Method for reducing defective waste quantity of ultrahigh-strength steel plate

Abstract

The invention relates to a method for reducing the defective waste quantity of an ultrahigh-strength steel plate, which comprises the following steps: S1, taking the yield strength of strip steel as a selection standard of the roll diameter of a working roll of a temper mill; s2, setting the meshing amount of a withdrawal and straightening machine according to the tensile strength of the strip steel; s3, the annealing temperature of the strip steel is determined; in the step S1, for strip steel of the same steel type, the width of the strip steel is inversely proportional to the convexity of a working roller; in the step S2, the meshing amount comprises the meshing amount of the bending roller, the meshing amount of the upper straightening roller and the meshing amount of the lower straightening roller; in the step S3, the annealing temperature is 780 DEG C; the method has the beneficial effects that the yield strength of the strip steel is used as the selection standard of the roller diameter of the working roller of the temper mill; under the condition that the plate shape and the ductility are guaranteed, a working roller with the large roller diameter is selected as much as possible, and the meshing amount of a withdrawal and straightening machine is set according to the tensile strength of strip steel; by adopting the scheme of high tension and small cut-in amount, the problem of transverse bending is solved; and the annealing temperature of the strip steel is determined to be 780 DEG C through setting, so that the plate shape is obviously improved.

Description

Technical Field

[0001] The invention belongs to the technical field of ultra-high-strength steel plate manufacturing, and specifically relates to a method for reducing the amount of defective and scrapped ultra-high-strength steel plates. Background Art

[0002] The lightweighting of automobiles has placed higher demands on the strength of steel plates, and the application of high-strength steel has become an important development direction. However, with the improvement of the strength of high-strength steel plates, the traditional cold stamping process is prone to cracking during the forming process and cannot meet the processing requirements of high-strength steel plates.

[0003] According to survey statistics, the application of high-strength steel in some car brands is constantly expanding, and the application of high-strength steel in the body frame of some models has reached 90%. According to the research of the Energy Department of the American Iron and Steel Institute, even if high-strength steel reduces some values, its stretching is still much more difficult than traditional cold plate. The elongation rate of high-strength steel is only half of that of ordinary steel.

[0004] At present, the statistics of defective high-strength plates of our company are as follows:

[0005]

[0006] It can be seen from the above table that the main quality problem of ultra-high strength steel is poor plate shape, and how to solve the plate shape problem is the main problem that needs to be solved.

[0007] After investigation and analysis, there are three main factors affecting poor plate shape: incorrect selection of leveling mill rolls, resulting in large waves in ultra-high-strength steel plates; unreasonable setting of straightening machine parameters, resulting in poor plate shape after the strip leaves the straightening machine; and unreasonable annealing temperature setting, resulting in poor plate shape after the strip leaves the annealing furnace.

[0008] In the existing patent, the application number is CN202110709725.8, and the name is short-process rolling 550MPa wide-width thin-gauge high-strength steel plate shape control method, which discloses the slab thickness and wedge control stage, the finish-rolled strip load distribution control stage, and the finish-rolled strip convexity and flatness control stage respectively; in the finish-rolled strip convexity and flatness control stage, the finished product convexity is controlled at 30um~50um. When the strip thickness is ≥1.5mm and ≤2.0mm, the actual convexity of the strip head 0~100m is controlled at 40um~45um; when the strip thickness is >2.0mm and ≤4.0mm, the actual convexity of the strip head 0~100m is controlled at 30um~35um. In the finish-rolled strip convexity and flatness control stage, the straightness compensation parameters of the F7 frame are corrected to regulate the straightness of the strip along the length direction. The application does not disclose how to adjust the convexity;

[0009] There is also an application number of 201410792776.1, entitled "Two-stage sparse layer for stress reduction of hot-rolled high-strength steel", which discloses the following method steps: (1) determining the strength of the steel grade and the product thickness; (2) determining the two-stage sparse cooling method; (3) setting and starting the first stage sparse cooling system; (4) setting and starting the second stage sparse cooling. This method can eliminate the bad plate shapes such as middle waves and edge waves of high-strength steel produced by the traditional one-stage cooling method; this method uses 16 groups of nozzles for cooling, which is complex in structure and difficult to control, and needs to be improved. Summary of the invention

[0010] In view of the above-mentioned deficiencies in the prior art, the technical problem to be solved by the present invention is: how to provide a method for reducing the amount of defective and scrapped ultra-high-strength steel plates, which can reduce the amount of scrap, improve the product qualification rate, and reduce enterprise losses.

[0011] The technical solution of the present invention is:

[0012] A method for reducing the amount of defective and scrapped ultra-high-strength steel plates comprises the following steps:

[0013] S1, the strip yield strength is used as the selection standard for the roller diameter of the skin-pass mill;

[0014] S2, set the engagement of the tension and straightening machine according to the tensile strength of the strip;

[0015] S3, determine the annealing temperature of the strip.

[0016] Furthermore, in step S1, for the strip steel of the same steel grade, the strip steel width is inversely proportional to the crown of the working roll.

[0017] Furthermore, in step S2, the engagement amount includes the engagement amount of the bending roller, the engagement amount of the upper straightening roller, and the engagement amount of the lower straightening roller.

[0018] Furthermore, when the tensile strength of the strip steel is 980-1100 MPa, the engagement of the bending roll is 17.5 mm, the engagement of the upper straightening roll is 14 mm, and the engagement of the lower straightening roll is 25 mm;

[0019] When the tensile strength of the strip is 1101-1300MPa, the engagement of the bending roll is 20mm, the engagement of the upper straightening roll is 14mm, and the engagement of the lower straightening roll is 33mm;

[0020] When the tensile strength of the strip steel is 1301-1500MPa, the engagement of the bending roll is 21mm, the engagement of the upper straightening roll is 14mm, and the engagement of the lower straightening roll is 25mm.

[0021] Furthermore, in step S3, the annealing temperature is 780 degrees.

[0022] Furthermore, it also includes an improved strip cooling system at the head and tail stages of the tension and leveling machine.

[0023] Furthermore, the strip cooling improvement system at the head and tail stages includes increasing the traveling speed of the straightening machine at the head stage of the steel strip by 20%; and increasing the traveling speed of the straightening machine at the tail stage of the steel strip by 25%.

[0024] The beneficial effects of the present invention are:

[0025] By setting the yield strength of the strip steel as the selection standard for the diameter of the working roll of the leveling mill, the diameter of the working roll can be correctly selected. Under the condition of ensuring the plate shape and elongation, the working roll with a large diameter is selected as much as possible, because the working roll with a large diameter is better than the small roll in terms of roughness transmission; at the same time, in terms of roll circulation, the large roll diameter is also better than the small roll;

[0026] By setting the meshing amount of the tension straightening machine according to the tensile strength of the strip steel, and by determining the reasonable meshing amount of the tension straightening machine, the problem of transverse bending is solved by adopting the solution of high tension and small cutting amount.

[0027] By setting the annealing temperature of the strip to 780 degrees, the part where the phase transformation occurs later also reaches the martensitic phase transformation temperature; the shape of the plate out of the furnace is significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a process diagram for annealing a steel strip according to the present invention;

[0029] Figure 2 It is a principle block diagram of the present invention;

[0030] Figure 3 Schematic diagram of the improved structure for water cooling;

[0031] Figure 4 for Figure 3 A magnified image of point A;

[0032] Figure 5 It is a schematic diagram of the structure of the special-shaped seam.

[0033] In the attached drawings, 1-steel belt, 2-first cold water pipe, 21-water spraying seam, 22-special-shaped seam, 3-second cold water pipe. DETAILED DESCRIPTION

[0034] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the specific implementation mode, structure, characteristics and effects of the present invention are described in detail below in combination with the accompanying drawings and preferred embodiments.

[0035] Example 1

[0036] A method for reducing the amount of defective and scrapped ultra-high-strength steel plates comprises the following steps:

[0037] S1, the strip yield strength is used as the selection standard for the roller diameter of the skin-pass mill;

[0038] In the step S1, for the strip steel of the same steel grade, the strip steel width is inversely proportional to the crown of the working roll.

[0039] The function of the leveling mill is to eliminate yield strength, transfer roughness and improve plate shape; the leveling mill is a four-roll mill with lower support roll transmission. Two working roll diameters of 400 and 620 can be selected for rolling, and the large and small rolls can be interchanged; the leveling mill has a hydraulic adjustment system, with two hydraulic cylinders arranged between the support roll bearing seat and the frame base to adjust the bottom roll height and increase the required roll pressure, while compensating for the roll diameter loss during rolling. The hydraulic adjustment system also includes a roll bending system, which adjusts the plate shape by adjusting the pressure and stroke of the oil cylinders on both sides to match the roll shape.

[0040] Through simulation calculation of strip steel, the selection principle of working rolls is determined. The yield strength of strip steel is used as the principle for selecting the diameter of working rolls of the leveler: as long as the strip shape and elongation are guaranteed, large roll diameters should be selected as much as possible, because large roll diameters are better than small rolls in terms of roughness transmission and roll flow; for the same type of steel, the wider the strip width, the smaller the required roll crown, and the narrower the strip width, the larger the required roll crown;

[0041] As shown in the following table,

[0042]

[0043] In the above table, the crown of the working roll can be selected according to the yield strength of the strip to ensure the flatness of the strip.

[0044] S2, set the engagement of the tension and straightening machine according to the tensile strength of the strip;

[0045] In the step S2, the meshing amount includes the meshing amount of the bending roll, the meshing amount of the upper straightening roll and the meshing amount of the lower straightening roll.

[0046] When the tensile strength of the strip steel is 980-1100MPa, the engagement of the bending roll is 17.5mm, the engagement of the upper straightening roll is 14mm, and the engagement of the lower straightening roll is 25mm;

[0047] When the tensile strength of the strip is 1101-1300MPa, the engagement of the bending roll is 20mm, the engagement of the upper straightening roll is 14mm, and the engagement of the lower straightening roll is 33mm;

[0048] When the tensile strength of the strip steel is 1301-1500MPa, the engagement of the bending roll is 21mm, the engagement of the upper straightening roll is 14mm, and the engagement of the lower straightening roll is 25mm.

[0049] The main function of the tension and straightening machine is to improve the plate shape and mechanical properties at the same time; the tension and straightening machine adopts two bends and one straightening to provide appropriate tensile force and straightening force to the strip, increase the elongation of the strip, eliminate the sickle bend, and improve the plate shape.

[0050] The meshing parameters before improvement are as follows:

[0051]

[0052] The above table shows the groups of strip steel by thickness before optimization, which pursues large elongation and has severe transverse bending.

[0053] As shown in the table below, the improved meshing parameters are grouped according to the yield strength of the strip steel. The high-tension and small-cut process solution solves the transverse bending problem and completely improves the plate shape.

[0054]

[0055] S3, determine the annealing temperature of the strip.

[0056] In step S3, the annealing temperature is 780 degrees.

[0057] like Figure 1 As shown in the strip annealing process diagram, after the strip is heated and heat-equilibrated, the high temperature will reduce the yield stress and yield critical stress value of the steel plate. The transverse temperature difference creates new uneven longitudinal fiber extension and superimposes the defects of the original plate shape, making the tension of the strip unevenly distributed along the width direction; and the uneven tension distribution is both the driving force for plastic deformation in the tensile area and the condition for bending deformation in the compressive area. When the strip is cooled, the cooling rate of the strip in the cooling section is uneven. Uneven cooling will cause the phase change time of various parts of the strip to be inconsistent. After the phase change, 3-4% of martensite will increase and the strip will expand linearly by 1%. Second, due to the poor plasticity of austenite itself, it cannot expand with the part where martensite has already occurred, and can only squeeze the martensite phase change part. In this way, the martensite phase change part will be compressed, so that the material enters the water quenching section at different temperatures of A and A+F. According to the cct curve, the determined annealing temperature ensures that AP transformation does not occur, and only partial AF phase transformation occurs.

[0058] When the part where the phase transformation occurs later also reaches the martensitic phase transformation temperature, it will follow linear expansion and eventually reach the same expansion result.

[0059] After repeated experiments, the slow cooling temperature was determined to be 780 degrees. This greatly improved the plate shape and did not affect the mechanical properties of the plate.

[0060] Example 2

[0061] At the beginning and end stages of the steel strip, after adopting the method of Example 1, the plate shape was not significantly improved. After analytical tests, the working parameters of the tension and straightening machine at the beginning and end stages were determined respectively.

[0062] It also includes an improved system for strip cooling at the head and tail stages of the tension leveler.

[0063] The strip cooling improvement system at the head and tail stages includes increasing the traveling speed of the drawing and straightening machine at the head stage of the steel strip by 20%; and increasing the traveling speed of the drawing and straightening machine at the tail stage of the steel strip by 25%.

[0064] Since there is no constraint on the ends of the plate at the head and tail stages, the plate deforms greatly during water cooling. After repeated tests and simulation calculations, the normal speed of 50 meters per minute in the head stage was increased to 60 meters per minute, and the plate deformation was reduced from the original 1mm to 0.1mm, which is within a reasonable range.

[0065] The normal speed of the steel strip at the tail stage was adjusted from the original 50m / min to 75m / min, and the change in plate shape at the tail stage was reduced from the original 1.5mm to 0.1mm, which also reached the upper limit of the qualified range.

[0066] Example 3

[0067] Based on Example 2, the water cooling stage of the steel strip is improved to avoid deformation caused by inconsistent internal stress of the steel strip due to uneven water cooling;

[0068] At present, water cooling is performed by setting two nozzles along the width direction of the steel plate, spraying water directly onto the surface of the steel plate, which is not very uniform, causing uneven stress in the steel plate and easy deformation.

[0069] like Figure 3 As shown, the water cooling improved structure includes two cold water pipes arranged at intervals along the traveling direction of the steel belt 1, one is a first cold water pipe 2, and the other is a second cold water pipe 3, and the second cold water pipe 3 has the same structure as the first cold water pipe 2.

[0070] like Figure 4 As shown, the length direction of the first cold water pipe 2 is consistent with the width direction of the steel strip 1, and a water spraying slot 21 is opened along the length direction of the first cold water pipe 2, and the length of the water spraying slot 21 is equal to the width of the steel strip 1; the angle between the water spraying slot 21 and the horizontal plane is 30-45 degrees, so that the water outlet of the water spraying slot 21 forms a water surface in contact with the surface of the steel strip 1, and the water spraying slot 21 replaces the original nozzle, so that the contact between the surface of the steel strip and the water is more uniform, and the steel plate is not easy to deform.

[0071] As a further improvement, Figure 5As shown, the water spray outlet on the first cold water pipe 2 is set as a shaped seam 22 which is wide in the middle and narrow at both ends. Since the temperature in the middle of the steel strip 1 is high and the heat dissipation at both ends is fast, after the water spray outlet is set as a shaped seam which is wide in the middle and narrow at both ends, the water volume in the middle of the steel plate is large and the water volume on both sides is small, which is consistent with the actual operation of the steel plate, facilitates the temperature of the steel plate along the width direction to be consistent, the surface of the steel plate is cooled more evenly, basically no stress is generated, and the plate shape is improved.

[0072] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technical personnel in this field can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any brief modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for reducing the amount of defective and scrapped ultra-high-strength steel plates, characterized in that: The following steps are included: S1, the strip yield strength is used as the selection standard for the roller diameter of the skin-pass mill; S2, set the engagement of the tension and straightening machine according to the tensile strength of the strip; S3, determine the annealing temperature of the strip.

2. A method for reducing the amount of defective and scrapped ultra-high-strength steel plates according to claim 1, characterized in that: In the step S1, for the strip steel of the same steel grade, the strip steel width is inversely proportional to the crown of the working roll.

3. The method for reducing the amount of defective and scrapped ultra-high-strength steel plates according to claim 1, characterized in that: In the step S2, the meshing amount includes the meshing amount of the bending roll, the meshing amount of the upper straightening roll and the meshing amount of the lower straightening roll.

4. The method for reducing the amount of defective and scrapped ultra-high-strength steel plates according to claim 3, characterized in that: When the tensile strength of the strip steel is 980-1100MPa, the engagement of the bending roll is 17.5mm, the engagement of the upper straightening roll is 14mm, and the engagement of the lower straightening roll is 25mm; When the tensile strength of the strip is 1101-1300MPa, the engagement of the bending roll is 20mm, the engagement of the upper straightening roll is 14mm, and the engagement of the lower straightening roll is 33mm; When the tensile strength of the strip steel is 1301-1500MPa, the engagement of the bending roll is 21mm, the engagement of the upper straightening roll is 14mm, and the engagement of the lower straightening roll is 25mm.

5. The method for reducing the amount of defective and scrapped ultra-high strength steel plates according to claim 1, characterized in that: In step S3, the annealing temperature is 780 degrees.

6. The method for reducing the amount of defective and scrapped ultra-high strength steel plates according to claim 1, characterized in that: It also includes an improved system for strip cooling at the head and tail stages of the tension leveler.

7. A method for reducing the amount of defective and scrapped ultra-high strength steel plates according to claim 6, characterized in that: The strip cooling improvement system at the head and tail stages includes increasing the traveling speed of the drawing and straightening machine at the head stage of the steel strip by 20%; and increasing the traveling speed of the drawing and straightening machine at the tail stage of the steel strip by 25%.

Images