JP2000087144A - Manufacture of high tensile strength hot rolled steel plate with excellent material uniformity and high workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To uniformize a structure before heat treatment and to improve ductility and stretch-flange formability as well as material uniformity in the longitudinal and width directions by hot rolling a steel of specific composition, cooling the resultant steel plate while avoiding the formation of ferrite, and controlling reheating conditions after coiling and cooling conditions, respectively. SOLUTION: A steel composition, where 0.08-0.20%, by mass, C, 0.80-1.80% Si, 1.0-2.0% Mn, and 0.005-0.04% P are incorporated, is provided. This steel is hot rolled at a finish rolling temperature not lower then the Ac3 point, cooled so that the temperature does not come to the ferrite transformation region, and coiled at 400 to 550 deg.C. The resultant steel plate is subjected to reheating consisting of holding at a temperature between the Ac1 and the Ac3 point for >=1 sec and then cooled at >=10 deg.C/sec cooling rate. In the course of this cooling, the steel plate is held at 350 to 500 deg.C for >=30 sec and coiled after cooling. By this method, the high tensile strength hot rolled steel plate of about 590 to 780 MPa class, containing >=5% retained austenite, can be stably obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet having a tensile strength of 590 to 780 MPa for use in structural members of automobiles and the like.
% Or more, and relates to a method for producing a high workability, high tensile strength hot rolled steel sheet having excellent material uniformity in a steel strip.

[0002]

2. Description of the Related Art As a structural member for a vehicle body for the purpose of improving fuel efficiency of an automobile, a 590-780 MPa class high-strength hot-rolled steel sheet excellent in workability that simultaneously satisfies both weight reduction and collision safety is required. In order to improve the workability of a 590-780 MPa class high-tensile hot-rolled steel sheet, it is necessary to improve the balance between strength and ductility and have good stretch flangeability. Conventionally, as a means of manufacturing a high-tensile hot-rolled steel sheet having an excellent balance between strength and ductility, residual austenite is contained, and TR during deformation is reduced.
A method of improving ductility by the IP phenomenon has been proposed.

For example, JP-A-63-4017, JP-A-6-40
4-79345 includes: C: 0.15 to 0.4%, Si:
Containing 0.5 to 2.0%, Mn: 0.5 to 2.0%,
The steel consisting of the balance Fe and inevitable impurities is hot-rolled at a finish rolling temperature of Ar ₃ ± 50 ° C. and a total draft of 80% or more,
A method for producing a high-tensile hot-rolled steel sheet which is cooled at 350 to 500 ° C., contains 5% or more of retained austenite, and the remainder is ferrite and bainite is disclosed. This technology regulates the reduction ratio of hot rolling, finish rolling temperature, runout cooling conditions and winding temperature, and enriches C in austenite by promoting the formation of polygonal ferrite.
Further, C concentration is advanced by bainite transformation to stabilize austenite, and finally, a mixed structure of ferrite and bainite containing 5% or more of retained austenite is obtained.

However, hot-rolled steel sheets containing this type of retained austenite are excellent in workability, but have large fluctuations in mechanical properties in the steel strip, resulting in poor yield. In the case of a 590 MPa class high-strength steel sheet that needs to be within the range of 640 MPa, the fluctuation in the longitudinal direction and the width direction may exceed 100 MPa. Fluctuations in the material are caused by temperature fluctuations in the length and width directions of the steel strip in the hot rolling process and differences in the cooling rate of each part such as the outer circumference and inner circumference after coil winding, resulting in uneven microstructure. It is believed that.

In order to solve such problems, Japanese Patent Application Laid-Open No. Hei 5-271664 discloses that C: 0.10 to 0.25%, Si:
0.8-3.0%, Mn: 1.0-3.0%, S: 0.
005% or less, Al: 0.01 to 0.10%,
A hot-rolled steel sheet comprising the balance of Fe and unavoidable impurities is subjected to the highest temperature: Ac ₁ + (Ac ₃ −Ac ₁ ) / 6 to Ac ₃ −
(Ac ₃ -Ac ₁ ) / 6, reheating for a holding time of 1 sec or more, then cooling at an average cooling rate of 20 ° C./S or more, winding in a temperature range of 350 to 450 ° C., or this temperature range A method for producing a high-strength hot-rolled steel sheet containing retained austenite with a volume ratio of 6% or more by cooling to 50 ° C. or less after winding for 100 seconds or more (hereinafter, Prior Art 1) is disclosed. .

[0006]

However, even in the method according to Prior Art 1, since the maximum heating temperature is limited to the center of the two-phase region of ferrite + austenite, the microstructure of the hot-rolled steel strip before reheating is reduced. If it fluctuates greatly, it is difficult to make it uniform, and there is a problem in material stability. Therefore, in the present invention, the material uniformity in the longitudinal direction and the width direction is extremely excellent, and 590 to 780M containing 5% or more of retained austenite is used.
Provided is a method for producing a Pa-class high-tensile hot-rolled steel sheet.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies on the effects of the steel composition and hot rolling conditions on the material uniformity of a steel strip containing retained austenite, and aimed to form retained austenite. It was found that the above object was achieved when the structure before heat treatment was a uniform bainite single phase structure.

That is, the present invention provides: In mass%, C: 0.08 to 0.20%, Si:
0.80 to 1.80%, Mn: 1.0 to 2.0%, P:
A steel containing 0.005 to 0.04% is hot-rolled at a finish rolling temperature Ar of ₃ points or more, cooled so as not to cover the ferrite transformation region, wound up at 400 to 550 ° C, and then reheated. After holding at a temperature of Ac _{1 to} Ac ₃ for 1 sec or more, while cooling at a cooling rate of 10 ° C./sec or more, holding at a temperature of 350 to 500 ° C. for 30 sec or more, winding, and cooling. This is a method for producing a high workability, high tensile strength hot rolled steel sheet containing 5% or more of retained austenite.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the component composition, hot rolling step and continuous annealing of the present invention will be described in detail. 1. Component composition C enhances the stability of austenite, but if it is less than 0.08%, 5% or more of retained austenite cannot be obtained, and if it exceeds 0.20%, weldability such as spot weldability deteriorates. 08 to 0.20%.

[0010] Si is a ferrite-forming element, promotes the formation of ferrite in cooling after heating in the two-phase region of Ac _{1 to} Ac ₃ in continuous annealing, enriches C in austenite, and further reduces 350% during cooling. The action of delaying the precipitation of cementite from austenite during holding at ~ 500 ° C ensures retained austenite.
If it is less than 0.80%, 5% or more of retained austenite cannot be obtained, and if it exceeds 1.8%, the weldability is deteriorated, and it becomes difficult to perform flash welding at the pickling line entrance side during pickling of steel strip. Therefore, it is set to 0.80 to 1.80%.

Mn enhances the stability of austenite and suppresses pearlite and martensitic transformation in cooling after heating in the two-phase region of Ac _{1 to} Ac ₃ in continuous annealing.
If it is less than 1.0%, 5% or more of retained austenite cannot be obtained. If it exceeds 2.0%, a layered structure is formed due to segregation, and stretch flangeability and bending workability deteriorate. 2.0%.

P, like Si, forms a solid solution in ferrite to increase the strength of the steel sheet and suppresses the generation of red scale due to the addition of Si. However, if less than 0.005%, the effect is not sufficient.
If added in excess of 04%, the segregation deteriorates the stretch flangeability, so the content is 0.005 to 0.04%.
In addition, the effect of increasing the steel sheet strength of P and suppressing the generation of red scale due to the addition of Si becomes significant when added at 0.02% or more, and particularly when the effect is expected, 0.02% or more is added. I do.

In the present invention, in addition to the above-mentioned elements, in order to improve stretch flangeability, 0.1% or less of Ca and 0.1% of Mg are used.
1% or less, REM 0.1% or less, Cr 1.5% or less, Ni 1.5% or less, W1.0% for improving hardenability, etc.
Hereinafter, Mo can be added in a range of 1.5% or less. In addition, for the purpose of strengthening precipitation or preventing HAZ softening of the welded portion, Nb
0.2% or less, Ti 0.2% or less, B0.01% or less,
V 0.2% or less, Zr 0.1% or less, Sn can be added in a range of 0.1% or less and Cu 0.5% or less for the purpose of improving corrosion resistance. In addition, sol. A
l, an element such as S, which is an impurity, may be contained within a range that does not impair the effects of the present invention. In the present invention, the method of smelting steel is not particularly limited, and any method such as a converter method, an electric furnace method, or a combination of both methods may be used.

2. Hot Rolling Step The point of the present invention is to make the structure before continuous annealing a uniform single-phase bainite. The finish rolling of hot rolling is performed in the austenitic single-phase region, and in the subsequent cooling of the run-out table, the nose of ferrite transformation occurs. It is necessary to cool the product and to wind it while avoiding the formation of ferrite. The slab may be formed by a continuous casting method or an ingot forming method, and is not particularly limited.

2.1 The finish rolling temperature is in the austenitic single phase region of Ar ₃ points or more. In the case of a two-phase region having less than the Ar ₃ point, a layered structure is formed, and even if the reheating temperature during the heat treatment in the continuous annealing is increased to just above the Ac ₃ point, the structure does not become sufficiently uniform and the stretch flangeability deteriorates.

2.2 Cooling at the run-out table
It is important to cool without affecting the nose of ferrite transformation and to suppress the formation of proeutectoid ferrite.
Pre-cooling using pre-bank, cooling rate 20 ° C
/ Sec or more.

2.3 Winding is performed while cooling while avoiding the formation of ferrite, and the bainite transformation temperature is 400 to 550.
Perform at ° C. When the winding temperature is 400 ° C. or lower, martensite is generated, and when it is 550 ° C. or higher, pearlite is generated. Therefore, the uniformity after the heat treatment is reduced, and it becomes difficult to obtain 5% or more of retained austenite.

[0018] Under the above conditions, the steel strip has a uniform bainite structure in the longitudinal and width directions in the hot rolling process, a very uniform microstructure is formed after heat treatment in a continuous annealing furnace, and the uniformity of mechanical properties is improved. Achieved.

3. Heat Treatment Conditions in Continuous Annealing Equipment A steel strip having a bainite single phase structure by pre-runout cooling after hot rolling is pickled and then continuously annealed to form a ferrite + bainite + retained austenite structure.

3.1 In the heat treatment, reheating is performed in the temperature range of two phases Ac ₁ to Ac ₃ , and the holding time is set to 1 second or more.
The reheating temperature is Ac ₁ or less not to two-phase region of ferrite + austenite, can not generate the residual austenite. In the case of Ac ₃ or more, there is no problem on the microstructure, but it is not preferable from the viewpoint of energy saving.
Less than ₃ . The holding time is set to 1 sec or more to uniformly heat the entire steel sheet and generate austenite.

3.2 Cooling after reheating is performed at a rate of 10 ° C./sec or more in order to generate residual austenite. After the temperature is maintained in the temperature range of 350 to 500 ° C. for 30 sec or more during the cooling, further cooling and winding are performed. Cooling is preferably performed to 200 ° C. The cooling rate is set at 1 to suppress pearlite transformation during cooling and promote bainite transformation.
0 ° C./sec or more. Austenite is transformed into pearlite when the temperature exceeds 500 ° C., and austenite is transformed into martensite when the temperature is less than 350 ° C., and the volume ratio of retained austenite is 5%.
Since it is difficult to secure the above, the temperature is set to 350 to 500 ° C. If the holding time is less than 30 sec, the bainite transformation does not proceed sufficiently, the concentration of C in the austenite becomes insufficient, and it becomes impossible to secure 5% or more of the retained austenite.

The structure of the hot-rolled steel sheet according to the present invention mainly comprises ferrite and 5% or more of retained austenite, but bainite, martensite and pearlite may be mixed in some cases. In addition, when the shape of the hot-rolled steel strip is poor and skin pass rolling is performed, it is desirable to reduce the rolling reduction as much as possible.

[0023]

EXAMPLES Inventive steels A to F and comparative steels G to M shown in Table 1 were melted in a converter and made into slabs by continuous casting, and then hot-rolled, pickled, heat-treated, and reduced by 1% or less. Skin pass rolling was sequentially performed to produce a hot-rolled steel sheet having a thickness of 1.6 mm. Table 2 shows the conditions of hot rolling and heat treatment and the evaluation results of the steel strip. The steel strip was evaluated by a tensile test, a hole expanding test, and a measurement of a retained austenite volume ratio by X-ray. The tensile test was performed by cutting out a JIS No. 5 test piece over the entire width from the center to the edge in the width direction of the plate, and the change in mechanical properties was evaluated.

Table 2 shows the difference between the maximum value and the minimum value of the yield strength, the tensile strength, the elongation, and the tensile strength in the width direction.
In the hole expansion test, 1 * was placed at the center of a 150 * 150 mm test piece.
A hole of 0 mmΦ was punched out, the burr was formed on the punch side, and this was pushed out with a conical punch with a vertex angle of 60 °, and the hole diameter was measured at the point when a crack penetrating the thickness of the hole edge was formed. The hole spread rate (λ) was determined by the equation. Hole spread rate: λ =
(Df-do) / do * 100 (%) where do: initial hole diameter, df: hole diameter at break The residual austenite volume fraction was measured by X-ray diffraction at a 1/4 plate thickness position.

As shown in Table 2, the present invention example No. 1
Nos. 10 to 10 have a residual austenite of 5% or more, have a good elongation and a good hole spreading rate, and have a very good mechanical property uniformity with a fluctuation in tensile strength in the width direction of 25 MPa or less.

On the other hand, in Comparative Example No. 1 and No. 2 is
Although the composition of the present invention was not subjected to pre-stage cooling by hot rolling, the structure was uneven and the mechanical properties in the width direction varied greatly. Comparative Example No. No. 3 has a non-uniform structure in which ferrite and pearlite are formed in the hot-rolling process because the hot-rolling temperature is high outside the range of the present invention, the hole expansion ratio is low, and the fluctuation of mechanical properties in the width direction is small. It was big. Comparative Example N where reheating temperature in continuous annealing is low outside the range of the present invention
o. 4, Comparative Example No. 4 in which the intermediate holding temperature of the heat treatment was high outside the range of the present invention. Comparative Example No. 5, which is low outside the scope of the present invention. 6
In each case, no residual austenite was obtained, the elongation was low, and sufficient moldability could not be secured.

Comparative Example No. In No. 7, the C content was small outside the range of the present invention, and the amount of retained austenite was insufficient, and good elongation was not obtained. Comparative Example No. In No. 8, the C content was large outside the range of the present invention, retained austenite was formed at 5% or more, and the mechanical properties were almost good, but the spot weldability was poor and not practical. Comparative Example No. In No. 9, the Si content was low outside the range of the present invention, no retained austenite was formed, and the elongation was low.
Comparative Example No. Sample No. 10 had a large amount of Si outside the range of the present invention and a residual austenite amount of 5% or more, but had poor weldability and caused problems in production.

Comparative Example No. No. 11 has a low Mn content outside the range of the present invention, does not yield retained austenite, has low elongation,
Comparative Example No. In No. 12, the Mn content was high outside the range of the present invention, a strong layered structure was formed, and the hole spreading rate was low. Comparative Example N
o. In No. 13, the P content was high outside the range of the present invention, and the hole spread rate was low due to the segregation of P. Comparative Example No. In No. 14, since the heat treatment in the continuous annealing furnace was not performed, the material difference between the central portion and the edge portion of the steel strip became very large.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

As described above, according to the present invention,
Effective for weight reduction of various structures such as automobiles, contains 5% or more of retained austenite, has good ductility and stretch flangeability, and has extremely high material uniformity in a steel strip of 590 to 780M.
It is possible to stably produce a Pa-class high-tensile hot-rolled steel sheet, which is extremely useful industrially.

Continuation of the front page (72) Inventor Akihide Yoshitake 1-2-1, Marunouchi, Chiyoda-ku, Tokyo F-term in Nihon Kokan Co., Ltd. 4K037 EA05 EA06 EA15 EA23 EA27 EA28 EB11 FC07 FE01 FF02 FF03

Claims (1)

[Claims] 1. A mass% of C: 0.08 to 0.20%,
Si: 0.80 to 1.80%, Mn: 1.0 to 2.0
%, P: steel containing 0.005 to 0.04% is hot-rolled at a finish rolling temperature Ar of ₃ or more points, cooled so as not to cover the ferrite transformation region, and wound at 400 to 550 ° C. After reheating at a temperature of Ac ₁ to Ac ₃ for 1 sec or more, cooling is performed at a cooling rate of 10 ° C./sec or more.
A method for producing a high workability, high tensile strength hot-rolled steel sheet containing 5% or more of retained austenite, wherein the hot-rolled steel sheet is retained for at least 0 sec and further cooled and then taken up.