CN114411055A - A 220MPa grade bake hardening high-strength steel and its production method - Google Patents

Abstract

The invention discloses 220 MPa-grade bake-hardening high-strength steel and a production method thereof, wherein the steel comprises the following chemical components in percentage by mass: less than or equal to 0.0013 percent of C, 0.031-0.037 percent of Si, 0.53-0.59 percent of Mn, less than or equal to 0.010 percent of P, less than or equal to 0.003 percent of S, 0.067-0.079 percent of Al, less than or equal to 93C/12 percent of Nb, 0.0030-0.0060 percent of Ti, 0.0045-0.0065 percent of N, and the balance of Fe and inevitable impurities. The high-strength steel avoids secondary processing brittleness caused by high P content, avoids cost increase caused by high microalloy element content, reasonably utilizes Si element remained in molten iron and N element remained in molten steel, obtains strength, and ensures bake-hardening performance and good aging performance; the performance indexes of the high-strength steel are as follows: the yield strength is 240-252 MPa, the tensile strength is 350-375 MPa, the elongation is more than or equal to 41%, the plastic strain ratio r90 is more than or equal to 2.0, the n90 is more than or equal to 0.21, and the BH value is 40-45 MPa.

Description

A kind of 220MPa grade bake hardening high-strength steel and production method thereof

technical field

The invention relates to a bake hardening steel and a production method thereof, in particular to a 220MPa grade bake hardening high-strength steel and a production method thereof.

Background technique

Bake Hardening steel refers to this type of steel sheet that has a lower yield strength in the supply state after annealing and hot-dip galvanizing, and is suitable for stamping; the stamping parts produced are in the subsequent painting and baking process. , the yield strength is increased under the influence of temperature; this increased value is called the bake hardening value, or BH value for short. Bake hardening steels have received extensive attention in automotive exterior panels due to their high strength, good dent resistance and cold forming properties. The basic principle of bake hardening steel is to use the strain aging phenomenon of the steel plate. A certain amount of free C and/or N atoms is maintained in the steel plate. After the steel plate is stamped into an outer cover, after the surface is painted, the free C and/or N atoms diffuse into dislocations in the microstructure, and the formation of Coriolis air mass increases the yield strength of the part, increases the dent resistance of the part, and realizes the organic combination of product strength, deep drawing performance and dent resistance of the part.

In addition to certain strength, formability and dent resistance, automobile outer covers are also required to have good bake hardening properties and anti-aging properties, and need to avoid secondary processing brittleness. With the continuous development of lightweight manufacturing of automobiles, for example, the four-door and two-cover of automobiles have been thinned from 0.7mm to 0.65mm or even 0.60mm. Due to the gradual thinning of the outer cover, the bake-hardening steel plate also gradually transitions from 180MPa to 220MPa. For the research on 220MPa grade bake hardening steel sheet, the patent publication number CN101497959A (Wuhan Iron and Steel Group Corporation) discloses "220MPa grade cold-rolled bake hardening steel with yield strength and its production method", and its chemical composition C content is 0.05～0.10%, It can be seen that it is a low-carbon bake hardening steel; however, due to the high C content of this steel, the amount of fixed C element microalloy is insufficient, and it is difficult to stably control the solid solution carbon content in the industrial production process, resulting in an unstable bake hardening value. The aging period can only be guaranteed within 3 months, which is easy to cause stamping cracks and tensile strain marks on the surface of the parts when the steel plate is stamped. Patent Publication No. CN109161814A (Tangshan Iron and Steel Group Co., Ltd.) discloses "an ultra-low carbon bake-hardening steel plate and its production method", and its chemical composition has a relatively high content of P (0.045% to 0.060%); this material is difficult to use In the environment of extremely cold weather in northern China, especially in northeast China, in the case of extremely cold weather, the segregation of microstructure grain boundary of P element can easily lead to secondary processing brittleness and cause parts to crack, thereby causing automobile safety accidents; the material is fixed with a single Nb A part of C will lead to large fluctuation of r value, low n value, high yield strength, and high recrystallization temperature, which leads to unstable performance. At the same time, high recrystallization temperature causes energy waste, and industrial mass production cannot be stable and The connection and transition of steel leads to a decrease in the product qualification rate; at the same time, combined with the range of its solid solution C content (0.0008 to 0.0015%), it can be predicted that the aging period of the product will not exceed 3 months, and when it exceeds 3 months, it is easy to stamp again. To cause stamping orange peel or cracking, it is very unfavorable for market application that the current development direction of automobiles, especially the manufacturing demand of new energy vehicles, requires a guarantee period of 6 months. Patent Publication No. CN105970094A (Wuhan Iron and Steel Co., Ltd.) discloses "a production method of electro-galvanized bake-hardening steel sheet for automobile outer panels", in its chemical composition system, a single Nb microalloying is also used, resulting in large fluctuations in strength , and the plastic strain ratio (r value) is low, the coating is only electro-galvanized coating, and the production of cold-rolled continuous annealed products and hot-dip galvanized products through continuous annealing production lines and hot-dip galvanizing production lines is not introduced.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to provide a 220MPa grade bake hardening high-strength steel with stable bake hardening value, strong aging resistance and good comprehensive performance; the present invention also provides a 220MPa grade bake with stable and controllable process. Production method of hardened high-strength steel.

In order to solve the above-mentioned technical problems, the chemical composition and mass percentage content adopted by the present invention are: C≤0.0013%, Si 0.031%～0.037%, Mn 0.53%～0.59%, P≤0.010%, S≤0.003% , Al 0.067%～0.079%, Nb≤93C/12, Ti 0.0030%～0.0060%, N 0.0045%～0.0065%, the balance is Fe and inevitable impurities; the 93C/12 is the carbon content×93÷12 .

The mass percentages of the chemical components in the present invention are: C 0.0010%～0.0013%, Si 0.031%～0.037%, Mn 0.53%～0.59%, P≤0.010%, S≤0.003%, Al 0.067%～0.079% , Nb≤93C/12, Ti0.0030%～0.0060%, N 0.0045%～0.0065%, the balance is Fe and inevitable impurities.

The chemical composition of the present invention also includes V 0.03%-0.05% and/or B 0.001%-0.004%.

The method of the invention is as follows: adopting steel making, hot rolling, cold rolling and continuous annealing procedures to obtain cold rolled steel sheets, or adopting steel making, hot rolling, cold rolling, continuous annealing and galvanizing procedures to obtain galvanized steel sheets.

The continuous annealing process of the cold-rolled steel sheet according to the method of the invention is as follows: annealing at a soaking temperature of 770-780°C, and then cooling to 300-320°C at a cooling rate of 20-25°C/s in the cooling section for aging treatment for 150-200s , and finally cooled to room temperature.

The continuous annealing process of the galvanized steel sheet according to the method of the present invention is as follows: annealing at a soaking temperature of 780-790°C, and then cooling to 460-480°C at a cooling rate of 30-35°C/s in the cooling section for galvanizing.

The beneficial effects produced by the above technical solutions are: (1) The present invention is implemented through reasonable chemical composition design and process design, especially, the Nb and Ti composite microalloying of the steel plate of the present invention is supplemented by a certain amount of C, N, Solid solution strengthening and precipitation strengthening of Si, Mn and other elements make full use of a certain amount of low-cost Si element without affecting the performance of galvanizing and make full use of N element contained in molten steel, reducing the cost of production raw materials and the process of denitrification Cost; use a certain amount of solid solution N to obtain a stable bake hardening value, that is, the BH value, instead of relying entirely on a single solid solution C to obtain the BH value in the prior art; (2) Use the annealing temperature to adjust the high temperature of NbC The amount of decomposition can be used to obtain an appropriate amount of free carbon, and a lower heat treatment temperature is adopted, which saves energy and meets the national "double carbon" emission reduction requirements; (3) The product has achieved good mechanical properties, bake hardening properties and surface 220MPa grade bake-hardening high-strength steel with coating performance and corrosion resistance, and has good aging resistance, no aging occurs in 6 months or even 8 months, which ensures that the stamping parts of car companies will not produce aging surfaces for a long time. No orange peel phenomenon occurs; (4) High plastic strain ratio and high strain hardening index are obtained to ensure ideal formability and no cracking phenomenon; (5) The size of the parts remains stable after deformation and has good dent resistance after painting. Corrosion resistance; (6) It can be used to manufacture car hood outer panels, fender panels and side wall outer panels, which shows a good market application prospect.

The invention avoids secondary processing brittleness caused by high content of P, avoids the substantial increase in cost caused by high content of microalloying elements, and rationally utilizes residual Si element in molten iron and residual N element in molten steel to obtain strength while ensuring Baking hardening performance and good aging performance; the performance indicators of the present invention are: yield strength 240-252MPa, tensile strength 350-375MPa, elongation ≥41%, plastic strain ratio r90≥2.0, n90≥0.21, BH value 40～ 45MPa.

According to the method of the invention, through reasonable chemical composition design and matching process, the obtained steel plate has good mechanical properties, bake hardenability and aging resistance, and ensures that the stamping parts of the steel plate have good concave resistance and aging resistance. The parts have strong dimensional stability.

Description of drawings

The present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

Fig. 1 is the metallographic microstructure diagram of the obtained cold-rolled steel sheet of the present invention;

Fig. 2 is a metallographic microstructure diagram of the galvanized steel sheet obtained by the present invention.

Detailed ways

Examples 1-8: The 220MPa grade bake hardening high-strength steel and its production method are as follows.

(1) Examples 1-4 adopt the processes of steelmaking, hot rolling, cold rolling and continuous annealing to obtain cold-rolled steel sheets; Examples 5-8 adopt the processes of steelmaking, hot rolling, cold rolling, continuous annealing and galvanizing , to obtain a galvanized steel sheet; the cold-rolled steel sheet and the galvanized steel sheet have the same processes for steelmaking and hot-rolling.

1) The steelmaking process: using converter, LF refining and RH refining process; the oxygen at the end of converter steelmaking is 300-500ppm, and the oxygen at the end of RH decarburization is 200-300ppm, which is detected by TSO probe; , ferromanganese, aluminum particles and vanadium and/or boron) are added according to the content of carbon and nitrogen, that is, 1000*weight of molten steel*([target composition]-[component of molten steel])/(alloy yield*alloy content).

2) The hot rolling process: precipitation behavior of second phase particles, deformation and recrystallization of austenite, and ferrite transformation occur.

3) The cold rolling process: a high cold rolling reduction ratio is used to obtain a favorable {111} plane texture, thereby obtaining a high plastic strain ratio r value. The cold rolling reduction ratio of the cold rolled steel sheet is ≥81%; the cold rolling reduction ratio of the galvanized steel sheet is ≥75%.

4) The continuous annealing process: heat treatment is a key link in determining the microscopic grain size, the shape, distribution and size of the steel plate material, and finally determines the mechanical properties, bake hardening properties and aging properties of the steel plate.

① The continuous annealing process of the cold-rolled steel sheet is as follows: annealing at a soaking temperature of 770-780°C, then cooling to 300-320°C at a cooling rate of 20-25°C/s for aging treatment for 150-200s, and finally After cooling to room temperature, the flat rolling reduction rate is 1.50-1.70%; the thickness of the obtained cold-rolled steel sheet is 0.60-0.70 mm.

②The continuous annealing process of the galvanized steel sheet is as follows: annealing at a soaking temperature of 780-790°C, and then cooling to 460-480°C at a cooling rate of 30-35°C/s in the cooling section for galvanizing.

5) The galvanizing process: the galvanized steel sheet is cooled to room temperature at 10-20°C/s, and the smoothing reduction rate is 1.50-1.70%; the thickness of the obtained galvanized steel sheet is 0.60-0.70 mm.

According to the above process, the specific production process of each embodiment is shown in Table 1, wherein the embodiment 1-4 is the cold rolling continuous annealing treatment delivery product process, and the embodiment 5-8 is the hot-dip galvanizing delivery product process.

Table 1: the specific production process of each embodiment

(2) The chemical composition of the products obtained in each embodiment is shown in Table 2. The remainder in Table 2 is Fe and inevitable impurities, and the present invention is not limited to the chemical composition in Table 2.

Table 2: Chemical composition of the products obtained in each example

(3) It can be seen from Figure 1 and Figure 2 that the metallographic structure of the cold-rolled steel sheet and the galvanized steel sheet obtained by this method are all ferrite, and the grain size is both grade 8; the performance indicators of the obtained cold-rolled steel sheet and galvanized steel sheet All are: yield strength 240～252MPa, tensile strength 350～375MPa, elongation ≥41%, plastic strain ratio r90≥2.0, n90≥0.21, BH value 40～45MPa. The mechanical properties, bake hardening properties and aging properties of the steel sheets obtained in each example are shown in Table 3.

Table 3: Mechanical properties, bake hardening properties and aging properties of the steel sheets obtained in each example

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Claims (6)

1. A 220MPa grade bake hardening high-strength steel, characterized in that its chemical composition and its mass percentage are: C≤0.0013%, Si 0.031%～0.037%, Mn 0.53%～0.59%, P≤0.010 %, S≤0.003%, Al 0.067%～0.079%, Nb≤93C/12, Ti 0.0030%～0.0060%, N 0.0045%～0.0065%, the balance is Fe and inevitable impurities. 2. A 220MPa grade bake hardening high-strength steel according to claim 1, characterized in that: the mass percentages of the chemical components are: C 0.0010%-0.0013%, Si 0.031%-0.037%, Mn 0.53 %～0.59%, P≤0.010%, S≤0.003%, Al 0.067%～0.079%, Nb≤93C/12, Ti 0.0030%～0.0060%, N 0.0045%～0.0065%, the balance is Fe and unavoidable impurities. 3 . The 220MPa grade bake hardening high-strength steel according to claim 1 or 2 , wherein the chemical composition further comprises V 0.03%-0.05% and/or B 0.001%-0.004%. 4 . 4. the production method of the described 220MPa grade bake hardening high-strength steel of claim 1,2 or 3, it is characterized in that: adopt steelmaking, hot rolling, cold rolling and continuous annealing procedure to obtain cold rolled steel plate, or adopt steelmaking, heat The rolling, cold rolling, continuous annealing and galvanizing processes obtain galvanized steel sheets. 5. The method for producing a 220MPa grade bake-hardening high-strength steel according to claim 4, wherein the continuous annealing process of the cold-rolled steel sheet is: annealing at a soaking temperature of 770-780°C, and then Cooling to 300-320°C at a cooling rate of 20-25°C/s in the cooling section for aging treatment for 150-200s, and finally cooling to room temperature. 6 . The method for producing a 220MPa grade bake-hardening high-strength steel according to claim 4 , wherein the continuous annealing process of the galvanized steel sheet is: annealing at a soaking temperature of 780-790° C. Galvanizing is carried out by cooling to 460-480°C at a cooling rate of 30-35°C/s in the cooling section.

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