阅读说明：本技术 140MPa级高烘烤硬化特性冷轧超低碳钢及生产方法 (140MPa grade high-bake-hardening-characteristic cold-rolled ultra-low-carbon steel and production method thereof ) 是由 程帅 于晓飞 吕超杰 王风强 王田惠 安亮 姜丽梅 于 2020-04-17 设计创作，主要内容包括：本发明涉及一种140MPa级高烘烤硬化特性冷轧超低碳钢,其组分及重量百分含量为：C:0.002-0.003%,Si≤0.03%,Mn:0.25~0.35%,P:0.008-0.018%,S≤0.012%,Als:0.025~0.055%,N≤0.003%,Nb:0.007~0.013%,余量为Fe及不可避免的杂质。本发明一种140MPa级高烘烤硬化特性冷轧超低碳钢的生产方法,包括炼钢、热轧、酸轧及连续退火工序；所述退火工序中,均热温度840~850℃,保温时间75-94s。本发明生产的140MPa级冷轧超低碳钢,在常温下具有较好的成形能力,且具有＞50Mpa的稳定的高烘烤硬化值,不仅满足了零件的成型需求,并且满足了零件成型后的刚度需要,表现出良好的综合性能。(The invention relates to a 140MPa grade cold-rolled ultra-low carbon steel with high baking hardening characteristic, which comprises the following components in percentage by weight: 0.002-0.003% of C, less than or equal to 0.03% of Si, 0.25-0.35% of Mn, 0.008-0.018% of P, less than or equal to 0.012% of S, 0.025-0.055% of Als, less than or equal to 0.003% of N, 0.007-0.013% of Nb, and the balance of Fe and inevitable impurities. The invention relates to a production method of 140MPa grade high-bake-hardening characteristic cold-rolled ultra-low carbon steel, which comprises the working procedures of steel making, hot rolling, acid rolling and continuous annealing; in the annealing procedure, the soaking temperature is 840-850 ℃, and the heat preservation time is 75-94 s. The 140 MPa-grade cold-rolled ultra-low carbon steel produced by the method has good forming capability at normal temperature, has a stable high baking hardening value of more than 50MPa, meets the forming requirement of parts, meets the rigidity requirement of the parts after forming, and shows good comprehensive performance.)

1.140MPa grade high bake hardening characteristic cold-rolled ultra-low carbon steel, which is characterized in that: comprises the following components in percentage by weight: 0.002-0.003% of C, less than or equal to 0.03% of Si, 0.25-0.35% of Mn, 0.008-0.018% of P, less than or equal to 0.012% of S, 0.025-0.055% of Als, less than or equal to 0.003% of N, 0.007-0.013% of Nb, and the balance of Fe and inevitable impurities.

The production method of the 2.140MPa grade cold-rolled ultra-low carbon steel with high baking hardening characteristic comprises the working procedures of steel making, hot rolling, acid rolling and continuous annealing; the method is characterized in that: in the annealing procedure, the soaking temperature is 840-850 ℃, and the heat preservation time is 75-94 s.

3. The method for producing a 140MPa grade high bake-hardening cold-rolled ultra-low carbon steel according to claim 2, wherein: in the annealing procedure, the slow cooling temperature is 640-660 ℃, the cooling speed is 6.5-10 ℃/s, the fast cooling end temperature is 330-370 ℃, the cooling speed is 45-69 ℃/s, the overaging end temperature is 280-320 ℃, the aging time is 275-345s, and the leveling elongation is 0.8-1.2%.

4. The method for producing a 140MPa grade high bake-hardening cold-rolled ultra-low carbon steel according to claim 2, wherein: in the hot rolling procedure, the hot rolling is divided into a heating section and a soaking section, the furnace time is 200-; the laminar cooling adopts a mode of front-section 1/2 cooling, and the coiling temperature is 703-.

5. The method for producing a 140MPa grade high bake-hardening cold-rolled ultra-low carbon steel according to claim 2, wherein: the total reduction rate of the acid rolling process is 70-80%.

6. The method for producing the 140MPa grade cold-rolled ultralow carbon steel with the high bake-hardening characteristic according to claim 1, wherein the cold-rolled ultralow carbon steel has a solid-solution C atom content of 1.0 × 10-5 to 2.4 × 10-5, and has mechanical properties of yield strength Rp0.2: 140-190 MPa, tensile strength Rm of 270-320 MPa, elongation after fracture A80mm of not less than 37%, plastic strain ratio r90 of not less than 2.0, work-hardening index n90 of not less than 0.20, bake-hardening value BH of more than 50MPa, and no natural aging for 3 months.

Technical Field

The invention relates to an ultra-low carbon steel and a production method thereof, in particular to a 140MPa grade cold-rolled ultra-low carbon steel with high baking hardening property and a production method thereof.

Background

In order to achieve the purpose of light weight and safety improvement of automobiles, the modern automobile industry mostly adopts high-strength steel to manufacture automobile bodies. Bake-hardened steel has both deep drawability similar to mild steel and high strength after bake hardening, and therefore, bake-hardened steel is widely used in many automobiles to replace ordinary mild steel for the manufacture of automobile body outer covers. On the one hand, the outer plate can be thinned to achieve the aim of lightening the automobile body; on the other hand, the dent resistance of the vehicle body outer covering can be improved by the bake hardening property thereof.

At present, patent and literature reports about cold-rolled ultra-low carbon bake-hardening steel focus on products with strength of 180MPa, 220MPa and higher, and the products generally obtain higher strength than 140MPa grade cold-rolled ultra-low carbon steel by adding more alloy elements such as C, Mn, P and the like and assisting with lower annealing temperature, but the products with higher strength are difficult to form complicated parts because automobile covering parts have higher requirements on material forming capability.

The lower limit of the requirement of the baking hardening value in the current national standard is 30MPa, and the baking hardening steel at 140MPa level has better forming capability, but the rigidity of the formed part is still lower, so that the use of common low-carbon steel or interstitial free steel and the like is more considered when forming the complex part, and the actual application range of the product is smaller, so that if the 140MPa level cold-rolled ultra-low-carbon steel with high baking hardening property can be developed, the rigidity of the formed part is improved, and the method has important significance for complex automobile body parts which simultaneously require material forming capability and rigidity.

Disclosure of Invention

The invention aims to solve the technical problem of providing 140MPa grade high-bake-hardening cold-rolled ultra-low carbon steel and a production method of the 140MPa grade high-bake-hardening cold-rolled ultra-low carbon steel.

The technical scheme for solving the technical problems is as follows:

the 140MPa grade cold-rolled ultra-low carbon steel with high baking hardening characteristic comprises the following components in percentage by weight: 0.002-0.003% of C, less than or equal to 0.03% of Si, 0.25-0.35% of Mn, 0.008-0.018% of P, less than or equal to 0.012% of S, 0.025-0.055% of Als, less than or equal to 0.003% of N, 0.007-0.013% of Nb, and the balance of Fe and inevitable impurities.

The production method of the 140MPa grade high-bake-hardening characteristic cold-rolled ultra-low carbon steel comprises the working procedures of steel making, hot rolling, acid rolling and continuous annealing; in the annealing procedure, the soaking temperature is 840-850 ℃, and the heat preservation time is 75-94 s.

In the production method of the 140MPa grade high-bake-hardening cold-rolled ultralow-carbon steel, in the annealing process, the slow cooling temperature is 640-660 ℃, the cooling speed is 6.5-10 ℃/s, the fast cooling finishing temperature is 330-370 ℃, the cooling speed is 45-69 ℃/s, the overaging finishing temperature is 280-320 ℃, the aging time is 275-345s, and the leveling elongation is 0.8-1.2%.

In the production method of the 140MPa grade high-bake-hardening-characteristic cold-rolled ultra-low-carbon steel, in the hot rolling procedure, the hot rolling is divided into a heating section and a soaking section, the furnace time is 200-230min, the temperature of the soaking section is 1195-1250 ℃, and the finish rolling temperature is 906-934 ℃; the laminar cooling adopts a mode of front-section 1/2 cooling, and the coiling temperature is 703-.

According to the production method of the 140MPa grade high-bake-hardening-characteristic cold-rolled ultra-low-carbon steel, the total reduction rate of the acid rolling process is 70-80%.

The above cold-rolled ultra-low carbon steel having a high bake hardening property of 140MPa grade, which is cold-rolledThe content of solid-solution C atoms in the rolled ultra-low carbon steel is 1.0 × 10^-5～2.4×10^-5(ii) a The mechanical properties are as follows: yield strength R_p0.2140 to 190MPa, tensile strength R_m: 270-320 MPa, elongation after break A_80mmNot less than 37 percent and plastic strain ratio r₉₀Not less than 2.0, work hardening index n₉₀Not less than 0.20, the baking hardening value BH more than 50MPa, and no natural aging occurs in 3 months.

In the continuous annealing process, the proper soaking temperature is an important parameter for controlling solid solution C atoms in the ultra-low carbon bake-hardening steel, and the NbC is subjected to redissolution decomposition at the annealing heating temperature, wherein NbC ↔ Nb + C is subjected to dissolution and precipitation of NbC in a ferrite matrix, which is a reversible chemical reaction process, and the equilibrium solid solution amount of Nb and C and the amount of NbC are changed along with the change of the strip steel temperature. The solid solubility product formula of NbC in ferrite adopts an empirical formula derived by the professor Yongqi dragon, and in addition, the mass of precipitated elements must meet the ideal chemical proportion of the precipitated elements in a second phase, and the two formulas are as follows:

in the formula:、mass fraction of niobium and carbon dissolved in ferrite under balanced condition.、Mass fraction of niobium and carbon elements in steel is percent; t represents temperature, K.

The intermediate values of the present invention for the components give a mass fraction of 1.35 × 10 for the carbon element solid-dissolved in ferrite under equilibrium conditions at 790 ℃ and 840 ℃ respectively^-5，1.6×10^-5That is, when soaking temperature is increased from 790 to 840 ℃, the content of solid solution C atoms is increased by 2.5 × 10^-6The increase of the solid solution C atoms can increase the bake hardening properties of the material.

On the other hand, the higher soaking temperature is favorable for fully growing the recrystallized grains, increasing the grain size, reducing the total area of the grain boundary and reducing the total amount of carbon and nitrogen atoms stored in the grain boundary position, thereby increasing the content of solid-solution carbon and nitrogen in the matrix, the soaking temperature of the invention is executed at 840-850 ℃, and the theoretical calculation can control the content of solid-solution C atoms in the steel to be 1.0 × 10 by combining the upper limit value and the lower limit value of the C, Nb element content in the steel^-5To 2.4 × 10^-5In the method, the lower limit of solid solution C atoms in the steel is mainly controlled, the bake hardening value of the material can be more than 50MPa at 840 ℃, and the natural aging resistance of the material is reduced along with the increase of the solid solution C atoms in the steel, so the upper limit of the annealing temperature is comprehensively considered to be 850 ℃.

The acid rolling reduction rate provides a driving force for continuous annealing recrystallization, determines the number of recrystallization nucleation points and the final grain size, and the total reduction rate of the acid rolling process is 70-80%.

The slow cooling section of the continuous annealing process mainly has the effect that the transition stage of the material from the soaking section to the rapid cooling is carried out, the phenomenon that the plate shape quality of a product is influenced by direct cooling at a higher temperature is avoided, but the solid solubility of NbC is obviously reduced due to the reduction of the temperature, and the Nb atom diffusion control process is considered in the NbC precipitation process, so that the NbC precipitation can be inhibited by increasing the cooling speed of the slow cooling section, and the solid-solution carbon content and the baking hardening performance in a matrix are improved. The slow cooling finishing temperature is 640-660 ℃, and the cooling speed is 6.5-10 ℃/s.

The rapid cooling section of the continuous annealing process has the effect that solid-solution C atoms in the steel are reserved in a final product to a large extent through rapid cooling, the baking hardening characteristic of the final product can be obviously improved, the rapid cooling finishing temperature is 330-370 ℃, and the cooling speed is 45-69 ℃/s.

The effects of the chemical components are as follows:

c, the ultra-low C content can reduce the strength of the steel and improve the plasticity of the steel, but in the bake hardening steel, certain solid solution C is needed, so that the steel obtains higher strength after baking, namely has better bake hardening characteristics;

nb: nb mainly forms NbC compounds with C in the bake hardening steel, is easy to dissolve at high temperature, and is beneficial to retaining solid-solution C after rapid cooling, thereby realizing the bake hardening characteristic of the steel. In order to obtain better bake hardening characteristics, the solid solution amount of C in the steel is controlled, and the C content is controlled within the range of 0.002-0.003wt% and the Nb element is controlled within the range of 0.007-0.013wt% by matching with a high-temperature annealing process at 840-850 ℃.

The bake hardening characteristics of the product of the invention are mainly related to the mass fractions of Nb and C elements of the steel, so the Nb and C elements need to be controlled within a reasonable range to ensure that certain solid solution C atoms exist in the steel, thereby the steel obtains the bake hardening characteristics, and the mass fraction of the solid solution C atoms in the 140 MPa-grade ultra-low carbon bake hardening steel ferrite can be controlled to be 1.0 × 10 by theoretical calculation at the annealing temperature of 840 ℃ and 850 DEG C^-5To 2.4 × 10^-5In the invention, the bake hardening value of the material is stabilized to be more than 50MPa by accurately controlling the process and elements, and the alloy elements such as Nb, C and the like and the subsequent annealing temperature need to be comprehensively considered and calculated in a complex way, so that the steel has excellent forming capability and obtains higher bake hardening characteristics.

The 140 MPa-level bake-hardening steel provided by the invention adopts Nb microalloy ultra-low carbon components, adopts high-temperature finish rolling and high-temperature coiling processes, and can be used as nucleation particles of subsequent grains, so that recrystallization is promoted, the development of {111} texture of the material is facilitated, the dislocation density is increased due to large cold rolling reduction, the distortion energy is increased, the driving force of recrystallization is larger, and the deep drawing performance of an annealed steel plate is facilitated to be improved. The proper soaking temperature is executed in the continuous annealing unit, so that full recrystallization is facilitated, the number of solid solution atoms in the steel is increased, more C, N solid solution atoms are reserved in the steel after rapid cooling, and sufficient flattening elongation is given in the subsequent flattening process, which is the most direct means for resisting natural aging of the bake-hardened steel.

As the steel grade utilizes solid solution atoms to increase the baking hardening characteristic, the solid solution atoms are deviated to dislocation at normal temperature, so that the strength of the material is increased, namely the aging property exists. The product produced by the invention can ensure that the natural aging does not occur within 3 months.

The invention has the beneficial effects that:

the invention controls the theoretical value of solid solution C atoms in steel to be 1.0 × 10 by accurately controlling alloy elements such as Nb, C and the like and assisting a high-temperature annealing process^-5～2.4×10^-5The produced 140 MPa-grade cold-rolled ultra-low carbon steel has good forming capability at normal temperature, has a stable high baking hardening value of more than 50MPa, meets the forming requirement of parts, meets the rigidity requirement of the parts after forming, and shows good comprehensive performance.

Detailed Description

The present invention will be described in further detail with reference to specific examples 1 to 10 below: