JP4626484B2 - Ferritic stainless steel cold-rolled steel sheet excellent in press formability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a ferritic stainless steel cold-rolled steel sheet, and more particularly to improvement of elongation characteristics that affect press formability.

  Ferritic stainless steel sheets are excellent in corrosion resistance and inexpensive, and are used in a wide range of applications such as building exterior materials, kitchen appliances, and chemical plants. Conventionally, in these applications, the ferritic stainless steel sheet is only required to have excellent corrosion resistance, and has not been required to be excellent in press formability. In recent years, however, even in these applications, the thin steel plate is often press-formed into a complicated three-dimensional shape, and it has been strongly demanded that the ferritic stainless steel plate be excellent in press formability.

  In response to such a request, for example, in Patent Document 1, C: 0.08% or less, Si: 0.70% or less, Mn: 1.00% or less, Cr: 15 to 20%, N: 0.04 to 0.12% (provided that C + N: A ferritic stainless steel plate comprising Fe and unavoidable impurities: remaining is proposed, and the use of this ferritic stainless steel plate is said to prevent the occurrence of stretch strain during press forming. However, the technique described in Patent Document 1 can prevent the generation of stretch strain and can eliminate the surface polishing after molding, but has a large C and N content and lowers the elongation itself. For this reason, there is a problem that it is difficult to say that the steel sheet is suitable for press forming.

  Patent Document 2 discloses a composite of ferrite and martensite by heating and hot rolling a slab of stainless steel composition containing C: 0.02 to 0.05%, N: 0.02 to 0.05%, and Al: 0.10 to 0.30%. A hot-rolled sheet having a structure, and then the obtained hot-rolled sheet is subjected to high-temperature and short-term annealing in a temperature range of 850 to 980 ° C. to produce AlN, followed by cold rolling and finish annealing, ridging resistance A method for producing a ferritic stainless steel sheet excellent in the above has been proposed. However, since the steel sheet obtained by the technique described in Patent Document 2 is annealed for a short time on the martensite generated in the hot-rolled sheet, the martensite stretched in the rolling direction is not equiaxed and is notable. The steel sheet has anisotropy and is not suitable for three-dimensional press forming.

  Patent Document 3 describes that ferritic stainless steel containing Cr: 10-23% and having a γ potential of 23% or less is included in the mother molten steel, and the γ potential is increased at the unsolidified center in the mold during continuous casting. As the steel slab in which the particles containing the element to be injected are melted and dissolved and the γ potential of the slab inner layer is increased by 5% or more from the value of the mother molten steel, hot rolling, cold rolling and annealing are performed. There has been proposed a method for producing a ferritic stainless steel sheet, which is a ferritic stainless steel sheet having excellent ductility and ridging properties by the process of including. However, in the steel sheet obtained by the technique described in Patent Document 3, although the generation of ridging is reduced, the range of the region where the γ potential at the center of the slab is high is not constant, so that a stable effect cannot be obtained and the elongation is not achieved. There was a problem that the r value was significantly lowered and it was difficult to say that the steel sheet was suitable for press forming.

Patent Document 4 includes hot rolling of a steel material including C: 0.01 to 0.12%, N: 0.01 to 0.12%, Cr: 11 to 18%, and V: 0.03 to 0.15%. After forming the plate, the hot-rolled sheet is subjected to 2-15% cold or warm pre-rolling to promote recrystallization and enable hot-rolled sheet annealing for a short time, followed by cold rolling and finish annealing. A method for producing a ferritic stainless steel cold-rolled steel sheet having excellent ridging resistance has been proposed.
JP 59-80753 A JP-A-9-111354 JP-A-10-99952 Japanese Patent Laid-Open No. 2001-107149

However, the steel sheet manufactured by the method described in Patent Document 4 has a high yield point and sufficient press formability because V carbonitride precipitates and refines crystal grains to harden the material. There was a problem that it was difficult to say that it was a steel plate.
An object of the present invention is to advantageously solve the above-described problems of the prior art, and to propose a ferritic stainless cold-rolled steel sheet having high elongation and excellent press formability and a method for producing the same.

  In order to achieve the above-mentioned problems, the present inventors have made a detailed study on the relationship between the elongation and the structure that greatly affects the press formability of the ferritic stainless cold-rolled steel sheet, particularly the stretch formability. As a result, the present inventors have found that the elongation of the stainless cold-rolled steel sheet is increased by activating the dislocation motion during deformation, and for that purpose, by suppressing the fine precipitation of AlN and by the fine AlN While reducing precipitation strengthening, reducing the amount of Cr carbonitride, coarsening the Cr carbonitride to limit the number of Cr carbonitrides present in the ferrite grains, and between dislocations generated from the Cr carbonitride / ferrite interface It has been found that it is better to suppress the interaction.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) A composition comprising mass%, C: 0.03% or less, N: 0.05% or less, Mn: 1% or less, Cr: 13 to 20%, Al: 0.01% or less, the balance being Fe and inevitable impurities It has excellent press formability characterized by having a ferrite single structure in which Cr carbonitride having an average grain size of 0.6 μm or more is dispersed in ferrite grains at 20 or less per ferrite grain Ferritic stainless steel cold rolled steel sheet.

(2) In (1), the composition includes mass%, C: 0.03% or less, N: 0.05% or less, Mn: 1% or less, Cr: 13 to 20%, Al: 0.01% or less, Si: 0.4% or less, P: 0.05% or less, S: 0.010% or less, a ferritic stainless steel cold-rolled steel sheet comprising a balance Fe and inevitable impurities.
(3) A stainless steel material is subjected to a hot rolling process, a hot rolled sheet annealing process for performing hot rolled sheet annealing and pickling, a cold rolled process, and a cold rolled sheet annealing process, In the manufacturing method of ferritic stainless steel cold rolled steel sheet, the stainless steel material is mass%, C: 0.03% or less, N: 0.05% or less, Mn: 1% or less, Cr: 13-20%, Al: 0.01% A steel material having a composition comprising the balance Fe and inevitable impurities, including the following, after the hot rolling step is heated to 1000 ° C. or higher, hot rolling is finished at a temperature of 900 ° C. or higher to obtain a hot rolled sheet The hot-rolled sheet is wound at a coiling temperature of 650 to 850 ° C., and the hot-rolled sheet annealing process is a process of performing a hot-rolled sheet annealing process at an annealing temperature of 800 ° C. or more. The cold rolling reduction: 90% or less of the cold rolled sheet as a process to be cold rolled sheet, the cold rolled sheet annealing process, Cold-rolled sheet to annealing temperature: 800 to 900 ° C. and a manufacturing method excellent ferritic stainless cold-rolled steel sheet in press formability, characterized by the step of performing continuous annealing of.

  (4) In (3), the stainless steel material in mass%, C: 0.03% or less, N: 0.05% or less, Mn: 1% or less, Cr: 13-20%, Al: 0.01% or less, Further, a ferritic stainless steel cold-rolled steel sheet characterized by comprising a steel material containing Si: 0.4% or less, P: 0.05% or less, and S: 0.010% or less, the balance being Fe and inevitable impurities. Production method.

  According to the present invention, it is possible to easily and inexpensively manufacture a stainless cold-rolled steel sheet having increased elongation and significantly improved press formability, particularly stretch formability, and has a remarkable industrial effect.

First, the reason for limiting the composition of the stainless cold-rolled steel sheet of the present invention will be described. Hereinafter, mass% in the composition is simply expressed as%.
C: 0.03% or less C dissolves in steel and contributes to stabilization of the austenite phase during hot rolling, and combines with Cr to form Cr carbide or Cr carbonitride in the crystal grains and grain boundaries. It precipitates in etc. If the content exceeds 0.03%, the amount of Cr carbide or Cr carbonitride increases excessively, and the steel sheet becomes harder and the elongation decreases. For this reason, C was limited to 0.03% or less. In addition, Preferably it is 0.025% or less.

N: 0.05% or less N, like C, contributes to the stabilization of the austenite phase during hot rolling by solid solution in steel and combines with Cr to form crystals as Cr nitride or Cr carbonitride Precipitates within grains and at grain boundaries. If the content exceeds 0.05%, the amount of Cr nitride or Cr carbonitride increases excessively, the steel sheet becomes hard, and the elongation decreases significantly. For this reason, N was limited to 0.05% or less. In addition, Preferably it is 0.03% or less.

Mn: 1% or less,
Mn is an element having an effect of strengthening the steel by solid solution in the steel, and it is desirable to contain 0.6% or more in the present invention for the coarsening of Cr carbonitride, but it contains more than 1%. Then, steel is hardened and press formability falls. For this reason, Mn was limited to 1% or less. In addition, Preferably it is 0.7 to 0.9%, More preferably, it is 0.75 to 0.85%. Moreover, it is preferable to make Mn / Si 2 or more from a viewpoint of form control of a deoxidation product.

Cr: 13-20%
Cr is an element that solidifies and strengthens steel and contributes to an improvement in corrosion resistance, and is an essential element for a stainless steel plate. However, if the Cr content is less than 13%, the corrosion resistance as stainless steel cannot be maintained. On the other hand, if the content exceeds 20%, the steel becomes too hard and the elongation decreases. For this reason, Cr was limited to the range of 13 to 20%.

Al: 0.01% or less
Al acts as a deoxidizer and combines with N in the steel and precipitates as AlN to harden the steel. AlN precipitates earlier than Cr carbonitride, and also precipitates finely, thus hardening the steel and lowering the elongation. Therefore, in the present invention, Al is limited to 0.01% or less. In addition, Preferably it is 0.005% or less.

In the present invention, except for the components described above, it is preferable to limit to Si: 0.4% or less, P: 0.05% or less, and S: 0.010% or less.
Si: 0.4% or less
Si is an element that acts as a deoxidizing agent. In the present invention that limits the Al content to a low level, deoxidation is performed with Si. Si is a solid solution strengthening element, and a large amount of it hardens the steel. Si is a ferrite-forming element and increases the ferrite phase fraction during hot rolling. For this reason, the generation of ridging becomes significant when a large amount of Si is contained. In the present invention, Si is preferably limited to 0.4% or less, more preferably 0.20% or less, from the viewpoints of preventing ridging and preventing hardening of steel.

P: 0.05% or less P has an effect of solid strengthening and reinforces the steel remarkably. However, P is an element that segregates at the ferrite crystal grain boundary and embrittles the steel. % Is acceptable. In addition, in order to ensure a high elongation value, it is preferable to limit to 0.03% or less. In addition, More preferably, it is 0.02% or less.

S: 0.010% or less S forms sulfides in steel. When Mn is contained, it binds to Mn to form MnS. MnS expands by hot rolling or the like and exists as precipitates (inclusions) at ferrite grain boundaries. Such sulfide-based precipitates (inclusions) lower the elongation and particularly have a great influence on the occurrence of cracks during bending, so it is desirable to reduce S as much as possible, but it is acceptable up to 0.010%. In addition, Preferably it is 0.005% or less.

The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include Ni: 0.5% or less, Cu: 0.05% or less, Mo: 0.05% or less, V: 0.05% or less, Nb: 0.01% or less, Ti: 0.01% or less, Ca: 0.01% or less, Mg: 0.01% or less is acceptable, but it goes without saying that the smaller the amount of inevitable impurities, the better.
Next, the structure of the ferritic stainless steel cold rolled steel sheet according to the present invention will be described.

  The structure of the ferritic stainless steel cold rolled steel sheet of the present invention is a single ferrite structure. As used herein, “ferrite single structure” refers to a structure composed of a ferrite phase and precipitates precipitated mainly in the ferrite crystal grains or at the ferrite crystal grain boundaries, mainly Cr carbonitride. When a second phase such as a martensite phase, a bainite phase, or an austenite phase is mixed in addition to the ferrite phase, the steel becomes hard and a desired elongation cannot be obtained.

Incidentally, the coarsened Cr carbonitride having an average particle size of 0.6 μm or more dispersed and precipitated in the ferrite grains is limited to 20 or less per ferrite grain. When the number of coarse Cr carbonitrides dispersed in ferrite grains exceeds 20 per ferrite grain, dislocations generated from the Cr carbonitride / ferrite interface occur, and the dislocations occur between the Cr carbonitrides. You will not be able to exercise without obstacles, and your elongation will decrease. For this reason, the number of coarse Cr carbonitrides dispersed and precipitated per ferrite grain is limited to 20 or less. Here, “Cr carbonitride” is a general term for Cr carbide and Cr nitride, and when these are combined, the composite is counted as one. Here, Cr carbide mainly refers to Cr ₂₃ C ₆ , and Cr nitride mainly refers to Cr ₂ N. Even if a part of Cr ₂₃ C ₆ and Cr ₂ N is substituted with other elements such as Fe and Mn, there is no difference in effect as long as the crystal structures are the same. Further, the “number of Cr carbonitrides” herein includes only those dispersed in ferrite grains, and does not include those precipitated at grain boundaries.

Note that the number of Cr carbonitrides dispersed and precipitated per ferrite grain is determined from a structural photograph of a plate thickness section parallel to the rolling direction. As for the number of Cr carbonitrides, an arithmetic average value of values measured for at least 20 grains adjacent in the cross-sectional structure is obtained.
Below, the preferable manufacturing method of the ferritic stainless steel cold-rolled steel plate of this invention is demonstrated.

The ferritic stainless steel cold-rolled steel sheet according to the present invention includes a hot-rolling process, a hot-rolled sheet annealing process, a cold-rolling process, a cold-rolling process, It is manufactured by applying a sheet annealing process.
After the molten stainless steel having the above composition is melted by a known melting method, a stainless steel material such as a slab is obtained by a known casting method, preferably a continuous casting method. In the present invention, the melting method and the casting method are not particularly limited, and any known method can be applied.

The obtained stainless steel material is subjected to a hot rolling process.
In the hot rolling process, the stainless steel material is preferably heated to a temperature of 1000 ° C. or higher. When the heating temperature is less than 1000 ° C, the deformation resistance increases, the rolling load during hot rolling increases, and an austenite phase having a hardness different from that of the ferrite phase is formed in the coarse ferrite. Cracks are likely to occur during hot rolling. For this reason, deterioration of surface quality such as fine cracks, shavings, and surface gloss deterioration occurs. For this reason, it is preferable to limit the heating temperature of the steel material to 1000 ° C. or higher.

The heated stainless steel material is hot-rolled by finishing hot rolling at a temperature of 900 ° C. or higher. In the hot rolling, winding at a winding temperature of 650 to 850 ° C. is preferable from the viewpoint of coarsening of Cr carbonitride.
If the end temperature of hot rolling is less than 900 ° C., the rolling load becomes high and the rolling roll becomes rough, and this is transferred to the steel sheet, so the surface roughness of the hot rolled sheet also increases, leading to deterioration of the surface quality of the steel sheet. For this reason, it is preferable that the completion | finish temperature of hot rolling shall be 900 degreeC or more.

Further, when the coiling temperature is less than 650 ° C., Cr carbonitride becomes fine. On the other hand, when it exceeds 850 ° C., Cr carbonitride precipitates in the form of flakes at the grain boundaries of the hot-rolled sheet. This piece of Cr carbonitride is pulverized and refined in the cold rolling process. For this reason, it is preferable to limit the winding temperature of a hot-rolled sheet to 650-850 degreeC. In addition, More preferably, it is 700-800 degreeC.
Subsequently, the hot-rolled sheet is subjected to a hot-rolled sheet annealing step for performing hot-rolled sheet annealing and pickling treatment. In the present invention, the hot-rolled sheet annealing and pickling treatment are not particularly limited, and it is preferable to use known hot-rolled sheet annealing conditions and pickling treatment conditions. As normal pickling conditions, a mixed acid aqueous solution is used, and preferable hot-rolled sheet annealing can be box-annealed from the viewpoint of homogenizing the hot-rolled sheet structure. The annealing temperature is preferably 800 ° C. or higher, more preferably 830 ° C. or higher. When the annealing temperature is less than 800 ° C, Cr carbonitride does not aggregate and coarsen.

The hot-rolled sheet that has been subjected to the hot-rolled sheet annealing process is then subjected to a cold-rolling process to obtain a cold-rolled sheet. The cold rolling conditions are preferably set to a cold reduction ratio of 90% or less in order to prevent the Cr carbonitride from being broken and refined. More preferably, it is 80% or less.
Next, the cold-rolled sheet is subjected to a cold-rolled sheet annealing process. In the present invention, from the viewpoint of coarsening of Cr carbonitride, the cold-rolled sheet annealing step is a step of applying a continuous annealing for 10 seconds or more at an annealing temperature of 800 ° C or higher, more preferably 850 to 900 ° C. Is preferred. When the annealing temperature is less than 800 ° C, Cr carbonitride does not grow sufficiently. On the other hand, when the annealing temperature is higher than 900 ° C., a part of the ferrite is transformed into austenite and becomes martensite after cooling, so that the steel becomes hard.

  Note that after the cold-rolled sheet annealing step, temper rolling with an elongation of 1.5% or less, preferably 0.3% or more may be performed for the purpose of shape correction.

  Molten steel having the stainless steel composition shown in Table 1 was melted to form a slab. Subsequently, these slabs were subjected to a hot rolling process under the conditions shown in Table 2 to obtain hot rolled sheets. The obtained hot-rolled sheet was subjected to a box-annealing that was soaked at 830 ° C. for 8 hours as a hot-rolled sheet, followed by a hot-rolled sheet annealing step in which pickling treatment was performed. Subsequently, the hot-rolled sheet was subjected to a cold-rolling process for performing cold rolling under the conditions shown in Table 2 to obtain cold-rolled sheets having a thickness shown in Table 2. Then, the cold-rolled sheet was subjected to a cold-rolled sheet annealing step under the conditions shown in Table 2 to obtain a cold-rolled annealed sheet.

About the obtained cold-rolled annealing board, the structure test and the tensile test were implemented and the structure | tissue and the tensile characteristic were investigated. The test method was as follows.
(1) Microstructure test A test piece was taken from a cold-rolled annealed plate, the central portion of the plate thickness was polished with a cross-sectional thickness parallel to the rolling direction, and corroded with aqua regia to reveal the structure. The obtained tissue was observed using a scanning electron microscope (magnification: 2000 times) to identify the tissue.
(2) Tensile test JIS 13B tensile test specimens were collected from the obtained cold-rolled annealed sheet so that the rolling direction was the tensile direction, and the tensile test was conducted in accordance with the provisions of JIS Z 2241. Properties (yield strength YS, tensile strength TS, elongation El) were determined.

  The obtained results are shown in Table 3.

  It can be inferred that each of the inventive examples has a ferrite single structure, has an El of 30% or more, has excellent ductility, and is excellent in press formability. On the other hand, in the comparative example that is out of the scope of the present invention, when El is less than 30%, ductility deteriorates and press formability deteriorates.

Claims (4)

mass%
C: 0.03% or less, N: 0.05% or less,
Mn: 1% or less, Cr: 13-20%,
Al: 0.01% or less of Cr carbonitride having a composition composed of the balance Fe and inevitable impurities and having an average particle size of 0.6 μm or more is dispersed in the ferrite grains by 20 or less per ferrite grain. A ferritic stainless cold-rolled steel sheet excellent in press formability, characterized by having a single ferrite structure. The composition is mass%,
C: 0.03% or less, N: 0.05% or less,
Mn: 1% or less, Cr: 13-20%,
Al: Including 0.01% or less,
Si: 0.4% or less, P: 0.05% or less,
2. The ferritic stainless steel cold-rolled steel sheet according to claim 1, wherein the ferritic stainless steel cold-rolled steel sheet comprises S: 0.010% or less, the balance being Fe and inevitable impurities. Ferrite made into a stainless steel cold-rolled steel sheet by subjecting a stainless steel material to a hot-rolling process, a hot-rolled sheet annealing process consisting of hot-rolled sheet annealing and pickling treatment, a cold-rolling process, and a cold-rolled sheet annealing process Stainless steel cold-rolled steel sheet manufacturing method, the stainless steel material in mass%,
C: 0.03% or less, N: 0.05% or less,
Mn: 1% or less, Cr: 13-20%,
Al: A steel material having a composition containing 0.01% or less and the balance Fe and inevitable impurities, the hot rolling process is heated to 1000 ° C or higher, hot rolling is finished at a temperature of 900 ° C or higher, and hot rolling is performed. After making the plate, the hot-rolled plate is a step of winding at a coiling temperature: 650 to 850 ° C., and the hot-rolled plate annealing treatment step is a step of performing a hot-rolled plate annealing treatment with an annealing temperature: 800 ° C. or more, The cold rolling process is a process of performing cold rolling at a cold rolling rate of 90% or less to obtain a cold rolled sheet, and the cold rolled sheet annealing process is continuously performed at an annealing temperature of 800 to 900 ° C. The manufacturing method of the ferritic stainless steel cold-rolled steel plate excellent in press formability characterized by setting it as the process of annealing. The composition is mass%,
C: 0.03% or less, N: 0.05% or less,
Mn: 1% or less, Cr: 13-20%,
Al: Including 0.01% or less,
Si: 0.4% or less, P: 0.05% or less,
The method for producing a ferritic stainless steel cold-rolled steel sheet according to claim 3, wherein the composition contains S: 0.010% or less and the balance is Fe and inevitable impurities.