JP3433698B2 - Ferritic stainless steel sheet with excellent weldability - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is suitable for household electric appliances, kitchen appliances, etc., which have a beautiful surface gloss and excellent weldability. a related ferritic stainless steel plate <br/>. 2. Description of the Related Art Ferritic stainless steel sheets have been used in various applications such as automobiles, household electric appliances (home electric appliances), kitchen appliances, western dishes, etc., because of their beautiful appearance and excellent cleanliness. . In the process of being used in various applications, materials that can meet various quality requirements such as formability, weldability, and beautiful surface finish are required. In response to this, for example, in order to improve the formability, a high-purity ferritic stainless steel in which a carbon-nitrogen fixing element such as Nb or Ti is contained in an ultra-low C steel is used. [0003] It is used as a member (visible member) whose product surface is directly visible to the human, such as an automobile molding material, a washing machine tub body plate and a bottom plate, an automatic dishwasher, a rice cooker inner pot, and an electromagnetic cooking pot. In such a case, a high-purity ferritic stainless steel containing Nb as a stabilizing element is particularly preferred in order to ensure beautiful surface. This is because, when Nb is used as a carbon-nitrogen fixing element, a steel having a fine-grained crystal structure can be obtained as compared with the case where Ti is used, so that the steel has excellent surface roughness resistance after press forming, and Due to good surface gloss. In some cases, further processing is performed after the ferritic stainless steel sheet is welded. For example, in the case of automobile exhaust system components, steel pipes obtained by welding steel plates are subjected to processing such as bending, overhanging, and hole expansion. At that time, there is a problem that a coarse columnar structure develops in the welded portion and its heat-affected zone, the ductility and toughness of the welded portion are reduced, and the formability is difficult. A ferritic stainless steel excellent in formability is required. [0005] In order to improve the workability of a welded portion of a material used for applications in which the surface gloss is not so important, such as for the above-mentioned automobile exhaust system parts, it is common to include Ti alone. This is intended to refine the crystal structure by the action of Ti-based precipitates precipitated in a high temperature range. Japanese Unexamined Patent Publication (Kokai) No. 58-67851 discloses a ferritic stainless steel containing 2 to 3% of Si and having excellent weldability in formability. The steel disclosed herein contains a large amount of Si, so that when the steel is melted and solidified at the time of welding, no austenite phase is generated and the ferrite single phase is suppressed, thereby suppressing martensitic transformation and thereby improving the weldability. To prevent the curing of [0007] JP-A-63-157837 discloses S
i, Mn, Cr, Ni, Ti, N, Al, C,
A heat-resistant ferritic stainless steel excellent in welding workability and workability of a welded portion in which the N and Ti contents are adjusted to satisfy a specific relational expression is disclosed. [0008] Recently, in applications where the appearance of the surface is important, such as household electrical appliances or members for kitchen appliances, there has been a case where a welded portion of a ferritic stainless steel plate is subjected to more severe processing than before. Is increasing. For example, in a component having an opening for attaching another component to the bottom of a cylindrical container, such as a bottom plate of a washing machine tub, conventionally, a circular blank is press-formed into a cup shape, and the bottom is punched to form an opening. However, this method requires many processing steps and the material yield is not good. As a method of reducing the manufacturing cost of such a part, there is a method in which a steel plate is welded into a tube and both ends thereof are press-formed to form a container bottom and a flange. FIG. 1 is a conceptual diagram showing one example of such a processing method. Hereinafter, thickness 1 mm, width 300 mm, length 6
A case where a 00 mm steel plate is processed will be described as an example. As shown in FIG. 1A, the steel plate having the above dimensions has a height of 300 mm.
Then, as shown in FIG. 3 (b), the tube is welded at the center except for 20 mm at both ends. [0010] As shown in FIG.
Press-mold 0 mm and bend 90 ° inside the tube to form the bottom of the cylindrical container. The bending radius at that time is 20mm
It is. At this time, the unwelded portions at the ends are not constrained in the circumferential direction, so they overlap, so that there is no excess material in the circumferential direction, and a good shape can be secured. The other 25 mm portion of the tube is 90 outside the tube.
° Bend to form a cylindrical container flange. Its bending radius is 10 mm. At this time, since the unwelded portion at the end opens in the circumferential direction, there is no shortage of material in the circumferential direction,
A good shape can be secured. According to such a processing method, a cylindrical container having a circular opening at the bottom can be obtained without punching the bottom, so that the material yield is improved as compared with the conventional case. However, when the flange is formed by the above method, the circumferential tensile stress and the bending stress simultaneously act on the flange welded portion,
In order to realize this processing method, it is necessary to stably obtain a sound welded portion capable of withstanding such a stress state and capable of being formed. However, according to the study results of the present inventors, Japanese Patent Application Laid-Open No. 58-67851 and Japanese Patent Application Laid-Open
Even the ferritic stainless steel disclosed in Japanese Patent No. 7837 breaks from the welded end at the time of press forming, and it is difficult to stably work with a good yield. In addition, the stainless steels disclosed in these publications do not always provide sufficient surface gloss, and there is a problem in this respect as a material for visible parts where beauty is important. An object of the present invention is to solve such problems.
An object of the present invention is to provide a ferritic stainless steel having excellent surface gloss required for appearance-oriented components such as home appliances and kitchen appliances, and having a workability of a welded part, particularly, its formability and toughness superior to conventional materials. . Means for Solving the Problems The inventors of the present invention have made a high-purity ferritic stainless steel containing Nb in order to ensure press formability and surface gloss, and to weld such a stainless steel. As a result of various studies on the effects of the properties of the weld and the chemical composition of the steel on the workability of the weld, the following findings were obtained. A. The conventional welded portion of high-purity ferritic stainless steel does not harden because it does not transform into martensite when cooled, but coarse columnar crystals develop and workability and toughness are poor. If the crystal structure of the welded portion can be a crystal structure mainly composed of equiaxed grains, the workability of the welded portion can be improved. The crystal structure that contributes to the improvement of workability is the ratio of the length in the long axis direction to the length in the short axis direction of a crystal grain generally used as a shape factor of a crystal grain (aspect ratio; the long axis direction of the crystal grain). The crystal grains have a length / length in the short axis direction of 1.5 or less (hereinafter, such crystal grains are referred to as “equiaxed grains”). Observing the cross section perpendicular to the weld line, if the ratio of equiaxed crystals to the weld region is 5% or more, it can be determined that the weld is a weld that can be formed into a desired shape. Here, the welded region refers to a region from the boundary between the base material and the heat-affected zone to the other boundary. B. In a high-purity ferritic stainless steel containing Nb, by adding an appropriate amount of Ti and N in addition to Nb, a weld having equiaxed grains of 5% or more can be obtained. The region where desired equiaxed grains are obtained is arranged by Ti × N, and the value may be any value as long as the value is a certain amount or more. This is because TiN precipitates at high temperature and TiN becomes a nucleus for the growth of equiaxed grains. C. If an appropriate amount of Al is further contained in addition to Ti and N, the range of Ti × N at which equiaxed crystals can be obtained is expanded to a lower alloy side, and an equiaxed grain structure can be obtained more stably. The above-mentioned effect by containing Al is as follows.
This is because TiN is non-uniformly deposited starting from the Al oxide. The invention, such has been completed based on the findings and has as its gist in ferritic stainless steel plate excellent in steel machining of the weld described below. Chemical composition in weight%, C: 0.02% or less, S
i: 1.0% or less , Mn: 1.0% or less , Cr: 11 to 20%, Cu:
0 to 0.7%, Ti: 0.05 to 0.3%, Nb: 0.1 to 0.7%, A
l: 0.090% or less, N: 0.003 to 0.03%, the balance consists of Fe and unavoidable impurities, Ti, Nb and N contents satisfy the relationship of the following formula (1), and further contain Ti and N When the Al content is less than 0.003%, the following formula (2)
Satisfy the relationship represented by the following formula when the Al content is 0.003 to 0.090% (3) is to satisfy the relation from the welding, characterized in workability of the weld to be subjected to press molding excellent ferritic stainless steel plate. [Equation 2] Embodiments of the present invention will be described below in detail. In addition, the following percentages of the chemical composition mean% by weight. C: C combines with Cr to form a Cr carbide at grain boundaries, which causes intergranular corrosion and reduced workability of welds. Therefore, the smaller the C content, the better. In the steel of the present invention, by containing a stabilizing element such as Ti or Nb, the adverse effect is mitigated.
%, The content of the necessary stabilizing element increases and the cost of steel increases, so the C content is set to 0.020% or less. Preferably it is 0.015% or less. Si: Si is not an essential element, but may be contained because it can deoxidize molten steel and efficiently reduce Cr. However, if the Si content exceeds 1.0%, the formability of the steel material deteriorates. Therefore, even when the Si content is contained, the upper limit is set to 1.0%. Mn: Mn is not an essential element but may be contained because it has an action of deoxidizing molten steel. In order to surely obtain the deoxidizing effect of Mn, its content is preferably 0.1% or more. On the other hand, if the content exceeds 1.0%, the cost of the steel increases and it is economically disadvantageous, so the Mn content is set to 1.0% or less. Cr: Cr is an element effective in ensuring the corrosion resistance and oxidation resistance of ferritic stainless steel. However, if the content is less than 11%, these effects are insufficient, so that Cr is contained at 11% or more. It is preferably at least 13%. On the other hand, if the Cr content exceeds 20%, the toughness of the steel is significantly reduced.
The following is assumed. Preferably it is 19% or less. Cu: Cu is not an essential element but may be included because it has the effect of improving the formability of ferritic stainless steel. However, its content is 0.7
If the content exceeds 0%, the hot workability deteriorates. Therefore, even when the content is contained, the upper limit is set to 0.70%. Ti: In order to ensure the workability of the welded portion so that it can withstand severe processing in which a tensile bending stress acts, the welded portion must have a crystal structure rich in equiaxed crystals. There is. Specifically, it is preferable that the equiaxed grains having an aspect ratio of 1.5 or less have an area ratio of 5% or more, more preferably 10% or more, with respect to a coarse grain region generated in a welded portion. The size of the equiaxed grains is preferably 200 μm or less in the major axis direction in order to secure the toughness of the welded portion. Ti is an element effective for forming precipitates by combining with C and N in steel at a high temperature to form equiaxed grains in a crystal structure of a welded portion. Further, since the amount of solid solution of C and N is reduced, there is also an effect that the formability and corrosion resistance of steel are improved. In the present invention, 0.05% or more of Ti is contained in order to obtain an equiaxed graining effect of the weld crystal structure. Preferably it is 0.08% or more. Further, in order to effectively precipitate TiN,
The relationship between Ti and N content is adjusted according to the Al content.
Specifically, when the Al content is less than 0.003%, Ti × N ≧ 1.3 × 10 ⁻³ is satisfied. It is preferably 1.8 × 10 ⁻³ or more. Al content 0.003%
In this case, Ti × N ≧ 6 × 10 ⁻⁴ . It is preferably at least 9 × 10 ⁻⁴ . If a large amount of Ti is contained alone, Ti oxides are likely to agglomerate, and these become the starting points of surface flaws, which may impair the surface gloss of the product. T to avoid this
The i content is 0.30% or less. Preferably 0.25
% Or less. Nb: In the present invention, the lower limit of the Ti content is specified in order to ensure the workability of the welded portion, and the upper limit is controlled in order to ensure excellent surface gloss. Therefore, C and N may remain in a solid solution state and the workability of the base material may be poor. Further, when Ti is solely contained, the precipitates are coarsened, the crystal structure of the base material is liable to become a coarse-grained structure, and the surface is likely to be roughened during press molding. To avoid these, Nb is contained. That is, Nb combines with C and N and has the effect of improving the formability, corrosion resistance and high-temperature oxidation resistance of steel.
In addition, the precipitate when Nb is contained is finer and dispersed as compared with the case where Ti is contained alone.
For this reason, Nb has a surface roughness improving action at the time of press molding more than Ti does, and the surface gloss is improved. In order to surely obtain such an effect, Nb is contained in an amount of 0.10% or more. Preferably it is 0.20% or more. On the other hand, if the Nb content exceeds 0.70%, the above effect is saturated and only the cost of steel increases, so the Nb content is set to 0.70% or less. Preferably it is 0.60% or less. Further, in order to sufficiently fix C and N as precipitates and to secure the workability of the steel itself, the contents of Ti, Nb and N are contained within a range satisfying the following formula (1). [Equation 3] Al: Al oxide combines with Ti carbonitride to promote the equiaxed grain formation of the crystal structure of the weld. When Al is contained, the Ti × N oxide required for the equiaxed crystallization is formed. This has the effect of expanding the area in a small number of directions. In addition, Al has a deoxidizing effect on molten steel and can be expected to have an effect of improving the yield of Ti. Therefore, although Al is not an essential element, it is preferable to contain Al in an amount of 0.003% or more in order to effectively obtain the above effects. More preferably, it is 0.005% or more.
On the other hand, if the Al content exceeds 0.090%, the deoxidizing action by Al becomes excessive, and the aggregation and coarsening of Al oxide, Ti oxide and nitride are promoted, and flaws are formed on the surface of the hot-worked steel material. Frequently occur. Therefore, the upper limit of the Al content is 0.090.
%. Preferably it is 0.050% or less. N: N combines with Ti to form Ti-based inclusions (main components of which are Ti and N) and has an effect of promoting equiaxed grain formation of the welded structure. . On the other hand, if the N content exceeds 0.03%, the workability and toughness of the base material deteriorate, so the N content is set to 0.03% or less. Preferably it is 0.015% or less. Other than the above, the chemical composition of the ferritic stainless steel of the present invention is Fe and inevitable impurities. The ferritic stainless steel of the present invention can be easily produced by subjecting a steel having the above chemical composition to hot rolling by a conventional method or cold rolling and heat treatment. The ferritic stainless steel of the present invention is effective in known welding methods such as TIG welding, plasma welding, laser welding, and electron beam welding. EXAMPLE A ferritic stainless steel having various chemical compositions is melted by a vacuum melting method to form a steel ingot, and subjected to hot forging and machining to form a steel having a thickness of 40 mm, a width of 140 mm and a length of 85 mm. It was a piece. This was heated to 1150 ° C. and held for 1 hour, and then hot-rolled to a thickness of 5 mm and a width of 160 m.
m and a length of 595 mm. The hot-rolled steel sheet is held at 980 ° C. for 10 minutes and soft-annealed, and both surfaces are ground by 0.7 mm in a direction parallel to the rolling direction to remove an oxide film and smoothen the surface. The rolls were cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.0 mm. Thereafter, recrystallization annealing was performed by heating to 980 ° C. and holding for 5 minutes, and an oxide film on the surface was removed by an alkali molten salt and a pickling treatment to obtain a steel sheet sample. The surface gloss and the presence or absence of surface flaws of the above steel sheet sample were visually observed and judged according to the following criteria, and ◎ and ○ were evaluated as good. Good gloss: 、, slightly whitish but without barbed defects: 欠 陥, slightly whitish with barbed defects: Δ, whitish: x The formability of the steel sheet sample was investigated by the following method. Cut out a strip whose thickness direction is 1.0mm, length 160mm, width 25mm and the length direction is parallel to the width direction, and JIS-Z
Machined into No. 13B tensile test specimens specified in 2201. Thereafter, a tensile test was performed in accordance with the method specified in JIS-Z2241 to determine the elongation at break, and evaluated according to the following criteria. Elongation at break 30% or more: 、, less than 30%, 28% or more: 、, less than 28%, 22% or more: Δ, less than 22%: x. The properties of the welds were evaluated by the following methods. FIG. 2 is a process chart showing a procedure for producing a test piece for evaluating weldability. 150mm length, 70mm width from steel plate sample
A test piece whose length direction is parallel to the width direction is cut out, and two test pieces are abutted on the long side surface as shown in FIG.
About 10 mm of the end on the welding line was stopped halfway so as not to be joined, and TIG welding was performed. The welding conditions were welding current 60A,
The voltage was set to 12 V, the welding speed was set to 25 cm / min, and Ar gas was blown onto each of the front and back surfaces at 10 L / min to shield. The structure of the weld metal (fused portion) was observed in a section perpendicular to the welding direction of the steady welding portion, and the area ratio of equiaxed grains was measured. The equiaxed grain area ratio was calculated according to (average width of equiaxed grain structure / average width of welded region) × 100 (%).
The “width” described in the denominator and the numerator is an average width in the thickness direction. The workability of the weld was examined by the following method.
As shown in FIG. 2B, the welded plate was machined to obtain a disk having a diameter of 110 mm including an unjoined portion. Using a cylindrical punch having a diameter of 50 mm, the disk was deep drawn into a cylinder having a flange with a depth of 25 mm. Both the punch shoulder radius and the die shoulder radius are 5 mm, using a commercially available lubricating oil, the wrinkle holding force is 1.0 ton, and the punch press-in speed is 50 mm.
/ Min. The occurrence of cracks at the weld end at the flange was evaluated according to the following criteria. No cracks in welds, no break: 、, slight cracks in welds, no problem in use: ○, cracks in welds: Δ, fractures in welds: x. Table 1 also shows the chemical composition of the steel and the results of the performance investigation on the cold-rolled steel sheet. [Table 1] As can be seen from Table 1, the ferritic stainless steels of Test Nos. 1 to 10 satisfying the conditions specified by the present invention have good surface properties, good elongation at break of the steel and good formability of the weld. there were. The crystal structure of the weld had sufficient equiaxed grains. On the other hand, in Test Nos. 11 and 12, the Ti content was too small, so that the equiaxed crystal ratio was low and the workability of the weld was not good. Test Nos. 14 and 15 with too low Nb content and Test No. 17 with too high Ti content
Had poor surface gloss. In Test No. 16, the Ti content was too small, and the value determined by Expression (1) was less than 7, and the elongation at break of the steel and the workability of the welded portion were poor. In Test Nos. 13, 16, and 17, in which the value of Ti × N was small, there were few equiaxed grains in the welded portion, and the workability was poor. The ferritic stainless steel of the present invention
Because the surface gloss is good and the workability of the weld is excellent
It is required to have a beautiful surface gloss such as for household appliances and kitchen appliances, and can sufficiently withstand use even in applications in which a weld is to be subjected to strong working. Therefore, the product yield can be improved and the productivity of product processing can be significantly improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process chart showing a method of welding a steel plate into a tube and press-forming both ends thereof to form a container bottom and a flange. FIG. 2 is a process chart showing a procedure for producing a test piece for evaluating workability of a weld portion.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-78751 (JP, A) JP-A-9-227999 (JP, A) JP-A-3-264652 (JP, A) JP-A-8-8 144021 (JP, A) JP-A-10-17937 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (1)

(57) [Claims] [Claim 1] The chemical composition is% by weight, C: 0.02% or less,
Si: 1.0% or less , Mn: 1.0% or less , Cr: 11 to 20%, Cu:
0 to 0.7%, Ti: 0.05 to 0.3%, Nb: 0.1 to 0.7%, A
l: 0.090% or less, N: 0.003 to 0.03%, the balance consists of Fe and unavoidable impurities, Ti, Nb and N contents satisfy the relationship of the following formula (1), and further contain Ti and N When the Al content is less than 0.003%, the following formula (2)
Satisfy the relationship represented by the following formula when the Al content is 0.003 to 0.090% (3) is to satisfy the relation from the welding, characterized in workability of the weld to be subjected to press molding excellent ferritic stainless steel plate. [Equation 1] 87.5Ti (%) + 45.2Nb (%) − 300N (%) ≧ 7 ・ ・ ・ (1) Ti (%) × N (%) ≧ 1.3 × 10 ^-3・ ・ ・ (2) Ti (%) × N (%) ≧ 6 × 10 ^-4 (3)