JP4852857B2 - Ferritic / austenitic stainless steel sheet with excellent stretch formability and crevice corrosion resistance - Google Patents

Description

The present invention of stainless steel, more particularly stretch forming property and crevice corrosion resistance excellent ferrite-austenite stainless steel plate (hereinafter simply referred to as "ferrite-austenite stainless steel").

  For wheel caps for automobiles, etc., a material that has both high overmoldability and resistance to crevice corrosion is required. Stainless steel is widely known as a material having corrosion resistance, and is used in a wide range of applications such as automobile parts, building parts, and kitchen appliances. Stainless steel is roughly classified into four types: austenitic, ferritic, martensitic, and ferrite-austenitic. Among these, austenitic stainless steel plates are most commonly used as stainless steel plates for automobile wheel caps.

  However, the austenitic stainless steel represented by SUS301 has some problems such as corrosion in the gap between the wheel and the cap due to incoming salt in the gulf area, and due to snow melting salt in the snowfall area. It has been pointed out that the corrosion resistance is insufficient. In addition, there is a problem that it is difficult to apply to a member having a complicated shape because a crack may occur when molding is performed up to the vicinity of the molding limit. Further, since it generally contains 6% or more of Ni, there is also a problem that it is expensive.

  Ferritic stainless steel, on the other hand, can improve crevice corrosion resistance by increasing the amount of Cr, but it has a problem that it is much less stretchable and difficult to form than austenitic stainless steel. is there. Further, martensitic stainless steel is insufficient in both stretch formability and crevice corrosion resistance.

  In contrast, ferrite and austenitic stainless steel sheets are considered to be promising as stainless steel for automobile hole caps because they are relatively excellent in corrosion resistance and stretch formability, but generally the Ni content is 4%. There is a problem that it is expensive because it is extremely high.

  In order to solve this problem, for example, Patent Document 1 discloses that Ni content is limited to more than 0.1 and less than 1%, and the stability of austenite is enhanced by taking the IM index defined below to 40 to 115, and Ni content is increased. A ferrite-austenitic stainless steel with a low amount and excellent tensile elongation is disclosed. Here, IM = 551-805 (C + N)%-8.52Si% -8.57% Mn-12.51Cr% -36Ni% -3.45Cu% -14Mo%.

  In addition, in order to reduce the Ni content of austenitic stainless steel and ferritic / austenitic stainless steel, an attempt has been made to contain a large amount of N instead of Ni. A method for producing austenitic stainless steel and ferrite-austenitic stainless steel substantially free of Ni by adding a large amount of nitrogen by the method is disclosed.

JP 11-71643 A Yasuyuki Katada “Manufacture of High Concentration Nitrogen Steel by Pressurized ESR Method”, Vol.7 (2002) p.848

  However, although the ferrite-austenitic stainless steel disclosed in Patent Document 1 is recognized to have high tensile elongation, since it contains a large amount of Mn, the corrosion resistance of the gap portion is insufficient, and the overhanging is also difficult. There is a problem that the moldability is unknown. On the other hand, the means disclosed in Non-Patent Document 1 requires a large facility for pressure dissolution even if it is simply considered as Ni saving means, and it is necessary to prepare an electrode for melting raw material in advance. It contains many cost-up factors. Furthermore, simply replacing Ni with N does not provide a material having both stretch formability and crevice corrosion resistance.

  An object of the present invention is to solve the problems associated with the prior art described above, and to provide a ferritic / austenitic stainless steel having both high formability and resistance to crevice corrosion while reducing the amount of Ni. .

The ferrite-austenitic stainless steel according to the present invention has a mass ratio of C: 0.111% or less, Si: 1.2% or less, Mn: 1.91% or less, P: 0.1% or less, S: 0.03% or less, Cr: 15% or more and 35% or less, Ni: 0.83% or less, N: 0.108% or more and 0.511% or less, balance Fe and inevitable impurities, and has a metal structure with an austenite phase fraction of 29 vol% or more and 82 vol% or less. Excellent formability and crevice corrosion resistance.

The above ferritic / austenitic stainless steels further include V: O.028% or more and 0.5% or less, Al: 0.003% or more and 0.1% or less, Mo: 4% or less, Cu: 0.32% or more and 4% or less , B: One or more selected from 0.01% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.1% or less, Ti: 0.1% or less can be optionally contained. The ferritic / austenitic stainless steel here is a stainless steel containing a ferrite phase and an austenitic phase, and may contain a martensite phase or the like.

  The ferritic / austenitic stainless steel of the present invention is excellent in overhang property and crevice corrosion resistance despite being relatively inexpensive because of its low Ni content. Thereby, it becomes possible to economically manufacture a workpiece having a complicated shape such as an automobile wheel cap without risk of cracking.

  The composition (%, mass%) of the ferrite-austenitic stainless steel according to the present invention is as follows.

C: 0.111% or less
C, together with N, has an effect of increasing the austenite phase fraction and concentrating in the austenite phase to increase its stability. Furthermore, it is effective for increasing the strength of the steel. In order to express these effects, C is preferably contained in an amount of 0.003% or more. However, if it exceeds 0.2%, the heat treatment temperature for solid solution becomes remarkably high, which hinders mass production. In addition to this, based on the examples shown later, in the present invention, the C content is 0.111% or less. Preferably, it should be limited to 0.05% or less.

Si: 1.2% or less
Si is an effective element as a deoxidizing material and should be contained in an amount of 0.01% or more. However, if it exceeds 1.2%, the hot workability deteriorates, so 1.2% or less, preferably 1.0% or less. In addition, when considering deterioration of corrosion resistance due to further sensitization (deterioration of corrosion resistance due to formation of chromium carbide and chromium nitride at grain boundaries), it is preferably 0.4% or less.

Mn: 1.91% or less
The Mn content is particularly important in achieving excellent stretch formability and crevice corrosion resistance. FIG. 1 is a graph showing the influence of the Mn content on the stretch formability (Ericsen value) in a ferritic / austenitic stainless steel having an Ni content of 1% or less and an austenite phase fraction of 40 to 50 vol%. As shown here, Mn greatly affects the stretch formability, and the stretch formability is significantly improved at 1.91% or less. The reason for this is not deterministic and does not affect the outer extension of the present invention, but when the Mn content is low, the Mn concentration in the ferrite phase is significantly reduced, resulting in a significant increase in the ductility of the ferrite phase. It can be improved.

FIG. 2 is a graph showing the influence of the Mn content on the outdoor exposure test results in a ferritic / austenitic stainless steel having an Ni content of 1% or less and an austenite phase fraction of 40 to 50 vol%. When the Mn content is about 2% or less, good crevice corrosion resistance is obtained. The reason for this is not deterministic and does not affect the extension of the present invention. However, when the Mn content is low, inclusions that adversely affect the corrosion resistance of gaps such as MnS are reduced. Can be mentioned. Based on the knowledge shown in FIG. 1 and FIG. 2, and based on the examples described later , the Mn content is 1.91% or less, preferably in order to obtain sufficient properties with respect to stretch formability and crevice corrosion resistance. Limited to 1.5% or less.

P: 0.1% or less, S: 0.03% or less
P is an element that is harmful to the corrosion resistance of the gap-resistant portion. Particularly, when P exceeds 0.1%, the effect becomes significant, so it is limited to 0.1% or less, preferably 0.05% or less. On the other hand, S is an element that is harmful to hot workability. Particularly, if it exceeds 0.03%, the influence becomes significant, so it is limited to 0.03% or less, preferably 0.02% or less.

Cr: 15% to 35%
Cr is a main component that imparts corrosion resistance. If the content is less than 15%, sufficient crevice corrosion resistance cannot be obtained. On the other hand, if the content is less than 15%, the stability of the austenite phase is low, and many of the austenite phases are transformed into the martensite phase at the initial stage of processing, so that excellent stretch formability cannot be obtained. However, if the Cr content exceeds 35%, it becomes difficult to form a ferrite-austenite structure having an austenite phase. Therefore, the Cr content is limited to 15 to 35%, preferably 17 to 30%, and more preferably 18 to 28%.

Ni: 0.83% or less
Ni is an element that promotes the formation of the austenite phase, but when the content is high, excellent stretch formability cannot be obtained. For example, SUS329 ferritic / austenitic stainless steel contains about 50% austenitic phase, but when Ni content exceeds 1%, the stretch formability deteriorates remarkably. Ni is an expensive alloy element, and its content is required to be reduced as much as possible in order to produce a ferrite-austenite structure from the viewpoint of economy and resource saving. In the present invention, from this viewpoint , the Ni content is further limited to 0.83% or less, preferably less than 0.5% , based on the examples described later . However, if the Ni content is 0.10% or less, the toughness of the steel decreases in both the base metal and the welded portion. Therefore, it is preferable to contain Ni at least over 0.10% in order to improve toughness including the welded portion.

N: 0.108% or more and 0.511% or less In the present invention, the Ni content is limited to 0.83% or less. Therefore, N is contained as an austenite phase formation promoting element in order to form an austenite phase in an amount sufficient to form a ferrite-austenite structure. If its content is less than 0.05%, a sufficient amount of austenite phase is not formed, while if it exceeds 0.6%, economically disadvantageous means such as pressure dissolution must be employed. In the present invention, in addition to the above viewpoint, the N content is limited to 0.108 to 0.511% based on examples described later . In addition, it is preferable to restrict | limit to 0.18% or more within the said range from a viewpoint of austenite phase production | generation, and to 0.34% or less within the said range from a viewpoint of hot workability improvement.

  In the present invention, in addition to the above elements, the following elements can be contained as required.

V: 0.028% to 0.5%
V refines the steel structure and increases the strength. In order to exhibit the effect effectively, it is preferable to contain 0.005% or more. However, if it exceeds 0.5%, it becomes difficult to reduce the precipitation of the V compound even if the annealing temperature is raised, and the stretch formability deteriorates . In the present invention, in addition to the above viewpoint, the V content is set to 0.028% or more and 0.5% or less based on examples described later.

Al: 0.003% to 0.1%
Al can be used as a deoxidizer and can be contained as much as necessary as a deoxidizer. This effect as a deoxidizing material is recognized at 0.003% or more, but when it exceeds 0.1%, nitrides are formed, which causes the steel sheet to become wrinkled. In the present invention, in addition to the above viewpoint, the Al content is set to 0.003% to 0.1% based on examples described later.

Mo: 4% or less, Cu: 0.32% or more and 4% or less These elements are effective in improving the corrosion resistance, and in order to effectively exhibit the effects, it is preferable to contain 0.1% or more. However, even if the Mo content exceeds 4%, the effect of improving the corrosion resistance is saturated and the economic efficiency is impaired. Therefore, the maximum content is 4% or less, preferably 2% or less. On the other hand, with respect to Cu, in addition to the viewpoint that hot workability is remarkably deteriorated when it exceeds 4% , Cu content is set to 0.32% or more and 4% or less based on examples described later .

B: 0.01% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.1% or less, Ti: 0.1% or less These elements are useful for improving the hot workability of steel. It can be contained in a range that achieves the object and does not cause harmful effects due to excessive inclusion. In order to effectively express these effects, B, Ca, and Mg should be 0.0003% or more, and REM and Ti should be 0.002% or more. However, since corrosion resistance deteriorates due to excessive content of B, the upper limit of the content is preferably 0.01% or less, and preferably 0.005% or less. For the same reason, Ca, Mg and REM are each 0.01% or less, preferably 0.005% or less, Mg is 0.01% or less, preferably 0.005% or less, and REM is 0.1% or less, preferably 0.05% or less. . If Ti is added excessively, it will cause flaws in the steel sheet due to the formation of nitrides, so it should be limited to 0.1% or less, preferably 0.05% or less.

Nb: 2% or less
Nb is effective in suppressing sensitization, and is preferably contained in an amount of 0.01% or more in order to effectively exhibit the effect. However, if the content of Nb exceeds 2%, a large amount of Nb carbonitride is generated, and as a result, solute C and solute N are consumed, so the upper limit is preferably limited to 2%.

Inevitable impurities except the remaining Fe Components other than the above components are Fe except for inevitable impurities. Inevitable impurities include deoxidation products such as O (oxygen). It is desirable to reduce these as much as possible, including cases where they inevitably remain.

  In the present invention, by using the above composition, the crevice corrosion resistance superior to that of austenitic stainless steel or ferritic stainless steel containing 15 to 35% of Cr equivalent to that of the present invention is exhibited. It is speculated that in ferrite and austenitic stainless steels, the concentration of Cr in the ferrite phase and the concentration of N in the austenite phase strengthens the passive film of each phase.

  The ferrite-austenitic stainless steel according to the present invention has the above composition, and its metal structure needs to have an austenite phase fraction in the structure of 10 vol% or more and 85 vol% or less.

FIG. 3 is a graph showing the relationship of the austenite phase fraction on the stretch formability of a ferritic / austenitic stainless steel sheet having an Mn content of 2% or less and an Ni content of 1% or less. As shown here, the stretch formability is improved by an increase in the austenite phase fraction, and excellent stretch formability is exhibited when the austenite phase fraction is 10 vol% or more, particularly 15 vol% or more. However, in the present invention, the Ni content is limited to 1% or less from the economical aspect, and in that case, it becomes difficult for the austenite phase fraction to exceed 85 vol%. In the present invention, in addition to the above viewpoint, the austenite phase fraction is further limited to 29 to 82 vol% based on examples described later .

The austenite phase fraction is the volume fraction of austenite in the structure. Typically, the steel structure is observed under a microscope, and the proportion of austenite in the structure is determined by the line segment method or the surface segment method. It can be determined by measuring. Specifically, after polishing the sample, the red blood salt solution (potassium ferricyanide (K ₃ [Fe (CN) ₆ ]): 30 g + potassium hydroxide (KOH): 30 g + water (H ₂ O): 60 ml) Under the optical microscope, the ferrite phase is identified as gray, and the austenite phase and martensite phase are identified as white.Therefore, the proportion of the gray portion and the white portion is obtained by image analysis, and the ratio of the white portion is determined as the austenite phase fraction. It is a rate. Strictly speaking, since the austenite phase and the martensite phase cannot be distinguished in this method, not only the austenite phase but also the martensite phase may be included in the white portion. Even when a phase is included, the object effect of the present invention can be obtained if the austenite phase fraction measured by this method and other conditions are satisfied.

  Ferrite and austenitic stainless steels with the above basic composition and an austenite phase fraction in the metal structure of 29 vol% or more and 82 vol% or less are relatively low cost and aim to save resources of Ni resources. Even though it is a material, it has excellent stretch formability and crevice corrosion resistance.

  The effect of the present invention can be obtained by any of a hot rolled sheet, a hot rolled annealed sheet, and a cold rolled annealed sheet. In addition, the effect of the present invention can be obtained in any surface finishing state such as No. 2A, No. 2B, BA, and polishing finishing regardless of the finishing state. The effect of the present invention can be obtained regardless of the shape of the product. For example, the effect can be obtained even in the shape of a wire, a steel bar, a shape steel, a steel pipe, or the like.

  Various steels having the compositions shown in Table 1 were melted in an atmosphere in which vacuum melting or nitrogen partial pressure was controlled within a range of up to 0.9 atm (882 hPa) to form a steel slab (or steel ingot, ingot). According to a conventional method, hot rolling, annealing, and cold rolling were performed, and then finish annealing was performed at a temperature of 900 to 1300 ° C. to obtain a cold-rolled annealed plate having a thickness of 1.25 mm. The obtained cold-rolled annealed plate was measured for austenite phase fraction, stretch formability and crevice corrosion resistance.

  Here, the austenite phase fraction was measured by observing the metal structure of a plate thickness section parallel to the rolling direction of the cold-rolled annealed plate. That is, after polishing the sample, it was etched with the above-mentioned red blood salt solution, and after taking a tissue photograph in the range of 400 to 1000 times the total thickness (1.25 mm thickness) x 0.15 mm length, these entire photographs were taken. The austenite phase fraction was measured by the surface segment method using the white part as the austenite phase. The stretch formability is determined by the Erichsen test, and the punch indentation length until cracking occurs is the Erichsen value. At this time, the test piece was a square plate having a size of 80 mm × 80 mm, and was lubricated by applying graphite grease, and was performed under the conditions of a punch diameter of 20 mm and a wrinkle holding force of 15.7 kN. In addition, as shown in Fig. 4, the crevice corrosion resistance test is a cold-rolled annealed plate of 3cm width x 4.5cm length from which the surface scale of the same material is removed from a cold-rolled annealed plate of 8cm width x 12cm length without the surface scale. (Small plates) are stacked, and these are fixed tightly with Teflon (registered trademark) bolts and Teflon (registered trademark) washers, and an outdoor exposure test is conducted at a location approximately 0.7 km from the coast for 7 months. After that, the test piece was disassembled, and the presence or absence of occurrence of corrosion in the gap portion and the base material portion was visually observed.

  The measurement results are shown in Table 2. As is clear from Tables 1 and 2, the ferrite-austenitic stainless steel sheet satisfying the present invention has an Erichsen value of 12 mm or more and high stretchability, and no gap-resistant part was observed in the exposure test. In Table 2, the evaluation of the corrosion resistance of the gap-resistant portion is the case where the mark “◯” indicates no corrosion and the mark “X” indicates corrosion.

  In the same manner as in Example 1, a steel having the composition shown in Table 3 was melted to form a steel slab (or steel ingot, ingot), followed by hot rolling, annealing, cold rolling according to a conventional method, Thereafter, finish annealing was performed at a temperature of 900 to 1300 ° C. to obtain a cold-rolled annealed sheet having a thickness of 1.25 m. About the obtained cold-rolled annealing board, the austenite phase fraction, the overhang formability (Ericsen value), and the crevice corrosion resistance were measured in the same manner as in Example 1. The results are shown in Table 4. As compared with the comparative examples shown in Table 1 and Table 2, it is clear that the stretch formability (Ericsen value) and the crevice corrosion resistance are excellent.

It is the graph which showed the influence of Mn content with respect to the overhang formability in the ferrite austenitic stainless steel plate whose Ni content is 1% or less and an austenite phase fraction is 40-50 vol%. It is a graph which shows the influence of Mn content on the outdoor exposure test result of the ferrite austenitic stainless steel plate whose Ni content is 1% or less and whose austenite phase fraction is 40-50 vol%. 3 is a graph showing the relationship of the austenite phase fraction on the stretch formability of a ferrite-austenitic stainless steel sheet having an Mn content of 2% or less and an Ni content of 1% or less. It is a conceptual diagram which shows a clearance-resistant part corrosion test apparatus structure.

Claims (5)

By mass ratio, C: 0.111% or less, Si: 1.2% or less, Mn: 1.91% or less, P: 0.1% or less, S: 0.03% or less, Cr: 15% or more and 35% or less, Ni: 0.83% or less, N : 0.108% or more and 0.511% or less, balance Fe and inevitable impurities, austenite phase fraction in the metal structure is 29vol% or more and 82vol% or less. ferrite-austenite stainless steel plate was. Furthermore V: O.028% or more bulging formability and crevice corrosion resistance excellent ferrite-austenite stainless steel plate according to claim 1, characterized in that it contains 0.5% or less. Furthermore Al: claim 1 or 2 bulging resistance and crevice corrosion resistance excellent ferrite-austenite stainless steel sheet according to, characterized in that it contains 0.003% to 0.1% or less. Further, Mo: 4% or less, Cu: 0.32% or more and 4% or less, or both of them, characterized by excellent stretch formability and crevice corrosion resistance according to any one of claims 1 to 3 ferrite-austenite stainless steel plate. Further, B: 0.01% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.1% or less, Ti: 0.1% or less selected from Ti or 0.1% or less is contained stretch forming property and crevice corrosion resistance excellent ferrite-austenite stainless steel plate according to any one of to 4.

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