JP4285843B2 - Ferritic stainless steel with excellent shape freezing property during bending and its manufacturing method - Google Patents

Description

【００４０】
【表２】

【００４１】
【表３】

【００４２】
【発明の効果】
本発明により、建築構造用或いは車両構造用として、曲げ加工時のスプリングバックが少なく、形状凍結性に優れたフェライト系ステンレス鋼板を安価に供給することが可能となり、工業的に極めて高い価値がある。
【図面の簡単な説明】
【図１】形状凍結性を評価するために実施した曲げ試験、および形状測定方法の概念図である。[0001]
BACKGROUND OF THE INVENTION
INDUSTRIAL APPLICABILITY The present invention has bending workability suitable for a structural member of a building such as a detached house, an apartment house, a large building, a building or a bridge, or a vehicle structure, and has excellent shape freezing property during bending, that is, a springback The present invention relates to a ferritic stainless steel for welded structures and a method for producing the same.
[0002]
[Prior art]
Due to stricter safety standards for buildings and the pursuit of functionality, steel materials for pillars and beams are required to have higher functionality.
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 stainless steel for building structures that eliminates rust. For structural use, SUS304 (18Cr-8Ni), which has excellent corrosion resistance and toughness, has many uses.
[0003]
However, since stainless steel requires a large amount of expensive elements such as Cr and Ni, the material cost and manufacturing cost are high, and although it is functionally superior, there is a problem in its economic efficiency. On the other hand, ferritic stainless steel added with Cr exceeding 10%, such as SUS430 steel, is excellent in corrosion resistance, but the metal structure of the hot-rolled steel sheet is a coarse ferrite grain structure elongated in the rolling direction. The workability is poor, and the ferrite structure of the weld heat affected zone is coarsened, and the weld toughness is significantly reduced.
Ferritic stainless steel for general building structures that require welding, because the toughness of the welded part is a serious problem with thick materials used for structures, etc., and cracks may occur during cooling after welding. Was not used.
[0004]
In addition, 11% Cr SUS410L steel precipitates the γ phase at high temperatures, so it is difficult for the weld heat-affected zone to become coarse, and by reducing the C content, weld cracking and weld toughness are also improved. It is partly used for structural applications such as container aggregates. However, since the hot-rolled steel sheet has a high yield strength, it has a drawback that the spring back during bending is large, that is, the shape freezing property during bending is poor.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a hot-rolled steel sheet and a method for producing the same, which have less spring back at the time of bending and have excellent shape freezing properties for use in building structures or vehicle structures.
[0006]
[Means for Solving the Problems]
The present inventors control the phase transformation in the hot-rolled zone by adjusting the component balance, and optimize the hot-rolling conditions and the subsequent heat treatment conditions, so that the metal structure and the texture are excellent in bending workability and less spring back. Aimed to realize.
In view of the above problems, the present inventors studied a texture (ratio to random crystal orientation, which will be described below) having excellent bending workability and less spring back. As a result of examining the bending characteristics of stainless steel plates having various textures, when the {100} planes are accumulated parallel to the steel plate surface, the springback is reduced, and the <011> direction is accumulated parallel to the bending direction. It became clear that the effect became remarkable.
[0007]
Since the bending of the steel sheet is generally in the rolling direction or a direction perpendicular thereto, the {100} planes are accumulated in parallel with the steel sheet surface, and the <011> direction is accumulated in parallel with the rolling direction or the sheet width direction. Therefore, it can be expected that a steel plate having a small spring back can be produced when it is bent in the rolling direction or the plate width direction.
[0008]
In general, such a texture is observed in a stretched processed structure after hot rolling or cold rolling, but the shape freezing property is sufficient because more {111} faces that increase the spring back accumulate. It was not improved. Further, a texture that accumulates {100} faces cannot be obtained by recrystallization by annealing after rolling, and a stainless steel sheet having the texture as described above is not manufactured.
[0009]
The present inventors have hot rolled various components of stainless steel under various conditions, and as a result of investigating the metal structure, texture and mechanical properties, the strain introduced during hot rolling and the subsequent γ → α phase transformation It has been found that the texture as described above can be obtained under the limited conditions of properly using the material, and excellent shape freezing property can be obtained.
That is, rolling in the γ phase region, finish 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 with the strain introduced by hot rolling left, The transformation texture as described above is formed. Furthermore, by optimizing the wetting temperature after finish rolling, the transformation can be completed and the crystal grain size can be adjusted to give the strength required as a structural material and the ductility required during bending.
[0010]
The gist of the present invention is as follows.
The amount of Cr and the amount of other components are adjusted so that a γ phase is obtained in a hot rolling region temperature range of 850 ° C. or higher. The amount of austenite phase generated in the high temperature range can be predicted from the component content, and if the components are adjusted so as to satisfy the following formula, a sufficient amount of austenite phase can be generated in the hot rolling temperature range.
Cr (%) + Mo (%) + 1.5 Si (%) + 6 Ti (%) + 3 Nb (%)
-Mn (%)-2Ni (%)-0.5Cu (%)-30C (%)-20N
(%) ≦ 11.5
[0011]
And hot rolling is mainly carried out in the austenite region, transformed into a ferrite phase during cooling after hot rolling, forming a texture as described above, and a hot rolled steel sheet having an appropriate ferrite grain size and yield strength Can be manufactured. At this time, if the transformation to ferrite phase is insufficient and part of the austenite phase is transformed to martensite phase, or if the grain growth after 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 cracking may occur during bending.
[0012]
If the above formula is not satisfied and a sufficient amount of austenite phase is not formed at a high temperature, formation of the texture is inhibited and ferrite crystal grains are coarsened. If the ferrite crystal grains exceed 50 μm on average, rough skin irregularities corresponding to the ferrite grains are generated on the outer surface during bending, which not only impairs the appearance but also causes cracks due to local deformation. Further, when the yield strength is less than 235 MPa due to the coarsening of ferrite grains, the yield strength is lower than that of general structural steel materials, and a special design is required, which is inappropriate as a general structural steel material.
More in particular to optimize the metal structure of the component and the hot-rolled steel sheet as described above, for general structural ferritic stainless steel can be realized.
[0013]
The specific configuration of the present invention is as follows.
(1) In mass %,
C: 0.1% or less, Si: 1.5% or less,
Mn: 1.5% or less, Cr: 8-18%,
P: 0.04% or less, S: 0.05% or less,
N: 0.05% or less, C + N: 0.1% or less ,
further,
Mo: 0.1-2.5%, Cu: 0.1-2.5%,
Ni: 0.1-2.5%, Ti : 0.01-0.5 %,
Nb: 0.01 to 0.5%, V: 0.05 to 0.5%
A ferrite structure containing one or more of the following , with the balance being Fe and unavoidable impurities, wherein the metal structure of the base metal part has an average crystal grain size of 5 to 50 μm, der reflected X-ray intensity ratio of parallel to the plate plane {100} plane is 2 or more in the texture of the thick central portion is, further reflected X-ray intensity ratio in the rolling direction or the plate and the vertical width direction {011} plane characterized in that it is 1.2 or more, bending excellent shape fixability at the time of processing ferritic stainless steels.
[0014]
( 2 ) Ferritic stainless steel excellent in shape freezing property during bending as described in (1) above , wherein the following formula is satisfied and the yield strength or 0.2% proof stress is 235 to 500 MPa.
Cr (%) + Mo (%) + 1.5 Si (%) + 6 Ti (%) + 3 Nb (%)
-Mn (%)-2Ni (%)-0.5Cu (%)-30C (%)-20N (%)
≦ 11.5
[0015]
( 3 ) When hot-rolling a steel having the components described in (1) or (2) above , the total reduction amount is 30% or more in the austenite region of 850 to 950 ° C. A method for producing a ferritic stainless steel having excellent shape freezing property during bending , characterized by performing finish rolling as described above , and cutting in a temperature range of 600 to 800 ° C.
(4) after hot rolling finished, and performing a heat treatment of from 0.1 to 10 hours at a temperature range of 600 to 850 ° C., ferrite with excellent bending when the shape fixability according to (3) Of manufacturing stainless steel.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
The reasons for limiting the component range of the steel of the present invention will be described below.
C is an effective element for improving the strength of steel. However, excessive addition exceeding 0.1% causes cracking during bending due to ductility reduction and weld cracking due to reduction in toughness of the weld heat affected zone, so the upper limit was made 0.1%.
[0017]
N is an unavoidable impurity element and is effective in improving the strength of steel, but it causes work cracks and weld cracks in the same manner as C, so its upper limit is set to 0.05%. The upper limit of the total content of N was set to 0.1%.
[0018]
Si is an unavoidable impurity element and has the effect of reducing dissolved oxygen in steel as a deoxidizer, but if added over 1.5%, the toughness of the base metal and the welded part is impaired, and bending cracking occurs. In order to cause weld cracking, the upper limit was set to 1.5%.
[0019]
Mn is an inevitable impurity element and is effective in improving the strength of steel, but if added over 1.5%, the toughness of the base metal and the welded part is impaired, causing bending cracks and weld cracks. Therefore, the upper limit was made 1.5%.
[0020]
P is an unavoidable impurity element, but if present in a large amount, P not only impairs weldability but also a phenomenon of promoting rust generation. Therefore, it was limited to 0.04% or less.
[0021]
S is an unavoidable impurity element, but mainly as a sulfur-based inclusion such as MnS, it not only serves as a starting point for rust but also increases the corrosion rate. Furthermore, it segregates at the grain boundaries and harms hot workability. Therefore, it is necessary to regulate to 0.05% or less. S is more preferable as an impurity.
[0022]
Cr is an additive element that is indispensable for suppressing the occurrence of corrosion and reducing the corrosion rate in the atmospheric environment, and an addition amount of 8% or more is necessary to achieve its effect. However, if added over 18%, a large amount of Ni or the like must be added in order to obtain a sufficient amount of γ phase in the hot rolling region, and the transformation from the γ phase to the ferrite phase is significantly suppressed after hot rolling. Even if annealing is performed after hot rolling, it does not become a ferrite phase but remains as a martensite phase or a part of austenite phase, and the workability and the like are significantly deteriorated. Therefore, the upper limit of the Cr content is 18%.
[0023]
In the present invention, the content of the above components is essential, but the following components are further added as necessary.
Ni, Cu and Mo have the effect of suppressing the occurrence of corrosion and reducing the corrosion rate in the atmospheric environment, as well as Cr, and also have the effect of developing the texture according to claim 1 which is excellent in shape freezing property. However, if the amount is small, the effect is weak, and if added excessively, the transformation from the γ phase to the ferrite phase after hot rolling is remarkably suppressed, and even after annealing after hot rolling, the ferrite phase does not become a martensite phase or A part of the austenite phase remains and the workability and the like are remarkably deteriorated. Accordingly, in both cases, the lower limit is set to 0.1% and the upper limit is set to 2.5%.
[0024]
Ti, Nb and V have the effect of fixing C or N and improving the ductility during bending, but if the amount is small, the effect is weak, 0.01% or more for Ti and Nb, 0.05 for V % Or more must be added. However, if added excessively, ductility is adversely reduced, so the upper limit was 0.5%.
[0025]
Other provisions of the individual components within the above range, the hot rolling temperature range in to precipitate a sufficient amount of γ-phase, in order to implement the hot rolling at a precipitation temperature zone of γ phase, shown in claim 1 It is necessary to satisfy the value calculated from the content of each component at 11.5 or less. If it exceeds 11.5, the amount of the ferrite phase present in the hot-rolled zone increases, and the texture that improves the shape freezing property does not sufficiently develop.
[0026]
In order to exhibit excellent shape freezing properties, by producing a steel satisfying the above component range under the conditions described in claims 3 and 4 , accumulation of {100} planes parallel to the plate surface in the texture Need to increase the degree. If the reflected X-ray intensity ratio of the {100} plane parallel to the plate surface is not 2 or more in the texture at the center of the plate thickness, it has an advantage over conventional steel or structural steel with similar strength. The shape freezing property cannot be obtained.
[0027]
The reflected X-ray intensity ratio (also referred to as pole density) shown here is the ratio between the X-ray diffraction intensity measured on the sample and the diffraction intensity measured on the non-directional standard sample (random sample). When the intensity ratio is 1, it indicates that the crystal orientation is not accumulated and exists at the same frequency as the random sample.
[0028]
Furthermore, 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 the {011} plane is accumulated perpendicular to the rolling direction, the {011} plane is necessarily accumulated perpendicular to the plate width direction due to crystal symmetry, so the X-ray intensity ratio is measured in any direction. Even it is effective. If it is difficult to directly measure the X-ray intensity ratio of the surface perpendicular to the rolling direction or the plate width direction, a method of calculating from the results measured on a surface parallel to the plate surface, or a measuring method using electron beam diffraction It may be adopted.
[0029]
Assembly in which 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} plane parallel to the plane perpendicular to the rolling direction or the plate width direction is 1.2 or more By realizing a texture, that is, a texture with {100} on the plate surface and a developed <011> orientation in the rolling direction, the shape freezing property during bending, particularly the shape freezing when bent in the rolling direction or the plate width direction It is possible to manufacture a steel sheet having excellent properties.
[0030]
In order to use it for a general building structure, the metal structure of the base material portion is further made into a substantially ferrite phase, adjusted to an average crystal grain size of 5 to 50 μm, and yield strength or 0.2% proof stress is 235 MPa or more and 500 MPa or less. And shall be. When the crystal grain size is less than 5 μm, the strength is increased, but the elongation required for bending is reduced. On the other hand, if the crystal grain size exceeds 50 μm, the strength decreases and surface irregularities occur during bending, which not only impairs the appearance but also decreases toughness and may cause cracks 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, and cracks may occur during processing.
[0031]
The ferritic stainless steel can be produced by a hot rolling process or a heat treatment after hot rolling. During the production, in order to obtain a texture having excellent shape freezing property, finish rolling is performed such 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 that takes up in the temperature range.
[0032]
The reason why finish hot rolling is performed at 850 ° C. or more and 950 ° C. or less is to accumulate strain due to hot rolling in the temperature range of the γ phase and to transform into the ferrite phase in that state. In the subsequent texture, the {100} <011> orientation as described above develops. When finish hot rolling is performed at less than 850 ° C, it transforms into a ferrite phase before hot rolling, and when finish hot rolling is performed at over 950 ° C, strain in the γ phase decreases due to recovery or recrystallization before transformation. Disappear.
[0033]
Further, unless the total reduction amount during finish hot rolling is 30% or more, the accumulated strain amount is insufficient, and the texture described in claim 1 does not develop. Furthermore, after finishing hot rolling, it is necessary to transform the austenite phase in which strain is accumulated into a ferrite phase by cutting at 600 ° C. or more and less than 800 ° C. When the temperature is reduced below 600 ° C., most of the austenite phase is transformed into a martensite phase, and the development of the texture is inhibited even if the annealing is transformed into a ferrite phase. Further, when the temperature is higher than 800 ° C., the strain in the γ phase is reduced by the recovery or recrystallization phenomenon before transformation, and the development of the texture is suppressed. Moreover, in order to ensure a finishing hot rolling temperature range, it is desirable that the heating before hot rolling be 1100 ° C. or higher and 1250 ° C. or lower.
[0034]
By satisfying the above hot rolling conditions, it is possible to develop a texture that is advantageous for shape freezing, but by carrying out annealing after hot rolling, a partially untransformed γ phase (martensitic phase) Disappearance can be adjusted to the optimum crystal grain size. This annealing makes it possible to stably manufacture a hot-rolled steel sheet having optimum strength, ductility, and bending workability for general structures.
[0035]
The annealing conditions must be not less than 0.1 hours and not more than 10 hours in a temperature range of not less than 600 ° C. and not more than 850. If the annealing is less than 600 ° C. or less than 0.1 hour, the above effect is not exhibited, and if the annealing exceeds 850 ° C., the ferrite phase is transformed again into the γ phase and the developed texture is destroyed. Moreover, even if it anneals over 10 hours, the effect will be saturated and a cost rise will be caused.
In addition, 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, and after cold annealing, it may be mild cold rolled to correct the shape of the steel sheet or adjust the surface properties. Doing so does not affect the effects of the present invention.
[0036]
【Example】
Hereinafter, the present invention will be described in detail based on examples.
Steels having various compositions shown in Table 1 were melted and cast into slabs having a thickness of 250 mm. After heating to 1200 ° C., a hot-rolled sheet having a thickness of 2 to 6 mm was produced by hot rolling. Some hot-rolled sheets were annealed after hot rolling at 750 ° C. 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.
[0037]
Furthermore, a JIS No. 5 tensile test piece was produced in parallel with the rolling direction of the hot-rolled sheet, and the yield strength (0.2% proof stress when no yield point was produced), 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 tensile test results.
It can be seen that the hot-rolled steel sheets satisfying the components and hot-rolling conditions described in the present invention all have an average crystal grain size of 5 to 50 μm and exhibit a texture with developed {100} <011> orientation.
[0038]
Next, a test piece having a width of 500 mm is cut out from the steel sheet having a thickness of 2 mm in the hot rolled steel sheet in the rolling direction (L) or the perpendicular direction (C), and the shape as shown in FIG. A bending test was conducted. The wrinkle holding force during the bending test was 10 tons, and the presence or absence of cracking and the shape after processing were measured after bending at room temperature.
The shape freezing property was evaluated from the measurement result of the difference ΔL in length between L1 and L2 shown in FIG. The smaller ΔL, the better the shape freezing property. Table 3 shows the bending test results. The steel of the present invention having a texture with developed {100} <011> orientation has an extremely small ΔL, excellent shape freezing property during bending, and no bending cracks.
[0039]
[Table 1]

[0040]
[Table 2]

[0041]
[Table 3]

[0042]
【The invention's effect】
According to the present invention, it is possible to supply a ferritic stainless steel sheet having a low spring back at the time of bending and having an excellent shape freezing property for an architectural structure or a vehicle structure 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 property.

Claims (4)

% By mass
C: 0.1% or less, Si: 1.5% or less,
Mn: 1.5% or less, Cr: 8-18%,
P: 0.04% or less, S: 0.05% or less,
N: 0.05% or less, C + N: 0.1% or less ,
further,
Mo: 0.1-2.5%, Cu: 0.1-2.5%,
Ni: 0.1 to 2.5%, Ti: 0.01 to 0.5%,
Nb: 0.01 to 0.5%, V: 0.05 to 0.5%
Containing <br/> 1 or two or more, the balance being a steel comprising Fe and unavoidable impurities, in ferritic structure where the metal structure of the base metal has an average grain size of 5 to 50 [mu] m, the plate der reflected X-ray intensity ratio of parallel to the plate plane {100} plane is 2 or more in the texture of the thick central portion is, further reflected X-ray intensity ratio in the rolling direction or the plate and the vertical width direction {011} plane characterized in that it is 1.2 or more, bending excellent shape fixability at the time of processing ferritic stainless steels. Satisfies the following formula, yield strength or 0.2% proof stress, characterized in that a 235～500MPa, claim 1, wherein bending during shape fixability excellent ferritic stainless steel.
Cr (%) + Mo (%) + 1.5 Si (%) + 6 Ti (%) + 3 Nb (%)
-Mn (%)-2Ni (%)-0.5Cu (%)-30C (%)-20N (%)
≦ 11.5 When hot-rolling a steel having the components described in claim 1 in order to obtain the steel according to claim 1 or 2, finish rolling is performed so that the total reduction amount is 30% or more in an austenite region of 850 to 950 ° C. A method for producing a ferritic stainless steel having excellent shape freezing property during bending , characterized in that the method is performed at a temperature range of 600 to 800 ° C. The ferritic stainless steel excellent in shape freezing property during bending according to claim 3 , wherein heat treatment is performed at a temperature range of 600 to 850 ° C for 0.1 to 10 hours after the end of hot rolling. Production method.

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