JP4325243B2 - Stainless steel plate for welded structure with excellent intergranular corrosion resistance and workability - Google Patents

Description

【００５４】
【表２】

【００５５】
【表３】

【００５６】
【表４】

【００５７】
表４から明らかなように、本発明鋼は、耐食性、溶接熱影響部の靭性および耐粒界腐食性に優れ、さらに母材の強度が低く、かつ伸びが高く、加工性が優れていることがわかる。比較鋼は、そのいずれかが本発明例に比べて劣っている。
【００５８】
［実施例２］
次に、冷延焼鈍板の特性を調査した。前述の実施例１の表１における供試材番号11として示した３mmの熱延焼鈍板を、リバース圧延機による冷間圧延により板厚1.5mm になるまで圧延し、750 ℃×１分の焼鈍を行った後、60℃の混酸（硝酸10質量％＋ふっ酸３質量％）に浸漬して脱スケールして、冷延焼鈍板を得た。かくして得られた冷延焼鈍板について、前述の実施例１の熱延板と同様の試験を実施した。
ただし、溶接部靱性を調べるための溶接は、TIG 溶接（電流：95A ，電圧：11Ｖ，溶接速度：400mm ／min ，シールドガス：表（電極）側20リットル／分、 裏側20リットル／分）を用いた。
【００５９】
その結果、耐食性は、CCT 発錆面積率が15％であった。焼なまし後の金属組織は、フェライトであり、溶接部のマルテンサイト組織比率は０％であった。溶接熱影響部の特性として、靱性は−50℃におけるシャルピー衝撃値（ vＥ_-50 ）が90Ｊ／cm² で、粒界腐食試験では割れがなかった。機械的特性は、引張強さが485MPa、破断伸びが35％であった。このように、冷延焼鈍板も、熱延焼鈍板とほぼ同等の特性を有しており、車両構造用鋼板としての特性の目標を達成していることが確認できた。
【００６０】
【発明の効果】
以上述べたように、 この発明によれば、母材の耐食性および加工性と、溶接熱影響部の耐粒界腐食性および靭性とを兼ね備えたステンレス鋼板を提供することが可能である。従って、本発明鋼板は、例えば鉄道、自動車およびバスなどの車両構造部品や、土木建築構造物にとりわけ適している。
【図面の簡単な説明】
【図１】 マルテンサイト組織比率２％のミクロ組織写真である。
【図２】 MIG 溶接部の粒界腐食試験片の形状を示す図である。
【図３】 MIG 溶接部のシャルピー衝撃試験片の形状を示す図である。[0001]
BACKGROUND OF THE INVENTION
This invention is welded with excellent intergranular corrosion resistance and workability suitable for vehicle structural applications such as railways, automobiles and buses, and civil engineering and construction, where welding and bending are frequently used and corrosion resistance is required. The present invention relates to a structural stainless steel plate.
[0002]
[Prior art]
Conventionally, from the viewpoint of preventing strength reduction due to thickness reduction due to corrosion or from the viewpoint of aesthetics, vehicle structural members that require corrosion resistance, for example, structural members of railway vehicles, such as SUS301L specified in Japanese Industrial Standards (JIS) Austenitic stainless steel plates such as SUS 304 are used. These austenitic stainless steel sheets are excellent in workability and weld zone toughness, but as shown in Non-Patent Document 1, the weld zone during vehicle production is sensitized (when heated to a high temperature, Cr carbide) (Cr _{twenty three} C ₆ ) Formed at the grain boundaries, and a Cr-deficient layer formed around this Cr carbide), which sometimes caused grain boundary corrosion. In addition, in ferritic stainless steel such as SUS430, the crystal grains are coarsened at the welded portion, so the toughness of the welded portion is low, and Cr carbonitride precipitates at the coarsened grain boundaries, causing intergranular corrosion. there were.
[0003]
In addition, martensitic stainless steels for welded structures, represented by SUS410 specified in JIS, have the lowest Cr content of about 12% by mass, and the addition of Ni and Mo that improve corrosion resistance. For this reason, it has been a problem that the corrosion resistance is insufficient for use in an area that is visible to the public, such as a pool shed.
[0004]
With regard to this problem, for example, Patent Document 1 discloses that a martensitic structure having a volume ratio of 50% or more is formed in a weld heat affected zone (a part of an unmelted base material whose structure or hardness has been changed by welding heat). In general, stainless steel for general building structures has been proposed that improves toughness by refining crystal grains, is inexpensive, and has excellent corrosion resistance, weldability, and weld properties in a residential environment. However, if a martensite structure is generated at the grain boundary of the ferrite structure in the weld heat affected zone, the martensite structure is selectively corroded depending on the environment. In some cases, grain boundary cracking may occur. In addition, high corrosion resistance martensitic stainless steels used in oil well pipes and line pipes usually contain 3% by mass or more of Ni, so the corrosion resistance is excellent, but Ni increases the annealing softening resistance. As a result of having a martensite structure instead of a ferrite structure after annealing, the strength is increased to 800 MPa or more, which is not suitable for applications that require processing such as bending.
[0005]
[Non-Patent Document 1]
The 89th Symposium on Corrosion and Corrosion Prevention (held on March 19, 1992, published, “Case Analysis Method: Corrosion Cases of Railway-Related Stainless Steel Vehicles”, pages 84-89)
[Patent Document 1]
Japanese Patent Laid-Open No. 11-302795
[0006]
[Problems to be solved by the invention]
Thus, no steel type has been found so far that has simultaneously established the corrosion resistance and workability of the base metal, and further the intergranular corrosion resistance and toughness of the weld zone.
[0007]
Therefore, the object of the present invention is to solve the above-mentioned problems, particularly to improve the intergranular corrosion resistance of the weld heat affected zone significantly, toughness of the weld heat affected zone, and to work with low strength and high elongation. The object is to provide a structural stainless steel plate having excellent properties.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the inventors have investigated in detail the influence of the components of stainless steel on the corrosion resistance and mechanical properties of the base metal and the intergranular corrosion resistance and toughness of the weld. As a result, (a) the addition of Cr and Ni (Mo, Cu, Co if necessary) significantly improves the corrosion resistance, (b) Ni, Mn, C (or more necessary to suppress ferrite transformation) The amount of Mo and Cu added accordingly is limited to reduce annealing softening resistance, and the microstructure of the base material after annealing is ferrite + carbide (hereinafter, ferrite includes carbide) A low strength of 600 MPa or less and high elongation can be obtained, and (c) C + N <0.015% by mass C and N, and (d) Cr, Mn, Ni , C, N (Mo, Cu if necessary) and optimized martensite formation in the weld heat affected zone less than 5% by volume, excellent intergranular corrosion resistance and toughness in the weld zone Has been found to be compatible.
[0009]
The present invention has been completed based on the above findings, and the configuration thereof is as follows.
1. C: Less than 0.008% by mass, Si: 1.0% by mass or less, Mn: 1.5% by mass or less, Cr: 11% by mass to 15% by mass, Ni: More than 1.0% by mass and 2.5% by mass %: Less than 0.10% by mass, N: 0.009% by mass or less, P: 0.04% by mass or less and S: 0.01% by mass or less, the balance Is Fe And a stainless steel plate for welded structure, which consists of inevitable impurities and has excellent intergranular corrosion resistance and workability satisfying the following formulas (1), (2), (3) and (4).
Record
[Cr] + 1.2 × [Ni] ≧ 15.0 −−−− (1)
[Ni] + 0.5 × [Mn] + 30 × [C] ≦ 3.0 −−−− (2)
[C] + [N] <0.015 ---- (3)
[Cr]-[Mn] -1.7 × [Ni] −27 × [C] −100 × [N] ≧ 9.0 −−−− (4)
Here, [Cr], [Ni], [Mn], [C] and [N] are the contents (% by mass) of Cr, Ni, Mn, C and N, respectively.
[0010]
2. C: Less than 0.008% by mass, Si: 1.0% by mass or less, Mn: 1.5% by mass or less, Cr: 11% by mass to 15% by mass, Ni: More than 1.0% by mass and 2.5% by mass %: Less than 0.10% by mass, N: 0.009% by mass or less, P: 0.04% by mass or less and S: 0.01% by mass or less, and Mo: 2.0% by mass or less Containing the balance Is Fe And a stainless steel plate for welded structure, which consists of inevitable impurities and has excellent intergranular corrosion resistance and workability, satisfying the following formulas (3), (5), (6) and (7).
Record
[C] + [N] <0.015 ---- (3)
[Cr] + 1.2 × [Ni] + 1.5 × [Mo] ≧ 15.0 −−− (5)
[Ni] + 0.5 × ([Mn] + [Mo]) + 30 × [C] ≦ 3.0 −−−− (6)
[Cr] + 0.8 × [Mo] − [Mn] −1.7 × [Ni] −27 × [C] −100 × [N] ≧ 9.0 −−− (7)
Here, [Cr], [Mo], [Ni], [Mn], [C] and [N] are the contents (% by mass) of Cr, Mo, Ni, Mn, C and N, respectively.
[0011]
3. C: Less than 0.008% by mass, Si: 1.0% by mass or less, Mn: 1.5% by mass or less, Cr: 11% by mass to 15% by mass, Ni: More than 1.0% by mass and 2.5% by mass %: Less than 0.10% by mass, N: 0.009% by mass or less, P: 0.04% by mass or less and S: 0.01% by mass or less, further Cu: 2% by mass or less and Co : Contains 1 or 2 mass% or less, the balance Is Fe And a stainless steel plate for welded structure which has excellent intergranular corrosion resistance and workability, which consists of inevitable impurities and satisfies the following formulas (3), (8), (9) and (10).
Record
[C] + [N] <0.015 ---- (3)
[Cr] + 1.2 × [Ni] + 0.5 × [Cu] + 0.3 × [Co] ≧ 15.0 −−−− (8)
[Ni] + 0.5 × ([Mn] + [Cu]) + 30 × [C] ≦ 3.0 −−−− (9)
[Cr] − [Mn] −1.7 × [Ni] −27 × [C] −100 × [N] −0.3 × [Cu] ≧ 9.0 −−− (10)
Here, [Cr], [Ni], [Mn], [Cu], [Co], [C] and [N] are the contents of Cr, Ni, Mn, Cu, Co, C and N, respectively ( mass%)
[0012]
4). C: Less than 0.008% by mass, Si: 1.0% by mass or less, Mn: 1.5% by mass or less, Cr: 11% by mass to 15% by mass, Ni: More than 1.0% by mass and 2.5% by mass %: Less than 0.10% by mass, N: 0.009% by mass or less, P: 0.04% by mass or less, S: 0.01% by mass or less, and Mo: 2.0% by mass or less, Furthermore, Cu: 2% by mass or less and Co: 2% by mass or less of one or two types, the balance Is Fe And a stainless steel plate for welded structure, which consists of inevitable impurities and has excellent intergranular corrosion resistance and workability satisfying the following formulas (3), (11), (12) and (13).
Record
[C] + [N] <0.015 ---- (3)
[Cr] + 1.2 × [Ni] + 1.5 × [Mo] + 0.5 × [Cu] + 0.3 × [Co] ≧ 15.0 −−− (11)
[Ni] + 0.5 × ([Mn] + [Mo] + [Cu]) + 30 × [C] ≦ 3.0 −−−− (12)
[Cr] + 0.8 × [Mo] − [Mn] −1.7 × [Ni] −27 × [C] −100 × [N] −0.3 × [Cu] ≧ 9.0 −−− ( 13)
Here, [Cr], [Mo], [Ni], [Mn], [Cu], [Co], [C] and [N] are Cr, Mo, Ni, Mn, Cu, Co and C, respectively. And N content (% by mass)
[0013]
5. Any one of the above 1 to 4, further comprising one or two of B: 0.0050 mass% or less and Ca: 0.0050 mass% or less, a stainless steel plate for welded structure excellent in intergranular corrosion resistance and workability.
[0014]
6). In any one of 1 to 5 above, Ti: 0.2% by mass or less, Nb: 0.2% by mass or less, V: 0.2% by mass or less, Zr: 0.2% by mass or less, and Ta: 0.2% by mass or less A stainless steel plate for welded structures containing the above and having excellent intergranular corrosion resistance and workability.
[0015]
6 . 1 to above 5 In any of the above, a stainless steel plate for welded structure excellent in intergranular corrosion resistance and workability, further containing one or two of W: 0.10 mass% or less and Mg: 0.01 mass% or less.
[0016]
7 . 1 to above 6 In any of the above, the steel sheet has a tensile strength of 600 MPa or less, and is provided for a welded structure, and is a stainless steel sheet for welded structure excellent in intergranular corrosion resistance and workability.
[0017]
8 . 1 to above 7 In any of the above, the volume ratio of the martensite structure generated in the weld heat affected zone is less than 5%, and the Charpy impact value of the weld heat affected zone is 30 J / cm at −50 ° C. ² A stainless steel plate for welded structures having excellent intergranular corrosion resistance and workability as described above.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the component composition of the stainless steel plate of the present invention (hereinafter referred to as the present steel plate) will be described in detail.
C: Less than 0.008% by mass
C is an element that increases the strength of the steel and decreases the workability, and also reduces the intergranular corrosion resistance and toughness of the welded portion. When the content is 0.008% by mass or more, the adverse effect becomes significant. Therefore, it is limited to less than 0.008% by mass. In particular, from the viewpoint of weld zone toughness, it is desirable that the amount be less than 0.0050 mass%.
[0019]
Si: 1.0% by mass or less
Si is an element necessary as a deoxidizer. However, if it exceeds 1.0% by mass, the steel is remarkably embrittled and the toughness of the welded portion is also reduced, so it is limited to 1.0% by mass or less. From the viewpoint of weld zone toughness, it is desirable that the content be 0.3% by mass or less.
[0020]
Mn: 1.5% by mass or less
Mn is limited to 1.5% by mass or less because it increases the strength of the steel and lowers the workability as well as the corrosion resistance. In particular, from the viewpoint of corrosion resistance, the addition is preferably 1.0% by mass or less, more preferably 0.5% by mass or less.
[0021]
Cr: 11% to 15% by mass
Cr is an effective component for improving the corrosion resistance that is a characteristic of stainless steel, and 11 mass% or more is necessary to obtain sufficient corrosion resistance. From the viewpoint of corrosion resistance, Cr is preferably added in an amount of 12% by mass or more, and more than 13% by mass. On the other hand, if the content exceeds 15% by mass, the toughness is remarkably lowered, so this is the upper limit. In addition, from the viewpoint of toughness, Cr is desirably 14% by mass or less.
[0022]
Ni: More than 1.0% by mass and less than 2.5% by mass
Ni improves the corrosion resistance that is a characteristic of stainless steel, and improves the toughness of the base metal and the weld, which is a characteristic of structural steel. For that purpose, it is necessary to add exceeding 1.0 mass%. In addition, it is preferable to add exceeding 1.5 mass% from a viewpoint of weld zone toughness. More preferably, it is desirable to add more than 1.6% by mass. On the other hand, even if added over 2.5% by mass, the effect of improving weld toughness is saturated and only the cost of the material is increased, so it is limited to 2.5% by mass or less. In addition, since the necessary improvement effect of the weld zone toughness can be obtained even with an addition amount of 2.2% by mass or less, it is advantageous to make it 2.2% by mass or less in order to further reduce the cost.
[0023]
Al: Less than 0.10% by mass
Al is necessary as a deoxidizer for steelmaking. In order to obtain the effect, it is necessary to add 0.001% by mass or more. However, excessive addition reduces toughness, so it is limited to less than 0.10% by mass.
[0024]
N: 0.009% by mass or less
N, like C, is an element that reduces the intergranular corrosion resistance and toughness of the weld. If the content exceeds 0.009% by mass, the adverse effect becomes significant, so it is limited to 0.009% by mass or less. More preferably, it is limited to less than 0.008% by mass. In particular, from the viewpoint of weld zone toughness, the upper limit is preferably 0.005 mass%.
[0025]
P: 0.04 mass% or less
P is an element that reduces hot workability, and the lower the possible, the better the productivity. However, if it is too low, the cost of steelmaking will increase, so 0.04% by mass should be the upper limit. From the viewpoint of hot workability, the content is preferably 0.02% by mass or less.
[0026]
S: 0.01% by mass or less
S, like P, when the content is high, the hot workability is lowered, but is 0.01% by mass or less due to the economic limitation on the de-S treatment at the time of steelmaking. From the viewpoint of hot workability, the content is preferably 0.005% by mass or less.
[0027]
In the steel of the present invention, it is essential that the above-described basic component composition satisfies the above four formulas (1), (2), (3) and (4).
First, in order to obtain excellent corrosion resistance of the base metal and intergranular corrosion resistance of the weld, which is one of the features of the steel of the present invention, the addition of Cr and Ni is effective, and the effect is reliably obtained. For this purpose, the empirical formula (1) described above, that is,
[Cr] + 1.2 × [Ni] ≧ 15.0 ── (1) {Same as the meaning of the following formula (5), (8) or (11)}
It is necessary to satisfy. In particular, when importance is attached to corrosion resistance, it is advantageous that the formula (1) satisfies 16.0 or more, more preferably 17.0 or more.
[0028]
Next, in order to improve the workability of the base material, it is necessary to transform all martensite structures into soft ferrite structures by annealing heat treatment. In this respect, the addition of Ni, Mn and C increases the resistance of this ferrite transformation, so that the above-described formula (2), that is,
[Ni] + 0.5 × [Mn] + 30 × [C] ≦ 3.0 ─ (2) {same as the meaning of the expression (6), (9) or (12) described later}
It is necessary to satisfy. The left side of the above formula (2) is a calculation formula of Ni equivalent in the Schaeffler's organization chart, and Mo and Cu in the formula (6), (9) or (12) described later do not exist in this calculation formula. , Found in the experiment and put in the formula.
In particular, from the viewpoint of workability of the base material, the formula (2) is preferably 2.6 or less. As a result, it becomes possible to make the metal structure of the base metal ferrite and to reduce the tensile strength to 600 MPa or less by a normal annealing method.
[0029]
Here, if the tensile strength is 600 MPa or less, if it exceeds 600 MPa, the force required for processing such as bending becomes remarkably large, making the processing difficult, and the elongation becomes 25% or less. This is because it becomes easier. In order to further improve the workability, the tensile strength is preferably 550 MPa or less.
[0030]
On the other hand, reduction of the amount of C and N is particularly effective for improving the toughness of the heat affected zone, and the above empirical formula (3), that is,
[C] + [N] <0.015 ---- (3)
By satisfying the above, the toughness of the weld heat affected zone is remarkably improved. Further, the formula (3) is preferably 0.012 or less. In particular, if the formula (3) is 0.010 or less, the toughness is further improved. Reduction of the amount of C and N is also effective for improving workability by softening.
[0031]
Mn, Ni, C and N increase the austenite equivalent (austenite phase ratio occurring at 1000 to 1100 ° C.), generate a martensitic structure at the ferrite grain boundary of the weld heat affected zone that is quenched from a high temperature, Refinement improves toughness, but in some environments this promotes intergranular corrosion. If the volume ratio of the martensitic structure of the weld heat affected zone is 5% or less, excellent intergranular corrosion resistance can be obtained. For this purpose, the contents of Mn, Ni, C and N are regulated to suppress an excessive increase in austenite equivalent, while Cr is added to increase the ferrite equivalent (ferrite phase ratio generated at 1000 to 1100 ° C.). For that purpose, the above empirical formula (4), that is,
[Cr]-[Mn] -1.7 × [Ni] -27 × [C] -100 × [N] ≧ 9.0 ─ (4) {Same as the meaning of the following formula (7), (10) or (13) }
It is necessary to satisfy.
[0032]
In the steel sheet of the present invention, it is possible to add Mo described later in addition to the basic components described above.In this case, instead of the above formulas (1), (2) and (4), It is important to satisfy the above formulas (5), (6) and (7). However, the significance represented by the formulas (5), (6) and (7) is the same as the above formulas (1), (2) and (4). Furthermore, it is possible to add one or two of Cu and Co described later to the basic component described above or a component added with Mo. In this case, the above formulas (1), (2 ) And (4), it is important to satisfy the above-mentioned formulas (8), (9) and (10), or formulas (11), (12) and (13). Also in this case, the significance represented by the formulas (8), (9) and (10) or the formulas (11), (12) and (13) is the same as the above formulas (1), (2) and (4 ). In addition, when adding one of Cu and Co, when the addition amount of the one not added is less than 0.02% by mass, (8), (9), (10), (11), (12) and In equation (13), it is treated as 0% by mass.
[0033]
In the steel of the present invention, in addition to the basic components described above, the following components can be added as necessary.
Mo: 2.0% by mass or less
Mo is an effective component for improving corrosion resistance, and can be added to obtain a sufficient effect of improving corrosion resistance. From the viewpoint of corrosion resistance, it is preferable to add more than 0.5% by mass. On the other hand, if the content exceeds 2.0% by mass, not only the corrosion resistance improving effect is saturated, but also the annealing softening resistance is increased to harden the steel and the workability is lowered, so 2.0% by mass is made the upper limit. In order to obtain the effect of improving corrosion resistance, addition of 1.5% by mass or less is sufficient.
[0034]
One or two of Cu: 2% by mass or less and Co: 2% by mass or less
Cu and Co are elements that are effective in improving the corrosion resistance like Mo, so they are added as necessary. In order to obtain sufficient improvement in corrosion resistance and intergranular corrosion resistance, it is preferable to add 0.3% by mass or more. On the other hand, if each content exceeds 2% by mass, not only the effect is saturated, but also the workability such as bending is reduced by hardening the steel, so 2% by mass is made the upper limit.
[0035]
One or two of B: 0.0050 mass% or less and Ca: 0.0050 mass% or less
B and Ca have the effect of improving the hot workability of steel with a small amount of addition, and are added as necessary. In order to acquire the effect, 0.0005 mass% or more is added respectively. However, adding 0.0050% by mass in each case not only saturates the effect but also decreases the corrosion resistance, so 0.0050% by mass is made the upper limit. In addition, since the addition effect of B and Ca can be obtained by adding a trace amount, it is not necessary to set a lower limit.
[0036]
One, two or more of Ti, Nb, V, Zr and Ta are each 0.2% by mass or less
Ti, Nb, V, Zr, and Ta are elements that improve the workability of steel by adding a small amount, and are added up to 0.2% by mass as necessary. However, if added over 0.2% by mass, it hardens excessively and the workability deteriorates, so one or more of Ti, Nb, V, Zr and Ta are added at 0.2% by mass or less.
[0037]
One or two of W: 0.10 mass% or less and Mg: 0.01 mass% or less
W and Mg have the effect of improving the corrosion resistance of steel, and are added as necessary. In order to obtain the effect, W is added at 0.01% by mass or more, and Mg is added at 0.001% by mass or more. However, when W and Mg are contained in amounts exceeding 0.10% by mass and 0.01% by mass, respectively, one or two of W: 0.10% by mass or less and Mg: 0.01% by mass or less are added.
[0038]
In addition, the steel plate of the present invention is other than the above components , Fe And inevitable impurities. However, in the balance, in addition to Fe, alkali metals, alkaline earth metals, rare earth elements, transition metals and the like may be contained in small amounts of 0.01% by mass or less, respectively. Inclusion of a small amount of these elements of 0.01% by mass or less does not hinder the effects of the present invention.
[0039]
When the above formula (4), (7), (10) or (13) is satisfied, the martensite content of the weld heat affected zone is less than 5% by volume, and the intergranular corrosion resistance of the weld zone is excellent. It will be a thing. When the left side of the above formula (4), (7), (10) or (13) is less than 9.0, the martensite amount in the weld heat affected zone becomes 5% by volume or more, and the martensite structure formed at the ferrite grain boundary Along the intergranular corrosion becomes remarkable. From the viewpoint of intergranular corrosion resistance of the weld, the left side of the above formula (4), (7), (10) or (13) should be 9.5 or more, and no martensite structure should be generated in the weld heat affected zone. Is preferred.
[0040]
Here, in order to obtain the weld zone toughness required for welded structure applications, the Charpy impact value (vE) of the weld heat affected zone at −50 ° C. _-50 ) 30J / cm ² It is necessary to do it above. The conventional knowledge was to secure toughness by generating 50% by volume of martensite structure in the weld heat affected zone and making the crystal grains fine, but the steel of the present invention is coarse in the weld heat affected zone. Even if ferrite grains are formed, excellent toughness is obtained by adding more than 1% by mass of Ni and satisfying the above formula (3). By the way, vE _-50 30J / cm ² Below this, there is a risk that the structure will brittlely break in the cold. From the viewpoint of preventing brittle fracture of this structure, vE _-50 50 J / cm ² Or more, more preferably 80 J / cm ² The above is good.
[0041]
Here, the method for producing the steel of the present invention is not particularly limited, and may be in accordance with general production of stainless steel. For example, in steelmaking, the basic components and components added as necessary are melted in a converter or electric furnace, and then secondary refining is performed by VOD (Vacuum Oxygen Decarburization) or AOD (Argon Oxygen Decarburization). The method is preferred. Although the molten steel can be made into a steel material according to a known casting method, it is preferable to apply a continuous casting method from the viewpoint of productivity and quality.
[0042]
Next, the steel material obtained by continuous casting is heated to 1000 to 1250 ° C., hot rolled under normal conditions, for example, a sheet bar having a thickness of 20 to 40 mm by a reverse rolling mill, and then by a tandem rolling mill. It is a hot-rolled sheet having a desired sheet thickness of 1.5 to 8.0 mm. Moreover, it is good also as a hot-rolled board of desired board thickness 1.5-8.0 mm only with a reverse rolling mill. This hot-rolled sheet is subjected to batch annealing at 600 to 800 ° C., and then descaled by pickling or the like as necessary to obtain a product. Moreover, depending on a use, after cold-rolling and pickling after continuous annealing of 650-850 degreeC and making it a cold-rolled annealing board, it becomes a sheet product.
[0043]
Using the hot-rolled sheet annealed product or the cold-rolled annealed sheet product obtained as described above, processing such as bending or welding according to each application is performed, and the product is formed into, for example, a pipe or a panel. And it is used for structural members of railway vehicles and structural members such as automobiles and buses, such as pillars, belts and beams. The method for welding these structural members is not particularly limited, and a normal arc welding method such as MIG (Metal Inert Gas), MAG (Metal Active Gas) and TIG (Tangsten Inert Gas), or a spot Resistance welding methods such as welding and seam welding, and high-frequency resistance welding and high-frequency induction welding such as an electric resistance welding method are applicable.
[0044]
The steel of the present invention has a sufficiently low C content and prevents weld cracking, so post-heat treatment after welding is unnecessary, and it can be used as a structural material as it is welded, but for strength adjustment etc. In addition, post-heat treatment may be performed.
[0045]
【Example】
[Example 1]
Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples.
First, a 50 kg steel ingot having the chemical composition shown in Table 1, Table 2 and Table 3 was melted in a vacuum melting furnace, and a hot rolled sheet having a thickness of 3 mm was obtained by ordinary hot rolling. Then, after annealing at 650 ° C. for 15 hours in an argon atmosphere, annealing annealing was performed, followed by descaling by pickling to obtain a test material. Using the specimens thus obtained, the ratio of rusting area after the Cyclic Corrosion Test (CCT), the base metal structure and the martensite structure volume fraction of the heat affected zone after welding, toughness and resistance Intergranular corrosion properties, and the tensile strength and elongation in the rolling direction of the base metal were investigated.
[0046]
First, CCT was a cycle test in which salt spray in accordance with JIS Z2371 was combined with drying and wetting. That is, two plates with dimensions of 70 mm x 150 mm were taken from the test material, and one side was used as the test surface, spraying with salt water: 35 ° C for 2 hours, drying: 60 ° C for 4 hours, and wetness: 2 at 50 ° C. After 30 cycles of a total of 8 hours, calculate the rusting area of the test surface by image analysis using a computer, divide this by the test area, calculate the rusting area rate, and the number of tests Is the CCT rust area ratio.
[0047]
The microstructure of the base metal after annealing is a microstructure after etching the plate thickness section parallel to the rolling direction of the specimen with aqua regia (mixed hydrochloric acid and concentrated nitric acid in a volume ratio of 2: 1) Was observed at a magnification of 1000 times, and the presence or absence of a ferrite structure and a martensite structure was investigated.
When the martensite structure was fine and difficult to confirm, the Vickers hardness was measured with a test load of 5 kgf in accordance with JIS Z2371, and if it was Hv 190 or less, all the structures were judged to be ferrite. Hv190 or less is converted to a tensile strength of 600 MPa or less according to the hardness conversion table {Table 1 of SAE (Society of Automotive Engineers) J417}.
[0048]
In addition, MIG butt welding (wire JIS Y308, current: 150 A, voltage: 19 V, welding speed: 9 mm / s, shielding gas: 20% / min, shielding gas: 20 liter / min, route gap: 1 mm). The microstructure of the weld heat-affected zone 1 mm away from the bond part (boundary line between the weld metal and the base metal) with a thickness cross section perpendicular to the welding direction is observed with a microscope (100x) after etching with aqua regia, and computer The volume ratio of the martensite structure was measured by image analysis using a martensite structure ratio. Here, as shown in FIG. 1, a microstructural photograph having a martensite structure ratio of 2% was observed at the grain boundaries of the ferrite crystal grains.
[0049]
Furthermore, in order to investigate the intergranular corrosion resistance, the weld was placed in a boiling sulfuric acid / copper sulfate solution and then subjected to a bending test to examine the presence of intergranular corrosion cracking in the weld heat affected zone. The test aqueous solution is 1.8% by mass H ₂ SO _Four +6.4 mass% CuSO _Four The copper piece was added so as to remain even after the test was completed. The test piece is the size shown in Fig. 2, and after grinding the weld overlay, the dimension of 25mm width x 70mm length is cut out with the weld heat affected zone (1mm from the bond part) as the center of the test piece longitudinal direction. It was. After placing the specimen in the test solution and conducting a boiling test for 16 hours, remove it from the test solution, perform 180 ° bending with a bending radius of 3.0mm around the weld heat affected zone, and observe the outside of the bend with a magnifying glass. Then, the presence or absence of cracks due to intergranular corrosion was examined.
[0050]
In addition, in order to evaluate the weld zone toughness, a test piece of the size shown in FIG. 3 was collected in the same manner as the test piece shown in FIG. 2 and after grinding the weld overlay, the weld heat affected zone (1 mm from the bond zone) Was subjected to notch processing and a Charpy impact test in accordance with JIS Z2242. The Charpy impact test specimen had a thickness H of 10 mm (2 mm of which was V-notched), a width w of 3 mm (welding was removed by grinding), and a length L of 55 mm. The Charpy impact test was performed on five test pieces. The absorbed energy at -50 ° C was divided by the cross-sectional area of the test piece at the notch (0.8cm x 0.3cm) to determine the Charpy impact value. Heat affected zone vE _-50 As evaluated.
[0051]
Furthermore, JIS Z2201 No. 13 B-shaped test specimens were collected from the test materials, and the tensile strength and breaking elongation in the rolling direction were measured by a tensile test based on JIS Z2241. Table 4 shows the above measurement and evaluation results.
[0052]
The rust area ratio of the CCT test is 30% or less, the annealed metal structure is all ferrite, the martensite structure ratio of the weld heat affected zone is less than 5% by volume, and the effect of welding heat at a test temperature of -50 ° C. Charpy impact value (vE _-50 ) 30J / cm ² As described above, if there is no crack in the intergranular corrosion test of the weld heat affected zone, and the elongation at break is 30% or more, the characteristics for vehicle structures and the like are excellent, and the expected performance of the present invention is obtained. Become.
[0053]
[Table 1]

[0054]
[Table 2]

[0055]
[Table 3]

[0056]
[Table 4]

[0057]
As is apparent from Table 4, the steel of the present invention is excellent in corrosion resistance, toughness of weld heat affected zone and intergranular corrosion resistance, and further has low base metal strength, high elongation, and excellent workability. I understand. Any of the comparative steels is inferior to the examples of the present invention.
[0058]
[Example 2]
Next, the characteristics of the cold-rolled annealed sheet were investigated. The 3 mm hot-rolled annealed plate shown as specimen number 11 in Table 1 of Example 1 is rolled to a thickness of 1.5 mm by cold rolling with a reverse rolling mill, and annealed at 750 ° C. for 1 minute. Then, it was dipped in a mixed acid of 60 ° C. (nitric acid 10 mass% + hydrofluoric acid 3 mass%) and descaled to obtain a cold-rolled annealed sheet. The cold rolled annealed sheet thus obtained was subjected to the same test as the hot rolled sheet of Example 1 described above.
However, TIG welding (current: 95A, voltage: 11V, welding speed: 400mm / min, shielding gas: front (electrode) side 20 liters / minute, back side 20 liters / minute) is used to check the weld toughness. Using.
[0059]
As a result, the corrosion resistance was CCT rusting area ratio was 15%. The metal structure after annealing was ferrite, and the martensite structure ratio of the weld was 0%. As a characteristic of weld heat affected zone, toughness is Charpy impact value at -50 ℃ (vE _-50 ) Is 90J / cm ² In the intergranular corrosion test, there were no cracks. Mechanical properties were a tensile strength of 485 MPa and an elongation at break of 35%. As described above, the cold-rolled annealed plate also has almost the same characteristics as the hot-rolled annealed plate, and it has been confirmed that the target of the properties as a steel plate for vehicle structure has been achieved.
[0060]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a stainless steel plate having both the corrosion resistance and workability of the base material and the intergranular corrosion resistance and toughness of the weld heat affected zone. Therefore, this invention steel plate is especially suitable for vehicle structural components, such as a railway, a motor vehicle, and a bus, and a civil engineering building structure, for example.
[Brief description of the drawings]
FIG. 1 is a microstructure photograph of a martensite structure ratio of 2%.
FIG. 2 is a diagram showing the shape of an intergranular corrosion test piece of a MIG weld.
FIG. 3 is a diagram showing the shape of a Charpy impact test piece of a MIG weld.

Claims (8)

C: less than 0.008% by mass,
Si: 1.0 mass% or less,
Mn: 1.5% by mass or less,
Cr: 11 mass% or more and 15 mass% or less,
Ni: more than 1.0% by mass and 2.5% by mass or less,
Al: less than 0.10% by mass,
N: 0.009 mass% or less,
P: 0.04% by mass or less and S: 0.01% by mass or less, the balance being Fe and inevitable impurities, and the following formulas (1), (2), (3) and (4) Stainless steel sheet for welded structures with excellent intergranular corrosion resistance and workability.
[Cr] + 1.2 × [Ni] ≧ 15.0 −−−− (1)
[Ni] + 0.5 × [Mn] + 30 × [C] ≦ 3.0 −−−− (2)
[C] + [N] <0.015 ---- (3)
[Cr]-[Mn] -1.7 × [Ni] −27 × [C] −100 × [N] ≧ 9.0 −−−− (4)
Here, [Cr], [Ni], [Mn], [C] and [N] are the contents (% by mass) of Cr, Ni, Mn, C and N, respectively. C: less than 0.008% by mass,
Si: 1.0 mass% or less,
Mn: 1.5% by mass or less,
Cr: 11 mass% or more and 15 mass% or less,
Ni: more than 1.0% by mass and 2.5% by mass or less,
Al: less than 0.10% by mass,
N: 0.009 mass% or less,
P: 0.04% by mass or less and S: 0.01% by mass or less, further containing Mo: 2.0% by mass or less, the balance consisting of Fe and inevitable impurities, and the following formula (3), A stainless steel plate for welded structure which satisfies (5), (6) and (7) and is excellent in intergranular corrosion resistance and workability.
[C] + [N] <0.015 −−−− (3)
[Cr] + 1.2 × [Ni] + 1.5 × [Mo] ≧ 15.0 −−− (5)
[Ni] + 0.5 × ([Mn] + [Mo]) + 30 × [C] ≦ 3.0 −−−− (6)
[Cr] + 0.8 × [Mo] − [Mn] −1.7 × [Ni] −27 × [C] −100 × [N] ≧ 9.0 −−− (7)
Here, [Cr], [Mo], [Ni], [Mn], [C] and [N] are the contents (% by mass) of Cr, Mo, Ni, Mn, C and N, respectively. C: less than 0.008% by mass,
Si: 1.0 mass% or less,
Mn: 1.5% by mass or less,
Cr: 11 mass% or more and 15 mass% or less,
Ni: more than 1.0% by mass and 2.5% by mass or less,
Al: less than 0.10% by mass,
N: 0.009 mass% or less,
P: 0.04% by mass or less and S: 0.01% by mass or less, Cu: 2% by mass or less and Co: 1% or 2% by mass or less, the balance being Fe and inevitable A stainless steel plate for welded structure made of impurities and excellent in intergranular corrosion resistance and workability satisfying the following formulas (3), (8), (9) and (10).
[C] + [N] <0.015 ---- (3)
[Cr] + 1.2 × [Ni] + 0.5 × [Cu] + 0.3 × [Co] ≧ 15.0 −−−− (8)
[Ni] + 0.5 × ([Mn] + [Cu]) + 30 × [C] ≦ 3.0 −−−− (9)
[Cr] − [Mn] −1.7 × [Ni] −27 × [C] −100 × [N] −0.3 × [Cu] ≧ 9.0 −−− (10)
Here, [Cr], [Ni], [Mn], [Cu], [Co], [C] and [N] are the contents of Cr, Ni, Mn, Cu, Co, C and N, respectively ( mass%) C: less than 0.008% by mass,
Si: 1.0 mass% or less,
Mn: 1.5% by mass or less,
Cr: 11 mass% or more and 15 mass% or less,
Ni: more than 1.0% by mass and 2.5% by mass or less,
Al: less than 0.10% by mass,
N: 0.009 mass% or less,
P: 0.04 mass% or less,
S: 0.01% by mass or less and Mo: 2.0% by mass or less, Cu: 2% by mass or less and Co: 1% or 2% by mass or less, the balance being Fe and inevitable A stainless steel plate for welded structure which is made of impurities and has excellent intergranular corrosion resistance and workability which satisfies the following formulas (3), (11), (12) and (13).
[C] + [N] <0.015 −−−− (3)
[Cr] + 1.2 × [Ni] + 1.5 × [Mo] + 0.5 × [Cu] + 0.3 × [Co] ≧ 15.0 −−− (11)
[Ni] + 0.5 × ([Mn] + [Mo] + [Cu]) + 30 × [C] ≦ 3.0 −−−− (12)
[Cr] + 0.8 × [Mo] − [Mn] −1.7 × [Ni] −27 × [C] −100 × [N] −0.3 × [Cu] ≧ 9.0 −−− ( 13)
Here, [Cr], [Mo], [Ni], [Mn], [Cu], [Co], [C] and [N] are Cr, Mo, Ni, Mn, Cu, Co and C, respectively. And N content (% by mass) The welded structure according to any one of claims 1 to 4, further comprising one or two of B: 0.0050 mass% or less and Ca: 0.0050 mass% or less, and having excellent intergranular corrosion resistance and workability. Stainless steel plate. The welded structure according to any one of claims 1 to 5 , further comprising one or two of W: 0.10% by mass or less and Mg: 0.01% by mass or less and having excellent intergranular corrosion resistance and workability. Stainless steel plate. In any one of claims 1 to 6, the steel sheet has a tensile strength of not more than 600 MPa, intergranular corrosion resistance and excellent formability for welded structures stainless steel, supplied for welding structures. In any one of Claim 1 thru | or 7 , the volume ratio of the martensite structure | tissue produced | generated in a welding heat affected zone is less than 5%, and the Charpy impact value of this welded heat affected zone is 30 J / cm < ² > or more at -50 degreeC. A stainless steel plate for welded structures with excellent intergranular corrosion resistance and workability.

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