JP2000297349A - High tensile strength hot rolled steel plate excellent in elongation flanging property and fatigue characteristic and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength hot rolled steel plate having high formability of being excellent in elongation flanging formability and fatigue characteristics and moreover being good in bulging formability and shape freezability. SOLUTION: A steel slab contg., by weight, 0.010 to 0.10% C, 0.50 to 1.50% Si, 0.50 to 2.50% Mn, <=0.05% P, <=0.05% S and 0.005 to 0.03% Ti is held in the temp. region of 900 to 1300 deg.C, is subjected to continuous hot rolling in which the draft in the final stand is controlled to <20%, and the rolling finishing temp. is controlled to 870 to 980 deg.C, is cooled at a cooling rate of 50 to 200 deg.C/sec after the completion of the rolling and is coiled in th temp. range of 300 to 650 deg.C to form a structure composed of a ferritic phase of 70 to 97% volume ratio, and the balance low temp. transformed phase of a bainitic phase, and the intraplane anisotropy Δr of (r) value is controlled to <=0.2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet mainly used as a part for an automobile, which is subjected to processing such as press forming, and more particularly to a member of an automobile body, a structural member such as a frame, or a foot such as a suspension or a wheel. The present invention proposes a high-strength hot-rolled steel sheet used for a member requiring stretch flangeability and fatigue properties, such as a surrounding member, and a method for producing the same.

[0002]

2. Description of the Related Art One of the most effective methods for reducing the weight of an automobile body is to use a high-tensile steel sheet. Above all,
Hot rolled steel sheets, which are excellent in economic efficiency, have been used for parts that do not require very good surface quality, such as strength members or suspensions of bodies. Under such circumstances, the use of hot-rolled high-tensile steel sheets is being expanded more and more. By the way, conventionally, the tensile strength is 490 ~ 780
As a method for strengthening the material of hot-rolled steel sheets with high tensile strength of about MPa, 1)
A transformation structure strengthening method in which a martensite phase, a pearlite phase or a bainite phase is precipitated in a ferrite phase,
2) There is a precipitation strengthening method using Ti, Nb and V carbonitrides. These strengthening methods are selected and applied in consideration of the formability of the steel sheet and the characteristics required for the member. For example, inexpensive general-purpose high-strength hot-rolled steel sheets include a steel (HSLA) having a structure composed of a precipitation-strengthened ferrite phase and a pearlite phase or a bainite phase. Dual Phase steel is precipitation strengthened Dual Pha when stretch flange formability is required
For example, se steel is selected. As a production technique of these hot-rolled high-tensile steel sheets, for example, JP-B-62-37089, JP-B-58-27328, JP-B-62-39230, etc., by winding a Si-added steel sheet at a low temperature A method for improving stretch flangeability has been proposed.

[0003]

However, all of the above-mentioned conventional methods are mainly aimed at improving only one processing characteristic, and the manufacturing process requires a very narrow temperature range. Since it is necessary to control the temperature within the range, there is a problem that the quality of the steel sheet varies greatly and the productivity is poor. In such a situation, the inventors have:
We have proposed a technology to stably obtain a high-strength hot-rolled steel sheet having both stretch formability (ductility), shape freezing property (yield ratio), and stretch flange formability (hole expanding property) (Japanese Patent Application No. 9-281567).
issue). Although the above technology has greatly improved the ability to cope with various molding conditions, automobile members, especially those having high strength, require even higher stretch flange formability and excellent fatigue properties. However, the above invention does not always respond sufficiently.

[0004] Therefore, an object of the present invention is to provide a stretch moldability (hole expanding property) and fatigue properties, and also excellent stretch formability (ductility) and shape freezing property (yield ratio). It is an object of the present invention to provide a tension hot-rolled steel sheet. Another object of the present invention is to set a heating temperature, a finish rolling temperature, a winding temperature, and the like of a steel material in a wide range, except for cooling at an extremely high speed after completion of rolling in a hot rolling process, and furthermore, a stable material. And a method for producing a high-tensile hot-rolled steel sheet which can drastically improve productivity. The specific material target of the present invention is a tensile strength: 5
40-640 MPa, yield strength: 400-500 MPa, elongation: 28-38
%, Hole expansion rate: 115% or more, notch fatigue strength: 200 MPa or more.

[0005]

Means for Solving the Problems In order to achieve the above objects, the present inventors have focused on low-carbon steel containing Si and Mn and focused mainly on the component composition, metal structure, and hot rolling process. I did diligent research. As a result, the carbonitride forming element Ti
By adding a small amount of, and appropriately controlling the hot rolling and the cooling process after hot rolling, the ferrite transformation after finish rolling is activated, and the in-plane anisotropy of the Rankford value (r value) is reduced. As a result, it was found that the stretch flangeability and the fatigue properties were improved, and the present invention was completed.
Here, the in-plane anisotropy of the Rankford value (r value) is Δ Δ
r = (r ₀ + r ₉₀ -2r ₄₅ ) / 2 (where r ₀ ,
r ₉₀ and r ₄₅ are evaluated as absolute values of r values in directions of 0 °, 90 °, and 45 ° in the rolling direction, respectively. The gist configuration of the present invention is as follows.

(1) C: 0.010 to 0.10 wt%, Si: 0.50 to 1.50 wt%, Mn: 0.50 to 2.50 wt%, P: 0.05 wt% or less, S: 0.005 wt% or less, Ti: 0.005 to 0.03 wt% %, The balance is composed of Fe and unavoidable impurities, and the metal structure has a ferrite phase of 70 to 97% by volume and a low-temperature transformation phase mainly composed of a bainite phase. A high-strength hot-rolled steel sheet having excellent stretch flangeability and fatigue properties, characterized in that the absolute value of the in-plane anisotropy Δr is 0.2 or less.

(2) C: 0.010 to 0.10 wt%, Si: 0.50 to 1.50 wt%, Mn: 0.50 to 2.50 wt%, P: 0.05 wt% or less, S: 0.005 wt% or less, Ti: 0.005 to 0.03 wt% %, And one or two types of Ca: 0.0005 to 0.01 wt% and REM: 0.0005 to 0.1 wt%, and the balance is composed of Fe and unavoidable impurities, and 70 to 97% by volume. Has a metal structure composed of a low-temperature transformation phase mainly composed of a bainite phase, and the absolute value of the in-plane anisotropy Δr of the r value is
A high-strength hot-rolled steel sheet having excellent stretch flangeability and fatigue properties, characterized by being 0.2 or less.

(3) The steel sheet according to the above (1) or (2) further contains one or two of V: 0.003 to 0.02 wt% and Nb: 0.003 to 0.02 wt% in addition to the above components. High-strength hot-rolled steel sheet with excellent stretch flangeability and fatigue properties.

(4) The steel sheet according to any one of the above (1) to (3), further comprising: B: 0.0005 to 0.005 wt% in addition to the above components. High-strength hot-rolled steel sheet with excellent heat resistance and fatigue properties.

(5) C: 0.010 to 0.10 wt%, Si: 0.50 to 1.50 wt%, Mn: 0.50 to 2.50 wt%, P: 0.05 wt% or less, S: 0.005 wt% or less, Ti: 0.005 to 0.03 wt% % Of the steel material (slab) containing steel at a temperature of 900 to 1300 ° C, and then subjected to continuous hot rolling at a final stand with a rolling reduction of less than 20% and a rolling end temperature of 870 to 980 ° C. After rolling, it is cooled at a cooling rate of 50 to 200 ° C / sec and wound up into a coil at a temperature of 300 to 650 ° C. Method.

[0011]

FIG. 1 shows an example of an experimental result on which the present invention is based. That is, 0.08 wt% C, 0.7 wt%
Si, 1.25 wt% Mn, 0.01 wt% P, 0.001 wt% S, 0.0025 wt
% N, 0.018wt% Ti, a steel sheet composed of a ferrite phase with a volume ratio of 90% and a low-temperature transformation phase mainly composed of a bainite phase, Δr indicates the hole expansion rate (stretch flangeability).
And the effects on notch fatigue strength are summarized. In addition, these test methods are the same as the method mentioned later. From FIG. 1, it can be seen that both the stretch flangeability and the fatigue properties can be improved by reducing the absolute value of Δr to 0.2 or less. Although the cause of such a tendency is not necessarily clear, the following may be considered. That is, in a hole expansion test of a hot-rolled steel sheet, cracks are likely to occur in a direction perpendicular to the rolling direction or in the rolling direction.
These directions correspond to directions in which the r value is small.
Therefore, it is considered that by reducing the anisotropy of the r value, the concentration of stress at these crack initiation sites was reduced, and as a result, the hole expansion rate was improved. Previous studies on hot-rolled steel sheets indicated that the effect of r value on stretch flangeability was small, but when the ferrite fraction was increased by controlling the structure as in the present invention, the effect of Δr was Is considered to appear remarkably. Further, regarding the notch fatigue characteristics, the presence or absence of burrs at the time of punching and the influence of the surface condition on the hole end face greatly affect. By reducing the absolute value of Δr as in the present invention, when the deformability is homogenized, the occurrence of burrs at the time of forming a punched hole and the properties of the punched surface are improved, and the fatigue characteristics are improved. It is thought that it was done.

Hereinafter, the reason why the constituent elements in the present invention are limited to the above range will be described. C: 0.010 to 0.10 wt% When the content of C is less than 0.010 wt%, not only the low-temperature transformation phase precipitates but sufficient strength cannot be obtained, but also carbides decrease during heating of the steel material, resulting in a coarse structure. And the workability decreases. On the other hand, when the content exceeds 0.10 wt%, carbides are coarsened during heating of the steel material, and the structure is coarsened, so that the workability is similarly reduced. Therefore, the C content is from 0.010 to
0.10 wt%, preferably in the range of 0.06 to 0.08 wt%.

Si: 0.50-1.50 wt% When the content of Si is less than 0.50 wt%, the volume ratio of the ferrite phase decreases and the formability decreases, but when it exceeds 1.50 wt%, the ferrite phase hardens and the ductility increases. Decrease. Therefore, the Si content is in the range of 0.50 to 1.50 wt%.

Mn: 0.50 to 2.50 wt% When the content of Mn is less than 0.50 wt%, the volume ratio of the second phase is reduced and sufficient strength cannot be obtained. The volume fraction of the second phase increases and the ductility decreases.
Therefore, the Mn content is 0.50 to 2.50 wt%, preferably 1.0
To 2.0 wt%.

P: 0.05 wt% or less If the content of P exceeds 0.05 wt%, the spot weldability deteriorates. Therefore, the content of P is limited to 0.05 wt% or less.

S: 0.005 wt% or less S increases the amount of non-metallic inclusions such as MnS and reduces workability, particularly stretch flangeability. Therefore, S is limited to 0.005 wt% or less, preferably 0.003 wt% or less.

Ti: 0.005 to 0.03 wt% Ti is a particularly important element in the present invention. Ti, Nb,
Elements such as V form carbonitrides, suppress austenite grain growth during hot rolling and reduce the grain size, thereby promoting ferrite transformation and ensuring good ductility. Have. However, on the other hand, these elements also suppress the recrystallization of austenite during rolling,
It also has the effect of increasing Δr due to ferrite transformation from unrecrystallized austenite. Also,
When these elements form a solid solution in steel, the ferrite transformation tends to be delayed. Among the carbonitride forming elements having such an effect, Ti has relatively weak austenite recrystallization inhibiting effect as compared to other elements, and also compares the effect of solid solution on ferrite transformation delay. Target small. For this reason, in the present invention, Ti is added as a carbonitride forming element, especially Ti as a basic component. 0.005 Ti content
If the content is less than wt%, the amount of carbonitride deposited is insufficient, and it is difficult to obtain the effect of suppressing austenite grain growth and the effect of promoting ferrite transformation. On the other hand, if the content exceeds 0.03 wt%, the precipitates tend to be coarsened, and the amount of dissolved Ti during heating of the steel material also increases, making it difficult to obtain the effect of promoting the ferrite transformation by Ti. Therefore, the Ti content is in the range of 0.005 to 0.03 wt%, preferably 0.015 to 0.025 wt%.

Ca: 0.0005 to 0.01 wt%, REM: 0.0005 to 0.
1 wt% Ca, REM is an element useful for controlling the morphology of nonmetallic inclusions and improving stretch flangeability. In order to exert such an effect, Ca: 0.0005 wt% or more and REM: 0.00
It is necessary to add at least one kind of at least 05 wt%. On the other hand, if the addition amount of these elements is too large, not only the effect is saturated, but also the inclusions due to Ca or REM increase and the burring property decreases. Therefore, the upper limit of the addition of these elements is 0.01 wt% for Ca and 0.1 wt% for REM.

V: 0.003-0.02 wt%, Nb: 0.003-0.02
wt% V and Nb are elements forming carbonitrides and have an effect of suppressing the growth of austenite grains, but these elements have a greater effect of suppressing the transformation of veferite than Ti in a solid solution state.
The effect of suppressing recrystallization of austenite is also large. However, by adding at least one of these elements in combination with Ti, the above-mentioned adverse effects can be reduced, the final structure can be refined, and the stretch flangeability can be improved. In order to obtain such effects, addition of 0.003 wt% or more is required. On the other hand, if any of the elements is added in excess of 0.02 wt%, the above-mentioned adverse effects are rather increased. Therefore, the added amounts of V and Nb are each in the range of 0.003 to 0.02 wt%. A more preferred range is 0.005 to 0.01 wt%.

B: 0.0005 to 0.005 wt% B is an element useful for improving hardenability, increasing the strength of the ferrite phase itself, and improving burring. Such an effect can be obtained by adding 0.0005% by weight or more. However, even if it exceeds 0.005% by weight, the effect is not only saturated, but also has an adverse effect of greatly reducing elongation. Therefore, B is added in the range of 0.0005 to 0.005 wt%.

Next, the organization will be described. Volume fraction of ferrite phase and remaining structure Ferrite phase is a structure necessary to secure ductility and stretch flangeability. If the volume ratio is less than 70%, sufficient ductility and stretch flangeability cannot be obtained. , 97
%, The second phase is hardened and the stretch flangeability decreases. For this reason, the volume ratio of the ferrite phase is set to 70 to 97
%, More preferably in the range of 80 to 97%. The remainder (second phase) needs to be a low-temperature transformation phase mainly composed of a bainite phase. This is because if the second phase has a structure mainly composed of martensite or pearlite, stretch flangeability and formability (ductility) are reduced. Note that the bainite-based structure means that the volume fraction of the bainite phase in the second phase is more than 50%.

Heating of steel material (slab) The heating of the steel material is performed in a temperature range of 900 to 1300 ° C. This is because if the heating temperature is lower than 900 ° C., the surface quality of the steel sheet is degraded, while if it exceeds 1300 ° C., the carbonitride is dissolved or condensed and the ferrite phase becomes coarse. The preferred heating temperature is 1050 to 1200 ° C.

Hot Rolling In hot rolling consisting of rough rolling and finish rolling, finish rolling is performed at a rolling end temperature of 870 to 980 ° C. If the finish temperature of the finish rolling is less than 870 ° C, ferrite transformation from unrecrystallized austenite is likely to occur, and the in-plane anisotropy of the r value increases, resulting in a reduction in stretch flangeability and fatigue properties. . On the other hand, if the temperature exceeds 980 ° C., the structure becomes coarse, ferrite transformation is delayed, and the formability decreases. The reason why the rolling reduction in the final stand of continuous hot rolling is set to less than 20% is that if the rolling reduction in the final stand is too large, ferrite from unrecrystallized austenite will be removed, as in the case of rolling at a relatively low temperature. This is because transformation tends to occur and the in-plane anisotropy of the r value increases. However, if the rolling reduction in the final stand is too low, the structure becomes mixed and the material tends to fluctuate. Therefore, the rolling reduction in the final stand is preferably 10% or more.

Cooling after completion of hot rolling After the completion of the hot rolling, cooling is performed at a cooling rate of 50 to 200 ° C / sec. This is because if the cooling rate is less than 50 ° C / sec, the volume fraction of the ferrite phase will exceed 97%, while if it exceeds 200 ° C / sec, the volume fraction of the ferrite phase will be less than 70%. is there. The preferred cooling rate is 60 to 120 ° C / sec.

The coil is wound at a temperature of 300 to 650 ° C. If the winding temperature is lower than 300 ° C., the structure of the second phase is mainly composed of martensite, and if it exceeds 650 ° C., the structure of the second phase is mainly composed of pearlite, and the stretch flangeability and the formability are reduced. The preferred winding temperature is 4
00-450 ° C.

[0026]

EXAMPLES Steels having various chemical compositions shown in Table 1 were melted in a converter to obtain continuous cast slabs. The slab was heated, hot-rolled, cooled under the conditions shown in Table 2, and then wound around a coil to produce a hot-rolled steel sheet having a thickness of 2.3 mm. From the obtained center position in the coil longitudinal direction of the hot-rolled steel sheet coil, JI
S5 test piece was sampled and subjected to a tensile test to determine elongation, yield ratio, and the like.
JIS No. 5 test pieces in each of the directions of 45 °, 45 ° and 90 ° were sampled, and the r value and Δr were measured. Here, the r value and Δr are respectively obtained by the following equations. r = (r ₀ + r ₉₀ + 2r ₄₅ ) / 4 Δr = (r ₀ + r ₉₀ −2r ₄₅ ) / 2 Here, r ₀ , r ₉₀ , and r ₄₅ are each 0 in the rolling direction.
R values in the directions of °, 90 °, and 45 °. In addition, usually, the r value is measured by measuring the sheet width after tensile strain is applied at 15%. However, since the material of the present invention has a relatively small uniform elongation, the material is stretched by ２〜 to ３ of the uniform elongation. The r value was determined by measuring the width and elongation of the sheet after the heat treatment.

Further, a test piece was sampled from the same position as in the tensile test, and a hole expansion test was performed to evaluate formability. The hole expansion test was calculated based on the Japan Iron and Steel Federation Standard JFST1001-1996. Notched fatigue test, punching clearance 10% of the shape shown in FIG. 2, by using a test piece having a pierced hole of 10 mm [phi, performs plane bending fatigue test of Schenk, and the notch fatigue strength sought 10 ⁷ times fatigue limit strength did. Furthermore, a specimen for microstructure observation was sampled from the center position in the coil longitudinal direction, and the cross-sectional microstructure in the rolling direction was observed with an optical microscope.
The volume fraction of the ferrite phase at the center in the thickness direction was determined. The volume fraction of the ferrite phase is obtained by calculating the number and average diameter of the ferrite phase and the remaining phase by image processing, converting the average diameter into a three-dimensional diameter by the following formula, and calculating the number of the ferrite phase and the remaining phase and the average three-dimensional. The volume ratio was determined from the diameter. The structure of the remaining phase was examined with an electron microscope. D = 1.128 L where D: average diameter (two-dimensional), L: average three-dimensional diameter

Table 3 summarizes the results of these tests.
From these results, each of the invention examples of Nos. 1 to 16, 19, 22, 25, 27, and 29 has stretch formability and shape freezing property, and particularly has excellent stretch flangeability and notch fatigue properties. You can see that. On the other hand, in Nos. 17 and 18, since the amount of Ti added was large, the ferrite volume ratio was small, and the elongation, hole expandability, and notch fatigue properties were poor. No. 20 has a large rolling reduction in the final pass, and No. 21 has a low rolling end temperature, so that Δr
Is large, and the hole expansion ratio and the notch fatigue characteristics are inferior. N
In o.23, since the cooling rate is low, the volume ratio of ferrite is too large, and the hole expandability and notch fatigue properties are poor. Also,
In No. 24, since the cooling rate was too high, the volume ratio of ferrite was too small, and the hole expandability, notch fatigue properties, and elongation properties were inferior. Also, No. 26 has a small amount of Ti added,
Poor elongation characteristics, hole expansion ratio, notch fatigue strength. No.
In Nos. 28 and 30, since the winding temperature is not within the range of the present invention, the second phase has a structure mainly composed of martensite and pearlite, and is inferior in elongation characteristics and hole expansion ratio.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

As described above, according to the present invention,
By optimizing the conditions of the chemical composition of the steel sheet, hot rolling, and subsequent cooling and winding, in addition to stretch formability and shape freezing properties, it has high formability with excellent stretch flange formability and notch fatigue properties. It is possible to provide a tension hot-rolled steel sheet. Further, according to the present invention, the need for strict control of the manufacturing conditions is eliminated, and a steel sheet having the above-mentioned material can be manufactured stably with high productivity.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of Δr on hole expansion characteristics and notch fatigue characteristics.

FIG. 2 is a view showing a test piece shape used for a notch fatigue test.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term in Technical Research Laboratory, Kawasaki Steel Co., Ltd. 4K037 EA02 EA05 EA09 EA15 EA16 EA19 EA23 EA25 EA27 EA28 EA31 EA32 EA36 EB05 EB08 EB09 EB11 FA01 FA02 FA03 FB08 FC04 FD04 FE01 FE02 HA03 HA04 JA06

Claims (5)

[Claims] 1. C: 0.010 to 0.10 wt%, Si: 0.50 to 1.50 wt%, Mn: 0.50 to 2.50 wt%, P: 0.05 wt% or less, S: 0.005 wt% or less, Ti: 0.005 to 0.03 wt% The balance is composed of the components of Fe and unavoidable impurities. The ferrite phase has a volume fraction of 70 to 97%, and the remainder has a metal structure composed of a low-temperature transformation phase mainly composed of a bainite phase. A high-strength hot-rolled steel sheet having excellent stretch flangeability and fatigue properties, wherein the absolute value of the in-plane anisotropy Δr is 0.2 or less. 2. C: 0.010 to 0.10 wt%, Si: 0.50 to 1.50 wt%, Mn: 0.50 to 2.50 wt%, P: 0.05 wt% or less, S: 0.005 wt% or less, Ti: 0.005 to 0.03 wt% And one or two of Ca: 0.0005 to 0.01 wt% and REM: 0.0005 to 0.1 wt%, and the balance is composed of Fe and unavoidable impurities. The ferrite phase and the remainder have a metal structure composed of a low-temperature transformation phase mainly composed of a bainite phase, and the absolute value of the in-plane anisotropy Δr of the r value is
A high-strength hot-rolled steel sheet having excellent stretch flangeability and fatigue properties, characterized by being 0.2 or less. 3. The steel sheet according to claim 1, wherein
A high-strength hot-rolled steel sheet having excellent stretch flangeability and fatigue properties, characterized by further containing one or two of V: 0.003 to 0.02 wt% and Nb: 0.003 to 0.02 wt% in addition to the above components. . 4. The steel sheet according to claim 1, further comprising, in addition to the above components, B: 0.0005 to 0.005 wt%, and stretch flangeability and fatigue properties. High-strength hot-rolled steel sheet with excellent resistance. 5. C: 0.010 to 0.10 wt%, Si: 0.50 to 1.50 wt%, Mn: 0.50 to 2.50 wt%, P: 0.05 wt% or less, S: 0.005 wt% or less, Ti: 0.005 to 0.03 wt% Is maintained in a temperature range of 900 to 1300 ° C, and then subjected to continuous hot rolling at a final stand with a rolling reduction of less than 20% and a rolling end temperature of 870 to 980 ° C. Cool at a cooling rate of 50 to 200 ° C / sec.
A method for producing a high-strength hot-rolled steel sheet having excellent stretch flangeability and fatigue properties, characterized in that it is wound around a coil in a temperature range of up to 650 ° C.