JP2001032050A - Ferritic stainless steel excellent in shape fixability at the time of bending and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide ferritic stainless steel small in springback at the time of bending and excellent in shape fixability. SOLUTION: This steel has a compsn. contg., by weight, <=0.1% C, <=1.5% Si, <=1.5% Mn, <=0.04% P, <=0.05% S, <=0.05% N, <=0.1% (C+N) and 8 to 18% Cr, and the balance Fe with inevitable impurities, in which the metallic structure of the base metal part is composed of a ferritic phase having 5 to 50 μm average crystal grain size, and the reflected X-rays intensity ratio in the 100} plane parallel to the sheet face in the texture of the center part of the sheet thickness is >=2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a bending property suitable for a structural member of a detached house, an apartment house, a large building, a building such as a building or a bridge, or a vehicle structure, and has a shape freezing during bending. TECHNICAL FIELD The present invention relates to a ferritic stainless steel for welded structures having excellent properties, that is, having little springback, and a method for producing the same.

[0002]

2. Description of the Related Art Due to stricter safety standards and the pursuit of functionality for buildings, steel materials for columns and beams are required to have higher functions. In particular, corrosion resistance is an important factor that affects 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 production cost are high. Although it is excellent in function, it has a problem in economical efficiency. On the other hand, ferritic stainless steel to which Cr is added in excess of 10%, for example, SUS430 steel has excellent corrosion resistance, but the metal structure of the hot-rolled steel sheet is a coarse ferrite grain structure elongated long in the rolling direction. The workability is poor, and the ferrite structure in the heat affected zone of the weld 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.

[0004] SUS410L steel of 11% Cr is
Since the γ phase precipitates at high temperatures, the microstructure of the heat affected zone of the weld is unlikely to become coarse, and the C content is reduced to improve weld cracking and weld toughness. Department has been used. However, since the hot-rolled steel sheet has a high yield strength, it has a disadvantage that springback at the time of bending is large, that is, the shape freezing property at the time of bending is poor.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a hot-rolled steel sheet which has little springback during bending and has excellent shape freezing properties, and a method for producing the same, for use in building structures or vehicle structures. And

[0006]

Means for Solving the Problems The present inventors have achieved excellent bending workability by controlling the phase transformation in the hot rolling zone by adjusting the component balance and optimizing the hot rolling conditions and the subsequent heat treatment conditions. The aim was to realize a metal structure and a texture with little springback. In view of the above problem, the present inventors have studied a texture (ratio to random crystal orientation, which is excellent in bending workability and low springback). As a result of examining the bending characteristics of stainless steel sheets having various textures, when the {100} planes are accumulated parallel to the steel sheet surface, the springback is reduced, and when the <011> direction is accumulated parallel to the bending direction. It became clear that the effect became remarkable.

Since bending of a steel sheet is generally performed in a rolling direction or a direction perpendicular to the rolling direction, {100} planes are accumulated in parallel with the sheet surface, and the <011> direction is parallel with the rolling direction or the sheet width direction. Can be expected to be able to produce a steel plate with small springback when bent in the rolling direction or the plate width direction.

[0008] In general, such a texture is recognized in the expanded texture after hot rolling or cold rolling,
The shape freezing property was not sufficiently improved because more {111} planes for increasing the springback were accumulated. In addition, a texture for accumulating {100} planes by recrystallization by annealing after rolling was not obtained, and a stainless steel sheet having the above texture has not been manufactured.

The present inventors hot rolled stainless steels of various components under various conditions and investigated the metallographic structure, texture and mechanical properties. As a result, the strain introduced during hot rolling and the subsequent γ → It has been found that the above-described texture can be obtained under limited conditions for appropriately utilizing the α-phase transformation, and that excellent shape freezing properties can be obtained. That is, rolling in the γ phase region, performing a final hot rolling at a temperature that does not cause recrystallization immediately after rolling, and transforming from the γ phase to the α phase after hot rolling while leaving the strain introduced by the hot rolling, A transformation texture as described above is formed. Further, by adjusting the winding temperature after the finish rolling, the transformation can be completed and the crystal grain size can be adjusted, so that the strength required as a structural material and the ductility required during bending can be imparted.

The gist of the present invention is as follows. Cr
The amount and the amount of other components are adjusted so that the γ phase is formed in the hot-rolling temperature range of 850 ° C. or higher. The amount of the austenite phase generated in the high temperature range can be predicted from the component content, and a sufficient amount of the austenite phase can be generated in the hot rolling temperature range by adjusting the components so as to satisfy the following formula. Cr (%) + Mo (%) + 1.5Si (%) + 6Ti
(%) + 3Nb (%)-Mn (%)-2Ni (%)-
0.5Cu (%)-30C (%)-20N (%) ≦ 1
1.5

[0011] The hot rolling is mainly performed in the austenite region, transforms into a ferrite phase upon cooling after hot rolling, forms the above-described texture, and has an appropriate ferrite grain size and yield strength. Hot rolled steel sheet can be manufactured. At this time, if the transformation to the ferrite phase is insufficient and a part of the austenite phase is transformed to the martensite phase, or if the grain growth after the ferrite transformation is insufficient and the average grain size is less than 5 μm, the yield strength exceeds 500 MPa. In addition, the shape freezing property is deteriorated, and cracks may occur during bending.

If the above formula is not satisfied and a sufficient amount of austenite phase is not formed at a high temperature, the formation of the above-mentioned texture is inhibited and ferrite crystal grains are coarsened. When the average ferrite crystal grain exceeds 50 μm, rough surface irregularities corresponding to the ferrite grain occur on the outer surface during bending,
In addition to spoiling the appearance, cracks may occur due to local deformation. Further, when the yield strength is less than 235 MPa due to coarsening of ferrite grains, the yield strength becomes lower than that of a general structural steel material, and a special design is required, which is unsuitable as a general structural steel material. By optimizing the components and the metal structure of the hot-rolled steel sheet as described above, a ferritic stainless steel for general structure has become feasible.

The specific constitution of the present invention is as follows. (1) By weight%, C: 0.1% or less, Si: 1.5% or less, Mn: 1.5% or less, Cr: 8 to 18%, P: 0.04% or less, S: 0. Steel containing at most 0.05%, N: at most 0.05%, C + N: at most 0.1%, the balance being Fe and unavoidable impurities, and the base material having a metal structure of 5 to 50 μm in average grain size In the ferrite structure having a diameter, the reflected X-ray intensity ratio of the {100} plane parallel to the plate surface in the texture at the center of the plate thickness is 2
A ferritic stainless steel having excellent shape freezing during bending, characterized by the above. (2) By weight%, Mo: 0.1 to 2.5%, Cu: 0.1 to 2.5%, Ni: 0.1 to 2.5%, Ti: 0.01 to 0.5% , Nb: 0.01 to 0.5%, and V: 0.05 to 0.5%. The method according to (1), further comprising one or more of the following: Ferritic stainless steel with excellent shape freezing properties.

(3) In the texture at the center of the sheet thickness, the {100} plane parallel to the sheet surface has a reflected X-ray intensity ratio of 2 or more, and {011} perpendicular to the rolling direction or the sheet width direction.
The ferrite stainless steel according to the above (1) or (2), which has excellent shape freezing property at the time of bending, wherein the reflection X-ray intensity ratio of the surface is 1.2 or more. (4) Satisfies the following expression and yield strength or 0.2% proof stress is 2
The ferritic stainless steel according to any one of the above (1) to (3), which is excellent in shape freezing property during bending, which is 35 to 500 MPa. Cr (%) + Mo (%) + 1.5Si (%) + 6Ti
(%) + 3Nb (%)-Mn (%)-2Ni (%)-
0.5Cu (%)-30C (%)-20N (%) ≦ 1
1.5

(5) The steel containing the component described in (1) or (2) above is subjected to hot rolling or heat treatment after hot rolling, and the steel described in any one of (1) to (4) above is used. A method for producing a ferritic stainless steel having excellent shape freezing properties during bending, characterized by obtaining characteristics. (6) Finish rolling is performed so that the total reduction amount is 30% or more in the austenite region of 850 to 950 ° C, and 600 to
The method for producing a ferritic stainless steel excellent in shape freezing property during bending according to the above (5), wherein the ferrite stainless steel is wound in a temperature range of 800 ° C. (7) After the completion of hot rolling, the temperature is set at 0.degree.
The method for producing a ferritic stainless steel having excellent shape freezing property during bending according to the above (6), wherein heat treatment is performed for 1 to 10 hours.

[0016]

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, an excessive addition exceeding 0.1% causes cracking during bending due to a decrease in ductility and weld cracking due to a decrease in toughness of the weld heat affected zone, so the upper limit was set to 0.1%.

N is an unavoidable impurity element and is effective in improving the strength of steel. However, N causes work cracks and weld cracks similarly to C, so its upper limit is 0.05%.
And the upper limit of the total content of C and N was set to 0.1%.

Si is an unavoidable impurity element and has an effect of reducing solid solution oxygen in steel as a deoxidizing agent.
If added in excess of 5%, the toughness of the base metal and weld is impaired,
Since it causes bending cracks and welding cracks, the upper limit is 1.
5%.

Mn is an unavoidable impurity element and is effective in improving the strength of steel. However, if added in excess of 1.5%, the toughness of the base material and the welded part is impaired, bending cracking, welding The upper limit is set to 1.5% because it causes cracks.

P is an unavoidable impurity element, but when present in a large amount, not only impairs weldability but also promotes rust generation. Therefore, it was limited to 0.04% or less.

S is an unavoidable impurity element, but mainly as a sulfur-based inclusion such as MnS, not only serves as a starting point of rust, but also increases the corrosion rate. further,
Segregates at grain boundaries and impairs hot workability. Therefore, 0.05
% Must be regulated. S is preferably as small as an impurity.

Cr is an indispensable additive element for suppressing the occurrence of corrosion and reducing the corrosion rate in the atmospheric environment, and its addition amount of 8% or more is necessary for the manifestation of its effect. However, if it exceeds 18%, a large amount of Ni or the like must be added in order to obtain a sufficient amount of the γ phase in the hot-rolled region, and the transformation from the γ phase to the ferrite phase after hot rolling is significantly suppressed. However, even after annealing after hot rolling, it does not become a ferrite phase but remains as a martensite phase or a part of an austenite phase, resulting in marked deterioration in workability and the like. Therefore Cr
The upper limit of the content was set to 18%.

In the present invention, the above component contents are essential, but the following components are further added as necessary. Ni,
Cu and Mo, like Cr, have the effect of suppressing the occurrence of corrosion and reducing the corrosion rate in the atmospheric environment,
It also has the effect of developing the texture according to claims 1 and 2 having excellent shape freezing properties. However, if the amount is small, the effect is weak, and if added excessively, the transformation from the γ phase to the ferrite phase is significantly suppressed after hot rolling, and it does not become a ferrite phase even when subjected to annealing after hot rolling, and the martensite phase or Part of the steel remains as an austenite phase, and the workability and the like deteriorate significantly. Therefore, in each case, the lower limit is 0.1% and the upper limit is 2.5%.
%.

Ti, Nb and V have the effect of fixing C or N and improving the ductility at the time of bending, but the effect is weak if the amount is small, and 0.01% by Ti and Nb.
As described above, V must be added in an amount of 0.05% or more. However, if added excessively, on the contrary, the ductility is reduced, so the upper limit is set to 0.5% in all cases.

In addition to the above-mentioned individual component ranges, a sufficient amount of γ phase is precipitated in a hot rolling temperature range, and hot rolling is performed in a γ phase precipitation temperature range. It is necessary to satisfy the value calculated from the content of each component shown in 3 to 11.5 or less. If it exceeds 11.5, the amount of the ferrite phase existing in the hot-rolled region increases, and the texture for improving the shape freezing property is not sufficiently developed.

In order to exhibit excellent shape freezing properties,
It is necessary to increase the degree of integration of the {100} plane parallel to the plate surface in its texture by producing a steel satisfying the above component range under the conditions described in claims 5 to 7. Unless the reflection X-ray intensity ratio of the {100} plane parallel to the plate surface is set to 2 or more in the texture in the central portion of the plate thickness, there is an advantage over conventional steel or ordinary structural steel having similar strength. Shape freezing property cannot be obtained.

The reflected X-ray intensity ratio (or pole density) shown here is the ratio of the X-ray diffraction intensity measured for the sample to the diffraction intensity measured for the non-directional standard sample (random sample). It is. When the intensity ratio is 1, it indicates that the crystal orientations are not integrated and exist at the same frequency as the random sample.

Further, by setting the X-ray intensity ratio of the {011} plane perpendicular to the rolling direction or the sheet width direction to 1.2 or more, further excellent shape freezing property can be obtained. When {011} planes are integrated perpendicular to the rolling direction, {011} planes are also necessarily integrated perpendicular to the sheet width direction due to crystal symmetry.
The measurement of the X-ray intensity ratio is effective regardless of the measurement in any direction. If it is difficult to directly measure the X-ray intensity ratio of the plane perpendicular to the rolling direction or the sheet width direction, use a method of calculating from the result measured on a plane parallel to the sheet surface, or a measurement method using electron beam diffraction. You may adopt it.

The X-ray reflection intensity ratio of the {100} plane parallel to the plate surface is 2 or more, and the X-ray reflection intensity ratio of the {011} surface parallel to the plane perpendicular to the rolling direction or the plate width direction is 1.2 or more. Ie, {100} on the sheet surface and <01 in the rolling direction
1> By realizing a texture with a developed orientation, it is possible to produce a steel sheet having excellent shape freezing properties during bending, particularly excellent shape freezing properties when bent in the rolling direction or the sheet width direction.

For use in general building structures,
Furthermore, the metal structure of the base metal part is substantially a ferrite phase,
The average crystal grain size is adjusted to 50 μm, and the yield strength or
The 2% proof stress must be 235 MPa or more and 500 MPa or less. If the crystal grain size is less than 5 μm, the strength increases but the elongation required for bending decreases. Also, 50
If the crystal grain size exceeds 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. If the yield strength or 0.2% proof stress is less than 235 MPa, it cannot be designed with the same specifications as general structural carbon steel, and cannot be used as a general-purpose structural material.
On the other hand, if it exceeds 500 MPa, bending becomes difficult,
Cracks may occur during processing.

The ferritic stainless steel can be produced by a hot rolling step or a heat treatment after hot rolling. At the time of its production, in order to obtain a texture having excellent shape freezing properties, finish rolling is performed so that the total reduction amount is 30% or more in an austenite region of 850 ° C or more and 950 ° C or less, and 600 ° C or more and 800 ° C or less. It is necessary to perform hot rolling in a temperature range.

The reason why the finish hot rolling is performed at a temperature of 850 ° C. or more and 950 ° C. or less is to accumulate strain due to hot rolling in a γ phase temperature range and to transform into a ferrite phase in that state.
The {100} <011> orientation as described above develops in the texture after transformation due to the strain existing before transformation. 8
When the finish hot rolling is performed at less than 50 ° C, it is transformed into a ferrite phase before hot rolling, and when the finish hot rolling is performed at more than 950 ° C,
Before the transformation, the strain in the γ phase decreases or disappears due to the recovery or recrystallization phenomenon.

If the total reduction during the hot rolling is not 30% or more, the amount of strain to be accumulated is insufficient, and the texture described in claims 1 and 2 does not develop. Furthermore, it is necessary to transform the austenite phase in which strain has accumulated into a ferrite phase by winding at 600 ° C. or more and less than 800 ° C. after the finish hot rolling. Most of the austenite phase is transformed into a martensite phase when coiled at a temperature lower than 600 ° C., and the development of the texture is inhibited even if the phase is transformed into a ferrite phase by subsequent annealing. Further, if the coiling is performed at a temperature exceeding 800 ° C., the strain in the γ phase is reduced by the recovery or recrystallization phenomenon before the transformation, and the development of the texture is suppressed. In order to secure the finishing hot rolling temperature range, heating before hot rolling is performed at 1100 ° C.
The temperature is desirably set to not less than 1250 ° C.

By satisfying the above hot rolling conditions, a texture advantageous for shape freezing can be developed,
By performing the annealing after the hot rolling, it is possible to eliminate the partially untransformed γ phase (martensite phase) and to adjust the crystal grain size to the optimum value. This annealing makes it possible to stably produce a hot-rolled steel sheet having optimal strength, ductility, and bending workability for general structures.

The annealing conditions must be in the temperature range of 600 ° C. to 850, for 0.1 to 10 hours. If the annealing is performed at a temperature of less than 600 ° C. or less than 0.1 hour, the above effect is not exhibited, and when the annealing is performed at a temperature exceeding 850 ° C., the ferrite phase is transformed into the γ phase again and the developed texture is broken. Further, even if annealing is performed for more than 10 hours, the effect is saturated, and the cost is increased. The steel sheet after hot rolling or heat treatment may be subjected to a treatment such as pickling or polishing from the viewpoint of corrosion resistance or surface design. Also, after annealing, mild cold rolling is performed to correct the shape of the steel sheet or adjust the surface properties. Does not affect the effect of the present invention at all.

[0036]

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
Cast into thick slabs. After heating to 1200 ° C, a hot rolled sheet having a thickness of 2 to 6 mm was produced by hot rolling. Some of the hot rolled sheets were annealed at 750 ° C. after hot rolling. The texture of the hot rolled sheet was measured by X-ray, and the average crystal grain size of the ferrite phase was measured from the metal structure.

Further, the JIS is parallel to the rolling direction of the hot rolled sheet.
A No. 5 tensile test piece was prepared, and its yield strength (0.2% proof stress when no yield point was generated), tensile strength, and elongation at break were measured. Table 2 shows the hot rolling conditions and texture of each hot rolled steel sheet, the measurement results of the ferrite crystal grain size, and the results of the tensile test. Each of the hot-rolled steel sheets satisfying the components and the hot-rolling conditions described in the present invention has an average crystal grain size of 5 to 50 μm, and
It can be seen that the texture shows a texture with a developed 00 ° <011> orientation.

Next, from the above-mentioned hot-rolled steel sheet, a width of 50 mm in a rolling direction (L) or a direction perpendicular to the rolling direction (C) from a 2 mm thick steel sheet.
A test piece of 0 mm was cut out, and a bending test was performed using a square punch into a shape as shown in FIG. The wrinkle pressing force at the time of the bending test was 10 tons, and the presence or absence of cracks after bending at room temperature and the shape after processing were measured. L1 shown in FIG.
The shape freezing property was evaluated from the measurement results of the difference ΔL between the length of L2 and L2. The smaller the ΔL, the better the shape freezing property. Table 3 shows the bending test results. $ 10
The steel of the present invention having a texture with a developed 0 ° <011> orientation has a remarkably small ΔL, is excellent in shape freezing property during bending, and does not generate bending cracks.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

[0042]

According to the present invention, for a building structure or a vehicle structure, there is little springback during bending.
It is possible to supply a ferritic stainless steel sheet having excellent shape freezing properties at a low cost, which is extremely valuable industrially.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a bending test and a shape measuring method performed for evaluating shape freezing properties.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masao Kikuchi 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division (72) Inventor Osamu Akizue 20-1 Shintomi Futtsu City Nippon Steel Corporation Technology Development Division F term (reference) 4K032 AA04 AA12 AA13 AA14 AA15 AA16 AA19 AA20 AA21 AA22 AA23 AA24 AA27 AA29 AA31 AA32 AA35 AA36 BA01 CB02 CC03 CC04 CE02 CF02 CF03

Claims (7)

[Claims] 1. In weight%, C: 0.1% or less, Si: 1.5% or less, Mn: 1.5% or less, Cr: 8 to 18%, P: 0.04% or less, S: Steel containing 0.05% or less, N: 0.05% or less, C + N: 0.1% or less, the balance being Fe and unavoidable impurities, and the metal structure of the base material is 5 to 5.
A ferrite structure having an average crystal grain size of 50 μm, wherein the texture at the center of the plate thickness has a reflection X-ray intensity ratio of {100} plane parallel to the plate surface of 2 or more, at the time of bending. Ferritic stainless steel with excellent freezing properties. 2. Mo: 0.1-2.5%, Cu: 0.1-2.5%, Ni: 0.1-2.5%, Ti: 0.01-0. The bending process according to claim 1, further comprising one or more of 5%, Nb: 0.01 to 0.5%, and V: 0.05 to 0.5%. Ferritic stainless steel with excellent shape freezing properties. 3. The texture in the central part of the sheet thickness has a reflection X-ray intensity ratio of {100} plane parallel to the sheet surface of 2 or more, and furthermore, reflection of the {011} plane perpendicular to the rolling direction or the sheet width direction. 3. The ferritic stainless steel having an excellent shape freezing property during bending according to claim 1 or 2, wherein the X-ray intensity ratio is 1.2 or more. 4. The yield strength or 0.2 which satisfies the following expression.
The ferritic stainless steel according to any one of claims 1 to 3, wherein the ferritic stainless steel has excellent shape freezing property during bending. Cr (%) + Mo (%) + 1.5Si (%) + 6Ti
(%) + 3Nb (%)-Mn (%)-2Ni (%)-
0.5Cu (%)-30C (%)-20N (%) ≦ 1
1.5 5. A steel having the composition according to claim 1 or 2, which is subjected to hot rolling or heat treatment after hot rolling.
5. A method for producing a ferritic stainless steel having excellent shape freezing property during bending, which obtains the characteristics described in any one of (4) to (4). 6. A finish rolling is performed so that the total reduction amount is 30% or more in an austenite region of 850 to 950 ° C.
Characterized by winding in a temperature range of 600 to 800 ° C,
A method for producing a ferritic stainless steel according to claim 5, which is excellent in shape freezing during bending. 7. The method according to claim 6, wherein heat treatment is performed in a temperature range of 600 to 850 ° C. for 0.1 to 10 hours after completion of the hot rolling. Manufacturing method of ferritic stainless steel.