JP3458955B2 - Ferritic stainless steel with excellent cold workability, corrosion resistance, machinability and weldability - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing joints such as various bolts and various sensors, and parts for home electric appliances and parts for machinery. The present invention relates to a ferritic stainless steel used as a material for various parts that require cold workability, require machinability and weldability depending on a manufacturing process, and require corrosion resistance during use. [0002] Conventionally, materials for the above-mentioned various parts include:
Austenitic stainless steel represented by SUS304 is used. SUS304 has excellent corrosion resistance, but has remarkable work hardening and poor cold workability. Therefore, most parts are processed by cutting. [0003] In some cases, austenitic stainless steel SUS303 having improved machinability is used.
Due to poor corrosion resistance, it is only applied to limited uses. Parts machining using only cutting processing requires a large cutting allowance,
The yield is significantly reduced, and the cost of parts is increased. In addition, yield reduction should be avoided as much as possible from the viewpoint of effective use of resources. Under such circumstances, ferritic stainless steel represented by SUS430, which is relatively excellent in cold workability, has been used as a material for the above-mentioned various components. [0004] However, it is difficult to say that SUS430 or the like has sufficient cold workability. Specifically, since the deformation resistance during cold working is high, the load on the working equipment is high, the life of the mold and the like are deteriorated, and the cold working of the near net shape is made difficult. Also, SUS430 is
Corrosion resistance is lower than that of US304, and its application is limited. In order to solve such problems, the inventions disclosed in Japanese Patent No. 2906445 and Japanese Patent Application Laid-Open No. 03-180449 are disclosed. [0005] Patent No. 2906445 discloses C, Si, M
An object of the present invention is to provide a ferritic stainless steel excellent in cold workability, toughness, and corrosion resistance by suppressing n, S, N, and O and adding Nb. Also, Japanese Patent Application Laid-Open
-180449 has excellent cold workability, toughness, corrosion resistance, and machinability by suppressing C, Si, Mn, N, and O, adding Nb, and further adding Pb, Bi, and Se. To provide ferritic stainless steel. [0006] In the above-described processing steps of various parts, it is rare that parts are processed only by cold working, and in many cases, cutting is performed after cold working. The material is also required to have machinability. Further, processed parts are often joined to other parts by welding, and weldability is also required. That is, a material excellent in cold workability, corrosion resistance, machinability and weldability is required. More specifically, the hardness after heat treatment is 80 HRB or less, and the critical work ratio is 50% or more, which is excellent in cold workability.
Shows sufficient corrosion resistance superior to US430, SUS43
It has a machinability of 0 or more, and for various welding methods,
Materials with excellent welding workability are required. The above-mentioned Japanese Patent No. 2906445 is an invention relating to a ferritic stainless steel excellent in cold workability, toughness and corrosion resistance, and does not consider weldability and machinability at all. Further, Japanese Patent Application Laid-Open No.
49 is an invention relating to a ferritic stainless steel excellent in machinability in addition to cold workability, toughness and corrosion resistance, but does not consider weldability at all. Also,
In the case of the present invention, Pb and Bi are added to impart machinability, but these low melting point metals impair weldability. That is, in the prior art, although knowledge has been obtained on the improvement of cold workability and corrosion resistance, no study has been made on weldability. The present invention, which has been obtained as a result of intensive studies in order to solve such problems, provides a ferritic stainless steel having good cold workability, corrosion resistance, good machinability, and excellent weldability. Things. The present inventors have investigated the effects of various elements on weldability and obtained the following findings. First, Si that is reduced as impairing cold workability
It has been clarified that the addition of a small amount of increases the weldability. Secondly, with the addition of Nb, C and N are reduced to 0.
By setting the lower limit to 010% or more, it is clear that it is possible to secure the precipitation amount of Nb carbonitride and to achieve the refinement of the crystal grains in the welded portion and the prevention of the coarsening of the crystal grains in the weld heat affected zone. became. Thereby, it becomes possible to secure the toughness of the welded portion and the weld heat affected zone. According to the present invention, in addition to the above-mentioned knowledge on weldability, in order to reduce deformation resistance during cold working, the amounts of C and N are kept low, and the amount of dissolved C and N is reduced by adding Nb. And Si, Cu, Ni, Cr, Mo,
Excellent corrosion resistance is imparted by limiting the contents of Al and O, and machinability is imparted by limiting the contents of Mn and S. Therefore, the cold workability of the present invention, corrosion resistance,
Ferritic stainless steel with excellent machinability and weldability
In terms of weight ratio, C: 0.020% or less, Si: more than 0.30 to 1.00
%, Mn: more than 0.50 to 1.00%, P: 0.040% or less, S: 0.005
0.020%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1
3.0 to 20.0%, Mo: more than 0.50 to 2.00%, Al: 0.020% or less, N: 0.020% or less, Nb: 0.20 to 0.50%, O: 0.010%
Hereinafter, C + N contains 0.010% or more, with the balance being Fe and unavoidable impurities. Next, the reasons for limiting the chemical components will be described. C: 0.020% or less C is an unavoidable element, but is an element that significantly deteriorates cold workability, toughness, and corrosion resistance, particularly the corrosion resistance of the heat affected zone by welding. The upper limit is made 0.020%. Si: more than 0.30 to 1.00% Si is an element necessary as a deoxidizing agent in the production of steel, and is also an element that increases the stability of the arc during welding, suppresses the amount of spatter generated, and further improves the flowability of molten metal. , It is also an element effective for corrosion resistance, and its effect is 0.30
It is necessary to contain more than%. However, when the content increases, the hot workability is impaired, and the hardness increases due to solid solution strengthening, which impairs the cold workability. Therefore, the upper limit is set to 1.00%, more preferably 0.80%. Mn: more than 0.50% to 1.00% Mn is an element necessary as a deoxidizing agent in the production of steel, and is an element that forms nonmetallic inclusions MnS together with S to enhance machinability. Must contain more than 0.50%. However, when the content is increased, corrosion resistance and cold workability are deteriorated, and hot workability is further impaired. Therefore, the upper limit is set to 1.00%, more preferably 0.90%. P: 0.040% or less Although P is an unavoidable element, it is an element that significantly deteriorates cold workability and also an element that easily causes weld cracking. Therefore, the upper limit is made 0.040%. S: 0.005 to 0.020% S is an element that forms nonmetallic inclusions MnS with Mn to improve machinability, and its effect requires an addition of 0.005% or more. However, when the content increases,
The upper limit is set to 0.020%, more preferably 0.015%, in order to impair corrosion resistance and cold workability and easily cause weld cracking. Cu: 1.0% or less Cu is an element that improves corrosion resistance and cold workability. However, when the content is large, the hardness increases due to solid solution strengthening and the cold workability is impaired, so the upper limit is 1.0%. , More preferably 0.60%. Ni: 1.0% or less Ni is an element that improves corrosion resistance and cold workability. However, if the content is too large, hardness increases due to solid solution strengthening, which impairs cold workability. Is set to 1.0%, more preferably 0.60%. Cr: 13.0 to 20.0% Cr is an essential element for imparting corrosion resistance. In order to bring out its effects, it is necessary to contain 13.0% or more, more preferably 15.0% or more, and still more preferably 17.0% or more. It is. However, when the content is increased, the cold workability and the hot workability are impaired, so the upper limit is set to 20.0%, more preferably 19.0%. Mo: more than 0.50 to 2.00% Mo is known as an element for improving corrosion resistance.
In order to achieve the effect, the content needs to be more than 0.50%. However, when the content increases, the hardness increases due to solid solution strengthening, which impairs the cold workability and further increases the material cost. Therefore, the upper limit is set to 2.00%, more preferably 1.00%. Al: 0.020% or less Al is an element necessary as a deoxidizing agent in the production of steel, but it is also an element that forms an oxide to deteriorate cold workability and corrosion resistance. It is also an element that causes spattering, and its upper limit is made 0.020%. N: 0.020% or less N, like C, is an unavoidable element.
It is an element that significantly deteriorates the toughness and corrosion resistance, particularly the corrosion resistance of the weld heat affected zone. The upper limit is made 0.020%. Nb: 0.20 to 0.50% Nb is an element having a strong affinity for C and N, and forms a carbonitride to reduce the amount of dissolved C and N, thereby lowering the hardness and increasing the cold workability. And it is an element that improves corrosion resistance. In addition, during welding, carbonitride is formed in the welded portion to refine the crystal grains in the welded portion, and in the weld heat affected zone, the carbonitride prevents the crystal grains from becoming coarse,
Increases ductility and toughness of weld zone and weld heat affected zone. For that effect, the content must be 0.20% or more. However, when the content is increased, conversely, to impair cold workability and hot workability, the upper limit is 0.50%, more preferably.
0.40%. O: 0.010% or less O is an element which is present in the steel as oxide inclusions typified by alumina, and significantly deteriorates cold workability and corrosion resistance. , Since it is also an element that impairs toughness, its upper limit is made 0.010%, more preferably 0.008%. C + N: 0.010% or more C and N are elements that significantly deteriorate cold workability and corrosion resistance. Therefore, it is preferable that each of them is 0.020% or less. In order to effectively precipitate Nb carbonitride, the lower limit of C + N must be set to 0.1 in order to effectively deposit this carbonitride.
010%. DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the steel of the present invention are summarized in comparison with a comparative steel and a conventional steel. Table 1 shows the chemical components of these test materials. [Table 1] In Table 1, A to J are the chemical components of the steel of the present invention, K to V are the chemical components of the comparative steel, and SUS430 and SUS304 are the conventional steels. For the test materials in Table 1, hardness after heat treatment,
The critical working rate, corrosion resistance, machinability, and weldability were evaluated and summarized in Table 2. [Table 2] The heat treatment conditions were as follows: rapid cooling from 1000 ° C. for the steel of the present invention and the comparative steel;
It was based on the IS standard. Regarding the hardness, Rockwell hardness tester (B
Scale), and the critical working ratio is 14 mm in diameter and 21 mm in height based on the cold forging test method based on the standards of the Cold Forging Subcommittee of the Japan Society for Technology of Plasticity.
The compression test was performed using a notched cylindrical test piece with
The upsetting rate at a crack generation rate of 50% was measured. The corrosion resistance was 96% in 5% NaCl, 35 ° C. aqueous solution.
A salt spray test was conducted for a long time, and those having an area ratio of rust generation of 2% or less were evaluated as ○, and those exceeding 2% were evaluated as x. The machinability was evaluated by a piercing test using a 10 mm thick test piece, a 5 mm straight SKH51 drill, a rotation speed of 759 rpm and a load of 72 kg. At the time of evaluation, based on the perforation time of the comparative material SUS430, those with a shorter perforation time were judged as ○, and those with a longer perforation time were judged as x. Regarding the weldability, under the argon gas shield, current 60A, voltage 10V, welding speed 80cm /
Butt TIG welding (without welding rod) was performed under the conditions of min, and the weldability was evaluated from the bead appearance, arc stability, and spatter generation status. It was determined as ×, and the crystal grain size of the welded portion was measured based on the JIS method and converted into a crystal grain size No. The steel of the present invention has a low hardness of 80 HRB or less and is soft, and has a high critical working ratio of 54% or more and is excellent in cold workability. In addition, corrosion resistance, machinability and weldability are good. On the other hand, comparative steel K
r is low, comparative steel L has high S, and comparative steel N has C +
N is low, the grain size of the weld is coarse,
The ductility and toughness of the weld are insufficient. Comparative steel M has a high C, a high hardness, a low critical working ratio, poor cold workability, and insufficient corrosion resistance. Further, the comparative steel P has a low Nb, similarly has poor cold workability and corrosion resistance, has a coarse crystal grain size at the welded portion, and has insufficient ductility and toughness at the welded portion. . Comparative steel O has a high Mo, a high hardness, a low critical working ratio, and poor cold workability. The comparative steel Q has a high Al content, a low critical working ratio, a poor cold workability, a high corrosion resistance,
The machinability and weldability are also insufficient. Comparative steel R has high O, low critical working ratio, inferior cold workability, and insufficient corrosion resistance and machinability. Comparative steel S has a low Mn and is inferior in machinability. Comparative steel T
Has low Si, and the weldability is insufficient. Comparative steel U has low Si, Mn, and S, and is excellent in cold workability and corrosion resistance, but is inferior in machinability and weldability. Comparative steel V has low Si, Mn, and S, and Pb is added as a free-cutting element, and is excellent in cold workability, corrosion resistance, and machinability, but is inferior in weldability. Therefore, the present invention is a ferritic stainless steel excellent in cold workability, corrosion resistance, machinability, and weldability. Although the embodiments of the present invention have been described in detail above, these are merely examples, and various parts requiring cold workability, corrosion resistance, machinability, and weldability without departing from the intent of the present invention. Applicable to classes. As described above, according to the present invention, a ferritic stainless steel excellent in cold workability, corrosion resistance and machinability and excellent in weldability can be obtained. Specifically, the hardness after heat treatment is 80 HRB or less, and the critical work ratio is 50% or more, which is excellent in cold workability, and shows sufficient corrosion resistance superior to SUS430 in a corrosion test simulating the atmospheric environment such as a salt spray test. And SUS430 or more, and ferritic stainless steel excellent in welding workability with various welding methods can be obtained.

Claims (1)

(57) [Claims] [Claim 1] In terms of weight ratio, C: 0.020% or less, Si: 0.
Over 30 to 1.00%, Mn: over 0.50 to 1.00%, P: 0.040% or less,
S: 0.005 to 0.020%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 13.0 to 20.0%, Mo: more than 0.50 to 2.00%, Al:
0.020% or less, N: 0.020% or less, Nb: 0.20 to 0.50%,
A ferritic stainless steel containing O: 0.010% or less and C + N: 0.010% or more, with the balance being Fe and unavoidable impurities and having excellent cold workability, corrosion resistance, machinability, and weldability.