WO2007079625A1

WO2007079625A1 - An ultrafine grain steel sheet produced by continuous casting and rolling a medium-thin slab and its manufacture process

Info

Publication number: WO2007079625A1
Application number: PCT/CN2006/001097
Authority: WO
Inventors: Jianlun Sun; Wanshan Zhang; Ju Guang; Xu Yang; Xin Chen; Qun Sun; Keqiang Wang; Liqun Liu; Yongbin Zhai; Guoli Fu; Jun Lu; Mingkai Yang
Original assignee: Angang Steel Company Limited
Priority date: 2006-01-06
Filing date: 2006-05-25
Publication date: 2007-07-19
Also published as: CN1995431A; CN100503864C; TR200804980T2; WO2007079625A8

Abstract

An ultrafine grain steel sheet produced by continuous casting and rolling a medium-thin slab and its manufacture process. The composition of the steel is (in wt%): C 0.12-0.18%, Si 0.15-0.35%, Mn 0.65-1.20%, P≤0.02%, S≤0.015%, balance Fe and impurities. The manufacture process includes: hot loading the cast ingot directly at a temperature above 800°C, heating to 1100-1280°C; rough rolling the slab to 30-60mm; finish rolling in multi-passes with large accumulated deformation with an inlet temperature of 920-1050°C and an outlet temperature of 750-850°C, in which the average deformation rate of the steel sheet between finish rolling mills is 1-10/s; laminar cooling at a rate of 10-45°C/s, reeling at 400-620°C. The hot-continuous-rolling steel sheet is 2-18mm in thickness, and have an average grain size of 3-7μm, yield strength up to above 400MPa, tensile strength of 510-580MPa, and elongation beyond 26%.

Description

Ultra-fine grain steel plate produced by medium and thin slab continuous casting and rolling and production method thereof

Technical field

The invention belongs to the technical field of rolling steel, relates to a hot-rolled steel plate and a production method thereof, in particular to a hot-rolled steel plate with ultra-fine grain in a short-flow hot-rolled wide strip steel production line of medium and thin slab continuous casting and rolling. And its production methods.

Background technique

Superfine grain processing is a cost-effective way to improve the physical and mechanical properties of steel materials. It can increase the strength of the material while reducing the ductile-brittle transition temperature of the material. At present, based on a simple C-Mn system, the grain of the steel material is remarkably refined by an effective process, and the performance is remarkably improved, so that the final product achieves ultra-thin weight reduction and long life, and has become an international steel material research. One of the most concentrated hot spots. Since the establishment of the "973" program in China, relevant research institutions and institutions in China and metallurgical enterprises have been conducting research in this area and have made significant progress. At present, the domestically formed hot-rolled steel sheet with ultra-fine grain is rolled on the continuous casting and rolling line of conventional (slab thickness greater than 230mm) based on the composition of Q235 type carbon steel. Mature production technology. It is well known that the production process of ultra-fine grain steel is characterized by low temperature and large deformation rolling (requiring a certain slab thickness) and rapid cooling after rolling. Generally, only ultra-fine grain steel plates with a thickness of 8 mm can be produced, in medium and thin slabs ( Thickness of 100~200mm) Continuous casting and rolling short-flow hot-rolled wide-band steel production line. Due to the thin slab thickness and small compression ratio, it is very difficult to produce ultra-fine grain steel (thickness of 2~18mm). Summary of the invention

The present invention has been made to solve the above problems, and an object thereof is to provide a continuous casting and rolling short-flow hot-rolled wide-bar steel production line for producing ultra-fine grain steel sheets, and achieving high strength, high plastic toughness, and reasonable yield. Excellent overall performance such as strong ratio. Another object of the present invention is to produce a method of the product, which can significantly reduce energy consumption,

1

Confirmation This reduces production time and reduces production and time costs.

In order to achieve the above object, the specific technical solution of the present invention is as follows:

In order to obtain ultra-fine grain hot-rolled steel sheet in the continuous casting and rolling short-flow hot-rolled wide strip steel production line, the chemical composition of the ordinary SS400 steel is adjusted and optimized, and the specific composition is weight: C : 0.12% to 0.18%, Si: 0.15% to 0.35%, Μη: 0·65% to 1.20%, P: <0.02%, S: <0.015%, and the balance is Fe and other impurities.

The microstructure of the steel sheet is mainly ferrite and pearlite, and the average grain size of the ferrite is 3 to 7 μm.

The rolling process of the present invention takes the following production process steps:

a. The molten steel is cast into a slab having a specification of 100~200mmX 800~2000mm according to the requirements of the composition;

b direct hot loading, the temperature of the hot charging furnace is above 800 °C, and the heating temperature of the casting blank is controlled between 1100~1280 °C;

After the billet is heated and discharged, the high-pressure water descaling device is used for descaling;

d rough rolling on a two-roll or four-roll reversible roughing mill to reduce the slab to 30~60mm _; e-rolling and rolling inlet temperature is controlled between 920~1050 °C, the billet is between the finishing stands Multi-pass, large deformation strain accumulation, finishing mill F3 ~ F5 to apply 20% ~ 55% reduction, control the average deformation rate of the steel strip between the finishing stands between l~10 / s, while in the rack Spray water cooling to ensure the finish rolling outlet temperature is between 750~850 °C;

f Immediately after the final rolling, the laminar flow is cooled, the cooling rate is 10~45 ° C / s, and the temperature is controlled to be taken up between 400 and 620 ° C.

The basis of the above specific technical solutions is - I. Composition adjustment and optimization: In order to reduce production costs, and at the same time improve the toughness of steel, to achieve the ultimate product ultra-thin weight loss and extended service life, the chemical composition of ordinary SS400 steel has been adjusted and optimized.

C plays an important role in ferritic-pearlite steel and is the most economical and most effective strengthening element. As the content of c increases, ferrite decreases, cementite increases, and cementite strengthens second phase. The strength of steel increases, but the ductility and toughness decrease. Taking into account the combined effects of strength and toughness, the adjustment C is 0.12~0.18%.

Si is soluble in ferrite and acts as a solid solution strengthening agent to increase the strength and hardness of steel, but the plasticity and toughness will decrease as the silicon content increases. At the same time, if Si exceeds 0.5%, red rust will be generated, which will deteriorate the surface shape, coating property and weldability of the steel sheet. To improve the strength and avoid the surface shape of the steel sheet, the silicon content is adjusted to 0.15% to 0.35%.

Mn can reduce the Ar ₃ transformation point, expand the austenite phase region, and have a solid solution strengthening effect. At the same time, Mn can inhibit the generation of pearlite during the cooling process after hot rolling, but in order to save the alloy, the amount of manganese is adjusted to 0.65% to 1.20%.

FeS synthesized by S and Fe forms a low melting point eutectic with Fe. Its melting point is only 985 ° C and is distributed on the grain boundary. When the steel is rolled at 800~1200 ° C, the eutectic is melted. The relationship between the grains is weakened, the steel is cracked, and the heat is brittle. In order to avoid the hot brittleness and the formability and weldability of the steel sheet, the sulfur content in the steel is strictly controlled ≤0.015%.

Since P in carbon steel is low in solubility in x-Fe, the tendency to segregation is severe, and the low-temperature brittleness of steel is increased, especially in the case of more than 0.06%, the toughness of the welded portion is deteriorated, and P is more than 0.02% to promote equiaxedness. The formation of ferrite reduces the effect of the yield ratio, so the P content is adjusted to ≤ 0.02%.

Second, the process control:

The medium-thin slab solidifies faster, and it is easy to obtain finer grains and smaller composition segregation, so that it can be further improved. Steps to refine the grains and reduce the segregation and looseness in the central region of the slab, so that the internal quality of the product is improved. The dynamic soft reduction process is adopted on the continuous casting unit to improve the slab structure and facilitate the formation of the final ultrafine grains. Direct hot charging and 1100~1280 °C steel burning temperature, effectively saving energy and preventing the grain boundary and crystal enthalpy caused by excessive growth of austenite grains, so that the grain refining effect Weakened. After the slab is heated and discharged, the high-pressure water is sprayed by the upper and lower double-row spray headers to remove the scale to improve the surface quality of the slab, and then rolled by a four-roll reversible roughing mill to reduce the slab to 30-60 mm. In order to achieve the adjustable range of the roll gap of the head finishing mill, the thickness of the billet before the finishing rolling should be reasonably set according to the thickness of the finished product, and the thickness of the intermediate billet should be increased as much as possible under the premise of the rolling capacity to ensure the finish rolling. The shape variable helps to form a large amount of ultrafine ferrite grains. Controlling the temperature, deformation and deformation rate during rolling is very effective for refining the formed steel structure and reducing the deformation resistance. Therefore, it is rolled and rolled at 920 to 1050 ° C to make it in austenite. Multi-pass large deformation strain accumulation in the non-recrystallization zone, and 20%~55% reduction in the finishing mill F3~F5, effectively increasing the defects such as grain boundary, deformation zone and dislocation of the deformed austenite, thereby Improve the effective grain boundary area of ferrite nucleation and the nucleation rate of ferrite, strain-induced ferrite transformation to achieve the purpose of refining ferrite grains, and control the average deformation rate of steel strip between finishing stands. Between ~10/s to relieve the load on the mill. Spray cooling is used between the finishing stands to ensure the final rolling temperature is between 750 and 850 ° C to retain the deformed structure, inhibit the austenite recrystallization and grain growth.

Forced cooling can increase the degree of subcooling of the phase change, increase the nucleation rate, and slow down the grain growth at lower temperatures, thereby refining the grains and improving the microstructure. Choosing a suitable coiling temperature has a great influence on the toughness of the steel. If the coiling temperature is too low, it will cause difficulty in coiling, and more bainite will be produced to increase the strength and toughness. When the coiling temperature is too high, the microstructure of the steel is coarse, resulting in a decrease in plasticity and toughness. Therefore, the laminar cooling rate at a cooling rate of 10 to 45 ° C / s ensures that the coiling temperature is Between 400 and 620 ° C.

The invention has the following advantages and effects: Through the adjustment and optimization of the chemical composition of the ordinary SS400 steel, and the tissue refinement control technology, the specification is produced in the short-flow hot-rolled wide-band steel production line of the medium-thin slab continuous casting and rolling process. It is a hot-rolled steel plate of 2~18mm. The microstructure of this steel plate is mainly ferrite and pearlite. The average grain size of ferrite is 3~7μηι, the yield strength is above 400MPa, and the tensile strength is 510. Between 580 MPa and an elongation of more than 26%, the chemical composition of this steel is very simple and easy to smelt. The ultra-fine grain hot-rolled steel sheet rolled has excellent comprehensive properties such as high strength, high plastic toughness and reasonable yield ratio, and achieves ultra-thin weight loss and long service life of the final product. The hot charging of the rolling process significantly reduces energy consumption, shortens production time and reduces production cost and time cost. The steel does not contain special alloying elements, which can replace the low-alloy steel structure of the same grade, saving The alloy resources can significantly improve the economic and social benefits of the company.

DRAWINGS

Figure 1 is a photograph of the metallographic structure of a 2 mm thick hot-rolled steel sheet having ultrafine grain.

Figure 2 is a photograph of the metallographic structure of a 6 mm thick hot-rolled steel sheet with ultrafine grain.

Figure 3 is a photograph of the metallographic structure of a 12 mm thick hot-rolled steel sheet with ultrafine grain.

Figure 4 is a photograph of the metallographic structure of a hot-rolled steel sheet having an ultrafine grain of 18 mm thick. detailed description

The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

The present invention employs the following chemical components (% by weight): C: 0.12% to 0.18%, Si: 0.15% to 0.35%, Mn: 0.65% to 1.20%, P: <0.02%, S: <0.015%, The amount of Fe and inevitable impurities, the microstructure of this steel plate is mainly ferrite and pearlite, the average crystal of ferrite The particle size is 3 to 7 μm.

The chemical composition of the ultra-fine grain steel set in the above four ranges is used to smelt separately on the converter, and cast into a slab of the specification of 100~200mmx800~2000mm, which is directly hot-packed, and the temperature of the hot-packing furnace is above 800 °C. The heating temperature is controlled between 1100 and 1280 ° C; after the billet is heated and discharged, the double-row high-pressure water descaling device is used for descaling; the rough rolling is performed on the two-roll or four-roll reversible roughing mill to thin the slab Up to 30~60mm; the inlet temperature of the finish rolling and rolling is controlled between 920~1050 °C, 7 times and large deformation strain accumulation are performed on the steel slab between the finishing stands, and the finishing mill F3~F5 is applied 20%~55% The amount of reduction, the average deformation rate of the steel strip between the finishing stands is between l~10/s, and the water is cooled between the racks to ensure the finishing temperature is between 750~850 °C;

Immediately after the final rolling, laminar cooling is used, the cooling rate is 10 to 45 ° C / s, and the temperature is controlled to be taken up between 400 and 620 ° C.

In the medium-thin slab continuous casting and rolling short-flow hot-rolled wide-bar steel production line, controlled rolling is carried out according to the requirements of the present invention to obtain an ultra-fine grain hot-rolled steel sheet. The specific experiments and results are shown in the following tables.

Table 1 shows the chemical composition of steel, Table 2 shows the process parameters during the rolling process, and Table 3 shows the corresponding mechanical properties and ferrite grain size. The metallographic photos are shown in Figures 1-4. Table 1 Chemical composition of the example steel (% by weight)

Example C Si Mn P s Fe and impurities

1 0. 12 0. 20 0. 65 0. 011 0. 008 balance

2 0. 14 0. 23 0. 87 0. 012 0. 006 balance

3 0. 16 0. 32 1. 01 0. 010 0. 005 Balance

4 0. 18 0. 26 1. 19 0. 010 0. 005 Balance Table 2 Control parameters of the rolling process of the example steel

EXAMPLES Slab heating Intermediate billet Finishing inlet Finishing mill exit Coiling cooling rate Finishing mean deformation

Ό mm V °C Ό °C/s rate l/s

1 1150 1100 35 38 980 920 850 800 620- -510 10 20 6 10

2 1200 1150 35 40 1000 940 830-780 600- -500 25 33 5 8

3 1250 1200 38 43 Painting ~960 800 755 475- -420 35 40 7 3

4 1280 1250 38 43 1050 980 790-750 460- -400 38 45 3 1

Table 3 Mechanical properties and ferrite grain size of the example steel Example Rel Rm A Cold bend B=35 Ferrite grain Specification

(MPa) (MPa) (%) D=0.5a Size (μιπ) (mm)

1 480 450 580 545 36- '30 Qualified 4.2-3.0 2X1950X*

2 455 420 550 535 36- '33 Qualified 5.3 4· 1 6X1850X*

3 440 415 535 525 33- ^30 Qualified 5.8 5.0 12X1550X*

4 435—400 530 510 29- '27 Qualified 7.0 5.6 18 X 1500 X* The above embodiment uses the technical method described in the present invention to produce a short-flow hot-rolled wide strip steel production line for medium and thin slab continuous casting and rolling Hot-rolled steel sheet with ultra-fine grain structure, yield strength

(Rel) is between 400 460 MPa, tensile strength (Rm) is between 510 580 MPa, elongation (A) is between 27% and 36%, and ferrite grain size is between 3.0 and 7.0 μm.

Claims

Rights request

1. An ultra-fine grain steel plate produced by continuous casting and rolling of medium and thin slab, characterized in that: chemical composition of the steel plate, weight percentage: C: 0.12%~0.18%, Si: 0.15%~0.35%, Mn: 0.65% to 1.20%, P: <0.02%, S: <0.015%, and the balance is Fe and other impurities.

2. The ultrafine grain steel plate produced by continuous casting and rolling of medium and thin slab according to claim 1, wherein the microstructure of the steel plate is mainly ferrite and pearlite, and ferrite is The average grain size is 3 to 7 μm.

3. A method for producing an ultra-fine grain steel sheet produced by continuous casting and rolling of medium and thin slab according to claim 1, wherein the process steps are as follows:

a cast steel according to the range of composition requirements into a slab of 100~200mmX 800~2000mm;

b Direct hot charging, the temperature of the hot charging furnace is above 800 °C, and the heating temperature of the casting blank is controlled between 1100~1280 °C;

d rough rolling on a two-roll or four-roll reversible roughing mill to reduce the slab to 30~60mm; e-rolling and rolling inlet temperature is controlled between 920~1050 °C, the billet is between the finishing stands Multi-pass, large deformation strain accumulation, finishing mill F3 F5 implementation of 20% ~ 55% reduction, control steel strip between the finishing frame between the average deformation rate between l~10 / s, while in the rack Spray water cooling to ensure the finish rolling outlet temperature is between 750~850 °C;

f After the final rolling, laminar cooling is used, the cooling rate is 10~45 °C / s, and the temperature control is taken up between 400~ 620 °C.