CN106868414A - Ultra-fine grained ferrite/low temperature bainite two-phase mild steel and preparation method thereof - Google Patents

Abstract

The invention discloses an ultra-fine-grained ferrite/low temperature bainite dual-phase low-carbon steel and a preparation method thereof. The chemical composition thereof is: C 0.18-0.22, Si 1.5-1.7, Mn 0.9-1.2, Cr 0.4~0.6, Mo 0.18~0.22, P≤0.02, S≤0.02, and the rest are Fe and inevitable impurities; among them, the grain size of ultrafine-grained ferrite is 0.5~3μm, and the volume content is 50~ 70%, the lath size of low temperature bainite is 95~212 nm. It quenches the martensite structure of low-carbon silicon-containing low-alloy steel, heats it to the temperature at which tempered troostite structure is obtained, rolls and deforms it after heat preservation, air cools it to room temperature, and then reheats it to the "α+γ" two-phase region for Partially austenitized, and then placed in a salt-bath furnace whose temperature is slightly higher than the starting point of martensite in the two-phase region austenite for isothermal bainite transformation, and then air-cooled to room temperature to obtain ultrafine-grained ferrite/ Low temperature bainite duplex structure.

Description

Ultra-fine-grained ferrite/low temperature bainite dual-phase low-carbon steel and preparation method thereof

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 an ultrafine-grained ferrite/low temperature bainite dual-phase low-carbon 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.), construction 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. First, the ferrite structure is refined to refine the ferrite grain size to the micron level (1-4 μm), that is, the ultra-fine ferrite structure. 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 combined with 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 an ultra-fine-grained ferrite/low temperature bainite dual-phase low-carbon steel with simple process, high preparation efficiency, easy precise control and high quality stability and its preparation method, which will reduce Carbon-silicon-containing low-alloy steel quenched martensite structure, heated to the temperature to obtain tempered troostite structure, rolled and deformed after heat preservation, air-cooled to room temperature, and then reheated to the "α+γ" two-phase region for partial austenitic Then put it into the salt bath furnace whose temperature is slightly higher than the martensite starting point of austenite in the two-phase region for isothermal bainite transformation, and then air-cool to room temperature to obtain ultra-fine-grained ferrite/low temperature bainite body duplex organization. The process is simple, and the prepared dual-phase steel has 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:

An ultra-fine-grained ferrite/low-temperature bainite dual-phase low-carbon steel, the chemical composition of which is: C 0.18~0.22, Si 1.5~1.7, Mn 0.9~1.2, Cr 0.4~0.6, Mo 0.18~ 0.22, P≤0.02, S≤0.02, and the rest are Fe and inevitable impurities; the grain size of ultrafine-grained ferrite is 1~4μm, the volume content is 57%, and the lath size of low-temperature bainite It is 95~212nm.

The present invention also provides a method for preparing ultra-fine-grained ferrite/low temperature bainite dual-phase low-carbon steel, comprising the following steps:

A. Steelmaking: Calculate the feeding ratio according to the design requirements of the steel, smelt, pour into steel ingots, and then carry out vacuum self-consumption refining;

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: heating the hot-rolled slab to 800-1000° C., keeping it warm for 20-40 minutes, quenching, and cooling to room temperature to obtain a quenched slab;

D. Warm rolling: heat the quenched slab at 500-550°C for 1 hour, then carry out rolling deformation with a total reduction of 30-50% at 500-600°C for 5-7 passes, and air-cool to room temperature to obtain a warm-rolled plate;

E. Isothermal transformation treatment: heat the warm-rolled sheet at 760-780°C for 3-5 hours, then quickly put it in a salt bath furnace at 260-380°C for 1.5-10 hours, and then air-cool to room temperature.

The chemical composition of the above steel is: C 0.18~0.22, Si 1.5~1.7, Mn 0.9~1.2, Cr0.4~0.6, Mo 0.18~0.22, P≤0.02, S≤0.02, the rest is Fe and not necessary Avoid impurities.

Preferably, the annealing condition for the steel ingot in step B is: heating the steel ingot to 1200°C-1250°C for 2-5 hours for homogenization treatment; the condition for hot rolling is: cooling the annealed steel ingot to 1150-1180°C in air Rolling, rolling 4~7 times, the final rolling temperature is not lower than 880 ℃, and the total reduction of hot rolling is 30~50%.

The physical metallurgical principle of the above-mentioned technical scheme is: the quenched martensite is heated to a certain high temperature and kept warm to obtain the tempered troostite structure, and after warm rolling and deformation at this temperature, it is reheated in the "α+γ" two-phase region Recrystallization of ferrite and partial formation of austenite occurred in the Since tempered troostite maintains the refined microstructure of quenched martensite, it will form fine ferrite equiaxed grains and obtain ultra-fine grain ferrite after heating and recrystallization after warm rolling deformation; at the same time, When the heating temperature exceeds A _{c 1} , austenite transformation occurs. Since the heating temperature is in the two-phase region, the growth of austenite grains is greatly restricted, so fine-grained austenite is formed. 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 thin film-like retained austenite distributed between the bainite laths. Low temperature can obtain low temperature bainite structure. Then, after cooling to room temperature, an ultra-fine-grained ferrite/low temperature bainite dual-phase low-carbon 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 low-carbon steel of the present invention has been refined, and has high strength, high plasticity, and low yield ratio and high-strength plastic product, with good comprehensive mechanical properties, 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 operate, and easy to control, which is conducive to the realization of industrial production, and only requires precise control The temperature of the heat treatment can control the product quality, and the preparation efficiency is high.

Description of drawings

Fig. 1 is the scanning electron micrograph of the microstructure of ultra-fine-grained ferrite/low temperature bainite dual-phase low-carbon steel prepared in Example 1;

Fig. 2 is the tensile curve of the ultrafine-grained ferrite/low temperature bainite dual-phase low-carbon steel sample prepared in Example 1;

3 is a scanning electron micrograph of the microstructure of the ultrafine-grained ferrite/low temperature bainite dual-phase low-carbon steel prepared in Example 2.

detailed description

Example 1

A. According to the mass percentage of C 0.21, Si 1.63, Mn 0.94, Cr 0.51, Mo0.2, P 0.006, S 0.001, the rest is the ratio of Fe and unavoidable impurities, calculate the feeding ratio, in the vacuum medium frequency induction furnace It is melted in a medium and poured into a cylindrical steel ingot with a diameter of 170 mm, and refined in a vacuum.

B. Annealing and hot rolling: heat the steel ingot to 1220°C for 2 hours for homogenization treatment, and then air-cool it to 1150°C to start rolling. The final rolling temperature is 920°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.

C. Quenching: reheat the hot-rolled slab in the furnace to 950° C., keep it warm for 40 minutes, then quickly take it out of the furnace and put it in water to quench and cool to room temperature to obtain a quenched slab.

D. Warm rolling: put the quenched sheet into a furnace at 550°C for 1 hour, and then perform multi-pass warm rolling deformation with a total reduction of 40% to obtain a 12mm thick warm-rolled sheet.

E. Isothermal transformation treatment: put the warm-rolled sheet into a furnace at 780°C, keep it warm for 0.5h, then quickly put it into a salt-bath furnace at 280°C for 1.5h, 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 produced ultra-fine-grained ferrite and low-temperature bainite dual-phase steel, Among them, the grain size of ultrafine-grained ferrite is 1-3 μm, and the volume content is about 57%, and the lath size of low-temperature bainite is 95-212 nm.

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.

In this embodiment, the hard phase of traditional dual-phase steel is replaced by low-temperature bainite, and then the grains of soft-phase ferrite are refined to obtain fine-grained ferrite/low-temperature bainite dual-phase steel. This will further improve the performance of dual-phase steel. 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 944MPa, yield strength ( ) is 516 MPa, elongation ( ) is 33%, the calculated yield ratio is 0.55, and the strength-plastic product is 31100 MPa·%. See the data in Table 1 for details.

Table 1 Microstructure and mechanical properties of dual-phase low-carbon steel in Examples 1-3

The above results show that the ultra-fine-grained ferrite/low temperature bainite dual-phase low-carbon steel produced in this example has high strength, high plasticity, low yield-tensile ratio and 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

A. According to the mass percentage of C 0.18, Si 1.52, Mn 1.02, Cr 0.50, Mo 0.21, P 0.01, S 0.01, the rest is the ratio of Fe and inevitable impurities, calculate the feeding ratio, in the vacuum medium frequency induction furnace It is melted and poured into a cylindrical steel ingot with a diameter of 170 mm, and refined in a vacuum.

B. Annealing and hot rolling: heat the steel ingot to 1200°C for 5 hours for homogenization treatment, and air-cool it to 1150°C to start rolling. The final rolling temperature is 920°C. After 6 passes of rolling, it is finally rolled into a 20 mm thick hot Roll the slab and air cool to room temperature after rolling.

C. Quenching: reheat the hot-rolled slab in the furnace to 950° C., keep it warm for 40 minutes, then quickly take it out of the furnace and put it in water to quench and cool to room temperature to obtain a quenched slab.

D. Warm rolling: put the quenched sheet into a furnace at 570°C for 1 hour, and then perform multi-pass warm rolling deformation with a total reduction of 40% to obtain a 12mm thick warm-rolled sheet.

E. Isothermal transformation treatment: put the warm-rolled sheet into a furnace at 780°C, keep it warm for 0.5h, then quickly put it into a salt-bath furnace at 290°C for 1.0h, 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 3

A. According to the mass percentage of C 0.22, Si 1.68, Mn 1.12, Cr 0.45, Mo 0.18, P 0.01, S 0.01, the rest is the ratio of Fe and inevitable impurities, calculate the feeding ratio, in the vacuum medium frequency induction furnace It is melted and poured into a cylindrical steel ingot with a diameter of 170 mm, and refined in a vacuum.

B. Annealing and hot rolling: heat the steel ingot to 1220°C for 2 hours for homogenization treatment, and air-cool it to 1150°C to start rolling. The final rolling temperature is 920°C. After 6 passes of rolling, it is finally rolled into a 20 mm thick hot Roll the slab and air cool to room temperature after rolling.

C. Quenching: reheat the hot-rolled slab in the furnace to 950° C., keep it warm for 1 hour, then quickly take it out of the furnace and put it in water to quench and cool to room temperature to obtain a quenched slab.

D. Warm rolling: put the quenched sheet into a furnace at 560°C for 1 hour, and then perform multi-pass warm rolling deformation with a total reduction of 40% to obtain a 12mm thick warm-rolled sheet.

E. Heat treatment: put the warm-rolled sheet into a furnace at 780°C, keep it warm for 0.5h, then quickly put it into a salt bath furnace at 280°C for 1.5h, 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

The difference with embodiment 1 is:

Step E, isothermal transformation treatment: put the warm-rolled sheet into a furnace at 780°C, keep it warm for 0.5h, then quickly put it into a salt-bath furnace at 290°C for 1h, 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.

Claims (10)

1. a kind of ultra-fine grained ferrite/low temperature bainite two-phase mild steel, it is characterised in that its chemical composition is by weight percentage For：C 0.18 ~ 0.22, Si 1.5 ~ 1.7, Mn 0.9 ~ 1.2, Cr 0.4 ~ 0.6, Mo 0.18 ~ 0.22, P≤0.02, S≤0.02, Remaining is Fe and the impurity that can not be avoided；Wherein the crystallite dimension of ultra-fine grained ferrite be 1 ~ 4 μm, volume content be 57%, it is low The slat dimension of warm bainite is 95 ~ 212 nm.

2. ultra-fine grained ferrite according to claim 1/low temperature bainite two-phase mild steel, it is characterised in that the two-phase Mild steel tensile strength is 888 ~ 944 MPa, and yield strength is 458 ~ 516 MPa, and elongation percentage is 33 ~ 37%.

3. the preparation method of a kind of ultra-fine grained ferrite/low temperature bainite two-phase mild steel, it is characterised in that comprise the following steps：

A, steel-making：Design requirement according to steel calculates ingredient proportion, melting, pours into steel ingot；

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

C, quenching：The hot rolling slab is heated to 800 ~ 1000 DEG C, 20 ~ 40min of insulation, quenching is cooled to room temperature, must quench Slab；

D, warm-rolling：By it is described quenching slab 1h is incubated under the conditions of 500 ~ 550 DEG C, then 500 ~ 600 DEG C through 5 ~ 7 passages, enter Row overall reduction is 30 ~ 50% rolling deformation, is air cooled to room temperature, obtains warm-rolling sheet material；

E, isothermal transformation treatment：By the warm-rolling sheet material under the conditions of 760 ~ 780 DEG C, then insulation is put into rapidly 260 ~ 380 DEG C Salt bath furnace in the h of isothermal 1.5 ~ 10, then be air cooled to room temperature.

4. the preparation method of ultra-fine grained ferrite according to claim 3/low temperature bainite two-phase mild steel, its feature exists Ingot annealing condition is in step B：The Heating Steel Ingots to 1200 DEG C ~ 1250 DEG C 2 ~ 5h of insulation are carried out into Homogenization Treatments； The condition of hot rolling is：Steel ingot after insulation of annealing is air cooled to 1150 ~ 1180 DEG C of open rollings, 4 ~ 7 passages are rolled, finishing temperature is not low In 880 DEG C, the overall reduction of hot rolling is 30 ~ 50%.

5. the preparation method of the ultra-fine grained ferrite according to claim 3 or 4/low temperature bainite two-phase mild steel, it is special Levy is that the chemical composition of the steel is by weight percentage：C 0.18~0.22、Si 1.5~1.7、Mn 0.9~1.2、Cr 0.4 ~ 0.6, Mo 0.18 ~ 0.22, P≤0.02, S≤0.02, remaining is Fe and the impurity that can not be avoided.

6. the preparation method of ultra-fine grained ferrite according to claim 4/low temperature bainite two-phase mild steel, its feature exists The step of also including that carrying out vacuum consumable refines after pouring into steel ingot in step A.

7. the preparation method of ultra-fine grained ferrite according to claim 4/low temperature bainite two-phase mild steel, its feature exists Finishing temperature is 920 DEG C in step B, is finally rolled into the thick hot rolling slabs of 20 mm.

8. the preparation method of ultra-fine grained ferrite according to claim 4/low temperature bainite two-phase mild steel, its feature exists In step E under the conditions of 780 DEG C, 0.5-3 h, the isothermal 1.5h in 275 ~ 290 DEG C of salt bath furnaces are incubated.

9. the preparation method of ultra-fine grained ferrite according to claim 8/low temperature bainite two-phase mild steel, its feature exists In step E under the conditions of 780 DEG C, 0.5 h is incubated.

10. the preparation method of ultra-fine grained ferrite according to claim 4/low temperature bainite two-phase mild steel, its feature It is that the quenching slab is incubated 1h under the conditions of 560 ~ 570 DEG C in step D, it is 30 ~ 50% that overall reduction is carried out immediately after Rolling deformation.