CN111408625A

CN111408625A - Flat coil control method for hot rolling advanced high-strength steel

Info

Publication number: CN111408625A
Application number: CN202010183210.4A
Authority: CN
Inventors: 刘洋; 王成; 宋波; 何金平; 孙宜强; 汪水泽
Original assignee: Wuhan Iron and Steel Co Ltd
Current assignee: Wuhan Iron and Steel Co Ltd
Priority date: 2020-03-16
Filing date: 2020-03-16
Publication date: 2020-07-14
Anticipated expiration: 2040-03-16
Also published as: CN111408625B

Abstract

The invention discloses a flat coil control method for hot-rolled advanced high-strength steel, which is characterized by mainly comprising four control stages: the method comprises a steel strip finish rolling surface roughness control stage, a laminar cooling austenite phase change control stage, a coiling tension control stage and a rotation control stage after coiling. The invention increases the surface roughness of the finished hot-rolled strip steel product, increases the interlayer friction force and the equivalent shearing rigidity of the steel coil, and then increases the capability of resisting the flat coil of the steel coil; the austenite phase change proportion of the strip steel on the laminar flow roller way is increased, the phase change expansion, the latent heat expansion and the phase change plastic deformation after coiling are reduced, the conversion of the gravitational potential energy and the total elastic storage energy of the steel coil is coordinated, the diameter change of the inner ring of the steel coil after coil unloading is in a controllable range, and the additional production cost increase and the yield loss caused by cutting the inner ring of the steel coil are avoided. The invention is suitable for hot rolling advanced high-strength steel grades with different components, is particularly suitable for thin hot rolling strip steel, and has wide application value.

Description

Flat coil control method for hot rolling advanced high-strength steel

Technical Field

The invention relates to a method for manufacturing strip steel, in particular to a method for controlling flat coiling of hot-rolled advanced high-strength steel, belonging to the field of manufacturing hot-rolled strip steel in ferrous metallurgy.

Background

With the increasing requirements of the society on environment, energy, safety and the like, the automobile steel is developing in the direction of light weight, high strengthening and low cost, and the production of thin Advanced High Strength Steel (AHSS) by adopting a thin slab continuous casting and rolling process becomes a new development direction of the industry. The typical production process of the continuous casting and rolling process of the sheet billet generally comprises the stages of molten steel smelting, continuous slab casting, soaking pit furnace heating, finish rolling, strip steel laminar cooling, coiling and warehousing cooling, and is shown in the attached figure 1. It is important to note that some Advanced High Strength Steels (AHSS) are prone to hot-rolled flat coil failure during production, and of these thin gauge (< 2.5mm) is particularly severe.

The hot rolling flat coil failure (flat coil for short) means that after the strip steel is coiled, when a winding drum of a coiling machine is drawn out, a horizontal steel coil collapses under the action of gravity to form an ellipse. This collapsing process can occur rapidly after the coil is unwound or can occur slowly during the cooling process. After the strip steel coil is flatly rolled, the minimum inner diameter of the strip steel coil is smaller than the minimum diameter allowed by subsequent processing equipment, so that the subsequent procedures of a steel mill or a steel customer cannot be carried out smoothly. For a slightly flat-rolled steel coil, a flame gun can be used for cutting off part of an inner ring with serious deformation, and then the steel coil is produced on a machine, but the measure can increase extra production cost and influence the production efficiency and the yield. In addition, the elliptical steel coil has inconsistent circumferential speed in the leveling process, the tension fluctuation is severe, the surface damage of the steel coil is easily caused, and the product degradation or the waste judgment is caused.

In order to solve the problem of the flat rolling of the strip steel, the Chinese patent with the publication number of CN1506174A discloses a coiling method for preventing the flat rolling of hot rolled steel strip, and the coiling temperature of the steel strip with the carbon content of more than 0.25 percent is controlled to be Ar₁The range of +/-10 to +60 ℃ makes the phase change of the steel coil gradually proceed from the core part contacted with the outer layer and the winding drum to the middle layer, thereby eliminating the flat coil. Chinese patent publication No. CN108754104A discloses a method for eliminating defects of 590MPa grade dual-phase steel flat coils: the hot rolling finishing temperature is controlled according to the transformation starting temperature of the ferrite of austenite, so that the transformation from austenite to ferrite is promoted; the hot rolling cooling section is divided into a fast cooling section and a slow cooling section, the hot rolled plate is cooled to the intermediate temperature of laminar cooling in the fast cooling section to promote the transformation of austenite to ferrite, the transformation of austenite to pearlite is promoted in the slow cooling section, and the retained austenite is reducedThe ratio of (a) to (b). The Chinese patent with publication number CN107904376A provides a method and a device for preventing flat coiling of dual-phase steel, which can accumulate energy for the phase change of strip steel to lead the phase change of the strip steel to occur in advance and prevent the strip steel from generating phase change to form flat coiling after being coiled into coils by controlling the temperature of a finish rolling outlet to be 860-900 ℃ and the cooling speed to be 70-100 ℃/s. Chinese patent publication No. CN102335681A discloses a coiling method for preventing the flat coiling of hot-rolled strip steel, the coiling temperature is 500-600 ℃, and then the coiled steel coil stays on a coiler mandrel for 20-60 s. Chinese patent publication No. CN107812789A discloses a method for preventing flat rolling of hot rolled coil: carrying out laminar cooling on the hot-rolled strip steel, rapidly cooling to 650-700 ℃ or 500-550 ℃ for coiling, and staying the coiled hot-rolled steel coil on a coiling machine for 5-20 s; compared with the conventional coiling tension, the coiling tension is improved by 5-10%.

The mechanism of the flat rolling of the hot rolled strip steel is related to the phase change process of the steel coil. Advanced high-strength steel such as DP, TRIP, QP steel and the like generally adds more high-hardenability alloy elements such as C, Mn, Cr and the like based on the requirements on the structure and the performance, and delays the occurrence of diffusion phase change, as shown in figure 2. Because the strip steel does not change the phase or the phase change is not finished on the laminar cooling roller way, the phase change of the untransformed austenite after coiling can generate the phase change expansion and the phase change latent heat thermal expansion, so that the steel coil is loosened and flat coiled; on the other hand, due to the phase transformation plasticity mechanism of the non-transformed austenite, the steel coil is plastically deformed under the action of the gravity field stress to cause flat coiling. Therefore, the austenite transformation completeness of the strip steel on the laminar flow roller way is one of the most important factors for the flat rolling. In addition, the thickness of the hot-rolled strip steel is also an important factor influencing flat rolling, and the thinner the strip steel is, the lower the rigidity of the strip steel is, the strip steel is not enough to support self weight, and the flat rolling defect is easily caused. The hot rolling flat coil control method related to the patent technology has the defects that the coiling temperature is too high, the surface problems such as internal oxidation and the like are easily caused to part of steel types, and the subsequent coating is difficult; some control methods (such as overlong waiting time of a winding drum) influence the production efficiency, or the problem of flat winding of the thin-specification steel coil is not obviously improved.

Disclosure of Invention

The invention aims to provide a method for controlling a flat coil of hot-rolled advanced high-strength steel, aiming at the problem of the flat coil of the hot-rolled advanced high-strength steel and the defects of the prior art. The core idea of the invention is to coordinate the conversion of the gravitational potential energy and the total elastic storage energy of the steel coil by controlling the surface roughness state of the finished strip steel product and the phase change process of the strip steel on the laminar flow roller way, so that the steel coil is prevented from being flatly rolled or the degree of flatly rolling is in an acceptable range under the action of gravity.

The invention is realized in such a way that:

a method for controlling a flat coil of hot-rolled advanced high-strength steel mainly comprises four control stages: the method comprises a steel strip finish rolling surface roughness control stage, a laminar cooling austenite phase change control stage, a coiling tension control stage and a rotation control stage after coiling.

The further scheme is as follows:

in the stage of controlling the surface roughness of the finish rolling strip, the surface of a working roll of a last stand of a finishing mill group is subjected to laser texturing treatment, and the surface roughness Rz of the working roll is controlled to be 80-100 mu m; the number RPc of roughness peaks per centimeter is 50-70 per cm.

The further scheme is as follows:

and in the control stage of the surface roughness of the finish rolling of the strip steel, the rolling reduction of the last stand is 15-20%.

The further scheme is as follows:

a control stage of the finish rolling surface roughness of the strip steel, wherein the finish rolling temperature (FDT) is controlled, and the FDT is Ar₃+(80～100)℃； Ar₃Calculated according to the following formula, Ar3 ═ 901-325 + 325 × C% +33 × Si% +287 × P% +40 × Al% -92 × (Mn% + Mo% + Cu%) -46 × (Cr% + Ni%).

The further scheme is as follows:

the austenite phase change control stage of laminar cooling adopts a two-stage cooling mode, and the strip steel is quickly cooled (70-100 ℃/s) to the intermediate air cooling temperature T after being discharged from the finishing mill group_mI.e. proeutectoid ferrite transformation nose point temperature (T)_n) Nearby, T_m＝T_nAnd +/-10 ℃, controlling the time t of the intermediate air cooling section in the laminar flow stage to be 4-8 s, and then rapidly reducing the temperature to the Coiling Temperature (CT), wherein the CT is (400-580) +/-30 ℃.

The further scheme is as follows:

and in the coiling tension control stage, the coiling unit tension of the strip steel is controlled to be Tu equal to 50-60 MPa, and the post-tension distribution coefficient of the pinch roll is 0.25-0.35.

The further scheme is as follows:

and a rotation control stage after coiling, namely after the coiling of the strip steel is finished, the steel coil rotates for 30-40 s at the rotating speed of 20-30 rpm along with the coiling mandrel in the coiling machine, and then is unloaded out of the coiling machine.

The further scheme is as follows:

the steel strip comprises the following chemical components in percentage by weight: 0.02-0.4%; si: 0.2-2.0%; mn: 0.8-3.0%; cr is less than or equal to 1.0 percent; cu is less than or equal to 0.5 percent; mo is less than or equal to 1.0 percent; ti + Nb + V is less than or equal to 1.0 percent; p is less than or equal to 0.03 percent; s is less than or equal to 0.01 percent; n is less than or equal to 0.01 percent; b is less than or equal to 0.005 percent, and the balance of Fe and inevitable impurities.

The reason for adopting the control process is that: when the influence of temperature and phase change on the hot rolled steel coil is not considered, the steel coil stores total elastic energy TEE (initial total elastic energy TEE) under the action of coiling tension when being coiled₀T L, where T is the total tension of the coil, L is the length of the strip steel), and the coil itself has the gravitational potential energy GPE (initial gravitational potential energy GPE0 of the coil-off is MgD/2, where M is the mass of the coil, g is the gravitational constant, and D is the diameter of the coil), both of which are functions of the collapse amount x of the coil flat coil, and the coil flat-coiling process is a competitive conversion process of the two potential energies, as shown in fig. 3(a)₀When the gravity potential energy GPE is reduced to be equal to the total elastic energy TEE, the steel coil reaches an energy balance state, and the flat coiling process is stopped (as shown by GPE1 and TEE2 curves in the attached figure 3 (b)); when the take-up tension is set properly, gpe (x) and TEE (x) intersect near x-0, indicating that there is substantially no flat roll. Therefore, increasing the winding tension is an effective means of controlling the edgewise roll. However, the hot rolled steel coil also has the influence of temperature and phase change factors on the flat coil, and the thermal expansion caused by phase change expansion and phase change latent heatThe steel coil is loosened, so that the interlayer pressure p is reduced, the interlayer contact area is reduced, and the friction coefficient mu is also reduced.

Therefore, in addition to increasing the residual radial pressure p between steel coil layers after coil unloading by increasing the coiling tension T, in the finish rolling stage, the surface roughness of the strip steel is increased by performing texturing treatment on the surface of the working roll of the finish rolling final frame and increasing the reduction of the final frame, so that the friction coefficient mu between the steel coil layers is increased, the interlayer friction force f and the equivalent shearing rigidity G of the steel coil are increased to be mu p, and the capacity of resisting the flat coil of the steel coil is further increased.

In the finish rolling stage, a relatively low finish rolling temperature FDT is used in consideration of the rolling pressure, and FDT ═ Ar₃And +/-80-100 ℃, wherein the temperature range can ensure that the finish rolling is carried out in an austenite single-phase region, and the lower temperature ensures that austenite grains of a finish rolling tail frame are finer and ferrite nucleation points are increased through accumulated deformation. In addition, the low-temperature finish rolling can reduce the rolling speed and ensure longer intermediate air cooling time.

In the laminar cooling stage, the strip steel is cooled to the nose tip temperature T of the ferrite transformation C curve through a faster cooling speed (70-100 ℃/s)_nNearby (T)_m＝T_nAnd + -10 ℃) to start ferrite transformation, wherein the intermediate air cooling time t is 4-8 s, so that austenite generates pro-eutectoid transformation, a large amount of pro-eutectoid ferrite is generated, the austenite is consumed as much as possible, and the cooling route is shown as a route ② in the attached figure 2.

And in the strip steel coiling stage, the coiling unit tension of the strip steel is controlled to be Tu 50-60 MPa. And setting the post-tension distribution coefficient of the pinch roll to be 0.25-0.35, and setting the tension of the strip steel between the last stand of the rolling mill and the pinch roll to be 0.65-0.75 Tu. On one hand, the temperature reduction phase change of the steel is expansion phase change, and the ferrite phase change can be promoted to occur under a larger tensile stress state; on the other hand, the residual radial pressure p between the steel coil layers after coil stripping is increased by increasing the coiling tension T, and the equivalent shearing rigidity G of the steel coil is increased.

And in the post-strip steel coiling stage, the steel coil is rotated for 30-40 s in the coiling machine along with the coiling mandrel at the rotating speed of 20-30 rpm and then is unloaded out of the coiling machine, on one hand, austenite which is not converted yet can be fully converted within 30-40 s, and on the other hand, the stress state and interlayer gaps of the steel coil can be uniformized by the slow rotation of the steel coil, so that the stability of the steel coil is improved.

The invention has the following beneficial effects:

the method for controlling the flat coil of the hot-rolled advanced high-strength steel increases the surface roughness of a hot-rolled strip steel finished product, increases the interlayer friction force and the equivalent shear stiffness of a steel coil, and further increases the capability of resisting the flat coil of the steel coil; the austenite phase change proportion of the strip steel on the laminar flow roller way is increased, the phase change expansion, the latent heat expansion and the phase change plastic deformation after coiling are reduced, the conversion of the gravitational potential energy and the total elastic storage energy of the steel coil is coordinated, and the diameter change of an inner ring of the steel coil after coil unloading is in a controllable range: the length b of the short shaft of the inner ring of the hot-rolled steel coil is more than or equal to 700mm, and the length a of the long shaft and the length b of the short shaft meet the requirement that a-b is less than or equal to 80mm, so that the additional production cost increase and yield loss caused by cutting off the inner ring of the steel coil are avoided.

In addition, the method for controlling the flat coil of the hot-rolled advanced high-strength steel is suitable for hot-rolled advanced high-strength steel grades with different components, is particularly suitable for thin hot-rolled strip steel, and has wide application value.

Drawings

FIG. 1 is a schematic view of a production line of the thin slab continuous casting and rolling process of the present invention;

FIG. 2 is a schematic diagram of the influence of the major alloying elements on the phase change mechanics of typical advanced high strength steels and the laminar cooling route involved in the method of the present invention;

FIG. 3 is a schematic diagram illustrating the competitive transformation process of gravitational potential energy and elastic potential energy of a steel coil;

in fig. 1, 1 is slab continuous casting, 2 is soaking furnace heating, 3 is finish rolling, 4 is strip steel laminar cooling, 5 is strip steel coiling, and 6 is steel coil warehousing cooling.

Detailed Description

The present invention will be described below with reference to specific examples. It should be noted that the embodiments are only for further illustration of the present invention, and other insubstantial modifications or adaptations made in accordance with the present invention are intended to be within the scope of the present invention.

The method for controlling the flat coil of the hot-rolled advanced high-strength steel mainly comprises four control stages: the method comprises a steel strip finish rolling surface roughness control stage, a laminar cooling austenite phase change control stage, a coiling tension control stage and a rotation control stage after coiling.

In the finish rolling stage, the surface of a working roll of a last stand of a finish rolling unit is subjected to laser texturing treatment, and the surface roughness Rz of the working roll is controlled to be 80-100 mu m; the number RPc of roughness peaks per centimeter is 50-70 per cm.

In the finish rolling stage, the final frame rolling reduction is 15-20%.

In the finish rolling stage, the finish rolling temperature (FDT) is controlled, where FDT is Ar₃+(80～100)℃；Ar₃Calculated according to the following formula: ar (Ar)₃901-one + 325 × C% +33 × Si% +287 × P% +40 × Al% -92 × (Mn% + Mo% + Cu%) -46 × (Cr% + Ni%). where C% is the mass percentage of C, if the mass percentage of C is 0.4%, the value of C% in the formula is 0.4, and the meaning of Si%, P%, Al%, Mn%, Mo%, Cu%, Cr%, Ni% in the formula is similar to C%.

And (2) the strip steel is cooled in two stages in the laminar cooling stage, the strip steel is quickly cooled (70-100 ℃/s) to an intermediate air cooling temperature Tm after being discharged from a finishing mill set, namely the temperature of a eutectoid ferrite transformation nose point (Tn) is close to, Tm is Tn +/-10 ℃, the time t of the intermediate air cooling section in the laminar cooling stage is controlled to be 4-8 s, and then the temperature is quickly reduced to a Coiling Temperature (CT), and the CT is (400-580) +/-30 ℃.

In the strip steel coiling stage, the coiling unit tension of the strip steel is controlled to be Tu equal to 50-60 MPa, and the post tension distribution coefficient of the pinch roll is 0.25-0.35. And after the strip steel is coiled, the steel coil rotates for 30-40 seconds in the coiling machine along with the coiling mandrel at the rotating speed of 20-30 rpm, and then is discharged out of the coiling machine.

The method can be suitable for hot rolled strip steel with different specifications and varieties. Preferably, the method for controlling the flat coil of the hot-rolled advanced high-strength steel is particularly suitable for thin-gauge hot-rolled advanced high-strength steel. The steel strip comprises the following chemical components in percentage by weight: 0.02-0.4%; si: 0.2-2.0%; mn: 0.8-3.0%; cr is less than or equal to 1.0 percent; cu is less than or equal to 0.5 percent; mo is less than or equal to 1.0 percent; ti + Nb + V is less than or equal to 1.0 percent; p is less than or equal to 0.03 percent; s is less than or equal to 0.01 percent; n is less than or equal to 0.01 percent; b is less than or equal to 0.005 percent, and the balance of Fe and inevitable impurities.

The following are specific examples of the present invention:

example one

The hot-rolled QP steel has the thickness of a steel strip from 1.0mm to 4.0mm, the chemical components of the steel strip in percentage by weight (wt.%) are shown in the table 1, and the balance of Fe and inevitable impurities. The relevant process of the strip steel is shown in tables 2a and 2 b.

Table 1: chemical composition of strip steel (wt.%)

TABLE 2a

TABLE 2b

Table 3 below shows the result of the degree of the edgewise winding of the steel strip coil of example 1 and the comparison with the comparative example.

Table 3: effects of the implementation

Example 2

The thickness of the hot rolled raw material coil of the cold rolled DP steel is from 1.0mm to 4.0mm, the chemical components of the strip steel in percentage by weight (wt.%) are shown in Table 4, and the balance is Fe and inevitable impurities. The relevant process for the strip is shown in tables 5a, 5 b.

Table 4: chemical composition of strip steel (wt.%)

TABLE 5a

TABLE 5b

Table 6 below shows the result of the degree of the edgewise winding of the steel strip coil of example 2 and the comparison with the comparative example.

Table 6: effects of the implementation

Example 3

Hot-rolled hot-formed steel having a strip thickness of from 1.0mm to 4.0mm, the chemical composition in weight percent (wt.%) of the strip being as shown in table 7, with the balance being Fe and unavoidable impurities. The relevant process for the strip is shown in tables 8a, 8 b.

Table 7: chemical composition of strip steel (wt.%)

TABLE 8a

TABLE 8b

Table 9 below shows the result of the degree of the edgewise winding of the steel strip coil of example 3 and the comparison with the comparative example.

Table 9: effects of the implementation

Although the present invention has been described herein with reference to the illustrated embodiments thereof, which are intended to be preferred embodiments of the present invention, it is to be understood that the invention is not limited thereto, and that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure.

Claims

1. A method for controlling a flat coil of hot-rolled advanced high-strength steel is characterized by mainly comprising four control stages: the method comprises a steel strip finish rolling surface roughness control stage, a laminar cooling austenite phase change control stage, a coiling tension control stage and a rotation control stage after coiling.

2. The method for controlling the edgewise coiling of the hot-rolled advanced high-strength steel as claimed in claim 1, wherein:

3. The method for controlling the edgewise coiling of the hot-rolled advanced high-strength steel as claimed in claim 1, wherein:

4. The method for controlling the edgewise coiling of the hot-rolled advanced high-strength steel as claimed in claim 1, wherein:

the control stage of the surface roughness of the finish rolling of the strip steel controls the finish rolling temperature FDT, wherein the FDT is Ar₃+(80～100)℃；Ar₃Calculated according to the following formula, Ar3 ═ 901-325 + 325 × C% +33 × Si% +287 × P% +40 × Al% -92 × (Mn% + Mo% + Cu%) -46 × (Cr% + Ni%).

5. The method for controlling the edgewise coiling of the hot-rolled advanced high-strength steel as claimed in claim 1, wherein:

the laminar cooling austenite phase change control stage adopts a two-stage cooling mode, and the strip steel is quickly cooled to the middle after exiting the finishing mill groupAir cooling temperature T_mI.e. proeutectoid ferrite transformation nose tip temperature T_nNearby, T_m＝T_nAnd +/-10 ℃, controlling the time t of the intermediate air cooling section in the laminar flow stage to be 4-8 s, and then rapidly reducing the temperature to the coiling temperature CT which is (400-580) +/-30 ℃.

6. The method for controlling the edgewise coiling of the hot-rolled advanced high-strength steel as claimed in claim 5, wherein:

the cooling speed of the rapid cooling is 70-100 ℃/s.

7. The method for controlling the edgewise coiling of the hot-rolled advanced high-strength steel as claimed in claim 1, wherein:

8. The method for controlling the edgewise coiling of the hot-rolled advanced high-strength steel as claimed in claim 1, wherein:

9. The method for controlling the edgewise coiling of the hot-rolled advanced high-strength steel as claimed in claim 1, wherein: