CN115125454B - Cold-rolled automobile outer plate with low waviness after forming and production method thereof - Google Patents

Abstract

The invention provides a cold-rolled automobile outer plate with low waviness after forming and a production method thereof, and the cold-rolled automobile outer plate comprises the following components: c:0.0002 to 0.003wt% of Si: less than or equal to 0.030 weight percent, mn:0.08 to 0.15 weight percent, als:0.015 to 0.050 weight percent, less than or equal to 0.015 weight percent of P, less than or equal to 0.015 weight percent of S, less than or equal to 0.06 weight percent of Nb+Ti, and the balance of Fe and unavoidable impurities. The composition design is matched with the cooperative control of the procedures of steelmaking, hot rolling, acid rolling and annealing, so that the fine and uniform structure of the final finished product is realized, and the waviness after deformation is ensured to be at a lower level. The waviness Wsa after 5% plastic deformation is less than or equal to 0.30 mu m, and the low waviness property after forming is good.

Description

Cold-rolled automobile outer plate with low waviness after forming and production method thereof

Technical Field

The invention belongs to the technical field of cold-rolled automobile steel, and particularly relates to a cold-rolled automobile outer plate with low waviness after forming and a production method thereof.

Background

In recent years, more and more vehicle enterprises start to use short process flows without middle coating, also called 2C1B processes, due to environmental protection, energy consumption and cost. This results in a decrease in the thickness of the paint film and an increase in the transfer of the steel sheet surface structure to the paint surface.

Under the novel coating process, how to control the low waviness of the steel plate and to obtain good appearance quality after the steel plate is formed and painted is a core technology which is required to be mastered by a steel mill. At present, under the condition that the deformation of a low-waviness outer plate is 5%, wsa is less than or equal to 0.35 mu m, and the like, the requirements of mass automobiles and the like are met.

The invention discloses a production method of a hot dip galvanized IF steel plate with controlled surface waviness, which is characterized in that comprehensive measures such as reasonably designing the chemical components of the steel plate, adjusting the parameters of hot continuous rolling and annealing processes, controlling the surface roughness of a finishing roller and the like are adopted, so that the improvement of the surface texture of the hot dip galvanized IF steel plate after stamping is finally realized, and the requirement of a 2C1B painting process is met. The waviness Wsa of the original steel plate produced by the method is less than or equal to 0.35 mu m, the waviness value is higher, the waviness value after forming is not disclosed, the waviness after forming is possibly not meeting the standard requirement, and the influence of the grain structure on the waviness after deformation of the finished product is not researched by the method.

The patent with publication number CN 109022736A published 12 in 2018 and 13 discloses a cold rolling process control method of a gapless atomic steel continuously annealing sheet, when the carbon content of the gapless atomic steel is in the range of 0.0005-0.0050%, the total rolling reduction of the cold rolling is set to 68.0-77.2% and the thickness of a hot rolled coil adopted is 2.5-3.5mm, and the cold rolling is completed in five passes. The method only takes measures for the steel plate with the thickness of 0.8mm, the thickness of the outer plate is basically between 0.6 and 0.8mm due to the light weight requirement at present, the influence factors of the waviness are numerous, and the stable control of the low waviness of the formed steel plate is difficult to realize by controlling the acid rolling reduction rate only. The patent realizes that the texture of the final finished product is tiny and uniform through the cooperative control of each working procedure of steelmaking, hot rolling, acid rolling and annealing, and the waviness after deformation is ensured at a lower level.

Disclosure of Invention

The invention aims to provide a cold-rolled automobile outer plate with low waviness after forming and a production method thereof, which solve the problem that the waviness of a steel plate is too high after stamping deformation to cause the long wave exceeding standard generated by adopting a middle-coating-free 2C1B coating process at a client position and can greatly improve the vividness of the steel plate after forming and coating.

The specific technical scheme of the invention is as follows:

the cold-rolled automobile outer plate with low waviness after being formed comprises the following components in percentage by mass:

c:0.0002 to 0.003wt% of Si: less than or equal to 0.030 weight percent, mn:0.08 to 0.15 weight percent, als: 0.015-0.050 wt%, P is less than or equal to 0.015wt%, S is less than or equal to 0.015wt%, and the balance is Fe and unavoidable impurities, wherein the product is an outer plate product, excessive microalloy addition can cause the temperature rise of recovery recrystallization, the recovery recrystallization is realized by the higher annealing temperature required by subsequent annealing, the surface quality of the finished product is poor due to the higher annealing temperature, and the increase of the waviness of the steel plate after deformation is increased, so that Nb+Ti is required to be less than or equal to 0.06wt%.

The cold-rolled automobile outer plate with low waviness after being formed has fine and uniform structure crystal grains, the structure is a pure ferrite, the grain size level is 8.5-9.5, the waviness Wsa of the product after 5% plastic deformation is less than or equal to 0.30 mu m, and more preferable: the waviness Wsa after 5% plastic deformation is less than or equal to 0.26 mu m, and the low waviness property after forming is good.

The cold-rolled automobile outer plate with low waviness after being formed has the following properties: the yield strength of the product is 140-180 MPa, the tensile strength is 270-350 MPa, and the elongation A is ₈₀ More than or equal to 38, n is more than or equal to 0.21, and r is more than or equal to 2.1.

The invention provides a method for producing a cold-rolled automobile outer plate with low waviness after forming, which comprises the following process flows: smelting, continuous casting, casting blank hot continuous rolling, pickling and cold rolling, continuous annealing, leveling and obtaining a finished product.

The smelting comprises the following steps: the method is suitable for smelting in a converter, an electric furnace and an induction furnace to obtain target molten steel;

the continuous casting comprises the following steps: producing a casting blank by continuous casting, forming molten steel under the action of a crystallizer, and rapidly solidifying and crystallizing to form a plate blank;

and (3) hot continuous rolling of the casting blank: carrying out hot rolling on the continuous casting slab to obtain a hot rolling slab; in the hot rolling process, the heating temperature is controlled to 1160-1200 ℃, and the final rolling temperature is controlled to 930+/-20 ℃. The adoption of lower slab heating temperature is easy to obtain coarse precipitates, is beneficial to the improvement of the forming performance of products, and the adoption of higher finishing temperature is beneficial to avoiding the rolling of a two-phase zone, so that mixed crystals are generated. Coiling the hot-rolled plate blank to obtain a hot-rolled steel coil; the coiling temperature is controlled at 630+/-20 ℃, and the lower coiling can obtain a relatively fine and uniform hot rolling structure and can avoid the generation of a coarse grain structure on the surface layer of the strip steel;

the pickling cold rolling: the conventional pickling process is adopted to fully remove the oxidized iron scale on the surface of the strip steel, then the strip steel is cold-rolled to the thickness specification required by a finished product, the cold rolling reduction rate is controlled to be 78-82%, the shear texture proportion of the surface layer is reduced by adopting the lower acid rolling reduction rate, meanwhile, the lower acid rolling reduction rate is lower in storage energy in the subsequent annealing process, the driving force of grain growth is reduced, and the grains of the finished product tissue are finer;

the continuous annealing: the steel plate treated in the pickling and cold rolling steps is controlled to be annealed at 780-810 ℃;

the flattening: the roughness of the roller used for leveling is controlled between 1.6 and 2.5 mu m, the leveling elongation is controlled between 0.6 and 1.0 percent, and the required steel plate is obtained after leveling.

The component design thought of the invention is as follows:

c: the most effective strengthening element is the most economical and strengthening element. When the C content is low, the formability and the welding performance are good; the trace solid solution carbon can enhance the grain boundary strength, improve the secondary processing brittleness, and is unfavorable for the forming property due to the excessively high carbon content. In the present invention, the content of C is 0.0002 to 0.003wt%.

Si: the steel has stronger strengthening effect, but Si is easy to form oxide, is unfavorable for acid washing and easy to form oxidation color on the surface after annealing, so that the invention adds little Si or does not add Si, and the upper limit is controlled to be 0.03 weight percent.

Mn: the alloy steel has a solid solution strengthening effect in the steel (Mn dissolved into the steel causes lattice distortion, which increases the resistance to dislocation movement, makes sliding difficult, and thus increases the strength and hardness of the alloy steel), and simultaneously reduces the yield ratio (toughness and plasticity) of the steel; in addition, the S in the steel acts to prevent the steel from generating thermal embrittlement. In the patent, mn mainly acts with S to prevent the hot embrittlement of steel, and simultaneously plays a role in solid solution strengthening, and the upper limit of the Mn is controlled to be 0.15%.

Als: al mainly plays a deoxidizing role in steel, and meanwhile, al can also form AlN precipitation to play a role in refining grains to a certain extent. The existence of a small amount of Al can improve the ductility of the steel on the premise of ensuring the strength performance. However, the Al content is too high, the crystallizer is easy to be blocked, and simultaneously, the casting blank is easy to generate defects such as cracks and the like, and the cost is increased. Therefore, the Al weight percentage content of the invention is controlled to be 0.015-0.050%.

P: the P content in the steel should be strictly controlled as an unavoidable harmful impurity in the steel, which has adverse effects on the stamping performance, cold brittleness, secondary working brittleness, etc. of the steel.

S: the S content in the steel is strictly controlled to increase the smelting cost of the steel, and therefore the upper limit of the S content is controlled to be 0.015 percent.

The balance being Fe and unavoidable impurity elements, the less the impurity elements are, the better the cost is not increased.

The invention realizes the fixation effect on the gap atoms C, N, S by adding Ti and Nb, ensures the complete fixation of the gap atoms by controlling the proper adding amount of Ti and Nb, and does not cause the deterioration of the surface state and the increase of the waviness after forming due to excessive addition. And because Ti is added, ti (CN) can be dissolved in the heating process of the hot-rolled plate blank, the dissolution of Ti (CN) can be effectively reduced by adopting a lower plate blank heating temperature, and finally undissolved carbonitride of a finished product is necessarily thicker, so that the improvement of the r value of the finished product is facilitated, and the deep drawing performance of the product is facilitated. The addition of Nb is beneficial to grain refinement and obviously improves the reduction of the waviness increment after forming.

The invention realizes the fine and uniform structure of the final finished product through the ingredient design and the cooperative control of the procedures of steelmaking, hot rolling, acid rolling and annealing, and ensures the waviness after deformation to be at a lower level. The waviness Wsa after 5% plastic deformation is less than or equal to 0.26 mu m, and the low waviness property after forming is good.

Drawings

FIG. 1 is a view showing the structure of an exemplary embodiment of the steel according to the present invention;

FIG. 2 shows the structure of a typical comparative example of the steel according to the present invention.

Detailed Description

Example 1 to example 6

The cold-rolled automobile outer plate with low waviness after being formed comprises the following components in percentage by mass: as shown in table 1, the balance not shown in table 1 is Fe and unavoidable impurities.

Table 1 steel compositions of examples

The production method of each embodiment comprises the following process flows: smelting, continuous casting, casting blank hot continuous rolling, pickling and cold rolling, continuous annealing, leveling and obtaining a finished product.

The smelting comprises the following steps: the method is suitable for smelting in a converter, an electric furnace and an induction furnace to obtain target molten steel;

the continuous casting comprises the following steps: producing a casting blank by continuous casting, forming molten steel under the action of a crystallizer, and rapidly solidifying and crystallizing to form a plate blank;

and (3) hot continuous rolling of the casting blank: carrying out hot rolling on the continuous casting slab to obtain a hot rolling slab; in the hot rolling process, the heating temperature is controlled to 1160-1200 ℃, and the final rolling temperature is controlled to 930+/-20 ℃. Coiling the hot-rolled plate blank to obtain a hot-rolled steel coil; the coiling temperature is controlled at 630+/-20 ℃, and the lower coiling can obtain a relatively fine and uniform hot rolling structure and can avoid the generation of a coarse grain structure on the surface layer of the strip steel;

the pickling cold rolling: the conventional pickling process is adopted to fully remove the oxidized iron scales on the surface of the strip steel, then the strip steel is cold-rolled to the thickness specification required by a finished product, the cold rolling reduction rate is controlled to be 78-82%, the reduction rate of the surface layer shearing texture is prevented from being reduced due to the fact that the moderate acid rolling reduction rate is adopted, meanwhile, the moderate acid rolling reduction rate is lower in storage energy in the subsequent annealing process, the driving force of grain growth is reduced, and the grains of the finished product tissue are finer;

the continuous annealing: the steel plate treated in the pickling and cold rolling steps is controlled to be annealed at 780-810 ℃;

the flattening: the roughness of the roller used for leveling is controlled between 1.6 and 2.5 mu m, the leveling elongation is controlled between 0.6 and 1.0 percent, and the required steel plate is obtained after leveling.

The specific process parameters of each example are shown in Table 2

Table 2 production process parameters of each example

The foregoing embodiments have been described in some detail for purposes of clarity of understanding, it will be appreciated that the foregoing embodiments are merely illustrative of the invention, and that the invention is not limited to the details of construction and practice of the invention, but is to be accorded the full scope of the invention.

Claims (4)

1. The cold-rolled automobile outer plate with the low waviness after forming is characterized by comprising the following components in percentage by mass:

c:0.0002 to 0.003wt% of Si: less than or equal to 0.030 weight percent, mn:0.08 to 0.15wt%, als: 0.015-0.050 wt%, P is less than or equal to 0.015wt%, S is less than or equal to 0.015wt%, nb+Ti is less than or equal to 0.06wt%, and the balance Fe and unavoidable impurities;

the production method of the cold-rolled automobile outer plate with low waviness after forming comprises the following process flows: smelting, continuous casting, casting blank hot continuous rolling, pickling and cold rolling, continuous annealing, leveling and obtaining a finished product;

and (3) hot continuous rolling of the casting blank: the heating temperature is controlled between 1160 and 1220 ℃;

and (3) hot continuous rolling of the casting blank: the final rolling temperature is controlled to be 930+/-20 ℃, and the coiling temperature is controlled to be 630+/-20 ℃;

the pickling cold rolling: the cold rolling reduction rate is controlled to be 78-82%;

the continuous annealing: the steel plate treated in the pickling and cold rolling steps is controlled to be annealed at 780-810 ℃;

the flattening: the roughness of a roller used for leveling is controlled to be 1.6-2.5 mu m; the flattening elongation is controlled to be 0.6-1.0%.

2. The cold-rolled automobile outer plate with low waviness after forming according to claim 1, wherein the cold-rolled automobile outer plate with low waviness after forming has fine and uniform structure grains, the grain size grade reaches 8.5-9.5, and the waviness Wsa of the product after 5% plastic deformation is less than or equal to 0.26 μm.

3. Cold rolled automobile outer panel with low waviness after forming according to claim 1 or 2, characterized in that the properties of the cold rolled automobile outer panel with low waviness after forming are: the yield strength of the product is 140-180 MPa, the tensile strength is 270-350 MPa, and the elongation A is ₈₀ More than or equal to 38, n is more than or equal to 0.21, and r is more than or equal to 2.1.

4. A method for producing a cold rolled automotive outer sheet having a low waviness after forming according to any one of claims 1 to 3, characterized in that the method comprises the following process steps: smelting, continuous casting, casting blank hot continuous rolling, pickling and cold rolling, continuous annealing, leveling and obtaining a finished product;

and (3) hot continuous rolling of the casting blank: the heating temperature is controlled between 1160 and 1220 ℃;

and (3) hot continuous rolling of the casting blank: the final rolling temperature is controlled to be 930+/-20 ℃, and the coiling temperature is controlled to be 630+/-20 ℃;

the pickling cold rolling: the cold rolling reduction rate is controlled to be 78-82%;

the continuous annealing: the steel plate treated in the pickling and cold rolling steps is controlled to be annealed at 780-810 ℃;

the flattening: the roughness of a roller used for leveling is controlled to be 1.6-2.5 mu m;

the flattening: the flattening elongation is controlled to be 0.6-1.0%.