JPH08257607A - Method for hot-rolling stainless steel for suppressing occurrence of edge crack - Google Patents

Abstract

PURPOSE: To suppress edge cracks which are generated at the time of hot-rolling and to obtain hot-rolled stainless steel sheet at high yield. CONSTITUTION: The constituents of stainless steel to be hot-rolled is adjusted so that the content X of δ ferrite which is defined by the next equation is 0-12% and, after roughly rolling at (edging amount -10 to 25)mm, hot-rolled to the target thickness. X=3.0×(Cr%+1.5×Si%+Mo%)-2.8×(Ni%+0.5×Mn%+0.5-×Cu%+30×C%+30×N%)-19.8%. After executing rough rolling at the edging amount of 5-25mm when the δ ferrite content X is 9-12% and at the edging amount of -10 to 25mm when the δferrite content is 0-8%, it is preferable and the stainless steel is hot-rolled to the target thickness.

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 stainless steel hot-rolled sheet having excellent surface quality without causing defects such as cutting edges or reducing the degree of cutting edges.

[0002]

Duplex stainless steels are superior in stress corrosion cracking resistance to austenitic stainless steels and are superior in weldability to ferritic stainless steels. This two-layer stainless steel, when using austenitic stainless steel as a welding material such as wire,
In order to prevent cracks from occurring in the formed weld beads, the composition is adjusted so that the welding material itself contains some δ-ferrite. Since this duplex stainless steel contains the austenite (γ) phase and the ferrite (δ) phase with different mechanical properties at the same time, the γ phase / δ phase is used during hot working such as slabbing and hot rolling. Cracks are likely to occur from the interface with and the hot workability is extremely poor. This is because stress during hot rolling is concentrated on the boundary between austenite matrix and δ ferrite, which have different deformation resistance and deformability, and cracks and fractures tend to occur at the phase interface. Defects appear as edges when hot-rolling and reduce the product yield.

In order to improve the hot workability of austenitic or duplex stainless steel containing δ ferrite, various methods have been conventionally proposed. For example,
JP-A-7-15660 suppresses the content of impurities such as S and O that adversely affect the deformability of the austenite phase,
The concentration of impurities segregated at the phase interface is reduced. In Japanese Examined Patent Publication No. 59-14099, the S content, which causes segregation at the grain boundaries and causes cracking, is reduced to 0.005% by weight or less, and hot workability is improved by adding B.
In JP-A-4-88151, by quantitatively adjusting the balance between the ferrite phase and the austenite phase in relation to the S content, the hot workability is stably improved and the stainless steel is hot-rolled with good yield. A method of rolling is disclosed. Further, Japanese Patent Laid-Open No. 5-255737 discloses that a stainless steel whose S content is regulated to 0.001% by weight or less is width-killed and rolled to prevent edge cutting during hot rolling.

[0004]

Rare earth elements are usually added in order to reduce the S content, but the rare earth elements not only increase the manufacturing cost, but also improve manufacturability such as clogging of the tundish nozzle during slab casting. It also causes the deterioration. Further, since the manufacturing variation of the slab width and the width-canceling amount in the hot rolling also vary, the necessary width-canceling amount may not be obtained when the width-canceling range is narrow. The present invention has been devised to solve such a problem, and it is necessary to perform heat treatment during rough rolling by performing width reduction for a predetermined amount of δ ferrite in the hot rolling temperature range. The purpose is to hot-roll slabs, billets and other slabs with high yield.

[0005]

In order to achieve the object of the hot rolling method of the present invention, stainless steel to be hot rolled has a δ ferrite content X defined by the formula (1) of 0 to 12%. It is characterized in that the components are adjusted so that the width is reduced to a rough rolling amount of -10 to 25 mm, and then hot rolling is performed to a target plate thickness. X = 3.0 × (Cr% + 1.5 × Si% + Mo%) −
2.8 x (Ni% + 0.5 x Mn% + 0.5 x Cu% +
30 × C% + 30 × N%)-19.8% δ defined by the equation (1) of the hot rolled stainless steel.
When the ferrite amount X is 9 to 12%, the width reduction amount is 5 to 2
It is preferable to perform rough rolling at 5 mm, rough rolling at a width reduction amount of −10 to 25 mm when the δ ferrite amount X is 0 to 8%, and then hot rolling to a target plate thickness. As hot rolled stainless steel, C: 0.030% by weight or less,
N: 0.040 wt% or less, Si: 0.60 wt% or less, Mn: 2.00 wt% or less, Cr: 18.0-2
1.50 wt%, Ni: 9.5 to 12.8 wt%, M
o: 0.50 wt% or less, Cu: 0.20 wt% or less, and if necessary, one or two types of Nb and Ta: 0.50 to
Stainless steel containing 1.10% by weight is used.

[0006]

The hot rolled slab S is rolled down between the upper roll 1 and the lower roll 2 as schematically shown in FIG. The plastic deformation during hot rolling is constrained by the frictional force between the roll and the rolled material in the central portion of the slab S, and is mainly a two-dimensional deformation of the slab S along the rolling direction. Since both end portions in the width direction of the slab S are open free ends, in addition to the two-dimensional deformation extending in the longitudinal direction of the slab S, there is a three-dimensional plastic deformation extending laterally as indicated by the arrow. . Therefore, plastic flow becomes large at both ends in the width direction, and rolling defects such as cracks and fractures are likely to occur at crystal grain boundaries. In the process of investigating and studying the mechanism of rolling defects on both sides in the width direction, the inventors of the present invention reduced the slab by a predetermined amount in the sheet width direction for a predetermined δ ferrite amount in the hot rolling temperature range. By doing so, it was found that the occurrence of rolling defects is effectively suppressed.

The width reduction is performed by pressing a vertical roll arranged in a direction orthogonal to or inclined to the upper and lower rolls 1 and 2 against the widthwise end of the slab. Due to the processing history by the combination of width reduction and horizontal reduction, dispersion of δ-ferrite is promoted by promoting dynamic recrystallization of the coarse structure at both ends of the slab, and the deformability and ductility are improved. δ Ferrite amount is 9
%, It is difficult to accumulate sufficient strain to cause dynamic recrystallization under a light pressure of less than 5 mm, and the ductility of the material is not sufficiently improved. But 2
In order to apply a width reduction of more than 5 mm, a very large load is applied in the width direction, which causes a problem in operation due to a buckling phenomenon in the rough hot rolling process. Duplex stainless steel is used to impart the given properties of hot-rolled sheet.
It is necessary to contain a predetermined amount or more of δ ferrite. Usually, in the case of an austenite single phase or a ferrite single phase, the bond strength of the crystal grain boundaries is strong and the effect of promoting recrystallization accompanying deformation is large. However, the bonding strength between the grain boundaries of the austenite phase and the ferrite phase is very weak, and the effect of promoting recrystallization accompanying deformation is also small.

Some hot-coils manufactured by adjusting the components so that the δ-ferrite amount X defined by the formula (1) is 8% or less can produce a good hot coil without performing width reduction. Further, as shown in FIG. 2, in the case where the amount of δ ferrite X defined by the formula (1) is adjusted to exceed 8%, cracks occur at the coil ends and the depth of edge cutting increases. To do. In this case, by performing the width reduction, the effect of promoting the dynamic recrystallization is enhanced, and a hot coil with improved degree of edge cutting can be manufactured. However, when the amount of δ ferrite X defined by the equation (1) exceeds 13%, the edge depth sharply increases, and even if width-killing rolling of 5 to 25 mm is performed, the edge depth of the hot coil is increased. Although the thickness is improved to some extent, it is not possible to manufacture a good hot coil that is completely free of sharp edges. From the above points, in the present invention, the amount of δ ferrite X is defined in the range of 0 to 12%.
Although the restriction and the width reduction amount for the δ ferrite phase are effective alone as a means for preventing edge disconnection, the combination of both in this way synergistically prevents defects. Particularly in the range where the effect is remarkable, the amount of δ ferrite defined by the formula (1) is 6 to 10%.

It is known that the balance of the ferrite phase and the austenite phase of the duplex stainless steel is not uniquely determined by the composition but changes with the heat treatment temperature. That is, at high temperature, the structure close to the ferrite single phase is 8
Austenite precipitates with a nose in the temperature range of about 00 to 900 ° C to form a two-phase structure. This is more advantageous in consideration of the difference in S solid solubility and the deformability of the two-phase structure when it is extracted at a high temperature of 1200 ° C. or higher where hot workability is high and ferrite is added, and it is more advantageous. is there. On the other hand, it becomes disadvantageous to perform hot rolling under the condition of less than 900 ° C at which austenite precipitates. Further, in order to make the slab have a uniform structure up to the central portion, it is necessary to perform sufficient heating for 150 minutes or more before hot rolling. As for the slab heating conditions, it is advantageous for hot workability to heat the duplex stainless steel at a high temperature for a long time at which the amount of ferrite produced becomes high. When the heat treatment is performed at a temperature of less than 1200 ° C, austenite is generated from the initial stage of hot rolling, and hot workability is deteriorated. Therefore, the slab extraction temperature is preferably 1200 ° C or higher. Also, in order to make the slab have a uniform structure up to the center, heating before hot rolling is performed at 15
It is necessary to perform it sufficiently for 0 minutes or more.

Next, the components of the stainless steel hot-rolled according to the present invention and the content thereof will be described. C: 0.030 wt% or less An element inevitably contained in steel. When the C content is reduced, the generation of carbides is reduced and the workability is improved. Further, the corrosion resistance and intergranular corrosion cracking resistance are also improved. Such an effect becomes remarkable when the C content is 0.030% by weight or less. N: 0.040% by weight or less Maintains the corrosion resistance of duplex stainless steel in a well-balanced manner, suppresses the precipitation of sigma phase, and exhibits the action of improving toughness.
However, the N content exceeding 0.040% by weight exceeds the solid solubility, so that an excessive amount of nitride is generated in the steel, causing defects in the product. Si: 0.60 wt% or less An element added to steel as a deoxidizer. But Si
If the content exceeds 0.60% by weight, the ability to form a sigma phase increases, and the toughness and corrosion resistance deteriorate.

Mn: 2.00% by weight or less An alloying element useful for improving weldability. However, when the Mn content exceeds 2.00% by weight, the corrosion resistance decreases. Cr: 18.0 to 21.50% by weight It is an essential alloying element for improving the corrosion resistance. If the Cr content is less than 16% by weight, the corrosion resistance is not sufficient. Particularly in a severe corrosive environment where duplex stainless steel is used, if the Cr content is less than 18.00% by weight, the corrosion resistance is insufficient and the ratio to the ferrite phase is lowered. On the contrary, a large amount of Cr exceeding 21.50% by weight
When included, the sigma phase is likely to precipitate and the toughness and weldability deteriorate. Ni: 9.5 to 12.8 wt% It is an alloying element that effectively acts to improve corrosion resistance and is essential for forming austenite or a two-phase structure. Regarding the Ni content, in order to obtain an appropriate structure in relation to the Cr content and the like, the content range is 9.5 to 12.
It was 8% by weight.

Mo: 0.50% by weight or less It is an alloying element that enhances resistance to local corrosion in a corrosive environment containing Cl ⁻ ions in cooperation with Cr. However, Mo is an expensive element, and if added in excess of 0.50% by weight, embrittlement due to sigma phase precipitation occurs in the duplex stainless steel, deteriorating workability and toughness. Cu: 0.20 wt% or less It is an effective alloying element for improving the stress corrosion cracking resistance and the sulfur dioxide corrosion resistance. However, when a large amount of Cu exceeding 0.20% by weight is contained, the hot workability deteriorates. In the stainless steel hot-rolled according to the present invention, in addition to the above alloy elements, one or two of Nb and Ta can be added in a total amount of 0.50 to 1.10% by weight. When Nb and / or Ta is added within this range, the edge-cutting inhibiting action is not impaired, and the actions and effects peculiar to each alloying element are exhibited.

[0013]

【Example】

Example 1: About 40-80 tons of duplex stainless steel was melted to produce a slab. After removing the surface flaws of the obtained slab, the slab was heated in a heating furnace under heating conditions of 1250 ° C. for 170 minutes, extracted, and hot-rolled under the conditions of width reduction shown in Table 1. The obtained hot-rolled steel sheet was observed, and the occurrence of edge cutting at both ends in the sheet width direction was investigated. As can be seen in FIG. 2 showing the investigation result, the amount of occurrence of ear-cutting increases as the amount of δ-ferrite X defined by the formula (1) increases, and the depth of ear-cutting increases sharply with the increase. There is.
Further, in the case of width reduction rolling, the edge depth is reduced as compared with the case of width-width rolling. Even if width reduction rolling is performed, the amount of δ ferrite defined by the formula (1) is 10
If the ratio exceeds%, the depth of ear loss does not become zero even if the occurrence of ear loss is reduced, so trimming is necessary, leading to a decrease in yield.

[0014]

[Table 1]

[0015]

As described above, in the present invention, the δ ferrite scale X defined by the formula (1) is 0 to 0.
Adjust the composition so that it will be 12%, and
By performing a width reduction of 25 mm, it is not necessary to reheat during the rough rolling, and it is possible to suppress or reduce the cutting edge that tends to occur at both ends in the plate width direction during hot rolling of duplex stainless steel. You can Therefore, the obtained hot rolled steel sheet,
It is possible to reduce the trimming margin for removing the edge-cutting portion, resulting in a product with high yield.

[Brief description of drawings]

FIG. 1 Metal flow at both ends in the sheet width direction during hot rolling

[Fig. 2] Effects of δ ferrite content and width reduction on ear cutting

[Explanation of symbols]

 1: Upper roll 2: Lower roll S: Slab during hot rolling

Claims (5)

[Claims] 1. A hot rolled stainless steel is represented by the formula (1):
The component is adjusted so that the δ-ferrite amount X defined by is 0 to 12%, rough rolling is performed with a width reduction amount of -10 to 25 mm, and then hot rolling is performed to a target plate thickness. Hot rolling method for stainless steel with suppressed generation. X = 3.0 × (Cr% + 1.5 × Si% + Mo%) − 2.8 × (Ni% + 0.5 × Mn% + 0.5 × Cu% + 30 × C% + 30 × N%)-19. 8% ・ ・ ・ ・ (1) 2. The hot rolled stainless steel is represented by the formula (1):
After adjusting the components so that the amount of δ ferrite X defined by is 9 to 12%, and roughly rolling with a width reduction amount of 5 to 25 mm,
A hot rolling method for stainless steel, which suppresses the occurrence of edge cutting, characterized by hot rolling to a target plate thickness. X = 3.0 × (Cr% + 1.5 × Si% + Mo%) − 2.8 × (Ni% + 0.5 × Mn% + 0.5 × Cu% + 30 × C% + 30 × N%)-19. 8% ・ ・ ・ ・ (1) 3. The hot rolled stainless steel is represented by the formula (1):
The composition is adjusted so that the amount of δ ferrite X defined by is 0 to 8%, rough rolling is performed with a width reduction amount of -10 to 25 mm, and then hot rolling is performed to a target plate thickness. Hot rolling method for stainless steel with suppressed generation. X = 3.0 × (Cr% + 1.5 × Si% + Mo%) − 2.8 × (Ni% + 0.5 × Mn% + 0.5 × Cu% + 30 × C% + 30 × N%)-19. 8% ・ ・ ・ ・ (1) 4. C: 0.030% by weight or less, N: 0.0
40% by weight or less, Si: 0.60% by weight or less, Mn:
2.00% by weight or less, Cr: 18.0 to 21.50% by weight, Ni: 9.5 to 12.8% by weight, Mo: 0.50% by weight or less and Cu: 0.20% by weight or less The hot rolling method according to claim 1, wherein the stainless steel is hot rolled. 5. The stainless steel according to claim 4, further comprising one or two of Nb and Ta: 0.50 to 1.10% by weight.
The hot rolling method according to any one of claims 1 to 3, wherein a stainless steel containing is used.