CN106868398B - 1300MPa grades of ultra-fine grained ferrites/low temperature bainite dual-phase steel and preparation method thereof - Google Patents

Abstract

The invention discloses a 1300MPa ultra-fine-grained ferrite/low temperature bainite dual-phase steel and a preparation method thereof, belonging to the field of iron and steel material engineering. The chemical composition of the dual-phase steel is as follows: C 0.28~0.32, Si 1.7~2.1, Mn 1.5~1.7, Cr1.1~1.4, Ni 0.8~1.2, W 0.6~0.8, P≤0.02, S≤ 0.02, the rest is Fe and inevitable impurities; its metallographic structure is ultra-fine-grained ferrite and low-temperature bainite, of which the grain size of ultra-fine-grained ferrite is 0.5~2μm, and the volume content is 20~ 35%. Heat the quenched martensitic structure of medium-carbon silicon-containing low-alloy steel to a temperature below A _{c 1} , heat it out of the furnace for rolling deformation, and then heat it to the "α+γ" two-phase region to partially austenitize, and then put Isothermal bainite transformation is carried out in a salt bath furnace slightly above the martensite start point of austenite in the two-phase region, and air cooled to room temperature.

Description

1300MPa class ultra-fine grain ferrite/low temperature bainite dual phase steel and its preparation method

technical field

The invention belongs to the field of iron and steel material engineering, and relates to a dual-phase steel and a preparation method thereof, in particular to a 1300MPa ultrafine-grained ferrite/low temperature bainite dual-phase steel and a preparation method thereof.

Background technique

The traditional ferrite/martensitic dual-phase steel has low yield ratio, high initial work hardening rate and good plasticity and toughness. However, due to the large difference in strength between ferrite and martensite, microcracks tend to expand along the ferrite/martensite phase interface, resulting in poor hole expansion performance, and cracks often occur in the hole expansion and flanging process. The ferrite/bainite dual-phase steel replaces martensite with bainite with better toughness, and has better flanging and hole expansion performance than ferrite/martensitic dual-phase steel, and With better tensile properties and impact toughness, it is more suitable for the manufacture of complex-shaped auto parts (such as wheels, chassis, suspension, etc.), engineering machinery parts and large-strain pipelines.

At present, ferritic/bainitic dual-phase steel is mainly prepared from low-carbon low-alloy steel (including microalloyed and non-microalloyed) through controlled rolling and controlled cooling. The preparation method has been disclosed in a number of patents. For example, the Chinese patent application number 200910169738.X discloses a high tensile strength hot-rolled ferritic bainite dual-phase steel and its manufacturing method. The tensile strength is between 514 and 535 MPa, and the yield strength ratio is 0.63 Above, but the tensile strength is still relatively low, the toughness is not enough. In order to obtain ferrite/bainite dual-phase steel with better comprehensive mechanical properties, researchers have carried out in-depth research and improvement on its ferrite and bainite structures. Firstly, the ferrite structure is refined, so that the ferrite grain size is refined to the micron level (1-4 μm), that is, the ultra-fine ferrite structure, and the ultra-fine ferrite is made due to its high strength. The performance of dual-phase steel is improved; secondly, the bainite structure is also continuously improved, and bainite structures with excellent strength and toughness such as carbide-free bainite and low-temperature bainite are gradually applied to dual-phase steel. The dual-phase steel combining ferrite and high-strength bainite has become the focus and hotspot of research scholars today.

Contents of the invention

The technical problem to be solved by the present invention is to provide a 1300MPa ultra-fine-grained ferrite/low temperature bainite dual-phase steel with simple process, high preparation efficiency, easy precise control and high quality stability and its preparation method. The quenched martensite structure of carbon-containing silicon-containing low-alloy steel is heated to a temperature below A _{c 1} , and after heat preservation, it is taken out of the furnace for warm rolling or cold rolling deformation, and then heated until the "α + γ" two-phase region is partially austenitized, and then placed in Isothermal bainite transformation is carried out in a salt bath furnace at a temperature slightly higher than the martensite starting point of austenite in the two-phase region, and air-cooled to room temperature to obtain an ultrafine-grained ferrite/low temperature bainite dual-phase structure. The process is simple, and the formed dual-phase steel has ultra-high strength and good plasticity.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A 1300MPa grade ultra-fine-grained ferrite/low temperature bainitic dual-phase steel, the chemical composition of which is: C0.28~0.32, Si 1.7~2.1, Mn 1.5~1.7, Cr1.1~1.4, Ni 0.8~1.2, W 0.6~0.8, P≤0.02, S≤0.02, the rest is Fe and inevitable impurities; its metallographic structure is ultrafine grain ferrite and low temperature bainite, among which ultrafine grain ferrite The grain size of bainite is 0.5~2μm, the volume content is 20~35%, and the lath size of low temperature bainite is 75~300 nm.

The tensile strength of the 1300MPa ultra-fine-grained ferrite/low temperature bainitic dual-phase steel is not less than 1300 MPa, the yield ratio is 0.50-0.65, and the elongation is not less than 12%.

The present invention also provides two preparation methods of the above-mentioned 1300MPa ultra-fine-grained ferrite/low temperature bainite dual-phase steel, wherein one method includes the following steps:

A. Steelmaking: Calculate the feeding ratio according to the design requirements of the steel, smelt it, and cast it into steel ingots;

B, annealing, hot rolling: the steel ingot is annealed, hot rolled, and air cooled to room temperature after hot rolling to obtain a hot rolled slab;

C. Quenching: heat the hot-rolled slab to 800-1000°C, keep it warm for 20-40 minutes, then quickly put it into oil and quench to cool to room temperature to obtain a quenched slab;

D. Warm rolling: heat the quenched slab at 450-550°C for 1 hour to obtain a tempered troostite slab, then heat the tempered troostite slab at 500-600°C for 5 ~7 passes, carrying out rolling deformation with a total reduction of 30~50%, air cooling to room temperature, to obtain warm rolled plates;

E. Heat treatment: keep the warm-rolled plate at 760-780°C for 3-5 hours, then quickly put it in a salt bath furnace at 260-320°C for 1.5-10 hours, and then take it out of the furnace and air-cool it to room temperature.

Another method includes the following steps:

A. Steelmaking: Calculate the feeding ratio according to the design requirements of the steel, smelt it, and cast it into steel ingots;

B. The steel ingot is heated to 1200~1250°C and kept for 3~5 hours for homogenization treatment; then it is air-cooled to 1180°C for rolling, and the final rolling temperature is not lower than 880°C. After 5~6 passes of rolling, it is finally rolled into 18 ~20 mm thick hot-rolled slab, hot-rolled slab air-cooled to room temperature;

C. Quenching and tempering: reheat the hot-rolled slab treated in step B to 850~950°C for 20~40 minutes, then quickly put it in oil to quench and cool to room temperature, and then put it into a furnace at 480~520°C Keep it warm for 1 h, take it out of the furnace and cool it to room temperature in air, and prepare a slab with tempered troostite structure;

D, cold rolling: the slab after tempering is subjected to multi-pass cold rolling deformation with a total reduction of 40%, to obtain a cold-rolled plate with a thickness of 12 mm;

E. Heat treatment: Put the cold-rolled sheet into a furnace with a temperature of 760~780°C, keep it warm for 3~5 hours, then quickly put it into a salt bath furnace with a temperature of 260~320°C and keep it at a constant temperature for 1.5~10 hours, then take it out of the furnace and air cool it to room temperature to obtain an ultra-fine-grained ferrite/low temperature bainite dual-phase steel plate.

Low-temperature bainite is a structure composed of extremely thin lath bainite and thin film-like retained austenite developed at the beginning of this century. It is a high-carbon alloy steel with a silicon content of more than 1.5% by weight. It is obtained by low-temperature isothermal bainite transformation at the starting point temperature of bainite. Due to the low transformation temperature, the thickness of the obtained bainite laths is relatively thin, even reaching tens of nanometers; and due to the inhibitory effect of silicon on the precipitation of carbides during the isothermal transformation process, a bainite lath is formed between the bainite laths. Retained austenite without carbide precipitation. Therefore, the organization is also called carbide-free nanostructure bainite. The ultra-fine lath bainite leads to high strength, and the transformation-induced plasticity effect of film-like retained austenite can further improve the plasticity and toughness, reduce the yield ratio and improve the formability. Then, replace the hard phase of traditional dual-phase steel with low-temperature bainite, and refine the grains of soft-phase ferrite to obtain fine-grained ferrite/low-temperature bainite dual-phase steel, which will make The performance of dual-phase steel is further improved.

The physical metallurgical principle of the above-mentioned technical scheme is: martensite is heated to a temperature below A _{c 1} and kept warm to obtain a tempered troostite structure. The matrix recrystallizes and forms part of austenite. Since the tempered troostite maintains the refined microstructure of the quenched martensite, it will form fine ferrite equiaxed grains after heating and recrystallization after rolling deformation, and obtain ultrafine-grained ferrite; at the same time, When the heating temperature exceeds A _{c 1} ( A _{c 1} is the lowest temperature in the so-called "α+γ" two-phase region), austenite transformation will occur. Since the heating temperature is in the two-phase region, the growth of austenite grains is more affected Large limit, so the formation of fine-grained austenite. In this way, the "ultrafine-grained ferrite + fine-grained austenite" structure will be formed during the heating and holding process in the two-phase region, and then the low-temperature (slightly higher than the martensite of the fine-grained austenite) in the salt bath furnace will start Point) During the isothermal process, the fine-grained austenite undergoes bainite transformation, while the ultra-fine-grained ferrite basically does not change. Since the higher silicon content inhibits the precipitation of carbides during the isothermal bainite transformation process, the austenite transforms into a carbide-free bainite structure with film-like retained austenite distributed between the bainite laths, that is, low temperature Bainite organization. Then, after cooling to room temperature, an ultrafine-grained ferrite/low temperature bainite dual-phase steel is obtained.

The beneficial effects produced by adopting the above technical solution are: (1) The microstructure of the ultra-fine-grained ferrite/low temperature bainite dual-phase steel of the present invention has been ultra-fine, and its tensile strength is not less than 1300MPa, even as high as 1500MPa, The yield ratio is 0.48~0.63, and the elongation rate is not less than 12%. It has high strength, high plasticity, low yield ratio and high strength-plastic product, and has good comprehensive mechanical properties. It can be used to manufacture energy-absorbing and anti-collision components with high formability requirements; (2) The preparation process of the present invention is simple, easy to implement and easy to control, which is beneficial to the realization of industrialized production. It only needs to precisely control the temperature of the heat treatment to control the product quality, and the preparation efficiency is high.

Description of drawings

Fig. 1 is the scanning electron micrograph of the duplex steel microstructure that embodiment 1 prepares;

Fig. 2 is the tensile curve of the dual-phase steel sample prepared in embodiment 1;

Fig. 3 is the scanning electron micrograph of the duplex steel microstructure prepared in embodiment 2;

Fig. 4 is the scanning electron micrograph of the duplex steel microstructure that embodiment 5 prepares;

Fig. 5 is the tensile curve of the dual-phase steel sample prepared in embodiment 5 ~ 7;

Fig. 6 is the tensile curve of the dual-phase steel samples prepared in Examples 8-10.

Detailed ways

Example 1

A. According to the mass percentage of C 0.3, Si 1.9, Mn 1.6, Cr 1.3, Ni 1.0, W 0.7, P 0.01, S0.01, the rest is the ratio of Fe and unavoidable impurities, calculate the feeding ratio, in vacuum Melted in a medium-frequency induction furnace and poured into a cylindrical steel ingot with a diameter of 170 mm.

B. Annealing and hot rolling: heat the steel ingot to 1220°C for 4 hours for homogenization treatment, and air-cool it to 1180°C to start rolling. The final rolling temperature is 880°C. After 6 passes of rolling, it is finally rolled into a 20 mm thick Hot rolled slab, air cooled to room temperature after rolling.

The smelting, annealing and hot rolling in the above step A and step C are all conventional steps.

C. Quenching: heat the hot-rolled slab to 900° C., keep it warm for 30 minutes, and then quickly put it into oil for quenching and cool it to room temperature to obtain a quenched slab.

D. Warm rolling: put the quenched slab into a furnace at 500° C. for 1 hour, and immediately perform multi-pass rolling deformation with a total reduction of 40% to obtain a 12 mm thick warm-rolled plate.

E. Heat treatment: put the warm-rolled sheet into a furnace at 760°C, keep it warm for 5 hours, then quickly put it into a salt bath furnace at 260°C for 10 hours, and then take it out of the furnace and air-cool it to room temperature.

Scanning electron microscope (SEM) analysis was carried out on the plate prepared in this example, and its microstructure photo is shown in Figure 1. It can be seen from the figure that: this example prepared ultra-fine-grained ferrite and bainitic dual-phase steel, in which The grain size of ultrafine-grained ferrite is 0.5~2μm, and the volume content is about 30%.

The plate of this embodiment is made into a sample, and the tensile test is carried out according to the GB/T228.1-2010 standard. The stress-strain curve is shown in Fig. 2, and the tensile strength of the sample is measured ( ) is 1383MPa, yield strength ( ) is 775 MPa, elongation ( ) is 14%, the calculated yield ratio is 0.56, and the strength-plastic product is 19362 MPa·%. Please refer to the data in Table 1 for details.

Table 1 Microstructure and mechanical properties of dual-phase steels in Examples 1-4

The above results show that the superfine-grained ferrite/low temperature bainite dual-phase steel produced in this example has high strength, high plasticity, low yield ratio, high strength-plastic product, and good comprehensive mechanical properties. It can be used to manufacture energy-absorbing anti-collision components with high formability requirements.

Example 2

Put the warm-rolled plate obtained according to Example 1 into a furnace with a temperature of 760°C, keep it warm for 5 hours, then quickly put it into a salt-bath furnace with a temperature of 290°C for 10 hours, and then take it out of the furnace and air-cool it to room temperature. See Figure 3 for its SEM photo, and Table 1 for the mechanical property test results.

Example 3

A. According to the mass percentage of C 0.28, Si 1.7, Mn 1.7, Cr 1.1, Ni 1.2, W 0.6, P 0.01, S0.01, the rest is the ratio of Fe and unavoidable impurities, calculate the feeding ratio, in a vacuum Melted in a medium-frequency induction furnace and poured into a cylindrical steel ingot with a diameter of 170 mm.

B. Annealing and hot rolling: heat the steel ingot to 1200°C for 5 hours for homogenization treatment, and air-cool it to 1180°C to start rolling. The final rolling temperature is 880°C. After 7 passes of rolling, it is finally rolled into a 20 mm thick Hot rolled slab, air cooled to room temperature after rolling.

C. Quenching: heat the hot-rolled slab to 850° C., keep it warm for 60 minutes, and then quickly put it into oil for quenching and cool it to room temperature to obtain a quenched slab.

D. Warm rolling: put the quenched slab into a furnace at 480° C. to keep it warm for 1 hour, and immediately carry out multi-pass rolling deformation with a total reduction of 35% to obtain a 13 mm thick warm-rolled plate;

E. Heat treatment: heat the warm-rolled plate at 780°C for 3 hours, then quickly put it in a salt bath furnace at 290°C for 3 hours, and then take it out of the furnace and air-cool it to room temperature.

A scanning electron microscope (SEM) analysis and a tensile test were carried out on the plates prepared in this example, and the results are shown in Table 1.

The above results show that the superfine-grained ferrite/low temperature bainite dual-phase steel produced in this example has high strength, high plasticity, low yield ratio, high strength-plastic product, and good comprehensive mechanical properties.

Example 4

A. According to the mass percentage of C 0.32, Si 2.1, Mn 1.5, Cr 1.4, Ni 0.8, W 0.8, P 0.01, S0.01, the rest is the ratio of Fe and unavoidable impurities, calculate the feeding ratio, in vacuum Melted in a medium-frequency induction furnace and poured into a cylindrical steel ingot with a diameter of 170 mm.

B. Annealing and hot rolling: heat the steel ingot to 1250°C for 2 hours for homogenization treatment, and then air-cool it to 1180°C to start rolling. The final rolling temperature is 880°C. After 7 passes of rolling, it is finally rolled into a 20 mm thick Hot rolled slab, air cooled to room temperature after rolling.

C. Quenching: heat the hot-rolled slab to 950° C., keep it warm for 20 minutes, and then quickly put it into oil to quench and cool to room temperature to obtain a quenched slab.

D. Warm rolling: heat the quenched slab at 550°C for 1 hour, then immediately perform multi-pass rolling with a total reduction of 50% to obtain a 10mm thick warm-rolled plate;

E. Heat treatment: Put the warm-rolled sheet into a furnace at 780°C for 3 hours, then quickly place it in a salt bath furnace at 320°C for 1.5 hours, and then take it out of the furnace and air cool it to room temperature.

A scanning electron microscope (SEM) analysis and a tensile test were carried out on the plates prepared in this example, and the results are shown in Table 1.

The above results show that the superfine-grained ferrite/low temperature bainite dual-phase steel produced in this example has high strength, high plasticity, low yield ratio, high strength-plastic product, and good comprehensive mechanical properties.

Example 5

A. According to the mass percentage of C 0.3, Si 1.9, Mn 1.6, Cr 1.3, Ni 1.0, W 0.7, P 0.01, S0.01, the rest is the ratio of Fe and unavoidable impurities, calculate the feeding ratio, in vacuum Melted in a medium frequency induction furnace and poured into a cylindrical steel ingot with a diameter of 170 mm;

B. The steel ingot is heated to 1220°C for 4 hours for homogenization treatment, and then air-cooled to 1180°C to start rolling. The final rolling temperature is 880°C. After 6 passes of rolling, the first and second pass reductions are 40 and 30 respectively. mm, the reduction in the last four passes is 20 mm, and finally rolled into a hot-rolled slab with a thickness of 20 mm, and air-cooled to room temperature after rolling;

C. Reheat the hot-rolled slab to 900°C and keep it warm for 30 minutes, then quickly put it into oil to quench and cool it to room temperature, then put it in a furnace at 500°C and keep it warm for 1 hour, and then take it out of the furnace and air cool it to room temperature to obtain tempered Qushi slabs of body tissue;

D, the slab after tempering is subjected to multi-pass cold-rolling deformation with a total reduction of 40%, to obtain a cold-rolled plate with a thickness of 12 mm;

E. Put the cold-rolled sheet into a furnace with a temperature of 760°C, keep it warm for 5 hours, then quickly put it into a salt-bath furnace with a temperature of 260°C for 10 hours, and then take it out of the furnace and air-cool it to room temperature to obtain ultra-fine-grained ferrite solid/low temperature bainite dual phase steel sheet. The scanning electron micrograph of its microstructure is shown in Figure 4. It can be seen from the figure that the grain size of ultrafine-grained ferrite is 0.5~2μm, and the volume content is about 35%.

The plate of this embodiment is made into a sample, and the tensile test is carried out according to the GB/T228.1-2010 standard. The stress-strain curve is shown in Figure 5. It can be seen from the figure that after the tensile load begins, the stress-strain curve First, it enters the elastic deformation stage, that is, as the strain increases, the tensile stress also increases linearly; with the increase of the tensile load action time, the curve gradually transitions to the yield stage, and at this stage, the curve does not appear Obvious platform or jagged shape, so it is inferred that the process is continuous yielding; further prolonging the loading time, the curve enters the stage of uniform plastic deformation, and the curve rises and reaches the tensile strength of the material; after that, the sample continues to produce uneven plastic deformation, at Necking occurs in the local area, and when the limit value is reached, the sample breaks.

According to the stress-strain curve of the sample, the tensile strength of the sample was measured ( ),Yield Strength( ) and elongation ( ) The results are shown in Table 2.

Table 2 Microstructure parameters and mechanical properties of samples prepared in Examples 5, 7, 8 and 10

This example shows that the ultrafine-grained ferrite/low temperature bainite dual-phase steel prepared in this example has high strength, high plasticity, low yield ratio and high strength-plastic product, good comprehensive mechanical properties, and can be used to manufacture formability Energy-absorbing anti-collision components with high requirements.

Embodiment 6~7

The difference from Example 1 is that in Step D, the cold-rolled sheet is put into a furnace at a temperature of 760°C, and after being kept for 5 hours, it is heated at a temperature of 275°C (Example 6) and 290°C (Example 7) respectively. Wait for 10 h in a salt-bath furnace, then take it out of the furnace and air-cool to room temperature. See Figure 5 and Table 2 for its tensile curve and test results.

Embodiment 8~10

The difference from Example 1 is that in step D, the cold-rolled sheet is placed in a furnace at a temperature of 780°C, kept for 3 hours, and then quickly placed in a salt-bath furnace at a temperature of 290, 305, and 320°C for 3 hours. , then take out of the oven and air cool to room temperature. The tensile test curve and results are shown in Figure 6 and Table 2, respectively.

Claims (6)

1. a kind of 1300MPa grades of ultra-fine grained ferrite/low temperature bainite dual-phase steel, it is characterised in that its chemical component is by weight Percentage is：C 0.28~0.32,Si 1.7~2.1,Mn 1.5~1.7,Cr1.1~1.4,Ni 0.8~1.2,W 0.6~0.8,P≤ 0.02, S≤0.02, remaining is Fe and inevitable impurity；Its metallographic structure be ultra-fine grained ferrite and low temperature bainite, The crystallite dimension of middle ultra-fine grained ferrite is 0.5 ~ 2 μm, volume content is 20 ~ 35%, the slat dimension of low temperature bainite is 75 ~ 300 nm。

2. 1300MPa grades of ultra-fine grained ferrite/low temperature bainite dual-phase steel according to claim 1, it is characterised in that institute Dual phase steel tensile strength is stated not less than 1300 MPa, yield tensile ratio is 0.50 ~ 0.65, and elongation percentage is not less than 12%.

3. a kind of preparation method of 1300MPa described in claim 1 grades of ultra-fine grained ferrites/low temperature bainite dual-phase steel, It is characterized in that including the following steps：

A, it makes steel：Ingredient proportion, melting are calculated according to the design requirement of steel and pour into steel ingot；

B, annealing, hot rolling：By the ingot annealing, hot rolling, it is air-cooled to room temperature after hot rolling, obtains hot rolling slab；

C, it quenches：The hot rolling slab is heated to 800 ~ 1000 DEG C, 20 ~ 40min of heat preservation, it is cold to be then put into oil quenching rapidly But to room temperature, quenching slab is obtained；

D, warm-rolling：The quenching slab is kept the temperature into 1h under the conditions of 450 ~ 550 DEG C, obtains the slab of secondary troostite tissue, then The rolling deformation for being 30 ~ 50% through 5 ~ 7 passages, progress overall reduction at 500 ~ 600 DEG C by the slab of secondary troostite tissue, it is empty It is cooled to room temperature, obtains warm-rolling plate；

E, it is heat-treated：By the warm-rolling plate under the conditions of 760 ~ 780 DEG C, 3 ~ 5h is kept the temperature, is then put into 260 ~ 320 DEG C rapidly 1.5 ~ 10 h of isothermal in salt bath furnace, then come out of the stove and be air-cooled to room temperature.

4. the preparation method of 1300MPa grades of ultra-fine grained ferrite/low temperature bainite dual-phase steel according to claim 3, It is characterized in that in step B that ingot annealing condition is：It is heated to 1200 DEG C ~ 1250 DEG C 2 ~ 5h of heat preservation；The condition of hot rolling is：It will annealing Steel ingot after heat preservation is air-cooled to 1150 ~ 1180 DEG C of open rollings, rolls 4 ~ 7 passages, and finishing temperature is not less than 880 DEG C, the stagnation pressure of hot rolling Lower amount is 30 ~ 50%.

5. the preparation method of 1300MPa grades of ultra-fine grained ferrite/low temperature bainite dual-phase steel according to claim 3, Be characterized in that hot rolled steel plate in step B with a thickness of 20mm, gained warm-rolling plate with a thickness of 10 ~ 14mm in step D.

6. a kind of preparation method of 1300MPa described in claim 1 grades of ultra-fine grained ferrites/low temperature bainite dual-phase steel, It is characterized in that including the following steps：

A, it makes steel：Ingredient proportion, melting are calculated according to the design requirement of steel and pour into steel ingot；

B, Heating Steel Ingots carry out Homogenization Treatments to 1200 ~ 1250 DEG C of 3 ~ 5 h of heat preservation；Then 1180 DEG C of open rollings, finish to gauge are air-cooled to Temperature is not less than 880 DEG C, by 5 ~ 6 passes, is finally rolled into the hot rolling slab of 18 ~ 20 mm thickness, hot rolling slab is air-cooled to Room temperature；

C, quenching and tempering：By after the hot rolling slab that step B is handled is reheated to 850 ~ 950 DEG C of 20 ~ 40 min of heat preservation, It is put into oil quenching rapidly to be cooled to room temperature, is then placed in 480 ~ 520 DEG C of stove and keeps the temperature 1 h, come out of the stove and be air-cooled to room temperature, make Obtain the slab of secondary troostite tissue；

D, cold rolling：The multi-pass cold-rolling deformation that slab after tempering is carried out to overall reduction 40%, obtains the cold-reduced sheet of 12 mm thickness Material；

E, it is heat-treated：Cold rolled sheet is put into the stove that temperature is 760 ~ 780 DEG C, after keeping the temperature 3 ~ 5h, being put into temperature rapidly is Constant temperature keeps 1.5 ~ 10 h in 260 ~ 320 DEG C of salt bath furnace, then comes out of the stove and is air-cooled to room temperature, obtains ultra-fine grained ferrite/low temperature shellfish Family name's body dual phase steel plate.