JP4398879B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a method for producing a steel slab containing B with a curved continuous casting machine or a vertical bending continuous casting machine.

  B improves the hardenability of steel by being added in small amounts in steel. For this reason, many high-strength steels contain B. However, when steel containing B is produced by the continuous casting method, BN precipitates in a line along the austenite grain boundary, which causes cracks at high temperatures in the peripheral corners of the slab. It has been made clear. Therefore, Japanese Patent Application Laid-Open No. 56-80354 (Patent Document 1) describes a method of preventing cracking by reducing N according to the amount of B added or adding Ti to suppress the precipitation of BN. ing. However, there are limits to the reduction of N in steelmaking operations, and the addition of Ti may not be an effective means. For example, in varieties that do not contain Al, even if Ti is added to the molten steel, Ti is consumed in the formation of oxides, and sufficient sol.Ti to fix N and suppress BN precipitation cannot be secured. is there. Thus, there is a limit to the component design to suppress cracking of the continuous cast slab of B-added steel.

  By the way, it is widely known that mild cooling in the secondary cooling zone is effective in preventing austenite grain boundary cracking in continuously cast slabs of steel to which Al, Nb, or V is added. In these steels, AlN, NbCN, and VN are formed, and the hot ductility is remarkably lowered at 700 to 900 ° C where film-like pro-eutectoid ferrite is formed at the austenite grain boundaries. For this reason, cracking is prevented by slow cooling that allows the bending and straightening points of the continuous caster, which deforms the slab, to pass at a temperature higher than the embrittlement temperature range.

  For example, Japanese Patent Laid-Open No. 56-80367 (Patent Document 2) describes that in a B-added steel, the cooling rate is controlled to a low cooling rate of 0.01 to 1 ° C./sec from the melting temperature to 900 ° C. However, in the high-speed casting that has been promoted in recent years, such slow cooling of the slab leads to underdevelopment of the solidified shell and induces abnormal operations such as breakout, so it is difficult to be a countermeasure.

  On the other hand, in Japanese Patent Laid-Open No. 9-225607 (Patent Document 3), as a method for preventing surface cracking of a low alloy steel containing Ni, Cu, V, Nb, etc., (1) the solidified shell thickness is 10 to 15 mm. Finish cooling with the mold in step 2 and start secondary cooling. (2) Temporarily lower the surface temperature of the slab to the range of 600 ° C or higher and below the Ar3 point. It is stated that the temperature is raised to 850 ° C. or higher by heat. In this method, the structure in which the γ grain boundary is obscured by (2) is obtained, and then the slab is deformed in the temperature range (3) where recuperation is performed and embrittlement is unlikely to occur. For recuperation, the condition (1) that defines cooling in the mold is necessary because the heat capacity is small and recuperation is insufficient unless the solidified shell thickness in the mold is 10 to 15 mm. Become.

  Japanese Patent Laid-Open No. 58-187251 (Patent Document 4) discloses an invention that regulates the thickness of the solidified shell in the mold to prevent cracks. However, cracks between dendrite trees such as vertical cracks and internal cracks directly below the surface are known. Austenite grain boundary cracking due to B occurring in the secondary cooling zone cannot be prevented simply by controlling the cooling in the mold.

JP 56-80354 A JP 56-80367 A Japanese Patent Laid-Open No. 9-225607 JP 58-187251 A

  As described above, countermeasures such as adjustment of components and slow cooling have been proposed for surface cracks in continuous cast slabs with B added, but these are difficult to achieve in actual operations. Often.

  An object of the present invention is to provide a method capable of preventing surface cracking while maintaining high productivity without incurring an increase in cost when steel containing B is continuously cast.

  As a result of repeated metallurgical studies, the present inventors found that the surface cracks that occur during continuous casting of B-added steel are coarse MnS formed in a row at austenite grain boundaries as shown in FIG. It was found that there are two factors, namely, precipitation of BN and coarse BN with the core and precipitation of film-like ferrite at the austenite grain boundary as shown in FIG. Among these, the precipitates of BN and MnS shown in Fig. 1 promote the formation of microvoids at the grain boundaries during the bending and straightening of the continuous cast slab, and embrittle the grain boundaries. The reason why BN precipitates with MnS as a nucleus is that the lattice matching between BN and MnS is very good. Therefore, to prevent coarse precipitation of BN at grain boundaries, it is necessary to prevent the formation of coarse precipitates including MnS. In this case, it is possible to suppress the precipitation of MnS by greatly reducing the S concentration in the steel during the refining process, but this will significantly increase the refining cost and processing time.

  Here, Fig. 3 shows the results of equilibrium calculations for the precipitation amounts of MnS and BN in steel with Mn = 0.55%, S = 0.01%, B = 12ppm, and N = 25ppm. BN begins to precipitate at 1000 ° C, and MnS starts to precipitate at a higher 1200 ° C. Therefore, in order to suppress these coarse precipitations, it is necessary to cool at a high cooling rate from 1200 ° C where MnS begins to precipitate. The cooling rate at this time needs to be 100 ° C./sec or more. MnS precipitation is most promoted at 1000 ° C in consideration of diffusion and nucleation (that is, nose of precipitation is about 1000 ° C in CCT and TTT diagrams). Therefore, it is necessary to maintain this cooling rate until the temperature is below 1000 ° C where MnS is most promoted.

  Further, since the film-like grain boundary ferrite shown in FIG. 2 has a lower strength than austenite, strain concentrates and causes cracking. In general, film-like ferrite is likely to be generated at a low cooling rate, but this tendency is particularly noticeable in steels containing B. Since B increases the hardenability, the B-added steel has a bainite structure relatively easily when the cooling rate is high. However, if the cooling rate is low, BN precipitates at the austenite grain boundary as shown in FIG. 1, and sol.B near the grain boundary decreases and the hardenability decreases, so that ferrite precipitates at the grain boundary. It becomes easy. Therefore, in order to suppress the formation of film-like ferrite, it is necessary to suppress the precipitation of BN and further the precipitation of MnS which is the nucleus of BN.

  Based on the above metallurgical studies, the present inventors have cooled the slab at a cooling rate of 100 ° C./sec or higher until the temperature of the slab is reduced from 1200 ° C. to 1000 ° C. or lower when casting the B-added steel. Thus, it was found that precipitation of MnS and BN can be avoided and cracking of B-added steel can be suppressed without requiring refining costs and processing time for reducing the S concentration. In actual steel production, it is necessary to satisfy this cooling condition by continuous casting. The present inventors pay attention to the fact that the cooling capacity of the secondary cooling is much larger than that of the mold in the continuous casting method, shortening the stay time of the slab in the mold, It has been found that the above cooling condition can be satisfied by spraying a large amount of cooling water on the slab at the upper part of the cooling zone.

  Based on the above findings, according to the present invention, a steel slab containing C: 0.06-0.20% and B: 0.0005-0.003% by mass% is obtained by a curved continuous casting machine or a vertical bending continuous casting machine. When manufacturing, cooling at a position 0.5mm to 2mm deep from the surface of the peripheral corner of the slab is performed at a cooling rate of 100 ° C / sec or more between 1200 ° C and 1000 ° C. , A method for continuous casting of steel is provided.

The thickness of the solidified shell at the corner of the peripheral surface of the slab is 7 to 10 mm at the lower end of the mold of the curved continuous casting machine or the vertical bending continuous casting machine. It may be cooled at a cooling rate of 100 ° C / sec or higher until the temperature at the depth of 0.5mm to 2mm from the surface of the face corner reaches 1000 ° C.
According to the present invention, when a steel slab containing C: 0.06 to 0.20% and B: 0.0005 to 0.003% by mass% is produced by a curved continuous casting machine or a vertical bending continuous casting machine, The casting speed Vc (m / min) is 3.24 × L <Vc <6.61 × L (L is the length below the mold surface of the mold), and the depth from the surface of the peripheral surface corner of the slab. There is provided a continuous casting method of steel, characterized in that the water density of cooling water is 3000 l / min / m ² or more until the temperature at a position of 0.5 mm to 2 mm reaches 1000 ° C.

  According to the present invention, it is possible to produce a cast slab having excellent surface properties without generating grain boundary cracks on the surface of B-added steel.

  Based on the above findings, the present inventors have investigated the relationship between the structure near the slab surface and the occurrence of cracks. As a result, in the slab where cracks occurred, precipitation of MnS and BN was always observed at the austenite grain boundary from the surface to the inside. On the other hand, in the slab where cracks did not occur, precipitation of MnS and BN was observed in the range from the surface to 0.5 mm, but at a deeper position (from 0.5 mm to 2 mm depth), MnS and BN were observed. There was no precipitate of BN. Therefore, even if MnS and BN precipitate at a depth of 0.5 mm from the surface, cracks will not propagate and be harmless if there is no MnS or BN deeper than that.

  Further investigation revealed that the peripheral corners of the slab are particularly prone to cracking, but if MnS and BN are not precipitated at a depth of 0.5 mm to 2 mm from the surface of the peripheral corners, It has been found that surface cracking of the entire piece can be avoided.

  When performing continuous casting with a curved continuous casting machine or a vertical bending continuous casting machine, the solidified shell thickness at the corner of the peripheral surface of the slab is controlled to be 7 to 10 mm at the lower end of the mold. When heat transfer solidification calculation is performed in a continuous casting machine, if the shell thickness at the lower end of the mold exceeds 10 mm, the temperature at a depth of 0.5 mm from the surface of the slab falls to 1200 ° C or less. Since the cooling rate is lower than 100 ° C / sec in the mold, if the shell thickness at the lower end of the mold exceeds 10 mm, MnS precipitates at a depth of 0.5 mm or more from the surface of the slab. Coarse MnS is formed at the grain boundaries, and BN is precipitated with MnS as nuclei. For this reason, if the shell thickness of the peripheral corner portion of the slab at the lower end of the mold exceeds 10 mm, surface cracks will occur at the bending point or correction point of the continuous casting machine.

  If the shell thickness at the corner of the slab at the lower end of the mold is 10 mm or less, the temperature will not drop to 1200 ° C at a depth of 0.5 mm or more from the surface of the corner of the slab in the mold. As a result, coarse MnS precipitation is 0.5 mm deep from the surface, and surface cracks do not propagate. In terms of operation, it is effective to increase the casting speed in order to achieve such a shell thickness. However, if the shell thickness at the lower end of the mold is smaller than 7 mm, the rigidity of the shell becomes insufficient and the risk of breakout increases. For this reason, the thickness of the solidified shell at the peripheral corner portion of the slab is set to 7 to 10 mm at the lower end of the mold of the continuous casting machine. By setting the casting speed Vc (m / min) to 3.24 × L <Vc <6.61 × L (L is the length below the mold surface of the mold), at the lower end of the mold of the continuous casting machine, The thickness of the solidified shell at the peripheral corner of the slab can be 7 to 10 mm.

Although the shell thickness at the lower end of the mold of the continuous casting machine should be determined strictly by heat transfer solidification calculation as described above, it may be determined simply by the following equation (Equation 1).
Mold bottom shell thickness (mm) = 18√ Elapsed time at the mold bottom (min) = 18√ Mold length below the meniscus (m) / Casting speed Vc (m / min) (Formula 1)

In this way, the slab that was brought into the secondary cooling zone without depositing MnS at a depth of 0.5 mm or more from the surface of the peripheral corner kept at a high temperature in the mold was transferred to the upper part of the secondary cooling zone. Cool quickly. This prevents precipitation of MnS and BN, and avoids embrittlement of grain boundaries. The cooling rate in the secondary cooling zone required for this is 100 ° C / sec. 100 ° C / sec until the temperature reaches 1000 ° C at a depth of 0.5-2mm from the surface of the peripheral corner of the slab. Cool at the above cooling rate. In order to achieve such rapid cooling in the upper part of the secondary cooling zone directly under the mold, the water density should be about 3000 to 4000 l / min / m ² . In addition, since the peripheral corners of the slab are particularly susceptible to cracking, care should be taken to ensure that the peripheral corners are covered with cooling water.

  In JP-A-9-225607, although it is not a B-added steel, as a casting method of steel containing Nb, V, Ni, and Cu, in order to facilitate subsequent reheating, the bottom of the mold is 10 to 15 mm. It is said to be shell thickness. In the present invention, in order to more positively control the precipitates, the shell thickness is further reduced to 7 to 10 mm to increase the temperature of the slab at the lower end of the mold. As a result, the precipitate generation area in the mold can be reduced to 0.5 mm from the surface, preventing the propagation of large cracks that would result in product defects. Thus, the conditions in the mold in the present invention are intended to control the precipitates, and are essentially different from that disclosed in JP-A-9-225607 is intended for recuperation in the secondary cooling zone.

  In the present invention, the steel component range is mass%, and C: 0.06 to 0.20%, because high temperature embrittlement of the slab is likely to occur within this C concentration range. This is because the austenite grain size of the slab increases in the vicinity of the peritectic composition. In the other C range, cracking of the slab does not occur even if measures such as the present invention are not taken.

  Moreover, although B: 0.0005 to 0.003% in mass%, if less than 0.0005% (5 ppm), BN does not easily precipitate, so cracking of the slab does not occur. Also, for the purpose of improving the material, there is no need to add B in excess of 0.003% (30 ppm), so it was set to 0.0005 to 0.003%.

  As in the present invention, it is possible to easily reduce the shell thickness at the lower end of the mold and to strongly cool the secondary cooling even under high speed casting. Therefore, unlike the slow cooling described in JP-A-56-80367, for example, there is almost no adverse effect on productivity.

Next, examples of the present invention will be described.
Molten steels of components 1 and 2 (mass%) shown in Table 1 were melted in a converter and secondary refining equipment, and cast with a curved billet continuous casting machine. The cross-sectional size of the slab was 130 mm square. A nozzle capable of rapid cooling is placed directly under the mold, enabling cooling to a water density of 4000 l / min / m ² . Casting was performed by changing the casting speed and the secondary cooling condition for each strand with five strands. Table 2 shows the composition of each slab of Examples 1 to 5 within the scope of the present invention and Comparative Examples 1 to 5 outside the scope of the present invention, casting speed, mold bottom shell thickness, secondary cooling just below the mold. The cooling rate in the strip, the slab cracking index, and the baldness index in the wire were shown. The casting speed was 2.0 to 5.7 m / min. The slab cracking index was measured as follows. That is, the slab was pickled after the diagonal line including the peripheral corner of the slab was cut in the casting direction. The number of fine cracks present at the corners of the slab was carefully measured, and the number of cracks per slab unit length (m) was taken as an index. In addition, the hail index for a wire indicates the rejection rate (%) of the wire after rolling.

  FIG. 4 shows temperature histories (calculation results of two-dimensional heat transfer solidification calculation) at a depth of 0.5 mm from the surface of the slab peripheral surface and 2.0 mm from the surface in Example 1. The temperature at this position at the lower end of the mold was 1200 ° C. or more, and the shell thickness at this time was 9 mm (the shell thickness was obtained from (Formula 1) from the casting speed, the distance between the mold meniscus and the lower end of the mold). In the secondary cooling zone, the cooling rate was 160 ° C./sec at a stroke by controlling the amount of water from the nozzle arranged directly under the mold. In Comparative Example 1, the cooling conditions in the mold were the same as in Example 1, but the amount of secondary cooling water was reduced and cooling was performed at 70 ° C./sec.

  A 500 mm long sample cut out from the slab after casting was pickled and examined for the occurrence of cracks at the corners of the peripheral surface of the slab. As a result, as shown in Table 2, there was no crack in Example 1 of the present invention. On the other hand, in Comparative Example 1, there were many very fine and sharp cracks having a depth of about 2 mm.

  Further, the structure of the corner portion of the slab was observed with an optical microscope (corrosion was nital). At the corner portion of the slab of Comparative Example 1, film-like grain boundary ferrite as shown in FIG. 2 was observed, and cracks were generated along a part thereof. On the other hand, no film-like ferrite was produced in the slab of Example 1. In order to observe the precipitate, a sample for TEM was prepared by the extraction replica method. The slab of Comparative Example 1 was taken from the tip of the crack, but MnS of about 0.2 μm as shown in FIG. Many of these precipitates existed in a line along the prior austenite grain boundary extending from the crack tip. On the other hand, in the slab of Example 1, such precipitation of MnS and BN was slightly observed from the surface to the position of 0.5 mm, but no precipitate was present from 0.5 mm to the inner 2.0 mm. This is because the outermost layer of the slab (from the surface to a depth of 0.5 mm) is slightly affected by the deposition temperature of MnS in the mold, but MnS was deposited inside, but below that, the MnS deposition temperature was below the MnS deposition temperature. Therefore, it is interpreted that the precipitation of MnS was suppressed by the rapid cooling of the secondary cooling.

  Next, the billet slabs of Example 1 and Comparative Example 1 were hot-rolled and processed into 6 mmφ wires. As a result, the surface of the wire obtained from the slab of Example 1 was completely free from defects. On the other hand, in the wire material of Comparative Example 1, generation of wrinkle-like or double skin-like wrinkles was observed.

  In Comparative Example 2, the casting speed was reduced and the shell thickness at the lower end of the mold was 11 mm. In the secondary cooling, strong cooling was performed at 160 ° C./sec. Also in Comparative Example 2, when the crack of the slab was observed by the above-described method, there was a crack. In Comparative Example 2, the residence time in the mold is long, so the temperature at the lower end of the mold is low, and coarse MnS precipitates in a row at a small cooling rate to a considerable depth. Despite the strong cooling, the grain boundary embrittlement could not be prevented.

  As shown in Table 2, cracks on the surface of the slab could also be prevented in the other examples, whereas in the comparative example, cracks occurred or operation was not possible due to breakout. In Comparative Example 3, the residence time in the mold was long and coarse MnS was precipitated, and the cooling rate was small even in the secondary cooling, and the precipitation of MnS and BN was promoted. In Comparative Example 4, as in Comparative Example 1, cooling in the mold was appropriate, but MnS and BN were precipitated and cracked due to insufficient cooling in the secondary cooling. In Comparative Example 5, the casting speed was too high and breakout occurred.

  Although the embodiment of the present invention is for billet continuous casting, the present knowledge can be similarly applied to slab continuous casting. In particular, in the thin slab continuous casting method with a high casting speed, it is easy to reduce the shell thickness at the lower end of the mold to 7-10 mm, which is effective in preventing cracking of B-added steel.

It is explanatory drawing of precipitation of MnS and BN which generate | occur | produce in an austenite grain boundary when carrying out continuous casting of B addition steel. It is a microscope picture which shows the film-form ferrite which precipitated on the slab. It is a graph which shows the relationship (thermodynamic calculation) with the temperature and the precipitation amount of MnS and BN in steel of Mn = 0.55%, S = 0.01%, B = 12ppm, N = 25ppm. 3 is a graph showing a temperature history at a depth of 0.5 mm and 2 mm from a slab surface in Example 1. FIG.

Claims (3)

When manufacturing steel slabs containing C: 0.06-0.20% and B: 0.0005-0.003% by mass in a curved continuous casting machine or a vertical bending continuous casting machine,
Continuous cooling of steel, characterized in that cooling at a depth of 0.5 mm to 2 mm from the surface of the peripheral corner of the slab is performed at a cooling rate of at least 100 ° C / sec between 1200 ° C and 1000 ° C. Casting method. The thickness of the solidified shell at the corner of the peripheral surface of the slab is 7 to 10 mm at the lower end of the mold of the curved continuous casting machine or the vertical bending continuous casting machine.
In the secondary cooling zone directly under the mold, cooling is performed at a cooling rate of 100 ° C / sec or more until the temperature at the depth of 0.5mm to 2mm from the surface of the peripheral surface of the slab reaches 1000 ° C. The continuous casting method of steel according to claim 1. When manufacturing steel slabs containing C: 0.06-0.20% and B: 0.0005-0.003% by mass in a curved continuous casting machine or a vertical bending continuous casting machine,
The casting speed Vc (m / min) is 3.24 × L <Vc <6.61 × L (L is the length below the mold surface of the mold), and the depth from the surface of the peripheral surface corner of the slab. until the temperature at the location of 0.5mm~2mm is 1000 ° C., and wherein the making the water density of the cooling water 3000l / min / m ² or more, a continuous casting method of steel.

Images