CN113996651A

CN113996651A - Low-carbon steel and manufacturing method for improving performance uniformity of low-carbon steel rolled by ferrite in CSP process

Info

Publication number: CN113996651A
Application number: CN202111273680.0A
Authority: CN
Inventors: 胡学文; 王承剑; 彭欢; 李耀辉; 赵海山; 王海波; 史红林; 张云锦; 杨德伦; 王飞; 游慧超; 吴志文; 孙照阳; 赵虎; 石东亚
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2021-10-29
Filing date: 2021-10-29
Publication date: 2022-02-01
Also published as: CN114405996A; CN114192577A; CN114226453A; CN114226453B; CN114405996B; CN114192577B

Abstract

The invention provides a low-carbon steel and a manufacturing method for improving the performance uniformity of low-carbon steel rolled by ferrite in a CSP process, which comprises the following components: c: less than or equal to 0.070%; si: less than or equal to 0.050 percent; mn: 0.08-0.30%; p: less than or equal to 0.025 percent; s: less than or equal to 0.015 percent; al: 0.020-0.060%; the balance of Fe and inevitable impurities; compared with the prior art, the invention solves the problems of narrow process window of ferrite rolling temperature, large sensitivity of the material structure performance of the austenite-ferrite transformation critical area influenced by temperature and inconsistency of austenite-ferrite transformation by reasonable smelting, heating, hot rolling, cooling and coiling processes in the CSP process, and improves the uniformity of the material performance. The uniformity of the performance of the low-carbon steel plate in the width direction is improved by over 50 percent, and the problem of edge waves in the next cold rolling galvanizing procedure is solved.

Description

Low-carbon steel and manufacturing method for improving performance uniformity of low-carbon steel rolled by ferrite in CSP process

Technical Field

The invention relates to the technical field of production processes of sheet billet continuous casting and rolling steel, in particular to low-carbon steel and a manufacturing method for improving performance uniformity of low-carbon steel rolled by ferrite in a CSP process.

Background

Compared with the conventional process, the yield strength of the low-carbon steel with the carbon content less than 0.07 percent produced by the CSP ferrite rolling process is reduced by about 15 to 25 percent, the thickness of the iron scale is reduced by 30 to 50 percent, the load and the power consumption of a rolling mill can be obviously reduced, and the method is one of the leading-edge processes of the iron and steel industry in the hot rolling field in the world at present. However, due to the technical characteristics of the CSP ferrite rolling process, the CSP ferrite rolling temperature process window is narrow, the sensitivity of the material structure performance in the critical area of the transformation from austenite to ferrite is high due to the influence of temperature, the inconsistency of the transformation from austenite to ferrite leads to relatively poor performance uniformity in the width direction of ferrite rolling low-carbon steel, and edge waves are easily generated in the next cold rolling and galvanizing process, so that the batch popularization and application of the ferrite rolling process are restricted.

There are several patents reported on methods for producing low carbon steel by ferrite rolling in the CSP process. For example: in 2016, 12, 21, patent CN 106244921a discloses a method for producing low-carbon steel by ferrite rolling process on CSP production line, which comprises the following chemical components by weight percent: 0.02 to 0.06%, Si: 0.01-0.04%, Mn: 0.1-0.3%, and the balance of impurity elements such as P, S and Fe. The process is characterized in that low-temperature heating, low-temperature rolling and high-temperature coiling are adopted, the reduction rate of F1 and F2 frames, the cooling water amount between F1 and F3 frames and the temperature of an F4 inlet are controlled, and finally the thick polygonal ferrite of the low-carbon steel with the thickness of 2-6 mm is obtained, the yield strength is 210MPa, the tensile strength is 330MPa, the yield strength is reduced by about 27.33%, and the tensile strength is reduced by about 13.40%. The patent CN111633028A published in 9/8/2020 discloses a method for producing thin low-carbon steel by ferrite rolling in a CSP production line, which comprises the steps of casting blank thickness of 50-60 mm, heating temperature of 990-1030 ℃, finish rolling inlet temperature of 900-950 ℃, finish rolling temperature of 760-800 ℃ and coiling temperature of 680-720 ℃. And after the steel coil is off-line, the steel coil enters the heat-preserving cover for slow cooling for more than 48 hours, and the thickness of the product is 1.0-2.0 mm. At present, ferrite rolling process research on CSP flow mainly focuses on the aspects of reducing strength, improving surface quality of strip steel and the like.

A method for manufacturing low-carbon steel by ferrite rolling in an ESP process disclosed in 2018, 1 month and 19 days, for example, patent CN107597844A discloses a ferrite rolling method and a ferrite rolling device for a low-carbon steel coil for endless continuous casting and rolling deep drawing, wherein the thickness of a casting blank is 80-130 mm, a roughing mill set takes austenite at 950-1020 ℃ for rough rolling until the thickness of an intermediate blank is 7-25 mm, then the intermediate blank is rapidly cooled to 730-850 ℃ in the same cooling channel for finish rolling, the thickness of a product is 0.7-4 mm, and the coiling temperature is 580-710 ℃. The continuous casting and rolling production line is suitable for rough rolling and finish rolling and is provided with a cooling channel. Patent CN108994081A on 12/14/2018 discloses a method for producing low-carbon steel by ferrite rolling in an ESP production line, which comprises the steps of finish rolling of rough-rolled strip steel, wherein the strip steel sequentially enters a first rack, a second rack, a third rack, a fourth rack and a fifth rack during finish rolling, and cooling water is carried out on the strip steel which enters between the first rack and the second rack and between the second rack and the third rack, so that the strip steel is rolled in a ferrite area between the third rack and the fifth rack during finish rolling of the strip steel. The inlet temperature of finish rolling is 950-1050 ℃, the outlet temperature is 740-800 ℃, and the coiling temperature is 650-700 ℃. The continuous casting and rolling production line is suitable for a sheet billet continuous casting and rolling production line with rough rolling and finish rolling.

In summary, at present, research results on the production of low-carbon steel by ferrite rolling in a thin slab continuous casting and rolling production line mainly focus on how to realize ferrite rolling process, strength reduction and surface quality improvement, and no patent is yet made on a manufacturing method for improving the performance uniformity of low-carbon steel by ferrite rolling in a CSP process.

Disclosure of Invention

The invention aims to provide a low-carbon steel and a manufacturing method for improving the performance uniformity of the low-carbon steel rolled by ferrite in a CSP process. The uniformity of the performance of the low-carbon steel plate in the width direction is improved by over 50 percent, and the problem of edge waves in the next cold rolling galvanizing procedure is solved.

The specific technical scheme of the invention is as follows:

a manufacturing method for improving the performance uniformity of low-carbon steel rolled by ferrite in a CSP process comprises the processes of converter smelting, LF furnace refining, continuous casting, heating, rolling, laminar cooling and coiling.

Carrying out converter smelting, LF furnace refining and vibration curve according to formula components, and carrying out continuous casting to obtain a casting blank with the thickness of 60-70 mm by adopting a high-frequency low-amplitude principle;

heating, namely heating the casting blank in a heating furnace, controlling the heating temperature of the casting blank to be 1000-1050 ℃, and reducing the temperature of the edge part to be less than 50 ℃;

the rolling is carried out by a 7-stand finishing mill after high-pressure water descaling, and the inlet pressure is set to be less than or equal to 300 bar; the outlet pressure is set to be less than or equal to 300 bar; cooling among F1 racks, applying a secondary descaling water system F2 under the pressure of 10MPa, closing cooling water, descaling water and roll gap lubrication among other racks, and controlling the temperature fluctuation of the F4 rack in the width direction to be less than 20 ℃; the wedge degree of the rolling mill is controlled to be +/-30 mu m, the convexity is controlled to be +/-20 mu m, and the flatness is controlled to be +/-50I. The pass reduction rate of the F1, F2 and F3 frames is controlled to be 40-65%, and the reduction rate of the F4 frame is controlled to be less than 10%. The inlet temperature of the F4 stand is not more than 870 ℃, the finish rolling temperature of the F7 stand is controlled at 780-820 ℃, constant-speed rolling is adopted, and the rolling speed of the outlet of the F7 stand is not less than 6 m/s.

And coiling the rolled steel plate after laminar cooling, wherein the laminar cooling mode is rear-section laminar sparse cooling, the side spraying mode adopts symmetrical side spraying, the side spraying is water spraying or air spraying, and the coiling temperature of the cooled steel plate is controlled at 740 ℃ for plus materials, preferably at 710 ℃ for plus materials 730 ℃.

After being coiled, the hot-rolled low-carbon steel coil produced by the production method is put into a slow cooling pit or a heat preservation cover for slow cooling to room temperature.

The low-carbon steel produced by the invention comprises the following components in percentage by mass:

c: less than or equal to 0.070%; si: less than or equal to 0.050 percent; mn: 0.08-0.30%; p: less than or equal to 0.025 percent; s: less than or equal to 0.015 percent; al: 0.020-0.060%; the balance of Fe and inevitable impurities.

According to the invention, on the heating process design, the heating temperature of the casting blank is controlled to be 1000-1050 ℃, and the heating temperature fluctuation in the width direction is controlled to be +/-20 ℃. The air surplus coefficient of the heating roller table is 1.05-1.2, and the purpose of the method is to reduce the surface oxidation burning loss of the casting blank and prevent the excessive heating temperature from causing the hard removal of the iron scale on the surface of the casting blank; on the other hand, the temperature difference between the heating temperature and the finishing temperature is reduced, powerful conditions are brought to the follow-up 7-stand finish rolling to reduce cooling water as far as possible, and the temperature uniformity of the surface of the hot rolled plate is improved. In addition, under certain conditions such as rolling load distribution, cooling water between stands, finish rolling temperature and the like, the heating temperature is reduced, so that the temperature of a casting blank finish rolling inlet can be reduced, the rolling speed can be increased, and the production efficiency can be improved.

In the design of a rolling process, an F1-F3 rack is rolled at a high reduction ratio, the characteristic of high temperature and high reduction of a rolled piece is fully exerted, the rolling temperature of the F4 rack is in a two-phase region of transformation from an austenite phase to a ferrite phase, the material structure performance of a critical transformation region from the austenite to the ferrite phase is greatly influenced by temperature, in order to solve the problem of consistency of transformation from the austenite to the ferrite phase and improve the stability of the material performance, cooling water, descaling water and the like between all racks are closed after F3, and the rolling reduction ratio of the F4 rack is required to be less than 10%; the final rolling temperature is controlled at 780-820 ℃, so that the rolling is carried out in a ferrite phase transition temperature region, the rolling pressure is reduced, and the rolling stability is improved; a constant-speed rolling mode is adopted, the rolling speed of the F7 rack is controlled to be more than or equal to 6.0m/s, the main purpose is to reduce the rolling time of a rolled piece in a two-phase region and improve the stability of the product structure performance. The coiling temperature is controlled at 700-740 ℃, and the purpose is to make ferrite completely undergo static recrystallization and fully grow up grains, reduce the strength of the strip steel and improve the elongation percentage of the strip steel on the one hand; on the other hand, the temperature difference between the finishing temperature and the coiling temperature can be reduced by increasing the coiling temperature, the temperature nonuniformity caused by laminar cooling is improved, and the performance uniformity of the hot coil in the width direction is improved.

In the laminar cooling design, the laminar cooling mode is rear-stage laminar sparse cooling, and the side spraying mode adopts symmetrical side spraying, and the aim of the invention is mainly to reduce surface accumulated water and improve the temperature uniformity of a hot rolled plate, thereby improving the performance uniformity of ferrite rolling low-carbon steel. After being coiled, the steel is placed in a slow cooling pit or a heat preservation cover for slow cooling to room temperature, so that ferrite grains can be further fully grown on one hand; on the other hand, the uniformity of the structure properties in the length and width directions of the hot-rolled coil can be improved.

The metallographic structure of the low-carbon steel plate produced by the method is polygonal ferrite and a small amount of pearlite, wherein the volume of the polygonal ferrite accounts for 95-100%, and the volume of the small amount of pearlite accounts for 0-5%; the grain size of ferrite is 7.5-8.5 grade; the yield strength Rp0.2 is 210-239MPa, the tensile strength Rm is 322-356MPa, and the elongation A₈₀38-44%, yield strength fluctuation in width direction of 22MPa, tensile strength of 34MPa, hardness fluctuation of 12HRB, and hardness fluctuation of cold-rolled finished product of 4HR 30T; the yield strength Rp0.2 of the low-carbon steel plate produced by other ferrite rolling processes is 223-₈₀32-34%, yield strength fluctuation in width direction 41MPa, tensile strength 58MPa, hardness fluctuation 27HRB, and hardness fluctuation 10.5HR30T of cold rolled product. Compared with other ferrite rolling processes, the yield strength and the tensile strength of the low-carbon steel plate produced by the method are reduced by 20MPa, the elongation is improved by about 3 percent, the performance fluctuation of the hot-rolled and cold-rolled finished products in the width direction is reduced by 50 percent, the uniformity of the hot-rolled and cold-rolled galvanized finished products in the width direction can be obviously improved, and the problem of edge wave of the next cold-rolled galvanizing process is solved.

Drawings

FIG. 1 shows the widthwise hardness fluctuation of low carbon steel according to example 3 of the present invention;

FIG. 2 shows a metallographic structure of low carbon steel according to example 3 of the present invention;

FIG. 3 shows the widthwise hardness fluctuation of a cold-rolled low-carbon steel product according to example 3 of the present invention;

FIG. 4 shows metallographic structures of cold-rolled low-carbon steel products according to example 3 of the present invention;

FIG. 5 shows the widthwise hardness fluctuation of low carbon steel according to the conventional ferrite process of comparative example 1;

FIG. 6 is a metallographic structure of a low carbon steel corresponding to the conventional ferrite process of comparative example 1;

FIG. 7 shows the widthwise hardness fluctuation of a cold-rolled low carbon steel product corresponding to the conventional ferrite process of comparative example 1.

FIG. 8 shows metallographic structures of cold rolled low carbon steel products corresponding to the conventional ferrite process of comparative example 1.

Detailed Description

The technical solution of the present invention will be described below by way of specific examples.

Example 1 to example 3

A manufacturing method for improving the performance uniformity of low-carbon steel rolled by ferrite in a CSP process comprises the following chemical components in percentage by weight: c: less than or equal to 0.070%, Si: less than or equal to 0.050%, Mn: 0.08-0.30%, P: less than or equal to 0.025 percent, S: less than or equal to 0.015 percent, Als: 0.020-0.060% of Fe and the balance of inevitable impurities. The contents of the components in the examples are shown in table 1, and the balance not shown in table 1 is Fe and inevitable impurities.

The manufacturing method comprises the following steps: carrying out converter smelting, LF furnace refining, continuous casting, heating, rolling, laminar cooling and coiling.

The process control is as follows:

1) smelting and continuous casting, wherein converter smelting, LF furnace refining and vibration curve adopt the principle of high frequency and low amplitude according to the components in the requirement 1; and then casting into a casting blank with the thickness of 60-70 mm;

2) heating, wherein the casting blank enters a heating furnace for heating, the heating temperature of the casting blank is controlled to be 1050 ℃ at 1000-;

3) rolling, namely performing high-pressure water descaling and then rolling by using a 7-stand finishing mill, wherein the inlet pressure is set to be less than or equal to 300 (bar); the outlet pressure is set to be less than or equal to 300 (bar); cooling among the F1 racks, applying a secondary descaling water system (10MPa) in the F2 mode, closing cooling water, descaling water and roll gap lubrication among the rest racks, and controlling the temperature fluctuation of the F4 rack in the width direction to be less than 20 ℃; the wedge degree of the rolling mill is controlled to be +/-30 mu m, the convexity is controlled to be +/-20 mu m, and the flatness is controlled to be +/-50I. The pass reduction rate of the F1, F2 and F3 frames is controlled to be 40-65%, and the reduction rate of the F4 frame is controlled to be less than 10%. The inlet temperature of the F4 stand is not more than 870 ℃, the finish rolling temperature of the F7 stand is controlled at 780-820 ℃, constant-speed rolling is adopted, and the rolling speed of the outlet of the F7 stand is not less than 6 m/s. .

4) And (3) coiling the rolled steel plate after laminar cooling, wherein the laminar cooling mode is rear-section laminar sparse cooling, a symmetrical side spraying (water spraying or air spraying) mode is adopted as the side spraying mode, and the coiling temperature of the cooled steel plate is controlled at 740 ℃ for plus materials, preferably at 710 ℃ for plus materials 730 ℃.

5) After being coiled, the hot-rolled low-carbon steel coil produced by the method is put into a slow cooling pit or a heat preservation cover for slow cooling to room temperature.

Examples 1 to 3 were produced by the same method as above, with the thickness of the continuously cast slab controlled to 60 to 70mm, and the heating temperature, the finish rolling temperature, the laminar cooling rate, and the coiling temperature controlled. The process parameter control of the specific examples 1 to 3 is shown in tables 2 and 3.

Comparative example 1

A manufacturing method of low-carbon steel comprises the following chemical components in percentage by weight: as shown in table 1, the balance not shown in table 1 was Fe and inevitable impurities.

The method for producing a low-carbon steel according to comparative example 1 is the same as in example, except for the parameters shown in tables 2 and 3.

Table 1 examples and comparative examples the measured chemical composition (mass percent, wt%)

Numbering	C	Si	Mn	P	S	Al
							Example 1	0.031	0.020	0.11	0.018	0.004	0.024
Example 2	0.030	0.018	0.11	0.012	0.003	0.024
							Example 3	0.029	0.020	0.11	0.011	0.003	0.025
Comparative example 1	0.050	0.028	0.10	0.012	0.003	0.025

TABLE 2 Rolling Process parameters of examples and comparative examples

TABLE 3 Rolling Process parameters of examples and comparative examples

The product performance parameters produced in each example and comparative example are shown in tables 4 and 5.

TABLE 4 mechanical Properties of the examples and comparative examples

TABLE 5 mechanical Properties of the examples and comparative examples

The hardness fluctuation of the low-carbon steel produced by the method in the width direction is 10HRB, and the hardness fluctuation of a cold-rolled finished product is 4HR 30T. The metallographic structure of the low-carbon steel produced by the conventional ferrite rolling process is that the hardness fluctuation in the width direction is more than or equal to 25HRB, and the hardness fluctuation of a cold-rolled finished product is 10.5HR 30T. Typical width-direction hardness fluctuations for both processes are shown in fig. 1, fig. 3, fig. 5, and fig. 7, respectively.

Claims

1. The manufacturing method for improving the performance uniformity of low-carbon steel subjected to ferrite rolling in the CSP process is characterized by comprising the following steps of rolling; the rolling is specifically as follows: the pressure of a rolling inlet is set to be less than or equal to 300 bar; the outlet pressure is set to be less than or equal to 300 bar; and F1 cooling between frames and F2 applying a secondary descaling water system, closing cooling water, descaling water and roll gap lubrication among other frames, and controlling the temperature fluctuation of the F4 frame in the width direction of the inlet to be less than 20 ℃.

2. The manufacturing method according to claim 1, wherein the rolling, rolling mill wedge control ± 30 μm, crown control ± 20 μm, and flatness control ± 50I.

3. The manufacturing method according to claim 1 or 2, characterized in that the rolling is carried out by controlling the pass reduction of the F1, F2 and F3 stands to be 40-65% and the reduction of the F4 stand to be less than 10%.

4. The manufacturing method as claimed in claim 1 or 2, wherein the inlet temperature of the F4 stand is not more than 870 ℃ and the finishing temperature of the F7 stand is controlled at 780-820 ℃.

5. The manufacturing method according to claim 1 or 2, wherein the rolling is performed at a constant rolling speed, and the exit rolling speed of the F7 stand is more than or equal to 6 m/s.

6. The method according to claim 1 or 2, further comprising heating the ingot to a temperature of 1000-1050 ℃ and a temperature drop of the edge portion of less than 50 ℃.

7. The manufacturing method according to claim 1 or 2, further comprising laminar cooling, wherein the laminar cooling is rear-stage laminar sparse cooling, the side-spraying is symmetrical side-spraying, and the side-spraying is water spraying or air spraying.

8. The method according to claim 1 or 2, further comprising coiling at a coiling temperature of 700-.

9. A low carbon steel manufactured by the manufacturing method according to any one of claims 1 to 8, characterized by comprising the following components in percentage by mass: