JP2013173159A - CONTINUOUS CASTING METHOD OF HIGH-C HIGH-Mn NONMAGNETIC STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method by which the occurrence of crack of a cast metal of high-C high-Mn nonmagnetic steel can be suppressed, in particular, the occurrence of crack in a bloom can be made nil, and a maintenance-free cast slab can be produced.SOLUTION: There is provided a continuous casting method of high-C high-Mn nonmagnetic steel having a composition containing, by mass, C:0.85-1.0%, Si:0.10-0.50%, Mn:13.0-16.0%, P:≤0.050%, N:0.003-0.05%, and V:0.01-0.50%, and the balance comprising Fe with impurities. In the method, contents of P and C satisfy inequality [P]≤-0.1241×[C]+0.1557, molten steel in a mold is stirred by an electromagnetic stirring device, a specific flow rate of secondary cooling water is set to be 0.20-0.50 L/kg-steel, and a casting rate is set to be 0.60-0.90 m/min. Surface temperature of a cast metal when straightening the bend of the cast metal in a continuous casting machine, is preferably ≥850°C. The cast metal is preferably a bloom with an aspect ratio of its cross section being 1 to 1.7.

Description

  The present invention relates to a continuous casting method for high-C, high-Mn nonmagnetic steel, and more particularly, to a method capable of suppressing the occurrence of cracks on the surface and inside of a slab without greatly reducing the P content.

  High Mn steel containing 11.0 to 16.0% by mass of Mn is used as nonmagnetic steel. This high Mn non-magnetic steel has a thermal expansion coefficient about 1.5 times that of ordinary steel and becomes brittle at 800 ° C. or lower, resulting in reduced hot workability. Has the disadvantage of frequent cracking. Furthermore, when this crack is cared for and removed, a turtle shell-shaped crack may occur due to the influence of carbides precipitated at the grain boundaries.

  As a method for suppressing the occurrence of cracks in such high-Mn steel, in Patent Document 1, the slab obtained by continuously casting the high-Mn steel is then heated immediately and soaked in the range of 1180-1230 ° C. Then, after pre-rolling, if necessary, heat treatment is performed, and then a high-Mn steel manufacturing method in which a water toughness treatment is performed. In this case, the P content is set to 0.030% or less, and the secondary cooling ratio during continuous casting. A method is disclosed in which the amount of water is in the range of 0.7 to 1.1 L / kg-steel, and the heating rate in the temperature raising process of the slab is 30 to 35 ° C./h.

  In Patent Document 1, as the reason for setting the secondary cooling specific water amount during continuous casting in the above range, if the secondary cooling specific water amount is less than 0.7 L / kg-steel, an internal crack occurs due to bulging stress. If it exceeds 1 L / kg-steel, it is mentioned that surface cracks and subepidermal cracks due to thermal stress occur.

  However, in the technique disclosed in Patent Document 1, since the secondary cooling specific water amount is as large as 0.7 to 1.1 L / kg-steel, casting is performed at a low speed as in the case where the slab to be continuously cast is bloom. When performed, there is a problem that cracks occur in the slab due to thermal stress during cooling in the continuous casting machine. In addition, the literature does not describe a case where the slab was completely cracked. Therefore, it is considered that the slab manufactured by the technique disclosed in the literature requires some care. .

  Moreover, in the technique disclosed in Patent Document 1, the P content must be reduced to 0.030% or less, and a great amount of time and cost are required for refining.

Japanese Examined Patent Publication No. 63-39336

  The present invention has been made in view of the above problems, and can suppress the occurrence of cracks on the surface and inside of a slab of high Mn steel, and in particular, can eliminate the occurrence of cracks in bloom. An object of the present invention is to provide a continuous casting method capable of producing a slab that does not require maintenance.

  In order to achieve the above object, the present inventors conducted a continuous casting test using high-C, high-Mn nonmagnetic steels having different C contents, and examined casting conditions.

  As a result, the cause of the occurrence of grain boundary cracking, thermal stress cracking, etc. occurring in the cast slab of high Mn non-magnetic steel, in addition to the large thermal expansion coefficient, embrittlement due to the inclusion of C and P, and It was found that there was a thermal stress generated by secondary cooling. Furthermore, it has been found that if the C content and the P content are in a predetermined relationship, cracks do not occur on the surface and inside of the slab even if the P content is not reduced to 0.030% by mass or less. Details of this examination will be described later. Here, “high C high Mn nonmagnetic steel” refers to high Mn steel having a C content of 0.85 mass% or more.

  In addition, when using a continuous casting machine having a correction band for correcting the bending of the slab, such as a vertical bending type continuous casting machine, the surface temperature of the slab when passing through the correction band is 850 ° C. or more. If it exists, it discovered that a crack did not generate | occur | produce on the surface of a slab.

  This invention is made | formed based on these knowledge, The summary exists in the continuous casting method of the high C high Mn nonmagnetic steel of the following (1)-(3).

(1) By mass%, C: 0.85 to 1.0%, Si: 0.10 to 0.50%, Mn: 13.0 to 16.0%, P: 0.050% or less, N: A continuous casting method of a high C high Mn nonmagnetic steel containing 0.003 to 0.05% and V: 0.01 to 0.50% with the balance being Fe and impurities, and containing P and C The rate satisfies the following formula (1), the molten steel in the mold is stirred with an electromagnetic stirrer, the secondary cooling specific water amount is 0.20 to 0.50 L / kg-steel, and the casting speed is 0.60 to 0.00. A continuous casting method for high-C, high-Mn nonmagnetic steel, characterized by being 90 m / min.
[P] ≦ −0.1241 × [C] +0.1557 (1)
Here, [P] and [C] are the contents (mass%) of P and C in the molten steel, respectively.

(2) The continuous casting method for high-C, high-Mn nonmagnetic steel according to (1), wherein the surface temperature of the slab when correcting the bending of the slab in a continuous casting machine is 850 ° C. or higher. .

(3) The continuous casting method for high-C, high-Mn nonmagnetic steel according to (1) or (2), wherein the slab is a bloom having a cross-sectional aspect ratio of 1 to 1.7. .

  In the following description, “mass%” for the component composition of steel is simply expressed as “%”.

  According to the continuous casting method of the high C, high Mn nonmagnetic steel of the present invention, the occurrence of cracks on the surface and inside of the slab can be suppressed, and in particular, the occurrence of cracks in bloom can be eliminated. Further, the P content may not be reduced to 0.030% or less. Since the slab obtained by the method of the present invention is excellent in surface quality and internal quality, maintenance is unnecessary.

It is a figure which shows the relationship between C content rate and P content rate of a slab, and the presence or absence of generation | occurrence | production of a crack.

1. The range of the component composition of steel The range of the component composition of the high C high Mn nonmagnetic steel which this invention makes object, and the reason limited to this range are demonstrated.

C: 0.85-1.0%
The reason why the C content is set to 0.85% or more is that if the C content is less than 0.85%, the strength required for steel use cannot be obtained. Further, the reason why it is set to 1.0% or less is that if it exceeds 1.0%, embrittlement of steel due to precipitation of carbides becomes remarkable and hot workability is remarkably lowered.

Si: 0.10 to 0.50%
The reason why the Si content is 0.10 to 0.50% is to prevent a decrease in hot workability. The reason why the content is set to 0.50% or less is that if it exceeds 0.50%, surface cracking occurs and the surface quality deteriorates.

Mn: 13.0 to 16.0%
The reason why the Mn content is 13.0 to 16.0% is to satisfy the magnetic permeability required for nonmagnetic steel.

P: 0.050% or less When the P content is 0.050% or less, the hot workability is remarkably lowered when the P content is greater than 0.050% compared to 0.050% or less. Because. The P content is preferably greater than 0.030% and less than or equal to 0.050%. This is because when the P content of steel is greater than 0.030% and 0.050% or less, an Mn raw material with a high P content can be used, and the cost can be reduced.

N: 0.003-0.05%, V: 0.01-0.50%
The reason why the content ratios of N and V are in the above range is to satisfy the mechanical properties as a reinforcing bar.

  The balance other than the above components is Fe and impurities.

2. Relationship between P content and C content In the continuous casting method for high C and high Mn nonmagnetic steel of the present invention, the relationship between the P content and the C content in the steel is defined by the following equation (1).
[P] ≦ −0.1241 × [C] +0.1557 (1)
Here, [P] and [C] are the contents (mass%) of P and C in the molten steel, respectively.

  This is based on the knowledge obtained from the result of an example described later (see FIG. 1) that the upper limit of the P content at which slab cracking occurs depends on the C content in steel.

3. Electromagnetic stirring In the continuous casting method of the present invention, molten steel in a mold is stirred using an electromagnetic stirring device. This is for forming equiaxed crystals inside the slab and preventing the occurrence of internal cracks. Further, the thickness of the solidified shell can be made uniform by electromagnetic stirring.

4. Secondary cooling specific water amount The secondary cooling specific water amount is a value defined by the following equation (2).
R = m / (t × w × ρ × Vc) (2)
Here, R: secondary cooling specific water amount (L / kg-steel), m: secondary cooling water flow rate (L / min), t: slab thickness (m), w: slab spray width ( m), ρ: density of steel (kg / m ³ ), Vc: casting speed (m / min).

  In the continuous casting method of the present invention, the secondary cooling specific water amount is 0.20 to 0.50 L / kg-steel. This is to prevent occurrence of cracks in the slab. When the secondary cooling specific water amount is less than 0.20 L / kg-steel, bulging occurs in the slab between adjacent support rolls, and the high C high Mn nonmagnetic steel targeted by the present invention due to this bulging. Internal cracks occur in the slab. On the other hand, if the secondary cooling specific water amount is larger than 0.50 L / kg-steel, surface cracking due to thermal stress occurs.

5. Casting speed In the continuous casting method of the present invention, the casting speed is set to 0.60 to 0.90 m / min. This aims at reducing the influence of the thermal expansion coefficient and reducing bulging.

  If the casting speed is less than 0.60 m / min, the thermal expansion coefficient of the high-C, high-Mn nonmagnetic steel targeted by the present invention is too large, and the slab is cracked. In this case, since the correction temperature of the slab described later is 850 ° C. or higher, surface cracks occur when correcting the bending of the slab. The term “fire cracking” refers to a phenomenon in which steel with uneven secondary cooling cracks due to distortion caused by volume expansion.

  If the casting speed is higher than 0.90 m / min, internal cracks occur in the slab due to the bulging described above.

6). Correction temperature In the continuous casting method of the present invention, when a continuous casting apparatus having a correction band for correcting the bending of the slab is used, the surface temperature of the slab when passing through the correction band (hereinafter referred to as “correction temperature”). ) At 850 ° C. or higher. As a result of the investigation by the present inventors, when the correction temperature is 850 ° C. or more, the slab is formed by surface cracking and bulging due to thermal stress generated during the correction of the slab of high C high Mn nonmagnetic steel targeted by the present invention. It has been found that the internal cracking of is significantly less than when it is less than 850 ° C.

7). Preferred slab for the continuous casting method of the present invention The slab applied to the continuous casting method of the present invention has an aspect ratio of the cross section (the value obtained by dividing the length of the long side of the cross section by the length of the short side). Blooms of ~ 1.7 are preferred. Blooms are less susceptible to bulging than slabs with a cross-sectional aspect ratio greater than 1.7. Therefore, by applying the continuous casting method of the present invention to bloom casting, it is possible to eliminate the occurrence of cracks on the surface and inside.

  In addition, since the required amount of secondary cooling ratio water is small compared to a slab having a cross-sectional aspect ratio larger than 1.7, the bloom can also save secondary cooling water.

  In order to complete the continuous casting method of the high C high Mn nonmagnetic steel of the present invention, the following tests were conducted and the results were evaluated.

1. Test conditions A vertical bending type continuous casting machine was used to produce a bloom (a cross-sectional aspect ratio of 1.33) having a width of 400 mm and a thickness of 300 mm as a cast piece. During casting, the molten steel in the mold was stirred at a flow rate of 25 to 40 cm / s by electromagnetic stirring. The molten steel used for casting was a high C high Mn nonmagnetic steel having the composition shown in Table 1. These steels are high Mn steels that can use inexpensive Mn alloys and have a P content of 0.055% or less. The table also shows the maximum value of the P content defined by the above formula (1), the secondary cooling specific water amount, the casting speed, and the slab correction temperature.

  In each of Invention Examples 1 to 7 shown in Table 1, the composition of the molten steel, the secondary cooling specific water amount, and the casting speed satisfied the provisions of the present invention. In Comparative Example 1, the secondary cooling specific water amount was larger than the regulation of the present invention. In Comparative Example 2, the P content was a large value exceeding the specified range of the present invention. In Comparative Example 3, the C content was a large value exceeding the specified range of the present invention. In Comparative Example 4, the casting speed and the slab correction temperature were small values exceeding the limits of the present invention. In Comparative Example 5, the P content did not satisfy the above formula (1) and did not satisfy the definition of the present invention.

2. Test result About the slab produced on the said conditions, the occurrence condition of a surface crack and an internal crack was evaluated. The surface crack was evaluated by visual observation and color check of the slab surface. Visual observation was performed on the entire length of the slab, and color check was performed on a portion of the slab having a length of 1 m in the casting direction. Here, the color check refers to a dye penetrant inspection specified in JIS Z 2343-1. Internal cracks were evaluated by color-checking the cross section of the sample taken from the slab and ground.

  The evaluation results are shown in Table 2 together with the production conditions. The slabs of Invention Examples 1 to 7 had a sound surface over the entire length, and neither surface cracks nor internal cracks were observed. Moreover, when this slab was rolled to produce a steel slab, no cracks were observed in this steel slab, and no maintenance was required. As described above, according to the continuous casting method of the present invention, it is possible to produce a stable slab in the vertical bending type continuous casting method, and to obtain a healthy slab as a base material of a hot-rolled material. I can say that.

  On the other hand, in the cast pieces of Comparative Examples 1 to 5, either surface cracks or internal cracks occurred. Among these, the reason why the surface crack occurred in the slab of Comparative Example 3 is considered that the C content was a large value exceeding the specified range of the present invention. The reason why the internal crack occurred in the slab of Comparative Example 4 although the P content satisfied the provisions of the present invention is considered to be because the correction temperature was low. Moreover, it is thought that the reason why the surface crack occurred in the slab of Comparative Example 5 was that the P content was a large value exceeding the upper limit defined by the above formula (1).

FIG. 1 is a diagram showing the relationship between the C content and P content of a slab and the presence or absence of cracks. In the figure, the test results of Examples 1 to 7 of the present invention and Comparative Examples 2 and 3 in which the secondary cooling specific water amount, the casting speed and the correction temperature of the slab satisfy the provisions of the present invention are shown. . From the figure, the upper limit of the P content at which cracking of the slab occurs changes depending on the C content in the high C high Mn nonmagnetic steel, and the upper limit of the P content at which the cracking of the C content and the slab occurs It can be seen that the relationship can be expressed by the following equation (3).
[P] max = −0.1241 × [C] +0.1557 (3)
Here, [P] max is the upper limit (mass%) of the P content at which slab cracking occurs, and [C] is the C content (mass%).

  Further, FIG. 1 shows that when the P content is equal to or less than [P] max defined by the expression (3), cracks do not occur in the slab even if it is greater than 0.030% by mass.

  According to the continuous casting method of the high C high Mn nonmagnetic steel of the present invention, it is possible to suppress the occurrence of cracks on the surface and inside of the slab without reducing the P content to 0.030% by mass or less. In particular, the occurrence of cracks in the bloom can be eliminated. Since the slab obtained by the method of the present invention is excellent in surface quality and internal quality, maintenance is unnecessary.

Claims (3)

In mass%, C: 0.85 to 1.0%, Si: 0.10 to 0.50%, Mn: 13.0 to 16.0%, P: 0.050% or less, N: 0.003 A continuous casting method of high C high Mn non-magnetic steel containing 0.05% and V: 0.01-0.50%, the balance being Fe and impurities,
The contents of P and C satisfy the following formula (1),
Stir the molten steel in the mold with an electromagnetic stirrer,
A continuous casting method of high C high Mn non-magnetic steel, characterized in that the secondary cooling specific water amount is 0.20 to 0.50 L / kg-steel and the casting speed is 0.60 to 0.90 m / min.
[P] ≦ −0.1241 × [C] +0.1557 (1)
Here, [P] and [C] are the contents (mass%) of P and C in the molten steel, respectively.   The continuous casting method for high C, high Mn non-magnetic steel according to claim 1, wherein the surface temperature of the slab when correcting the bending of the slab in a continuous casting machine is 850 ° C or higher.   3. The continuous casting method for high C, high Mn nonmagnetic steel according to claim 1, wherein the slab is a bloom having a cross-sectional aspect ratio of 1 to 1.7. 4.

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