JP4281535B2 - Ferritic stainless steel plate with excellent surface strain resistance - Google Patents

Description

  The present invention relates to a ferritic stainless steel plate having excellent surface strain resistance, and in particular, in a normal ferritic stainless steel plate, which has been a problem particularly when stainless steel plates are applied to inner and outer plate panels of automobiles and buildings. The present invention relates to a ferritic stainless steel sheet having improved surface strain resistance, which is improved in that it is difficult to ensure surface accuracy of a press-formed product (that is, surface strain resistance is insufficient).

As a technique for improving the workability of a ferritic stainless steel sheet, a technique of adding Ti or Nb in addition to the reduction of C and N is generally known (for example, Patent Document 1). Moreover, in addition to hot rolling control by high temperature winding, by prescribing the P, S, C, and N contents in steel, precipitation of phosphide (FeTiP) that causes ductility reduction and hardening is suppressed, and annealing is omitted. A possible manufacturing method is known (Patent Document 2). Further, in general materials, not limited to stainless steel, the relationship between the yield stress (hereinafter abbreviated as YS as appropriate) and the crystal grain size is the Hall-Petch relationship: σ _y = σ ₀ + Kd ^−1/2 It is known that it can be well described by (σ _y : yield stress, σ ₀ : frictional force when dislocations move in the grains, d: average crystal grain size, K: constant) (for example, Non-Patent Document 1).
Japanese Patent Laid-Open No. 3-264652 JP-A-5-320772 Edited by the Japan Institute of Metals, “Steel Materials,” 1995, Volume 3, P.56

  Due to the need for weight reduction of automobiles, high-tensile interior panel panels such as side panels are being advanced. In addition, from the viewpoint of simplifying and omitting the painting process, studies are underway to use stainless cold-rolled steel sheets for automobile and building interior and exterior panels.

  The inner and outer plate panels are required to have surface strain resistance as an important characteristic in addition to the stretchability and drawability. In particular, improvement of surface accuracy is particularly important for press-molded products, and there is a strong demand for a material that is difficult to cause surface distortion during press molding, that is, a material excellent in surface distortion resistance. However, the conventional ferritic stainless steel sheet cannot be said to have sufficient surface strain resistance. In the techniques described in Patent Documents 1 and 2, both Ti and Nb are added alone or both are added together to fix C and N in steel as carbonitrides (IF conversion). The purpose is to improve the ductility and r value, and no means for improving the surface strain resistance is taken into consideration.

  Then, an object of this invention is to provide the ferritic stainless steel plate excellent in the surface distortion resistance which can be applied to the inside and outside board panel use of a motor vehicle or a building.

  It is known that the surface strain resistance is improved by lowering the YS and yield ratio (YR = yield stress / tensile strength (YS / TS)) of the material. Therefore, the present inventors examined means for achieving a low YR of the Nb-added ferritic stainless steel sheet, and based on the ferritic stainless steel added with Nb forming carbides and nitrides, Ti was added thereto. It was newly found that the YS of the cold-rolled sheet can be reduced by fixing a small amount of N in the steel as TiN.

  And, using cold rolled sheet made by hot rolling-cold rolling using small steel ingot of Ti-Nb added ferritic stainless steel made by melting Ti content in various ways, its mechanical properties The effect of adding a small amount of Ti was investigated in detail. As a result, by containing a small amount (0.003 to 0.03%) of Ti in the range of Nb / Ti ≧ 10, compared with the same crystal grain size, YS and YR are reduced (YR ≦ 60%). I found that I can achieve.

  In conventional YS reduction means such as high purity and coarsening of grains, YS does not decrease sufficiently because TS also decreases with YS, whereas YS reduction means by the addition of a small amount of Ti does not reduce TS. It is relatively small. The reason why the mechanical properties of the cold-rolled plate changed in this way is that Nb (C, N) precipitates in the steel with Nb added alone, but TiN is formed by adding a small amount of Ti, and Nb (C) precipitates. I believe that there is to do. On the other hand, the increase in YS due to the addition of excess Ti promotes not only TiN but also the formation of Ti (C, N) and TiC, and the excess of C and N as binding partners is deprived by Ti and cannot be precipitated. This is thought to be due to an increase in the amount of solute Nb.

The present invention is based on these new findings, and the gist of the present invention is as follows.
(First Item) By mass%, C: 0.02% or less, Si: 0.5% or less, Mn: 0.3% or less, P: 0.04% or less, S: 0.01% or less, Cr : 8-30%, Al: 1.0% or less, Nb: 0.05-0.5%, Ti: 0.003-0.03%, N: 0.02% or less, the balance Fe and A ferritic stainless steel sheet excellent in surface strain resistance, comprising inevitable impurities, satisfying 400 ≧ Nb / Ti ≧ 10, Nb ≧ 16 × (C + N), and having YR of 60% or less.
(Second Item) The ferritic stainless steel sheet having excellent surface strain resistance according to the first item, which contains Mo: 3.0% or less instead of a part of Fe.
(Section 3) Instead of a part of Fe, Ni: 1.0% or less, Cu: 1.0% or less, Co: 1.0% or less selected from one or more selected from The ferritic stainless steel sheet having excellent surface strain resistance according to item 1 or 2, characterized in that:
(Item 4) Ferritic stainless steel having excellent surface strain resistance according to any one of Items 1 to 3, which contains B: 0.005% or less instead of part of Fe steel sheet.
(Section 5) In place of a part of Fe, one or more selected from Ta: 0.2% or less, V: 0.2% or less, W: 0.2% or less The ferritic stainless steel sheet excellent in surface distortion resistance according to any one of items 1 to 4, wherein
(Sixth Item) Mg: 0.0005 to 0.010% is contained instead of a part of Fe, and the surface distortion resistance according to any one of the first to fifth items is excellent. Ferritic stainless steel sheet.
(Seventh Item) Ferritic stainless steel excellent in surface strain resistance according to any one of the first to sixth items, wherein Ca is contained in an amount of 0.05% or less instead of a part of Fe. steel sheet.
(Section 8) Ferritic stainless steel sheet having excellent surface strain resistance according to any one of sections 1 to 7, wherein the ferrite grain size number is 6.0 or more.

  Since the ferritic stainless steel sheet of the present invention is excellent in surface distortion resistance, it can be applied to interior and exterior panel panels of automobiles and buildings, as well as interior and exterior panel panels of home appliances and kitchens. This contributes to the expansion of the use of the stainless steel sheet and simplification and further omission of the painting process in the panel manufacturing field.

  The reason why the present invention is limited to the above range will be described below. In addition, all content of each element is the mass%, and shall represent only by% in the following description.

C: 0.02% or less When C is contained as solid solution C, the steel becomes hard (solid solution strengthening). Moreover, {111} texture formation of a hot-rolled sheet and a cold-rolled sheet is inhibited, and the r value improvement of a steel plate is inhibited. In particular, if it exceeds 0.02%, the effect becomes remarkable, so the content is limited to 0.02% or less. In addition, C hardens the steel by solid solution strengthening and precipitation strengthening and lowers the ductility, so it is preferable that it is as low as possible in the steel sheet used for forming, but on the other hand, if it is excessively reduced, the refining load is increased. At the same time, the crystal grains are remarkably coarsened, and it is difficult to control precipitates. From these viewpoints, the C content is preferably more than 0.0005% and not more than 0.008%.

Si: 0.5% or less Si is an element effective for improving oxidation resistance and corrosion resistance, and improves corrosion resistance in an atmospheric environment. Moreover, it is used for removing oxygen in steel as a deoxidizer. However, if it is excessively contained, the steel becomes harder and the ductility is lowered with the increase of solute Si, so the content is limited to 0.5% or less. In addition, Preferably it is 0.05-0.2%.

Mn: 0.3% or less Mn is an element effective for improving the oxidation resistance. However, if excessively contained, Mn deteriorates the toughness of the steel and also deteriorates the secondary work brittleness resistance of the welded portion. Limited to 0.3% or less.

P: 0.04% or less P, when dissolved, hardens the steel and significantly lowers the ductility, and precipitates as fine adjacency, thereby impairing workability and corrosion resistance. In particular, if the content exceeds 0.04%, the influence becomes significant, so the content is limited to 0.04% or less. In addition, although the one where P is low is desirable, since excessive reduction will raise the steelmaking cost, 0.005 to 0.025% is preferable from a viewpoint of the refining load of steel.

S: 0.01% or less Since S forms soluble MnS and lowers the corrosion resistance of steel, the lower one is preferable. However, the content is set to 0.01% or less in consideration of the economic load on the de-S treatment during steelmaking. In addition, Preferably it is 0.002 to 0.006%.

Cr: 8-30%
Cr is an element effective for improving corrosion resistance. However, in order to ensure sufficient corrosion resistance, it is necessary to contain 8% or more. In addition, in order to ensure the corrosion resistance including the coastal environment and the welded portion, the content is preferably 11% or more which makes the passive film stable. On the other hand, Cr is an element that deteriorates the workability of steel. Particularly, when it exceeds 30%, the effect becomes remarkable, and the addition of sigma (sigma) phase and chi (chi) phase due to complex addition with other elements. Precipitation makes the steel brittle, so it is limited to 30% or less. When used as an inner and outer panel of an automobile, 15 to 24% is preferable.

Al: 1.0% or less Al is necessary as a deoxidizing agent in steelmaking, but excessive addition generates excessive oxide inclusions and degrades the surface appearance and corrosion resistance. To do.

Nb: 0.05-0.5%
Nb has the effect of suppressing the formation of Cr-based carbonitride by precipitating and fixing C and N as Nb-based carbonitride, improving corrosion resistance and improving workability (elongation, r value). It is necessary to add a predetermined amount. However, when the content is less than 0.05%, C and N cannot be sufficiently fixed as precipitates, so that solid solution C and N remain in the steel, resulting in deterioration of workability and corrosion resistance. On the other hand, if the content exceeds 0.5%, the amount of solute Nb increases, resulting in steel hardening, ductility reduction, and toughness reduction. Therefore, the Nb content is limited to a range of 0.05 to 0.5%. Nb is preferably added so as to satisfy 16 ≦ Nb / (C + N) ≦ 40 from the viewpoint of forming stable carbides and nitrides with C and N. Here, the element symbol in the inequality represents the content (mass%) of the element in steel.

Ti: 0.003 to 0.03%
Ti is the most important element in the present invention. Ti forms C and N in steel and Ti-based carbonitrides, suppresses the formation of Cr-based carbonitrides like Nb, has the effect of improving corrosion resistance and improving workability (elongation r value). However, in the present invention, in the Nb-added ferritic stainless steel to which Nb is added as a main stabilizing element, by adding a small amount (0.003 to 0.03%) of Ti, low YS and low YR can be achieved. It was newly found that a stainless cold-rolled steel sheet that can be achieved and has excellent surface strain resistance can be obtained. Therefore, in the present invention, the Ti content is limited to a range of 0.003 to 0.03%.

  Although the mechanism that produces the above effect is not yet clear, Ti forms nitrides more easily than Nb. Therefore, N in the steel is fixed as Ti-based nitride (TiN), and Nb carbonitriding is performed. This is considered to change the composition of the product and affect the solubility of these precipitates in the steel.

N: 0.02% or less N, if contained in an appropriate amount, strengthens the grain boundary and improves toughness. However, if contained in an amount exceeding 0.02%, it becomes a nitride and precipitates at the grain boundary, leading to corrosion resistance. In addition, TiN is formed together with Ti and causes a scratch on a cold-rolled plate, particularly a glossy product, so the content is limited to 0.02% or less. In consideration of the refining load, the N content is preferably 0.005 to 0.02%.

Nb / Ti ≧ 10 and Nb ≧ 16 × (C + N)
Each content of C, N, Nb, and Ti is limited as described above, but it is not sufficient for improving the surface distortion resistance. One of the important factors. That is, it is important that Ti mainly contributes to TiN formation, and that solid solution C, N, solid solution Nb, and Ti do not remain as much as possible in the steel. Therefore, in the present invention, Nb / Ti (: Ti The ratio of Nb content to content) is 10 or more, and Nb / (C + N) (the ratio of Nb content to the total content of C and N) is limited to 16 or more.

  When Nb / Ti is less than 10, not only Nb (C, N) but also Ti (C, N) is formed, and solid solution Nb is formed. Nb / Ti is preferably 10 to 400. Further, when Nb / (C + N) is less than 16, Nb cannot sufficiently form carbonitride, and surplus carbon and nitrogen remain as solid solution C and N, resulting in deterioration of workability and corrosion resistance. It becomes difficult to reduce YR.

YR (= YS / TS) ≦ 60%
In order to improve the surface strain resistance, it is necessary that the buckling stress (ie, yield stress: YS) is low and has a predetermined strength (ie, tensile strength: TS). However, when TS is increased together with YS, the steel becomes hard, ductility is lowered, and particularly the stretch formability is lowered. In the present invention, YR is limited to 60% or less in order to improve surface distortion resistance while ensuring stretchability. In addition, YR is so preferable that it is low, since shaping | molding is easy and it is excellent in surface distortion resistance.

  In this invention, although the above-mentioned element is contained essential, you may add the following elements as needed.

Mo: 3.0% or less Mo is an element effective for improving the corrosion resistance of stainless steel. When used for inner and outer panel applications, ensuring corrosion resistance is important for aesthetic and functional reasons. Mo is an element effective for improving corrosion resistance and rust resistance, but if added over 3.0%, the steel sheet becomes hard and processing such as press molding becomes difficult, so 3.0% or less is preferable. .

One or more selected from Ni: 1.0% or less, Cu: 1.0% or less, Co: 1.0% or less Ni, Cu, and Co are all corrosion resistance and toughness of hot-rolled sheet It is an effective element for improvement. However, if the content exceeds 1.0%, the steel becomes hard and adverse effects on workability increase. Therefore, each content is preferably 1.0% or less.

B: 0.005% or less B is an element effective for improving secondary work brittleness resistance. However, addition over 0.005% hardens the steel and conversely inhibits the primary workability and secondary workability of the steel, so 0.005% or less is preferable. More preferably, it is 0.0003 to 0.005%.

One or more selected from Ta: 0.2% or less, V: 0.2% or less, W: 0.2% or less These elements form carbonitrides and contribute to improving corrosion resistance However, it also forms carbides and nitrides, affecting the formation of carbonitrides in steel. Moreover, when it exists as a solid solution element, steel will be hardened and the workability will fall. In order to avoid this harmful effect, the content is preferably 0.2% or less. In addition, More preferably, it is 0.01 to 0.10%.

Mg: 0.0005-0.010%
Mg dissociates from the deoxidizer and refractory and dissolves in the steel. Mg forms oxide-based inclusions of Mg—Al spinel, serves as a nucleation site for TiN, and promotes the TiN conversion to TiN. In addition, TiN effectively works as a nucleation site of ferrite (δ (delta) -ferrite), and thus effectively works to increase the equiaxed crystal ratio and refine the hot-rolled sheet structure. This effect becomes remarkable at 0.0005% or more. However, if added over 0.010%, it remains in the steel as oxide inclusions, causing a decrease in corrosion resistance. Therefore, the range of 0.0005 to 0.010% is preferable. More preferably, it is 0.002 to 0.005%.

Ca: 0.05% or less Ca can be added from the viewpoint of continuous casting of steelmaking, but if added excessively, the corrosion resistance is significantly impaired, so the content is preferably 0.05% or less.

  In the present invention, the balance excluding the above components from the entire composition is Fe and inevitable impurities.

Grain size number of ferrite crystal grains: 6.0 or more Generally, it is known that the crystal grain diameter is related to YS of steel, and according to the Hall-Petch equation, YS decreases as the ferrite crystal grains become coarser. (For example, Non-Patent Document 1). However, if the ferrite crystal grains are coarsened to a grain size number of less than 6.0, surface irregularities called orange peel are formed due to severe processing such as press formability, and the appearance is impaired and the molding limit is lowered. Since the inner and outer plate panels dislike such unevenness, in the present invention, the ferrite crystal grains may be as fine as a grain size number of 6.0 or more that does not bother the surface unevenness even when severe processing is performed. preferable. In addition, the particle size number said by this invention is measured by the cutting method prescribed | regulated to JISG0552, 5 field observations are carried out about the observation surface of x100 times in the plate | board thickness cross section parallel to a rolling direction (L direction), and the average value As sought.

  Next, a method for producing the ferritic stainless steel plate of the present invention (: steel plate of the present invention) will be described.

  The steel sheet of the present invention is obtained by sequentially processing a steel material that has been melted and cast in the steel making process so as to satisfy the composition requirements of the present invention in a hot rolling process, a hot-rolled sheet annealing process (for example, box annealing), and a pickling process. It is suitable to manufacture by a method of forming a hot-rolled sheet and further processing it in a cold-rolling step and a finish annealing step (for example, continuous annealing) to form a cold-rolled annealed plate.

  In addition, when assembling a pipe by welding using the steel plate of the present invention described above, all the usual welding methods such as arc welding including TIG, MIG, ERW, electric seam welding, and laser welding are applicable. is there.

A steel slab having the chemical composition shown in Table 1 was heated under conditions of 1150 ° C. × 1 hour, and then hot-rolled to obtain a hot rolled sheet having a thickness of 5.0 mm. Next, this hot-rolled sheet is annealed in a temperature range of 950 to 1100 ° C., and further cold-rolled to a plate thickness of 0.8 mm to form a cold-rolled sheet, which is finish-annealed in a temperature range of 850 to 1100 ° C., A cold-rolled annealed plate was obtained. The following characteristics of the cold-rolled annealed plate were examined.
-The grain number number of the ferrite crystal grain in the plate thickness section parallel to the rolling direction (L direction) was determined according to JIS G 0552 (cutting method).
-Using JIS No. 13 B test piece, YS, TS, El (L direction (rolling direction), D direction (45 ° direction with respect to rolling direction), C direction (90 ° direction with respect to rolling direction)) Elongation) was measured, and average YS, average TS, average El, and average YR were calculated according to the following formula (1), and an average value of n points of 3 points was obtained.

Average X = (X _L + 2 × X _D + X _C ) / 4 (1)
Here, X is any one of YS, TS, El, and YR (= YS / TS), and the subscript means a measured value in the subscript direction.
-Using JIS No. 13 B test piece, giving 15% uniaxial tensile pre-strain, obtaining r value in each direction of L, D, C according to the three-point method, and formula where X is r in the formula (1) The average r value was calculated by the above, and the average value of n number of 3 points was obtained.

These results are shown in Table 2. In addition, about some examples, the graph which plotted the relationship between Nb / Ti and YR (= average YR) is shown in FIG. In the present invention example, the average YS is as low as 300 MPa or less, the average YR is as low as 60% or less, and the surface distortion resistance is excellent. FIG. 2 is a graph showing the relationship between the grain size number of the ferrite crystal grains and the surface average roughness serving as a rough index for the steels 33 to 38 in Table 2. A JIS No. 5 test piece was collected from the rolling direction and pulled 25%, and then the surface average roughness was measured in the direction perpendicular to the rolling direction, and the relationship with the particle size number obtained by the above-described method was examined. When the particle size number is less than 6.0, the orange peel is remarkably deteriorated, and not only the appearance is impaired, but also the unevenness portion becomes a starting point of cracking, resulting in deterioration of workability. Therefore, the particle size number is preferably 6.0 or more.

  The present invention can be used for interior and exterior panels of automobiles, buildings, home appliances, and kitchens.

It is a graph which shows the relationship between Nb / Ti and YR. It is a graph which shows the relationship between the particle size number of a ferrite crystal grain, and surface average roughness Ra (micrometer) [bare].

Claims (8)

In mass%, C: 0.02% or less, Si: 0.5% or less, Mn: 0.3% or less, P: 0.04% or less, S: 0.01% or less, Cr: 8-30% , Al: 1.0% or less, Nb: 0.05-0.5%, Ti: 0.003-0.03%, N: 0.02% or less, the balance being Fe and inevitable impurities A ferritic stainless steel sheet excellent in surface strain resistance, characterized by satisfying 400 ≧ Nb / Ti ≧ 10, Nb ≧ 16 × (C + N), and YR being 60% or less.   The ferritic stainless steel sheet having excellent surface strain resistance according to claim 1, wherein Mo is contained in an amount of 3.0% or less instead of part of Fe.   Instead of a part of Fe, it contains one or more selected from Ni: 1.0% or less, Cu: 1.0% or less, Co: 1.0% or less The ferritic stainless steel sheet having excellent surface strain resistance according to claim 1 or 2.   The ferritic stainless steel sheet having excellent surface strain resistance according to any one of claims 1 to 3, wherein B is contained in an amount of 0.005% or less in place of part of Fe.   Instead of part of Fe, it contains one or more selected from Ta: 0.2% or less, V: 0.2% or less, W: 0.2% or less The ferritic stainless steel sheet excellent in surface distortion resistance according to any one of claims 1 to 4.   The ferritic stainless steel sheet having excellent surface strain resistance according to any one of claims 1 to 5, wherein Mg: 0.0005 to 0.010% is contained instead of a part of Fe.   The ferritic stainless steel sheet having excellent surface strain resistance according to any one of claims 1 to 6, wherein Ca is contained in an amount of 0.05% or less in place of a part of Fe.   The ferritic stainless steel sheet having excellent surface strain resistance according to any one of claims 1 to 7, wherein the ferrite crystal grains have a grain size number of 6.0 or more.

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