JPH11302795A - Stainless steel for building construction - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive stainless steel for general building construction, excellent in corrosion resistance in a residential environment, weldability, and characteristic in a weld zone. SOLUTION: The steel has a composition consisting of, by weight, 0.005-0.1% C, 0.05-1.5% Si, 0.05-1.5% Mn, <=0.04% P, <=0.05% S, <=0.05% N, 8-16% Cr, and the balance Fe with inevitable impurities and satisfying (C+N)<=0.1%. By forming the metallic structure of a base material part into ferritic phases having 5 to 50 μm average crystalline grain size, 235 to 440 MPa 0.1% proof stress can be provided. Further, by satisfying inequality Cr(%)+Mo(%)+1.5Si(%)-Mn(%)-2Ni(%)-0.5Cu(%)-30C(%)-20N(%)<=12, martensitic phases of 250% by volume ratio are precipitated in a weld heat-affected zone and the Charpy impact value at 0 deg.C in the weld heat-affected zone is regulated to >=2 kgm/cm<2> .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent corrosion resistance used as a structural member of a detached house, an apartment house, a large building, a building such as a building or a bridge.

[0002]

2. Description of the Related Art Steel materials for pillars and beams are required to have higher functions due to stricter safety standards and pursuit of functionality of buildings. In particular, corrosion resistance is an important factor affecting the service life of a structure, and its improvement is required. The ultimate example is rust-free stainless steel for building structures. SUS304 (18Cr-8Ni), which is excellent in corrosion resistance and toughness, is often used for structural purposes.

[0003] However, stainless steel requires a large amount of expensive elements such as Cr and Ni, so that the material cost and the manufacturing cost are high. Although it is excellent in function, there is a problem in economical efficiency. Thus, although rust and corrosion are inevitable, steel materials that are inexpensive, suppress the progress of corrosion, and minimize the amount of corrosion have been developed. For example, JP
JP-A-162507 and the like disclose a method for producing a black scale coating film having excellent adhesion and corrosion resistance, but the production process is complicated and there is a problem in economy.

Japanese Unexamined Patent Publication (Kokai) No. 8-199289 discloses an H-shaped steel having an oxide scale of 10 μm or less, which is produced by hot rolling a steel containing 0.50 to 1.50% of Cr. It has been disclosed. However, when corrosion progresses through the oxide layer, the effect of improving corrosion resistance is lost, and it is considered impossible to improve the long-term durability of the building.

On the other hand, a ferritic stainless steel to which Cr is added in excess of 16%, for example, SUS430 steel is excellent in corrosion resistance, but the metal structure of a hot-rolled steel sheet is a coarse ferrite grain structure elongated long in the rolling direction. In addition, the bending workability is poor, and the ferrite structure of the heat affected zone is coarsened, and the toughness of the weld is significantly reduced. For thick materials used for structures, etc., the toughness of the welded parts is a serious problem, and cracks may occur during cooling after welding, so ferritic stainless steels are required for general building structures that require welding. Was not used.

[0006]

When considering structural materials such as pillars and beams of a building, the corrosive environment is roughly divided into a period until the exterior material is completed and a period thereafter. The latter is not so severe as a corrosive environment because the free inflow of outside air is regulated by exterior and interior materials. Rather, the corrosiveness of the environment is more severe in the former period, although the time is short, but the period is directly exposed to the elements such as wind and rain. In addition, if rust is generated on the surface of the steel material by the time the construction of the exterior material is completed, there is a problem that the corrosion easily progresses under the rust layer even after the exterior material is formed. That is, it is the corrosion resistance against rust generation that substantially governs the durability of the structural material, and it is sufficient to include the necessary amount of Cr in the steel material.

Further, by controlling the amount of Cr to a necessary minimum and adjusting other components, a sufficient amount of austenite phase is generated at a high temperature to prevent the ferrite microstructure of the welded portion from being coarsened. It is also considered possible to use the phase transformation of the ferrite phase to refine the ferrite structure while hot rolling. In other words, by effectively utilizing the balance of components and phase transformation, making the ferrite microstructure of a ferritic hot-rolled steel sheet moderately fine, and preventing the ferrite microstructure in the weld heat affected zone from becoming coarse, the general building structure It can have mechanical properties that can be used for applications.

[0008]

In order to solve the above problems, the present inventors have produced various component steels. By investigating the transformation behavior, they produced ferritic stainless steel that can be used for general building structures and has excellent corrosion resistance in residential environments. That is, the gist of the present invention is as follows. C
Adjust the amount of r and the balance of other components, 1000 ℃
An austenite phase is formed in a high temperature range around -1200 ° C. The amount can be predicted from the component content. If the component is adjusted so as to satisfy the following formula, a sufficient amount of austenite phase is generated at a high temperature, and the minimum required 50% martensite in the weld heat affected zone is obtained. It is possible to leave a phase. Cr (%) + Mo (%) + 1.5Si (%)-Mn (%)-2Ni (%)-0.5Cu (%)
−30C (%) − 20N (%) ≦ 12

[0009] Hot rolling is mainly performed in the austenite region, and is transformed into a ferrite phase upon cooling after hot rolling,
A hot-rolled steel sheet having an appropriate ferrite grain size and 0.1% proof stress can be manufactured. At this time, if the transformation into the ferrite phase is insufficient and a part of the austenite phase is transformed into the martensite phase, or if the grain growth after the ferrite transformation is insufficient and the average grain size becomes 5 μm or less, the 0.1% proof stress is reduced. 4
If it exceeds 40 MPa, the bending workability and ductility of the steel sheet decrease.

In addition, if the above formula is not satisfied and a sufficient amount of austenite phase is not formed at a high temperature, or if ferrite grains are grown to an average of 50 μm or more during cooling or heat treatment after hot rolling, external ferrite grains may not be formed during bending. Rough surface irregularities corresponding to ferrite grains are generated on the surface, which not only impairs aesthetic appearance but also may cause cracks due to local deformation. ☆
Further, the 0.1% proof stress is 23 due to the coarse ferrite grains.
If it is less than 5MPa, it will have lower yield strength than general structural steel,
It requires special design and is unsuitable for general structural steel.

By optimizing the components and the metal structure of the hot-rolled steel sheet as described above, a ferritic stainless steel for general structure can be realized. That is, the configuration of the present invention is as follows. (1) By weight%, C: 0.005% to 0.1%, Si: 0.05% to 1.5%, Mn: 0.05% to 1.5%, P: 0.04% or less , S: 0.05% or less, N: 0.05% or less, (C + N): 0.1% or less, Cr: 8 to 16%, the balance being Fe and inevitable impurities, The metal structure of the base metal part is a ferrite phase having an average crystal grain size of 5 to 50 μm, and the 0.1% proof stress is 235.
Not less than MPa and not more than 440 MPa, and by satisfying the following formula, a martensite phase having a volume ratio of not less than 50% is precipitated in the weld heat affected zone, and the Charpy impact value at 0 ° C of the weld heat affected zone is 2 kgm / A stainless steel for building structures that is excellent in corrosion resistance, weldability and weld characteristics in a residential environment, characterized in that the thickness is at least ² cm2. Cr (%) + Mo (%) + 1.5Si (%)-Mn (%)-2Ni (%)-0.5Cu (%)-30C (%)-
20N (%) ≦ 12 (2) Mo: 0.1 to 2.5%, Cu: 0.1 to 2.5%, Ni: 0.1 to 2.5% The stainless steel for building structures according to the above (1), further comprising: (3) A hot-rolled stainless steel strip for a building structure, wherein the steel according to (1) or (2) is manufactured by hot rolling or heat-treating and then pickling after hot rolling.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the composition range of the steel of the present invention will be described below. C is an element effective for improving the strength of steel. However, 0.005
If it is less than 10%, the strength required for structural steel cannot be obtained. Further, an excessive addition exceeding 0.1% hardens the martensite phase, significantly lowers the toughness of the weld heat affected zone, and may cause cracking during welding. For this reason,
The lower limit was 0.005% and the upper limit was 0.1%.

[0013] N is an unavoidable impurity element and is effective in improving the strength of steel. However, an excessive addition exceeding 0.05% hardens the martensite phase and causes the toughness of the weld heat affected zone. , And cracking may occur during welding. The upper limit was set to 0.05%.

Both C and N have the effect of hardening the martensite phase appearing in the heat affected zone, and if the hardness is excessively hardened, the martensite phase becomes the starting point of cracking.
Conversely, the toughness of the heat affected zone is reduced, and the effect of the ferrite phase coarsening by the martensite phase is reduced. Therefore,
The upper limit of (C + N) was set to 0.1% or less.

Si must be added to the molten steel as a deoxidizing agent in order to reduce dissolved oxygen in the steel and ensure hot workability. If it is less than 0.05%, the deoxidizing effect is weak. On the other hand, 1.5
%, The lower limit is set to 0.05% and the upper limit is set to 1.5%.

Mn needs to be added to molten steel as a deoxidizing agent and a desulfurizing agent. If it is less than 0.05%, the desired effect cannot be obtained. On the other hand, if added in excess of 1.5%, the toughness and cracking properties of the base metal and the weld are impaired.
% And the upper limit was 1.5%.

When P is present in a large amount, it not only impairs the weldability but also promotes the generation of rust. Therefore, it was limited to 0.04% or less.

[0018] S mainly as a sulfur-based inclusion such as MnS not only serves as a starting point of rust, but also causes an increase in corrosion rate. Further, they segregate at grain boundaries and impair hot workability. Therefore, it is necessary to regulate to 0.05% or less.
S is preferably as small as an impurity.

Cr has the effect of suppressing the occurrence of corrosion and reducing the corrosion rate in an atmospheric environment. In addition, even when corrosion occurs once and a rust layer is formed, there is an effect of reducing the rate of overall corrosion of the steel material under the rust layer. But C
If the amount of r is small, that is, if it is less than 8%, the effect of suppressing the generation of rust and reducing the corrosion rate sharply decreases. On the other hand, if an amount exceeding 16% is added, it becomes virtually impossible to generate a martensite phase in the weld heat affected zone of 50% or more within the above component range.

Mo, Cu, and Ni, like Cr, have the effect of suppressing the occurrence of corrosion and reducing the corrosion rate in an atmospheric environment. However, if the amount is small, the effect is weak,
Excessive addition increases raw material costs, production costs, etc., and reduces economic efficiency. Therefore, the lower limit is 0.1% and the upper limit is 2.5%
And

For use in general building structures,
The metal structure of the base metal part is substantially a ferrite phase,
m, the average grain size is adjusted to 235MPa
Must be 440 MPa or less. If the crystal grain size is less than 5 μm, the strength increases but the elongation required for bending decreases. On the other hand, if the crystal grain size exceeds 50 μm, the strength is reduced and surface irregularities are generated during bending, which not only impairs aesthetic appearance but also decreases toughness and may cause cracking during bending. 235MP with 0.1% proof stress
If it is less than a, it cannot be designed with the same specifications as general structural carbon steel, and cannot be used as a general-purpose building structural material. On the other hand, with a 0.1% proof stress exceeding 440 MPa, bending becomes difficult, and workability or workability deteriorates due to springback or the like.

The metal structure and characteristics of the base material are produced,
In order to satisfy the weldability and weld characteristics required for general structures, the balance of components must be adjusted and the austenite phase formed at high temperatures must be controlled. That is, by satisfying the following expression, a martensite phase having a volume ratio of 50% or more is precipitated in the weld heat affected zone, and the Charpy impact value at 0 ° C. of the weld heat affected zone is set to ² kgm / cm ² or more. Cr (%) + Mo (%) + 1.5Si (%)-Mn (%)-2Ni (%)-0.5Cu (%)
-30C (%)-20N (%) ≦ 12

If the left side of the above equation exceeds 12, it is difficult to precipitate a martensite phase (austenite phase at a high temperature) of 50% or more by volume in the heat-affected zone of the weld, making the ferrite structure of the hot-rolled steel sheet finer. It is not possible to prevent the ferrite grains in the heat affected zone from becoming coarse. In order to secure the weld toughness for general building structures and prevent cracking during welding, the component balance is adjusted within the above formula range while satisfying the above component range, and 50 % Or more of the relatively soft martensite phase, so that the Charpy impact value at 0 ° C. must be ² kgm / cm ² or more. If the Charpy impact value is less than ² kgm / cm ² , low-temperature cracking may occur after welding, and there is a risk that the structural material may be brittlely broken under load. The scope of the present invention is a range necessary for not only expressing the characteristics of the base material necessary for general building structures but also providing the weldability and the welded portion characteristics.

In addition, since the steel of the present invention is used for general building structures, from the viewpoint of manufacturing cost and productivity,
It is desirable to produce the steel to a predetermined thickness by a hot rolling process. However, the effects of the present invention can be maintained even if a heat treatment step or a pickling step for ensuring the stability of the surface design or mechanical properties is provided.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on embodiments. Melting steels of various compositions shown in Table 1
Thick ingots were cast. After heating this to 1200 ° C., a hot-rolled sheet having a thickness of 6 mm was produced by hot rolling. About a part, annealing after hot rolling was implemented. The metal structure of the hot rolled sheet was observed, and the average crystal grain size of the ferrite phase was measured. Further, JIS No. 5 tensile test pieces were prepared in parallel with the rolling direction of the hot-rolled sheet, and 0.1% proof stress, tensile strength, and elongation at break were measured. In addition, a test piece having a width of 500 mm was cut out, bent at room temperature with a bending radius of 12 mm in parallel with the rolling direction, and the occurrence of cracks and the surface of the bent portion were observed. The conditions of hot rolling and annealing after hot rolling, observation results of the metal structure, results of the tensile test,
Table 2 shows the bending test results.

Next, a test piece for welding test was cut out from the hot-rolled sheet so as to be welded in parallel with the rolling direction. Two test pieces cut out from the same grooved material were welded by TIG. SUS410 or SUS30 for welding
A test piece fixed to a welding table was welded without preheating using an 8 series wire. A test piece for observing the metal structure was prepared at right angles to the welding direction, and the area ratio of the martensite phase generated in the heat affected zone was measured.

A 5 mm thick Charpy test piece (sub-size of JIS No. 4 test piece) was cut out from the vicinity of the portion where the metallographic structure was observed at a right angle to the welding direction, and the notch was located at the boundary between the weld metal and the base metal. It was processed so as to be a welding heat affected zone from a 0.5 mm base material. A Charpy test was performed at 0 ° C. using the test piece, and a Charpy impact value was measured from the absorbed energy.

Table 3 shows the measurement results of the area ratio of the wire and martensite phase used for welding and the Charpy impact value. Further, after a part of the welded test piece was exposed to the outdoors outdoors for 3 weeks, the state of rust was observed, but no clear rust was formed on the steel of the present invention.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

From the above results, the steel of the present invention has sufficient mechanical properties in the base metal part for building structures, excellent bending workability, and also excellent toughness of the weld heat affected zone. It was confirmed that. Although it is a short time, there is no rust during outdoor exposure, so it is predicted that rust will not occur for a long time in a relatively rust-resistant environment in a house. It can be said that.

[0033]

According to the present invention, it is possible to supply inexpensively general steel materials having excellent corrosion resistance, which are used as structural members of detached houses, apartment houses, large buildings, buildings and bridges.

Claims (3)

[Claims] C: 0.005% to 0.1%, Si: 0.05% to 1.5%, Mn: 0.05% to 1.5%, P: 0.04% by weight. %, S: 0.05% or less, N: 0.05% or less, (C + N): 0.1% or less, Cr: 8 to 16%, the balance being Fe and unavoidable impurities. Is a ferrite phase having a metal structure of a base material having an average crystal grain size of 5 to 50 μm, and a 0.1% proof stress of 235
Not less than MPa and not more than 440 MPa, and by satisfying the following formula, a martensite phase having a volume ratio of 50% or more is precipitated in the heat affected zone, and the Charpy impact value at 0 ° C. of the heat affected zone is 2 kgm / A stainless steel for building structures that is excellent in corrosion resistance, weldability and weld characteristics in a residential environment, characterized in that the thickness is at least ² cm2. Cr (%) + Mo (%) + 1.5Si (%)-Mn (%)-2Ni (%)-0.5Cu (%)-30C (%)-
20N (%) ≦ 12 2. The composition further contains at least one of Mo: 0.1 to 2.5%, Cu: 0.1 to 2.5%, and Ni: 0.1 to 2.5% by weight. The stainless steel for a building structure according to claim 1, wherein: 3. A hot-rolled stainless steel strip for a building structure, wherein the steel according to claim 1 or 2 is manufactured by hot rolling or heat-treating after hot rolling and pickling.