JP5338245B2 - Stainless cold-rolled steel sheet with good strength-elongation balance and small ridging and method for producing the same - Google Patents

Description

  The present invention relates to a ferritic stainless steel cold-rolled steel sheet, and more particularly to a stainless steel cold-rolled steel sheet having a good strength-elongation balance and low ridging and a method for producing the same.

  Ferritic stainless steel cold-rolled steel sheets are inexpensive and have excellent corrosion resistance, so they are used in various applications such as building materials, transportation equipment, home appliances, kitchen equipment, chemical plants, water tanks, and automobile parts. However, it is difficult for ferritic stainless steel cold-rolled steel sheets to achieve both high strength and high elongation, and when processed, there is a problem that irregularities appear in parallel with the rolling direction called ridging.

Therefore, conventionally, techniques for improving the elongation of ferritic stainless steel cold-rolled steel sheets and reducing ridging have been studied. For example, in Patent Document 1, ferritic stainless steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and then subjected to hot rolled sheet annealing and pickling, followed by cold rolling and finish annealing, and stainless steel. In the method of manufacturing the band, γp represented by the following formula (1) calculated from the composition of the ferritic stainless steel is 20 to 80, and at least one pass of the rough rolling is performed at a temperature of 970 to 1150. ℃, friction coefficient of 0.3 or less and reduction ratio of 40 to 75%, and further, hot-rolled sheet annealing is maintained at a temperature of 750 to 950 ℃ for 1 hour or more, and then at a temperature of 600 to 750 ℃ for 1 hour or more A method for producing a ferritic stainless steel strip having low in-plane anisotropy and excellent strength-elongation balance, characterized in that it is carried out in step (b), has been proposed.
γp = 288 × [C] + 350 × [N] + 22 × [Ni] + 7.5 × [Mn] -18.75 × [Cr] -54 × [Si] +338.5 ... (1)
However, [M] represents the content (mass%) of the element M.
JP-A-8-253818

  However, the stainless steel strip manufactured by the method described in Patent Document 1 is extremely excellent in TS × El ≧ 18000PMa ·% (TS: strength, El: elongation) and strength-elongation balance, but the ridging height is high. There is a problem that ridging is large at 3 μm or more.

  An object of the present invention is to provide a stainless cold-rolled steel sheet having a good strength-elongation balance and a small ridging and a method for producing the same.

  As a result of intensive studies on a stainless cold-rolled steel sheet having a good strength-elongation balance and a small ridging, the present inventors have found the following.

  i) When the composition is optimized, the area ratio of the ferrite phase in the entire structure is 80 to 97%, and the ferrite particle size is 5 to 20 μm, a good strength-elongation balance can be obtained and the ridging can be reduced.

  ii) In order to obtain such a microstructure, after hot rolling at a finishing temperature of 800 to 1000 ° C., without hot-rolled sheet annealing, hold at least 20 seconds at an annealing temperature of 800 to 950 ° C. after cold rolling. It is effective to anneal at a cooling rate of 10 ° C./sec or more.

  The present invention was made based on such findings, and in mass%, C: 0.01 to 0.03%, Si: 0.02 to 0.30%, Mn: 0.45 to 1.0%, P: 0.05% or less, S: 0.01% Hereinafter, Al: 0.01 to 0.20%, N: 0.01 to 0.06%, Cr: 16.0 to 18.0%, the composition comprising the balance Fe and inevitable impurities, the area ratio of the ferrite phase occupying the entire structure is 80 to There is provided a stainless cold-rolled steel sheet characterized by having a microstructure with an average grain size of 97% and a ferrite phase of 5 to 20 μm.

  The stainless cold-rolled steel sheet of the present invention is, for example, a slab having the above composition, heated at a heating temperature exceeding 1050 ° C., hot-rolled at a finishing temperature of 800 to 1000 ° C., pickled, and cold-rolled. It can be manufactured by a method of annealing at an annealing temperature of 800 to 950 ° C. for 20 seconds or more and cooling at a cooling rate of 10 ° C./sec or more.

  According to the present invention, it is possible to produce a stainless cold-rolled steel sheet having a good strength-elongation balance with TS × El ≧ 15000 PMa ·% and a small ridging with a waviness (riding height) of less than 2.4 μm.

  Below, the detail of the stainless steel cold-rolled steel plate and its manufacturing method which are this invention is demonstrated.

1) Component composition (“%” indicating the unit of the following component content represents “mass%”.)
C: 0.01-0.03%
Since C is an austenite-forming element, an appropriate amount of martensite phase is dispersed in the ferrite phase to prevent the ferrite grain colony from becoming coarse, and it has the effect of reducing ridging and also contributes to high strength. . In order to obtain such effects, the C content needs to be 0.01% or more. On the other hand, if the amount of C exceeds 0.03%, a large amount of Cr carbide precipitates or the formation of martensite phase becomes excessive, resulting in a decrease in elongation. Therefore, the C amount is 0.01 to 0.03%.

Si: 0.02 ~ 0.30%
Si is an element used as a deoxidizer in the steel melting stage. For deoxidation, the amount of Si needs to be 0.02% or more, but when the amount exceeds 0.30%, it becomes hard and elongation decreases. Therefore, the Si content is 0.02 to 0.30%.

Mn: 0.45-1.0%
Like C, Mn is an austenite-generating element, so an appropriate amount of martensite phase is dispersed in the ferrite phase to prevent ferrite grain colonies from coarsening, reducing ridging and contributing to higher strength. To do. In order to obtain such an effect, the Mn content needs to be 0.45% or more. On the other hand, if the amount of Mn exceeds 1.0%, the amount of MnS produced increases and the corrosion resistance decreases. Therefore, the Mn content is 0.45 to 1.0%.

P: 0.05% or less
When the amount of P exceeds 0.05%, it hardens and the elongation decreases significantly. Therefore, the P content is 0.05% or less.

S: 0.01% or less
When the amount of S exceeds 0.01%, the corrosion resistance is remarkably lowered. Therefore, the S content is 0.01% or less.

Al: 0.01-0.20%
Al, like Si, is an element used as a deoxidizer in the steel melting stage. Further, it has an effect of binding to N at the time of annealing and suppressing a decrease in elongation due to solute N. In order to suppress deoxidation and decrease in elongation, the Al content needs to be 0.01% or more. On the other hand, when the Al content exceeds 0.20%, Al ₂ O ₃ inclusions increase and the surface properties tend to deteriorate. Therefore, the Al content is 0.01 to 0.20%.

N: 0.01-0.06%
N, like C and Mn, is an austenite-forming element, so an appropriate amount of martensite phase is dispersed in the ferrite phase to prevent the ferrite grain colony from becoming coarse, reducing ridging and increasing strength. Also contribute. In order to obtain such effects, the N content needs to be 0.01% or more. On the other hand, when the N content exceeds 0.06%, a large amount of nitride precipitates or the formation of martensite phase becomes excessive, and the elongation decreases. Therefore, the N content is 0.01 to 0.06%.

Cr: 16.0-18.0%
Cr is an element that improves the corrosion resistance by forming a passive film on the steel sheet surface. In order to obtain such an effect, the Cr amount needs to be 16.0% or more. On the other hand, if the Cr content exceeds 18.0%, the formation of martensite phase is suppressed and ridging cannot be reduced. Therefore, the Cr content is 16.0 to 18.0%.

  The balance is Fe and inevitable impurities, but the amount of inevitable impurities is preferably reduced as much as possible.

2) Micro structure
2.1) Ferrite phase area ratio: 80-97%
If the area ratio of the ferrite phase is less than 80%, the ferrite phase becomes hard and the elongation decreases. On the other hand, when the area ratio of the ferrite phase exceeds 97%, the ferrite grain colonies become coarse and ridging becomes large. Therefore, the area ratio of the ferrite phase is 80 to 97%.

  Here, the area ratio of the ferrite phase was determined by image analysis of an image obtained by corroding a cross section of an arbitrary steel sheet with aqua regia and observing it at 100 times using an optical microscope.

2.2) Average grain size of ferrite phase: 5-20μm
If the average particle size of the ferrite phase is less than 5 μm, the elongation decreases, and if it exceeds 20 μm, orange peel and ridging are likely to occur during processing. Therefore, the average particle size of the ferrite phase is 5 to 20 μm.

  Here, the average particle diameter of the ferrite phase is an ASTM (ASTM Designation E 112-82) nominal particle diameter measured using an image observed in the same manner as described above.

3) Manufacturing conditions As described above, the stainless cold-rolled steel sheet of the present invention is, for example, a slab having a composition within the scope of the present invention, heated at a heating temperature exceeding 1050 ° C., and then a finishing temperature of 800 to 1000 ° C. It can be manufactured by a method of hot rolling with, pickling, cold rolling, holding at an annealing temperature of 800 to 950 ° C. for 20 seconds or more, annealing, and cooling at a cooling rate of 10 ° C./sec or more.

  First, steel adjusted to a composition within the range of the present invention is melted in a converter or an electric furnace, and further decarburized. Although the obtained molten steel is made into a slab by a continuous casting method or an ingot-making method, it is preferable to use a continuous casting method with high productivity.

  Next, the slab is heated at a heating temperature exceeding 1050 ° C. and hot-rolled at a finishing temperature of 800 to 1000 ° C. to obtain a hot-rolled steel sheet. This is because heating at a heating temperature of 1050 ° C or less or hot rolling at a finishing temperature of less than 800 ° C tends to cause surface defects such as rough skin on the hot-rolled steel sheet, and heating at a finishing temperature exceeding 1000 ° C. This is because hot rolling expands the structure after hot rolling and increases ridging.

Finally, the hot-rolled steel sheet is descaled by pickling, cold-rolled, annealed at an annealing temperature of 800 to 950 ° C for 20 seconds or more, cooled at a cooling rate of 10 ° C / sec or more and cooled. It is a rolled steel sheet. At this time, if the annealing temperature is less than 800 ° C. or the holding time is less than 20 seconds, recrystallization becomes insufficient and elongation decreases. On the other hand, if the annealing temperature exceeds 950 ° C., the ferrite phase grains become coarse, the expanded grains increase, and the ridging increases. Furthermore, when the cooling rate is less than 10 ° C./sec, a large amount of carbonitride precipitates during cooling, and the elongation decreases.

  In addition, after annealing, it is preferable to perform skin pass rolling at an elongation of 0.3 to 2.0% in order to correct the shape and remove the elongation at the yield point.

  Stainless steels Nos. 1 to 6 having the compositions shown in Table 1 were melted in a converter-secondary refining process and made into slabs by a continuous casting method. These slabs were heated to 1200 ° C. and hot-rolled at finishing temperatures of 850 ° C. and 1050 ° C. to obtain hot-rolled steel sheets having a thickness of 4 mm. The obtained hot-rolled steel sheet is pickled and cold-rolled to obtain a cold-rolled steel sheet having a thickness of 0.8 mm and annealed at 830 ° C and 980 ° C (retained at 800 ° C or higher and 950 ° C or higher for 30 seconds, respectively). Steel plates No. 1 to 10 were produced by cooling to 200 ° C. at a cooling rate of 20 ° C./sec. And the sample was extract | collected from the width direction center part of the obtained steel plate, and the microstructure was evaluated by said method. In addition, a JIS 13B tensile test piece was taken in parallel with the rolling direction, and a tensile test was performed at a strain rate of 10 mm / min in accordance with JIS Z 2241 to obtain TS and El, and TS × El was calculated. If TS × El ≧ 15000 PMa ·%, the strength-elongation balance was considered good. In addition, two JIS No. 5 tensile test specimens were collected in parallel with the rolling direction, and after polishing one side of the specimen with No. 600 abrasive paper in accordance with JIS Z 2201, 20% simple tensile pre-strain The waviness (riding height) was measured in accordance with JIS B 0601 using a roughness meter at the center of the test piece, and the average waviness of the two test pieces was calculated. If the average waviness is less than 2.4 μm, the ridging is small enough not to cause a problem.

  The results are shown in Table 2. Steel plates No. 1 and 4, which are examples of the present invention, are TS × El ≧ 15000PMa ·%, swell is less than 2.4, strength-elongation balance is good, and it is understood that this is a stainless cold-rolled steel plate with small ridging. .

Claims (2)

  In mass%, C: 0.01 to 0.03%, Si: 0.02 to 0.30%, Mn: 0.45 to 1.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.01 to 0.20%, N: 0.01 to 0.06% , Cr: 16.0 to 18.0%, with the balance consisting of Fe and inevitable impurities, the area ratio of the ferrite phase occupying the entire structure is 80 to 97%, and the average grain size of the ferrite phase is 5 to 20 μm A stainless cold-rolled steel sheet characterized by having a microstructure that is The slab having the composition according to claim 1 is heated at a heating temperature exceeding 1050 ° C, hot-rolled at a finishing temperature of 800-1000 ° C, pickled, cold-rolled, and annealed at 800-950 ° C. Hold for 20 seconds or more at temperature, anneal , cool at a cooling rate of 10 ° C / sec or more, the area ratio of the ferrite phase occupying the whole structure is 80-97%, and the average grain size of the ferrite phase is 5-20μm method for producing a cold-rolled stainless steel sheet, which comprises perforated microstructure is.