JP2003253385A - Cold-rolled steel sheet superior in high-velocity deformation characteristic and bending characteristic, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet superior in high-velocity deformation characteristics and bending characteristics, which has both such excellent high-velocity deformation characteristics as to have large absorbing energy when deformed at high velocity up to a strain amount of 0.1%, and excellent bending characteristics, and to provide a manufacturing method therefor. <P>SOLUTION: This manufacturing method is characterized by cold-rolling a hot-rolled steel sheet which comprises 0.05-0.5% C, 2.5% or less Si, 0.5-3.5% Mn, 0.015% or less P, 0.01% or less S, 0.04% or less Al, and the balance Fe with unavoidable impurities, in a particular condition, and subsequently heat- treating it in a particular condition into a steel sheet having a structure in which old γ grains have diameters of 20 μm or less, and two of 30% or more bainite and 10% or more tempered martensite occupy 90% or more in total. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet excellent in high-speed deformation characteristics and bending characteristics and a manufacturing method thereof. The cold-rolled steel sheet is preferably applied to parts that require high-speed deformation characteristics, such as automobile bumpers and door reinforcements. "Bainite" in the present invention means 300 to 600
It means a structure of ferrite transformed from austenite at ℃, that is, bainitic ferrite or a mixture of bainitic ferrite and carbide. Further, "tempered martensite", in addition to what was heated and tempered what was martensite at room temperature, so-called what was tempered by room temperature after transformation from austenite to martensite during cooling. "Auto tempered martensi
te ”is also included.

[0002]

2. Description of the Related Art Cold-rolled steel sheets (hereinafter also simply referred to as steel sheets) used for parts that require high-speed deformation characteristics, such as automobile bumpers and door reinforcements, are conventionally subjected to normal tensile tests. High strength (tensile strength (: TS):
980MPa or more) is required, and for steel plates of this strength level, the high-speed deformation characteristics that are important for evaluating the collision characteristics of automobiles are hardly known.

On the other hand, the reduction of the weight of the vehicle body is an extremely important subject in relation to the regulation of exhaust gas due to the recent global environmental problems. At the same time, ensuring the safety of passengers and passengers is an equally important issue. In order to solve these problems at the same time, not only the strength of the steel sheet to be used is increased, but also improvement in rigidity and collision resistance is aimed at by devising the shape of parts. The material used for this improvement is required to be a steel sheet having both the former formability and high-speed deformation characteristics.

In response to this demand, for example, Japanese Patent Laid-Open No. 10-19
Japanese Patent No. 5588 discloses a technique for manufacturing a steel sheet having a high absorbed energy up to a true strain of 0.3. However, as the above-mentioned parts, those that largely deform at the time of collision and absorb the energy of the collision are not preferable from the viewpoint of ensuring safety of passengers and passengers, and high-speed deformation characteristics with a small amount of deformation are important. It is hard to say that the disclosed technology is sufficient. Regarding the moldability, there is a technique disclosed in JP-A-60-100630, but no study has been made on high-speed deformation characteristics.

[0005]

As described above, the conventional techniques do not provide a cold rolled steel sheet having both sufficient formability and high-speed deformation characteristics. In view of such a background, the present invention provides a high-speed deformation property and a bending property that combine excellent high-speed deformation property that the absorbed energy at the time of high-speed deformation up to a strain amount (: true strain) of 0.1 and excellent bending property are combined. It was an object to provide an excellent cold rolled steel sheet together with its manufacturing method.

[0006]

Means for Solving the Problems As a result of intensive studies for solving the above-mentioned problems, the present inventors have found that, with regard to the structure of a steel sheet, the former γ (γ: austenite) grain size, bainite, and tempered martensite. We obtained a new finding that by precisely controlling the site fraction within a specific range, the absorbed energy during high-speed deformation up to a strain of 0.1 is increased and the bending properties are also improved.

The present invention was made based on this finding, and the gist thereof is as follows. [1] C: 0.05 to 0.5% by mass%, Si: 2.5% or less, M
n: 0.5 to 3.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.04% or less, the balance: composition consisting of Fe and inevitable impurities, and the old γ particle size is 20 μm or less, A cold rolled steel sheet excellent in high-speed deformation characteristics and bending characteristics, which has a structure containing bainite of 30% or more and tempered martensite of 10% or more in terms of phase fraction in a total of 90% or more of these two types.

[2] If the composition is further mass%, Cr: 1.5
% Or less, Mo: 1.5% or less, B: 0.01% or less, one or two
The cold-rolled steel sheet according to [1], which comprises at least one kind. [3] If the composition is further mass%, Ti: 0.3% or less, Nb:
One or more of 0.3% or less, V: 0.2% or less, and REM: 0.01% or less are included [1] or
The cold rolled steel sheet according to [2].

[4] C: 0.05 to 0.5% by mass%, Si: 2.5
% Or less, Mn: 0.5 to 3.5%, P: 0.015% or less, S:
0.01% or less, Al: 0.04% or less, or
Cr: 1.5% or less, Mo: 1.5% or less, B: 0.01% or less 1
One or more, and / or Ti: 0.3% or less,
Nb: 0.3% or less, V: 0.2% or less, REM: 0.01% or less, a hot rolled steel sheet having a composition containing one or more
A method for producing a cold-rolled steel sheet excellent in high-speed deformation characteristics and bending characteristics, which comprises cold-rolling at a rolling reduction of not less than%, and treating the obtained cold-rolled steel sheet in the following steps 1 to 4 in this order.

Step 1: After heating to the ferrite-austenite two-phase region or the austenite single-phase region with a ferrite fraction of 10% or less,
Hold for more than 30 seconds 2: Cooling rate of 10 ℃ / s or more up to temperature of 300-600 ℃ 3: Hold for 10-600 seconds at temperature of 300-600 ℃ 4: Cooling rate of less than 30 ℃ / s At room temperature or at a cooling rate of 30 ° C / s or more to 200 ° C or less and then 150
10 seconds to 10 hours held at a temperature of ° C. to Ac ₁ point

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the steel composition in the present invention will be explained. C: 0.05 to 0.5% C needs to be 0.05% or more in order to set TS to 980 MPa or more. However, if C exceeds 0.5%, the weldability deteriorates significantly and it cannot withstand actual use, so the upper limit is made 0.5%.
The content is preferably 0.08 to 0.3%.

Si: 2.5% or less Si is added to facilitate the formation of bainite.
This effect is observed at 0.05% or more, but if it is added in a large amount, the cold rolling property deteriorates, so the upper limit is made 2.5%.
It is preferably 0.3 to 2.0%. Mn: 0.5-3.5% Mn is added to increase strength, and TS ≧ 98 in the present invention.
0.5% or more is contained to achieve 0 MPa. However, excessive addition significantly reduces weldability, so the upper limit is 3.5.
%. It is preferably 1.5 to 3.0%.

P: 0.015% or less P segregates at the old γ grain boundaries to deteriorate the low temperature toughness of the steel, and since it tends to segregate in the steel, it increases the anisotropy of the steel sheet and reduces the workability. . Therefore, it is preferable to reduce it as much as possible, but in the present invention, up to 0.015% is allowed. S: 0.01% or less S is preferable to be reduced as much as possible because S segregates at the old γ grain boundary or a large amount of MnS is generated, which deteriorates low temperature toughness and makes it difficult to use in cold regions. To be done. It is preferably 0.003% or less.

Al: 0.04% or less Al is an element that is added as a deoxidizing agent for steel and is effective for improving the cleanliness of steel, and is also an element that is desirable for refining the structure of steel. . 0.001 for this effect
% Or more is preferably contained. However, if it exceeds 0.04%, a large amount of inclusions are generated, which may cause a flaw in the cold-rolled steel sheet, so the upper limit was made 0.04%.

Cr: 1.5% or less, Mo: 1.5% or less, B: 0.
One or two or more of 01% or less of Cr, Mo, and B can be contained in order to improve hardenability and easily obtain bainite and martensite. In order to exert this effect, Cr, Mo and B are respectively Cr: 0.2% or more, Mo: 0.2% or more, B: 0.0005%.
It is preferable to contain the above. On the other hand, when it is contained in a large amount, Cr has a problem that it deteriorates the coating property of electrodeposition coating performed after molding into parts, and Mo has a problem that it makes cold rolling difficult, and B precipitates as a nitride or the like. , The effect is saturated. Therefore, the upper limits of the contents of Cr, Mo and B are Cr: 1.
5%, Mo: 1.5%, B: 0.01%. In addition, Cr, Mo,
The preferable content range of each B is Cr: 0.2 to 1.0
%, Mo: 0.2 to 1.0%, B: 0.0005 to 0.005%.

Ti: 0.3% or less, Nb: 0.3% or less, V: 0.
2% or less, REM: 0.01% or less, one or more of Ti, Nb, V, and REM can be contained in order to refine the old γ grain size. In order to exert this effect, Ti, Nb,
V and REM are Ti: 0.005% or more and Nb: 0.005%, respectively.
As described above, it is preferable to contain V: 0.005% or more and REM: 0.001% or more. On the other hand, the effect is saturated even if a large amount is contained. Therefore, the upper limits of the Ti, Nb, V, and REM contents are Ti: 0.3%, Nb: 0.3%, V: 0.2%,
REM: 0.01%. The preferable content ranges of Ti, Nb, V and REM are Ti: 0.005 to 0.2% and Nb: 0.
005 to 0.2%, V: 0.005 to 0.15%, REM: 0.001 to 0.
It is 007%.

Next, the reasons for limiting the structure of the steel sheet will be described. Old γ grain size: 20 μm or less The smaller the old γ grain size, the larger the absorbed energy during high-speed deformation, but the effect manifests from 20 μm, so it was set to 20 μm or less. It is preferably 0.1 μm or less.

Phase fraction bainite: 30% or more and tempered martensite: 10% or more Bainite less than 30% or tempered martensite
Since the absorbed energy during high-speed deformation is low for structures less than 10%, the lower limit is 30% bainite, tempered martensite.
It was set to 10%. Bainite is preferably 50% or more. Sum of bainite and tempered martensite by phase fraction:
If the total of bainite and tempered martensite is 90% or more and the total is less than 90%, the bending property deteriorates, so the lower limit of the total phase fraction of both structures was set to 90%. It is preferably 95% or more.

Next, the reasons for limiting the conditions relating to the manufacturing method will be described. In the present invention, a steel material having the above composition is hot-rolled by a generally known method, the hot-rolled steel sheet obtained is cold-rolled under the following conditions, and the obtained cold-rolled steel sheet is subjected to the following steps 1 to 4: In this order, the cold-rolled steel sheet excellent in high-speed deformation property and bending property is manufactured.

Cold rolling at a rolling reduction of 30% or more and cold rolling at a rolling reduction of less than 30% tend to increase the old γ grain size, and a stable old γ grain size of 20 μm or less cannot be obtained. Therefore, cold rolling is performed at a reduction rate of 30% or more. Step 1: Heating in the ferrite-austenite two-phase (: α + γ) region with a ferrite fraction of 10% or less, that is, in the α + γ region where the ferrite percentage is 10% or more, or in the austenite single phase (: γ) region. After that, it is a necessary condition for obtaining the final structure in which the total of bainite and tempered martensite held for 30 seconds or more becomes 90% or more. The temperature range corresponding to the α + γ range with a ferrite fraction of 10% or less is
A sample obtained by heating the steel to various temperatures in an electric furnace for 600 seconds and then cooling it with water was subjected to mirror polishing, nital etching, and then the structure was observed to determine the ferrite fraction.

Step 2: Cooling up to a temperature of 300 to 600 ° C. at a cooling rate of 10 ° C./s or more. If the cooling rate is less than 10 ° C./s, pearlite may be formed, and the required high-speed deformation characteristics cannot be satisfied. The cooling end temperature in step 2 is set to 300 to 600 ° C. in order to match the holding temperature in the next step 3. Step 3: Hold at a temperature of 300 to 600 ° C. for 10 to 600 seconds This is a necessary condition for obtaining 30% or more of bainite. The holding at a temperature of 350 to 550 ° C. for 30 to 300 seconds is preferable.

Step 4: (4-1 :) Cooling to room temperature at a cooling rate of less than 30 ° C./s After treatment in step 3, cooling to room temperature and then holding treatment at a predetermined temperature as shown in 4-2 If not done, cooling rate
The bending rate deteriorates at 30 ° C / s or more, so the cooling rate is 30
Less than ℃ / s. It is preferably 10 ° C / s or less.

(4-2 :) After cooling at a cooling rate of 30 ° C./s or more to 200 ° C. or less, 150 ° C. to a temperature of ₁ point of Ac for 10 seconds to 10 hours. After the treatment in step 3, the cooling rate is 30 ° C. When cooled to 200 ° C. or lower at / s or more, bending characteristics can be ensured by holding the temperature at 150 ° C. to Ac ₁ point for 10 seconds to 10 hours. If this holding condition is not satisfied, the bending characteristics will deteriorate. The holding here may be performed during cooling to room temperature,
After cooling to room temperature, reheating may be performed. After the holding, it may be cooled to room temperature at an arbitrary cooling rate.

[0024]

EXAMPLES Steels having the compositions shown in Table 1 were melted in a converter and formed into slabs by continuous casting. The slabs were hot-rolled under the conditions shown in Table 2, then cold-rolled, and further heat-treated. This was done to obtain a steel plate having a thickness of 1.6 mm (cold rolled annealed plate). The cooling rate in step 2 is the average cooling rate from the annealing temperature in step 1 to the holding temperature in step 3. The cooling rate of step 4 is the average cooling rate from the holding temperature of step 3 to 200 ° C when the holding treatment is performed in step 4, and the cooling rate is 200 ° C or less. . Process 4
For the cooling rate of, when the holding process is not performed in step 4,
It is the average cooling rate from the holding temperature in step 3 to room temperature.
The Ac ₁ point of the steel sheet used this time was 690 to 710 ° C.

For this steel sheet, the microstructure, tensile properties, absorbed energy during high-speed deformation up to a strain amount of 0.1%, and bending properties were investigated. The absorbed energy during high-speed deformation up to a strain amount of 0.1 is at a strain rate of 2000 / s (Ref.
It was determined by integrating the stress-strain curve obtained by the tensile test by the method shown in 8) in the strain amount range of 0 to 0.1. The bending characteristics were evaluated by a 180 ° U bending test specified in JIS Z 2248 using a JIS No. 3 test piece specified in JIS Z 2204, and evaluated by the minimum radius of curvature without cracking. The old γ particle size is
The old γ grain boundary was revealed using Picral's etchant, and 1.128 times the grain size measured by the cutting method was evaluated as the actual grain size. The fraction of bainite and tempered martensite is 10% of the surface layer of the L section (section parallel to the rolling direction).
With respect to the part excluding, the magnified image of the corrosion-exposed structure was visually classified into each structure, and each area was color-coded and then quantified by image analysis to obtain the area ratio, which was taken as the phase fraction of the structure. Tensile properties are measured using JIS No. 5 test pieces
The method was performed according to JIS Z 2241.

The results are shown in Table 2. In the present invention example, the strain rate
The absorbed energy when deformed to a strain of 0.1 at 2000 / s is 60.
MJ / m ³ or more and a limit bending radius of 2 mm or less, which is much higher than the comparative example.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

According to the present invention, the strain rate is 2000 / s and the strain amount is 0.
A cold-rolled steel sheet that has excellent high-speed deformation characteristics and bending characteristics with absorbed energy of 60 MJ / m ³ or more when deformed up to 1 and a critical bending radius of 2 mm or less is obtained, thus reducing the weight of automobiles (reducing the thickness of parts) ) And improved collision safety, and has an excellent effect of greatly contributing to high performance of automobile bodies.

Continued front page    (72) Inventor Kei Sakata             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. F-term (reference) 4K037 EA01 EA05 EA06 EA11 EA15                       EA16 EA17 EA23 EA25 EA27                       EA28 EA31 EA32 EA36 EB05                       EB09 EB11 EC01 FG01 FH01                       FJ05 FJ06 FK03 FL01 FL02                       FL03

Claims (4)

[Claims] 1. C: 0.05 to 0.5% and Si: 2.5% by mass%
Below, Mn: 0.5-3.5%, P: 0.015% or less, S: 0.01
% Or less, Al: 0.04% or less, balance: Fe and inevitable impurities, old γ grain size is 20 μm or less, phase fraction bainite: 30% or more, tempered martensite: 10 %, Which has a structure containing 90% or more in total of these two types, a cold-rolled steel sheet excellent in high-speed deformation characteristics and bending characteristics. 2. The cold rolling according to claim 1, wherein the composition further contains, in mass%, one or more of Cr: 1.5% or less, Mo: 1.5% or less, and B: 0.01% or less. steel sheet. 3. The composition further comprises, by mass, one or more of Ti: 0.3% or less, Nb: 0.3% or less, V: 0.2% or less, and REM: 0.01% or less. Item 3. The cold rolled steel sheet according to Item 1 or 2. 4. C: 0.05 to 0.5%, Si: 2.5% by mass%
Below, Mn: 0.5-3.5%, P: 0.015% or less, S: 0.01
% Or less, Al: 0.04% or less, or even Cr:
1.5% or less, Mo: 1.5% or less, B: 0.01% or less, one or more, and / or Ti: 0.3% or less, Nb:
Cold-rolled steel sheet obtained by cold rolling a hot-rolled steel sheet having a composition containing one or more of 0.3% or less, V: 0.2% or less, and REM: 0.01% or less at a rolling reduction of 30% or more. Is processed in the following steps 1 to 4 in this order. A method for producing a cold-rolled steel sheet having excellent high-speed deformation characteristics and bending characteristics. Step 1: After heating to a ferrite-austenite two-phase region or austenite single-phase region with a ferrite fraction of 10% or less,
Hold for more than 30 seconds 2: Cooling rate of 10 ℃ / s or more up to temperature of 300-600 ℃ 3: Hold for 10-600 seconds at temperature of 300-600 ℃ 4: Cooling rate of less than 30 ℃ / s At room temperature or at a cooling rate of 30 ° C / s or more to 200 ° C or less and then 150
10 seconds to 10 hours held at a temperature of ° C. to Ac ₁ point