JP3404215B2 - Steel plate with low yield strength - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having an extremely low yield strength. Due to such low yield strength, cold-rolled steel sheet has excellent workability which is useful for applications such as automobile steel sheets. In addition, in the hot rolled steel sheet, it is intended to propose a steel sheet capable of absorbing vibration energy as plastic strain energy to enhance the vibration damping property of a building when an earthquake occurs.

[0002]

2. Description of the Related Art Excellent workability is required for cold-rolled steel sheets used for automobile panels and the like. Among these workability, in order to improve the deep drawability, high r value (Rankford value) and high ductility (El) are required as mechanical properties of the steel sheet. In addition, a low yield strength (YS) is required to secure the stretch formability and shape fixability, and specifically, a yield strength of 120 MPa or less is required. As such a cold-rolled steel sheet for deep drawing, after hot-rolling at an Ar ₃ transformation point or higher, a cold-rolled sheet having a final thickness is obtained.
A cold-rolled steel sheet produced by subsequent recrystallization annealing has been used.

It is known that the yield strength generally depends on the crystal grain size, and the yield strength decreases as the crystal grain size increases. Therefore, in the conventional method for obtaining a low yield strength cold-rolled steel sheet, it is mainly intended to obtain coarse grains by promoting crystal grain growth by making the steel highly pure or annealing at a high temperature. It was technology.

However, if the crystal grains of the steel sheet become coarser than a certain size, problems such as reduction of ductility and roughening of the surface at the time of pressing occur. Therefore, in the conventional method, low yield strength of cold-rolled steel sheet is required. There was a limit. Therefore, it has been desired to further reduce the yield strength of the cold-rolled steel sheet without adverse effects.

Next, the demand for lowering the yield strength of hot-rolled steel sheets will be described. In the field of seismic structure design of buildings, the steel material arranged in the vibration control part is intentionally plastically deformed against shaking caused by an earthquake. Therefore, there is an idea that the vibration input energy at the time of earthquake is absorbed by the plastic deformation energy of the steel material to improve the earthquake resistance. The steel material used for this seismic structure is called so-called damper steel material and has a low yield strength (low YS) as a characteristic required for such applications. Specifically, it has a low yield strength of 120 MPa or less. Steel materials are required.

The demand for the damper steel material having the low yield strength as described above has been strong in the field of thick plate until now, but recently, also in the field of hot-rolled thin plate, the demand for a steel sheet having a low yield strength is increasing. . In the field of this thick plate, various studies have been made on a method for manufacturing a thick steel plate having a low yield strength, for example, JP-A-3-219018 and JP-A-5-218018.
However, in the field of hot-rolled thin plates, there are almost no examples examined for at least construction.

By the way, a technique of cold rolling and recrystallization annealing after performing cold rolling on a cold rolled steel sheet in a ferrite region below the Ar ₃ transformation point for the purpose of achieving an extremely high r value is disclosed in Japanese Patent Publication No. -61727 gazette. This method gives a higher r-value than the conventional method, but does not suggest the above-mentioned yield strength. Also, regarding hot rolled steel sheets, for the purpose of increasing the r-value, for example, as disclosed in Japanese Patent Publication No. 3-49967, Ar ₃
A technique of lubricating rolling in the ferrite region below the transformation point is disclosed. According to this method, the r-value of the hot-rolled steel sheet can be increased, and the hot-rolled steel sheet having the deep drawability comparable to that of the conventional cold-rolled steel sheet can be manufactured. However, in the ferrite area lubrication rolling, how good is the deep drawability {11
1) Only research has been done from the viewpoint of whether a recrystallized texture can be formed, and there is no suggestion of low yield strength.

[0008]

DISCLOSURE OF THE INVENTION The present invention proposes a cold rolled steel sheet and a hot rolled steel sheet having extremely low yield strength, which is particularly advantageous for uses of steel sheets for automobiles and damper steel materials for construction. To aim.

[0009]

The present invention provides C: 0.01 wt.
% Or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, Al: 0.
01 to 0.20 wt%, P: 0.15 wt% or less, S: 0.05 wt% or less and N: 0.01 wt% or less, and Ti: 0.01 to 0.2 wt% and Nb: 0.001 to 0.2 wt%, 1 or 2 The balance containing seeds consists of Fe and unavoidable impurities, and the area ratio is 30 to 100% in the cross section including the rolling direction and the thickness direction in the steel sheet.
The steel sheet having a low yield strength is characterized in that it has colonies in the range of, and the average colony size is 80 μm or more and 1000 μm or less.

Further, according to the present invention, C: 0.01 wt% or less, S
i: 2.0 wt% or less, Mn: 3.0 wt% or less, Al: 0.01 to 0.20w
t%, P: 0.15 wt% or less, S: 0.05 wt% or less, and N: 0.0
Contains 1 wt% or less, Ti: 0.01 to 0.2 wt% and Nb: 0.0
01-0.2 wt% of 1 or 2 types,
B: 0.0001 to 0.008 wt% and the balance Fe and unavoidable impurities, and the steel sheet had a colony having an area ratio of 30 to 100% in a cross section including the rolling direction and the thickness direction. And the average colony size is 80 μm or more 10
It is a steel sheet having a low yield strength characterized by having a thickness of 00 μm or less.

The steel sheet according to the present invention includes both a cold rolled steel sheet and a hot rolled steel sheet. Also,
A colony generally refers to a group of crystal grains in which adjacent crystal grains have substantially the same orientation. Since the colony is an important factor in the present application and a more strict definition is required, it is defined as a crystal grain group in which the orientation difference between adjacent crystal grains is within 20 °. In the present application, the colony size refers to the diameter of a normally flat colony converted into a sphere.

FIG. 1 shows the steel sheet of the present invention.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have conducted intensive studies to obtain cold-rolled steel sheets and hot-rolled steel sheets having a lower yield strength, and newly focused on colonies defined by the orientation of crystal grains of the steel sheet.
It was found that a cold-rolled steel sheet and a hot-rolled steel sheet having a yield strength of 120 MPa or less can be obtained when this colony satisfies predetermined requirements.

The research that has become the basis for obtaining the present invention will be described below. C: 0.002 wt%, Si: 0.02 wt%, Mn: 0.
1 wt%, P: 0.01 wt%, S: 0.005 wt%, Al: 0.04 wt%
%, N: 0.002 wt%, Ti: 0.035 wt%, Nb: 0.005 wt%
A sheet bar containing Fe and unavoidable impurities in the balance is heated to 1050 ° C-soaking, hot rolled at a finishing temperature of 980 to 750 ° C, and coiled at 700 ° C for 1 hour. Was applied. Subsequently, cold rolling was performed at a rolling reduction of 75%, and then recrystallization annealing was performed at 830 ° C. for 20 seconds. For the cold-rolled steel sheet thus obtained, while measuring the yield strength,
EBSD (Electron Back Scattering Diffraction Pat
tern), the crystal orientation of the plate thickness section is measured for each crystal grain, and the crystal grain group in which the orientation difference between adjacent crystal grains is within 20 ° is regarded as a colony, and the size of this colony is flattened in the rolling direction. The colonies were evaluated as the diameter when converted into spheres. The result is shown in FIG.

From the figure, the yield strength of cold-rolled steel sheet strongly depends on the colony size of the steel sheet, and the average colony size is 80
Ultra-low yield strength steel sheets with a yield strength of 120 MPa or less can be manufactured with a micrometer or more.

Next, an experiment leading to a lower yield strength of the hot rolled steel sheet will be described. C: 0.002 wt%, Si: 0.02 wt%, M
n: 0.1 wt%, P: 0.01 wt%, S: 0.005 wt%, Al: 0.0
4 wt%, N: 0.002 wt%, Ti: 0.035 wt%, Nb: 0.005 w
A sheet bar containing t% and the balance of Fe and unavoidable impurities is heated to 1050 ° C.-after soaking, 980 to 750
After hot rolling at a finishing temperature of ℃, the coil was wound at 700 ℃ for 1 hour. Regarding the hot rolled steel sheet thus obtained,
The yield strength and the colony size of the steel sheet were examined in the same manner as in the previous experiment, and the results are shown in FIG.

From the figure, the yield strength of hot-rolled steel sheet strongly depends on the colony size of the steel sheet, and the average colony size is 80
Ultra-low yield strength steel sheets with a yield strength of 120 MPa or less can be obtained at μm or more.

Next, the reason why the composition of the steel sheet is limited in the invention of this application will be explained. C: 0.01 wt% or less; The smaller the content of C, the lower the yield strength and the better the extrusion formability, which is preferable. However, if the content of C is 0.01% or less, there is no significant adverse effect, so C is 0.01 wt% or less. Limited

Si: 2.0 wt% or less; Si has the effect of strengthening steel, and the necessary amount is added according to the desired strength. If the added amount exceeds 2.0 wt%, the yield strength increases. However, it was limited to 2.0 wt% or less because of poor stretchability.

Mn: 3.0 wt% or less; Mn has an action of strengthening steel, and the necessary amount is added according to the desired strength. If the added amount exceeds 3.0 wt%, the yield strength increases. However, it was limited to 3.0 wt% or less because of poor stretchability.

Al: 0.01 to 0.20 wt%; Al is added in a necessary amount for deoxidizing and improving the yield of carbonitride forming elements, but if the content is less than 0.01 wt%, the effect of addition is On the other hand, even if added over 0.20 wt%, no further deoxidizing effect can be obtained, so the range was limited to 0.01 to 0.20 wt%.

P: 0.15 wt% or less; P has the action of strengthening steel, and the necessary amount is added according to the desired strength. If the amount of addition exceeds 0.15 wt%, the yield strength increases. However, it was limited to 0.15 wt% or less because of poor stretchability.

S: 0.05 wt% or less; S is preferable because the smaller the content is, the lower the yield strength and the better the swelling moldability and the like. However, if the content of S is 0.05 wt% or less, it does not cause much adverse effects. Limited to wt% or less.

N: 0.01 wt% or less; N is preferable because the smaller it is, the lower the yield strength and the better the extrusion formability, but if the N content is 0.01 wt% or less, it does not cause much adverse effects. Limited to wt% or less.

Ti: 0.01 to 0.2 wt%; Ti is an important component in the present invention, and solid solution C in steel is precipitated and fixed as a carbide to reduce it, and prevents increase in yield strength due to solid solution C. effective. If the addition amount is less than 0.01 wt%, there is no addition effect. On the other hand, if the addition amount exceeds 0.2 wt%, no further effect is obtained, and rather the yield strength rises. Limited

Nb: 0.001 to 0.2 wt%; Nb is an important component in the present invention, and solid solution C in steel is precipitated and fixed as a carbide to reduce it and prevent an increase in yield strength due to solid solution C. effective. If the addition amount is less than 0.001 wt%, there is no addition effect.On the other hand, if the addition amount exceeds 0.2 wt%, no further effect is obtained, and rather the yield strength increases, so 0.001 to 0.2 wt% Limited to the range. The above Ti and Nb are components having the same effect, and in the present invention, one or two selected from Ti and Nb are contained.

B: 0.0001 to 0.0080 wt%; B is useful for improving the secondary work embrittlement resistance of steel, and can be added for the purpose of obtaining this effect. The addition amount is 0.0001wt
%, There is no effect of this addition, while addition in excess of 0.0080 wt% leads to an increase in yield strength.
It was limited to the range of 0.0001 to 0.0080 wt%.

Colony size of steel sheet: In the present invention, the colony size of the steel sheet is the most important, and is necessary for improving the stretch formability of cold rolled steel sheet and the seismic resistance when a hot rolled steel sheet is used as a damper steel sheet. , Yield strength 120 MP
a Average colony size of 80μ
Must be at least m. If the steel sheet colony size is less than 80 μm, the yield strength is 12
It is not possible to obtain below 0 MPa.

The influence of the colony size on the yield strength is considered as follows. Traditionally,
It is known that the yield strength of a steel sheet depends on the crystal grain size, and the yield strength decreases as the crystal grain size increases. However, there is a limit to the coarsening of the crystal grains of the cold-rolled steel sheet and the hot-rolled steel sheet, and when the crystal grains are coarsened to a certain extent or more, on the contrary, the ductility is rapidly deteriorated. Further, in the cold-rolled steel sheet, when the crystal grain size is 80 μm or more, a roughening phenomenon occurs during press forming, and the surface appearance is significantly deteriorated.

The inventors of the present invention have adopted the EBSD (Electron Back Sc
attering Diffraction Pattern)
A crystal grain group (colony) with an orientation difference between adjacent crystal grains within 20 ° is newly deformed as if it were one crystal grain when the product steel sheet is deformed (worked). Furthermore, from this finding, it was further found that a yield strength of 120 MPa or less can be obtained when the colony size is 80 μm or more. The larger the colony size, the more effective it is for lowering the yield strength.However, when the size exceeds 1000 μm, a rapid decrease in ductility is observed, so the colony size is 80 μm or more and 1000 μm or less. It is preferably 100 μm or more and 1000 μm or less.

This colony exists almost uniformly in the steel sheet, and the existence ratio of the colony is
In the cross section including the rolling direction and the thickness direction, the area ratio of colonies needs to be in the range of 30 to 100%. The size of each colony differs slightly in the thickness direction,
In the present invention, the average size of these may be 80 μm or more.

Next, a method for producing the cold rolled steel sheet and hot rolled steel sheet according to the present invention will be described. Regarding the steps up to hot rolling, it may be manufactured according to a conventional method as a cold-rolled steel sheet, hot-rolled steel sheet, for example, molten steel prepared to have a predetermined component composition is formed into a slab by a continuous casting method, etc. After reheating or continuous casting, immediately after the temperature is not lowered to below the Ar ₃ transformation point, or after heat retention, the product is subjected to hot rolling.

Regarding the slab heating temperature, low temperature heating of 1100 ° C. or lower is preferable for lowering the yield strength. That is, the coarsening of precipitates by the low temperature heating of the slab and the refinement of the structure before hot rolling are advantageous for the progress of recrystallization after hot rolling, and are also preferable for forming colonies.

This hot rolling step is performed by the colony of the present invention.
Is important to get. Conventional Ar ₃Temperature above the transformation point
In the region rolling, no matter how much rolling is performed, the γ → α transformation causes
Since the texture is randomized,
To obtain a crystal grain group (colony) with an orientation difference within 20 °
Have difficulty. Therefore, Ar₃In the temperature range below the transformation point
Finishing hot rolling is effective for colony formation.
It

Since the colony size is considered to mainly depend on the structure before finish rolling, it is possible to obtain colonies of 80 μm or more and 1000 μm or less by controlling the rough rolling temperature. For example, the rolling end temperature is (Ar ₃ transformation point −50
℃) or more and 950 ℃ or less, and in the latter stage of rough rolling
Applying a strong reduction of 25% or more is an effective means for obtaining a suitable colony size.

Finish rolling needs to be finished in a temperature range below the Ar ₃ transformation point, and in order to obtain a ferrite structure with grain size regulation, the rolling reduction below the Ar ₃ transformation point is 50% or more. Is preferred. Further, since it is necessary for the hot-rolled sheet to have a recrystallized structure for lowering the yield strength, the coil winding temperature is preferably 650 ° C. or higher. It is also possible to obtain a recrystallized structure of the hot rolled sheet by performing a recrystallization process after the hot rolling is completed. In this case, the coil winding temperature is
It may be 650 ° C or lower. Strong reduction (25% or more at the final stand) in the latter stage of finish rolling that promotes recrystallization of hot-rolled sheet,
It is an effective means for lowering the yield strength. Lubrication rolling during finish rolling is effective in making the rolling structure uniform and reducing the rolling load, and does not hinder the object of the present invention.

Summarizing the above preferable hot rolling conditions, slab heating temperature: 1100 ° C. or less Rough rolling finish temperature: (Ar ₃ transformation point −50 ° C.) or more and 950 ° C. or less Stage reduction after rough rolling: 25% or more Finish rolling Finishing temperature: Ar ₃ transformation point or less Finish rolling total reduction: 50% or more Post-rolling post-stage reduction: 25% or more Coil winding temperature: 650 ℃ or more

By such hot rolling, in the case of a hot rolled steel sheet, it is directly produced as a product, but temper rolling of 10% or less is added to the steel strip after hot rolling for shape correction and surface roughness adjustment. May be. The hot-rolled steel sheet obtained by the present invention can also be applied to the raw material of the surface-treated steel sheet for processing and the raw material of the cold-rolled steel sheet. As surface treatment, zinc plating (including alloy type),
There are tin plating, enamel, etc. Further, the hot-rolled steel sheet of the present invention is subjected to a special treatment (for example, after annealing or galvanizing).
Ni plating, etc.) may be applied to improve chemical conversion treatability, weldability, press formability, corrosion resistance and the like.

In the case of a cold rolled steel sheet, cold rolling is performed after the hot rolling step described above. This step is essential to obtain a high r value, and it is essential that the cold rolling reduction rate is 50 to 95%. When the cold rolling reduction is less than 50% or more than 95%, excellent deep drawability cannot be obtained.

The cold rolled steel strip that has undergone the cold rolling process needs to be subjected to recrystallization annealing. The annealing method may be either a box-type annealing method or a continuous-type annealing method. The annealing temperature shall be in the range of 650-950 ° C. If the annealing temperature is less than 650 ° C, good properties cannot be obtained because recrystallization is not completed. On the other hand, if it exceeds 950 ° C, the texture becomes random due to the α → γ transformation and a high r
I can't get the value.

After annealing, the steel strip may be subjected to temper rolling of 10% or less in order to correct the shape and adjust the surface roughness. The cold-rolled steel sheet obtained by the present invention can also be applied to an original plate of a surface-treated steel sheet for processing. The surface treatment includes zinc plating (including alloy system), tin plating, enamel and the like. Further, the cold rolled steel sheet of the present invention may be subjected to special treatment (for example, Ni plating) after annealing or galvanizing to improve chemical conversion treatability, weldability, press formability, corrosion resistance and the like. .

[0042]

【Example】

(Example 1) A steel slab having the composition shown in Table 1 was heated and soaked at 1050 ° C, and a hot-rolled steel strip having a plate thickness of 3.5 mm was formed under the hot-rolling conditions shown in Table 2 and then cold-rolled. A cold-rolled steel strip having a thickness of 0.8 mm was prepared, and then recrystallization annealing was performed at 830 ° C for 20 seconds, and then 0.8% temper rolling was performed. The hot rolling was non-lubricating rolling, and the hot-rolled sheet was not recrystallized. With respect to the cold-rolled steel sheet thus obtained, the colony size and material properties of the steel sheet were investigated. This tensile property was measured using a JIS No. 5 tensile test piece. Further, after r values that gave 15% strain tensile pre, measured by 3-point method, r value r _D direction L r value r _L, D direction (45 ° direction to the rolling direction) of the (rolling direction) And C
The average value (average r value) of the r values r _{C in} the direction (90 ° direction to the rolling direction) was determined as the average r value = (r _L + 2r _D + r _C ) / 4. The colony size is EBSD (Electron
Back Scattering Diffraction Pattern) is used to measure the crystal orientation of the steel sheet with a thickness cross section for each crystal grain, and the crystal grain groups with an orientation difference between adjacent crystal grains within 20 ° are regarded as the same colony and The diameter when flattened colonies were converted into spheres was evaluated as the average size. The material properties of the final product are also shown in Table 2.

[0043]

[Table 1]

[0044]

[Table 2]

From Table 2, it can be seen that the cold rolled steel sheet according to the present invention has a lower yield strength (YS) than that of the comparative example.

Example 2 A slab having the composition shown in Table 1 above was heated and soaked at 1050 ° C., and then a hot rolled steel sheet having a thickness of 3.5 mm was formed under the hot rolling conditions shown in Table 3. . The hot-rolled sheet was not temper-rolled. The colony size and material properties of the hot rolled steel sheet were investigated. The hot rolling was non-lubricating rolling, and the hot-rolled sheet recrystallization treatment was not performed. The method for measuring the colony size is the same as in Example 1.
The results of this investigation are also shown in Table 3.

[0047]

[Table 3]

From Table 3, it can be seen that the hot-rolled steel sheet according to the present invention has a lower yield strength (YS) than that of the comparative example.

[0049]

Industrial Applicability The steel sheet of the present invention is a steel sheet that satisfies the specific conditions for the colony of the steel sheet, and thus has a lower yield strength than before and has excellent characteristics as a steel sheet for working and as a damper steel material. A cold rolled steel sheet and a hot rolled steel sheet are obtained.

[Brief description of drawings]

FIG. 1 is an explanatory view of a steel plate of the present invention.

FIG. 2 is a diagram showing the influence of the average colony size on the yield strength of a cold-rolled steel sheet.

FIG. 3 is a diagram showing the influence of the average colony size on the yield strength of hot-rolled steel sheet.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Osamu Furukun, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba, Kawasaki Steel Corporation Technical Research Institute (56) Reference JP-A-63-76849 (JP, A) JP Japanese Patent Laid-Open No. 5-320760 (JP, A) Japanese Patent Laid-Open No. 7-286232 (JP, A) Japanese Patent Laid-Open No. 5-202444 (JP, A) Japanese Patent Laid-Open No. 2-305937 (JP, A) Japanese Patent Laid-Open No. 8-60242 (JP , A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 301 C21D 8/02 C22C 38/14

Claims (4)

(57) [Claims] 1. C: 0.01 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, Al: 0.01 to 0.20 wt%, P: 0.15 wt% or less, S: 0.05 wt% or less and N: It includes the following 0.01 wt%, and Ti: 0.01 to 0.2 wt% and Nb: 0.001 to 0.2 remainder containing wt% of one or consists of Fe and unavoidable impurities, in the steel sheet, the rolling direction In the cross section including the thickness direction,
A steel sheet having a low yield strength, which has colonies having an area ratio in the range of 30 to 100% and has an average colony size of 80 μm or more and 1000 μm or less . 2. C: 0.01 wt% or less, Si: 2.0 wt% or less, Mn: 3.0 wt% or less, Al: 0.01 to 0.20 wt%, P: 0.15 wt% or less, S: 0.05 wt% or less and N: It contains 0.01 wt% or less and contains one or two of Ti: 0.01 to 0.2 wt% and Nb: 0.001 to 0.2 wt%, and further contains B: 0.0001 to 0.008 wt% with the balance being Fe. And an unavoidable impurity, in the steel plate, in the cross section including the rolling direction and the thickness direction,
A steel sheet having a low yield strength, which has colonies having an area ratio in the range of 30 to 100% and has an average colony size of 80 μm or more and 1000 μm or less . 3. A steel sheet having a low yield strength according to claim 1, wherein the steel sheet is a cold rolled steel sheet. 4. A steel sheet having a low yield strength according to claim 1 or 2, wherein the steel sheet is a hot rolled steel sheet.