CN113235011B

CN113235011B - Method for reducing blackening defect of surface of 550 MPa-level low-alloy high-strength steel

Info

Publication number: CN113235011B
Application number: CN202110405663.1A
Authority: CN
Inventors: 张亮亮; 于洋; 马壮; 王朝斌; 王林; 王畅; 刘文鑫; 高小丽; 任秋红; 史震; 李晓军; 李欢; 陈斌
Original assignee: Shougang Group Co Ltd; Shougang Jingtang United Iron and Steel Co Ltd
Current assignee: Shougang Group Co Ltd; Shougang Jingtang United Iron and Steel Co Ltd
Priority date: 2021-04-15
Filing date: 2021-04-15
Publication date: 2022-05-20
Anticipated expiration: 2041-04-15
Also published as: CN113235011A

Abstract

The embodiment of the invention discloses a method for reducing blackening defects on the surface of 550 MPa-level low-alloy high-strength steel, which comprises the following steps: smelting and continuously casting molten steel to obtain a casting blank; adding continuous casting covering slag containing 5-6% of F element in mass fraction into the continuous casting; the total mass fraction of Nb and Ti in the casting blank is controlled to be less than or equal to 0.04 percent; heating the casting blank before rolling, rough rolling, finish rolling and cooling after rolling to obtain a finish rolled plate; coiling the finish rolling plate at 520-580 ℃ to obtain a hot rolling coil; and stretching, bending and straightening the hot-rolled coil, and then pickling, cold rolling and finishing to obtain 550 MPa-grade low-alloy high-strength steel with good surface quality. According to the invention, the total mass fraction of Nb and Ti is controlled to be less than or equal to 0.04% of Nb + Ti, and the coiling temperature is controlled to be 520-580 ℃, so that the problem of blackening of 550MPa low-alloy high-strength steel can be solved, and the surface quality of 550MPa low-alloy high-strength steel can be improved.

Description

Method for reducing blackening defect of surface of 550 MPa-level low-alloy high-strength steel

Technical Field

The invention relates to the technical field of ferrous metallurgy, in particular to a method for reducing blackening defects on the surface of 550 MPa-level low-alloy high-strength steel.

Background

The low-alloy high-strength steel is developed on the basis of common carbon steel, and is widely applied to the construction of basic facilities such as automobiles, ships, ocean engineering, bridges and the like due to moderate strength, good processability and economical applicability.

The whole process from steel making to cold rolling has a plurality of working procedures, which causes the control difficulty of the surface quality of products to be increased, and the cold rolling and pickling process can remove the surface iron scale of the hot rolling raw material, thereby exposing various pickling defects, wherein the phenomenon of blackening of the surface strip of the low-alloy high-strength steel after pickling is shown in figure 1, which causes huge threat to the product quality, the subsequent cold rolling and annealing can not be effectively eliminated, and the conventional method only focuses on the improvement of the hot rolling surface iron scale, but can not be completely eliminated.

Therefore, how to develop a method for reducing the blackening defect of the surface of the 550 MPa-level low-alloy high-strength steel becomes a technical problem to be solved urgently.

Disclosure of Invention

The invention aims to provide a method for reducing the surface blackening defect of 550 MPa-level low-alloy high-strength steel and reducing the surface blackening defect of a 550MPa low-alloy high-strength steel pickling plate.

In order to achieve the above object, the present invention provides a method for reducing blackening defects on the surface of 550 MPa-grade low-alloy high-strength steel, the method comprising:

smelting and continuously casting molten steel to obtain a casting blank; adding continuous casting covering slag containing 5-6% of F element in mass fraction into the continuous casting; the total mass fraction of Nb and Ti in the casting blank is controlled to be less than or equal to 0.04 percent;

heating the casting blank before rolling, rough rolling, finish rolling and cooling after rolling to obtain a finish rolled plate;

coiling the finish rolling plate at 520-580 ℃ to obtain a hot rolling coil;

and stretching, bending and straightening the hot-rolled coil, and then pickling, cold rolling and finishing to obtain 550 MPa-grade low-alloy high-strength steel with good surface quality.

Further, the chemical components of the steel are as follows in mass fraction: c: 0.06-0.1%, Mn: 1.2-1.6%, Si: 0.08-0.13%, Nb + Ti is less than or equal to 0.04%, and the balance is Fe and inevitable impurities.

Further, the pre-rolling heating comprises: the temperature of the heating section is increased to the soaking temperature of 1180-1220 ℃ from room temperature, and the temperature of the soaking section is kept at the soaking temperature of 1180-1220 ℃; and the total heating time before rolling is 130-160 min.

Further, in the rough rolling, 3+3 passes are adopted for rolling, the rolling speed is controlled to be 2-5 m/s, the total deformation of the rough rolling is controlled to be 75-85%, and the inlet temperature of the rough rolling is controlled to be 1140-1170 ℃.

Further, in the precision rolling, 6 passes of rolling are adopted, the rolling speed is controlled to be 8-12 mm/s, the total deformation of the precision rolling is controlled to be 85-95%, the inlet temperature of the precision rolling is controlled to be 1000-1040 ℃, and the final temperature of the precision rolling is controlled to be 900-920 ℃.

Further, the post-rolling cooling adopts a sparse cooling process, and the cooling is carried out at the speed of 30-50 ℃/s to 500-530 ℃.

Further, in the stretching, bending and straightening, a two-bending one-straightening machine is adopted, and the straightening amount is 10-15 mm; the two-bending one-straightening machine comprises a first bending unit and a second bending unit, wherein the insertion depth of the first bending unit is 15-20 mm, and the insertion depth of the second bending unit is 20-25 mm.

Further, in the acid washing, the acid washing temperature is 75-90 ℃, and the acid washing speed is 40-220 m/min.

Further, in the cold rolling, the cold rolling reduction is controlled to be 60-95%.

Further, the finishing elongation is 0.2% -0.4%.

The invention also provides 550MPa low-alloy high-strength steel prepared by the method.

One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

the invention provides a method for reducing blackening defects on the surface of 550 MPa-level low-alloy high-strength steel, which comprises the following steps: smelting and continuously casting molten steel to obtain a casting blank; adding continuous casting covering slag containing 5-6% of F element in mass fraction into the continuous casting; the total mass fraction of Nb and Ti in the casting blank is controlled to be less than or equal to 0.04 percent; heating the casting blank before rolling, rough rolling, finish rolling and cooling after rolling to obtain a finish rolled plate; coiling the finish rolling plate at 520-580 ℃ to obtain a hot rolling coil; and stretching, bending and straightening the hot-rolled coil, and then pickling, cold rolling and finishing to obtain 550 MPa-grade low-alloy high-strength steel with good surface quality. According to the embodiment of the invention, the total mass fraction of Nb and Ti is controlled to be less than or equal to 0.04% and the coiling temperature is controlled to be 520-580 ℃, so that the problem of blackening of 550MPa low-alloy high-strength steel can be easily reduced under the condition of not adding equipment, and the surface quality of the 550MPa low-alloy high-strength steel is improved; the method is simple, economical and efficient.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.

FIG. 1 is a prior art topographical view of a surface blackening defect;

FIG. 2 is a diagram showing the surface morphology of the steel slab of comparative example 1 after pickling when Nb + Ti is 0.045%;

FIG. 3 is a graph of the acid-washed surface morphology of a steel slab composition of one embodiment of example 1 with Nb + Ti of 0.035%;

FIG. 4 is a surface morphology diagram of acid washing in the case where the content of F-in the mold flux in comparative example 2 is 12%;

FIG. 5 is a diagram showing the surface morphology of an acid-washed mold flux in the case where the F-content is 6% in the mold flux according to one embodiment of example 1;

FIG. 6 is a graph showing the surface morphology of the acid-washed film in comparative example 3 at a take-up temperature of 660 ℃;

FIG. 7 is a graph of the acid-washed surface topography at a coiling temperature of 560 ℃ for one embodiment of example 1;

FIG. 8 is a graph of the acid-washed surface topography for the case of the withdrawal straightening process H1 > H2 in comparative example 4;

FIG. 9 is a graph of the acid-washed surface topography for one embodiment of the withdrawal and straightening process of example 1, H1 < H2;

FIG. 10 is a flowchart of a method for reducing blackening defects on the surface of 550 MPa-grade low-alloy high-strength steel according to an embodiment of the invention.

Detailed Description

The present invention will be described in detail below with reference to specific embodiments and examples, and the advantages and various effects of the present invention will be more clearly apparent therefrom. It will be understood by those skilled in the art that these specific embodiments and examples are illustrative of the invention and are not to be construed as limiting the invention.

Throughout the specification, unless otherwise specifically noted, terms used herein should be understood as having meanings as commonly used in the art. Accordingly, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. If there is a conflict, the present specification will control.

Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be prepared by existing methods.

According to an exemplary embodiment of the present invention, there is provided a method for reducing blackening defects on a surface of 550MPa grade low-alloy high-strength steel, as shown in fig. 10, including:

s1, smelting and continuously casting the molten steel to obtain a casting blank; adding continuous casting covering slag containing 5-6% of F element in mass fraction into the continuous casting; the total mass fraction of Nb and Ti in the casting blank is controlled to be less than or equal to 0.04 percent;

specifically, the chemical components of the steel are as follows in mass fraction: c: 0.06-0.1%, Mn: 1.2-1.6%, Si: 0.08-0.13%, Nb + Ti is less than or equal to 0.04%, and the balance is Fe and inevitable impurities.

The application discovers through experiments that:

the continuous casting covering slag viscosity influences the surface quality of a casting blank, the insufficient covering slag viscosity causes uneven slag film between the wall of a crystallizer and a primary blank shell, the heat conduction is uneven, the thickness and the strength of the primary blank shell are uneven, the acting force applied to the blank shell is unstable in the vibration process of the crystallizer, the local micro-area of the primary blank shell is broken, and finally the defect of the casting blank is caused. By adding F in the continuous casting mold flux^-The content of the casting powder is controlled to be 5-6%, the viscosity of the casting powder is improved, liquid slag is enabled to uniformly flow into a gap between an initial blank shell and the wall of a crystallizer, and a slag film with uniform distribution thickness is formed, so that the surface quality of a casting blank is improved. The main components of the continuous casting mold flux can include CaO: 30-35%, MgO: 6.2 to 7.2 percent of Na₂O：8.1～9.1％，SiO₂:24～28％，Li₂O：0.4～1％，F：4.2～7.1％，Al₂O₃：5～6％，C_free:4～5％。

Secondly, if the content of the alloy elements Nb and Ti in the low-alloy high-strength steel in the continuous casting process is too high, the casting blank quality problems such as the generation of casting blank cracks, the segregation of internal components of the casting blank and the like are easy to occur especially when the content of Nb and Ti in the steel blank exceeds 0.045%. The total mass fraction of Nb and Ti is controlled to be less than or equal to 0.04 percent;

in conclusion, F in the casting powder is obtained by controlling the alloy components of the low-alloy high-strength steel and the chemical components of the casting powder^-The viscosity of the steel is reduced to below 8 percent, the uniformity of a slag film is improved, and the surface defects of a casting blank are obviously reduced, so that the strip defects on the surface of the strip steel in the subsequent rolling process are greatly reduced.

S2, heating the casting blank before rolling, roughly rolling, finely rolling, cooling after rolling to obtain a finely rolled plate;

as an alternative embodiment, the pre-rolling heating comprises: the heat-preserving and heat-preserving device comprises a heating section and a soaking section, wherein the heating section is heated to a soaking temperature of 1180-1220 ℃ from room temperature, and the soaking section is heat-preserving at the soaking temperature of 1180-1220 ℃; the total heating time before rolling is 130-160 min. Is favorable for ensuring the uniformity of the head and tail structures of the plate blank.

In an optional embodiment, in the rough rolling, 3+3 passes are adopted for rolling, the rolling speed is controlled to be 2-5 m/s, the total deformation of the rough rolling is controlled to be 75-85%, and the inlet temperature of the rough rolling is controlled to be 1140-1170 ℃. The arrangement is favorable for controlling the temperature drop in the rough rolling process and ensuring the uniformity of the hot coil head and tail structure performance.

In an optional embodiment, in the fine rolling, 6 passes of rolling are adopted, the rolling speed is controlled to be 8-2 mm/s, and the total deformation amount of the fine rolling is controlled to be 85-95%. The arrangement is favorable for controlling the temperature drop in the finish rolling process and ensuring the uniformity of the hot coil head and tail structure performance.

As one optional implementation mode, the post-rolling cooling adopts a sparse cooling process, and the cooling is carried out at the speed of 30-50 ℃/s to 500-530 ℃. This application discovers at hot rolling process research, is belted steel laminar cooling device between finish rolling export to the coiling machine, to low-alloy high-strength steel, controls the segregation degree that different laminar cooling mode also influenced belted steel surface texture, leads to the appearance of banded structure, and the segregation degree is more serious, changes the defect that blackens that appears in the pickling process. Actual industrial trial production finds that compared with front section concentrated cooling, sparse cooling is relatively more uniform, and segregation can be effectively reduced by matching with proper coiling temperature.

S3, coiling the finish rolling plate at 520-580 ℃ to obtain a hot rolling coil;

the application finds that: the coiling temperature has great effect on controlling the iron scale, and if the coiling temperature is lower than 520 ℃, the precipitation of microalloy carbide is not facilitated; if the coiling temperature is too high, the iron sheet structure is unreasonable, the coiling temperature exceeds 600 ℃, the non-metallic oxide on the interface of the iron sheet and the substrate is obviously increased, in addition, the interface of the iron sheet is more uneven, in the acid pickling process, the acid pickling is more difficult, and the blackening defect is easy to occur.

S4, straightening the hot-rolled coil by stretching and bending, and then pickling, cold rolling and finishing to obtain 550 MPa-grade low-alloy high-strength steel with good surface quality.

In the stretching bending straightening, a two-bending one-straightening machine is adopted, and the straightening amount is 10-15 mm; the two-bending one-straightening machine comprises a first bending unit and a second bending unit, wherein the insertion depth of the first bending unit is 15-20 mm, and the insertion depth of the second bending unit is 20-25 mm.

The stretch bending straightener is positioned in front of a pickling tank, the pickling quality is improved by stretching and bending an iron oxide skin layer on the surface of a scaled steel strip, the stretch bending straightener adopts a two-bending one-straightening process, the reasonable arrangement of the insertion depths of the two groups of bending rollers has a great influence on the descaling effect of the stretch bending straightener under the condition that the total insertion depth is relatively fixed, the number 2 insertion amount is greater than the number 1 insertion amount, and the optimal descaling effect can be obtained when the difference value of the insertion depths of the two groups of bending rollers is less than 10 mm. The specific parameters are that the insertion depth H1 of the No. 1 bending roller is 15-20 mm, the insertion depth H2 of the No. two bending rollers is 20-25 mm, and the straightening amount is 10-15 mm.

In an optional embodiment, in the acid washing, the acid washing temperature is 75-90 ℃, and the acid washing speed is 40-220 m/min.

As an alternative embodiment, the finishing elongation is 0.2% to 0.4%.

The method for reducing the blackening defect of the surface of the 550 MPa-grade low-alloy high-strength steel is described in detail in the following by combining the examples, the comparative examples and the experimental data.

S1, smelting the molten iron in a converter, and obtaining a casting blank of 550MPa low-alloy high-strength steel by adopting a continuous casting mode; the actual chemical composition is shown in table 1.

TABLE 1-550MPa Low-alloy high-strength steel chemical composition (wt%)

Group of	C	Mn	Si	Nb+Ti
					Example 1	0.090％	1.38％	0.08％	0.035％
Example 2	0.060％	1.20％	0.10％	0.040％
					Example 3	0.010％	1.60％	0.13％	0.030％
Comparative example 1	0.082％	1.46％	0.09％	0.045％
					Comparative example 2	0.090％	1.38％	0.08％	0.035％
Comparative example 3	0.090％	1.38％	0.08％	0.035％
					Comparative example 4	0.090％	1.38％	0.08％	0.035％

S2, heating the casting blank before rolling, rough rolling, finish rolling, cooling after rolling to obtain a finish rolling plate; heating the casting blank before rolling, rough rolling, finish rolling and cooling after rolling to obtain a finish rolled plate;

s3, coiling the finish rolling plate to obtain a hot rolling coil;

The parameters for each example and each comparative example are specifically shown in table 2, and the other parameters not listed are approximately the same.

TABLE 2 Process parameters

Statistics are carried out on the surface blackening severity of the pickled surface of the 550MPa low-alloy high-strength steel plate of each group, and the results are shown in Table 3. Wherein the degree of severity of blackening is divided into severe blackening, light blackening and no blackening in sequence from high degree of severity to low degree of severity.

TABLE 3

Group of	Severity of blackening
		Example 1	Does not blacken
Example 2	Does not blacken
		Example 3	Does not blacken
Comparative example 1	Severe blackening
		Comparative example 2	Blackening is more serious
Comparative example 3	Blackening is more serious
		Comparative example 4	Blackening is more serious

From the data in table 3, it can be seen that:

in comparative example 1, Nb + Ti is 0.045%, which is greater than the range of Nb + Ti of the inventive example being not more than 0.04%, and the surface topography is shown in fig. 2, with severe surface blackening;

in comparative example 2, the content of F in the continuous casting mold flux was 12%, which is larger than the range of 5 to 6% in the examples of the present invention, and the surface topography was as shown in fig. 4, and the surface was severely blackened;

in comparative example 3, the coiling temperature was 660 ℃ which is higher than the temperature range of 520 ℃ to 580 ℃ in the example of the invention, the surface topography is shown in fig. 6, and the surface blackening is serious;

in comparative example 4, the withdrawal and straightening process was: h1 is more than H2, the insertion depth H1 of the No. 1 bending roller is 23mm, the insertion depth H1 of the No. two bending roller is 18mm, and the straightening amount is 12 mm; the surface topography is shown in FIG. 8, the surface is severely blackened;

the black defect problem does not occur in the embodiment 1-the embodiment 3, and the surface quality of the 550MPa low-alloy high-strength steel is good.

FIGS. 2-9 illustrate:

2-3 are the same situation of other craft, adopt two kinds of different alloy compositions in example 1 and comparative example 1, compare the surface situation after the hot coil acid cleaning, through comparing, can see, with the reduction of microalloy Nb/Ti element, the stripe defect severity of the acid cleaning surface is obviously reduced, demonstrate that the control microalloy composition can lighten the acid cleaning and turn black the defect effectively;

FIGS. 4-5 show the surface conditions of the casting powder with 12% F content and the casting powder with 6% F content obtained by comparing the casting powder with the steel billet Nb + Ti content of less than 0.04% in example 1 and comparative example 2, and the comparison shows that the severity of surface blackening can be reduced by reducing the F content of 6% in the casting powder;

FIGS. 6 to 7 show the surface conditions of example 1 and comparative example 3, which are obtained by comparing the precipitation cooling mode for layer cooling and the two coiling temperatures of 660 ℃ and 560 ℃ respectively under the condition that the Nb + Ti content of the steel billet is less than 0.04% and other processes are the same, and the severity of surface blackening can be further reduced by reducing the coiling temperature to 560 ℃ through comparison.

FIGS. 8-9 show a comparison of two withdrawal straightening processes, process 1: h1 is more than H2, the insertion depth H1 of the No. 1 bending roller is 23mm, the insertion depth H1 of the No. two bending roller is 18mm, and the straightening amount is 12 mm; and (2) a process: h1 is less than H2, the insertion depth H1 of the bending roller No. 1 is 16mm, the insertion depth H2 of the bending roller No. 1 is 24mm, and the surface condition obtained by the straightening amount of 14mm can be seen through comparison.

Finally, it should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.

It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.

Claims

1. A method for reducing blackening defects on the surface of 550 MPa-level low-alloy high-strength steel is characterized by comprising the following steps:

smelting and continuously casting molten steel to obtain a casting blank; adding continuous casting covering slag containing 5-6% of F element in mass fraction into the continuous casting; the casting blank comprises the following chemical components in percentage by mass: c: 0.06-0.1%, Mn: 1.2-1.6%, Si: 0.08-0.13%, Nb + Ti is less than or equal to 0.04%, and the balance is Fe and inevitable impurities;

heating the casting blank before rolling, rough rolling, finish rolling and cooling after rolling to obtain a finish rolling plate, wherein the cooling after rolling adopts a sparse cooling process and is cooled to 500-530 ℃ at the speed of 30-50 ℃/s;

coiling the finish rolling plate at 520-580 ℃ to obtain a hot rolling coil;

stretching, bending and straightening the hot-rolled coil, then pickling, cold rolling and finishing to obtain 550 MPa-grade low-alloy high-strength steel with good surface quality, wherein a two-bending one-straightening machine is adopted in the stretching, bending and straightening, and the straightening amount is 10-15 mm; the two-bending one-straightening machine comprises a first bending unit and a second bending unit, wherein the insertion depth of the first bending unit is 15-20 mm, and the insertion depth of the second bending unit is 20-25 mm.

2. The method for reducing the blackening defect of the surface of the 550MPa grade low-alloy high-strength steel as claimed in claim 1, wherein the heating before rolling comprises: the temperature of the heating section is increased to the soaking temperature of 1180-1220 ℃ from room temperature, and the temperature of the soaking section is kept at the soaking temperature of 1180-1220 ℃; the total heating time before rolling is 130-160 min.

3. The method for reducing the blackening defect of the surface of the 550 MPa-grade low-alloy high-strength steel as claimed in claim 1, wherein in the rough rolling, 3+3 passes are adopted for rolling, the rolling speed is controlled to be 2-5 m/s, the total deformation of the rough rolling is controlled to be 75-85%, and the inlet temperature of the rough rolling is controlled to be 1140-1170 ℃.

4. The method for reducing the blackening defect of the surface of the 550 MPa-grade low-alloy high-strength steel as claimed in claim 1, wherein in the finish rolling, 6 passes are adopted for rolling, the rolling speed is controlled to be 8-12 mm/s, the total deformation amount of the finish rolling is controlled to be 85-95%, the inlet temperature of the finish rolling is controlled to be 1000-1040 ℃, and the finish rolling temperature of the finish rolling is controlled to be 900-920 ℃.

5. The method for reducing the blackening defect on the surface of the 550 MPa-grade low-alloy high-strength steel as claimed in claim 1, wherein the pickling temperature is 75-90 ℃ and the pickling speed is 40-220 m/min in the pickling.

6. The method for reducing the blackening defect on the surface of the 550 MPa-grade low-alloy high-strength steel as claimed in claim 1, wherein in the cold rolling, the cold rolling reduction is controlled to be 60-95%.

7. The method for reducing the blackening defect on the surface of the 550 MPa-grade low-alloy high-strength steel as claimed in claim 1, wherein the finishing elongation is 0.2-0.4%.