Hot stamping process: forming quenching is performed on the die, the forming temperature range is 550-650°C; the holding time during hot stamping is 5-19s, the holding pressure is 400-1200t, and the cooling rate is 40-60°C / s. The hot stamping process parameters of each embodiment are shown in Table 2.

[0050] The chemical composition and weight percentage of the substrate of the zinc-based hot-formed steel product of Examples 1-8 are shown in Table 3, and the properties of the zinc-based hot-formed steel are shown in Table 4.

[0051] Table 1 Process control parameters of hot rolling, cold rolling, continuous hot dip galvanizing and skin finishing in various embodiments

[0052]

[0053] Table 2 Hot stamping process control parameters of various embodiments

[0054]

[0055] Table 3 Chemical composition of zinc-based coating substrates in various embodiments by weight percentage (%)

[0056]

[0057]

[0058] Table 4 Performance of zinc-based hot-formed steel products in various embodiments

[0059] Example Tensile strength / Mpa Yield strength / Mpa Elongation / % Surface cracks 1 1663 1231 6.0 No cracks 2 1742 1289 5.5 No cracks 3 1750 1295 5.0 No cracks 4 1682 1245 5.5 No cracks 5 1600 1100 6.5 No cracks 6 1615 1195 6.0 No cracks 7 1705 1262 5.0 No cracks 8 1635 1210 5.5 No cracks

[0060] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. Zinc-based coated hot-formed steel suitable for direct hot forming production, characterized in that: The chemical composition and mass percentage of the substrate of the zinc-based hot-formed steel are: C: 0.19-0.22%, Mn: 1.92-2.50%, Si: 0.20-0.28%, B: 0.0030-0.0050%, Ti: 0.020-0.030%, S≤0.005%, P≤0.022%, Als: 0.030-0.050%, Cr: 0.15-0.30%, N≤0.0050%, and the rest are Fe and unavoidable impurities.

2. The zinc-based coated hot-formed steel suitable for direct hot forming production according to claim 1, characterized in that: The yield strength of the zinc-based coated hot-formed steel is ≥1100MPa, the tensile strength is ≥1600MPa, and the elongation A80 is ≥5%.

3. The zinc-based coated hot-formed steel suitable for direct hot forming production according to claim 1, characterized in that: The specification of the zinc-based hot-formed steel is 1.4-1.8 mm.

4. The method for producing zinc-based coated hot-formed steel suitable for direct hot forming production according to any one of claims 1 to 3, characterized in that: The production method includes a hot-dip galvanizing process and a hot stamping forming process, and the specific steps are as follows: 1) Hot dip galvanizing process: The production steps of galvanized steel strip for hot stamping steel are as follows: (1) Smelting and casting according to the composition requirements to form a casting billet; (2) Hot rolling process: the ingot is heated and then rolled, with the final rolling temperature of 860-900°C and the coiling temperature of 620-650°C; (3) Cold rolling process: cold rolling the steel coil to 1.4-1.8 mm, with a cold rolling reduction rate of 45-60%; (4) Continuous hot-dip galvanizing process: soaking zone temperature 770-790°C, strip entering zinc pot temperature 460-480°C, zinc pot temperature 457-463°C; (5) Finishing process: the elongation of the finishing machine is 0.7-1.0%; 2) Hot stamping process: Using galvanized steel strip, the production steps of producing zinc-based coated hot-formed steel include blanking, preheating, heating, precooling, and hot stamping forming processes; (1) Blanking process: blanking the zinc-coated steel sheet and cutting the steel sheet according to the shape and size of the finished parts; (2) Preheating process: Place the fallen sheet into the annealing furnace for heating at a preheating temperature of 500-700°C for 260-380 seconds; (3) Heating process: reheat to 890-910°C and keep warm for 120-240s to make the steel plate austenitized evenly; (4) Pre-cooling process: After the steel plate comes out of the heating furnace, the heated steel plate is quickly transferred to the forming mold, and then pre-cooled to 620-690°C at a cooling rate of ≥35°C / s; (5) Hot stamping forming process: forming quenching is performed on the die, and the forming temperature range is 550-650°C; the holding time during hot stamping is 5-19s, the holding pressure is 400-1200t, and the cooling rate is 40-60°C / s.

5. The method for producing zinc-based coated hot-formed steel suitable for direct hot forming production according to claim 4, characterized in that: The (2) preheating process: first heat to 400-430°C, with a heating rate V1≤18°C / s, and after the temperature reaches 430°C, heat to a preheating temperature of 500-700°C at a heating rate V2≤8°C / s.

6. The method for producing zinc-based coated hot-formed steel suitable for direct hot forming production according to claim 4, characterized in that: The (5) hot stamping forming process: the holding time during hot stamping is 5 to 19 seconds, and the holding pressure is 400 to 1200 tons.

7. The method for producing zinc-based coated hot-formed steel suitable for direct hot forming production according to claim 4, characterized in that: The zinc-based coated hot-formed steel has a yield strength of ≥1100MPa, a tensile strength of ≥1600MPa, and an elongation A80 of ≥5%.

8. The method for producing zinc-based coated hot-formed steel suitable for direct hot forming production according to claim 4, characterized in that: The zinc-based coated hot-formed steel has no cracks on the surface through metallographic observation.

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