WO2008004684A1

WO2008004684A1 - Stainless steel with low chromium content excellent in the corrosion resistance of repeatedly heat-affected zones and process for production thereof

Info

Publication number: WO2008004684A1
Application number: PCT/JP2007/063622
Authority: WO
Inventors: Masuhiro Fukaya; Akihiko Takahashi; Shinichi Teraoka; Shunji Sakamoto
Original assignee: Nippon Steel & Sumikin Stainless Steel Corporation
Priority date: 2006-07-04
Filing date: 2007-07-03
Publication date: 2008-01-10
Also published as: AU2007270326B2; AU2007270326A1

Abstract

A stainless steel with low chromium content which is little lowered in the corrosion resistance of a weld formed by repeated welding and is excellent in the intergranular corrosion resistance of a weld even under severe corrosive atmosphere and which is freed from preferential corrosion in the heat affected zone near a weld fusion line and is excellent in productivity. The stainless steel contains by mass C: 0.03% or below, N: 0.004 to 0.02%, Si: 0.2 to 1%, Mn: more than 1.5% to 2.5%, P: 0.04% or below, S: 0.03% or below, Cr: 10 to 15%, Ni: 0.2 to 3.0%, Al: 0.005 to 0.1%, and Ti: 4 × (C% + N%) to 0.35% with the balance consisting of Fe and unavoidable impurities and satisfies the requirements: γp(%) of 80 or above as represented by a prescribed numerical formula and Ti% × N% < 0.004.

Description

Description Low-chromium stainless steel with excellent corrosion resistance in heat-affected zone of multiple times and its manufacturing method Technical Field

The present invention improves the intergranular corrosion resistance in the heat-affected zone in the vicinity of the weld when multiple welding is performed (multi-pass), and further avoids preferential corrosion that occurs in the vicinity of the fusion line in the weld. It relates to a low chromium-containing stainless steel with excellent corrosion resistance of welds that can be used for a long period of time in applications where the corrosive environment is severe as structural steel. Background art

Chromium-containing stainless steel with low chromium content and low nickel content is extremely advantageous in terms of price compared to austenitic stainless steel such as SUS304 steel. It is suitable for applications that are used in large quantities. Such a chromium-containing steel has a ferrite structure or a martensite structure according to its composition. Generally, fillite or martensite stainless steels are inferior in the low temperature toughness or corrosion resistance of welds. For example, in the case of martensitic stainless steel represented by SUS4 10, the C content is as high as about 0.1 nias s%, so the weld toughness is poor and the weldability is inferior. Therefore, the welding workability is also inferior, so that problems remain in the application to parts that require welding.

As a means for preventing such deterioration of the properties of the welded portion, a martensite structure formed by the welded portion as described in Japanese Patent Publication No. 51-13463 and Japanese Patent Publication No. 61-23259 is used. Corrosion resistance and A method for preventing a decrease in low temperature toughness is disclosed. JP-B-51-13463 proposes Cr: 10-18%, Ni: 0.1-3.4%, Si: 1.0% or less, and Mn: 4.0% This is a method of generating a mashite martensite structure in the heat affected zone of the weld with a steel component reduced to C: 0.030% or less and N: 0.020% or less. We are proposing welded martensitic stainless steel with improved weld performance.

Low chromium-containing stainless steel using martensite transformation in such welds is actually used as an aggregate in marine containers, and examples of corrosion resistance or low-temperature toughness in welds have been problems until now. Absent. However, when used in severe corrosive environments (long steel wetting time, high chloride concentration, high temperature, low pH, etc.), the corrosion resistance of welds may be insufficient. I've come to understand. For example, it has been reported that intergranular corrosion occurs in the heat-affected zone when used in the platform of a railway freight car that carries coal or iron ore.

As a method to improve the corrosion resistance and weld toughness of the weld heat-affected zone of low chromium-containing stainless steel, the above-mentioned high purity, and in addition, elements for fixing carbon and nitrogen as carbides and nitrides Various steels produced by such means are disclosed because the addition of is effective. For example, Japanese Patent Laid-Open No. 2002-32725 1 discloses intergranular corrosion resistance of welds of chromium-containing steel using martensite transformation by adding appropriate amounts of carbon and nitrogen stabilizing elements Nb and Ti. In addition, chromium-containing steels that are excellent in low temperature toughness are disclosed. Similarly, Japanese Patent No. 349 1625 discloses a Fe—Cr alloy that has improved the corrosion resistance of welds by adding carbonitride-forming elements Ti, Nb, Ta, and Zr. . However, this patent contains Co, V and W. It is essential to improve the initial rust resistance.

Based on the above background, in recent years, as a countermeasure against intergranular corrosion in the weld heat affected zone, in the environment where coal and iron ore used for railway freight carriages are mined inland and transported to the coast by railway, There is an example in which low chromium-containing stainless steel added with Ti similar to the disclosure of Japanese Patent No. 327251 and Japanese Patent No. 34 9 1625 is applied.

However, in this example, the intergranular corrosion resistance of the weld heat-affected zone is improved, but the part along the interface between the weld zone and the mash martensite structure, which is the heat-affected zone closest to it (fusion fusion zone) The present inventor has newly found that there is a problem that preferential corrosion occurs in the vicinity. This phenomenon is called a knife line attack found in welds of SUS321 and SUS347 stable austenitic stainless steels as disclosed in Journal of the Japan Welding Society, Vol. 44, 1975, No. 8, page 679. This phenomenon is similar to the phenomenon, and the interface (fusion line) between the weld and heat-affected zone preferentially progresses in corrosion and the corrosion area expands, so this is an issue to be improved.

The cause of knife line attack is that when C is fixed with 1 ^ (nya ^ (: and stainless steel is welded, Ti C and NbC are dissolved in the region where the thermal history is raised to about 1200 or more. However, when passing through the sensitization temperature range in the subsequent cooling process, Cr carbide precipitates at the grain boundaries and the corrosion resistance decreases, but in the case of low chromium-containing stainless steel, what are the causes? Whether or not preferential corrosion occurs in Japan has not been fully examined and no measures have been taken.

In addition, the low-chromium stainless steel added with the C and N fixing elements described above is a component system with improved intergranular corrosion resistance in the weld zone, but the corrosion resistance of the heat-affected zone after multiple welds. However, it has been reported that corrosion may occur in the heat affected zone. Welding structure From the viewpoint of expanding the degree of freedom in construction design and improving the level of weld repair, a low-chromium stainless steel capable of multi-pass welding with excellent corrosion resistance in the heat-affected zone even after multiple welding has been awaited.

On the other hand, it is known that edge cracks during hot rolling are likely to occur in the production of low chromium-containing stainless steel. This is thought to be due to the fact that the phase stability of the austenite and Del Yuferai cocoon in the hot working temperature range is directly affected by the balance change of the contained elements. Therefore, there are issues to be solved from the viewpoint of optimizing the manufacturing process, and improvements were desired.

In addition, when coal or iron ore railcar freight trucks are used, it is desired to improve transportation efficiency by increasing the loading capacity and to reduce fuel costs by reducing weight. Since the total weight of railway wagons is fixed, it is essential to make the stainless steel sheet thinner in order to increase the loading capacity. In order to achieve this, it is indispensable to increase the strength of low chromium-containing stainless steel sheets, but no low chromium-containing stainless steel sheets with excellent balance of strength and ductility considering workability have yet been developed. Was expected. Disclosure of the invention

The present invention prevents deterioration of corrosion resistance at the welded part when multi-pass low-chromium stainless steel using martensite transformation is used, and it is difficult to use railway wagons of coal and iron ore. Even in a corrosive environment, the multi-pass welds have excellent intergranular corrosion resistance, and at the same time, there is no preferential corrosion that occurs near the weld fusion line. The first issue is to provide stainless steel. If necessary, the second is to provide high-strength, low-chromium stainless steel with an excellent balance of strength and ductility. Let it be an issue.

As a result of intensive studies to solve the above first problem, the present inventors have found that in order to prevent the occurrence of intergranular corrosion at and near the welded part when multiple welding is performed (multipass), This can be achieved by adding Ti and Nb, which stabilize the carbon and nitrogen that cause corrosion, while Ti and Nb add preferential corrosion near the weld fusion line. It was found that there is no effect in preventing the occurrence. Therefore, as a result of examining to prevent preferential corrosion of the heat affected zone adjacent to the weld zone, the heat affected zone where the mash martensite adjacent to the weld zone is exposed to very high temperature, Found that the scale is formed thick only in this part, the Cr concentration just below the scale is reduced, so-called Cr-deficient layer is formed, and as a result, the phenomenon of preferential corrosion similar to knife line attack occurs. . In addition, if the Ti content, which has an effect on the intergranular corrosion resistance of the multipass weld heat-affected zone, increases, it causes surface defects due to TiN crystallization. Clarified that there is a need to control. Furthermore, in addition to improving the corrosion resistance of the weld heat-affected zone, as a result of careful study to prevent a decrease in weld toughness, a component design was made to satisfy the following formula (A) describing the austenite stability, and the phase stability It was found that the objective can be achieved by optimizing.

r P (%) = 420x C% + 470 xN% + 23xNi% + 9 xCu

+ 7 XMn%-11.5XCr%-11.5xSi%-12xMo% -23xV% -47xNb% -49xTi%-52XA1% + 189≥80 (A) r P (gamma potential) It is an index that evaluates the degree, and at the same time, it is an index that represents the speed of formation of martensi cocoons.

In addition, in producing stainless steel with component design, It is possible to produce low-chromium stainless steel that does not cause edge cracking when the heating temperature in the hot rolling process of the slab is controlled to the austenite single-phase region or to a temperature at which the Dell evening ferrite amount exceeds 50%. I found it.

In addition, as a result of intensive studies to solve the above second problem, the present inventors have not been able to achieve a sufficiently high strength with an annealed ferrite structure in a low chromium-containing ferritic stainless steel. I found out.

Therefore, we studied to increase the strength of ferritic stainless steel with low chromium content. As a result, in the production of stainless steel with components designed to improve the corrosion resistance of the heat-affected zone adjacent to the weld, By appropriately selecting the heat treatment temperature and holding time, in the temper softening heat treatment process of the hot-rolled sheet, which is a martensite structure, the metal structure can be appropriately tempered into a two-phase structure of Ferai and martensite. It was found that high-strength chromium-containing stainless steel with an excellent balance of strength and ductility can be produced. In particular, it is effective and practical in the case of a component that appropriately contains Nb and Ni and has increased temper softening resistance. Practically, for example, the heat treatment temperature is 600 to 800, the holding time is 2 to 30 hours, and an appropriate temperature is set. Thus, a desired metal structure can be obtained.

The present invention has been completed based on such findings, and the gist thereof is as follows.

(1) By mass%, C: 0.03% or less, N: 0.004 to 0.02%, Si: 0.2 to 1%, Mn: more than 1.5 to 2.5%, P: 0.04% or less, S: 0.03% or less, Cr: 10 to 15%, Ni: 0.2 to 3.0%, A1: 0.005 to 0.1%, In addition, Ti: 4 X (C% + N%) or more and 0.35% or less, the balance is Fe and inevitable impurities, and the content of each element is expressed by the following formula (A) And a low-chromium stainless steel with excellent intergranular corrosion resistance in the heat-affected zone of the multipass weld and excellent corrosion resistance near the weld fusion line, characterized by satisfying the formula (B).

r P (%) = 420 x C% + 470 XN% + 23xNi% + 9 XCu%

+ 7 XMn%-11.5xCr%-11.5xSi%-12xMo%-23xV% -47xNb% -49xTi% -52xAl% + 189≥80-... (A) Ti% XN% <0.004 )

(2) The multipass welding heat effect according to (1), characterized by containing one or two of Mo: 0.05 to 3% and Cu: 0 · 05 to 3% in mass% A low chromium-containing stainless steel with excellent intergranular corrosion resistance in the weld zone and superior corrosion resistance in the vicinity of the welded fusion line.

(3) Multipass as described in (1) or (2), characterized by containing one or two of Nb: 0.01 to 0.5% and ¥: 0.01 to 0.5% in mass% Low chromium stainless steel with excellent intergranular corrosion resistance in weld heat affected zone and preferential corrosion resistance in the vicinity of weld zone fusion joint line

(4) Stainless steel composed of the components described in any one of (1) to (3), wherein the metal structure is a two-phase structure of a ferrite phase and a martensite phase. {110} The half-width broadening B defined by the following formula (C) of the diffraction line is 0.1 to 1.0, and has an excellent balance of strength and ductility, and the grain boundary of the heat-affected zone of the multipass welding A low-chromium stainless steel with excellent corrosion resistance and preferential corrosion resistance in the vicinity of the welded fusion line.

B = (W-Wo) / Wo · · · · (C)

Wo: Half width without internal distortion (deg)

W: Half width (deg)

(5) Heat of the flakes comprising the component according to any one of (1) to (3) The heating temperature in the hot rolling process is lower than the upper limit temperature Ac ₄ of the austenite single phase determined from the components of the flakes, or in the case of heating above Ac ₄ Production of low chromium-containing stainless steel with excellent intergranular corrosion resistance in heat-affected zone of multipass welding and preferential corrosion resistance in the vicinity of welded fusion line Method. Brief Description of Drawings

Figure 1 shows an example of the relationship between annealing temperature and hardness.

Fig. 2 shows the effect of Cu and Cr on the corrosion rate of a steel material of 0.25 mass% Ti and pH = 2 as a result of the sulfuric acid immersion test.

Figure 3 shows the cross-sectional metallographic structure of the heat affected zone after the improved Strauss test. a) Cross-sectional structure of MIG welding heat-affected zone of comparative steel No. 21 (without addition of Ti) b) Cross-sectional structure of TIG welding heat-affected zone of invention steel No. 1 c) MIG welding heat-affected zone of invention steel No. 1 D) Cross sectional structure of MIG welding heat affected zone of invention steel No. 11. (In Fig. 3, 1, 2 and 3 indicate the weld heat affected zone 1, 2 and 3)

Fig. 4 shows an example of the relationship between annealing conditions, strength, and ductility. BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in further detail. First, the reasons for limiting the ingredients will be explained.

C lowers the toughness of the martensitic structure of the weld and reduces the intergranular corrosion resistance. Therefore, its content is set to 0.03% by mass or less.

N precipitates as a nitride and deteriorates the intergranular corrosion resistance due to the formation of a Cr-deficient phase, so the upper limit of its content is 0.02 mass% or less. The However, in the composition range of the present invention, excessive N reduction not only increases the fertility load, but also softens, so that the desired material as a structural material cannot be obtained, so the lower limit of the content is 0.004. It was set as mass%.

Si is an element usually used as a deoxidizing material, but if the content is 0.2% by mass or less, a sufficient deoxidizing effect cannot be obtained. In addition, it may be positively added for the purpose of improving oxidation resistance. However, if its content exceeds 1% by mass, the manufacturability of the material deteriorates, so the content is 0.2 to 1% by mass. %.

Mn is an austenite phase (a phase) stabilizing element, and effectively contributes to the improvement of weld toughness by making the weld heat-affected zone structure a martensite structure. Mn is also useful as a deoxidizer, as is the case with Si, so it should be contained in a range of more than 1.5 mass%. However, if added excessively, sulfide inclusions that deteriorate the corrosion resistance of the steel material are formed, and the corrosion resistance of the material is deteriorated. Therefore, its content is limited to 2.5% by mass or less. More preferably, it is 2.0 mass% or less.

P is an element that segregates at the grain boundaries and is not only detrimental to hot workability, formability, and toughness, but is also harmful to corrosion resistance. In particular, when the content exceeds 0.04 mass%, the effect becomes significant. Therefore, the P content is limited to 0.04 mass% or less. More preferably, it is 0.025% or less.

S is an element that forms sulfide inclusions and degrades the corrosion resistance of steel. The upper limit of its content must be 0.03 mass%. The smaller the S content, the better the corrosion resistance, but the desulfurization load for reducing S is increased, so the lower limit is preferably set to 0.003 mass%.

Cr is an element effective for improving corrosion resistance, but if it is less than 10% by mass, it is difficult to ensure sufficient corrosion resistance. Cr is an element that stabilizes the ferrite phase (α phase). Addition of more than 15% by mass not only causes a decrease in workability, The stability of the austenite phase (a phase) decreases, and a sufficient amount of martensite phase cannot be secured during welding, leading to a decrease in weld strength and toughness. Therefore, in the present invention, Cr is contained in the range of 10 mass% or more and 15 mass% or less. A particularly preferable range for combining weather resistance, workability, and weldability is 11.0 to 13.0% by mass. Furthermore, not only the intergranular corrosion resistance of the heat-affected zone of multi-pass welding, but also to prevent the occurrence of preferential corrosion in the vicinity of the welded part fusion line. .

Ni is an indispensable element for improving the corrosion resistance and for forming martensite in the weld and improving the toughness of the weld. Its content must be at least 0.2% by mass. Become. However, if the content exceeds 3.0% by mass, the amount of martensite generated in the welded portion will increase significantly, so the content should be 0.2 to 3.0% by mass. In addition, Ni has the effect of increasing the temper softening resistance of the martensite structure of the hot-rolled sheet. Therefore, when manufacturing a high-strength material with a good balance of strength and ductility, it can be applied during tempering and annealing of the hot-rolled sheet. The range can be widened.

Ti is an essential element for preventing intergranular corrosion resistance in welds. The T i content must be at least four times the sum of the C and N contents. On the other hand, even if it exceeds 0.35% by mass, it is resistant to intergranular corrosion. The effect of improving the food texture is saturated, and as will be described later, the formation of cluster inclusions causes other properties such as surface flaws during hot rolling and deterioration of workability. Therefore, the lower limit of Ti content is set to 4 X (C mass% + N mass%) in terms of corrosion resistance, and the upper limit is set to 0.35 mass% in terms of surface properties.

A 1 is an effective additive as a deoxidizer, but if it is contained in a large amount, the surface quality of the steel deteriorates and the weldability also deteriorates, so its content is in the range of 0.005 to 0.1% by mass. And Preferably, from 0.005 to 0.03 mass% is there.

In addition to the above component concentration range, the component concentration is specified so that the following equation (A) is satisfied. With this rule, it is possible to obtain a chromium-containing steel with excellent weld toughness and intergranular corrosion.

% By mass

Ύ P (%) = 420 x C + 470xN% + 23xNi% + 9 xCu%

+ 7 XMn%-11.5XCr%-11.5XSi%-12xMo% -23xV% -47XNb% -49xTi%-52xAl% + 189≥80-, · · · · · · · · · · · It is an index that shows the stability of the site, and at the same time, an index that shows the speed of martensite formation. When the ap is 80% or more, the weld heat affected zone undergoes complete transformation via the high-temperature austenite single phase during cooling, and a sufficient martensite structure is formed in the weld heat affected zone. On the other hand, if it is less than 80%, the austenite becomes unstable and the formation of the martensite phase is insufficient. At the same time, it is necessary to satisfy the formula (A) in order to obtain a fine grain structure as it is while being rolled by complete transformation through a single phase during hot rolling. -'

A finer ferrite grain size is also advantageous in improving intergranular corrosion resistance and low temperature toughness by increasing the interfacial area. Therefore, the ferrite average particle size is preferably 6 or more in terms of ferrite particle size number according to JIS G 0522. The ferrite grain size number refers to that in the final product. Since the chromium-containing steel of the present invention is required to have a low cost as a structural material, the final product is exclusively a hot-rolled annealing material.

In addition to the above component concentration range and the above equation (A), the component concentration is defined so as to satisfy the following equation (B). Such regulations can prevent surface flaws on the hot rolled sheet. If the content of Ti and N is not satisfied and (B) is not satisfied, the molten steel will solidify, and a large number of coarse TiN will crystallize at the liquidus temperature, causing surface defects during hot rolling. As mentioned above, the final product is a hot-rolled annealed material and is often descaled and used as pickled skin. Therefore, it is necessary to regulate the components from the viewpoint of preventing surface wrinkles.

T i% X N% 0.0.004 (B)

The low chromium content stainless steel described above is excellent in weld toughness and intergranular corrosion resistance, but in order to improve the corrosion resistance in low pH solutions, addition of Mo or Cu to the steel Works effectively. In particular, Cu loading is effective for low pH dilute sulfuric acid environment with coal leachate when loading coal.

In order to improve the corrosion resistance of both Mo and Mo, it is necessary to add at least 0.05% by mass, respectively. However, if Mo exceeds 3% by mass and Cu exceeds 3% by mass, the effect of improving corrosion resistance is saturated. In addition, the upper limit is 3 mass% for Mo and 3 mass% for Cu. Preferably, both Mo and Cu are 0.1 to 1.5% by mass. Since it is an austenite stable element next to N and Ni, it is also an effective element for controlling the phase stability calculated from a in Eq. (A). Also, Cu is a solid solution strengthening element, so it is a useful element for increasing strength.

Nb and V are carbonitride forming elements and can be selectively added. In order to fix C and N, Nb requires a content of 0.01% by mass, but even if added over 0.5% by mass, the improvement effect of intergranular corrosion resistance is saturated. However, it may cause deterioration of other characteristics such as workability. Therefore, Nb is in the range of 0.01 to 0.5 mass%. Preferably, it is 0.03 to 0.3% by mass. For the same reason, V is also in the range of 0.01 to 0.5 mass%. Preferably, it is 0.03 to 0.3 mass%. Nb is the hot rolled sheet metal. Since it has the effect of increasing the temper softening resistance of the rutensite structure, when manufacturing high-strength materials with an excellent balance of strength and ductility, the range of application during tempering and annealing of hot-rolled sheets can be widened. .

High-strength materials with a tempered balance of strength and ductility have a resistance of 450 MPa or more and an elongation of 15% or more. It is desirable to have a resistance of 450MPa and an elongation of 20% or more.More preferably, it has a yield strength of 500MPa or more and an elongation of 20% or more. Is not a fully annealed ferrite single-phase structure, but a two-phase structure consisting of a Ferai phase and a martensite phase. It is a metal structure in the temper softening process of the martensite phase structure of hot-rolled sheets.

、マルテンテンしたした具備具備具備具備したした高高高した高したしたしたした. Also, a metal structure in which the precipitated austenite phase (reverse transformation, r phase) is transformed into a martensite phase transformed during cooling may be used.

Since the degree of softening of martensite cocoon due to tempering and the strength and ductility are correlated, controlling the fraction of martensite phase and ferrite phase is important in designing materials such as strength and ductility. is important. However, it is generally difficult to obtain the volume fraction by distinguishing between ferrite phase and martensite phase in the metal structure. Since both phases have the same crystal structure, the diffraction angles in X-ray diffraction are almost the same and are difficult to distinguish, and because both phases are ferromagnetic, they can be distinguished by the presence or absence of magnetic properties. difficult.

Therefore, in the present invention, as a method for measuring the degree of recovery of dislocations in the tempering process of the martensite structure, that is, the degree of recovery of the disorder of the crystal structure, the following (C ) Applying the half-value spread Β defined by did.ピーク α ΐ and Κ «2 peaks were separated, and the half width of the Κ α ΐ line was measured to obtain Β.

Β = (W— Wo) / Wo—— —— (C)

Wo: Half width without internal distortion (deg)

W: Half width (deg)

In the present invention, Cu is used as the X-ray source, but other X-ray sources may be used. In addition, B was evaluated using the value (Wo = 0.089 deg) of 11 mass% Cr ferritic stainless steel (steel material No. 1 in Example Table 1 described later).

This method is based on the Japan Iron and Steel Institute, Material and Structure Properties Subcommittee, Stainless Steel Formability and Utilization Technology Voluntary Forum, “Strengthening and Utilization Technology of Stainless Steel” September 29, 1998, p. 49 As disclosed in, is a general purpose evaluation method for evaluating the tempering behavior of steel.

The half width corresponds to the dislocation density. The definition of the half-value width is the width of the diffraction angle corresponding to the intensity of 1 2 of the peak intensity from the diffractive surface. The larger the half width, the greater the amount of strain (disturbance of the crystal structure) of the material. As the tempering progresses and the dislocation recovers and the strain amount decreases, the half width decreases. B = 0 means an annealed structure from which strain has been removed (tempered structure and ferrite single phase). In the present invention, B was less than 0.1. In the martensitic structure of hot-rolled steel sheets, the B value is approximately 2.0. In order to obtain a high-strength material with excellent strength ductility by controlling the martensite phase and ferri-phase to a two-phase structure during the tempering process, the B value is 0.1 to 1.0. It is preferably 0.3 to 0.8. If the B value is more than 1.0 and less than 2.0, tempering does not proceed and ductility is insufficient.

Next, a preferred method for producing a low chromium-containing stainless steel will be described. First, the molten steel adjusted to the above preferred component composition is melted in a commonly known melting furnace such as a converter or an electric furnace, and then vacuum degassing (RH method) The steel material is scoured by a known scouring method such as VOD method or AOD method, and then forged into a slab or the like by a continuous forging method or an ingot one-piece method. The steel material is then heated and made into a hot-rolled steel sheet by a hot rolling process. At that time, the selection of the heating temperature in the hot rolling process is very important from the viewpoint of avoiding edge cracks in the hot rolled sheet. In the case of austenitic stainless steel, during the hot working stage, in the phase state containing less than 50% of Dell evening ferrite, especially 10-30%, due to the difference in deformation resistance between both austenite and Dell evening ferrite, Distortion concentrates on the soft Dell evening ferrite phase, causing cracks at the interface between the two phases, and defects such as surface cracks and especially edge cracks are likely to occur, resulting in various problems in process, yield, and quality. The present inventor has found that the same applies to the hot working temperature range of the low chromium-containing stainless steel.

Therefore, the heating temperature in the hot rolling step of铸片is austenite phase in the case of heating in either the upper limit temperature Ac below ₄ austenite single phase deposition minutes determined in铸片, or Ac ₄ than Good hot workability can be obtained by selecting a temperature at which the amount of ferrite is greater than 50%. The temperature of Ac ₄ can be determined from the component values of the steel by phase diagram calculation using the integrated thermodynamic calculation system Thermo-C a 1 c (distributor: CRC Solutions). When the heating temperature is high, the deformability is improved as the amount of delta ferrite increases, but in the case of the Dell State ferrite main phase, if the heating temperature is too high, the crystal grains become coarse and during hot working. Wrinkle-like defects due to coarse crystal grains occur at the edge of the hot-rolled sheet, which causes problems in process, yield, and quality as with edge cracking. Therefore, the heating temperature is preferably 1300 or less. .

Moreover, in the hot rolling process, it is only necessary to obtain a hot rolled steel sheet having a desired thickness, and the hot rolling conditions are not particularly limited, but the hot rolling finishing temperature is 800: or more and 1000 or less. Can be strength, workability and ductility It is preferable from the viewpoint of ensuring. The coiling temperature is 800 or less, preferably 650 to 750 when tempering annealing is performed.

In addition, when increasing the strength of the two-phase structure of the ferrite phase and martensite phase, which will be described later, the hot rolling finishing temperature should be 900 or less and the coiling temperature should be 650 or less. Therefore, it is desirable to accumulate work strain and improve the temper softening resistance in order to widen the annealing condition range.

After the hot rolling is finished, it is preferable to subject the hardened structure to a martensite phase by hot-rolled sheet annealing to soften the martensite phase by tempering. The tempering temperature should be as high as possible in the ferrite temperature range. Although the transformation point, which is the upper limit temperature of the ferrite single phase, varies depending on the amount of Ni, etc., it is often adjusted to about 650-700 for practical steels, and annealing below this temperature is desirable. Therefore, in this hot-rolled sheet annealing, it is preferable to set the annealing temperature: 650 to 750 t and the holding time: 2 to 20 h from the viewpoint of improving workability and ensuring ductility.

In addition, it is more preferable in terms of softening that the temperature range between 600 and 750 is annealed at a cooling rate of 50 and Zh or less after hot-rolled sheet annealing.

If necessary, when providing a high-strength, low-chromium stainless steel with an excellent balance of strength and ductility, the temper softening process of the martensite phase structure of the hot-rolled sheet rather than the fully annealed ferrite phase structure Therefore, it is necessary to control the two-phase structure of the ferrite phase and martensite phase. For this reason, the heat treatment temperature of the hot-rolled sheet is set to 550 t: to 850. There is no particular restriction on the holding time, but it is desirable to set the heat treatment time considering practicality. Therefore, it is preferable that the heat treatment temperature is 600 to 800 t: and the holding time is 2 to 30 h. In the case of batch heat treatment, the cooling rate is usually controlled to 50t: Zh or less. Heat treatment temperature is (^ or less The above may be Ac, or the following.

Ac, in the following cases, is a metal structure in the temper softening process of the martensite phase, and the retention time needs to be shorter than that of a fully annealed ferrite single phase structure. This heat treatment condition can be obtained by creating a one-hour temperature map of the hot-rolled sheet structure for each individual component composition of steel.

In the above case, in addition to the metal structure obtained by the following heat treatment, it is a metal structure in which the precipitated austenite phase (reverse transformation phase) is combined with the martensite phase transformed during cooling. The holding time in this case is not particularly limited, but is practically 2 to 30 hours, preferably 2 to 15 hours.

In addition, the steel sheets after hot rolling or after hot rolling annealing were adjusted to the desired surface properties with the scale removed by shot blasting, pickling, etc. as necessary, or by polishing, skin pass, etc. Later, it may be a product plate. Further, the component steel according to the present invention can be applied to various steel materials that can be used as structural steels in fields such as thick steel plates, shaped steels manufactured by hot rolling, and bar steels. Example

(Example 1)

Tables 1 and 2 show invention examples and comparative examples related to the first problem. Table 1 shows the components in steel of the invention steel and comparative steel in mass%. Steel Nos. 1 to 20 are invention steels, and Steel Nos. 2 to 26 are comparative steels.

The pieces of the ingredients shown in Table 1 were melted into a 40 kg or 35 kg flat ingot by vacuum melting. After cleaning these steel surfaces, the ingots were heated for 1 hour at 1 1 50 to 1 250 to perform hot rough rolling consisting of multiple passes followed by hot finish rolling. Hot rolled The end temperature was 800 to 950. The hot-rolled sheet was air-cooled, held at a coiling temperature of 700 ° C for 1 hour, and then air-cooled and subjected to a simulated winding heat treatment to obtain a hot-rolled sheet with a thickness of 4 mm. Subsequently, in order to determine the annealing temperature of the hot-rolled sheet, the hot-rolled sheet of each component value was subjected to 600 to 775 for 5 hours, and then air-cooled heat treatment. The temperature at which it becomes the softest was the annealing temperature.

Figure 1 shows an example of the relationship between heat treatment temperature and hardness. As hot-rolled, it has high hardness but is softened by heat treatment. In this example, it becomes softest at 675 to 700. If heat treatment is performed at a higher temperature, the austenite phase precipitates and transforms into martensite during cooling, so it hardens. The Vickers hardness (Hv) of the L section was measured and evaluated at the center of the plate thickness with a load of 1 kg.

Finally, descaling by shot and pickling was performed to produce hot-rolled annealed sheets.

Table 2 shows the evaluation results of various characteristics of the inventive examples and comparative examples. Case Nos. 1 to 20 are examples of the present invention, and Case Nos. 2 to 27 are comparative examples.

The steel of the present invention not only has excellent weld corrosion resistance without occurrence of intergranular corrosion of multiple welds and preferential corrosion near the weld fusion line, but also has excellent impact characteristics of the welds. . Furthermore, the material of strength and ductility is also good, and it is possible to drastically improve sulfuric acid resistance by selectively adding elements. In addition, by designing the components of the steel material and devising the manufacturing conditions, it is possible to obtain a steel material with excellent manufacturability that is free from edge cracks and surface defects in the hot rolled sheet.

Comparative Example No. 21 is inferior in corrosion resistance of the heat affected zone because the soot content and T i / (C + N) are out of the scope of the present invention. In Comparative Example No. 22, since T i · N was outside the scope of the present invention, surface flaws occurred during hot rolling. In Comparative Example No. 23, T i deviated from the upper limit of the range of the present invention, so T i · N deviated from the range of the present invention, and surface flaws occurred due to hot rolling. Comparative example In case No. 24, the impact characteristics of the weld heat-affected zone are inferior because a is outside the scope of the present invention. In Comparative Example No. 25, Cr deviated from the upper limit of the scope of the present invention, so that p deviated from the scope of the present invention, and the impact characteristics of the weld heat affected zone were inferior. In addition, Ac, cannot be defined because there is no single-phase temperature range in this application. Comparative Example No. 26 is inferior in sulfuric acid resistance and corrosion resistance of the heat affected zone because Cr is outside the lower limit of the range of the present invention. In Comparative Example No. 27, edge cracking occurred because the amount of δ at the hot rolling heating temperature was out of the range of the present invention.

The following describes the evaluation test methods for various characteristics.

The components were analyzed by sampling test pieces from steel plates. Gas analysis method for C, S and N (N is an inert gas melting and thermal conductivity measurement method, C and S are combustion and infrared absorption method in oxygen stream), and X-ray fluorescence analyzer for other elements ( SH I MADZU, MXF-2 1 00). Judgment of the occurrence of cracks in the hot-rolled sheet was made based on the appearance observation. ◯ indicates no crack, △ indicates that there is no crack from the front surface to the back surface with a crack, and X indicates that there is a crack and the crack penetrates from the front surface to the back surface. In addition, when the hot rolling heating temperature is lower or higher than Ac ₄ (the upper limit temperature of the austenite single phase) calculated from each component value using the scientific calculation software: Thermocalc. Ear cracking did not occur only when the amount of Dell evening ferrite was above 50%.

Judgment on the occurrence of wrinkles, which is one of the surface defects of hot-rolled sheets, was made by visual observation of the presence or absence of wrinkles on the hot-rolled sheet surface. “No” indicates surface defects and “X” indicates presence.

0.2% resistance and elongation were determined from hot-rolled annealed plates; HSZ 2201 No. 13B test pieces were prepared and tested using an Instron type tensile tester according to the JISZ2241 test method. Measure the data in the L direction (parallel to the rolling direction) at n = 2. Set. ○ x in the table indicates 0.2% proof stress of 320MPa or more with ○, and less than 320MPa with X. Elongation of 20% or more is indicated by ○, and less than 20% is indicated by X. '

The sulfuric acid immersion test method is shown below. A 2 mm x 25 mm x 25 mm corrosion test piece was prepared from the hot rolled annealed pickling plate. The corrosive solution was 0.1, 0.01, 0.001N-sulfuric acid solution (pH = 1, 2, 3). The liquid volume was 50 OmL per test piece. The test temperature was 30.

As a typical example, when pH = 2, the corrosion rate is 3 g Zm ² / h or less, ◯, especially 2 g Zm ² Zh or less is indicated by ◎, and when it is more than 3 g Zm ² Zh Is indicated by X. Fig. 2 shows the effect of Cu and Cr on the corrosion rate when 0.25% by mass of steel is used and the pH is 2, as a result of the sulfuric acid immersion test. When Cu is added, the corrosion rate decreases. The addition of 0.3 to 0.5% by mass results in the lowest corrosion rate. Increasing the amount of Cu added further saturates the Cu effect. Even if Cr is increased, the corrosion rate can be reduced.

TIG welding was performed with tanning, welding speed ZOOcmZmiiK welding current 110 A, and the sealing gas was argon. MIG welding was performed by the following method.

Welding material is 309 LSi (C: 0.017%, Si: 0 · 74%, Mn: 1.55%, P: 0.024%, S: 0.001%, Ni: 13.68%, Cr: 23.22%), voltage 25 ~ 30V, current: 230 to 250A, shielding gas: 98% Ar + 2% ○ ₂ The welding machine used Daihen turbo-pulse. The test was conducted under sufficient conditions to penetrate the 4 mm plate thickness and reverse the wave. In the case of a butt weld joint, the root face is 2 mm (gap 0) at a 90 ° V groove, the heat input Q is about 12500 J Zcm, and in the case of cross welding, the seam weld is left about 1 mm thick. After deletion, welding was performed, and the Q was about 5600 J cm.

The intergranular corrosion test is basically copper sulfate monosulfate standardized by IIS. The test (G0575) (Strauss test) is generally used, and this test is appropriate for high chromium-containing stainless steels such as SUS304. However, stainless steel with a low chromium content in the steel (.about 12% low chromium stainless steel) is too corrosive, so tests were conducted using an evaluation method suitable for low chromium stainless steel. . In other words, a 24-hour immersion test in a solution (boiling) with a sulfuric acid concentration reduced to 0.5%

(Improved Strauss test) was conducted. Except for reducing the sulfuric acid concentration, tests were conducted in accordance with JIS, and the presence or absence of intergranular corrosion was determined by observing the metal structure of the cross section. The weld heat-affected zone was observed, and the case where no intergranular corrosion occurred was marked with ◯, and the case where it occurred was marked with X. In addition, ◎ indicates that no preferential corrosion has occurred in the heat-affected zone closest to the weld, and ◯ indicates that some of the multiple observation sites have been observed. Preferential corrosion has occurred in all of the multiple observation sites. In the case of, it is indicated by X. Figure 3 shows the cross-sectional metallographic structure of the weld heat-affected zone after the modified Strauss test. A) to d) are a) Comparative Steel No. 21 ( B) Cross-sectional structure of the TIG welding heat-affected zone of the invention steel No. 1; c) Cross-sectional structure of the MIG welding heat-affected zone of the invented steel No. 1 D) The cross-sectional structure of the heat-affected zone of MIG welded steel of invention steel No. 11 is shown.

In the weld zone, there are three different types of heat affected zone in addition to the raised weld metal zone. The heat-affected zone adjacent to the weld metal is 1, the heat-affected zone next to it—2 and the heat-affected zone—3. 1 and 2 are martensite and the base metal and metal structure are different. The heat affected zone 13 is affected by the heat of welding, but no martensite is formed. In Fig. 3 a), corrosion mainly consisting of intergranular corrosion has occurred in all the heat-affected zones 1 to 3 of several hundred meters from the surface. The corroded area is black near the surface Contrast part. The white deposit on top of it is a deposit of copper, corresponding to the occurrence of corrosion. The figure on the right is an enlarged view of the surface layer of the figure on the left. In Fig. 3 b), no corrosion has occurred.

In this example, since TIG welding is performed by tanning welding, unlike MIG welding, there is no weld metal part. Therefore, since the heat affected zone does not have an interface with the weld metal, preferential corrosion is unlikely to occur. In Fig. 3c), no corrosion of the heat affected zone 2 and 3 occurred, but the formation of wedge-shaped corrosion along the fusion line is observed in the heat affected zone 1 adjacent to the weld metal. In Fig. 3d), no corrosion occurred in the heat affected zone.

Impact characteristics were measured by Charpy test. A JIS4 2 mmV notch subsize (thickness 4 mm) test piece conforming to the HS standard was taken from the MIG weld and an impact test was performed at 20. The V-notch was inserted in the BOND part where the weld metal and base metal part were 1/2 each. When the impact value is 30 J Zcm ² or more, it is indicated as ◯. When the impact value is less than 30 J Zcni ^2, X is indicated.

table 1

r P (%) = 420 [C] +470 [N] +23 [Ni] +9 [Cu] +7 [Mn] -ll.5 [Cr] -11.5 [Si] -12 [Mo] -23 [ V] -47 [Nb] -49 [Ti] -52 [Al] +189

—: Indicates that it is outside the scope of the present invention.

Table 2

(Example 2)

Tables 3 and 4 show invention examples and comparative examples related to the second problem. Table 3 shows the mass% of components in the steel of the present invention steel (steel materials No. 27 to 35). The pieces of the ingredients shown in Table 3 were melted into 40 kg or 35 kg flat ingots by vacuum melting. After cleaning the surface of these steels, the ingot was heated at 1150 for 1 hour, followed by hot rough rolling consisting of multiple passes followed by hot finish rolling. The hot rolling end temperature was 800 to 900. The hot-rolled sheet was air-cooled, held at a coiling temperature of 500 for 1 hour, and then air-cooled and subjected to a simulated coiling heat treatment to obtain a hot-rolled sheet with a thickness of 4 mm. Subsequently, in order to determine the annealing temperature of the hot-rolled sheet, the hot-rolled sheet of each component value is held at 575: -850 at 5-50 hours, and then the batch heat treatment cooling process is simulated at 20 Controlled cooling with Zh and at 100 removed from the furnace.

Figure 4 shows an example of the relationship between heat treatment conditions, strength, and ductility. When steel No. 33 was held for 5 hours, it was softened by heat treatment with high strength and low elongation as it was hot rolled. In this example, there is a condition that the elongation is 15% or more with a proof stress of 450 MPa or more in a wide temperature range of 675 or more and 800 or less.

Table 4 shows the heat treatment conditions and various evaluation results of the inventive examples and comparative examples. Case Nos. 28 to 41 are examples of the present invention, and Case Nos. 42 to 50 are comparative examples. In the case of a metal structure corresponding to claim 4 of the present invention, a high-strength, low-chromium-containing stainless steel having an elongation of 15% or more or 20% or more and a high strength and ductility balance of 450 MPa or more is obtained. It is possible.

For example, the B value of case No. 36 is 0.36, 0.2% resistance to 538 MPa, elongation 2 1.5 %.

These steels also have excellent intergranular corrosion resistance in the heat-affected zone of multi-pass welding and preferential corrosion resistance in the vicinity of the weld fusion line.

Comparative Examples No.42 to 46, 49 and 50 are not suitable because the heat treatment conditions are low temperature or short time, and because the metal structure is outside the scope of the present invention, the elongation is less than 15% and the strength-ductility balance is inferior. . In Comparative Examples No. 47 and 48, the heat treatment takes a long time and is not suitable, so the metal structure deviated from the scope of the present invention. Therefore, the yield strength was less than 450 MPa, and the strength ductility balance was inferior.

The evaluation test was carried out by the following method. Except for the following, the determination of the metal structure in accordance with Example 1 was evaluated by the half-width broadening B of the Cu—Kal {110} diffraction line in X-ray diffraction. A B value of 0.1 to 1.0 is indicated by ◯, and a value of less than 0.1 and exceeding 1.0 is indicated by X.

The measurement methods of 0.2% resistance and elongation were the same as in Example 1, but the evaluation was as follows. 0.2% proof stress is 450MPa or more is indicated by ○, and less than 450MPa is indicated by X. In particular, the case where the 0.2% proof stress is 500 MPa or more is indicated by ◎. Elongation of 15% or more is indicated by ○, and less than 15% is indicated by X. In particular, the case where the elongation is 20% or more is indicated by ◎.

Table 3

r P (%) = 420 [C] +470 [N] +23 [Ni] +9 [Cu] +7 [Mn] -11.5 [Cr] -11.5 [Si] -12 [Mo] -23 [V] -47 [Nb] -49 [Ti] -52 [Al] +189

Table 4

Industrial applicability

The present invention does not contain an unnecessarily expensive element, can be used as a structural steel even in severe corrosion environments, does not cause preferential corrosion in the vicinity of the welded fusion line, and is resistant to grains in multipass welds. It is an industrially extremely valuable invention that can provide a low chromium-containing stainless steel having excellent corrosion resistance and can be provided as a high-strength material if necessary.

Claims

The scope of the claims

1. In mass%,

C: 0.03% or less,

N: 0.004 to 0.02%

Si: 0.2-1%,

Mn: more than 1.5 to 2.5%,

P: 0.04% or less,

S: 0.03% or less,

Cr: 10 to 15%,

Ni: 0.2-3.0%,

A1: 0.005 to 0.1% is contained, and

Ti: 4 X (C% + N%) or more and 0.35% or less, with the balance being Fe and inevitable impurities, and

The intergranular corrosion resistance of the heat-affected zone of multipass welding and the preferential corrosion resistance in the vicinity of the welded fusion line, characterized in that the content of each element satisfies the following formulas (A) and (B) Low chromium content stainless steel with excellent properties.

r P (%) = 420 x C% + 470 XN% + 23XNi% + 9 XCu%

+ 7 XMn%-11.5xCr%-11.5xSi%-12xMo%-23xV% -47XNb% -49XTi%-52XA1% + 189≥80 (A) Ti% XN% <0.004 )

2. In mass%,

Mo: 0.05-3%,

Cu: Intergranular corrosion resistance of the multipass weld heat-affected zone described in Claim 1 characterized by containing 0.05 to 3% of 1 type or 2 type, and priority for resistance in the vicinity of the welded fusion line Low chromium content with excellent corrosivity Stainless steel.

3. In mass%,

Nb: 0.01-0.5%

V: 0.01-0.5% of 1 type or 2 types, characterized by intergranular corrosion resistance of multipass weld heat affected zone and weld zone fusion line according to claim 1 or claim 2 Low chromium-containing stainless steel with excellent preferential corrosion resistance nearby.

4. Stainless steel comprising the component according to any one of claims 1 to 3, wherein the metal structure is a two-phase structure of a ferrite phase and a martensite phase, and in X-ray diffraction半 α {110} diffraction line half-width broadening で defined by the following formula (C) Β is 0 to 1.0 and has excellent strength-ductility balance and grain resistance in the heat-affected zone of multi-pass welding A low chromium-containing stainless steel with excellent interfacial corrosion resistance and preferential corrosion resistance in the vicinity of the weld fusion line.

Β = (W- Wo) / Wo (C)

Wo: Half width without internal distortion (deg)

W: Half width (deg)

5. The heating temperature in the hot rolling process of the flakes comprising the components according to any one of claims 1 to 3 is the upper limit temperature of the austenite single phase determined from the flake components Ac ₄ The intergranular corrosion resistance of the heat-affected zone in multi-pass welding is characterized by a temperature at which the amount of Dell ferrite in the austenite phase exceeds 50% when heating with Ac ₄ or more. Of low-chromium stainless steel with excellent corrosion resistance and preferential corrosion resistance in the vicinity of the weld line.