JP2010001504A - Stock for cold rolled stainless steel sheet capable of suppressing occurrence of roping and ear crack can be suppressed, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stock for a cold rolled stainless steel sheet capable of suppressing the occurrence of roping and ear cracks upon cold rolling, and to provide a method for producing the stock for the cold rolled stainless steel sheet. <P>SOLUTION: The stock for the cold rolled stainless steel sheet has a composition containing, by mass, 0.01 to 0.05% C, 0.02 to 0.30% Si, 0.40 to 1.0% Mn, ≤0.05% P, ≤0.01% S, ≤0.02% Al, 0.01 to 0.06% N and 16.0 to 18.0% Cr, and the balance Fe with inevitable impurities. The stock has a composition satisfying -12≤7.5×[%Mn]-54×[%Si]≤4 (wherein, [%Si] is the Si content and [%Mn] is the Mn content), and a structure having a martensitic phase with a Vickers hardness of ≤700 by 25 to 50%. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a material for a stainless cold-rolled steel sheet that is unlikely to cause roping or ear cracking due to cold rolling, and a method for manufacturing the same.

  Ferritic stainless steel sheets are excellent in corrosion resistance and can be easily processed, so they are used in various applications such as building materials, home appliances, kitchen equipment, chemical plants, water tanks, and automobile parts. However, the ferritic stainless steel sheet has a problem that unevenness (so-called roping) is likely to occur on the surface by cold rolling. When roping occurs, the surface gloss peculiar to stainless steel products is impaired, so a technique for suppressing the occurrence of roping is being studied.

  For example, in Patent Document 1, a slab whose component is adjusted so that an index γpot calculated using the content of each element satisfies the range of 20.0 to 40.0 is hot-rolled under predetermined conditions to obtain ferrite crystal grains. A technique for suppressing roping of a ferritic stainless steel sheet is disclosed by adjusting the crystal orientation and then cold rolling under predetermined conditions to further adjust the crystal orientation of the ferrite crystal grains.

In Patent Document 2, a slab whose components are adjusted so that the same index γpot as in Patent Document 1 is 40.0 or less is hot-rolled under predetermined conditions to adjust the ratio of the martensite phase, and then under predetermined conditions. A technique for suppressing roping of a ferritic stainless steel sheet by randomizing the crystal orientation of ferrite crystal grains by cold rolling is disclosed.
However, all of the ferritic stainless steel sheets obtained by these techniques have a roping height of 0.10 μm or more generated by cold rolling, and the effect of suppressing roping is not sufficient.
JP-A-8-49017 Japanese Patent Laid-Open No. 9-217124

  An object of this invention is to provide the raw material for stainless cold-rolled steel sheets which can suppress generation | occurrence | production of roping in cold rolling, and can suppress generation | occurrence | production of an ear crack, and its manufacturing method.

  In addition to Patent Documents 1 and 2, techniques for suppressing roping have been conventionally studied. For example, research on technology that suppresses the occurrence of roping by creating a high-hardness martensite phase in stainless steel cold-rolled steel material and dividing the expansion of ferrite crystal grains by cold rolling is underway. . However, it is difficult to greatly improve the effect of suppressing roping by such a conventional technique.

  The inventors conducted experiments by changing the setting conditions and the components of the stainless steel cold-rolled steel sheet variously until the slab was hot-rolled to obtain the stainless steel cold-rolled steel sheet material, and the mechanism of the occurrence of roping in the cold rolling Was examined. And it investigated in detail paying attention to the high hardness martensite phase producing | generating in the layer form in the raw material for stainless steel cold-rolled steel sheets. As a result, since the martensite phase was lamellar, it was found that the expansion of ferrite crystal grains by cold rolling could not be sufficiently divided, and the hard martensite phase itself could cause roping.

  On the other hand, in order to reduce the amount of ferrite phase colonies that are the main cause of roping, it is essential to secure a certain amount or more of martensite phase. Thus, it has been found that by generating a certain amount of a soft martensite phase, it is possible to effectively suppress the retention of ferrite phase colonies. And since the raw material for stainless steel cold-rolled steel sheets softens, it turned out that the ear crack by cold rolling can also be suppressed.

Next, the effects of the setting conditions until obtaining the stainless steel cold rolled steel sheet material and the components of the stainless steel cold rolled steel sheet material on roping and ear cracking were investigated. as a result,
(a) By specifying the contents of C, Si, Mn, P, S, Al, N, and Cr, and by specifying the finishing temperature and the coiling temperature when hot rolling the slab, it is relatively soft A material for a stainless cold-rolled steel sheet in which a martensite phase is generated can be obtained.
(b) The knowledge that the ratio of the soft martensite phase in the stainless cold-rolled steel sheet can be regulated to suppress roping and ear cracking in cold rolling.

The present invention has been made based on these findings.
That is, the present invention includes C: 0.01 to 0.05 mass%, Si: 0.02 to 0.30 mass%, Mn: 0.40 to 1.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.02 mass% or less, N: 0.01 to 0.06% by mass, Cr: 16.0 to 18.0% by mass, the balance being Fe and inevitable impurities, Si content is [% Si] and Mn content is [% Mn] -12 ≦ 7.5 × [% Mn] −54 × [% Si] ≦ 4
And a structure having a martensite phase with a Vickers hardness of 700 or less and a structure having 25 to 50% of martensitic phase.

The present invention also includes C: 0.01 to 0.05 mass%, Si: 0.02 to 0.30 mass%, Mn: 0.40 to 1.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.02 mass% or less, N: 0.01 to 0.06% by mass, Cr: 16.0 to 18.0% by mass, the balance being Fe and inevitable impurities, Si content is [% Si] and Mn content is [% Mn] -12 ≦ 7.5 × [% Mn] −54 × [% Si] ≦ 4
The slab having a composition satisfying the above conditions is hot-rolled at a finishing temperature of 800 to 1000 ° C., and when the C content is 0.01 mass% or more and less than 0.03 mass%, the temperature is 400 to 600 ° C., and the C content is 0.03 mass% or more and 0.05 It is a method for producing a material for a stainless cold-rolled steel sheet that obtains a structure having 25 to 50% of a martensite phase having a Vickers hardness of 700 or less by winding at 400 to 700 ° C. when the content is less than or equal to mass%.

  In addition, the ratio (%) of the martensite phase is a value (= 100 × M /%) indicating the ratio of the area M of the martensite phase in an arbitrary cross section (the area is A) of the stainless cold-rolled steel sheet material. A). That is, if the ratio of the martensite phase is 35 to 50%, the remainder (that is, 65 to 50%) is ferrite crystal grains.

  ADVANTAGE OF THE INVENTION According to this invention, the raw material for stainless steel cold-rolled steel plates which can suppress the roping and ear crack in cold rolling can be obtained.

First, the reason for limitation of the component of the raw material for stainless steel cold-rolled steel sheets of this invention is demonstrated.
C: 0.01-0.05 mass%
C is an important element that generates a soft martensite phase in a material for a stainless cold-rolled steel sheet. When the C content is less than 0.01% by mass, a soft martensite phase is not sufficiently generated, and thus the effect of suppressing roping in cold rolling cannot be obtained. On the other hand, if it exceeds 0.05% by mass, a high-hardness martensite phase is generated, so that it is easy for ear cracks to occur in cold rolling, which causes breakage of the ferritic stainless steel sheet. Therefore, C is in the range of 0.01 to 0.05 mass%.

Si: 0.02 to 0.30 mass%
Si is an important element that is used as a deoxidizer in the melting stage of ferritic stainless steel and that generates a soft martensite phase in a stainless steel cold rolled steel sheet material. When the Si content is less than 0.02% by mass, the effect of deoxidation cannot be obtained sufficiently. On the other hand, if it exceeds 0.30% by mass, the production of a soft martensite phase is reduced, and the effect of suppressing roping in cold rolling cannot be obtained. Therefore, Si is within the range of 0.02 to 0.30 mass%.

Mn: 0.40 to 1.0 mass%
Mn is an element that generates a soft martensite phase in the material for stainless cold-rolled steel sheets. When the Mn content is less than 0.40% by mass, a soft martensite phase is not sufficiently generated, and thus the effect of suppressing roping in cold rolling cannot be obtained. On the other hand, when it exceeds 1.0 mass%, it will combine with S mentioned later and precipitation of MnS will be accelerated | stimulated and the corrosion resistance of a ferritic stainless steel plate will fall. Therefore, Mn is in the range of 0.40 to 1.0 mass%.

7.5 × [% Mn] −54 × [% Si]: −12 to 4
When the Si content (% by mass) is [% Si] and the Mn content (% by mass) is [% Mn], the balance between [% Si] and [% Mn] is to generate a soft martensite phase. Is important. As already explained, Si decreases the martensite phase and Mn increases the martensite phase. When the value of 7.5 × [% Mn] −54 × [% Si] is less than −12, the amount of martensite phase generated is insufficient, and the effect of suppressing roping becomes small. On the other hand, if it exceeds 4, the amount of martensite phase produced becomes excessive, and the occurrence of ear cracks becomes remarkable. Accordingly, 7.5 × [% Mn] −54 × [% Si] is set within the range of −12 to 4.

P: 0.05 mass% or less P segregates at the grain boundary of ferrite crystal grains and induces brittle fracture. When the P content exceeds 0.05% by mass, in addition to the problem of segregation at the grain boundaries, the material for the stainless cold-rolled steel sheet is hardened by solid solution strengthening, and the ductility is significantly reduced. The drop in ductility of the stainless steel cold rolled steel sheet promotes roping and ear cracking in cold rolling. Therefore, P is 0.05 mass% or less.

S: 0.01% by mass or less S is an element that combines with Mn to generate MnS. MnS becomes the starting point of the ear crack in cold rolling. If the S content exceeds 0.01% by mass, a large amount of MnS precipitates and becomes the starting point of ear cracks, leading to breakage of the ferritic stainless steel sheet. Therefore, S is set to 0.01% by mass or less.
Al: 0.02 mass% or less
Al is an important element that is used as a deoxidizing agent in the melting stage of ferritic stainless steel and generates a soft martensite phase in the material for stainless cold-rolled steel sheets. When the Al content exceeds 0.02% by mass, the formation of a soft martensite phase decreases, and the effect of suppressing roping in cold rolling cannot be obtained. Therefore, Al is 0.02 mass% or less.

N: 0.01 to 0.06 mass%
N is an element that generates a soft martensite phase in the material for stainless cold-rolled steel sheets. When the N content is less than 0.01% by mass, a soft martensite phase is not sufficiently generated, and thus the effect of suppressing roping in cold rolling cannot be obtained. On the other hand, if it exceeds 0.06% by mass, a hard martensite phase is generated, and roping and ear cracking in cold rolling cannot be sufficiently suppressed. Therefore, N is in the range of 0.01 to 0.06 mass%.

Cr: 16.0-18.0% by mass
Cr is an element that forms a passive film on the surface of a ferritic stainless steel sheet to enhance corrosion resistance. If the Cr content is less than 16.0% by mass, sufficient corrosion resistance cannot be obtained. On the other hand, if it exceeds 18.0% by mass, the ferrite phase of the material for stainless cold-rolled steel sheet increases, and a soft martensite phase is not sufficiently generated, so that the effect of suppressing roping in cold rolling cannot be obtained. Therefore, Cr is in the range of 16.0 to 18.0 mass%.

The balance other than the above components is Fe and inevitable impurities. It is desirable to reduce inevitable impurities as much as possible.
Next, the structure of the stainless steel cold rolled steel sheet material will be described.
Soft martensite phase ratio: 25-50%
Here, the soft martensite phase refers to a martensite phase having a Vickers hardness of 700 or less. The roping and the ear crack in cold rolling are caused by the hardened material for the stainless cold-rolled steel sheet, which is a material for cold rolling. Therefore, in the present invention, the formation of a soft martensite phase is promoted by reducing the hard martensite phase. As the index, the ratio of the soft martensite phase (that is, Vickers hardness of 700 or less) to the stainless steel for cold-rolled steel sheets is defined. If the ratio of the soft martensite phase is less than 25%, the material for stainless cold-rolled steel sheets becomes hard, and roping and ear cracking in cold rolling cannot be sufficiently suppressed. On the other hand, if it exceeds 50%, the material for stainless cold-rolled steel sheet becomes hard and the occurrence of ear cracks becomes remarkable. Therefore, the ratio of the soft martensite phase is set in the range of 25 to 50%. The ratio (%) of the soft martensite phase is determined by dividing the area of the arbitrary section of the stainless cold rolled steel sheet material with A, the area of the soft martensite phase in the section with M, and dividing M by A. The value expressed as a percentage (= 100 × M / A).

Next, the manufacturing method of the raw material for stainless steel cold-rolled steel sheets of this invention is demonstrated.
Using a converter, electric furnace, etc., ferritic stainless steel having a predetermined component is melted (so-called primary refining) and further decarburized (so-called secondary refining). The secondary refining preferably employs a strong stirring vacuum oxygen decarburization method (so-called VOD method).
The obtained molten steel is made into a slab by a continuous casting method or an ingot-making method. However, it is preferable to employ a continuous casting method with high productivity.

  Next, the slab is reheated as necessary, and further hot-rolled at a finishing temperature of 800 to 1000 ° C. to form a hot-rolled steel strip. The hot-rolled steel strip has a C content of 0.01 mass% or more and less than 0.03 mass%. When the temperature is 400 to 600 ° C. and the C content is 0.03% by mass or more and 0.05% by mass or less, the coil is wound at 400 to 700 ° C. to obtain a hot rolled coil. In the component of the present invention, when hot rolling is terminated in the temperature range of 800 to 1000 ° C., the amount of austenite phase generated increases. Therefore, hot rolling is finished in this temperature range, and a large amount of austenite phase is generated in the hot-rolled steel strip. Thereafter, the hot-rolled steel strip is cooled to 400 to 700 ° C. (400 to 600 ° C. when the C content is low) and wound to suppress the transformation from austenite to ferrite, and a soft martensite phase is predetermined. Generate at the rate of

  When the finishing temperature of hot rolling is out of the range of 900 to 1000 ° C., the austenite phase of the hot-rolled steel strip is reduced, so that the soft martensite phase of the hot-rolled coil is reduced. Further, when the coiling temperature of the hot-rolled steel strip is less than 400 ° C., the deformation resistance of the hot-rolled steel strip increases, and various defects are likely to occur in the wound hot-rolled coil. On the other hand, when the coiling temperature exceeds 700 ° C. (600 ° C. when the C content is low), the transformation from austenite to ferrite proceeds during cooling of the hot rolled coil, and the formation of a soft martensite phase is inhibited. The

A material obtained by cooling the hot-rolled coil thus manufactured to room temperature is a material for a stainless cold-rolled steel sheet. This raw material for stainless cold-rolled steel sheet can suppress roping and ear cracking in cold rolling.
Next, a method for producing a ferritic stainless steel sheet by cold rolling the stainless cold rolled steel sheet material of the present invention will be described.

  The cold-rolled steel strip is pickled and cold-rolled to form a cold-rolled steel strip. The obtained cold-rolled steel strip is annealed and pickled in order to obtain a ferritic stainless steel plate. Cold rolling may be performed twice or more after intermediate annealing. Further, in order to eliminate the elongation of the yield point, skin pass rolling may be applied to the cold-rolled steel sheet. The rolling reduction of the skin pass rolling is preferably in the range of 0.5 to 1.5%.

  Using an experimental furnace, ferritic stainless steel (30 kg) having the components shown in Table 1 was melted in an argon atmosphere to obtain a slab. The slab was heated to 1200 ° C. and hot-rolled to obtain a hot-rolled steel sheet having a thickness of 4 mm and a width of 150 mm. The obtained hot-rolled steel sheet was cooled at a predetermined temperature for 1 hour and then cooled. The process of holding the hot-rolled steel sheet at a predetermined temperature for 1 hour is a process assuming a heat-retaining effect when the hot-rolled steel strip in actual operation is wound to form a hot-rolled coil. The conditions for hot rolling are as shown in Table 2.

  This experiment was performed using the experimental equipment without using the equipment used for actual operation, and the hot-rolled steel sheet thus obtained corresponds to the material for the stainless cold-rolled steel sheet.

  The obtained hot-rolled steel sheet was pickled and a structure observation specimen (width 10 mm, length 15 mm) was cut out from the center in the width direction of each hot-rolled steel sheet. The central part (1 mm × 1 mm) in the plate thickness direction of the cross section perpendicular to the rolling surface and parallel to the rolling direction of this structure observation specimen was corroded with aqua regia and Murakami reagent, and a photograph was taken with an optical microscope. The martensite phase was identified using the photograph, the area ratio which the martensite phase occupied was calculated by image processing, and the ratio of the martensite phase was calculated | required. The results are shown in Table 2.

Furthermore, the Vickers hardness (measurement load: 10 kg) of the martensite phase of each specimen for tissue observation was measured. 10 points of Vickers hardness were measured for each tissue observation specimen, and the average value of 8 points excluding the maximum and minimum values is shown in Table 2.
The remaining hot-rolled steel sheet from which the structure observation specimen was cut was cold-rolled to obtain a cold-rolled steel sheet having a thickness of 0.8 mm. The surface of the cold-rolled steel sheet was visually observed to investigate ear cracks. Ear cracks were evaluated as crack generation (x) when cracks with a length of 2 mm or more were observed in the width direction. The results are shown in Table 2.

  Further, in order to evaluate the roping of the cold rolled steel sheet, the surface roughness Rmax was measured. The surface roughness Rmax was measured according to JIS standard B0601 (2001), and was measured three times at a measurement length of 50 mm and perpendicular to the rolling direction. An average value of less than 0.10 was evaluated as good (◯) and 0.10 or more was evaluated as bad (×). The results are shown in Table 2.

The invention examples shown in Table 2 (namely, Experiment Nos. 1, 4, 7, 9, 11, 13, 15, 17) are examples in which the components, structure, and production conditions satisfy the scope of the present invention. Roping is small and no ear cracks occur.
Experiments Nos. 2, 3, 5, 6, 8, 10, 12, 14, 16, and 18 in Table 2 are examples in which the components satisfy the scope of the present invention but the manufacturing conditions deviate from the scope of the present invention. . In these examples, large ropings occurred because there were few soft martensite phases.

In Experiment No. 19 in Table 2, since the C content was small, the amount of martensite produced with high hardness was insufficient, and large roping occurred. In Experiment No. 20 in Table 2, since the C content was large, a large amount of high-hardness martensite was generated and ear cracks were generated.
In Experiments Nos. 21, 22, 23, and 26 in Table 2, the contents of Si, Mn, Al, and Cr were outside the scope of the present invention, so that there was little soft martensite phase and large roping occurred.

In experiment No. 24 in Table 2, since the N content was small, soft martensite was insufficient and large roping occurred. In Experiment No. 25, since the N content was large, a large amount of high-hardness martensite was generated and ear cracks were generated.
In Experiment No. 27 in Table 2, since the balance between the Si content and the Mn content is out of the range of the present invention, the martensite phase is small and large roping occurs.

Claims (2)

C: 0.01-0.05 mass%, Si: 0.02-0.30 mass%, Mn: 0.40-1.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.02 mass% or less, N: 0.01-0.06 % By mass, Cr: 16.0 to 18.0% by mass, the balance being Fe and inevitable impurities, Si content is [% Si], Mn content is [% Mn] -12 ≦ 7.5 × [% Mn] −54 × [% Si] ≦ 4
A material for a stainless cold-rolled steel sheet, characterized by having a composition satisfying the above requirement and a structure having a martensite phase with a Vickers hardness of 700 or less of 25 to 50%. C: 0.01-0.05 mass%, Si: 0.02-0.30 mass%, Mn: 0.40-1.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.02 mass% or less, N: 0.01-0.06 % By mass, Cr: 16.0 to 18.0% by mass, the balance being Fe and inevitable impurities, Si content is [% Si], Mn content is [% Mn] -12 ≦ 7.5 × [% Mn] −54 × [% Si] ≦ 4
The slab having a composition satisfying the above conditions is hot-rolled at a finishing temperature of 800 to 1000 ° C., and when the C content is 0.01 mass% or more and less than 0.03 mass%, the temperature is 400 to 600 ° C., and the C content is 0.03 mass% or more and 0.05 A method for producing a material for a stainless cold-rolled steel sheet, wherein a structure having 25 to 50% of a martensite phase having a Vickers hardness of 700 or less is obtained by winding at 400 to 700 ° C. when the content is less than or equal to mass%.