CN113215373B - A Method for Eliminating Thin Line Defects in Edges of Boron-Containing Steel - Google Patents

Abstract

The invention provides a method for eliminating thin line defects at the edge of boron-containing steel, which belongs to the technical field of steel rolling. The method includes obtaining a slab with a boron mass fraction of 0.0005-0.001%; heating and rough rolling the slab, Obtain an intermediate billet; finish rolling the intermediate billet at an inlet temperature of 1050-1070°C and an outlet temperature of 880-920°C to obtain a steel strip; coil the strip at a temperature of 620-660°C to obtain Boron-containing steel with good surface quality. The boron-containing steel provided by the method of the invention has no shape defects, and has no fracture and cracking problems when used in cold rolling; and the method is simple, has strong applicability and remarkable effect.

Description

A Method for Eliminating Thin Line Defects of Boron-Containing Steel Edge

technical field

The invention belongs to the technical field of steel rolling, and in particular relates to a method for eliminating thin line defects at the edge of boron-containing steel.

Background technique

Boron-containing steel is the product of hot-rolled slabs, which can be used as raw materials for cold-rolled sheets and subsequent processing. Therefore, the surface quality of boron-containing hot-rolled coils directly affects the subsequent treatment of the strip surface in the next process, including Oil coating, hot-dip galvanizing, electro-galvanizing, electro-tin plating, phosphating, passivation, anti-fingerprint, color coating and other processes. Cold-rolled sheets with high-grade surfaces are mainly used for panels of mid-to-high-end cars. With the advancement of science and technology and the improvement of people's living needs, the requirements for the surface quality of cold-rolled sheets in industries such as modern household appliances and building materials are also increasing. high. For the surface of high-grade cold-rolled sheet, it is not only required to have no defects in optical inspection, but also to be polished by oilstone without defects.

At present, linear defects often appear in the hot-rolled edge of boron-containing steel, which generally occur on the upper surface of the hot coil. Due to the difference in the alloy elements of the steel, the appearance of thin lines in the edge is also quite different. The appearance of thin line defects at the edge affects the user's use, and the defects need to be removed before subsequent processing. If cold rolling is required later, the risk of cracking or strip breaking during the cold rolling process will increase.

Contents of the invention

The invention provides a method for eliminating thin line defects at the edge of boron-containing steel to solve the technical problem in the prior art that the thin line defects at the edge of boron-containing steel affect the use of users.

The invention provides a method for eliminating the fine line defects at the edge of boron-containing steel, the method comprising:

Obtain a slab with a boron mass fraction of 0.0005-0.001%;

heating and rough rolling the slab to obtain an intermediate slab;

Finish rolling the intermediate slab at an inlet temperature of 1050-1070°C and an outlet temperature of 880-920°C to obtain strip steel;

The steel strip is coiled at a temperature of 620-660° C. to obtain boron-containing steel with good surface quality.

Further, in the finish rolling process, the reduction ratio of the second pass is 40-45%, and the reduction ratio of the third pass is 30-35%.

Further, the rolling force of the finish rolling F2 stand is 22000-28000KN, and the rolling force of the finish rolling F3 stand is 20000-26000KN.

Further, the temperature difference between the entrance of the rough rolling and the exit of the rough rolling is 150-170°C; the difference between the center temperature and the edge temperature in the width direction of the intermediate billet is 5-15°C.

Further, the rough rolling is carried out by using the first rough rolling mill and the second rough rolling mill; the rolling times of the first rough rolling mill is 1 pass; the rolling times of the second rough rolling mill are 5 passes, so When rolling in the second roughing mill, the width reductions of the first pass and the third pass are <15mm and <10mm respectively.

Further, the heating includes soaking, the soaking temperature is 1200-1230° C., the soaking time is 30-40 minutes; the total heating time is 160-200 minutes.

Further, the slab obtained with a boron mass fraction of 0.0005-0.001% includes,

Obtaining molten steel with a boron mass fraction of 0.0005-0.001%;

The molten steel is continuously cast to obtain a slab; in the continuous casting, the temperature of the straightening section is >950°C.

Further, in the continuous casting, the casting speed is 1.5-1.7m/min.

Further, in the molten steel, the mass fraction of C is 0.015-0.04%, the mass fraction of Al is 0.02-0.05%, the mass fraction of Mn is 0.2-0.3%, and the mass fraction of Si is 0.03-0.05%.

Further, the thickness of the slab is 230-250mm, and the width of the slab is 1000-1800mm.

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 eliminating thin line defects at the edge of boron-containing steel. Due to the segregation of boron at the grain boundary in the temperature range of 750-850°C, more fine BN in the steel is precipitated along the grain boundary And pin the grain boundary, thereby inhibiting the migration of the grain boundary, increasing the probability of intergranular fracture, reducing the high temperature plasticity of the steel, and making intergranular fracture more likely to cause linear defects; therefore, in the production process, by controlling the finishing rolling and The coiling temperature is to avoid the temperature range of 750-850°C, so as to avoid the problem of linear defects caused by intergranular fracture. The boron-containing steel provided by the method of the invention has no shape defects, and has no fracture and cracking problems when used in cold rolling; and the method is simple, has strong applicability and remarkable effect.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings that need to be used in the description of the embodiments. Obviously, the drawings in the following description are some embodiments of the present invention. For those skilled in the art, other drawings can also be obtained based on these drawings without creative effort.

Figure 1 is the macroscopic morphology of thin line defects in the edge of boron-containing steel;

Figure 2 is the microscopic appearance of thin line defects in the edge of boron-containing steel;

Figure 3 is a partially enlarged view of Figure 2;

Fig. 4 is the high-temperature plasticity curve of the boron-containing steel provided by the embodiment of the present invention;

Fig. 5 is the surface topography diagram of the boron-containing steel provided by the embodiment of the present invention after pickling;

Fig. 6 is a surface topography diagram of the cold-rolled boron-containing steel provided by the embodiment of the present invention.

Detailed ways

The present invention will be described in detail below in conjunction with specific embodiments and examples, and the advantages and various effects of the present invention will be presented more clearly. Those skilled in the art should understand that these specific implementations and examples are used to illustrate the present invention, not to limit the present invention.

Throughout the specification, unless otherwise specified, terms used herein should be understood as commonly used in the art. Therefore, unless otherwise defined, 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. In case of conflict, this specification shall take precedence.

Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

It should be noted that in this article, relative terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply these No such actual relationship or order exists between entities or operations.

The technical solutions provided by the embodiments of the present invention are to solve the above-mentioned technical problems, and the general idea is as follows:

An embodiment of the present invention provides a method for eliminating thin line defects at the edge of boron-containing steel, the method comprising:

S1, obtaining a slab with a boron mass fraction of 0.0005-0.001%;

Wherein, the boron mass fraction obtained is 0.0005-0.001% slab, which can be obtained by the following method:

S101, obtaining boron-containing molten steel with a boron mass fraction of 0.0005-0.001%;

Adding B element to steel can preferentially precipitate coarse BN high-temperature austenite, thereby inhibiting the precipitation of fine AlN to weaken grain coarsening; in addition, the austenite grain boundary segregation of B atoms can also inhibit ferrite nucleation to Control grain coarsening; moreover, the preferential combination of B and N can effectively reduce the free N content in steel and reduce the occurrence of aging problems.

As an implementation manner of the embodiment of the present invention, in the boron-containing molten steel, the mass fraction of C is 0.015-0.04%, the mass fraction of Al is 0.02-0.05%, and the mass fraction of Mn is 0.2-0.3%, The mass fraction of Si is 0.03-0.05%.

The boron-containing steel in the present invention is a low-carbon aluminum-killed steel, and its C content is 0.015-0.04%. Because the C content is relatively low, the temperature of the phase transition point is relatively high, and in the rolling process due to the width of the strip There is a temperature difference between the edge and the middle of the strip width direction, and it is easy to happen that the edge of the width direction of the strip enters the two-phase region of pro-eutectoid ferrite and austenite or the single-phase region of ferrite, resulting in edge and austenite rolling. There is a large difference in the middle structure, so low carbon steel is more prone to edge fine line defects.

S102, performing continuous casting on the boron-containing molten steel to obtain a slab; in the continuous casting, the temperature in the straightening section is >950°C.

The hot-rolled edge thin line defects of boron-containing steel generally occur on the upper surface of the hot coil. Due to the difference in the alloy elements of the steel grades, the appearance of the edge thin line is also quite different. The thin lines at the edge of boron-containing low-carbon hot coils are mainly distributed along the rolling direction, as shown in Figure 1. In the present invention, the linear defect refers to the longitudinal crack morphology of the innermost crenel-shaped edge within the range of 10 mm from the edge in the width direction. Observing the microscopic appearance (see Figure 2 and Figure 3), it can be seen that there are many small cracks on the surface , the depth of the crack is about 30-50um, most of the cracks have a certain angle with the vertical direction, and most of them are inclined to the right, but there are also individual characteristics that are inclined to the left and bidirectional; the linear defects in the present invention also include There are many surface cracks at the round edge of the boron-containing steel hot coil, and the crack depth is 48-52 μm. The occurrence of thin lines at the edge affects the user's use, and the defect needs to be removed before subsequent processing. If cold rolling is required later, the risk of cracking or strip breakage during the cold rolling process will increase. Using the high-temperature tensile test method, the high-temperature plasticity of boron-containing steel was analyzed, and it was found that the plasticity in the range of 750-850 ° C decreased significantly, down to below 70% (see Figure 4); while the boron element in boron-containing steel is at the grain boundary The characteristics of segregation make more fine BN in the steel precipitate along the grain boundary and pin the grain boundary, thereby inhibiting the migration of the grain boundary, increasing the probability of intergranular fracture, and reducing the high temperature plasticity of the steel. Therefore, at 750-850°C In the temperature range, boron-containing steel is more prone to intergranular fracture, which causes linear defects. Boron-containing steel slabs are prone to cracks mainly because the thermoplasticity of the steel decreases due to the influence of BN precipitation along the grain boundaries, and cracks occur when the stress and strain are too large during the continuous casting process. The continuous casting process adopts a weak cooling mode, and the straightening fracture temperature is controlled above 950°C, which can avoid a large amount of BN precipitation and brittle grain boundaries.

As an implementation manner of the embodiment of the present invention, in the continuous casting, the casting speed is 1.5-1.7 m/min.

The casting speed is controlled at 1.5-1.7m/min to ensure that the straightening section of the continuous casting process avoids the elongation reduction temperature range of 850°C during high temperature stretching, and the plasticity decreases around this temperature, and linear defects are prone to appear.

As an implementation manner of an embodiment of the present invention, the thickness of the slab is 230-250 mm, and the width of the slab is 1000-1800 mm.

S2, heating and rough rolling the slab to obtain an intermediate slab;

As an implementation manner of the embodiment of the present invention, the heating includes soaking, the soaking temperature is 1200-1230° C., the soaking time is 30-40 minutes; the total heating time is 160-200 minutes.

Through the above-mentioned heating process control, the temperature difference of the slab can be guaranteed to be within 20°C, thereby improving the temperature uniformity of each part of the strip during the rolling process. If the temperature difference of the slab is too large, the edge temperature in the width direction of the strip will drop more during the rolling process, which will cause the two phases of pro-eutectoid ferrite and austenite at the edge of the strip. Zone rolling makes the structure of the edge and the middle of the strip steel different, so that a mixed crystal structure appears in the transition zone between the edge and the middle, and a linear crack appears at a position about 30-50cm away from the edge of the strip. Then it evolves into a peeling defect (see Figure 2); in addition, ensuring that the temperature difference of the slab is within 20°C can also reduce the problems of metal flow and flanging in the edge rolling process. Therefore, it is necessary to ensure the uniformity of temperature in the middle and edge of the slab during the heating process, so as to reduce the width of the edge and the flow of metal as much as possible during the hot rolling process.

The above-mentioned heating process can ensure the soaking temperature and time of the slab, and ensure the steel grade is burned through. If the soaking temperature is too high, it will easily cause the austenite to become coarse and affect the quality of the finished product. If the soaking temperature is too low, it will easily cause the temperature of the rolling process to be too low and fall into the two-phase region. If the soaking time is too long, it will easily cause serious burning loss, and if the soaking time is too low, it will easily cause impenetrable burning. If the total heating time is too long, it is easy to cause serious burning loss, and if the total heating time is too low, it is easy to cause impenetrable burning.

As an implementation manner of the embodiment of the present invention, the temperature difference between the rough rolling entrance and the rough rolling exit is 150-170°C; 5-15°C.

After measuring the lateral temperature distribution of the strip after it leaves the R2 rolling mill (the second rolling mill), it is found that the temperature at the edge of the strip is about 50-100 ° C lower than that at the center, and the temperature at the operating side is lower than that at the driving side, which is consistent with the temperature at the edge Thin lines mostly appear on the edge of the strip, especially on the operation side;

Therefore, the temperature difference between the center and the edge of the width direction is guaranteed, and the edge in the width direction of the strip is prevented from falling into the two-phase area for rolling, thereby avoiding rolling cracking caused by structural differences; at the same time, controlling the temperature difference between the entrance and exit of the rough rolling can improve the intermediate temperature. The uniformity of the billet temperature distribution, especially the temperature uniformity of the head and tail of the intermediate billet, so as to avoid the cracking problem caused by the structural difference caused by the temperature drop of the edge of the strip width in the direction of the strip due to the temperature drop of the tail of the strip being too low.

As an implementation manner of the embodiment of the present invention, the rough rolling is carried out by using the first rough rolling mill and the second rough rolling mill; the rolling frequency of the first rough rolling mill is 1 pass; the rolling of the second rough rolling mill The number of times of production is 5 passes, and when rolling in the second roughing mill, the width reductions of the first pass and the third pass are <15mm and <10mm respectively.

Controlling the width reduction of the first pass and the third pass can reduce the metal flow and flanging problems at the edge, and ensure that there is no obvious large edge compression deformation, so as to avoid linear cracks at the edge of the hot-rolled sheet defect.

S3, performing finish rolling on the intermediate billet at an inlet temperature of 1050-1070°C and an outlet temperature of 880-920°C to obtain a steel strip;

In the temperature range of 750-850 ℃, along the width direction of the strip steel, the edges are thick strip ferrite, the center is a mixture of proeutectoid ferrite and austenite, the transition zone is a mixed crystal structure, and the mixed The crystal structure is easy to cause linear defects. Control the exit temperature of the finish rolling temperature and avoid the temperature range of 750-850°C, so as to avoid the problem of linear defects caused by intergranular fracture.

The entrance temperature of finish rolling is too high, the iron sheet is too thick, and the surface quality is reduced; the entrance temperature of finish rolling is too low, and the temperature of the edge falls into the two-phase region or single-phase region for rolling, resulting in abnormal structure, resulting in thin line defects such as cracking .

If the finish rolling exit temperature is too high, the material structure will be abnormally coarse.

As an implementation manner of the embodiment of the present invention, in the finish rolling process, the reduction ratio of the second pass is 40-45%, and the reduction ratio of the third pass is 30-35%.

During the finishing rolling process, the reduction rate of each pass should be reasonably distributed to avoid the cracking of the phase interface caused by a single pass being too large.

As an implementation manner of the embodiment of the present invention, the rolling force of the finish rolling F2 stand is 22000-28000KN, and the rolling force of the finish rolling F3 stand is 20000-26000KN. Start double-pass descaling in finishing rolling to ensure the surface quality of hot coils, reduce the rolling load of F2 and F3 in the finishing rolling process, and how to eliminate linear defects.

The main function of the finish rolling F1 stand is to prevent slipping and biting problems. Therefore, the finish rolling F1 stand generally does not carry out a large reduction. The main purpose of controlling the rolling force of the finish rolling F2 and F3 stands is to distribute the finish rolling reasonably. Each rolling pass avoids excessive rolling force in a single pass, which will cause phase interface cracking.

S4, coiling the steel strip at a temperature of 620-660° C. to obtain a boron-containing steel with good surface quality.

The coiling temperature below 660°C can avoid the temperature range of 750-850°C, thereby avoiding the problem of linear defects caused by intergranular fracture.

If the coiling temperature is too high, it is easy to cause coarse grain defects. If the coiling temperature is too low, the downstream rolling force will be too large, which will affect the cold rolling production line.

In addition, it should be noted that, said linear defect in the present invention refers to the crack along the rolling direction of strip steel in the range of 30-50 cm away from the edge portion in the width direction. When the crack was serious, the macroscopic appearance showed as peeling ( as shown in picture 2).

A method for eliminating boron-containing edge fine line defects of the present invention will be described in detail below in combination with examples, comparative examples and experimental data.

Embodiment 1-5 and comparative example 1

Embodiments 1-5 and Comparative Example 1 provide a method for eliminating linear defects in boron-containing low-carbon aluminum-killed steel, which method is specifically as follows:

1. The molten steel is obtained by smelting, and the molten steel is continuously cast to obtain a slab. The chemical composition and size of the slab are shown in Table 1. During the continuous casting process, the temperature of the straightening section is controlled, and the continuous casting casting speed is controlled, as shown in Table 2.

2. Place the slab in a heating furnace to heat. The heating includes a heating stage, a heating stage 2 and a soaking stage. The process control of each stage is shown in Table 2.

3. Rough rolling the heated slab in step 2 in a 1+5 mode to obtain an intermediate slab. Among them, the RT2 roughing mill is a reversing mill. When the RT2 roughing mill is used for rolling, the width reduction of the first pass and the third pass, as well as the roughing entrance temperature and roughing exit temperature are shown in Table 2.

4. Finish rolling the intermediate slab obtained in step 3. Finish rolling includes F1, F2, F3, F4, F5 and F6 stands, and controls the rolling force and reduction rate of F2 and F3 stands, as shown in Table 3 .

5. Laminar cooling the strip steel obtained after the finish rolling in step 4, and coiling to obtain boron-containing steel with good surface quality. The coiling temperature is shown in Table 3.

Table 1

Table 2

table 3

Table 4

Numbering Thickness of boron-containing steel/mm Surface Quality Cracking or broken belt ratio/% Example 1 2.5 Good surface quality, no defects 0 Example 2 2.5 Good surface quality, no defects 0 Example 3 2.5 Good surface quality, no defects 0 Example 4 2.5 Good surface quality, no defects 0 Example 5 2.5 Good surface quality, no defects 0 Comparative example 1 2.5 There are cracks, the crack depth is 35μm 5

The surface observation of the boron-containing steel rolled in Examples 1-5 of the present invention and Comparative Example 1 is shown in Table 4.

It can be seen from Table 4 that the boron-containing steel provided by Examples 1-5 of the present invention has a good surface quality and no defects; the boron-containing steel is cold-rolled, and the production process is smooth without cracking or broken strips. The boron-containing steel provided in Comparative Example 1 had linear crack defects, as shown in FIG. 3 , and it was used for cold rolling, and the rate of strip breakage or cracking was 5%.

It can be seen from Figure 5 that the boron-containing steel provided by the embodiment of the present invention has no fine line defects after pickling, and the surface quality is good; it can be seen from Figure 6 that the edge of the boron-containing steel provided by the present invention has no cracking defects after cold rolling, and the edge quality is good.

The invention provides a method for eliminating thin line defects at the edge of boron-containing steel, which belongs to the technical field of steel rolling. Due to the characteristic of boron element segregation at the grain boundary in the temperature range of 750-850 ° C, more fine lines in the steel BN precipitates along the grain boundary and pins the grain boundary, thereby inhibiting the migration of the grain boundary, increasing the probability of intergranular fracture, reducing the high-temperature plasticity of the steel, and making intergranular fracture more likely to cause linear defects; therefore, in the production process, By controlling the temperature of finish rolling and coiling to avoid the temperature range of 750-850°C, the problem of linear defects caused by intergranular fracture can be avoided. The boron-containing steel provided by the method of the invention has no shape defects, and has no fracture and cracking problems when used in cold rolling; and the method is simple, has strong applicability and remarkable effect.

Finally, it should also be noted that the term "comprises", "comprises" or any other variation thereof is intended to cover a non-exclusive inclusion such that a process, method, article or apparatus comprising a set of elements includes not only those elements, but also Other elements not expressly listed, or inherent to the process, method, article, or apparatus are also included.

While preferred embodiments of the invention have been described, additional changes and modifications to these embodiments can be made by those skilled in the art once the basic inventive concept is appreciated. Therefore, it is intended that the appended claims be construed to cover the preferred embodiment as well as all changes and modifications which fall within the scope of the invention.

Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and equivalent technologies thereof, the present invention also intends to include these modifications and variations.

Claims (9)

1. A method for eliminating the fine line defect at the edge of boron-containing steel is characterized by comprising the following steps,

obtaining a plate blank with the boron mass fraction of 0.0005-0.0009%;

heating and rough rolling the plate blank to obtain an intermediate blank; the difference between the temperature of the rough rolling inlet and the temperature of the rough rolling outlet is 150-170 ℃; the difference between the central temperature of the intermediate blank in the width direction and the edge temperature is 5-15 ℃;

performing finish rolling on the intermediate billet at an inlet temperature of 1050-1070 ℃ and an outlet temperature of 910-920 ℃ to obtain strip steel;

and coiling the strip steel at the temperature of 620-660 ℃ to obtain the boron-containing steel with good surface quality.

2. The method for eliminating the fine line defect on the edge of the boron-containing steel as claimed in claim 1, wherein the reduction ratio of the second pass is 40-45% and the reduction ratio of the third pass is 30-35% during the finish rolling.

3. The method for eliminating the fine line defect on the edge of the boron-containing steel according to claim 2, wherein the rolling force of the finish rolling F2 stand is 22000 KN to 28000KN, and the rolling force of the finish rolling F3 stand is 20000 KN to 26000KN.

4. The method for eliminating fine line defects on the edge of boron-containing steel according to claim 1, wherein the roughing is performed by using a first roughing mill and a second roughing mill; the rolling frequency of the first roughing mill is 1 pass; the rolling frequency of the second roughing mill is 5 passes, and the width reducing amount of the 1 st pass and the 3 rd pass is respectively less than 15mm and less than 10mm when the second roughing mill rolls.

5. The method for eliminating the fine line defect at the edge of the boron-containing steel according to claim 1, wherein the heating comprises soaking, the soaking temperature is 1200-1230 ℃, and the soaking time is 30-40min; the total heating time is 160-200min.

6. The method for eliminating fine line defects on the edge of boron-containing steel according to claim 1, wherein the slab with the boron mass fraction of 0.0005-0.0009% is obtained by the method comprising,

obtaining molten steel with the boron mass fraction of 0.0005-0.0009%;

continuously casting the molten steel to obtain a plate blank; in the continuous casting, the temperature of the straightening section is more than 950 ℃.

7. The method for eliminating fine line defects on the edge of a boron-containing steel according to claim 6, wherein the drawing speed in the continuous casting is 1.5-1.7m/min.

8. The method for eliminating fine line defects at the edge of a boron-containing steel as claimed in claim 6, wherein the molten steel contains C0.015 to 0.04 wt%, al 0.02 to 0.05 wt%, mn 0.2 to 0.3 wt%, and Si 0.03 to 0.05 wt%.

9. The method for eliminating fine line defects on the edge of a boron-containing steel according to claim 1, wherein the thickness of the slab is 230-250mm, and the width of the slab is 1000-1800mm.

Images