JPH10195588A - Hot rolled high tensile strength steel plate excellent in formability and collision resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a thin hot rolled high tensile strength steel sheet having the characteristics of 490 to 980MPa tensile strength, <=75% yield ratio, >=25% total elongation and >=250MJ/m<3> absorbed energy to 0.3 true strain in tensile testing at a strain rate of 2000sec<-1> and excellent in formability and collision resistance. SOLUTION: As for a steel stock having a componental compsn. contg., by weight, 0.02 to 0.2% C, 0.1 to 1.5% Si, 0.5 to 3.0% Mn and 0.010% S, furthermore contg. one or >= two kinds selected from 0.03 to 0.15% Mn, 0.1 to 2.0% Cr and 0.1 to 1.0% Mo, and the balance Fe with inevitable impurities, hot rolling is finished at the Ar3 transformation point or above, within 0.1 to 5.0sec after that, cooling is started, it is cooled to 620 to 800 deg.C at a cooling rate of >=50 deg.C/sec, is subjected to air cooling for 0.5 to 15sec, is next cooled to 300 to 600 deg.C at a cooling rate of >=30 deg.C/sec and is coiled to form its structure into the one in which ferritic phases having <=10μm average grain size occupy by 80 to 97vol.%, and the balance secondary phases essentially consisting of martensite in which the average diameter is regulated to 0.2 to 2.0 times the ferrite average grain size.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is mainly used as a material for a part of an automobile which requires high strength and high formability. The present invention relates to a hot-rolled high-strength steel sheet suitable for use as a material of a part where impact resistance (hereinafter simply referred to as “impact resistance”) is required, and a method for producing the same.

[0002]

2. Description of the Related Art Recently, momentum for global environmental protection has been increasing.
Against the background, CO from automobiles _TwoReduction of emissions
It has been demanded. CO_TwoSpecific measures to reduce emissions
Therefore, it is effective to measure the weight of the car body,
The effective method is to reduce the thickness of the steel sheet by increasing its strength.
It is believed that there is. In addition, recent car body installations
Based on the idea, it is not only a simple strengthening of steel sheet,
Excellent impact resistance in case of collision during running
Absorbed energy when deformed at high strain rate for steel plate
Development of high-quality steel sheets leads to improved vehicle safety
And also contributes effectively to reducing the weight of the body
It is attracting attention. On the other hand, cost reduction of recent members
The cold-rolled steel sheet used conventionally
To adopt a hot-rolled steel sheet with a thickness of 3.0 mm or less.
The motivation to do so is increasing. Is this a recent situation?
From the viewpoint of improving vehicle safety and reducing costs,
Development of hot-rolled high-strength steel sheets with excellent collision properties is eagerly awaited.
You.

[0003] Conventionally, in the ferrite single-phase structure, solid-solution strengthening by adding a substitution element such as Si, Mn or P, or carbonitride forming element such as Nb or Ti has been conventionally performed in a ferrite single-phase structure. In general, a method by precipitation strengthening by the addition of chromium is used. For example, in JP-A-56-139654, etc., ultra-low carbon steel is made to contain Ti and Nb in order to improve workability and aging property, and further contains a reinforcing component such as P as far as the workability is not impaired. We have proposed a steel sheet with high strength. Also, for example, see JP-A-59-19
Japanese Patent No. 3221 proposes a method for increasing the strength of ultra-low carbon steel by adding Si. In addition, JP-A-60-5
No. 2528 proposes a method for producing a high-strength steel sheet having excellent ductility by annealing a low-carbon steel at a high temperature and precipitating a martensite phase after cooling.

[0004] Such a dual phase steel sheet of ferrite and martensite is generally subjected to hot rolling at a temperature higher than the Ar ₃ transformation point, rapidly cooled to a temperature at which the ferrite precipitates, and then air-cooled to sufficiently reduce the ferrite. It is produced by precipitating, enriching carbon in the remaining austenite phase, transforming it into martensite after winding, and has excellent moldability.

[0005]

However, the sheet thickness is 2.6 mm.
When thinner extent below the temperature lowering rate of the steel sheet is increased, Ar ₃ hot rolling finishing temperature over coil entire length
In order to maintain the temperature above the transformation point, the rolling speed usually had to be increased. For this reason, even if the thin hot-rolled steel sheet is manufactured by a conventional technique, the air cooling time for precipitating ferrite becomes short, the carbon concentration in austenite becomes insufficient, and austenite becomes bainite after winding. Therefore, there was a problem that good strength-elongation balance could not be obtained.

[0006] Further, a steel sheet whose strength has been increased by the conventionally proposed method as described above, although the thickness of the automobile body can be reduced to some extent, the above-mentioned impact resistance characteristic is essentially maintained. It did not improve. Because
In these proposals, the yield strength or tensile strength, which is an index of steel sheet strength, is determined based on only a so-called static evaluation method in which the strain rate is extremely low, such as 10 ^-3 to 10 ^-2 (sec ^-1 ). . In contrast, in the actual automobile body design, rather than such a static strength, considering safety upon collision, strain rate shock deformation of 10~10 ⁴ (sec ^-1) This is because the strength based on the so-called dynamic evaluation method is more important. Therefore, the conventional proposals described above, which are developed focusing only on the static strength, have a problem that they cannot be used as a fundamental index for reducing the weight of an automobile body.

[0007] Japanese Patent Application Laid-Open No. 7-90482 proposes a two-phase structure steel sheet of martensite and ferrite for the purpose of improving the impact resistance of the steel sheet. However, although the ferritic and martensitic dual phase steel sheets have relatively good impact resistance, they have not yet met the higher levels of properties required by today's automakers. It is the current situation.

Accordingly, an object of the present invention is to provide a hot-rolled high-strength steel sheet having more excellent formability and impact resistance than conventional hot-rolled steel sheets. A specific object of the present invention is to provide a tensile strength of 490 to 980 MPa, a yield ratio (= yield strength /
Tensile strength) 75% or less, total elongation 25% or more, strain rate 20
It is an object of the present invention to provide a thin hot-rolled high-strength steel sheet excellent in formability and impact resistance, having a property of absorbing energy up to a true strain of 0.3 in a tensile test of 00 sec ^-1 and not less than 250 MJ / m ³ . Another object of the present invention is to provide a production method for stably obtaining a hot-rolled high-tensile steel sheet having the above characteristics up to a sheet thickness of 0.8 mm.

[0009]

Means for Solving the Problems The inventors of the present invention based on a two-phase structure steel consisting of ferrite and martensite,
As a result of intensive research to realize the above objectives,
By appropriately controlling the production conditions such as hot rolling, cooling after rolling, and winding in addition to the chemical composition, it is possible to produce hot-rolled high-tensile steel sheets with much better formability and impact resistance than before. And these properties can be adjusted to a thickness of 0.8
The present inventor has found that it is possible to stably obtain a thin material having a thickness of not less than mm, and arrived at the present invention. That is, the present invention has the following content as a gist configuration.

(1) C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.03 to 0.15 wt%, Cr: 0.1 to 2.0 wt%, Mo: 0.1 to 1.0 wt%, one or more selected from the group consisting of Fe and unavoidable impurities, the balance being a ferrite phase having an average particle size of 10 μm or less having a volume fraction of 80 to 80%. Hot rolling high tensile strength with excellent formability and impact resistance characterized by 97% occupation, the balance consisting of a second phase composed mainly of martensite with an average diameter of 0.2 to 1.5 times the average diameter of ferrite. steel sheet.

(2) C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.03 to 0.15 wt%, Cr: 0.1 ~2.0 wt%, Mo: 0.1 contain one or more selected from to 1.0 wt%, the balance being a steel material to become component composition of Fe and unavoidable impurities, hot at Ar ₃ transformation point or more After the rolling is completed, cooling is started between 0.1 and 5.0 seconds, and the cooling is started at a cooling rate of 50 ° C./sec or more.
Cool to ~ 800 ° C, air-cool for 0.5-15 seconds, then 30
At a cooling rate of at least 300 ° C / sec.
A method for producing a hot-rolled high-tensile steel sheet having excellent formability and impact resistance, characterized by winding.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the material properties targeted by the present invention will be described. -Tensile strength: 490-980 MPa, yield ratio (= yield strength / tensile strength): 75% or less, total elongation: 25% or more When tensile strength is 490MPa or more, and the yield ratio exceeds 75%, Since the springback at the time of press molding becomes large and a stable pressed product cannot be manufactured, the yield ratio needs to be 75% or less. Further, if the total elongation of the steel sheet is less than 25%, cracks are likely to occur during press forming, so the total elongation needs to be 25% or more. The reason why the upper limit of the tensile strength is set to 980 MPa is that if the tensile strength exceeds 980 MPa, it becomes difficult for the steel plate to absorb the collision energy at the time of a vehicle collision, and a large impact is transmitted to the passengers in the cabin.

In a tensile test at a strain rate of 2000 (sec ^-1 ), the true strain
Absorbed energy to 0.3: 250 MJ / m ³ strain rate of deformation over time vehicle collision member reaches 2000 (sec ^-1), the amount of deformation as an index of absorbed energy collision characteristic up 0.3 true strain is important. As a result of research, even if the strength of the steel sheet could be reduced, it was possible to reduce the thickness of automotive steel sheet, but the absorbed energy above was 250 MJ / m.
^If it is less than ³ , it was found that sufficient collision resistance could not be obtained in an automobile collision test. Therefore, the absorbed energy under the above conditions needs to be 250 MJ / m ³ or more.

Next, in the present invention, the chemical components of steel
The reason why the organization, manufacturing conditions, and the like are limited according to the gist configuration will be described. C: 0.02 to 0.2 wt% C is a component necessary for increasing the strength and volume fraction of martensite in the two-phase structure. If the C content is less than 0.02 wt%, a sufficient amount of carbide and a second phase mainly composed of martensite cannot be obtained. On the other hand, when the content exceeds 0.2 wt%, solid solution C is present in the ferrite, which impairs formability. Therefore,
The content of C is set to 0.02 to 0.2 wt%.

Si: 0.1-1.5 wt% Si improves the hardenability of steel by concentrating solid solution C in ferrite in austenite, and improves the formability of steel sheet by increasing the purity of ferrite. Has the effect of causing. This effect appears when added in an amount of 0.1 wt% or more, but when the content exceeds 1.5 wt%, the surface properties and surface treatment properties of the hot-rolled sheet are significantly deteriorated. Therefore, the content of Si is 0.1-1.5 wt%, preferably 0.3-1.
2 wt%.

Mn: 0.5 to 3.0 wt% Mn is an austenite-stabilizing element. If it is less than 0.5 wt%, hardenability decreases and it is difficult to obtain a two-phase structure. On the other hand, when the content exceeds 3.0 wt%, the steel sheet hardens, and the formability decreases. Therefore, the Mn content is 0.5-3.0 wt%, preferably 0.7-2.0 wt%.

S: 0.010 wt% or less S reduces the amount of S, thereby reducing precipitates in steel and contributing to improvement of workability. These effects are
It can be obtained by limiting the amount of S to 0.010 wt% or less.

P: 0.03 to 0.15 wt% P has the effect of reducing the critical cooling rate of martensite formation, but this effect appears when 0.03 wt% or more is added. On the other hand, if the content exceeds 0.15 wt%, the steel sheet is hardened, the formability is reduced, and the surface treatment property is also deteriorated. Therefore, the content of P is set to 0.03 to 0.15 wt%.

Cr: 0.1-2.0 wt% Cr has the effect of reducing the critical cooling rate of martensite formation, but its effect appears when added at 0.1 wt% or more. On the other hand, if the content exceeds 2.0 wt%, the effect is saturated and the production cost increases. Therefore, the content of Cr is set to 0.1 to 2.0 wt%.

Mo: 0.1 to 1.0 wt% Mo has the effect of reducing the critical cooling rate of martensite formation, but its effect appears when 0.1 wt% or more is added. On the other hand, if the content exceeds 1.0 wt%, the effect is saturated and the production cost is increased. Therefore, the content of Mo is set to 0.1 to 1.0 wt%.

As described above, the hot-rolled high-strength steel sheet of the present invention has a two-phase structure composed of a second phase mainly composed of ferrite and martensite.
The following ferrite phase occupies 80 to 97% by volume, and the remainder needs to be a second phase mainly composed of martensite having an average diameter of 0.2 to 1.5 times the average ferrite particle size. This is because if the volume fraction of the ferrite phase is less than 80%, the hard second phase increases, so that the yield ratio of 75% or less cannot be achieved. This is because, as shown in FIG. 1, the absorbed energy during high-speed deformation is reduced. The second phase must be mainly martensite, at least 50% of the second phase
Must be martensite. This is because when the amount of austenite or bainite in the second phase increases and the amount of martensite is less than 50%, the absorbed energy during high-speed deformation decreases. Further, the reason why the average particle diameter of the ferrite phase is 10 μm or less and the average diameter of the martensite phase as the second phase is 0.2 to 1.5 times the average particle diameter of the ferrite is out of this range, as shown in FIG. This is because the absorbed energy during high-speed deformation decreases.

The hot-rolled high-strength steel sheet according to the present invention is obtained by heating a steel material (slab) by a conventional method, performing continuous hot rolling including rough rolling and finish rolling, winding up a coil, and, if necessary, adding acid. It is produced by descaling by washing. In these manufacturing processes, in the present invention, in particular,
It is important to control the cooling conditions after hot rolling until winding. That is, after finishing hot rolling at the Ar ₃ transformation point or higher,
Then, between 0.1 and 5.0 seconds, start cooling, and
c Cool to 620 to 800 ° C at the above cooling rate (first forced cooling), then air-cool for 0.5 to 15 seconds, then 30 ° C / se
c At the above cooling rate, it is necessary to cool to 300-600 ° C (secondary forced cooling) and wind up. The reason will be described below.

Hot rolling end temperature: above the Ar ₃ transformation point If hot rolling is performed below the Ar ₃ transformation point, strain is accumulated in the ferrite in the hot-rolled sheet and the formability is significantly reduced.

Cooling and winding after hot rolling Between 0.1 and 5.0 seconds after hot rolling, cooling at 50 ° C./sec or more (hereinafter referred to as “primary forced cooling”) is started by hot rolling. If the elapsed time from the end of rolling to cooling is less than 0.1 second, it is difficult to control the temperature at the end of rolling, while if it exceeds 5.0 seconds,
The coarsening of austenite grains causes a delay in ferrite transformation, thereby inhibiting the concentration of carbon in austenite and transforming the second phase into pearlite or bainite, leading to deterioration in formability and impact resistance. is there. On the other hand, if the cooling rate is less than 50 ° C./sec, the nucleation rate of ferrite grains decreases, ferrite transformation is delayed, and the enrichment of carbon in austenite is inhibited. This is because bainite transformation is caused, and the formability and the impact resistance are reduced. In particular, in order to obtain a stable material with a thin hot-rolled steel sheet, it is most effective to increase the cooling rate in this temperature range.

The primary forced cooling to 620 to 800 ° C. is performed when the temperature at the end of cooling exceeds 800 ° C. because the transformation speed of ferrite is slow, so that the second phase having martensite as the main phase is used. Is not obtained, and the moldability deteriorates. Also, if the temperature at the end of cooling is less than 620 ° C,
This is because pearlite transformation starts from austenite, and a second phase having martensite as a main phase cannot be obtained, resulting in deterioration in moldability and impact resistance.

Next, the air cooling for 0.5 to 15 seconds is performed if the air cooling time is less than 0.5 second because the time for ferrite transformation is short, so that the second phase having martensite as the main phase cannot be obtained and the formability is deteriorated. On the other hand, if it exceeds 15 seconds, the ferrite grains are coarsened and the collision resistance is reduced.

The secondary forced cooling at a rate of 30 ° C./sec or more to 300 to 600 ° C. with the above air cooling interposed therebetween is performed at a cooling rate of less than 30 ° C./sec or a cooling stop temperature exceeding 600 ° C. This is because austenite starts pearlite transformation or bainite transformation, and the second phase of martensite as a main phase cannot be obtained, resulting in deterioration of formability. Also, if the cooling stop temperature is lower than 300 ° C., solid solution C remains in the ferrite and elongation is deteriorated.

In the above description, the case where the present invention is used exclusively for automobiles has been described. However, the technique according to the present invention is similarly effective for other uses which require strength under a high strain rate. Needless to say.

[0029]

EXAMPLES Steel having the chemical composition shown in Table 1 was melted in a converter. These slabs were hot-rolled and cooled under the conditions shown in Table 1 and then wound around a coil to obtain a thickness of 1.0 to 3.0 mm.
Of hot-rolled high-strength steel sheets. About the obtained steel plate,
The test material was sampled from the center position in the longitudinal direction of the coil, the constitutional structure and the crystal grain size were examined by an optical microscope, and the volume fraction of the second phase was determined. The volume ratio of the second phase is determined by image processing to determine the number and average diameter of the first and second phases,
The average diameter was converted into a three-dimensional diameter by the following formula, and the volume ratio was determined from the number of the first and second phases and the average three-dimensional diameter. D = 1.128 L, where D: average diameter (two-dimensional), L: average three-dimensional diameter Further, when tensile deformation is performed at a strain rate of 2000 (sec ⁻¹ ) by a tensile test at a normal strain rate. Absorbed energy up to a true strain of 0.3 was measured. Table 2 shows the results. FIG. 3 shows the relationship between the yield ratio and the total elongation from the obtained results, and FIGS. 4, 5, and 6 show the primary forced cooling rate, the yield ratio, the total elongation, and the absorbed energy during high-speed deformation, respectively. It shows the relationship with.

[0030]

[Table 1]

[0031]

[Table 2]

From the above test results, it can be seen that all of the present inventions are ferrite-martensite dual phase steel sheets having a predetermined grain size, and have good formability and impact resistance. As is clear from the results shown in Table 2, the properties are as follows: tensile strength 490 to 980 MPa, yield ratio (yield strength / tensile strength) 75% or less, total elongation 25% or more, strain rate
It has excellent characteristics that the absorbed energy up to a true strain of 0.3 when subjected to tensile deformation at 2000 sec ^-1 is 250 MJ / m ³ or more. In addition, the same test material was sampled and tested from the positions of the tip and tail ends in the coil longitudinal direction.
The values were within the range of ± 2% of the values shown in Table 2, and stable characteristics were also exhibited in the longitudinal direction of the coil.

[0033]

As described above, according to the present invention,
By making the two-phase structure with the chemical composition and the metal structure appropriately controlled, it is possible to provide a thin hot-rolled high-tensile steel sheet having more excellent formability and impact resistance than conventional ones. Moreover, according to the present invention, this steel sheet has a thickness of 0.
Up to 8 mm can be manufactured while maintaining a stable material in the coil longitudinal direction. Therefore, by applying the hot-rolled high-tensile steel sheet according to the present invention to an automobile, it is possible to more economically achieve a reduction in the weight of the automobile body and an improvement in safety without impairing the press formability. .

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the volume fraction of a second phase and the absorbed energy during high strain rate deformation.

FIG. 2 is a graph showing a relationship between (diameter of second phase) / (particle diameter of ferrite) and absorbed energy at the time of high strain rate deformation.

FIG. 3 is a graph showing the relationship between yield ratio and total elongation.

FIG. 4 is a graph showing the effect of the primary cooling rate on the yield ratio.

FIG. 5 is a graph showing the effect of the primary cooling rate on the total elongation.

FIG. 6 is a graph showing the effect of the primary cooling rate on the absorbed energy during high strain rate deformation.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Corp. (72) Takashi Ohara 1, Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corporation Technical Research Institute (72) Inventor Norio Kanemoto 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corporation Chiba Works (72) Inventor Kunihiko Kataoka Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture No. 1 Kawasaki Steel Corporation Chiba Works

Claims (2)

[Claims] C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.03 to 0.15 wt%, Cr: 0.1 -2.0 wt%, Mo: 0.1-1.0 wt%, one or more selected from the group consisting of Fe and unavoidable impurities, and the balance of the ferrite phase having an average particle size of 10 µm or less is 80-97 by volume. %, The balance being a second phase mainly composed of martensite having an average diameter of 0.2 to 1.5 times the average diameter of ferrite. . 2. C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.03 to 0.15 wt%, Cr: 0.1 ~2.0 wt%, Mo: 0.1 contain one or more selected from to 1.0 wt%, the balance being hot-rolled at a steel material comprising the component composition of Fe and unavoidable impurities, Ar ₃ transformation point or more After that, cooling is started between 0.1 and 5.0 seconds, and cooling is performed at a cooling rate of 50 ° C./sec or more.
Cool to ~ 800 ° C, air-cool for 0.5-15 seconds, then 30
At a cooling rate of at least 300 ° C / sec.
A method for producing a hot-rolled high-tensile steel sheet having excellent formability and impact resistance, characterized by winding.