JP2002220625A - Stainless steel strip with superior surface luster and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel strip with superior luster, and the manufacturing method which can save labor in annealing and a cold rolling process and abbreviate a polishing process, by adopting a condition for reducing surface roughness of metal/ oxidized scale interface during annealing a hot rolled sheet. SOLUTION: A manufacturing method for hot rolled and annealed sheet comprises; winding up the stainless steel strip at lower temperature than 600 deg.C after finishing hot rolling; soaking and keeping it at 700 deg.C or more and 900 deg.C or less, for 1 to 30 hours, in the sequent annealing process; then soaking and keeping it at 950 deg.C or more and 1200 deg.C or less, for one minute or less; and acid pickling. A method for manufacturing a cold rolled and annealed sheet comprises; winding up the stainless steel strip at less than 600 deg.C after finishing hot rolling; soaking and keeping it at 700 deg.C or higher and 900 deg.C or lower, for 1 to 30 hours, in the sequent annealing process; then acid pickling, cold rolling, and bright annealing it, or annealing/acid pickling it. Thereby, corroded grooves in grain boundaries, which are formed during acid pickling of the hot rolled sheet and cause reduction of the surface luster in a final product, are reduced; a surface micro cratering rate is reduced to 5% or less on a cold rolled, annealed, pickled, and polished material, and to 10% or less in a bright annealed material; and a product with the high luster is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a stainless steel strip having excellent surface gloss and having a small surface depression when manufacturing austenitic stainless steel, martensitic stainless steel or ferrite + austenite duplex stainless steel strip. And a method for producing the same.

[0002]

2. Description of the Related Art Hot-rolled steel strips of austenitic stainless steel, martensitic stainless steel and ferritic + austenite duplex stainless steel are usually wound at a temperature of 750 ° C. to 850 ° C. after hot rolling, and then annealed. Pickling is taking place. One purpose of this annealing is a solution heat treatment for eliminating a Cr-depleted layer caused by the formation of Cr-based carbide generated during hot rolling and winding.

However, when annealing is performed at around 1000 ° C., surface oxidation proceeds, and the Cr-deficient layer is not necessarily eliminated. The formation of the oxide scale promotes the formation of the Cr-deficient layer. During pickling due to the increase of Cr deficient layer,
Grain boundaries including a Cr-deficient layer on the surface are easily eroded, resulting in a wide and deep surface depression (see FIG. 1). The unevenness of the oxide scale and the matrix metal interface generated during the cooling process and the annealing from the hot-rolling winding is reflected in the surface roughness after annealing and pickling. If the surface roughness is large, it causes poor gloss. .

[0004] In order to remove the surface irregularities caused by the irregularities of the interface between the metal and the oxide scale, the sheet passing speed was reduced in order to lengthen the pickling time during annealing and pickling, or the pickled hot-rolled coil was ground. After that, a method of cold rolling or a method of reducing the surface roughness by increasing the rolling reduction per pass in cold rolling in order to reduce the depth of the surface roughness is adopted. I have. Further, a mechanical descaling method disclosed in Japanese Patent Application Laid-Open No. 2-73918, in which an abrasive such as iron sand is sprayed together with high-pressure water onto a hot-rolled steel strip immediately after hot rolling to smooth the scale / metal interface. Are known.

[0005]

However, the mechanical descaling method at the time of hot rolling also has a problem in the uniformity of the descaling, so that the stripping speed is reduced in the annealing step, the surface defects are removed in the grinding, and the cooling with a large reduction amount is performed. The operation of reducing the surface roughness by cold rolling increases the process load and significantly reduces productivity. In the case of a bright annealed product which is cold-rolled after removing surface defects due to grinding, the surface irregularities are not completely eliminated by the cold rolling by polishing. The present invention has been devised to solve such a problem. By adopting conditions for reducing the surface roughness of the interface between the metal and the oxide scale during annealing of a hot-rolled sheet, annealing and cold-rolling can be performed. An object of the present invention is to provide a stainless steel strip excellent in gloss and a method for producing the same, which enables labor saving in a rolling step and omission of a polishing step.

[0006]

According to the production method of the present invention, in order to achieve the object, a hot-rolled annealed sheet is prepared by winding a hot-rolled stainless steel strip at a temperature of less than 600 ° C. In the annealing step, after soaking at 700 ° C. to 900 ° C. for 1 hour to 30 hours, soaking at 950 ° C. to 1200 ° C. for 1 minute or less, and then pickling. Regarding the method for producing a cold-rolled annealed sheet, the stainless steel strip after hot rolling is wound at a temperature of less than 600 ° C.
After holding the soaking for 1 hour to 30 hours at ℃ to 900 ° C,
Pickling, cold rolling, bright annealing or annealing and pickling are performed.

The surface of the obtained hot-rolled annealed steel strip has an average grain boundary depth of 2.0 μm or less. The cold-rolled annealed steel strip is # 100 emery paper 50 in the direction perpendicular to the rolling direction.
Each of the bright annealed steel strips having a surface microdentation rate of 5% or less after repeated reciprocating polishing and having a surface microdentation rate of 10% or less is a steel strip excellent in surface gloss with few surface microdentations. In addition,
The present inventors, using an image analysis processing device, by using the image analysis processing device, based on the depth and area of the minute dents remaining on the surface of the final product, Defined as follows. In addition, this minute dent can be measured by a scanning probe microscope, and is mainly 0.2 to 3.0 μm.
m is observed in the range. (A) Surface micro-depression; depth of 0.2 μm or more, 0.7
Each area is 300 mm in a rectangular area of mm × 0.9 mm.
μm ² or more

[0008]

The present inventors have conducted various investigations on the causes of the reduction of the surface gloss of the stainless steel strip as to the surface unevenness of the oxide scale / metal interface of the hot-rolled and annealed material before pickling. Studied. As a result, it was clarified that the occurrence of surface depressions was mainly caused by grain boundary erosion at the time of pickling, which occurred at the recrystallized austenite grain boundaries formed after hot rolling. In other words, in the vicinity of this temperature, Cr-based carbides precipitate at the grain boundaries, and a Cr-deficient layer is generated due to the precipitates, and the scale of the surface layer reacts with the scale mainly composed of Cr, and at this time, the diffusion speed is high. In the vicinity of the grain boundary, a large amount of the Cr-deficient layer is generated. Therefore, the grain boundary erosion increases during pickling of the hot-rolled annealed material.

Therefore, it is desirable to avoid keeping the hot-rolled steel strip at a high temperature of 600 ° C. or higher in order to suppress the precipitation of Cr carbide and the formation of oxide scale in the cooling process after hot rolling. Furthermore, when annealing for a long time in the temperature and time range in the hot-rolled steel strip wound at a low temperature, the Cr-depleted layer generated at the grain boundaries due to the diffusion of Cr is reduced, and grain boundary erosion during hot-rolled annealing pickling is suppressed. It has been found that a stainless steel strip having excellent gloss can be obtained as a result.

The reasons for setting conditions in the method of the present invention will be described below. During the process of cooling the hot-rolled steel strip from the winding temperature to room temperature, oxide scale is generated in the hot-rolled steel strip, and a Cr-deficient layer is formed at the grain boundaries due to precipitation of Cr carbide. By keeping the temperature of the hot-rolled steel strip as low as possible,
Scale formation can be suppressed, and the amount of the Cr-deficient layer formed can be reduced. Therefore, the winding temperature is set low so that the hot-rolled steel strip is kept as low as possible.
Specifically, the hot-rolled steel strip is wound at a temperature of less than 600 ° C.
Suppress scale generation.

[0011] The winding temperature in hot rolling is regulated, and C at grain boundaries is controlled.
Although the formation of the r-deficient layer is suppressed as much as possible,
A depletion layer exists. In order to further reduce the Cr deficiency layer, it is effective to subject the hot-rolled steel strip to long-term annealing to form Cr carbides and then actively utilize the grain boundary diffusion of Cr from the parent phase. It is. In order to effectively utilize the grain boundary diffusion of Cr from the mother phase, a temperature of 700 ° C. or higher is required. However, when heating to a temperature exceeding 900 ° C., oxidation is promoted more than the grain boundary diffusion of Cr. A deficiency layer is generated. Cr
Heating is required for about one hour to completely precipitate carbides. Therefore, the heat treatment condition for achieving the above object is soaking at a temperature of 700 ° C. or more and 900 ° C. or less for 1 hour or more. The soaking time is 30 considering the productivity.
Less than an hour is desirable. For the hot-rolled steel strip, the ultimate material temperature was set to 95 in order to dissolve precipitated Cr carbide.
Continuous annealing is performed at a temperature equal to or higher than 0 ° C and equal to or lower than 1200 ° C for 1 minute or less. 950 ° C. to completely dissolve the Cr carbide
The above temperature is required, and at a temperature of 1200 ° C., a complete solid solution is formed, so that there is no need to heat further.
In order to reduce the amount of the oxide scale layer, the annealing time is preferably as short as 1 minute or less.

As described above, by setting the winding temperature of the steel strip after hot rolling and the annealing temperature and time of the long-time annealing of the hot-rolled steel strip within the ranges specified in the present invention, the vicinity of the grain boundary can be improved. The Cr-deficient layer region becomes smaller, and the grain boundary depth becomes significantly smaller. As a result, measures to increase the rolling speed during annealing of hot-rolled steel strip and to increase the rolling reduction per pass in cold rolling to reduce surface roughness, which were required by the conventional method to improve surface gloss, etc. Simplifies the cold rolling method due to the elimination of hot rolling, makes it possible to omit the step of grinding the hot-rolled coil after pickling, etc. It is possible to obtain a cold-rolled, annealed and pickled product or a bright annealed product.

[0013]

EXAMPLES Three types of austenitic stainless steel, two types of martensitic stainless steel, and one type of (ferrite + austenite) duplex stainless steel shown in Table 1 were melted in an electric furnace and a converter, Each slab was manufactured by a continuous casting method after degassing. 3.8mm each slab
After hot rolling at each temperature, annealing was performed for a long time at each temperature. The hot-rolled steel strip is further subjected to continuous annealing and pickling, and the cold-rolled steel sheet is further cold-rolled to 1.0 mm after pickling and then annealed (soaking at 1050 ° C. for 10 s) —pickling or bright annealing (soaking at 1030 ° C.). 10s) to obtain the final product. The winding temperature after hot rolling and the conditions for long-time annealing of the hot-rolled steel strip were varied as shown in Tables 2 and 3, and the grain boundary depth for the hot-rolled steel strip and the cold-rolled steel strip for the cold-rolled steel strip were changed. Investigated the surface micro-dent ratio.

[0014]

[0015]

[0016]

With respect to the product after the hot-rolled steel strip annealing, the depth of the surface grain boundary of the hot-rolled annealed-pickled material was set to 10 using a laser microscope.
Table 4 shows the results of the average depth measurement. In the method of the present invention, a steel strip having an average grain boundary depth of 2.0 μm or less is obtained. On the other hand, in the comparative method deviating from the present invention under the conditions of the winding temperature, the long-time annealing temperature / time, and the hot-rolled sheet annealing temperature / time, only those having an average crystal grain boundary depth exceeding 2 μm were obtained.

[0018]

Table 5 shows the percentage of surface micro-dents after the surface of the cold-rolled annealed pickling material was polished 50 times with # 1000 emery paper in the direction perpendicular to the rolling direction. Table 6 shows the surface micro-dents of the bright annealed material. It is a result of obtaining the ratio by an image analyzer. According to the method of the present invention, a stainless steel strip having less surface dents and a surface micro dent rate of less than 10% with a bright annealed material having a surface micro dent rate of 5% or less in a polished cold-rolled annealed pickling material, and having an excellent surface gloss. Obtained. On the other hand, in the comparative method which deviates from the present invention under the conditions of the winding temperature and the temperature and time of the long-time annealing, both the polished cold-rolled annealed pickling material and the bright annealed material have a fine surface dent ratio of more than 5% and 10%, respectively. ing.

FIG. 1 shows the relationship between the surface minute dent ratio of the bright annealed material and the relationship between the long annealing temperature and the holding time. When the temperature is from 700 ° C. to 900 ° C., which is the long-time annealing range in the present invention, and the soaking time is 1 hour or more, the surface minute dent is as small as 10% or less. In Test Nos. 21 and 32 in which the soaking time was less than 1 hour, and in Test Nos. 20 and 29 in which the soaking temperature was low, the diffusion rate of Cr was not sufficiently performed, and the rate of surface microdentation was large. In Test Nos. 23 and 30, in which the soaking temperature was high, the amount of oxide scale generated was larger than the grain boundary diffusion of Cr, and the generation of a Cr-depleted layer on the surface was promoted. Was. Further, the test number 25 having a high winding temperature is:
Perhaps because the generation amount of the Cr-deficient layer was too large and the crystal grain boundary depth was too deep, the rate of surface microdentation after cold rolling was large.
According to the method of the present invention, a stainless steel strip having less surface dents and a surface micro dent rate of less than 10% with a bright annealed material having a surface micro dent rate of 5% or less in a polished cold-rolled annealed pickling material, and having an excellent surface gloss. Obtained.

[0021]

[0022]

[0023]

As described above, in the present invention, by controlling the hot-rolling coiling temperature of stainless steel and the long-term annealing conditions of the hot-rolled steel strip within the range specified in the present invention, the surface of the steel sheet is controlled. The grain boundary erosion depth, which causes a decrease in gloss, is reduced, so that a hot rolled annealed material or a bright annealed material having excellent surface gloss can be manufactured. Moreover, such as increasing the passing speed during hot-rolled steel strip annealing and omitting the intermediate polishing step for bright annealed materials,
It is also very effective in improving productivity.

[Brief description of the drawings]

FIG. 1 Formation of oxide scale during annealing and grain boundary C
Schematic diagram (a) for explaining the appearance of the r-deficient layer and schematic diagram (b) for explaining the situation in which the grain boundary grooves after the pickling are depressed.

FIG. 2 is a surface observation simulated diagram of a plate material having a depression on the surface.

FIG. 3 is a graph in which the surface micro-dent ratio of the bright annealed material is arranged in relation to a long-time annealing temperature and a holding time.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kenichi Morimoto 4976 Nomura Minamicho, Shinnanyo-shi, Yamaguchi Prefecture F-term (reference) 4N037 EC01 FE02 FF02 FF03 FH01 HA05 JA06

Claims (8)

[Claims] When manufacturing a hot rolled steel strip of stainless steel,
After the completion of the hot rolling, the film is wound at a temperature of less than 600 ° C., and in the next annealing step, a soaking at a temperature of 700 ° C. or more and 900 ° C. or less for 1 hour or more and 30 hours or less is performed.
A method for producing a stainless steel strip having excellent surface gloss, wherein annealing is performed at a temperature of 00 ° C. or less for one minute or less and pickling is performed. 2. A stainless steel strip having an excellent surface gloss and having an average grain boundary depth of 2.0 μm or less on the surface of the steel strip produced by the method of claim 1. 3. A stainless steel strip having a grain boundary depth of 2.0 μm or less according to claim 2, wherein the stainless steel strip is subjected to cold rolling, annealing, and pickling. Excellent stainless steel strip manufacturing method. 4. A stainless steel strip having a grain boundary depth of 2.0 μm or less according to claim 2, which is subjected to cold rolling and bright annealing. Manufacturing method of stainless steel strip. 5. When manufacturing a cold rolled steel strip of stainless steel,
After the completion of the hot rolling, the film is wound at a temperature of less than 600 ° C., and in a subsequent annealing step, is soaked at a temperature of 700 ° C. or more and 900 ° C. or less for 1 hour or more and 30 hours or less.
A method for producing a stainless steel strip having excellent surface gloss, characterized by performing annealing and pickling. 6. When manufacturing a cold rolled steel strip of stainless steel,
After the completion of the hot rolling, the film is wound at a temperature of less than 600 ° C., and in a subsequent annealing step, is soaked at a temperature of 700 ° C. or more and 900 ° C. or less for 1 hour or more and 30 hours or less. A method for producing a stainless steel strip having excellent surface gloss, characterized by performing bright annealing. 7. A fine surface defined by the following (a) of the surface of the steel strip produced by the method according to claim 3 or 5 and polished 50 times reciprocally with # 1000 emery paper in a direction perpendicular to the rolling direction. Stainless steel strip with excellent surface gloss with dent ratio of 5% or less. (A) Surface micro-depression; depth of 0.2 μm or more, 0.7
Each area is 300 mm in a rectangular area of mm × 0.9 mm.
μm ² or more 8. A stainless steel strip produced by the method according to claim 4 or 6 and having excellent surface gloss having a surface minute dent ratio defined by the following (a) of 10% or less after bright annealing. (A) Surface micro-depression; depth of 0.2 μm or more, 0.7
Each area is 300 mm in a rectangular area of mm × 0.9 mm.
μm ² or more