JPS61199084A - Manufacture of cr stainless steel sheet - Google Patents

Abstract

PURPOSE:To obtain product having good surface even if surface grinding before cold rolling is omitted, by descaling mechanically and previously hot rolled Cr stainless steel plate, then regulating concn. and temp. of nitrofluoric acid, at descaling thereby. CONSTITUTION:Hot rolled steel plate (or strip) of Cr stainless steel is descaled mechanically and previously by shot blasting, etc. Thereafter, the plate is de scaled by dipping or spraying of aqueous soln. of nitrofluoric acid contg. 120-180g/l HNO3 and 40-120g/l HF at 50-90 deg.C liquid temp., cold rolled and bright annealed without coil grinder polishing. In this way, gold dust flaw after cold rolling is not caused and product having good surface can be obtd. Pickling process can be smplified.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is directed to the production of ferritic or martensitic stainless steel plates or steel strips (hereinafter collectively referred to as Cr stainless steel plates) for obtaining good surface properties. This relates to a manufacturing method.

[Conventional technology]

Cold-rolled products of general Ks Cr stainless steel sheets are manufactured as follows. That is, a continuously cast slab is heated and hot rolled to produce a hot rolled steel plate or copper strip (hereinafter collectively referred to as a hot rolled sheet) with a thickness of 2 to 8 mm, and then this is passed through a continuous annealing furnace as necessary. Alternatively, after annealing in a ZEX furnace, descaling is performed, and CG grinding (
Coil grinder grinding) and cold rolling to product plate thickness, followed by annealing, pickling, skin and base rolling to produce 2B products, or bright annealing and skin pass rolling to produce BAiR products. do.

Descaling of hot-rolled sheets is carried out by mechanical preliminary descaling such as shot blasting, followed by immersion in an inorganic acid bath such as sulfuric acid, hydrochloric acid, nitric hydrofluoric acid, etc., or by spraying or electrolysis. ing.

For example, a method for descaling long-time annealed copper strips using a box furnace for 5US430 stainless steel is to perform mechanical preliminary treatments such as shaving and doplast, then immerse them in sulfuric acid or hydrochloric acid solution, and then immerse them in nitric-fluoric acid. Alternatively, it is generally carried out to complete descaling by spraying, or to further immerse the material in a nitric acid solution to whiten and passivate the surface.

A SUS 43 Q steel plate that has been hot rolled, annealed in a Dex furnace, and pre-descaled by shot blasting is immersed in sulfuric acid or hydrochloric acid in the conventional process, then sprayed with nitric-fluoric acid, and then pickled in nitric acid. Figure 1 (
As shown in the scanning electron microscopic images of , ), the grain boundaries are eroded in the form of grooves, resulting in microscopic corrosion grooves. If such a hot-rolled sheet is cold-rolled without CG grinding, the corrosion grooves will be crushed to form an overlapping shape. Particularly in the case of BA products, when a vinyl film or the like applied to the surface to retain impurities is peeled off, an overlapping area is raised, causing a defect called gold dust that shines in the form of dots, which significantly impairs the surface quality of the product. As a countermeasure, after pickling, the surface was uniformly ground by CG grinding to remove corrosion grooves.

In recent years, in the manufacturing process of stainless steel sheets,
In order to reduce manufacturing costs, it is desired to omit the CG grinding process. For example, according to Japanese Patent Publication No. 54-72728, after descaling a hot rolled or hot rolled annealed stainless copper strip by shot peening, an acid solution is applied to the surface layer t-5 to tI14 before cold rolling. 40
A method has been proposed for preventing the generation of cold dust in cold-rolled products by removing it by dissolving it in a μ bath, and it is said that the acid bath solution is not involved in acid planting.

However, the present inventors found that with hot-rolled sheets or Dex furnace annealed hot-rolled sheets, 5 to 40
Even after removing the surface layer, corrosion grooves occur and significant gold dust appears in cold-rolled products, which corrodes with nitric-fluoric acid pickling? It has been found that there is an appropriate performance range that does not cause #.

[Problem that the invention seeks to solve]

An object of the present invention is to provide a manufacturing method for producing a Cr-based stainless steel sheet, in which good surface quality can be obtained even if the CG grinding step before cold rolling is omitted.

[Means for solving problems]

In the present invention, after performing mechanical preliminary descaling such as shot blasting on a hot-rolled Cr-based stainless steel plate, it produces 120 to 1801 vll of HNO3 and 40 to 120 vll of HF.
It is characterized by cold rolling without descaling by immersion or spraying in a nitric-fluoric acid bath solution containing 1/l at a liquid temperature of 5o to 90°C, and without CG grinding.

The target material of the present invention is a hot-rolled Cr-based stainless steel plate, which is either hot-rolled without being annealed or annealed in a Dex furnace after hot-rolling so that no Cr-deficient region exists. Perform preliminary descaling. Mechanical preliminary descaling is performed using known means such as shot blasting, slurry cylinder pressure projection, and roll bending. The subsequent descaling is carried out by immersing the hot-rolled sheet in the above-mentioned good quality nitric-fluoric acid solution or by spraying the solution onto the hot-rolled sheet. HNO3 less than 1201/l or HF 40F/
Below A, descaling is poor and it takes a long time to complete descaling. In addition, 1 (if the NO3 concentration exceeds 180 IA or the HF' concentration exceeds 120 gll,
As the surface of the stainless steel plate becomes rough, NOx generation increases dramatically. If the solution temperature is less than 50°C, it will take a long time to descale, and if it exceeds 90°C, water will evaporate, making it difficult to carry out industrially. Therefore, HNO3*gke120~
18011/l, HF yield 40~1201//l
, the temperature was limited to 50-90°C.

The descaled stainless steel sheet may be cold rolled to the product thickness without intermediate annealing, or it may be cold rolled after intermediate annealing at an intermediate thickness. The inventors conducted a detailed XIE on the mechanism of corrosion groove formation in various acid solutions using 5US430 hot-rolled sheets and gas furnace annealed hot-rolled sheets. It was found that the concentration and temperature of the hydrofluoric acid solution exist.

In general, the characteristics of acid solutions are that non-oxidizing acids such as hydrochloric acid and sulfuric acid are so-called fully active dissolving acid solutions that uniformly dissolve the inside of grains and grain boundaries, whereas highly oxidizing nitric acid and hydrofluoric acid The mixed acid has grain boundary selective solubility as used in grain boundary corrosion tests as JrSG0574 "Nitric acid-fluoride water accumulated acid corrosion test method for stainless steel". For this reason, as a countermeasure against corrosion grooves, hydrochloric acid or sulfuric acid pickling is commonly used for hot-rolled sheets annealed at high temperatures and short times in continuous annealing furnaces that have Cr-depleted layers at the grain boundaries, and in this case, products that do not generate gold dust can be obtained. It is being In the case of ♂Tux furnace annealed sheets, since the holding time and temperature rise/cooling time during annealing are long, the Cr-depleted layer that is once formed due to the precipitation of Cr carbonitrides at the grain boundaries is recovered by the diffusion of Cr, resulting in intergranular corrosion grooves. should not occur.

In addition, even for hot-rolled sheets that are not annealed, it is necessary to perform rapid cooling after hot rolling to prevent the precipitation of Cr carbonitrides, or to diffuse Cr even if Cr carbonitrides precipitate and a Cr-depleted layer is formed around it. If slow cooling is performed to recover the Cr
Since there is no depleted layer, intergranular corrosion grooves should not occur.

However, when these Cr-based stainless steels, which do not have a Cr-deficient layer, are pickled with hydrochloric acid or sulfuric acid, intergranular corrosion grooves occur at any concentration and temperature, and when pickled with nitric-hydrofluoric acid, corrosion grooves occur. Conventional thinking regarding corrosion groove susceptibility cannot be applied.

As a result of analyzing the grain boundary properties of DEX furnace annealed plates, the present inventors found that the appropriate concentration of nitric hydrofluoric acid for DEX annealed plates,
It has been shown that descaling can be achieved in a short time without forming corrosion grooves in a temperature range, and that gold dust does not occur even when cold rolling is performed without CG grinding.

〔Example〕

? 5US430 with a thickness of 4m annealed in a Tux furnace. Preliminary descaling with t% spread baking plate shot i-plast,
It was immersed in a solution containing a combination of concentrations ranging from 130 to 2009/l and a fluff of 30 to 2001/1. After pickling, CG
It was cold rolled to a plate thickness of 0.4 m without grinding, and the surface texture after bright annealing was measured. The presence or absence of corrosion grooves after hot rolling and temporary pickling was observed using a scanning electron microscope (magnification: 1000). As shown in the results in Fig. 91, stainless steel produced by the method of the present invention had no corrosion grooves, and even in the BA product, stainless steel with good surface quality was obtained without gold dust. A scanning electron micrograph of the surface after descaling by the method of the present invention is shown in FIG. 1(b).

〔Effect of the invention〕

According to the present invention, hot-rolled Cr stainless steel can be descaled without causing intergranular corrosion, and moreover, can be carried out in a single pickling bath compared to the conventional method.

As a result, even if the CG grinding step, which is costly after descaling, is omitted, cold-dust defects do not occur after cold rolling, and a product with a good surface can be obtained. Moreover, the pickling process can be simplified.

[Brief explanation of drawings]

FIG. 1 is a scanning electron micrograph showing the metal structure of the stainless steel surface after descaling by the conventional method (a) and the method (b) of the present invention. Figure 1 (xlooool

Claims (1)

[Claims] After mechanically descaling a hot-rolled Cr stainless steel plate, it was mixed with 120 to 180 g/l of nitric acid and 40 g/l of hydrofluoric acid.
Using an aqueous solution containing 0 to 120 g/l, the liquid temperature is 50 to 120 g/l.
A method for producing a Cr-based stainless steel sheet, which comprises descaling at 90°C, followed by cold rolling and annealing without surface grinding.