JP3018888B2 - Continuous casting method for stainless steel pipe material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of continuously casting a stainless steel pipe material having no center porosity and no slab center segregation, which causes defects in the inner surface of the pipe during pipe production.

[0002]

2. Description of the Related Art In continuous casting of a material for a stainless steel pipe, a reduction in yield due to a defect in the inner surface of the pipe at the time of pipe production becomes a problem. It is known that the defects at the inner surface of the pipe during pipe production are mainly caused by deterioration of hot workability due to ferrite present at the center segregation part generated in the continuous cast slab of stainless steel, center porosity defect of the slab. I have. It is known that electromagnetic stirring is effective in addition to low-temperature casting in order to reduce internal defects such as center segregation, center porosity, internal cracks and inclusions. It is known that the center porosity is effective to reduce the roll in the final stage of solidification at the center of the slab. However, the effect of electromagnetic stirring is not sufficient to reduce center segregation and center porosity, and the rolling condition of stainless steel greatly affects the roll reduction.

[0003] As a method of preventing center segregation and center porosity in the above continuous casting, as described above, after the slab is completely solidified, while the temperature at the center of the slab thickness is 1000 to 1400 ° C, the above-mentioned roll is pressed by a rolling roll. The slab is 0.3 to 1.0 mm per pair of rolls and 0.2 to 6.
A method of reducing the pressure by 0% (JP-A-53-57131);
A method of positively reducing the slab so as to gradually reduce the thickness of the slab as it goes downstream from the mold (Japanese Patent Application Laid-Open No.
JP-A-102225), the ratio of the amount of reduction / unsolidified thickness is set to 0.5 to 1.0, the solid phase ratio at the center of the slab at the time of reduction is set to 0.5 to 0.8, and the strain at the time of reduction is set. (Japanese Patent Laid-Open Publication No. 63-10855) has been proposed.

[0004]

Problems to be Solved by the Invention
The method disclosed in Japanese Patent Publication No. 131 requires a large reduction force to compress the center porosity after completion of solidification,
It is difficult to crimp the center porosity with normal rolling force. In addition, the large rolling force required to press the center porosity inevitably tends to induce internal cracks. Also,
In the method disclosed in Japanese Patent Application Laid-Open No. 53-102225, there is no description as to which range is appropriate for a reduction position and a reduction amount for pressing the center porosity. Further, the method disclosed in Japanese Patent Application Laid-Open No.
Although the amount of rolling and the timing of rolling are specified, in the case of continuous casting of stainless steel, the amount of rolling and the time of rolling are not enough.
Reduction of center segregation and center porosity in continuous casting is not sufficient.

An object of the present invention is to provide a method for continuously casting stainless steel slabs capable of reducing center segregation and center porosity for manufacturing a stainless steel seamless steel pipe having excellent inner surface quality.

[0006]

Means for Solving the Problems The present inventors have intensively studied and studied to achieve the above object. As a result, in continuous casting of stainless steel, by defining the rolling conditions according to the chemical composition of the steel, the center segregation and center porosity of the slab can be reduced, and a seamless stainless steel pipe with excellent inner surface quality can be manufactured. Investigated and arrived at this invention.

That is, according to the present invention, C: 0.10% or less, Si: 1.10% or less.
00% or less, Mn: 2.00% or less, P: 0.05% or less, S: 0.010% or less,
Cr: 15.0-20.0%, Ni: 6.0-13.0%, Mo: 3.00% or less, Al: 0.
From 001 to 0.100% N: comprises from 0.001 to 0.15%, further Ca: 0.05%
Hereinafter, B: 0.005% or less, Ti: 0.500% or less, Nb: upon continuous casting molten steel containing at least one or more of 1.00% or less, when the Ni equivalent which issued calculated by the following equation is 1.5 to 2.0 Is the center of the slab just before the reduction at the reduction position during continuous casting
0.5 the ratio of the slab rolling reduction against unsolidified thickness or less, 1.0 in the case of Ni equivalent is 1.0 to 1.5, if the Ni equivalent is less than 1.0
A continuous casting method for a material for a stainless steel pipe, wherein the material is set to 0.5 or more and 1.0 or less. Ni equivalent = 30 × (C + N) + 0.5 × Mn + Ni + 8.2-1.1 × (1.5 × Si + Cr + M
o + 0.5 × Nb) where C, N, Mn, Ni, Si, Cr, Mo and Nb are each
Element content (%), Ti ≧ 0.05% or Nb ≧ 0.100%
In this case, the item of N is omitted.

[0008]

First, the reasons for limiting the chemical components of stainless steel in the present invention will be described in detail. C is an element necessary for imparting strength to steel, but if it exceeds 0.10%, the acid and corrosion resistance deteriorates. Therefore, C is set to 0.10% or less. Si is an element effective for deoxidizing steel, but if it exceeds 1.00%, hot workability is deteriorated due to an increase in ferrite.
1.00% or less. Mn is a γ-forming element and is added as a substitute for Ni. However, if it exceeds 2.00%, steelmaking problems such as a decrease in yield occur, so Mn is set to 2.00% or less.
Since both P and S deteriorate corrosion resistance and hot workability, the upper limits are set to 0.050% and 0.010%, respectively. Cr
Is added in order to improve the corrosion resistance. However, if it is less than 15.0%, its effect is not sufficient, and if it exceeds 20.0%, σ
Since cracks due to phase precipitation are concerned, 15.0 to 20.
0%. Ni is added to improve the corrosion resistance, but if it is less than 6.0%, the effect is not sufficient, and 13.0% is used.
%, The deformation resistance during processing will increase too much.
It was set to 6.0 to 13.0%.

[0009] Al is used for deoxidation of steel in the refining process.
001% or more is necessary, but if it exceeds 0.100%, A
Since the formation of 1N adversely affects the corrosion resistance and hot workability, the content is set to 0.001 to 0.100%. N improves the hot workability as a γ-forming element. However, if it is less than 0.001%, its effect is not sufficient, and if it exceeds 0.150%, the hot workability is reduced due to an increase in deformation resistance. .001
-0.150%. Nb, Ti, and Mo may be added to maintain heat resistance, but the upper limits are 1.00%, 0.500%, and 3.00, respectively, due to manufacturing cost restrictions.
%. Ca is effective in controlling the sulfide morphology,
If the content exceeds 01%, the oxide-based inclusions become coarse.
It was set to 0.01% or less. B is an element effective for improving hot workability, but if it exceeds 0.005%, the corrosion resistance deteriorates.

In the present invention, Ni equivalent = 30 × (C + N) + 0.5 × Mn + Ni + 8.2-1.1 × (1.5 × Si + Cr + M
o + 0.5 × Nb) where C, N, Mn, Ni, Si, Cr, Mo, and Nb are
Element content (%). When Ti ≧ 0.05% or Nb ≧ 0.100%, the item of N is omitted . In case the calculated Ni equivalent is 1.5 to 2.0, the 0.5 ratio of the slab rolling reduction against pressure just before the slab center unsolidified thickness at pressing position during continuous casting or less, Ni
By setting the equivalent to 1.0 to 1.5 and 1.0 or less, and when the Ni equivalent is less than 1.0 to 0.5 or more and 1.0 or less, it is possible to prevent the occurrence of slab center cracking in the high Ni equivalent range and to reduce the low Ni equivalent. The segregation of the center of ferrite in the amount range is suppressed, and the deterioration of hot workability is suppressed. As a result, when the continuously cast stainless steel slab is used as a material for a seamless steel pipe, the occurrence of inner surface defects is prevented, and a reduction in product yield can be prevented.

[0011]

EXAMPLES Example 1 Stainless steels of steel Nos. 1 to 14 having the chemical components shown in Table 1 were melted in a converter, and the casting speed was 0.5 m / s using a 410 mm × 530 mm mold.
After continuous casting while reducing the area near the unsolidified end with ec to make a slab, it was slab-rolled to a diameter of 225 mm to form a round slab for producing a seamless steel pipe, and the round slab was subjected to 1230 in a rotary heating furnace. After heating to ℃, perforation using a Mannes mandrel mill,
Elongation rolling and diameter reduction were performed to produce a stainless steel seamless steel pipe having an outer diameter of 273 mm. Each seamless steel pipe obtained was inspected for inner surface quality. The result is shown in FIG. Also, a stainless steel seamless steel pipe was manufactured under the same conditions as above except that the roll reduction was increased in the continuous casting step. Each seamless steel pipe obtained was inspected for inner surface quality. The result is shown in FIG. Also, the Ni equivalent weight and the center of the slab just before the reduction at the reduction position during continuous casting
The relationship between the range without inner surface flaws ratio and seamless steel pipes of the slab rolling reduction against unsolidified thickness shown in FIG. The roll reduction during continuous casting was carried out by adjusting the solid phase ratio and the reduction amount at the center of the slab as disclosed in Japanese Patent Application Laid-Open No. 63-108955.
Further, R / L in FIG. 1, FIG. 2, pressure in the pressing position immediately before
2 shows the ratio of the reduction amount R to the unsolidified thickness L of the slab.

[0012]

[Table 1]

As shown in FIG. 1, when the Ni equivalent is low,
Hot workability deteriorates due to ferrite generation, and flaws occur on the inner surface of the raw tube during piercing and rolling. When the Ni equivalent is high, internal cracks occur in the center at the casting stage, and inner surface defects of the stainless steel seamless steel pipe occur. This is because the hot workability in the solid-liquid coexistence region around the solidus temperature is deteriorated.
However, when the roll reduction is increased during continuous casting, the inner surface quality is improved as shown in FIG. As a result, in the region where the Ni equivalent was low, the center segregation was improved by the roll reduction and the inner surface quality of the pipe was improved.
However, in the region where the Ni equivalent is high, as shown in FIG. 2, the quality of the inner surface of the pipe tends to deteriorate. This is presumably because the roll reduction reduced the cracks in the center of the slab. Also, as shown in FIG. 3, when the Ni equivalent is 1.5 to 2.0, the rolling position during continuous casting is reduced.
0.5 or less the ratio of the slab rolling reduction against pressure just before the slab center unsolidified thickness in the case Ni equivalent is 1.0 to 1.5, 1.0 to the ratio of the slab rolling reduction against slab center unsolidified thickness or less, Ni
If equivalent is less than 1.0, by 0.5 to 1.0 the slab rolling reduction ratio billet center unsolidified thickness,
It is possible to greatly suppress the occurrence of inner surface defects of the stainless steel seamless steel pipe.

[0014]

As described above, according to the method of the present invention, the occurrence of center segregation and center porosity which cause internal surface defects during continuous casting of a material for manufacturing a stainless steel seamless steel pipe can be greatly reduced. The occurrence rate of inner surface flaws of a stainless steel seamless steel pipe can be significantly reduced.

[Brief description of the drawings]

FIG. 1 shows an example in which Ni is reduced when the roll reduction amount is small.
Is a graph showing the relationship between the incidence of inner surface flaws of equivalents and steel pipes.

FIG. 2 shows a case where a roll reduction amount in an embodiment is large.
Is a graph showing the relationship between the incidence of inner surface flaws of equivalents and steel pipes.

[3] the inner surface of the ratio and seamless steel pipes of the slab rolling reduction against pressure just before the slab center Not coagulated <br/> solid thickness at Ni pressing position of equivalents and during continuous casting in the case of roll pressure in the embodiment It is a graph which shows the relationship with the range without a flaw.

──────────────────────────────────────────────────続 き Continued on front page (51) Int.Cl. ⁷ Identification code FI C22C 38/58 C22C 38/58

Claims (1)

(57) [Claims] [Claim 1] C: 0.10% or less, Si: 1.00% or less, Mn: 2.00%
Below, P: 0.05% or less, S: 0.010% or less, Cr: 15.0 to 20.0%, N
i: 6.0-13.0%, Mo: 3.00% or less, Al: 0.001-0.100%, N: 0.
It comprises 001 to 0.15%, further Ca: 0.05% or less, B: 0.005% or less, Ti: 0.500% or less, Nb: 1.00% at least 1 of the following
When continuously casting molten steel containing more than one kind ,
If Ni equivalent issued Sun of 1.5 to 2.0, reduction of the time of continuous casting
0.5 the ratio of the slab rolling reduction against pressure just before the slab center unsolidified thickness at position below, if Ni equivalent is 1.0 to 1.5 1.0
Hereinafter, a method for continuously casting a material for a stainless steel pipe, wherein the Ni equivalent is less than 1.0 and 0.5 or more and 1.0 or less. Ni equivalent = 30 × (C + N) + 0.5 × Mn + Ni + 8.2-1.1 × (1.5 × Si + Cr + M
o + 0.5 × Nb) where C, N, Mn, Ni, Si, Cr, Mo and Nb are each
Element content (%), Ti ≧ 0.05% or Nb ≧ 0.100%
In case N, omit