CN102747276A - Transformation induced plasticity steel and preparation method thereof - Google Patents

Abstract

The invention discloses transformation induced plasticity steel and a preparation method thereof and belongs to the technical field of cold rolling hot galvanizing. The transformation induced plasticity steel comprises by mass, 0.18%-0.21% of carbon, 0.6%-10.5% of silicon, 1.8%-2.0% of manganese, 0.03%-0.04% of niobium, no more than 0.01% of phosphorus, no more than 0.01% of sulphur, 0.8%-1.0% of all aluminum, no more than 0.005% of nitrogen and the balance ferrum. Based on a producing method of conventional jet cooling hot galvanizing, the transformation induced plasticity steel and the preparation method are used in 780 MPa automobile cold rolling hot galvanizing, and the transformation induced plasticity steel not only can meet design strength and has good welding and plastic performances.

Description

A kind of phase transformation induced plasticity steel and preparation method thereof

technical field

The invention belongs to the technical field of cold-rolling and hot-dip galvanizing, and relates to a phase transformation-induced plasticity steel and a preparation method thereof.

Background technique

In order to reduce fuel consumption and emission reduction, it is necessary to reduce the weight of automobiles. On the contrary, in order to improve automobile safety, it will be necessary to increase the quality of automobiles. An effective means to solve this contradiction is to use high-strength steel and advanced high-strength steel. On the other hand, the corrosion resistance of automobiles has also become an increasing requirement for high-end automobiles.

The microstructure of Transformation Induced Plasticity Steel (TRIP) consists of ferrite, bainite, retained austenite and a small amount of martensite. TRIP steels have well-matched strength and ductility due to the TRIP effect. The TRIP effect is that the retained austenite in the steel induces the martensitic transformation during the deformation process, thereby improving the strength and plasticity of the steel. At present, phase change induced plasticity steel for automobiles includes: hot rolled phase change induced plasticity steel, cold rolled phase change induced plasticity steel and cold rolled hot dip galvanized phase change induced plasticity steel.

The main production processes of cold-rolled hot-dip galvanized phase-change-induced plasticity steel include: steelmaking, hot rolling, cold rolling and CGL (continuous hot-dip galvanizing). The hot-dip galvanizing production line is very different from the continuous annealing process. The bainite temperature on the annealing production line can be adjusted, while the bainite temperature on the hot-dip galvanizing production line is limited to around 460°C; The bulk transformation time is sufficient, but the bainite transformation time on the hot-dip galvanizing production line is very short, which will lead to insufficient bainite transformation, and insufficient carbon enrichment of some austenite during the bainite transformation process , in the subsequent cooling process into martensite, which affects the mechanical properties of the steel. C is an important factor to stabilize austenite and increase the strength of steel, but excessive C will affect weldability and deteriorate elongation. Si element can strongly inhibit the formation of cementite, make the untransformed austenite rich in carbon, and greatly improve the stability of austenite, so adding Si can solve the problem caused by high C. However, although high Si tends to improve the stability of retained austenite, it leads to poor coatability. Adding Al instead of Si can solve the coating problem, but this will weaken a certain strength (70-120MPa).

Existing enterprises have specially built hot-dip galvanizing annealing production lines using open flame heating technology to solve the coating problems of steel types containing alloy elements such as Si and Mn. But for the generally used jet hot-dip galvanizing annealing production line at present, still can't solve this problem.

Contents of the invention

The technical problem to be solved by the present invention is to provide a phase transformation induced plasticity steel with high strength and excellent elongation and a preparation method thereof.

In order to solve the above technical problems, the present invention provides a phase transformation induced plasticity steel, the weight percentage of its chemical composition is respectively:

C 0.18%～0.21%, Si 10.5%～0.6%, Mn 1.8%～2.0%, Nb 0.03%～0.04%, P≤0.01%, S≤0.01%, Alt 0.8%～1.0%, N≤0.005%, The balance is Fe.

A preparation method of phase transformation induced plasticity steel, which includes adding scrap steel into a converter, smelting to obtain a continuous casting billet, hot rolling to obtain a hot rolled plate, cold rolling to obtain a chilled steel strip, and then continuous hot galvanizing. Wherein, the continuous hot-dip galvanizing method is:

A The chilled steel strip is first heated to 220°C, and the heating rate is 8°C/s-12°C/s;

B is further heated to 780°C-830°C, and the heating rate is 1.5°C/s-4°C/s;

C Then keep warm for 60s-100s at 780℃～830℃;

D. Cool the strip steel obtained after heat preservation to 720°C-760°C, and the cooling rate is 8°C/s-12°C/s;

E is quickly cooled to the temperature of the zinc pot by blowing air to 450°C-460°C, and then cooled to 420-430°C by air knife blowing after galvanizing;

F. The air cooling at the front end between the air knife and the top roller is combined with the air cooling at the rear end, and finally cooled to 250-300°C with a cooling rate of 6°C/s-9°C/s.

Further, in the smelting step, in the smelting step, the weight percentages of the chemical components of the hot-rolled sheet are respectively: C0.18-0.21%, S≤0.010%, P≤0.01%.

Further, in the smelting step, the end point temperature of the first furnace in the converter is 1670-1690°C.

Further, in the smelting step, the end point temperature of the continuous casting is 1660-1680°C.

Further, in the smelting step, the deoxidizer is Al-Fe alloy antioxidation powder.

Further, the addition amount of the deoxidizer is 4kg/t.

Further, in the smelting step, the slag material is 800kg/furnace of small-grained lime and 200kg/furnace of fluorite.

Further, in the hot rolling step, the heating temperature of the continuous casting slab is 1220-1280°C, the finishing rolling temperature is 860-900°C, and the coiling temperature is 640-700°C.

Further, the reduction ratio of cold rolling in the cold rolling process is 50%-70%.

The invention provides a phase-change-induced plasticity steel and a preparation method thereof. Based on the conventional air-jet cooling hot-dip galvanizing production method, the cold-rolled hot-dip galvanized phase-change-induced plasticity steel has better performance on the basis of meeting the design strength level. Weldability and plasticity.

Description of drawings

Figure 1 is a photo of the microstructure of a transformation-induced plasticity steel provided in an embodiment of the present invention.

Detailed ways

In order to deeply understand the present invention, the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

Example 1:

A kind of phase transformation induced plasticity steel, its chemical composition weight percent is respectively:

C: 0.19%, Si: 0.5%, Mn: 1.9%, P: 0.008%, S: 0.005%, Alt: 0.9%, N: 0.005%, Nb: 0.03%, and the balance is Fe and impurities.

The preparation method of phase transformation induced plasticity steel in this embodiment specifically includes the following steps:

1) Smelting: Add steel scrap to the converter for smelting. The end temperature of the first furnace during the smelting process is 1670-1690°C; the target temperature of the continuous pouring during the smelting process is 1660-1680°C; during the smelting process, Al-Fe alloy is used to resist Chemical powder deoxidation, the addition of Al-Fe alloy anti-chemical powder is 4kg/t; during the smelting process, slag is added during the tapping process, and the slag is added with the steel flow in the early stage of tapping, and the tapping amount reaches the total steel amount Before 1/5 of the slag, add all the slag materials, 800kg small-grained lime and 200kg fluorite per furnace; after that, use Si-Mn to adjust Si, so that the mass percentage of Si is 0.5%; use micro-carbon Mn-Fe to adjust Mn, so that Mn The mass percentage is 1.9%; the smelting process is in the tapping process, the amount of slag ≤ 80mm, and the tapping time ≥ 4 minutes. When the smelting is completed, the weight percentages of the chemical components of the hot-rolled sheet are C: 0.19%, Si: 0.5%, Mn: 1.9%, P: 0.008%, S: 0.005%, Alt: 0.9%, N: 0.005%, Nb: 0.03%, the balance is Fe and impurities.

2) Then hot rolling, during the hot rolling process, the heating temperature of the continuous casting slab is 1250°C; the finishing temperature of hot rolling is 890°C; the coiling temperature of hot rolling is 690°C. At the end of hot rolling, the thickness of the hot-rolled sheet obtained is 5.0mm. During this process, the coiling temperature has a great influence on the intermediate structure and mechanical properties of the cold-rolled galvanized steel hot-rolled. Considering the strength level, high-temperature final rolling and high-temperature coiling are adopted, so that the microstructure of the hot-rolled sheet is coarse polygonal ferrite grains and well-developed pearlite, and the microstructure has a relatively low yield strength. This reduces the rolling force during cold rolling deformation, which is beneficial to the cold rolling process.

3) Then cold rolling, during the cold rolling process, the reduction rate of cold rolling is 50%-70%, so as to facilitate the cold rolling process. At the end of the cold rolling, a chilled steel strip with a thickness of 1.8 mm was obtained.

4) After that, continuous hot-dip galvanizing annealing is carried out on the above-mentioned chilled strip steel, specifically:

A The chilled steel strip is first heated to 220°C, and the heating rate is 8°C/s-12°C/s. During this process, the cold deformed ferrite recovers. ;

B is further heated to 780°C-830°C, and the heating rate is 1.5°C/s-4°C/s. This process realizes the recrystallization of the cold-rolled ferrite structure, and the pearlite first transforms into austenite and then transforms to ferrite. body growth;

C Then hold at 780°C-830°C for 60s-100s, this process realizes partial austenitization, C and Mn elements in ferrite transfer to austenite and homogenize in austenite;

D. Cool the strip steel obtained after heat preservation to 720°C-760°C, and the cooling rate is 8°C/s-12°C/s. This process makes the austenite part transfer to ferrite, and C, Mn and other elements further Aggregation in austenite;

E Quickly cool to the temperature of the zinc pot by air blowing to 450°C-460°C. After galvanizing, it is cooled to 420-430°C by air knife blowing and scraping. During this process, try to increase the strip steel in the equalization section, furnace nose and zinc pot The residence time of carbon is further enriched in the retained austenite during the bainite transformation process, and the stability of austenite is improved;

F. After the front air cooling between the air knife and the top roller, combined with the rear air cooling, it is finally cooled to 250-300°C, and the cooling rate is 6°C/s-9°C/s. During this process, some unstable austenite phases transformed into martensitic phase.

After the above steps, the mass percentages of the chemical components of the obtained 780MPa grade cold-rolled hot-dip galvanized phase transformation induced plasticity steel are: C: 0.19%, Si: 0.5%, Mn: 1.9%, P: 0.008%, S: 0.005% , Alt: 0.9%, N: 0.005%, Nb: 0.03%, the balance is Fe and impurities, the specific microstructure is shown in Figure 1, in the figure, it is etched with picric acid sodium metabisulfite solution, and the gray black is iron Matrix matrix; bright white martensitic islands. Its mechanical properties are shown in Table 1:

Table 1 The cold-rolled hot-dip galvanized phase transformation-induced plasticity steel mechanical properties provided by the present invention

Rp0.2,N/mm2 Rm,N/mm2 A80,% this invention 430 795 29

As can be seen from Table 1, each index of the cold-rolled hot-dip galvanized trial-manufactured steel provided by the present invention can meet the requirements of enterprise standards (Rp 0.2, N/mm 2 at 420-550, Rm, N/mm ≥ 780, A80, %≥23).

Example 2:

A kind of phase transformation induced plasticity steel, its chemical composition weight percent is respectively:

C: 0.21%, Si: 0.6%, Mn: 2.0%, P: 0.01%, S: 0.01%, Alt: 0.8%, N: 0.0008%, Nb: 0.04%, and the balance is Fe and impurities.

The preparation method of phase transformation induced plasticity steel in this embodiment specifically includes the following steps:

1) Smelting: Add steel scrap to the converter for smelting. The end point temperature of the first furnace during the smelting process is 1690°C; the target temperature at the end point of continuous casting during the smelting process is 1660°C; during the smelting process, Al-Fe is used for deoxidation. Addition amount is 4kg/t; slag is added during the tapping process of the smelting process, and the slag is added along with the steel flow at the early stage of tapping, and all the slag is added before the tapping amount reaches 1/5 of the total steel amount. Furnace 800kg small-grain lime and 200kg fluorite; after that, use Si-Mn to adjust Si, so that the mass percentage of Si is 0.5%; use micro-carbon Mn-Fe to adjust Mn, so that the mass percentage of Mn is 1.9%; During the process, the amount of slag removed from the steel was 50 mm, and the time for tapping was 8 minutes. When the smelting is completed, the weight percentages of the chemical components are: C: 0.19%, Si: 0.5%, Mn: 1.9%, P: 0.008%, S: 0.005%, Alt: 0.9%, N: 0.005%, Nb: 0.03 %, the balance is Fe and impurities.

2) Then hot rolling, during the hot rolling process, the heating temperature of the continuous casting slab is 1280°C; the finishing temperature of hot rolling is 860°C; the coiling temperature of hot rolling is 650°C. At the end of hot rolling, the thickness of the hot-rolled sheet obtained is 5.0mm. During this process, the coiling temperature has a great influence on the intermediate structure and mechanical properties of the cold-rolled galvanized steel hot-rolled. Considering the strength level, high-temperature final rolling and high-temperature coiling are adopted, so that the microstructure of the hot-rolled sheet is coarse polygonal ferrite grains and well-developed pearlite, and the microstructure has a relatively low yield strength. This reduces the rolling force during cold rolling deformation, which is beneficial to the cold rolling process.

3) Cold rolling again, during the cold rolling process, the cold rolling reduction rate is 60%, so as to facilitate the cold rolling process. At the end of the cold rolling, a chilled steel strip with a thickness of 1.8 mm was obtained.

4) After that, continuous hot-dip galvanizing annealing is carried out on the above-mentioned chilled strip steel, specifically:

A The chilled steel strip is first heated to 220°C at a heating rate of 10°C/s. During this process, the cold deformed ferrite recovers. ;

B is further heated to 800°C, and the heating rate is 3°C/s. This process realizes the recrystallization of the cold-rolled ferrite structure, and the pearlite first transforms into austenite and grows toward ferrite;

C Then hold at 800°C for 80s, this process realizes partial austenitization, C and Mn elements in ferrite transfer to austenite and homogenize in austenite;

D. Cool the strip steel obtained after heat preservation to 740°C, and the cooling rate is about 10°C/s. This process makes the austenite part transfer to ferrite, and elements such as C and Mn further gather in the austenite;

E is quickly cooled to the temperature of the zinc pot by air blowing to 460°C, and then cooled to 420°C by air knife blowing and scraping after galvanizing. Carbon is further enriched in the retained austenite during the bainite transformation process, and makes the austenite more stable;

F. After air cooling at the front end between the air knife and the top roller, combined with air cooling at the rear end, and finally cooling to 280°C, the cooling rate is about 8°C/s. During this process, part of the unstable austenite phase transforms into martensite Mutually.

After the above steps, the obtained 780MPa grade cold-rolled hot-dip galvanized phase change induced plasticity steel

The cold-rolled hot-dip galvanized phase transformation-induced plasticity steel provided by the embodiment of the present invention can not only ease the coating property but also effectively improve the stability of austenite, so that the TRIP effect can be fully exerted, and cold-rolled hot-dip galvanized steel with good matching of strength and plasticity can be obtained. Transformation-induced plasticity in steels. At the same time, reduce the content of Si element and use Al to avoid the precipitation of cementite and drive C to enrich in austenite, improve the stability of austenite, and add Nb to compensate for the decrease in strength caused by adding Al.

Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention without limitation, although the present invention has been described in detail with reference to examples, those of ordinary skill in the art should understand that the technical solutions of the present invention can be carried out Modifications or equivalent replacements without departing from the spirit and scope of the technical solution of the present invention shall be covered by the claims of the present invention.

Claims (10)

1. a transformation induced plasticity steel is characterized in that, its chemical component weight per-cent is respectively:

C 0.18%-0.21%, Si 10.5%-0.6%, Mn 1.8%-2.0%, Nb 0.03%-0.04%, P≤0.01%, S≤0.01%, Alt 0.8%-1.0%, N≤0.005%, surplus is Fe.

2. the preparation method of transformation induced plasticity steel according to claim 1 is characterized in that, steel scrap is added converter; Smelting obtains the continuous casting embryo, after hot rolling, obtains hot-rolled sheet, obtains chill attitude band steel through cold rolling; Promptly get through continuous hot galvanizing then, wherein, said continuous hot galvanizing method is:

The said chill attitude band steel of A at first is heated to 220 ℃, 8 ℃ of its rate of heating/s-12 ℃/s;

B further is heated to 780 ℃-830 ℃, and its rate of heating is 1.5 ℃/s-4 ℃/s;

C is then at 780 ℃～830 ℃ insulation 60s-100s;

The band steel that D. will be incubated the back gained is cooled to 720 ℃-760 ℃, and speed of cooling is 8 ℃/s-12 ℃/s;

E is cooled to 450 ℃-460 ℃ of zinc pot temperatures soon through blowing, and zinc-plated end back experience air knife blows to scrape and is cooled to 420-430 ℃;

F. cooperate the rear end air-cooled through said air knife to the front end air cooling between the top roller, be cooled to 250-300 ℃ at last, speed of cooling is 6 ℃/s-9 ℃/s.

3. method according to claim 2 is characterized in that, in the said smelting step, the weight percent of the chemical ingredients of said hot-rolled sheet is respectively: C 0.18-0.21%, S≤0.010%, P≤0.01%.

4. method according to claim 2 is characterized in that, in the said smelting step, the terminal temperature of first stove is 1670-1690 ℃ in the said converter.

5. method according to claim 2 is characterized in that, in the said smelting step, the terminal temperature that said company waters is 1660-1680 ℃.

6. method according to claim 2 is characterized in that, in the said smelting step, said reductor is anti-ization of Al-Fe alloy powder.

7. method according to claim 6 is characterized in that, the add-on of said reductor is 4kg/t.

8. method according to claim 2 is characterized in that, in the said smelting step, said slag charge is the granule lime of 800kg/ stove and the fluorite of 200kg/ stove.

9. method according to claim 2 is characterized in that, in the said hot-rolled step, said continuous blank heating temperature is 1220-1280 ℃, and finishing temperature is 860-900 ℃, and coiling temperature is 640-700 ℃.

10. method according to claim 2 is characterized in that, cold rolling draft is 50%-70% in the said cold-rolled process.

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