JP3063591B2 - How to prevent intergranular corrosion of stainless steel sheets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for preventing surface corrosion of austenitic stainless steel sheets, and more particularly, to a steel sheet (AsRoll material) that has been rolled before a heat treatment step of stainless steel sheets is subjected to a corrosive environment such as seawater or oil. The present invention relates to a method for preventing intergranular corrosion when exposed to water.

[0002]

2. Description of the Related Art In a case where a stainless steel sheet is managed in an as-rolled state, if it is left in a state where seawater or oil adheres to the surface of an As Roll material, intergranular corrosion occurs. Further, if heat treatment is performed in this state, intergranular corrosion will further progress, and after heat treatment, the entire surface must be polished with a grinder or the like to remove intergranular corrosion. However, polishing the entire surface of the steel sheet is not preferable because it hinders delivery and productivity.

[0003] In general, as measures for preventing intergranular corrosion of austenitic stainless steel, a method of reducing the carbon content in the steel, or the addition of a stabilizing element such as Ti or Nb to reduce the carbon content of Ti or Nb. There is a method of fixing as carbide. However, low carbonization requires the addition of expensive decarburizing elements during scouring, and there is a problem that the decarburization scouring lowers the Cr yield. Further, when a stabilizing element is added, in addition to fixing carbon, it binds to elements such as N ₂ and O ₂ in molten steel and becomes nonmetallic inclusions, which causes surface flaws.

[0004] As a means for solving such a problem, for example, the surface of a semi-finished product of stainless steel is heated to a melting point of Cr carbide or nonmetallic inclusions or higher and melted to an appropriate depth to thereby prevent intergranular corrosion of stainless steel. A method for improving the cleanliness and surface cleanliness has been proposed (Japanese Patent Publication No. Sho 62-5).
No. 3569). However, this method consumes an enormous amount of energy to heat a stainless steel piece to 2000 ° C. or more by a plasma arc, requires expensive special equipment, and further steps to melt the surface layer by 1 to 15 mm. There are problems such as worsening of the stay.

As a means for improving the intergranular corrosion resistance of a B-containing austenitic stainless steel, a stainless steel containing Mo, Nb, or Ta has been proposed (see Japanese Patent Application Laid-Open No. 53-19915). ). But,
In the case of this intergranular corrosion-resistant stainless steel, Nb, Ta, and the like have a function of forming carbides and stabilizing Cr,
When used at 600 to 800 ° C. for a long time, embrittlement and reheat cracking may occur. In addition, material costs must be increased.

Further, there has been proposed a method of improving the intergranular corrosion resistance when used in a sensitized region by performing a specific shot peening on the surface of an austenitic stainless steel material and then performing an overaging treatment. (Japanese
No. 125811). However, this method is not preferable in terms of productivity because it requires a long time of 150 hours for the overaging treatment.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and does not require expensive special equipment, does not have the problem of material cost increase, and can shorten the heat treatment time. In addition, it is intended to propose a means for preventing surface corrosion of an austenitic stainless steel sheet, which does not require any overall polishing after heat treatment.

[0008]

The method for preventing intergranular corrosion of a stainless steel sheet according to the present invention is directed to a heat treatment step when water wet or oil adhesion is observed on the as-rolled steel sheet (As Roll material). It is characterized in that shot blasting and pickling are performed beforehand.

In the case of the As Roll material, if the steel sheet is left in a state where seawater or oil adheres to the steel sheet surface, as described above, grain boundary corrosion occurs in the base metal layer through the surface scale layer.
If heat treatment (solution heat treatment) is performed in this state, the intergranular corrosion further progresses. After the heat treatment, the surface of the steel sheet must be entirely polished to remove intergranular corrosion. Therefore, the present invention is intended to eliminate intergranular corrosion of the As Roll material before the heat treatment so that the entire surface is not polished after the heat treatment.
s Roll material is subjected to shot blasting and then pickling.

[0010] That is, when shot blasting is performed on the As Roll material, cracks are generated on the surface scale and a part of the scale falls off. When pickling is performed in this state, the acid is soaked from the cracks in the As Roll material, the base metal is melted and descaled, and at the same time, intergranular corrosion is almost completely removed. Further, since the As Roll material is commercialized through a shot blasting and pickling process after a heat treatment in a normal process, even if intergranular corrosion is not completely removed by shot blasting and pickling before the heat treatment, the Since the degree of residual is extremely small, it is completely removed by shot blasting and pickling after heat treatment. Therefore, it is not necessary to polish the entire surface of the steel sheet after the heat treatment.

Note that, in the present invention, As before the heat treatment is performed.
Shot blasting is used as a means for removing intergranular corrosion of the Roll material because a crack is generated on the scale surface of the As Roll material to facilitate the penetration of acid by pickling.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a manufacturing process diagram of a stainless steel plate showing a method for preventing intergranular corrosion of a stainless steel plate according to the present invention, and FIG. 2 is a drawing of the stainless steel plate in a grain boundary corrosion removing process (shot blast, pickling). It is explanatory drawing which shows a surface form, (a) is the state before shot blast, (b) is the state after shot blast, (c) is the state at the time of pickling, (d)
Is a state after pickling, 1 is an As Roll material, 2 is a base metal, 3 is a scale, 4 is intergranular corrosion, and 5 is a crack.

That is, according to the method of the present invention, first, as shown in FIG. 1, the appearance of an As Roll steel sheet is visually observed. Introduced to the heat treatment step, if water wet or oil adhesion is confirmed on the steel sheet surface, introduce the steel sheet to the shot blasting step, apply shot blasting to the steel sheet surface, and then introduce to the pickling step .

As shown in FIG. 2A, a scale (thickness: about 10 μm) 3 is usually formed on the surface of a base metal 2 as shown in FIG. In addition, in the case of a steel sheet with water wet or oil adhesion on the steel sheet surface, the grain boundary corrosion 4
And the depth 1 of the intergranular corrosion 4 is about 20 μm. When the steel sheet on which the intergranular corrosion 4 has occurred is subjected to shot blasting, as shown in FIG. 2B, cracks 5 enter the scale 3 on the steel sheet surface, and at the same time, a part of the scale is crushed and falls off. When pickling is performed in this state, as shown in FIG. 2 (c), the acid penetrates the cracks generated by the shot blast, soaks into the scale layer, dissolves the base metal 2, and is descaled. Therefore, intergranular corrosion 4 is almost completely removed at this time.

The As Roll steel sheet thus pre-treated (shot blasted, pickled) is then introduced into a normal solution heat treatment step, where it is subjected to a solution heat treatment. As is well known, the solution heat treatment is a heat treatment for removing internal stress generated by rolling, recrystallizing a worked structure, restoring ductility, and dissolving carbides and the like precipitated by rolling. And then quenching. Cooling is performed by rapid cooling by a suitable method such as blast, water spraying, or water cooling.

Thereafter, the product is produced through a usual shot blasting step and pickling step. According to the present invention, intergranular corrosion 4 is considerably reduced by shot blasting and pickling performed before the solution heat treatment. Therefore, the above-mentioned residual intergranular corrosion is completely removed by shot blasting and pickling after the heat treatment in the usual process.

In the conventional process, if intergranular corrosion is confirmed after the inspection, the entire surface is polished with a grinder or the like to remove the intergranular corrosion as described above.

[0018]

EXAMPLE A SUS304 material having the composition shown in Table 1 was used as a test steel sheet (2000 mm in width and 16.0 mm in thickness). The as-rolled steel sheet was subjected to the conditions shown in Table 2 before the solution heat treatment. After performing shot blasting and pickling, a steel plate (length 6000) subjected to ordinary solution heat treatment, straightening (leveler), shot blasting, cutting, and pickling treatments
mm) are shown in Table 3 in comparison with a case where the same test material was manufactured by the conventional method (without shot blasting and pickling before heat treatment).

As is clear from the results shown in Table 3, in the case of the conventional method in which the solution treatment was performed directly without performing shot blasting and pickling prior to the solution treatment, 6/100 of the treated steel sheets were used.
Zero grain boundary corrosion occurred and required overall polishing,
In the present invention, intergranular corrosion was not confirmed in 300 of the 400 treated steel sheets, and polishing was not required on the entire surface.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

As described above, according to the method of the present invention, intergranular corrosion can be almost completely removed by shot blasting and pickling before heat treatment.
Even if the ll material is left in a state where seawater or oil adheres to the surface, it is not necessary to perform full-surface polishing by a grinder or the like after the heat treatment, and the method of the present invention does not require any special equipment or equipment. Therefore, productivity and manufacturing cost can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a manufacturing process diagram of a stainless steel sheet showing a method for preventing intergranular corrosion of the stainless steel sheet according to the present invention.

FIG. 2 Grain boundary corrosion removal process (shot blasting, pickling)
FIG. 4 is an explanatory view showing the surface morphology of the stainless steel plate in FIG.
(A) shows a state before shot blasting, (b) shows a state after shot blasting, (c) shows a state at the time of pickling, and (d) shows a state after pickling.

[Explanation of symbols]

 Reference Signs List 1 As Roll material 2 Base metal 3 Scale 4 Intergranular corrosion 5 Crack

Claims (1)

(57) [Claims] 1. A solution heat treatment of a steel sheet as rolled,
In the process of straightening, shot blasting, cutting, and pickling to produce a product, if water wetting or oil adhesion is observed on the as-rolled steel sheet, the steel sheet is shot blasted before the heat treatment step. And a method for preventing intergranular corrosion of a stainless steel sheet, which comprises subjecting the steel sheet to a pickling treatment.