JP3471699B2 - Austenitic stainless steel slab that hardly generates hot rolling defects and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an austenitic stainless steel such as 25% Cr-20% Ni-0.1% C series which is mainly used as a welding material, at the time of hot rolling such as continuous wire rolling. Surface defects of rolling material that occurs (hereinafter,
The present invention relates to a slab that is unlikely to cause hot-rolling defects and a method for manufacturing the slab.

[0002]

2. Description of the Related Art High C austenitic stainless steels such as 25% Cr-20% Ni-0.1% C series are used for welding materials such as welding rods, and their composition depends on the composition balance and structure of the weld. It has been decided. A wire product is mainly used as a welding material and is manufactured by hot rolling a continuously cast slab. However, there is a problem that hot rolling peculiar to the surface of the rolled material is likely to occur during hot rolling, and the manufacturing yield is reduced. The present inventors have investigated the cause of this hot-rolled defect occurrence, and have arrived at the present invention as a countermeasure, but for steels of the same system as the target material of the present invention, no related art is found, and for steels of different systems, The following methods are known.

Regarding 9% Ni steel, Japanese Patent Publication No. 56-141
Japanese Patent Laid-Open No. 26-26 proposes a method of gradually cooling a cast steel product at a temperature of 400 to 300 ° C. at a cooling rate of 5 to 40 ° C./hr. And 9% N, which is known as a cryogenic material
For i steel, intergranular cracking occurs in the cooling process after casting, and in the case of thick wall, cracking occurs even after completion of casting, so cast products were hardly used, but due to phase transformation during cooling As a result of clarification that supersaturated hydrogen diffuses at the grain boundaries, grain boundary cracking occurs due to transformation stress and shrinkage stress, and that the martensite transformation rapidly starts near 350 ° C. even though the Ms point is 445 ° C., It is described that a sound cast steel product was obtained by the above method.

[0004]

As described above, 25%
In the hot rolling of austenitic stainless steel such as Cr-20% Ni-0.1% C system, there is a problem that a specific hot rolling defect occurs and the manufacturing yield is reduced. And, it cannot be solved by the method and the idea described in the above publication. Therefore, the problem to be solved by the present invention is to effectively reduce the characteristic hot-rolling defects that occur in the hot rolling of the austenitic stainless steel as described above, and to provide a continuous cast piece capable of improving the production yield. And a method for manufacturing the same.

[0005]

Means for Solving the Problems The cast slab of the present invention for solving the above problems is C: 0.07 to 0.3% in mass%,
Cr: An austenitic stainless steel containing 16 to 30%, having a C content and a Cr content as factors, and having a negative K value represented by the following formula (1), after continuous casting 10% based on JIS G 0571 in the state of
When the ratio of grain boundaries with a width of 2 μm or more to the total grain boundaries after oxalic acid etching is P (%) and the residual stress is Q (MPa),
It is an austenitic stainless steel slab that is resistant to hot-rolling defects and is characterized by satisfying any of the following expressions (2) to (4). K = 0.28-0.01 [Cr]-[C] (1) P ≦ 20 ………………………………………… (2) Q ≦ 50 ……… …………………………………… (3) R = 45P + 4Q-2900 ≦ 0 ………… (4) However, [Cr] and [C] are the contents of Cr and C, respectively. (mass%).

Further, the first invention method of the present invention contains C: 0.07 to 0.3% and Cr: 16 to 30% in mass%,
In addition, in the continuous casting of austenitic stainless steel composed of components having negative C values represented by the formula (1), with C content and Cr content as factors, 5 ° C / sec from a temperature of 1000 ° C or higher after casting A method for producing an austenitic stainless steel slab in which hot-rolling defects are less likely to occur is characterized by cooling at the above cooling rate.

The second invention method of the present invention contains C: 0.07 to 0.3% and Cr: 16 to 30% in mass%,
In addition, in the continuous casting of an austenitic stainless steel composed of a component having a negative K value represented by the formula (1), with C content and Cr content as factors, after casting, 800 ° C. to 4 ° C.
A method for producing an austenitic stainless steel slab, in which hot-rolling is less likely to occur, characterized by cooling a temperature range up to 00 ° C at a cooling rate of 0.4 ° C / sec or less.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have found that 25% Cr-20
% Ni-0.1% C system represented by austenitic stainless steel as a typical hot-rolled flaw observed and examined. As a result, the unique flaw present in the surface layer of the continuously cast slab was the cause. . When the hot rolling flaw was observed in detail, it was found that the grain boundary of the slab was cracked, and a large amount of Cr carbide was precipitated at the cracked grain boundary. Then, fine grain boundary cracks were observed in the surface layer of the cast piece before hot rolling, and a large amount of Cr carbide was precipitated at the grain boundaries. A high residual stress was found in the slab.

From these facts, it was speculated that the hot rolling defects of this steel may be caused by the intergranular stress cracking of the slab. That is,
With a large amount of Cr carbide precipitating in the grain boundaries during the cooling process of the cast piece after continuous casting and the susceptibility to intergranular stress cracking became high, the residual stress existing in the cast piece after cooling acted on the cast surface layer. It was thought that stress cracking occurred at the grain boundaries of slag and that this resulted in hot-rolled defects, so we observed the grain boundaries of the surface layer and measured the residual stress for many cast pieces. As a result, the target cast piece in which hot-rolling defects were likely to occur was a case where there were many Cr carbides at the grain boundaries of the surface layer and the residual stress was high.

The slab of the present invention is characterized by limiting the amount of grain boundary Cr carbide precipitated on the surface layer of the slab in the cooling process after continuous casting and the extent of residual stress after cooling, thereby providing a unique hot rolling property. This is what makes it difficult for defects to occur. The grain boundary Cr carbide amount was based on the ratio P (%) of grain boundaries having a width of 2 μm or more to all grain boundaries after 10% oxalic acid etching based on JIS G 0571 as an index. The composition of the target austenitic stainless steel is limited within the range required as a welding material.

The slab of the present invention is composed of C and C.
The r is limited, and the reason for the limitation is as follows.
C is a strong austenite forming element, and the lower limit was made 0.07% from the required characteristics as a welding material. However, if it is excessive, M ₂₃ C ₆ type Cr carbide is likely to precipitate during cooling after casting, and grain boundary stress cracking is likely to occur in the slab. Further, since the hot workability is deteriorated and Cr carbides are precipitated to deteriorate the oxidation resistance and the corrosion resistance during welding or when the welded product is used at high temperature, the upper limit was made 0.3%.

Cr is a basic element for maintaining the corrosion resistance of stainless steel, and the lower limit is set to 16% from the required characteristics as a welding material including the corrosion resistance of the welded portion. However, even if added over 30%, the effect of improving the corrosion resistance commensurate with the cost increase is not recognized, and the problem of hot workability during hot rolling occurs, so the upper limit was made 30%. In addition, since the C content and the Cr content are in the above ranges and the above-described surface flaws do not occur in the slab and the hot rolling flaw does not matter, the slab of the present invention is It is limited to the range in which the K value represented by the equation (1) is negative.

The amount of grain boundary Cr carbide precipitated on the surface layer of the cast slab should be reduced to a range where P is 20% or less as shown in the formula (2), and the residual stress Q after cooling is (3 It is sufficient that the pressure is 50 MPa or less as shown in the formula). Further, P and Q may be in the range of the expression (4), that is, R may be 0 or a negative range. Then, it suffices to satisfy any one of the expressions (2), (3), and (4).

Next, the first invention method of the present invention defines cooling conditions of the cast slab of the present invention for reducing the grain boundary Cr carbide precipitation amount as described above. That is, in continuous casting of an austenitic stainless steel containing C and Cr in the same composition as that of the cast slab of the present invention, cooling is performed from a temperature of 1000 ° C or higher to a cooling rate of 5 ° C / sec or higher after casting.

By cooling under these conditions, the temperature range where Cr carbide is likely to precipitate is rapidly cooled, so that C at the grain boundaries is cooled.
A cast product that has a small amount of r-carbide precipitates and satisfies the above formula (2) or (4) is obtained. That is, even if the residual stress is high and the formula (3) is not satisfied, the precipitation amount of the grain boundary Cr carbide is reduced to satisfy the formula (2), or the grain boundary C is reduced.
Even if the amount of precipitated r-carbon is large to some extent and the equation (2) is not satisfied, the residual stress is low and falls within the range of the equation (4).

Therefore, according to the first invention method, the grain boundary stress cracking is sufficiently suppressed in the cast slab after cooling, and the hot rolling defect when the slab is hot rolled does not become a problem. If this cooling condition is not satisfied, the amount of precipitation of grain boundary Cr carbides will not be sufficiently suppressed, neither equation (2) nor equation (4) will be satisfied, and yield loss due to hot-rolling defect will occur. The lower limit of the temperature range for cooling at a cooling rate of 5 ° C./sec or higher may be 400 ° C. or lower at which there is no risk of Cr carbide precipitation.

Next, the second invention method of the present invention defines cooling conditions of the slab for reducing the residual stress in the slab of the present invention as described above. That is, in the continuous casting of an austenitic stainless steel whose C and Cr contents are the same as those of the slab of the present invention, 8
The temperature range from 00 ° C to 400 ° C is cooled at a cooling rate of 0.4 ° C / sec or less.

By cooling under these cooling conditions, a slab having a low residual stress of the slab and satisfying the above formula (3) or (4) can be obtained. That is, even if the amount of grain boundary Cr carbide precipitation is large and the formula (2) is not satisfied, the residual stress becomes low so that the formula (3) is satisfied, or the residual stress is high to some extent and the formula (3) is satisfied. Even if it does not exist, the amount of precipitation of the grain boundary Cr carbide is small, and the range satisfies the formula (4).

Therefore, also according to the second invention method, grain boundary stress cracking is sufficiently suppressed in the cast slab after cooling, and the hot rolling defect when hot rolling is not a problem. If this cooling condition is not satisfied, the residual stress after cooling will be high, and neither formula (3) nor formula (4) will be satisfied, and yield loss due to thermal defect will occur.

[0020]

EXAMPLES Seven kinds of austenitic stainless steels A to G having the chemical components shown in Table 1 were continuously cast,
It cooled by changing the conditions. Steels A and G in Table 1 are within the scope of the present invention, and others are outside the scope of the present invention. The cooling after casting was performed in eight patterns a to h shown in FIG. 1, the conditions are as follows: a and c are the range of the first invention method of the present invention, and e is the range of the second invention method. Is. Others are outside the scope of the present method.

A: Cooling from 1050 ° C. to 6 ° C./sec b: Cooling from 800 ° C. to 6 ° C./sec c: Cooling from 100 ° C. to 10 ° C./sec d: Cooling from 800 ° C. to 10 ° C./sec e: 800 Cooling between 400 to 400 ° C at 0.3 ° C / sec f: Cooling between 800 to 400 ° C at 1.0 ° C / sec g: Cooling between 800 to 600 ° C at 0.4 ° C / sec,
Cooling from 600 ° C to 8 ° C / sec h: After cooling between 800 to 600 ° C at 8 ° C / sec, 60
Cooling from 0 ℃ to 0.4 ℃ / sec

The steels A to F in Table 1 were cooled under conditions a and b. The G steel was cooled under the conditions of c to h. 10% based on JIS G 0571 for cooled slabs
Oxalic acid etching was performed, and the ratio P (%) of grain boundaries having a width of 2 μm or more to all grain boundaries was determined by microscopic observation. The residual stress Q (MPa) was also measured. Table 2 shows P, Q and R of the equation (4) and the evaluation of the grain boundary stress cracking condition on the surface of the slab. O indicates that there is no crack and no hot-rolling defect occurs, and X indicates that there is a crack and hot-rolling defect may occur.

In Table 2, No. 1, 13 and 15 are the slabs of the present invention. In each case, the components of C amount, Cr amount and K value satisfy the conditions of the present invention, and No. 1 is R ≦ 0, N
o. 13 is P ≦ 20%, No. 13 No. 15 satisfied Q ≦ 50 MPa, and no intergranular stress cracking occurred.

Then, No. 1 and 13 are examples of the first invention method. When cooling after casting, No. Since No. 1 was performed under the condition a, the precipitation region of Cr carbide was rapidly cooled, so that the precipitation amount of grain boundary Cr carbide was small and the residual stress was low, so that R was low and grain boundary stress cracking was hard to occur. No. No. 13 was performed under the condition c, and although the Cr carbide precipitation region was further rapidly cooled, the residual stress was high, but the grain boundary Cr carbide precipitation amount was smaller and P was lower, so that grain boundary stress cracking was less likely to occur. No. 1
5 is an example of the second invention method. Cooling after casting is performed under the condition of e, and although the amount of grain boundary Cr carbide precipitation is large, 800-40
Since the temperature range of 0 ° C. is gradually cooled, the residual stress Q is low and grain boundary stress cracking is less likely to occur.

No. 2 is a component within the scope of the present invention,
The cooling condition is the b condition outside the scope of the present invention, the rapid quenching starting temperature is low, so the precipitation amount of grain boundary Cr carbide is large and P is high, and the residual stress Q and R values also deviate from the conditions of the present invention. Stress cracking occurred. No. In the B steels of 3 and 4, the C content was out of the lower limit of the range of the present invention, and No. The D steels of Nos. 7 and 8 had Cr contents outside the lower limit and K values, and No. 9 and 1
Since the K value deviates from 0, the precipitation amount of grain boundary Cr carbides is small even when cooled under b condition outside the range of the present invention.
It is not a target for which intergranular stress cracking does not occur.

No. The C steels of Nos. 5 and 6 had a C content outside the upper limit of the range of the present invention. 11 and 12 F
Since the Cr content of the steel is out of the upper limit of the range of the present invention, the grain boundary Cr carbide precipitation amount was large and grain boundary stress cracking occurred even when cooled in the condition a of the present invention.

No. No. 14 is a component within the range of the present invention, but has a large amount of grain boundary Cr carbide precipitation and a high P content due to cooling at d condition and a low quenching start temperature, and residual stresses Q and R.
The value was also high, and intergranular stress cracking occurred. No. 16-18
Is also a component within the scope of the present invention, but the cooling conditions are outside the scope of the method of the present invention. No. No. 16 is the condition f, and No. 17
Is a g condition, and No. No. 18 was cooled under the h condition, and the grain boundary stress cracking occurred because of the large amount of grain boundary Cr carbide precipitation, high P, and high residual stress.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

INDUSTRIAL APPLICABILITY The slab of the present invention is an austenitic stainless steel such as 25% Cr-20% Ni-0.1% C system which is mainly used as a welding material. The intergranular stress cracking that occurred in the layer is significantly reduced, hot defects are less likely to occur during hot rolling such as continuous wire rolling, and the manufacturing yield is improved, and the defect maintenance process is unnecessary. . Further, the method of the present invention defines the cooling conditions of the slab after continuous casting, and makes it difficult for the Cr carbide to precipitate at the grain boundaries or reduces the residual stress, so that the grain boundary stress cracking is effectively performed. Can be reduced.

[Brief description of drawings]

FIG. 1 is a graph showing a cooling pattern in an example.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-10-211548 (JP, A) JP-A-2-240219 (JP, A) JP-A-6-344079 (JP, A) JP-A-6- 87054 (JP, A) JP-A 2-133529 (JP, A) JP-A 5-57406 (JP, A) JP-A 7-268453 (JP, A) JP-A 1-168810 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) B22D 11/124 B22D 11/00 C22C 38/00 302 C22C 38/18

Claims (3)

(57) [Claims] 1. C: 0.07 to 0.3% by mass%, C
An austenitic stainless steel containing r: 16 to 30%, and having a C content and a Cr content as factors and having a negative K value represented by the following formula (1),
When P (%) is the ratio of grain boundaries with a width of 2 μm or more to the total grain boundaries after 10% oxalic acid etching based on JIS G 0571 in the state after continuous casting and Q (MPa) is the residual stress, An austenitic stainless steel slab that is less likely to cause hot-rolling defects, characterized by satisfying any of the expressions (2) to (4). K = 0.28-0.01 [Cr]-[C] (1) P ≦ 20 ………………………………………… (2) Q ≦ 50 ……… …………………………………… (3) R = 45P + 4Q-2900 ≦ 0 ………… (4) However, [Cr] and [C] are the contents of Cr and C, respectively. (mass%). 2. C: 0.07 to 0.3% by mass%, C
In the continuous casting of austenitic stainless steel containing r: 16 to 30%, and having a C content and a Cr content as factors and a K value represented by the following formula (1) being negative, after casting A method for producing an austenitic stainless steel slab, which is resistant to hot-rolling defects, characterized by cooling from a temperature of 1000 ° C or higher to a cooling rate of 5 ° C / sec or higher. K = 0.28-0.01 [Cr]-[C] (1) where [Cr] and [C] are the contents (% by mass) of Cr and C, respectively. 3. C: 0.07 to 0.3% by mass%, C
In the continuous casting of austenitic stainless steel containing r: 16 to 30%, and having a C content and a Cr content as factors and a K value represented by the following formula (1) being negative, after casting A method for producing an austenitic stainless steel cast slab that is unlikely to cause hot-rolling defects, characterized by cooling a temperature range from 800 ° C to 400 ° C at a cooling rate of 0.4 ° C / sec or less. K = 0.28-0.01 [Cr]-[C] (1) where [Cr] and [C] are the contents (% by mass) of Cr and C, respectively.