Ferritic stainless steel for roll forming

Abstract

To provide a ferritic stainless steel material for roll molding which is excellent in roll molding properties and shape freezing properties.SOLUTION: A ferritic stainless steel material for roll molding has a composition including not more than 0.08 mass% of C, not more than 0.10 mass% of N and 10.50-19.00 mass% of Cr. A maximum value of austenite transformation rate when heated to Ac1 point or above is 15% or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel material for roll forming. [Background technology]

[0002] Ferritic stainless steel has excellent formability and corrosion resistance, and is therefore used in a variety of applications, including kitchen components, building materials, automobile parts, and gas and electrical appliance parts. Commonly known methods for forming ferritic stainless steel materials include press forming, bending, roll forming, etc. Among these, roll forming has the advantage of being faster than press forming and allowing for the continuous production of long, seamless products.

[0003] However, ferritic stainless steel has a larger springback during roll forming than carbon steel, and therefore, when ferritic stainless steel is rolled using a roll forming apparatus for carbon steel under the same conditions as carbon steel, there is a problem that the springback tends to cause a decrease in dimensional accuracy and a defective shape.

[0004] Therefore, Patent Document 1 discloses a composition containing C: 0.10 mass% or less, Si: 1.0 mass% or less, Mn: 1.0 mass% or less, P: 0.050 mass% or less, S: 0.020 mass% or less, Ni: 2.0 mass% or less, Cr: 8.0 to 22.0 mass%, and N: 0.05 mass% or less in proportions such that the FM value is 0 or less, with the balance consisting of Fe and unavoidable impurities, and which has an in-plane anisotropy r max -r min is 0.80 or less, and 0.2% yield strength anisotropy σ max -σ min is 20N / mm 2 The following ferritic stainless steel material has been proposed. This ferritic stainless steel material is said to have little change in shape due to springback during forming and excellent shape fixability. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-332549 Summary of the Invention [Problem to be solved by the invention]

[0006] The ferritic stainless steel material described in Patent Document 1 has its in-plane anisotropy and 0.2% proof stress anisotropy controlled within a predetermined range during the cold rolling and annealing processes, and is primarily intended for cold-rolled and annealed materials with small thicknesses. Therefore, the technology of Patent Document 1 cannot be applied to hot-rolled materials or hot-rolled and annealed materials with large thicknesses. Therefore, there is a need for the development of a technology that can suppress springback during roll forming and improve shape fixability, regardless of the thickness of the ferritic stainless steel material.

[0007] The present invention has been made under the above-mentioned circumstances, and an object of the present invention is to provide a ferritic stainless steel material for roll forming which has excellent roll formability and shape fixability. [Means for solving the problem]

[0008] As a result of intensive research conducted by the present inventors to solve the above problems, the composition and A of a ferritic stainless steel material were found to be c1 The inventors have found that the maximum austenite transformation rate when heated to above this point is related to roll formability and shape fixability, and have completed the present invention.

[0009] That is, the present invention provides a composition containing C: 0.08% by mass or less, N: 0.10% by mass or less, Cr: 10.50 to 19.00% by mass, Ni: 2.00% by mass or less, Mn: 2.00% by mass or less, Cu: 2.00% by mass or less, Mo: 2.00% by mass or less, Al: 0.50% by mass or less, and Si: 1.00% by mass or less, with the balance being Fe and unavoidable impurities, The ratio of the area of ​​martensite to the total area of ​​the observation field measured by observing the cross-section structure after heating at 1100°C for 10 minutes and then quenching by water cooling. The maximum value of ferritic stainless steel for roll forming is 15% or more. [Effects of the Invention]

[0010] According to the present invention, a ferritic stainless steel material for roll forming having excellent roll formability and shape fixability can be provided. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram illustrating the cross-sectional shape of a ferritic stainless steel sheet that was roll-formed in a shape fixability test. DETAILED DESCRIPTION OF THE INVENTION

[0012] The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention.

[0013] A ferritic stainless steel material for roll forming according to an embodiment of the present invention (hereinafter abbreviated as "ferritic stainless steel material") has a composition containing C: 0.08 mass% or less, N: 0.10 mass% or less, and Cr: 10.50 to 19.00 mass%. Furthermore, the composition of the ferritic stainless steel material according to an embodiment of the present invention may further include one or more selected from Ni: 2.00 mass% or less, Mn: 2.00 mass% or less, Cu: 2.00 mass% or less, Mo: 2.00 mass% or less, Al: 0.50 mass% or less, and Si: 1.00 mass% or less, with the remainder being Fe and unavoidable impurities. Furthermore, the composition of the ferritic stainless steel material according to an embodiment of the present invention may further include one or more selected from Ti: 0.50 mass% or less, Nb: 1.00 mass% or less, Zr: 0.50 mass% or less, and Sn: 0.50 mass% or less. In this specification, the term "unavoidable impurities" refers to components that are difficult to remove, such as O, P, and S. Inevitable impurities are inevitably mixed in during the process of melting raw materials.

[0014] [C: 0.08% by mass or less] Generally, C deteriorates formability, corrosion resistance, and weldability, so a low C content is preferable. Therefore, from the viewpoint of ensuring these properties, the upper limit of the C content is set to 0.08 mass%, preferably 0.07 mass%. From the viewpoint of corrosion resistance, it is desirable to set the upper limit of the C content to 0.020 mass%. On the other hand, C is also an element that promotes and increases the formation of the austenite phase at high temperatures. In the ferritic stainless steel material according to the embodiment of the present invention, A is added to obtain a metal structure with small springback. c1 It is an important requirement to control the maximum austenite transformation rate at temperatures equal to or higher than this point to 15% or more, and C is an effective element in this regard. Moreover, since an excessive reduction in C content increases refining costs, the lower limit of the C content is preferably 0.002% by mass, more preferably 0.005% by mass.

[0015] In addition to C, elements that promote and increase the formation of the austenite phase at high temperatures include N, Ni, Cu, Mn, etc., and it is possible to obtain a desired metal structure by adjusting the contents of these elements. Furthermore, among these elements, C and N, which are usually treated as harmful elements, are the most economical elements, and by increasing the contents of C and N, it is possible to reduce the contents of expensive elements such as Ni, Cu, and Mn. In the ferritic stainless steel material according to the embodiment of the present invention, C and N, which are usually considered harmful elements that reduce workability, corrosion resistance, and weldability, can minimize springback and improve shape fixability even in the case of a large thickness by controlling the metal structure through the effect of promoting the formation of austenite phase, thereby enabling the production of formed products with high dimensional accuracy and few shape defects.

[0016] [N: 0.10% by mass or less] N is an element that increases the amount of austenite transformation, but an excessive N content reduces formability, corrosion resistance, and weldability, so the upper limit of the N content is set to 0.10 mass%. From the viewpoint of improving formability, particularly the effect of suppressing springback, the upper limit of the N content is preferably set to 0.05 mass%. On the other hand, in the ferritic stainless steel material according to the embodiment of the present invention, N is usually considered harmful like C, but for the same reasons as for C as described above, the lower limit of the N content is preferably 0.002 mass%, more preferably 0.005 mass% or more.

[0017] [Cr:10.50~19.00% by mass] Cr is an essential element for ensuring corrosion resistance. To enable the formation of a passive film in the expected environment, the lower limit of the Cr content must be 10.50% or more. On the other hand, if the Cr content is too high, workability and toughness at low temperatures will be reduced, so the upper limit of the Cr content is set to 19.00 mass%.

[0018] [Ni: 2.00% by mass or less] Ni is an element that is effective in suppressing the progression of pitting corrosion and also has the effect of increasing the amount of austenite transformation. However, because Ni is an expensive element, adding a large amount of Ni not only increases costs but also may increase susceptibility to stress corrosion cracking. Therefore, the upper limit of the Ni content is set to 2.00 mass%. In consideration of alloy costs, the upper limit of the Ni content is preferably 0.60 mass%. On the other hand, from the viewpoint of stably obtaining the above-mentioned effects of Ni, the lower limit of the Ni content is preferably set to 0.05 mass%. Moreover, since Ni is also an element effective in improving toughness, from the viewpoint of obtaining this effect as well, the lower limit of the Ni content is more preferably set to 0.10 mass%.

[0019] [Mn: 2.00% by mass or less] Mn is an element added as a deoxidizer. However, if an excessive amount of Mn is added, MnS is formed, which reduces corrosion resistance, so the upper limit of the Mn content is set to 2.00 mass%, preferably 0.50 mass%. On the other hand, from the viewpoint of stably obtaining the above-mentioned effects of Mn, the lower limit of the Mn content is preferably set to 0.01 mass %.

[0020] [Cu: 2.00% by mass or less] Cu is an element effective in improving corrosion resistance. However, excessive Cu content reduces hot workability and can cause edge cracks during hot rolling, so the upper limit of the Cu content is set to 2.00 mass%. On the other hand, from the viewpoint of stably obtaining the above-mentioned effects of Cu, the lower limit of the Cu content is preferably set to 0.05 mass %.

[0021] [Mo: 2.00% by mass or less] Mo is an element effective in improving corrosion resistance, but an excessive Mo content reduces workability, so the upper limit of the Mo content is set to 2.00 mass%, preferably 1.10 mass%. On the other hand, from the viewpoint of stably obtaining the above-mentioned effects of Mo, the lower limit of the Mo content is preferably set to 0.01 mass %, more preferably to 0.10 mass %, and even more preferably to 0.50 mass %.

[0022] [Al: 0.50% by mass or less] Al is a useful element as a deoxidizing element, but if added in excess, it will cause a decrease in room temperature ductility, so the upper limit of the Al content is set to 0.50 mass%. If deoxidation can be achieved with other elements such as Si, Al does not need to be added. However, from the viewpoint of stably obtaining the above-described effect of Al, it is preferable to set the lower limit of the Al content to 0.005 mass %.

[0023] [Si: 1.00% by mass or less] Silicon is useful as a deoxidizer and is an element effective in improving oxidation resistance. However, if excessive silicon is added, the ferritic stainless steel material becomes too hard, so the upper limit of the silicon content is set to 1.00 mass%, preferably 0.50 mass%. On the other hand, from the viewpoint of stably obtaining the above-mentioned effects due to Si, the lower limit of the Si content is set to 0.01 mass %.

[0024] [Ti: 0.50 mass% or less; Nb: 1.00 mass% or less] Ti and Nb have the effect of immobilizing C or N and improving ductility during processing. However, excessive addition of Ti and Nb may actually reduce ductility, increase costs, and reduce manufacturability, so the upper limit for the Ti content is set to 0.50 mass% and the upper limit for the Nb content is set to 1.00 mass%. On the other hand, from the viewpoint of stably obtaining the above effects of Ti and Nb, it is preferable to set the lower limit of each of the Ti content and the Nb content to 0.01 mass %.

[0025] [Zr: 0.50% by mass or less] Zr, like Nb and Ti, is an element that forms carbonitrides and suppresses the formation of Cr carbonitrides, thereby improving corrosion resistance. However, even if excessive Zr is added, the effect saturates and the formation of large oxides can cause surface defects, so the upper limit of the Zr content is set to 0.50 mass%. Furthermore, since Zr is an expensive element compared to Ti and Nb, taking into account production costs, the upper limit of the Zr content is preferably 0.30 mass%, more preferably 0.10 mass%. On the other hand, from the viewpoint of stably obtaining the above-mentioned effects of Zr, the lower limit of the Zr content is set to 0.02 mass %.

[0026] [Sn: 0.50% by mass or less] Sn is an effective element for improving corrosion resistance without significantly degrading mechanical properties at room temperature. However, excessive addition of Sn reduces manufacturability and weldability, so the upper limit of the Sn content is set at 0.50 mass%. On the other hand, from the viewpoint of stably obtaining the above-mentioned effects of Sn, the lower limit of the Sn content is preferably set to 0.005 mass %, and more preferably to 0.010 mass %.

[0027] [P: 0.04% by mass or less] P is an element with a high solid solution strengthening ability, but is an impurity element that is harmful to corrosion resistance, and corresponds to an unavoidable impurity in the ferritic stainless steel material according to the embodiment of the present invention. A lower P content is preferable, but the typical upper limit is 0.04 mass%. When better corrosion resistance is required, the upper limit of the P content is preferably set to 0.03 mass%. The lower limit of the P content is not particularly limited, but an excessive reduction would increase the dephosphorization load and the production cost due to the selection of low-P raw materials, so the lower limit can be set to 0.005 mass%.

[0028] [S: 0.01% by mass or less] S is an impurity element that forms sulfide-based inclusions and reduces the general corrosion resistance (general corrosion and pitting corrosion) of steel materials, and corresponds to an unavoidable impurity in the ferritic stainless steel material according to the embodiment of the present invention. Although a lower S content is preferable, the typical upper limit is set to 0.01 mass%, preferably 0.005 mass%. The lower limit of the S content is not particularly limited, but since reducing S content increases production costs due to increased desulfurization load and the selection of low-S raw materials, the lower limit can be set to 0.0001 mass % or 0.0005 mass %.

[0029] The ferritic stainless steel material according to the embodiment of the present invention is A C1 The maximum austenite transformation rate when heated above the C1 The maximum austenite transformation rate (maximum austenite transformation rate at temperatures above the temperature point) is 15% or more. By controlling the maximum austenite transformation rate within this range, springback after forming of ferritic stainless steel material can be significantly suppressed. Typically, when ferritic stainless steel is formed, elastic strain is released when the formed product is released from the mold, resulting in a product angle larger than the design angle. This springback characteristic is called shape fixability, and when the maximum austenite transformation rate is 15% or higher, shape fixability is significantly improved compared to when the maximum austenite transformation rate is less than 15%. Furthermore, the ferritic stainless steel material according to the embodiment of the present invention has superior shape fixability compared to carbon steel (such as SS400) that has been widely used in the past. The maximum austenite transformation rate can be controlled by designing the composition while taking into consideration the balance between ferrite-forming elements and austenite-forming elements. The main austenite-forming elements in the present invention are C, N, Ni, Mn, and Cu, and the main ferrite-forming elements are Cr, Si, Mo, and Al. It is essential for the ferritic stainless steel material according to the embodiment of the present invention to be designed within the above-mentioned composition ranges so that the maximum austenite transformation rate is 15% or more while ensuring corrosion resistance.

[0030] Here, the austenite transformation rate is ,centre Ferritic stainless steel material A C1 After heating above this point and rapidly cooling, the cross section is observed to measure the structure. And A C1 The austenite generated when heated above the annealing point transforms into martensite by rapid cooling. Therefore, the austenite transformation rate is determined by observing the cross-section structure and measuring the ratio of the area of ​​martensite to the total area of ​​the observation field. do. In addition, in this specification, the "maximum value of the austenite transformation rate" refers to the maximum value among the area ratios of martensite (austenite transformation rates) measured at a plurality of locations.

[0031] The thickness of the ferritic stainless steel material according to the embodiment of the present invention is not particularly limited and may be either a thin plate material or a thick plate material. For example, the thickness of the ferritic stainless steel material according to the embodiment of the present invention may be 0.1 mm or more. In particular, even when the ferritic stainless steel material according to the embodiment of the present invention has a thickness of 3.5 mm or more, it is possible to suppress springback during roll forming and improve shape fixability. The thickness of the ferritic stainless steel material according to the embodiment of the present invention is preferably 3.8 to 20 mm, more preferably 4.0 to 15 mm.

[0032] The type of ferritic stainless steel material according to the embodiment of the present invention is not particularly limited, and can be various steel materials such as hot-rolled material, hot-rolled annealed material, cold-rolled material, and cold-rolled annealed material, but is preferably a hot-rolled material or hot-rolled annealed material with a large thickness.

[0033] The ferritic stainless steel material according to the embodiment of the present invention can be produced by a method known in the art, except for using a steel type having the above-described composition. For example, when the ferritic stainless steel material according to the embodiment of the present invention is a hot-rolled material, it can be obtained by hot-rolling a slab having the above-described composition obtained by melting and casting raw materials. After hot rolling, pickling may be performed. Furthermore, when the ferritic stainless steel material according to the embodiment of the present invention is a hot-rolled annealed material, annealing may be performed after hot rolling. After annealing, pickling may be performed. The ferritic stainless steel material according to the embodiment of the present invention can be stored and transported after being wound into a coil, similar to ordinary stainless steel materials. The manufacturing apparatus used to manufacture the ferritic stainless steel material according to the embodiment of the present invention is not particularly limited, and any apparatus known in the art can be used.

[0034] The hot rolling conditions are not particularly limited and may be adjusted appropriately depending on the composition. For example, the slab heating temperature can be 1100°C to 1250°C, and the hot rolling finish temperature can be 850°C or higher. When the slab is wound into a coil after hot rolling, accelerated cooling to 350°C can be performed immediately after the finish hot rolling using spray water or steam cooling, as needed. Forced cooling after winding into a coil is not required. When annealing is performed after hot rolling, the annealing temperature is 680 to 930°C (A C1 It is preferable that the temperature is just below the A point. C1 If the annealing temperature exceeds this point, martensite is generated, making the steel brittle and hard, and the crystal grains are likely to become coarse. The annealing time is not particularly limited, and can be adjusted appropriately depending on the composition, etc. Depending on the composition, C1 Although the point fluctuates, annealing is A C1 Generally, in order to control the maximum austenite transformation rate to 15% or more, the annealing temperature in the case of BAF annealing is preferably set to about 680 to 900°C. [Example]

[0035] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0036] (Examples 1 to 8 and Comparative Examples 1 and 2) Raw materials were melted to produce ingots having the compositions shown in Table 1, and the ingots were cast to obtain slabs. The slabs were then heated to 1150 to 1230°C and hot-rolled to a thickness of 5 mm at a finishing temperature in the range of 880 to 980°C to obtain hot-rolled steel sheets. Some of the hot-rolled steel sheets were annealed at 700 to 900°C for 60 to 120 seconds to obtain hot-rolled annealed sheets.

[0037] [Table 1]

[0038] The ferritic stainless steel sheets obtained in the above examples and comparative examples were subjected to the shape fixability test, formability test, and toughness test shown below, and the results were compared with those of ordinary steel SS400 to evaluate them.

[0039] (Maximum austenite transformation rate: Maximum gamma transformation rate) The ferritic stainless steel plates obtained in the above examples and comparative examples were cut into 30 mm square test pieces and heated to 1100°C (A C1 The specimen was heated to a temperature above the rolling point for 10 minutes, and then rapidly cooled in water. Next, the specimen was cut in the thickness direction parallel to the rolling direction, and the cross section was etched with aqua regia for microstructural observation. Microstructural observation was carried out at five random locations in the center of the thickness (within a depth range of 2 to 4 mm from the surface). Since austenite transforms to martensite by rapid cooling, in the microstructural observation of the cross section, the ratio of the area of ​​martensite to the total area of ​​the observation field was measured, and this was defined as the A C1 The austenite transformation rate was determined as the austenite transformation rate when the specimen was heated above the 1000°C point. Image analysis was performed using the image processing software Image J. The maximum value of the martensite area ratio in the five microstructure observation points was determined as the maximum austenite transformation rate.

[0040] (shape freezing) Shape fixability was evaluated by roll-forming a ferritic stainless steel sheet specimen with a width of 55 mm ± 5 mm into the cross-sectional shape shown by the dotted line in Figure 1 and measuring the amount of springback. In Figure 1, the shape after springback occurs is shown by the solid line. In this roll-forming test, the bending ridge of the ferritic stainless steel sheet was parallel to the rolling direction (L direction). The amount of springback (ΔL2) was calculated using the following formula (1). ΔL2=L2'-L2 (1) In this evaluation, a smaller ΔL2 value compared to SS400 is represented by "++", an equivalent ΔL2 value is represented by "+", and a larger ΔL2 value is represented by "-".

[0041] (Moldability) To evaluate formability, three test pieces of ferritic stainless steel plates measuring 300 mm long and 60 mm wide were hand-polished with #80 sandpaper at the edges in the width direction, and then subjected to a 180° bending test in the width direction (C direction) at room temperature with a bending radius of 1t (t = plate thickness) mm. In this evaluation, a mark of ○ indicates that no cracks occurred in any of the test pieces, and a mark of × indicates that partial cracks occurred in any of the test pieces.

[0042] (toughness) The toughness was evaluated by a Charpy impact test in accordance with JIS Z2242:2018. The test specimens were V-notch sub-size specimens. In this evaluation, the Charpy impact value at 0°C was ≥ 30 J / cm. 2 If the above condition is satisfied, the Charpy impact value at 0°C is ≥ 30 J / cm 2 If the above condition is not satisfied, it is indicated as ×.

[0043] [Table 2]

[0044] As shown in Table 2, the ferritic stainless steel sheets of Examples 1 to 8, which satisfied the prescribed composition and had a maximum γ transformation rate of 15% or more, all had good results in terms of shape fixability, formability, and toughness. In contrast, the ferritic stainless steel sheet of Comparative Example 1, which had an excessively high Cr content and a maximum γ transformation rate of less than 15%, and the ferritic stainless steel sheet of Comparative Example 2, which had a maximum γ transformation rate of less than 15%, showed insufficient results in terms of shape fixability, formability, and toughness.

[0045] As can be seen from the above results, the present invention can provide a ferritic stainless steel material for roll forming that is excellent in roll formability and shape fixability.

Claims

1. A ferritic stainless steel material for roll forming having a composition containing 0.08 mass% or less C, 0.10 mass% or less N, 10.50 to 19.00 mass% Cr, 2.00 mass% or less Ni, 2.00 mass% or less Mn, 2.00 mass% or less Cu, 2.00 mass% or less Mo, 0.50 mass% or less Al, and 1.00 mass% or less Si, with the balance being Fe and unavoidable impurities, wherein when heated at 1100°C for 10 minutes and then quenched by water cooling, the maximum ratio of the area of ​​martensite to the area of ​​the entire observation field measured by structural observation of a cross section is 15% or more.

2. 2. The ferritic stainless steel material for roll forming according to claim 1, wherein the composition further contains one or more selected from the group consisting of Ti: 0.50 mass% or less, Nb: 1.00 mass% or less, Zr: 0.50 mass% or less, and Sn: 0.50 mass% or less.

3. 3. The ferritic stainless steel material for roll forming according to claim 1, having a thickness of 3.5 mm or more.

4. The ferritic stainless steel material for roll forming according to any one of claims 1 to 3, which is a hot-rolled material or a hot-rolled annealed material.

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