JP2002224798A - Manufacturing method of high chromium ferritic heat- resistant steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ingot manufactured by the continuous casting which has a high quality and does not crack as a material for the hot milling of a high-chromium and ferritic heat-resistant steel. SOLUTION: This is a manufacturing method of a steel containing C: 0.03-0.2%, Si: 0.05-0.7%, Mn: 0.1-1.5%, Cr: 8-14%, W: 0.8-4%, V: 0.1-0.3%, Nb: 0.01-0.2%, N: 0.005-0.2%, Al: 0.002-0.05% in mass %. By the condition that a relative water volume for the secondary cooling of an ingot is made to be 0.6 liter/kg-steel and the secondary cooling is completed before the center part of the thick side of the ingot begins to solidify, the ingot which is not less than 150 mm thick and of which cross-sectional shape is rectangle, is cast continuously, and as long as the surface temperature of the ingot is not less than 400 deg.C after the solidification of the center part of the thick side of the ingot has been completed, the primary rolling of the ingot is made by the condition of the pressure ratio 0.1-0.4, and then, the ingot to which the primary rolling is made is hot-rolled and is made to a steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high chromium ferrite heat-resistant steel material.

[0002]

2. Description of the Related Art Heat-resistant steel materials used in the fields of boilers, nuclear power and chemical industries are required to have excellent high-temperature strength and toughness, corrosion resistance, oxidation resistance, etc., as well as excellent workability and weldability. It is also required to be more economical.

[0003] As a heat-resistant steel material for steel pipes used in such fields, represented by 2 1 / 4Cr-1Mo steel, JI
Low alloy steel specified for alloy steel tubes for boilers and heat exchangers of SG 3462, SUS304HTB, SUS3
21HTB, etc., austenitic heat-resistant steel specified in JIS G 3463 stainless steel tubes for boilers and heat exchangers, and ferritic heat-resistant steel containing about 11% by mass or more of Cr and also specified in JIS G 3463. Is used.

Among these heat-resistant steel materials, heat-resistant ferritic steel materials are superior to low-alloy steels in strength, toughness, corrosion resistance, oxidation resistance, and the like. It is excellent in characteristics, stress corrosion cracking resistance, etc., and is widely used.

With respect to such a ferritic heat-resistant steel,
JP-A-59-140352 discloses a heat-resistant steel material having both heat resistance and low-temperature toughness with a Cr content of 8-12.
A heat-resistant steel material in which Mo, W, V, Nb, and the like are added in combination based on mass% is proposed. Also, JP-A-3-978
No. 32 discloses Mo, W, and Mo as a heat-resistant steel material having excellent oxidation resistance and weldability based on a Cr content of 8 to 14% by mass.
A heat-resistant steel material to which V, Nb, Cu, N, and the like are added in combination has been proposed. Further, JP-A-5-17850 discloses that
As a heat-resistant steel material excellent in oxidation resistance and weldability, based on a Cr content of 8 to 14% by mass, Ni, W, V, Nb, Cu,
A heat-resistant steel material to which N or the like is added in combination has been proposed. JP-A-59-140352 and JP-A-3-978.
The heat-resistant steel materials proposed in JP-A No. 32 and JP-A-5-17850 have excellent properties such as strength, toughness, corrosion resistance, oxidation resistance, weldability, and workability.

[0006] However, as a material for heat-resistant ferritic steel containing about 8 to 14% by mass of Cr, a steel slab obtained by dividing a steel ingot cast by an ingot method is conventionally used. . This is because a continuously cast slab is likely to have cracks on the slab surface that are difficult to remove by maintenance, and it is liable to crack near the center of the thickness called double cracks. Further, when the continuously cast slab is subjected to gas cutting, fine cracks are likely to occur on the cut surface.

In the method of obtaining a billet from a steel ingot by the ingot method, the yield from the steel ingot to the billet is extremely low, and the cost of producing a heat-resistant steel material is high. At present, it is required to establish a method.

[0008]

SUMMARY OF THE INVENTION The present invention provides a high chromium
Regarding a method for producing a heat-resistant ferritic steel material, as a material for hot rolling of the heat-resistant steel material, a method capable of obtaining a good quality slab without cracking by continuous casting, and gas cutting the slab At the time, an object of the present invention is to provide a method free from cracks.

[0009]

The gist of the present invention resides in a method for producing a high chromium / ferrite heat resistant steel as shown in the following (1) to (3). (1) In mass%, C: 0.03 to 0.2%, Si: 0.
05 to 0.7%, Mn: 0.1 to 1.5, Cr: 8 to 1
4%, W: 0.8-4%, V: 0.1-0.3%, N
b: 0.01 to 0.2%, N: 0.005 to 0.2%,
Al: 0.002 to 0.05%, Ni: 1% or less, M
o: 1.2% or less, Cu: 3.5% or less, Co: 4% or less, B: 0.02% or less, Ti: 0.05% or less, T
a: 0.05% or less, Hf: 0.05% or less, Nd:
0.05% or less, La: 0.05% or less, Ce: 0.0
5% or less, Y: 0.05% or less, Ca: 0.01% or less, Mg: 0.01% or less, with the balance being a method for producing a high chromium / ferrite heat resistant steel material comprising Fe and impurities. Then, the specific water amount of the secondary cooling of the slab is 0.1 to 0.1.
6 liters / kg-steel, continuous casting of a slab with a thickness of 150 mm or more and a rectangular cross section under the condition that secondary cooling is completed before solidification of the slab thickness starts. While the surface temperature of the slab after the center is completely solidified is 400 ° C. or higher, the slab is primarily rolled under the conditions of a reduction ratio of 0.1 to 0.4. A method for manufacturing high-chromium ferrite heat-resistant steel that is hot rolled to finish it into steel. (2) until the surface temperature of the slab after the primary rolling reaches 400 ° C., cooling at an average cooling rate of 50 ° C./hour or less;
Thereafter, the primary-rolled slab is hot-rolled to produce a steel material having a total reduction ratio of 0.67 or more from the slab before the primary rolling, thereby producing a high chromium / ferritic heat-resistant steel material according to the above (1). Method. (3) When the slab before or after the primary rolling is cut using a cutting gas, the slab is cut while the surface temperature of the slab near the cut portion is 350 ° C. or higher.
Or the method for producing a high chromium / ferritic heat-resistant steel material according to (2).

In the present invention, the timing at which "the center of the slab thickness starts to solidify" is determined by the chemical composition of the steel to be cast, the slab size, the casting speed, the specific water volume for the secondary cooling of the slab, and the like. For example, it can be calculated by a method based on ordinary solidification heat transfer analysis or the like. For example, the start time can be expressed by the distance from the meniscus of the molten steel in the mold.

The “surface temperature of the slab” defined in the present invention is a surface temperature that can be measured by, for example, a radiation thermometer, and means a temperature from the surface of the slab to just below the skin. The surface temperature of the slab is not always uniform in the width direction. In the process of cooling the slab, the vicinity of the short side of the slab cools faster than the center of the width, and the surface temperature tends to decrease. The surface temperature of the slab in the present invention means the lowest surface temperature in the width direction.

The term "cooling at a cooling rate of 50 ° C./hour or less on average until the surface temperature of the slab after the primary rolling reaches 400 ° C." It means cooling at a cooling rate.

[0013] The "reduction ratio" defined in the present invention is:
The reduction amount, that is, the ratio obtained by subtracting the thickness after rolling from the thickness before rolling and dividing the thickness by the thickness before rolling. Therefore, the "rolling ratio" at the time of primary rolling is the thickness obtained by subtracting the thickness of the slab after primary rolling (the material for hot rolling of steel) from the thickness of the slab as continuously cast, It is the ratio divided by the thickness of the slab as continuously cast. The `` total reduction ratio '' when hot rolling the primary rolled slab is defined as the thickness obtained by subtracting the thickness of the steel material from the thickness of the slab as continuously cast. It is the ratio divided by the thickness of the piece.

Further, the phrase “the surface temperature of the slab near the cut portion is 350 ° C. or more” as defined in the present invention means that, for example, when cutting the slab in the width direction for hot rolling, 50-100m in the length direction which is the casting direction including
It means that the surface temperature of the entire width portion of the slab of about m is 350 ° C. or more. Without lowering the surface temperature of the slab from a high temperature immediately after casting to less than 350 ° C., the slab may be cut, or the slab is once cooled to room temperature, and then the slab is cooled. The slab may be cut after heating so that the surface temperature of the region of the slab becomes 350 ° C. or higher.

[0015] The steel material to which the present invention is applied is, in mass%,
C: 0.03-0.2%, Si: 0.05-0.7%,
Mn: 0.1 to 1.5, Cr: 8 to 14%, W: 0.8
-4%, V: 0.1-0.3%, Nb: 0.01-0.
2%, N: 0.005 to 0.2%, Al: 0.002
0.05%, Ni: 1% or less, Mo: 1.2% or less, C
u: 3.5% or less, Co: 4% or less, B: 0.02% or less, Ti: 0.05% or less, Ta: 0.05% or less, H
f: 0.05% or less, Nd: 0.05% or less, La:
0.05% or less, Ce: 0.05% or less, Y: 0.05
% Or less, Ca: 0.01% or less, Mg: 0.01% or less, with the balance being high chromium.
Ferritic heat-resistant steel. In addition, among the above components,
Ni, Mo, Cu, Co, B, Ti, Ta, Hf, N
The contents of d, La, Ce, Y, Ca and Mg may be at the impurity level.

In such a continuous cast slab of a high chromium / ferritic heat-resistant steel material having a Cr content of 8 to 14% by mass, especially when the slab thickness is small, it is close to the short sides on both sides of the slab. Extremely large vertical cracks or horizontal cracks (hereinafter simply referred to as surface cracks) are likely to occur. 30 mm deep from the surface
Remarkable surface cracks may occur. Also,
These surface cracks tend to increase over time.

[0017] Since the Cr content is high, the crack sensitivity is high, the solidification structure is coarse, and martensitic transformation is involved in the cooling process of the slab. Surface cracks are likely to occur near the short sides on both sides. It is difficult to remove such surface cracks by care, and the slab having the surface cracks has to be debris-treated.

When the surface of the slab is continuously subjected to secondary cooling at the time when the center of the thickness starts to solidify, cracks tend to occur near the center of the thickness of the slab. This is because, when the slab surface is secondarily cooled, the slab surface usually contracts, so that a tensile stress acts on the central portion of the thickness. Since the slab of steel containing a large amount of Cr has high cracking susceptibility, when the surface of the slab is secondarily cooled at the stage of the start of solidification at the center portion of the thickness, the final solidified portion is cracked, and a double crack occurs.

Also, since it contains many alloying elements such as Cr, center segregation is likely to occur at the center of the thickness of the slab. A hot-rolled product steel material (hereinafter simply referred to as “steel material”) using such a slab has a center crack (hereinafter simply referred to as “center crack”) at the center of thickness. This is because the center segregation of the slab remains in the steel material, and a center crack is generated in that portion during rolling to the steel material or after rolling to the steel material.

Further, when a continuously cast slab is subjected to gas cutting, fine cracks are apt to occur on the cut surface. The reason is that the cut surface is rapidly heated and cooled during gas cutting,
This is because thermal stress acts on the cut surface, and at that time, martensitic transformation occurs in the process of cooling the cast slab after cutting.

In the method of the present invention, the slab thickness is set to 150 mm.
As described above, since the slab has a large amount of heat and the slab surface cools slowly, the surface temperature near the short sides on both sides of the slab gradually decreases. Therefore, the occurrence of surface cracks near the short sides can be suppressed.

The specific water amount of the secondary cooling of the slab is 0.1 to 0.6.
Liter / kg-steel, and the secondary cooling is completed before the center of the thickness of the slab starts to solidify. Since the secondary cooling is completed before the solidification of the center of the thickness of the slab starts, it is possible to suppress the occurrence of splits near the center of the thickness. In addition, since the secondary cooling is performed with the above-described appropriate specific water amount, a decrease in the surface temperature near the short sides on both sides of the slab can be suppressed, and the occurrence of surface cracks near the short sides can be suppressed. Further, since warpage of the slab is suppressed, it is possible to prevent a large force from acting in the thickness direction of the slab, and it is possible to suppress the occurrence of splits at the center of the thickness.

Further, while the secondary cooling of the slab is completed and thereafter the solidification is completed and the surface temperature of the slab is 400 ° C. or higher, the slab is subjected to a reduction ratio of 0.1 to 0.4. Primary rolling,
Thereafter, the primary rolled slab is hot-rolled to finish the steel. Primary rolling of the slab while maintaining the surface temperature of the slab at 400 ° C. or higher, that is, by adding a predetermined amount of primary rolling to the slab before the solidified slab undergoes martensitic transformation in the cooling process. The brittle solidified structure is recrystallized into a fine solidified structure, and the susceptibility of the slab to cracking is reduced. Since such a primary-rolled slab is hot-rolled into a steel material, a high-quality steel material can be obtained.

Further, in the method of the present invention, the slab is gradually cooled at an average cooling rate of 50 ° C./hour or less until the surface temperature of the slab after the primary rolling reaches 400 ° C., and then the slab is subjected to the primary rolling. It is desirable that the slab is hot-rolled and finished to a steel material having a total reduction ratio of 0.67 or more from the slab before the primary rolling.

Until the surface temperature of the slab after the primary rolling reaches 400 ° C., the slab is gradually cooled at an average cooling rate of 50 ° C./hour or less, so that the rate of martensitic transformation can be suppressed and crack susceptibility can be suppressed. Can be maintained. Also,
The thickness of the steel material is set to a thickness such that the total reduction ratio from the slab before the primary rolling becomes 0.67 or more, that is, the thickness of the slab is three times or more the thickness of the steel material. The center segregation generated in the piece can be suppressed from remaining in the steel material, and the generation of the center crack in the steel material can be effectively suppressed.

Further, in the method of the present invention, before the primary rolling,
That is, when cutting the slab immediately after casting or the slab after primary rolling using a cutting gas, cutting the slab while the surface temperature of the slab near the cut portion is 350 ° C. or higher. More desirable. Since the sudden heating and cooling of the cut surface is suppressed and the thermal stress acting on the cut surface is reduced, the occurrence of minute cracks in the cut surface can be more effectively suppressed.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION A high chromium / ferrite heat-resistant steel material to which the method of the present invention is applied is one in which a slab obtained by a continuous casting method is used as a material for hot rolling the steel material. The slab may be a slab or a bloom.
The heat-resistant steel material refers to a steel pipe used in fields such as a boiler, a nuclear power plant, and a chemical industry.

The chemical composition of the high chromium / ferritic heat resistant steel and the method for producing the same, which are targeted by the method of the present invention, are as follows:
This will be described in detail below. In addition,% display of the content of each component means mass%. (A) Chemical composition C: 0.03 to 0.2% C forms a carbide together with Cr, Fe, W, V, Nb and the like, and increases the high-temperature strength. C content is 0.03%
If it is less than 10, the amount of precipitated carbide is small and a large amount of δ ferrite is formed, so that the strength and the toughness decrease. However, if the C content exceeds 0.2%, carbides are excessively formed and the strength becomes too high, resulting in poor workability and weldability. Therefore, the C content is set to 0.03 to 0.2%.

Si: 0.05-0.7% Si is an element effective for deoxidizing steel and an element effective for improving steam oxidation resistance. In order to exhibit these effects, it is necessary to contain 0.05% or more. However, if the Si content exceeds 0.7%, the toughness is significantly reduced, and the creep strength is also reduced. Therefore, the Si content is set to 0.05 to 0.7%.

Mn: 0.1-1.5% Mn is an element effective for improving the strength. In order to exhibit this effect, it is necessary to contain 0.1% or more. However, when the Mn content exceeds 1.5%,
Workability and weldability deteriorate. Therefore, the Mn content is set to 0.1 to 1.5%.

Cr: 8 to 14% Cr is a basic element effective for improving oxidation resistance and high-temperature corrosion resistance. In order to exhibit these effects, it is necessary to contain 8% or more. However, Cr
When the content exceeds 14%, a large amount of δ ferrite is generated, and thus the strength and the toughness decrease. Therefore, Cr
The content is 8 to 14%.

W: 0.8-4% W is an element effective for improving creep strength as a solid solution strengthening element and a precipitation strengthening element for fine carbonitrides. In order to exhibit this effect, the content needs to be 0.8% or more. However, if the W content exceeds 4%, the toughness decreases and the workability deteriorates. Therefore, the W content is set to 0.8 to 4%. A more preferred range is 1.5 to
2.5%.

V: 0.1-0.3% V is an element effective for improving the creep strength by combining with C and N to precipitate fine nitrides and carbonitrides. To achieve this effect,
It is necessary to contain 0.1% or more. If the content is less than 0.1%, when the slab is heated, these nitrides and carbonitrides re-dissolve, so that the effect cannot be obtained. However, if the V content exceeds 0.3%, the amount of solid solution V increases, and on the contrary, the strength decreases. Therefore, the V content is 0.1 to 0.3.
%.

Nb: 0.01 to 0.2% Nb is effective for improving creep strength by combining with C and N like V to precipitate fine nitrides and carbonitrides. Element. In order to exhibit this effect, it is necessary to contain 0.01% or more. However, when the Nb content exceeds 0.2%, the amount of undissolved carbonitrides increases during the normalizing treatment, and the strength decreases and the weldability deteriorates. Further, fine precipitates are easily aggregated and coarsened at a high temperature, and the creep strength is reduced. Therefore, the Nb content is set to 0.01 to 0.2%.

N: 0.005 to 0.2% N combines with V, Nb and C to form a carbonitride,
It is an element effective for improving creep strength. In order to exhibit this effect, the content needs to be 0.005% or more. However, when the N content exceeds 0.2%, weldability and workability are significantly deteriorated. Therefore, the N content is set to 0.005 to 0.2%. If the N content exceeds 0.1%, blow holes may be formed in the slab, and the desirable range is 0.005 to 0.1%.

Al: 0.002 to 0.05% Al is an effective element for deoxidizing steel, and it is necessary to contain 0.002% or more in order to exert its effect. However, when the content exceeds 0.05%, the creep strength decreases. Therefore, the Al content is 0.00
2 to 0.05%.

Ni: 1% or less Ni does not have to be contained, but when it is contained, martensite is stabilized. However, if the Ni content exceeds 1%, the transformation point is remarkably lowered, and the tempering treatment is hindered, and the creep strength is lowered. Therefore, the content of Ni is preferably set to 1% or less. However, in the case where Cu described later is contained in an amount of 0.3% or more, in order to prevent the precipitation of Cu at the crystal grain boundary within the upper limit of Ni, the Cu content is 1/3 or more of the Cu content by mass ratio. Ni
Is desirably contained at the same time.

Mo: 1.2% or less Mo does not have to be contained, but when it is contained, it is effective for improving creep strength as a solid solution strengthening element and a precipitation strengthening element for fine carbonitride similarly to W. Element. However, when the Mo content exceeds 1.2%, a large amount of δ ferrite is generated, so that strength and toughness are reduced. Therefore, when Mo is contained, the Mo content is preferably 1.2% or less. When Mo is contained, it is more desirable that the content be less than the W content. The reason is that the effect of suppressing the growth and coarsening of carbonitride is smaller than W.

Cu: 3.5% or less, Co: 4% or less: C
Although u and Co do not have to be contained, if they are contained, both elements become δ as an austenite stabilizing element.
It is an effective element for suppressing the formation of ferrite and improving the strength and toughness. To obtain the effect, C
It is desirable to contain 0.1% or more of each of u and Co. However, when the Cu content exceeds 3.5%, the amount of Cu precipitated at the crystal grain boundary increases, the ductility and high-temperature strength decrease, and the weldability and workability deteriorate. Also, C
Even if the o content exceeds 4%, the effect is saturated. Therefore, in the case where Cu is contained, it is desirable that Cu: 3.5% or less and Co: 4% or less.
-3.5%, Co: 0.1-4% is more desirable.

B: 0.02% or less B does not need to be contained, but is an element effective to improve the strength by uniformly dispersing carbides by adding a small amount of B. 0.000 to get the effect
It is preferable to contain 1% or more. However, when the B content is 0.1%.
If it exceeds 02%, weldability and workability deteriorate. Therefore, when B is contained, the B content is desirably 0.02% or less, and more desirably 0.0001 to 0.02%.

Ti: 0.05% or less, Ta: 0.05%
Hf: 0.05% or less, Nd: 0.05% or less,
La: 0.05% or less, Ce: 0.05% or less, Y;
0.05% or less, Ca: 0.01% or less, Mg: 0.0
1% or less: Ti, Ta, Hf, Nd, La, Ce, Y,
Ca and Mg need not be contained, but by containing them, the effect of fixing and stabilizing impurity elements such as O (oxygen) and S in steel, that is, the effect of controlling the form of nonmetallic inclusions There is. Ti, Ta, Hf, Nd, La,
When Ce, Y, Ca and Mg are contained, one kind or two or more kinds may be used.

In order to obtain the effect, Ti, Ta, H
f and Nd are preferably contained in each of 0.005% or more. However, when each of Ti, Ta, Hf and Nd exceeds 0.05%, the amount of nonmetallic inclusions increases, and on the contrary, the strength and toughness decrease, and the corrosion resistance deteriorates. Therefore, when Ti is contained,
It is desirable that the respective contents of Ta, Hf and Nd are each 0.05% or less. Furthermore, it is more desirable to set each to 0.005 to 0.05%.

In the case where La, Ce and Y are contained in order to obtain the above-mentioned effects, if each exceeds 0.05%, nonmetallic inclusions increase, and strength and toughness decrease. , And the corrosion resistance deteriorates. Therefore, when they are contained, it is preferable that the respective contents of La, Ce and Y be 0.05% or less.

Furthermore, when Ca and Mg are contained in order to obtain the above-mentioned effects, if each contains more than 0.01%, nonmetallic inclusions increase, and on the contrary, strength and toughness decrease, and Corrosion resistance deteriorates. Therefore, when they are contained, the respective contents of Ca and Mg are preferably set to 0.01% or less.

P and S are typical impurities in the high chromium / ferritic heat-resistant steel material to which the method of the present invention is applied. All of these elements reduce strength and toughness and deteriorate weldability and workability. Therefore, it is preferable that P is 0.025% or less and S is 0.015% or less. More preferred is P: 0.015% or less,
S: 0.01% or less. (B) Manufacturing method In the method for manufacturing a steel material according to the present invention, the specific water volume of the secondary cooling of the slab is set to 0.1 to 0.6 liter / kg-steel, and until the center of the thickness of the slab starts to solidify. Under the condition of terminating the secondary cooling, continuously cast a slab having a thickness of 150 mm or more and a rectangular cross section, and while the surface temperature of the slab after solidification of the thickness center is 400 ° C. or more, In addition, reduction ratio 0.1-0.4
The slab is subjected to primary rolling under the conditions described above, and then the primary-rolled slab is hot-rolled to finish the steel.

Since the slab thickness is 150 mm or more, the occurrence of surface cracks near the short side of the slab can be suppressed. 150
It is also possible to cast a slab with a thickness of less than 1 mm and heat the slab in a continuous casting machine to keep the surface temperature near the short sides on both sides of the slab and keep it warm. It is overkill and unrealistic. When the thickness of the slab is less than three times the thickness of the steel material, the center segregation of the slab remains in the steel material, and the steel material is likely to have a center crack.

Since the secondary cooling is completed before the solidification of the center of the thickness of the slab starts, it is possible to suppress the occurrence of splits near the center of the thickness. If the specific water volume of the secondary cooling is less than 0.1 liter / kg-steel, the thickness of the solidified shell near the outlet of the mold becomes thin, and the solidified shell tends to bulge. Likely to happen. In an extreme case, the solidified shell may be broken and breakout may occur. On the other hand, when the specific water volume exceeds 0.6 liter / kg-steel, not only near the short sides on both sides of the slab, but also the entire slab is strongly cooled, and the surface temperature of the slab is excessively lowered. Cracks easily occur throughout. Furthermore, a large force acts in the thickness direction of the slab due to the warpage of the slab, etc., and a two-piece crack occurs near the center of the thickness.

While the surface temperature of the slab after the solidification of the center of the thickness is 400 ° C. or higher, the slab is subjected to primary rolling. In this case, even if the surface temperature of the slab is 400 ° C. or higher, However, to perform primary rolling as it is, the surface temperature of the slab may be too low, so the slab is once charged into a heating furnace, etc.
It is desirable to heat to a temperature at which primary rolling is possible. If there is a slab heat retaining facility in the continuous casting machine and the slab temperature is sufficient for rolling, the primary rolling can be performed without heating.

Before the primary rolling, the surface temperature of the slab is 40
When the temperature falls below 0 ° C., martensitic transformation occurs with the brittle solidified structure, and surface cracks of the slab and double cracks near the center of the thickness tend to occur due to the transformation stress. By performing the primary rolling before the martensitic transformation occurs, the coarse solidified structure is recrystallized into a fine structure, so that the susceptibility to cracking can be reduced.

If the reduction ratio of the slab during the primary rolling is less than 0.1, the effect of recrystallization of the coarse solidified structure to a fine structure is small, and the surface crack of the slab is likely to occur. Further, when the rolling reduction ratio of the slab exceeds 0.4, the effect of the coarse solidified structure recrystallizing into a fine structure is saturated,
Using the subsequently primary-rolled slab, the reduction ratio during hot rolling to steel may decrease, and the mechanical properties of the steel may decrease. Preferably, the reduction ratio of the slab is 0.2 to 0.3.
5 is assumed.

In the method for producing a steel material according to the present invention, the slab is gradually cooled at an average cooling rate of 50 ° C./hour or less until the surface temperature of the slab after the primary rolling reaches 400 ° C., and thereafter,
It is desirable that the primary-rolled slab is hot-rolled and finished to a steel material having a total reduction ratio of 0.67 or more from the slab before the primary rolling.

When the slab is cooled at a cooling rate exceeding the average of 50 ° C./hour, the surface temperature decreases particularly near the short sides on both sides of the slab, and large transformation stress during martensitic transformation is applied to the slab. Even if the slab is recrystallized and has small crystal grains, surface slabs are likely to occur in the slab near the short side.

The primary rolled slab is hot-rolled and finished to a steel material having a total reduction ratio of 0.67 or more from the slab before the primary rolling, so that the center crack of the steel material can be suppressed as described above. It is as follows.

Further, in the method for producing a steel material according to the present invention, when the slab before or after the primary rolling is cut by using a cutting gas, the surface temperature of the slab near the cut portion is 35%.
It is more desirable to cut the slab while it is at 0 ° C. or higher.
That is, when cutting the slab immediately after casting and when necessary before and after the primary rolling, the temperature at least in the vicinity of the cut portion is 3 ° C.
It is desirable to cut the slab at 50 ° C. or higher.

For example, when a slab is cut by a slab cutting device in a continuous casting machine immediately after casting, the surface temperature of the slab is usually 350 ° C. or higher. However, the surface temperature of the slab that has been gradually cooled after the primary rolling is usually lower than 350 ° C. In this case, it is desirable to heat the vicinity of the cut surface. It is preferable to heat the vicinity of the cut surface of the cast piece by sandwiching the cast piece after slow cooling using the piece. Moreover, you may heat using a heating furnace. It is possible to suppress rapid heating and cooling of the cut surface, reduce the thermal stress acting on the cut surface, and suppress the occurrence of fine cracks in the cut surface.

[0056]

EXAMPLE Using a vertical continuous casting machine, a high chromium / ferritic heat resistant steel material having the chemical composition shown in Table 1 was prepared with a thickness of 120 mm and a width of 800 mm, a thickness of 150 mm and a width of 800 mm, or a thickness of 280 mm and a width of 1000 mm. Speed 0.4
Cast at m / min or 0.6 m / min.

[0057]

[Table 1] The specific water volume of the secondary cooling of the slab is 0.09-0.7 liter /
It was varied in the kg-steel range. In some tests, secondary cooling was performed even after solidification started, but in others,
Secondary cooling was completed before the center of the slab thickness started to solidify. At that time, when the thickness center of the slab starts to solidify,
Based on the conditions of the chemical composition of the steel to be cast, the size of the slab, the casting speed, and the specific water content of the secondary cooling of the slab, it was obtained by a method based on ordinary solidification heat transfer analysis. The surface temperature of the slab and the surface temperature near the cut surface described below were measured by a radiation thermometer.

Immediately after casting, the slab is cut in a continuous casting machine,
Thereafter, the slab was mounted on a transport trolley, transported to a heating furnace with a steel cover. The surface temperature of the slab at the time of cutting the slab was about 780 ° C. By hanging the cover,
The surface temperature of the slab after solidification of the center of the thickness is 400
The cast slab could be charged to the heating furnace without dropping below ℃. In some tests, the surface temperature of the slab dropped to less than 400 ° C. because no cover was applied. 12 in heating furnace
After heating at 80 ° C. for 3 hours, primary rolling was performed. The surface temperature of the slab after the primary rolling was about 1020 to 1090 ° C. However, primary rolling was not performed in some tests.

After the primary rolling, the slab was placed in an annealing furnace and gradually cooled, and cooled for about 250 hours until the surface temperature of the slab reached 200 ° C., and the average cooling rate during that time was 3 ° C./hour. And Thereafter, the slab was taken out from the annealing furnace and cooled to room temperature. However, in some tests, the slab was cooled by adjusting the average cooling rate to 60 ° C./hour until the surface temperature of the slab reached 200 ° C.

The slab which has been subjected to the primary rolling and cooled to room temperature is heated by sandwiching it between two cast slabs of ordinary steel immediately after casting for about 10 to 15 hours, and thereafter the slab is cut. did. The surface temperature of the slab to be cut could be 350 ° C. or higher by sandwiching the slab between the high-temperature slabs. However, in some tests, the time for sandwiching the slab was shortened, and the surface temperature of the slab was set to less than 350 ° C. The cut slab was hot-rolled into a steel material, and the heating temperature, furnace time, and the like of the slab were the same as those of a normal high chromium / ferrite heat-resistant steel material.

The surface temperature of the slab when the slab is charged into the heating furnace, the amount of reduction in the primary rolling, the reduction ratio, and the vicinity of the cut surface when the slab is cut after slow cooling after the primary rolling. Surface temperature,
The test was performed by changing the test conditions such as the thickness of the steel material of the product within the ranges shown in Tables 2 and 3 described below.

The surface cracks of the slab after the primary rolling and the double cracks near the center of the thickness were visually observed. Further, after the slab was heated and cut after annealing, cracks in the cut surface of the slab were visually observed. Furthermore, the occurrence of surface cracks and center cracks in the product steel was investigated by magnetic flaw detection and ultrasonic flaw detection. Tables 2 and 3 show the test conditions and test results.

[0063]

[Table 2]

[Table 3] Test No. of the present invention example. 1 to Test No. 8, as shown in Table 2, steel a to steel f were used, the thickness was 280 mm, and the width was 10 mm.
A 00 mm slab was cast at a speed of 0.4 m / min and a secondary cooling specific water volume of 0.42 liter / kg-steel. Secondary cooling was completed before the center of thickness of the slab started to solidify. At the time of primary rolling, the slab is reduced by 60 to 80 mm in 3 to 5 passes so that the reduction ratio is about 0.2 to 0.3, and the thickness is 200 to
A 220 mm slab was used. The cast slab after the primary rolling was placed in an annealing furnace, and the test sample No. was cooled until the surface temperature reached 200 ° C. 1
~ Test No. In Test No. 7, the sample was gradually cooled at an average cooling rate of about 3 ° C./hour. In No. 8, cooling was performed at a cooling rate of about 60 ° C./hour on average. Thereafter, the surface temperature was lowered to near normal temperature.

Test No. 1 to Test No. 6 and test N
o. 8, a slab of 200 to 220 mm in thickness after slow cooling is cast on a normal steel slab 2 immediately after being cast by another continuous casting machine.
The slab was sandwiched for about 10 to 15 hours, and the slab was heated so that the surface temperature of the slab was 350 to 420 ° C., and then the slab was cut. Test No. 7, the thickness after slow cooling is 200
mm slab is sandwiched by another high temperature slab for only about 3 hours.
After the surface temperature of the slab reached 270 ° C., the slab was cut. The slabs after the primary rolling were hot-rolled into product steel materials having a thickness of 50 to 80 mm. At this time, the total reduction ratio from the slab at the time of casting is 3.5 to 5.6.

Test No. 1 to Test No. The conditions of the thickness of the slab, the specific water amount of the secondary cooling and the end time thereof, the surface temperature of the slab before the primary rolling, and the reduction ratio at the time of the primary rolling are all defined by the method of the present invention. It is within the range of the condition.

Test No. 1 to Test No. In No. 6, no surface cracks or double cracks occurred on the surface and the center of the thickness of the slab after the primary rolling. Also, no cracks occurred on the cut surface of the cast piece cut after the slow cooling. Further, the product steel did not have any surface cracks or center cracks.

Test No. In No. 7, no surface cracks or double cracks occurred on the surface and the center of the thickness of the cast slab after the primary rolling, but fine cracks occurred on the cut surface of the cast slab cut after slow cooling. However, the fine cracks on the cut surface were small enough not to hinder the subsequent hot rolling, and only slight cracks occurred at the end of the product steel material. Also,
No surface cracks or center cracks occurred in the product steel.

Test No. In No. 8, fine surface cracks occurred on the surface of the slab after the primary rolling. However, the fine surface cracks were cracks that would not hinder subsequent hot rolling, and no cracks occurred on the surface of the product steel material.

Test No. of the present invention example In No. 9, a slab having a thickness of 150 mm and a width of 800 mm was formed at a speed of 0.6 using steel f.
Casting was performed at a rate of 0.42 l / kg of steel at a specific water rate of m / min and secondary cooling, and the secondary cooling was completed before the center of the slab thickness started to solidify. The rolling during the primary rolling was reduced by 30 mm in two passes. The rolling reduction of this primary rolling is 0.2. Also,
After the primary rolling, the slab was placed in a lehr and the surface temperature was 200
The temperature was gradually cooled to a temperature of about 3 ° C./hour or less on average. According to the temperature measurement during cooling, the surface temperature was 400
The cooling rate to ° C. averaged about 6 ° C./hour. Thereafter, the surface temperature was lowered to near normal temperature. Thereafter, the slab was sandwiched between two ordinary steel slabs immediately after being cast by another continuous casting machine for about 12 hours, and the surface temperature of the slab was 350
After heating the slab to ℃, the slab was cut. The cut slab having a thickness of 120 mm was rolled into a product steel material having a thickness of 55 mm.

Test No. In 9, the specific water amount of secondary cooling and the end time thereof, the surface temperature of the slab before the primary rolling, and the test conditions of the rolling reduction during the primary rolling are within the conditions specified by the method of the present invention. is there. However, the total reduction ratio from the slab to the steel material is 2.7, which is lower than the desired condition.

Test No. In No. 9, no surface crack occurred in the slab after the primary rolling. Also, no cracks occurred on the cut surface of the cast slab after cutting. However, hot-rolled thickness 5
In the case of a 5 mm product steel material, a slight defect echo was observed by the ultrasonic flaw detection method, but the defect echo level was not a problem.

Test No. of Comparative Example 10 to Test No. Fifteen
Then, as shown in Table 3, a test was performed using steel f. Test No. 10 to Test No. In 14, the thickness 28
Test No. 0 was cast on a slab having a width of 1000 mm and a speed of 0.4 m / min. For 15, the thickness is 120 mm and the width is 800
mm was cast at a speed of 0.6 m / min.

Test No. In Test No. 10, except that the rolling reduction during the primary rolling was set to 20 mm in one pass. The same casting conditions as those of No. 1 were used, such as the conditions of primary rolling and slab cooling after the first rolling. The reduction amount of 20 mm during the primary rolling corresponds to a reduction ratio of 0.07 of the slab, and means a small reduction ratio excluding the lower limit of the condition specified by the method of the present invention. Test No.
In No. 11, when the slab was mounted on a transport trolley and transported to the heating furnace, a steel cover was not hung, and the surface temperature of the slab charged to the heating furnace before the primary rolling was set to 330 ° C. Is
Test No. The same casting conditions as those of No. 1 were used, such as the conditions of primary rolling and slab cooling after the first rolling. The surface temperature of the slab charged into the heating furnace means a low surface temperature excluding the lower limit of the condition specified by the method of the present invention.

Test No. In No. 10, the rolling reduction in the primary rolling was small, and the effect of recrystallizing a coarse solidified structure into a fine structure was small, so that a surface crack occurred on the short side of the slab after the primary rolling. Since these surface cracks were extremely large, it was judged that subsequent hot rolling was difficult, and cutting of the slab and hot rolling to the product steel material were not performed.

Test No. In the case of No. 11, when the slab was charged into the heating furnace at the time of the primary rolling, the surface temperature of the slab dropped to less than 400 ° C., so that martensitic transformation occurred, and the casting stress after the primary rolling was caused by the transformation stress. A surface crack occurred on one short side. Since these surface cracks were extremely large, it was judged that subsequent hot rolling was difficult, and cutting of the slab and hot rolling to the product steel material were not performed.

Test No. In Test No. 12, except that the specific water volume of the secondary cooling was 0.09 liter / kg-steel.
The same casting conditions as in No. 1 were used. This specific water amount means a small specific water amount excluding the lower limit of the condition specified by the method of the present invention. This test no. In No. 12, the level of the molten metal in the mold fluctuated greatly, and it became difficult to continue casting. Therefore, casting was stopped halfway, and the test after primary rolling was not performed. The surface of the cast slab obtained in the early stage of casting had a remarkable lateral crack due to the fluctuation of the molten metal level.

Test No. In Test No. 13, except that the specific water amount of the secondary cooling was set to 0.7 liter / kg-steel. 1
The same casting conditions as those described above, and the conditions of primary rolling and slab gradual cooling after the first rolling were used. This specific water amount means a large specific water amount excluding the upper limit of the condition specified by the method of the present invention. This test N
o. In No. 13, it was confirmed that the specific water amount of the secondary cooling was too large, and significant cracks were generated on the entire surface of the slab at the outlet side of the continuous casting machine. Furthermore, surface cracks were also remarkably generated on the entire surface of the cast slab after the primary rolling. Since these surface cracks were remarkable and it was judged that subsequent hot rolling was difficult,
Cutting of the slab and hot rolling to the product steel material were not performed.

Test No. In Test No. 14, except that the end time of the secondary cooling was after the start of solidification at the center of the slab thickness, the test N
o. The same casting conditions as those of No. 1 were used, and primary rolling and the slab gradual cooling after the primary rolling were performed. The end time of the secondary cooling means that the condition specified by the method of the present invention is not satisfied.
This test no. In 14, at the stage where the thickness center portion starts to solidify, the slab surface was secondarily cooled, so that the final solidification portion of the slab was cracked, and even in the slab after the primary rolling, two sheets were formed near the thickness center portion. Cracks occurred. Since the splitting was remarkable and it was judged that the subsequent hot rolling was difficult, the cutting of the slab and the hot rolling to the product steel material were not performed.

Test No. In No. 15, a slab having a thickness of 120 mm was cast with a secondary cooling specific water volume of 0.35 l / kg-steel, and the secondary cooling was terminated before the center of the slab thickness started to solidify. The slab thickness of 120 mm means that the slab thickness is thin except for the slab thickness condition specified by the method of the present invention. The test conditions for the specific water volume of secondary cooling and the end time are as follows:
It is within the range defined by the method of the present invention.

This test no. In No. 15, the cooling near the short side of the slab was fast, and remarkable surface cracking occurred on the short side of the slab on the exit side of the continuous casting machine. Since these surface cracks were extremely large, tests after the primary rolling were not performed.

[0081]

By applying the method of the present invention, a high quality cast slab free of cracks can be obtained as a material for hot rolling used in the production of high chromium / ferritic heat resistant steel.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int. Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/00 302 C22C 38/00 302Z 38/54 38/54

Claims (3)

[Claims] (1) C: 0.03 to 0.2% by mass%, S
i: 0.05 to 0.7%, Mn: 0.1 to 1.5, C
r: 8-14%, W: 0.8-4%, V: 0.1-0.
3%, Nb: 0.01 to 0.2%, N: 0.005 to
0.2%, Al: 0.002 to 0.05%, Ni: 1%
Mo: 1.2% or less, Cu: 3.5% or less, C
o: 4% or less, B: 0.02% or less, Ti: 0.05%
Hereinafter, Ta: 0.05% or less, Hf: 0.05% or less,
Nd: 0.05% or less, La: 0.05% or less, Ce:
0.05% or less, Y: 0.05% or less, Ca: 0.01
% Or less, Mg: 0.01% or less, the balance being a method for producing a high chromium / ferrite heat resistant steel material comprising Fe and impurities, wherein the specific water amount of the secondary cooling of the slab is 0.1 to
0.6 liter / kg-steel, 150 m under the condition that secondary cooling is completed before solidification of the center of the slab thickness starts.
continuous casting of a rectangular piece with a thickness of at least m
The surface temperature of the slab after solidification of the center of the thickness is 400
While the temperature is not less than 0 ° C., the slab is primarily rolled under a reduction ratio of 0.1 to 0.4, and then the primary-rolled slab is hot-rolled to finish it into a steel material. Manufacturing method of heat-resistant ferritic steel. 2. The surface temperature of the slab after the primary rolling is 400
Until the temperature reaches 50 ° C., the steel is cooled at an average cooling rate of 50 ° C./hour or less, and then the primary-rolled slab is hot-rolled, and the total reduction ratio from the slab before the primary rolling is 0.67 or more. The method for producing a high chromium / ferritic heat-resistant steel material according to claim 1, wherein the steel material is finished. 3. A method for cutting a slab before or after primary rolling using a cutting gas while cutting the slab while the surface temperature of the slab near the cut portion is 350 ° C. or higher. The method for producing a high chromium / ferritic heat-resistant steel material according to claim 1 or 2, wherein: