JPS63119910A - Continuous casting and rolling method for steel containing silicon - Google Patents

Abstract

PURPOSE:To manufacture the high quality hot coil without any modification by drawing a steel containing silicon from a continuous casting machine at a speed being near to the critical drawing speed and hot rolling the steel by specifying a temp. range and a cumulative draft after cutting. CONSTITUTION:A steel containing 4-7wt% silicon is drawn from a continuous casting machine at a speed being 70-100% of the critical drawing speed allowed by the length of the casting machine and then is cut. A high temp. billet is subjected to a temp. equalization treatment, is inserted into a hot rolling mill at a mean temp. >=1,050 deg.C and a min. temp. >=1,000 deg.C, is rolled with a cumulative draft of >=95% to obtain a hot coil whose final product thickness is <=2mm. In this method, a temp. of the rolled billet is always kept >=900 deg.C and the billet has small deformation resistance and generation of intergranular cracks when rolling forces are loaded is evaded.

Description

Detailed Description of the Invention [Industrial Field of Application] This invention is a method of continuously casting high-Si steel that contains 4 to 7% by weight of silicon (Si) and is easily hot-embrittled and difficult to roll. The present invention relates to a hot rolling method.

[Prior Art] Low C height 5in4, which has a SL content of 4 to 7% by weight (hereinafter simply abbreviated as %) and a low carbon (C) content, tends to form coarse crystal grains during the solidification process. Therefore, this steel type
It becomes brittle in hot conditions, and due to minute externally added strain (strain due to thermal stress, bulging stress, or straightening force, etc.) during continuous casting,
Intergranular cracks are likely to occur on the surface and inside of the slab. Also,
During hot rolling, intergranular cracks may occur in slabs due to external rolling forces.

This invention was made in view of the above circumstances, and it is possible to continuously cast high-Si steel, which is easily hot-embrittled and difficult to roll, and then directly hot-roll it, and to take care of the surface of the slab. An object of the present invention is to provide a method for continuous casting and rolling of silicon-containing steel that can produce high-quality hot coils with high efficiency.

[Means for Solving the Problems] The method for continuous casting and rolling of silicon-containing steel according to the present invention is a method in which silicon-containing steel having a weight of 4 to 7 weight 96 is cast by a continuous casting machine, and then rolled. , the slab is pulled out at a speed of 70 to 100% of the limit drawing speed allowed by the machine length of the continuous casting machine, the slab after cutting is transported to a hot rolling mill, and the average temperature of the slab is 1050 ° C. or higher, The cast slab is inserted into a hot rolling machine at a minimum temperature of 1000° C. or higher, and rolled so that the cumulative reduction ratio is 95% or higher and the final product thickness is 2 mm or less.

[Function] According to the present invention, the slab discharged from the mold is, for example, water-cooled and then heat-retained, so that the temperature of the slab becomes uniform and its surface temperature increases (recuperation). In this case, the slab is pulled out at a high speed of 70 to 100% of the limit drawing speed determined by the machine length, so the slab maintains a high temperature after cutting. The average temperature is 1050℃ or higher, and the minimum temperature is 1
If hot rolling of a slab is started at 000°C or higher, the temperature of the rolled material must always be maintained at 900°C or higher during the hot rolling process where the cumulative reduction rate is 95% or higher and the final product thickness is 2mm or less. I can do it. Therefore, this rolled material has excellent workability in the above-mentioned temperature range, has low deformation resistance, and avoids occurrence of intergranular cracks when rolling force is applied.

[Embodiments] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Figure 1 plots the average cooling rate and test temperature on the horizontal axis, and the grain size on the vertical axis, and shows the relationship between the cooling rate and grain size, and the relationship between grain size and the boundary temperature at which cracking occurs. FIG.

This data was obtained by conducting melting and solidification tests on slabs with a thickness of 150 to 300 mm in a small experimental furnace, and then performing high-temperature tensile tests on tensile test pieces taken from the slabs after solidification. It is something that FIG. 2 is a cross-sectional view of a high-Si continuous cast slab showing the ferrite grain distribution. High S
In steel i, ferrite crystal grains are formed during solidification, but the surface 1 of the ingot has a chill crystal structure due to the rapid cooling rate. The interior 2 of this slab has a columnar crystal structure extending from the surface 1 toward the axial center 3, and the axial center 3 is equiaxed. As described above, the crystal grain size includes a grain size (minor axis) in the direction along the surface of the slab and a grain size (long axis) in the direction perpendicular to the slab surface. In Figure 1, the solid line on the left shows the relationship between the particle size and the average cooling rate for the major axis, and the broken line shows the relationship between the particle size and the average cooling rate for the minor axis. .

As is clear from FIG. 1, there is a strong correlation between the major diameter particle size and the average cooling rate.

The right diagram in Figure 1 shows a constant strain application test at a strain rate of 10-3/sec and a strain amount of 10% under simple heating tensile conditions on a tensile test piece taken from the slab obtained by solidification. After the test, the specimen was observed under a microscope to investigate the occurrence of intergranular cracking.The results are shown below. In this figure, the solid line indicates the limit at which grain boundary cracking occurs; if the grain size is the same, the test temperature is lower than this solid line, and if the test temperature is the same, the grain size is lower than this solid line. If it is large, intergranular cracking will occur. That is, in the shaded area in the figure, intergranular cracking does not occur.

As can be seen from the left diagram in Fig. 1, at the cooling rate (0.1°C/sec) at the axial center 3 of the slab, the crystal grain size is 30°C.
+a (point A), and at the cooling rate (3°C/sec) on surface 1 of the slab, the crystal grain size is 10ml1 (point B)
It is. In a slab having such a grain size, the critical temperatures for cracking are points a and b in the right diagram of FIG. 1, which correspond to 1100°C and 600°C, respectively. Therefore, in order to prevent intergranular cracking, the average cooling rate of the surface of the slab should be increased by 3.
It is necessary to maintain the surface temperature at 600° C./second or higher and at the same time to maintain the surface temperature at 600° C. or higher.

In this way, in order to increase the cooling rate of the surface layer of the slab, it is necessary to increase the cooling rate of the slab in the mold.
For example, possible measures include reducing the wall thickness of the mold, using powder with a low melting point and high viscosity, or adjusting the oscillation mode of the mold. Furthermore, the cooling rate of the slab can be adjusted within a predetermined range by high-speed drawing or adjustment of secondary cooling water.

On the other hand, according to a high-temperature oxidation test of high-Si steel, when the temperature of the slab exceeds 1200°C, scale melts, grain boundary infiltration occurs on the surface of the slab, and intergranular cracking is promoted. For this reason, it is necessary to maintain the surface temperature of the slab at 1200°C or less.

FIG. 3 is a graph showing the relationship between the average deformation resistance σ and the tensile temperature. This graph shows that high-speed tensile tests are conducted at high temperatures on high-Si steel, austenitic stainless steel, and ordinary steel, and the average deformation resistance σ is determined from the obtained stress strain curves. ). As is clear from this figure, high-Si steel has low deformation resistance and is easy to roll at relatively high temperatures of 900°C or higher, but at temperatures lower than 900°C, its deformation resistance increases significantly and it suddenly becomes difficult to roll. . Therefore, during the rolling process, the temperature of the slab should be kept at 900°C.
It is necessary to maintain the temperature above ℃. Therefore, in the case of rolling where the cumulative reduction ratio is 95% or more and the thickness of the final product is 2fflI11 or less, the average temperature of the slab at the start of rolling is 10%.
It is necessary that the minimum temperature of the slab is 50°C or higher and the minimum temperature of the slab is 1000°C or higher.

Note that the head (top part) and rear part (bottom part) of the slab, as well as the side edges (parts of the corners) of the slab, are more susceptible to heat dissipation, so the temperature is usually lower than that of the central part of the slab. is 50 to 1
00℃ low. However, if this temperature difference exceeds 100°C, rolling defects will occur in this low temperature area, so the temperature difference will be 100°C.
It is necessary to maintain the temperature below 0°C. Therefore, if necessary, an adiabatic heat-retaining device that insulates or retains the slab, or a heating means that heats the slab using gas or electricity, etc. is installed in the continuous casting machine or on the conveyance route from the continuous casting machine to the rolling mill. Set up.

Under these conditions, after continuous casting, if the slab is conveyed to a hot rolling mill and hot rolled directly, the slab being rolled will have a temperature gradient from the surface to the inside of the slab. The temperature inside the slab is higher than the surface. For this reason, the axial center of the slab is more easily rolled down than the surface side, and therefore, compared to normal reheat-rolled material, directly hot-rolled material has smaller crystal grains in the axial center. promoted.

Next, the quality of the example slabs that were continuously cast and hot rolled under the conditions specified in the present invention will be explained together with the comparative example slabs. As shown in FIG. 4, the molten steel injected into the tundish 11 from the MIO is cooled by the mold 12 to form a solidified shell, and the slab containing unsolidified molten steel inside is guided by the roll band 13. Water cooled. This slab is heat-insulated and uniformly heated by a heat insulating device 14 and a heat insulating cover 15, and then cut by a cutting machine 16. After the cut slab is partially heated by an edge heater 17 and further uniformized, it is rolled by a rolling stand 18 of a hot rolling mill.

The slab is then heated by a heating device 19, further rolled by a rolling stand 18, and wound around a coiler 20.

FIG. 5 shows the temperature history of the slab surface during this continuous casting hot rolling process. The examples satisfy all the conditions specified in this invention; in Comparative Example 1, the slab surface temperature decreased to less than 1050°C during the conveyance process, and in Comparative Example 2, it exceeded 1200°C. . In addition, in Comparative Example 3, the slab underwent the same temperature history as Comparative Example 1 until immediately before rolling, and then was heated to raise the temperature of the slab to 1050°C or higher.
In Comparative Example 4, the piece inside the continuous casting machine was once cooled, then reheated and hot rolled.

These slabs were rolled at a cumulative reduction rate of 95% or more and a final product thickness of about 2H. As a result, the surface quality, internal quality, and crystal grain size on the surface and inside of the obtained hot coil are as shown in Table 1 below.

Table 1. However, the surface quality was visually observed after the coil was pickled in a warm environment, and the internal quality was determined by examining polarization cracks in the cross section of the coil. As described above, in the case of the embodiment of the present invention, the average crystal grain size is 100 μm or less even in the axial center, and both the surface quality and internal quality of the hot coil are excellent. On the other hand, Comparative Examples 1 to 4 have large average particle diameters and have poor surface quality or internal quality.

[Effects of the Invention] According to the present invention, a cast slab with good surface and internal quality can be obtained, and a high-quality hot coil can be manufactured without maintenance at low cost.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between grain size and average cooling rate and the relationship between grain size and cracking frequency, Figure 2 is a cross-sectional view of a slab showing ferrite crystal distribution, and Figure 3 is deformation resistance. FIG. 4 is a schematic diagram showing the continuous casting hot rolling process, and FIG. 5 is a graph diagram showing the temperature pattern when casting and rolling is performed according to an embodiment of the present invention together with a comparative example. It is.

Claims (1)

[Claims] In a method in which steel containing 4 to 7% by weight of silicon is cast by a continuous casting machine and then rolled, the maximum drawing speed of 70 to 100% is allowed by the machine length of the continuous casting machine.
The slab is pulled out at a speed of 100%, and the cut slab is transported to a hot rolling machine.The slab is transferred to a hot rolling machine with an average temperature of 1050°C or higher and a minimum temperature of 1000°C or higher. A method for continuous casting and rolling of silicon-containing steel, characterized by rolling the steel so that the thickness of the final product is 2 mm or less at a cumulative reduction rate of 95% or more.