JPH04253505A - Direct rolling method for continuous cast slab - Google Patents

Abstract

PURPOSE:To provide a direct rolling method for a continuous cast slab possible to reduce the generation of surface flow of the slab. CONSTITUTION:The steel slab drawn out of a continuous casting mold is cooled until the surface temperature comes to a range below point A1, then, cooling and heat insulation of the slab are adjusted, the surface of the slab is reheated up to more than 1000 deg.C by using sensible heat and latent heat of solidification held by the residual molten steel existing in the slab, then, the slab is heated uniformly in its width direction up to more than 1000 deg.C in an on-line heating zone installed at the rear stage of a continuous casting machine and then rolled. Then, in the same way, the slab is cooled until its surface temperature comes to >=point A1 and <=950 deg.C, the slab is heated uniformly until the temperature is its width direction exceeds 1000 deg.C as said above, then, is held for >=2min, then rolled.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for directly rolling slabs in continuous steel casting.

0002

[Prior Art] Direct rolling (HDR) of continuously cast slabs is
This is a method in which a slab cast in a continuous casting machine is kept warm or heated online without being cooled, and then rolled in a hot rolling mill. This HDR process realizes a significant streamlining of the process, and is expected to be further developed and expanded in the future. However, slabs manufactured by HDR have more surface defects than the conventional method (HCR), in which the slab is cooled after continuous casting, then reheated, and then rolled. , there is a concern that the yield will decrease.

[0003] Surface defects that generally occur in continuously cast slabs are well known to include vertical cracks, transverse cracks, and cracks caused by subsurface inclusions. Regarding vertical cracks, it has been revealed that the starting point of the crack is formed within the mold, and the crack progresses during the subsequent cooling process. Important countermeasures to prevent this include selection of mold powder, optimization of mold taper, control of fluctuations in the melt level within an appropriate range, and uniform cooling in the secondary cooling zone.

[0004] Furthermore, cracks caused by inclusions below the surface layer are
This is related to whether inclusions are trapped in the initial solidified shell in the mold, and countermeasures to this problem include cleaning the molten steel, reducing the amount of Al2O3 in the molten steel, and adjusting the level fluctuations in the mold to improve powder control. Efforts are being made to prevent entanglement.

[0005] Transverse cracking is thought to be caused by high-temperature embrittlement of the slab due to precipitation of impurity elements during solidification and cooling. The basic measure to prevent transverse cracking is to correct the surface temperature of the slab by avoiding the high temperature embrittlement temperature range when bending deformation (straightening) is applied in the continuous casting machine. Furthermore, one of the countermeasures is to reduce impurity elements such as S, P, and N, which are the causes of precipitates, to minimize high-temperature embrittlement.

[0006] These general surface flaw countermeasures are HD
It is also applied to slabs manufactured by R.

[0007]

[Problem to be Solved by the Invention] However, even if conventional surface flaw reduction measures are implemented, slabs produced by the HDR process have more surface flaws, especially horizontal cracks, than slabs produced by the conventional HCR process. The problem is not resolved.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for directly rolling continuously cast slabs, which can reduce the occurrence of surface defects on slabs.

[0009]

[Means for Solving the Problems and Effects] The direct rolling method for continuously cast slabs according to the present invention firstly involves rolling steel slabs pulled out from a continuous casting mold so that the surface temperature thereof is within a range of A1 point or lower. The slab is cooled by spray or mist spray, etc., until the slab reaches After reheating to 1000°C or higher, the cast slab is uniformly heated in the width direction to 1000°C or higher in an online heating zone installed at the latter stage of the continuous casting machine, and then rolled. .

Second, the steel slab drawn from the continuous casting mold is cooled until its surface temperature reaches a temperature above the A1 point and below 950°C, and then the cooling and heat insulation of the slab are adjusted. At the same time, the surface of the slab is heated to 10
After reheating to 00°C or higher, the temperature in the width direction of the slab is raised to 1000°C in the online heating zone installed at the rear stage of the continuous casting machine.
It is characterized in that it is heated uniformly above, held at this temperature for at least 2 minutes, and then rolled.

The inventors of the present invention have studied the causes of surface flaws peculiar to slabs produced by the HDR process, and have come to the following conclusion.

[0012] When steel is solidified and cooled, S in the austenite phase (hereinafter referred to as γ phase) segregates at γ grain boundaries. Therefore, S and Mn in the steel react, producing MnS and (Fe, Mn)S.
will precipitate out. When such steel is continuously cooled, the precipitation form of MnS and (Fe,Mn)S is γ.
It precipitates finely along the grain boundaries. Since the steel in this state has very low hot strength and is brittle, cracks are likely to occur when this slab is hot rolled. Therefore,
Many surface defects occur due to such cracks.

Therefore, in the present invention, firstly, after the surface and surface temperature of the slab are once lowered to a temperature below the A1 point (a range where the phyllite + pearlite phase is completely formed, hereinafter abbreviated as α + P phase), By further reheating, the surface layer of the slab is returned from the α+P phase to the γ phase, and the temperature in the width direction of the slab is uniformly heated to 1000° C. or higher in the online heating zone. Fine γ grains can be obtained by such treatment. Then, MnS and (Fe, Mn) precipitate at the γ grain boundaries.
At the same time as the concentration of S decreases, MnS and (Fe, Mn)S, which had been finely precipitated through the reheating process, become coarse. By coarsening MnS and (Fe, Mn)S in this way, the high-temperature ductility of the steel is restored, and cracking during rolling can be prevented.

[0014] Secondly, after the surface and surface temperature of the slab are once brought to a temperature range from the A1 point to 950°C (the range where the phyllite phase is completely formed, hereinafter abbreviated as α phase), the slab is further reheated. By doing this, the surface layer of the slab is returned from the α phase to the γ phase, and the temperature in the width direction of the slab is uniformly heated to 1000° C. or higher in the online heating zone, and maintained at this temperature for at least 2 minutes or more. By such processing, based on a principle similar to the above-mentioned principle, fine γ grains can be obtained and MnS and (Fe,Mn)S can be coarsened. Therefore, the high-temperature ductility of the steel is restored, and cracking during rolling can be prevented.

[0015]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0016] Figure 1 shows that among the surface flaws found on hot-rolled sheets manufactured from continuously cast slabs, surface flaws on the hot-rolled sheets are caused by cracks in the slab or cracks generated during the hot rolling process. This figure shows the relationship between the number of defects considered to be caused by hot embrittlement caused by sulfides (MnS or (Fe,Mn)S) and the Mn/S ratio in steel. As shown in this figure, it was confirmed that in the case of HDR, the number of defects generated was several times higher than in the case of HCR. The components of the hot rolled sheet at this time were as follows in weight percent.

C=0.01~0.05 Si=0
．． 01~0.03 Mn=0.25~0.35
P=0.010~0.025 S=0.012
~0.030 sol. Al=0.02~0.0
3 In Figure 1, the flaw occurrence rate increases as the Mn/S ratio decreases, but this is due to a decrease in high-temperature brittleness, and this figure shows that in order to prevent this, it is well known that This shows that it is effective to reduce S in steel. However, what causes the difference between HDR and HCR processes at the same S level?
The cause of this has not been elucidated until now. The inventors of the present invention investigated the cause of surface flaws in steel sheets manufactured from cast slabs produced by the HDR process, with the aim of reducing surface flaws.

[0018] HDR and HC
A simulation test was conducted on R. That is, in accordance with HDR, the test piece was melted, solidified, and cooled to a predetermined temperature in a predetermined time in the testing machine, and then a tensile test was conducted. Further, in correspondence with HCR, the test piece was melted and solidified in the test machine, cooled to 300°C, reheated, and then subjected to a tensile test. The test results at this time are shown in FIG. FIG. 2 is a diagram showing the relationship between the temperature of the tensile test and the ductility (restriction of area) at that time. As is clear from the figure, in the HDR simulation, the ductility decreases significantly at high temperatures of 800 to 1200°C.

Next, the fracture surfaces of the HDR-simulated test piece and the HCR-simulated test piece were observed using a scanning electron microscope (SEM). As a result, in the former specimen, many fine MnS and (Fe,
It was found that Mn)S had precipitated, and the latter had become coarse and its number was extremely small. Therefore, as shown in FIG. 2, the reason why there are many surface flaws on the hot-rolled sheet due to HDR is considered to be due to the difference in thermal history.

Regarding the test simulating HDR,
A high-temperature tensile test was conducted to investigate the thermal history and hot ductility improvement effect of preliminary processing. As a result, the following findings were obtained regarding steel with a small Mn/S ratio.

(1) After melting and solidifying the test piece, cooling it,
When a tensile test is performed while holding the temperature above 1000℃,
Hot ductility is very good (Figure 6).

(2) After melting and solidifying the test piece, cooling it,
Once the temperature is lowered to about 700℃ or less, then heated again to raise the temperature to 1000℃ or higher and a tensile test is performed.
Hot ductility becomes very good (Figure 7).

(3) After melting and solidifying the test piece, cooling it,
Once held in the range of approximately 750°C to 950°C, heated again, held at a temperature of 1000°C or more, held for 2 minutes or more, and then subjected to a tensile test, the elongation is significantly better (hot improved ductility).

Based on the results of these tensile tests, the conditions for direct rolling of continuously cast slabs were investigated, and hot direct charge (Hot Direct Charge)
) experiments were conducted. Example-1 A test was conducted in which steel having the composition described above was manufactured by the HDR process. The secondary cooling pattern for continuous casting was set as follows. Immediately after pulling the slab out of the continuous casting mold, it is cooled using spray or mist spray to lower the surface temperature of the slab to 700°C, and then, using the sensible heat and latent heat of the unsolidified part inside the slab. After reheating the surface of the slab, controlling the cooling intensity of the secondary cooling zone (using weak cooling), and installing a heat insulating cover inside the continuous casting machine, the surface temperature of the slab was controlled to over 1000℃. Using an edge heater installed at the latter stage of the continuous casting machine, the edge of the slab was actively heated (online heating), and then quickly transferred to a hot rolling mill for hot rolling. The surface of the steel plate (hot rolled plate) after hot rolling was observed and the number of surface flaws was counted. The slab surface temperature pattern used at this time is shown in FIG. 6 in comparison with a conventional pattern. Further, Table 1 shows the surface flaw index of hot rolled sheets by comparing the conventional HDR method and the HDR method of the present invention. Note that the surface flaw occurrence index was expressed as the number of plates with surface flaws divided by the number of observation plates x 100.

[0025]

[Table 1] As is clear from Table 1, by using the secondary cooling pattern within the scope of the present invention, when continuously casting and directly rolling low Mn/S steel, compared to the conventional example, It has been found that the incidence of flaws on the surface of steel sheets can be extremely reduced. Example 2 In this example as well, a test was conducted in which steel having the above-mentioned composition was manufactured by the HDR process. The secondary cooling pattern of continuous casting was different from Example 1 and was set as follows. Immediately after pulling out the slab from the continuous casting mold, cool it using spray or mist spray to bring the surface temperature of the slab to 90°C.
After that, the surface of the slab is reheated using the sensible heat and latent heat of the unsolidified part inside the slab, and the cooling intensity of the secondary cooling zone is controlled (weak cooling is adopted) and continuous cooling is performed. After installing a heat insulating cover inside the casting machine and controlling the surface temperature of the slab to over 1000℃, we actively heat the edge of the slab using the edge heater installed at the rear of the continuous casting machine. After holding at this temperature for at least 2 minutes (on-line heating), it was quickly transported to a hot rolling mill and hot rolled. Steel plate after hot rolling (hot rolled plate)
The number of surface flaws was counted. Figure 7 shows the slab surface temperature pattern used at this time. For comparison, we also counted the number of surface flaws on slabs that were pulled out from the continuous casting mold, held at a temperature of 1000°C or higher without cooling, and then hot-rolled (comparative example). . Incidentally, the slab surface temperature patterns of this comparative example and the above-mentioned conventional example are also shown in FIG. Further, Table 2 shows the surface flaw occurrence index of Example 2, which is within the scope of the present invention, together with the surface flaw occurrence index of the conventional example and the comparative example.

[0026]

[Table 2] As is clear from Table 2, in Example 2 using the secondary cooling pattern within the scope of the present invention, as in Example 1,
It was found that the incidence of defects on the surface of the steel sheet was extremely low compared to the conventional example. In addition, in the comparative example, as in Example 2, MnS
Although the incidence of surface flaws was low because of the coarsening achieved, internal cracks did occur. This is considered to be caused by coarsening of the γ grains.

[0027]

Effects of the Invention According to the present invention, by preliminary processing, Mn
The precipitation of S and (Fe/Mn)S is promoted, and the precipitation state of these precipitates becomes closer to the equilibrium state, so it is possible to coarsen MnS and (Fe, Mn)S in the subsequent reheating process. can. Therefore, hot ductility is improved and surface flaws can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between Mn/S in steel and surface flaw occurrence index.

FIG. 2 is a diagram showing the relationship between the temperature of a tensile test and the ductility at that time.

FIG. 3 is a diagram showing the relationship between holding time at high temperature and ductility.

FIG. 4 is a diagram showing the relationship between temperature and ductility after solidifying and cooling a slab.

FIG. 5 is a diagram showing the relationship between holding time before a tensile test and ductility.

FIG. 6 is a diagram showing a slab surface temperature pattern in an embodiment of the present invention.

FIG. 7 is a diagram showing a slab surface temperature pattern in another embodiment of the present invention.

Claims (2)

[Claims] Claim 1: Cooling a steel slab drawn from a continuous casting mold until its surface temperature falls within a range of A1 point or below, and then controlling the cooling and heat insulation of the slab, and
After reheating the surface of the slab to 1000℃ or higher using the sensible heat and latent heat of solidification held by the residual molten steel inside the slab, the slab is heated in an online heating zone installed at the rear of the continuous casting machine. A method for directly rolling continuously cast steel slabs, characterized by uniformly heating the temperature in the width direction to 1000° C. or higher, and then rolling. 2. Cool the steel slab pulled out from the continuous casting mold until its surface temperature reaches a temperature above the A1 point and below 950°C, and then adjust the cooling and heat insulation of the slab, and After reheating the surface of the slab to 1000℃ or higher using the sensible heat and latent heat of solidification held by the residual molten steel inside the slab, the width of the slab is heated in an online heating zone installed at the rear stage of the continuous casting machine. Uniformly heat the temperature in the direction to 1000 ° C. or more and maintain this temperature for at least 2 minutes,
A method for directly rolling continuously cast steel slabs, which is characterized in that a rolling process is then carried out.