JP2008274328A - Ferritic stainless steel sheet excellent in stretch-flanging formability and producing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel sheet excellent in stretch-flanging formability and producing method therefor. <P>SOLUTION: The ferritic stainless steel sheet excellent in stretch-flanging formability, has the composition composed by mass% of ≤0.015% C, ≤0.15% Si, ≤0.3% Mn, ≤0.04% P, ≤0.005% S, ≤0.08% Al, ≤0.015% N, 20.5-23.5% Cr, 0.3-0.7% Cu, ≤0.5% Ni, 0.2-0.4% Ti, ≤0.015% Nb and the balance Fe with inevitable impurities, and has no portion locally existing ≥10 pieces of spheroidal TiS and the aspect ratio of the ferrite grains (average grain diameter in the rolling direction/average grain diameter in the sheet thickness direction) is ≤2. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a ferritic stainless steel sheet, and more particularly to a ferritic stainless steel sheet excellent in stretch flangeability and a method for producing the same.

  Ferritic stainless steel sheets are excellent in design and corrosion resistance, and are therefore used in various applications such as buildings, transportation equipment, home appliances, and kitchen appliances. Since the ferritic stainless steel sheet is characterized by excellent surface properties, the requirements for its workability have not been severe so far. However, in recent years, when the application range of the ferritic stainless steel sheet is expanded and processed into a complicated shape, excellent press workability, particularly excellent stretch flange workability is required.

Therefore, in Patent Document 1, Cr: 8 to 50%, C and N content is reduced to 0.020% or less, C and N are formed into precipitates by box annealing after hot rolling, and cold rolling is performed at a high rolling rate. Thus, a ferritic stainless steel strip excellent in shape freezing property in which in-plane anisotropy of mechanical properties is reduced by randomizing the crystal orientation of ferrite grains is disclosed. In Patent Document 2, recrystallization is promoted by reducing the amount of C and N to 0.02% or less with Cr: 10 to 35% and by defining the distribution density of TiC and Nb (CN) precipitates of 1 μm or less. Ferritic stainless steel with excellent workability is disclosed. In addition, Patent Document 3 describes a ferritic stainless steel material for heat exchangers in which Cr: 8 to 35%, C and N contents are 0.1% or less, and the Cr, Mo, Si, and Al contents are defined by the plate thickness and operating environment temperature. Is disclosed.
Japanese Patent Laid-Open No. 2003-160846 JP 9-263903 A JP2003-328088

  However, in the ferritic stainless steel strip described in Patent Document 1, the amount of springback is reduced and the shape freezing property is excellent, but excellent stretch flangeability cannot be obtained. In addition, in the ferritic stainless steel described in Patent Document 2, the C and N precipitates are coarsened and softened to improve the stretchability and deep drawability, but excellent stretch flangeability is obtained. Absent. Furthermore, in the ferritic stainless steel material described in Patent Document 3, the steam oxidation resistance is improved, but cracking occurs when stretch flange processing is performed, which is not suitable for press processing of a complicated shape.

  This invention is made | formed in view of such a situation, and it aims at providing the ferritic stainless steel plate excellent in stretch flange workability, and its manufacturing method.

As a result of examining the stretch flangeability of the ferritic stainless steel sheet, the present inventors have found the following.
i) In general cold-rolled steel sheets, stretch flangeability is improved by reducing MnS expanded along the ferrite grain boundaries, but in high corrosion resistance stainless steel sheets such as ferritic stainless steel sheets, such MnS precipitation Despite the small amount, stretch flange workability is inferior.
ii) Cracks in stretch flange processing occur from the part where more than 10 spherical TiS are localized.
iii) If the aspect ratio of ferrite grains (average grain size in the rolling direction / average grain size in the plate thickness direction) of the ferrite grains exceeds 2, the cracks in the stretch flange process tend to occur in parallel to the plate surface.

  The present invention has been made based on such knowledge, and in Mass%, C ≦ 0.015%, Si ≦ 0.15%, Mn ≦ 0.3%, P ≦ 0.04%, S ≦ 0.005%, Al ≦ 0.08%, N ≦ 0.015%, Cr: 20.5-23.5%, Cu: 0.3-0.7%, Ni ≦ 0.5%, Ti: 0.2-0.4%, Nb ≦ 0.015%, with the balance being Fe and inevitable impurities The present invention provides a ferritic stainless steel sheet excellent in stretch flangeability, characterized in that there is no portion where 10 or more spherical TiS are localized in a group and the aspect ratio of ferrite grains is 2 or less.

  The ferritic stainless steel sheet of the present invention is a steel having the above composition, heated to a heating temperature of 1000 ° C or higher, hot-rolled at a finishing temperature of 800 ° C or higher, and wound at a winding temperature of 400 to 600 ° C. Thereafter, it can be produced by annealing at 900 ° C. or more for 100 seconds or less, pickling, cold rolling, and then recrystallization annealing at 850 ° C. or more.

  According to the present invention, a ferritic stainless steel sheet excellent in stretch flangeability can be produced.

  Details of the present invention will be described below.

1) Composition ("%" below represents "mass%")
C ≦ 0.015%
C combines with Cr to reduce solute Cr, thus deteriorating corrosion resistance. In addition, since Cr carbide becomes a starting point of cracking during plastic deformation, stretch flange workability is also deteriorated. Therefore, the C content is 0.015% or less.

Si ≦ 0.15%
Si is a solid solution strengthening element and makes steel hard and low ductile. Therefore, the Si content is 0.15% or less.

Mn ≦ 0.3%
Since Mn deteriorates the corrosion resistance, its content is made 0.3% or less.

P ≦ 0.04%
P remarkably solidifies and strengthens steel to deteriorate workability, and segregates at the grain boundary to promote brittle fracture at the grain boundary. Therefore, the P content is 0.04% or less.

S ≦ 0.005%
S forms spherical TiS, but conventional knowledge suggests that spherical TiS does not degrade stretch flangeability. However, as explained in ii) above, when spherical TiS is localized in a group of 10 or more, it acts in the same way as expanded MnS found in general cold-rolled steel sheets, and stretch flanges. Degradation of workability. In order to prevent such spherical TiS from being localized in a group, it is necessary that at least S content is 0.005% or less.

Al ≦ 0.08%
Al is a deoxidizer, and the amount is preferably 0.02% or more in order to improve the cleanliness of steel. However, if the Al content exceeds 0.08%, it precipitates as AlN, and ferrite grains having a large aspect ratio that are stretched in the rolling direction by inhibiting grain growth are formed, and stretch flangeability deteriorates. Therefore, the Al content is 0.08% or less, preferably 0.05% or less.

N ≦ 0.015%
N, like C, combines with Cr to reduce solute Cr, thus deteriorating corrosion resistance. In addition, since Cr nitride becomes a starting point of cracking during plastic deformation, stretch flange workability is also deteriorated. For this reason, the N content is 0.015% or less.

Cr: 20.5-23.5%
Cr is an element that forms a passive film on the steel sheet surface and improves corrosion resistance. Although excellent corrosion resistance can be obtained by containing together with Cu described below, the Cr content needs to be 20.5% or more. On the other hand, if the Cr content exceeds 23.5%, the recrystallization delay due to Cr becomes remarkable, and the ferrite grains tend to extend in the rolling direction and the stretch flangeability deteriorates. Therefore, the Cr content is 23.5% or less.

Cu: 0.3-0.7%
Cu has a function of improving corrosion resistance when the Cr content is 20.5% or more, so the content is 0.3% or more. On the other hand, if the Cu content exceeds 0.7%, CuS is liable to precipitate and the stretch flangeability is deteriorated. Therefore, the Cu content is 0.7% or less, preferably 0.5% or less.

Ni ≦ 0.5%
Ni is an element that improves the corrosion resistance, but if contained in a large amount, Ni hardens the steel and causes ductility deterioration. Therefore, the Ni content is 0.5% or less.

Ti: 0.2-0.4%
Ti combines with N, C, and S to form nitrides, carbides, and sulfides. If the Ti content is less than 0.2%, these elements cannot be fixed as precipitates, and as a result, Cr carbide is formed and the corrosion resistance deteriorates. For this reason, Ti amount is 0.2% or more. On the other hand, when the Ti content exceeds 0.4%, nucleation of TiN and TiS is promoted and fine precipitation occurs, and the steel becomes hard and ductile. As a result, stretch flange workability also deteriorates. For this reason, the Ti amount is set to 0.4% or less.

Nb ≦ 0.015%
Nb fixes C and N as precipitates, but also suppresses recrystallization at the same time, so that ferrite grains easily extend in the rolling direction and stretch flangeability deteriorates. For this reason, the Nb content is 0.015% or less.

  The balance is Fe and inevitable impurities, but the effect of the present invention is not limited even if it is contained within the range of B ≦ 0.001%, Mo ≦ 0.1%, V ≦ 0.05%, Mg ≦ 0.01%, Ca ≦ 0.01%. can get.

2) TiS presence status
When 10 or more TiS are localized at one location, they are localized at the ferrite grain boundary, so they are arranged in a film at the ferrite grain boundary. TiS lined up in the form of a film at the ferrite grain boundary becomes a starting point of cracking during stretch flange processing and deteriorates stretch flangeability. Therefore, it is necessary to prevent a portion where 10 or more TiS are localized.

3) Aspect ratio of ferrite grains The ferritic stainless steel sheet of the present invention is composed of ferrite grains and precipitates. The ratio of the average grain size in the rolling direction to the average grain size in the rolling direction and the average grain size in the plate thickness direction, that is, the aspect ratio exceeds 2, makes it easy for the precipitates, especially TiS, to be arranged in parallel to the plate surface. In addition, since stress during plastic deformation is concentrated on ferrite grain boundaries parallel to the plate surface, fractures are likely to occur starting from precipitates present at the ferrite grain boundaries parallel to the plate surface, particularly TiS. For this reason, since the stretch flangeability deteriorates, the aspect ratio of the ferrite grains must be 2 or less.

4) Manufacturing conditions Heating temperature prior to hot rolling: 1000 ° C or more When the heating temperature of steel (slab) prior to hot rolling falls below 1000 ° C, the rolled structure remains on the hot-rolled steel sheet, cold rolling and recrystallization annealing Since the later ferrite grains easily expand in the rolling direction, stretch flange workability deteriorates. Therefore, the heating temperature is 1000 ° C. or higher.

Hot rolling finishing temperature: 800 ° C. or more If the finishing temperature is below 800 ° C., the rolling load increases and the roll becomes rough. The rough surface of the roll is transferred to the steel sheet and roughens the steel sheet surface. Therefore, Cr on the convex portion of the steel plate surface is preferentially oxidized and deteriorates the corrosion resistance of the steel plate. Therefore, the finishing temperature is 800 ° C. or higher, preferably 850 ° C. or higher.

Winding temperature: 400 ~ 600 ℃
The coiling temperature is important for controlling precipitates in the hot rolled steel sheet. When the coiling temperature is below 400 ° C., TiC does not precipitate, and TiC precipitates at the ferrite grain boundary during the subsequent annealing of the hot-rolled steel sheet, so that the ferrite grains tend to expand, so that stretch flangeability is deteriorated. On the other hand, when the coiling temperature exceeds 600 ° C., the ferrite grains of the hot-rolled steel sheet are coarsened, and the crystal grains after cold rolling and recrystallization annealing are liable to be coarsely mixed and deteriorated in stretch flangeability. Therefore, the coiling temperature is 400 to 600 ° C.

Annealing of hot-rolled steel sheets: 900 ° C or more and 100 seconds or less If the annealing temperature is below 900 ° C, the ferrite structure that has developed in the hot rolling direction and expanded in the rolling direction will not be destroyed. However, since it becomes easy to expand in the rolling direction, stretch flangeability deteriorates. Therefore, the annealing temperature is 900 ° C. or higher. On the other hand, when the soaking time at 900 ° C. or higher exceeds 100 seconds, the precipitate once precipitated is redissolved and then re-precipitated during cold rolling and recrystallization annealing, and the ferrite grains are expanded. Since precipitates are formed collectively at the ferrite grain boundaries, stretch flangeability deteriorates. Therefore, the soaking time is 100 seconds or less.

  Pickling: After annealing a hot-rolled steel sheet, pickling is necessary for removing the scale, but a normal pickling method performed on stainless steel can be applied.

  Cold rolling: The steel plate after pickling is made into a cold rolled steel plate having a desired thickness by cold rolling.

Recrystallization annealing annealing temperature: 850 ° C. or higher If the recrystallization annealing annealing temperature is lower than 850 ° C., a cold rolled structure stretched in the rolling direction tends to remain, and stretch flangeability deteriorates. In addition, since recrystallization is insufficient, the elongation is extremely reduced. For this reason, the annealing temperature of recrystallization annealing shall be 850 degreeC or more.

  The cold-rolled steel sheet after recrystallization annealing can be subjected to temper rolling in the range of 0.5 to 1.5% elongation.

Steel Nos. 1 to 13 having the chemical composition shown in Table 1 were melted and hot rolled under the hot rolling conditions shown in Table 2 to produce hot rolled steel sheets having a thickness of 3 mm. Next, these hot-rolled steel sheets were annealed under the conditions shown in Table 2, pickled, and cold-rolled to obtain cold-rolled steel sheets having a thickness of 0.8 mm. Finally, these cold-rolled steel sheets were recrystallized and annealed under the conditions shown in Table 2 to produce steel sheets Nos. 1-13. The TiS precipitation state, ferrite grain aspect ratio, mechanical properties, and corrosion resistance were investigated.
Precipitation state of TiS: The steel sheet was ground in the thickness direction, and the precipitate was collected from the center of the thickness by the extraction replica method, and observed with 10 fields of view with a transmission electron microscope, and the number of TiS was counted. When 10 or more TiS were observed, the number of TiS was shown, and when it was less than 10, all were set to 0.
Aspect ratio of ferrite grains: Polish the center of the thickness of the section of the thickness parallel to the rolling direction, etch with aqua regia, observe the ferrite grains, find the average grain size in the rolling direction and the thickness direction, and The aspect ratio defined by the formula was calculated.
Aspect ratio = (Average grain size in rolling direction) / (Average grain size in sheet thickness direction)
Here, the average particle diameter was determined by a cutting method. Five lines with an actual length of 500 μm were drawn on the structure photograph in the plate thickness direction and five in the rolling direction, and the number of intersections of these line segments and grain boundaries was counted. By dividing the total length of the line segment drawn in the thickness direction by the number of intersections between the line segment and the grain boundary, the average length of the line segment cut at the grain boundary is calculated, and the average grain size in the thickness direction is calculated. It was. Similarly, the average particle size in the rolling direction was determined.
Mechanical properties: JIS 13B tensile test specimens were taken in parallel with the rolling direction and subjected to a tensile test to determine the tensile strength TS and total elongation El. In addition, a 10 mmφ hole was punched in the center of the sample cut into a 100 mm square, and this hole was widened from the opposite side of the burr by punching with a conical punch with a vertex angle of 60 °. And the hole diameter d when the generation | occurrence | production of the crack was confirmed visually was measured, and stretch flange workability was evaluated by the hole expansion rate defined by the following formula | equation.
Hole expansion rate = [(d-10) / 10] x 100 (%)
Corrosion resistance: evaluated by combined cycle test (CCT). That is, spraying of a 5% NaCl aqueous solution at 35 ° C. for 2 hours, 30% humidity at 60 ° C. for 4 hours for drying, 95% humidity, 2 hours at 50 ° C. for 2 hours, and 15 cycles. And generation | occurrence | production of rust was evaluated visually and the thing by which generation | occurrence | production of rust was recognized was set as x.

  The results are shown in Table 3. It turns out that the steel plate Nos. 1-3, 5-7, 10-12 which are the examples of this invention are ferritic stainless steel plates excellent in stretch flangeability. Moreover, these steel plates have excellent corrosion resistance.

  Steel plates No. 1 to 4 are examples when the amount of C is changed. Steel plates Nos. 1 to 3 having an amount of C within the range of the present invention are excellent in both hole expansion rate and corrosion resistance. Steel plate No. 4 having a C content outside the scope of the present invention has an aspect ratio of ferrite grains exceeding 2, has a low hole expansion rate, and is inferior in corrosion resistance.

  Steel plates Nos. 5 to 8 are examples when the S amount is changed. Steel sheets Nos. 5 to 7 having an S amount within the range of the present invention are excellent in both hole expansion ratio and corrosion resistance. Steel plate No. 8 having an S amount exceeding the range of the present invention has agglomeration of TiS exceeding 10 pieces, has a low hole expansion rate, and is inferior in corrosion resistance.

  Steel plates Nos. 9 to 13 are examples when the amount of Ti is changed. Steel plate No. 9 whose Ti amount is below the scope of the present invention has a small amount of TiS precipitation and TiS is not locally agglomerated, but coarse MnS is precipitated and the hole expansion rate is low, and the corrosion resistance is also inferior. is there. Steel plates Nos. 10 to 12 whose Ti amount is within the range of the present invention show an excellent hole expansion ratio exceeding 100% and are excellent in corrosion resistance. Steel plate No. 13 whose Ti content exceeds the range of the present invention has an El of less than 30%, high strength, an aspect ratio of ferrite grains of more than 2, and a hole expansion ratio of well below 100%.

Claims (2)

  Mass%, C ≦ 0.015%, Si ≦ 0.15%, Mn ≦ 0.3%, P ≦ 0.04%, S ≦ 0.005%, Al ≦ 0.08%, N ≦ 0.015%, Cr: 20.5 to 23.5%, Cu: 0.3 to Including 0.7%, Ni ≦ 0.5%, Ti: 0.2-0.4%, Nb ≦ 0.015%, the balance is composed of Fe and unavoidable impurities, and the portion where 10 or more spherical TiS are localized A ferritic stainless steel sheet excellent in stretch flangeability, characterized in that the ferrite grain aspect ratio (average grain size in the rolling direction / average grain size in the plate thickness direction) is 2 or less.   The steel having the composition according to claim 1 is heated to a heating temperature of 1000 ° C or higher, hot-rolled at a finishing temperature of 800 ° C or higher, and wound at a winding temperature of 400 to 600 ° C, and then 900 ° C or higher. A method for producing a ferritic stainless steel sheet excellent in stretch flangeability, characterized by annealing at 100 seconds or less, pickling, cold rolling, and then recrystallization annealing at 850 ° C. or higher.