JPH073381A - Thin steel sheet excellent in impact resistance and production thereof - Google Patents

Abstract

PURPOSE:To produce a thin steel sheet excellent in impact resistance by subjecting a low carbon steel slab to hot rolling, cold rolling and annealing under specified conditions. CONSTITUTION:A low carbon steel slab contg., by weight, 0.01 to 0.10% C, <1.5% Si, <3.0% Mn, <1.00% P, <0.10% S, <0.10% Al and <0.0050% N is subjected to hot rough rolling at <=1250 deg.C, is thereafter held to the temp. range of 950 to 1100 deg.C, is subjected to bending and rebending treatment and is successively subjected to hot finish rolling into a hot rolled sheet, which is cooled at >=10 deg.C/sec cooling rate and is thereafter annealed at 300 to 500 deg.C or, after the hot finish rolling, it is subjected to cold rolling, is subsequently heated to the temp. range of the recrystallization temp. to the AC3 transformation point for >=30sec, is rapidly cooled at 10 to 1000 deg.C/sec cooling rate and is thereafter annealed at 300 to 500 deg.C. When the grain diameter of ferrite in the structure is defined as Fdmum, the number of cementite in one piece of ferrite crystal is defined as Cn piece and the area per piece of cementite is defined as Camum<2>, (Cn+Ca)/Fd=0.10 to 2.00 is satisfied, by which the hot rolled steel sheet or cold rolled steel sheet excellent in impact resistance can be produced.

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 having excellent impact resistance and a method for producing the same, and is particularly used as a steel sheet for automobiles which is subjected to processing such as press forming,
In particular, it is a proposal regarding a thin steel sheet suitable as a material for a portion where a car should collide during traveling, that is, a shock resistance, and a manufacturing method thereof. Since the momentum for global environment conservation has increased recently, CO ₂ emitted from automobiles
Reduction of emissions is required. For this reason, the weight of automobile bodies has been reduced, which also means that the strength of the steel sheet is increased to reduce the thickness of the steel sheet, and the material is excellent in both press formability and strength. Is required. Furthermore, focusing on the design concept of an automobile body, it is not only the strength of the steel plate that is simply strengthened, but more importantly, the steel plate that has excellent impact resistance in the event of a collision during running, that is, the plate thickness is thin and high. It is necessary to develop a steel plate that has a large deformation resistance when deformed at a strain rate, and it is only when this is realized that the weight of the car body can be reduced while improving the safety of the automobile, and a more desirable automobile steel plate can be provided. be able to.

[0002]

2. Description of the Related Art Conventionally, a method for strengthening a material of a steel sheet for automobiles has been a solid solution strengthening method for a ferrite single-phase steel mainly by adding substitutional solid solution elements such as Si, Mn and P, or N.
A common method is precipitation strengthening by adding carbon and nitride forming elements such as b and Ti. For example, as described in JP-A-56-139654, etc., ultra-low carbon steel contains Ti and Nb in order to improve workability and aging property, and P is added within a range not impairing workability. Many steel sheets have been proposed that contain strengthening components such as to improve the strength. In addition to this, for example, in JP-A-59-193221,
A method of further strengthening by adding Si has also been proposed.

Certainly, by increasing the strength of the steel sheet by such a method, the thickness of the automobile body can be reduced to some extent. However, in these proposals, the yield rate or tensile strength, which is an index of steel plate strength, is determined by
^-3 to 10 ^-2 (s ^-1 ) based on a very slow static evaluation method. However, in actual car body design, rather than such "static" strength, impact deformation at a strain rate of 10 ^{4 to} 10 ⁴ (s ^-1 ), which takes into account safety during a collision, is involved. Because “dynamic” strength is more important,
The conventional proposals cannot be said to provide truly effective means for reducing the weight of the automobile body.

[0004]

Conventionally, the above-mentioned static strength and dynamic strength are uniquely treated as having the same tendency, and the static strength is mainly used as a standard. I was making a decision. However, according to the research conducted by the inventors, the dynamic strength does not always correspond to the static strength, so that the improvement of the static strength of various improved materials does not directly lead to the improvement of the dynamic strength. I knew that. And this tendency was remarkable especially in high-strength steel sheets.

That is, FIG. 1 shows the relationship between deformation rate and strength.
It shows the effect of mild steel and high-strength steel on the engagement.
As is clear from this figure, the deformation rate of mild steel sheet is 10^-3
~Ten ^-2(s^-1) Static strength and 10 to 10^Four(s^-1) Dynamic strength
Is not as high as the static strength of mild steel.
It This means that the strength of high-strength steel sheets for automobiles is
When the wall thickness is reduced based on the value, the dynamic strength, that is, the resistance
It means that the impact strength will be insufficient.
ing. And this is also based only on static strength values.
Conventional way of thinking that has attempted to reduce the thickness of high-strength steel sheets
Suggests that it must be reviewed. The present invention
Aims to overcome the problems with the above-mentioned conventional technology
In particular, the static strength of thin steel sheets
The value of dynamic strength is higher than that of mild steel plate.
The thin steel sheet for automobiles that exhibits excellent impact resistance
To provide.

[0006]

[Means for Solving the Problems] As a result of intensive research aimed at solving the above-mentioned problems, as a result, not only the strength under a low strain rate such as mild steel but also the strength under a high strain rate, that is, the impact resistance It has been found that merely a static strength having a high value is not sufficient for forming a thin steel sheet having excellent strength. This does not mean that it is sufficient to simply develop a material having a high strength under a high strain rate, that is, a material having a high dynamic strength (which is uneconomical),
It has been found that it is necessary for the so-called static strength and dynamic strength to be well balanced. In other words, a steel sheet with excellent press formability and impact strength at high strain rate has a static-dynamic ratio = (yield stress at strain rate 10 ² (s ^-1 )) / (strain rate 10
^-3 (s ^-1 ) .The yield stress at (s ^-1 ), if the steel plate has a static-dynamic ratio of 1.6 or more, it is equivalent to a mild steel plate even under high strain rate when used as an automobile part. Since it is possible to obtain high strain rate dependence of strength, it has been found that the improvement in safety of the automobile body can be achieved along with the realization of weight reduction.

Based on such knowledge, the inventors further studied in detail the influence of the chemical composition, the structure and the manufacturing conditions on the above-mentioned static-dynamic ratio, and made a thin steel plate having the following constitution and its manufacturing. Developed a method. That is, the present invention is: (1) C: 0.01 to 0.10 wt%, Si: 1.5 wt% or less, Mn: 3.
0 wt% or less, P: 1.00 wt% or less, S: 0.10 wt% or less, Al: 0.10 wt% or less, N: 0.0050 wt% or less, the balance being Fe and unavoidable impurities, and in any cross section Ferrite crystal grain size Fd (μm) is 10 ~
The relationship between the cementite number Cn (pieces) having a size of 50 μm and existing in one ferrite crystal grain and the area Ca (μm ² ) per cementite and Fd satisfies the following formula: A thin steel sheet with excellent impact resistance. (Cn × Ca) /Fd=0.10 to 2.00 (2) C: 0.01 to 0.10 wt%, Si: 1.5 wt% or less, Mn: 3.
A steel slab containing 0 wt% or less, P: 1.00 wt% or less, S: 0.10 wt% or less, Al: 0.10 wt% or less, N: 0.0050 wt% or less, and the balance of Fe and inevitable impurities at 1250 ° C. After performing hot rough rolling by heating to the following temperatures, and then performing bending and unbending treatment by holding in the temperature range of 950 to 1100 ° C, hot finish rolling is performed, and then 10 ° C / Cool at a cooling rate of more than 2 seconds, 300 to 500 ℃
A method for producing a thin steel sheet having excellent impact resistance, which comprises forming a hot-rolled steel strip by staying in the temperature range of 1 minute or more. (3) C: 0.01 to 0.10 wt%, Si: 1.5 wt% or less, Mn: 3.
A steel slab containing 0 wt% or less, P: 1.00 wt% or less, S: 0.10 wt% or less, Al: 0.10 wt% or less, N: 0.0050 wt% or less, and the balance of Fe and inevitable impurities at 1250 ° C. After performing hot rough rolling by heating to the following temperature, and then performing bending and unbending treatment by holding it in the temperature range of 950 to 1100 ° C, hot finish rolling and cold rolling are performed. From the recrystallization temperature to the Ac ₃ transformation point in the temperature range of 30 seconds or more, and cooled at a cooling rate of 10 to 1000 ° C / sec, and then retained in the temperature range of 300 to 500 ° C for 20 seconds to 10 minutes. A method for producing a thin steel sheet having excellent impact resistance, which comprises subjecting a cold rolled steel strip to continuous annealing.

[0008]

The inventors of the present invention have made it possible to improve the static-dynamic ratio without impairing the formability and manufacturing characteristics of the thin steel sheet described above.
First, based on a high-strength low-carbon steel containing Si, Mn, N, and P, the influence of metallurgical factors on the static-dynamic ratio, particularly the chemical composition, structure, and manufacturing conditions were investigated. As a result, first, regarding the chemical composition, the alloy design should be such that the solute C and solute N in the steel can be reduced as much as possible, and regarding the structure, the ferrite crystal grain size and the cementite number should be controlled, and Regarding the manufacturing conditions, it was found that it was necessary to examine the slab heating temperature, intermediate processing, cooling, and annealing conditions.

In particular, in order for the above-mentioned static-dynamic ratio in the high-strength thin steel sheet for automobiles to show a static-dynamic ratio equivalent to that of mild steel sheet: 1.6 or more, if the above component composition, structure and manufacturing conditions are made appropriate. It was also found that since solid solution C and solid solution N can be effectively precipitated, they have a particularly effective effect on the improvement of each strength under both high and low strain rates. The desirable conditions will be described below.

The chemical components and their contents constituting the thin steel sheet of the present invention are limited for the following reasons in order to improve the formability as well as the static-dynamic ratio. C: 0.01 to 0.10 wt% The amount of C requires delicate control in order to obtain a steel sheet excellent in both press formability and static-dynamic ratio. Its content is 0.01wt%
At less than 1, the supersaturation degree required for solid solution C cannot be obtained even when rapid cooling is performed during continuous annealing. Therefore, even if overaging treatment is performed, cementite does not precipitate in ferrite crystal grains and an improvement in static-dynamic ratio is expected. become unable. On the other hand, if it exceeds 0.10 wt%, elongation and r value, which are indicators of press formability, are reduced, and ferrite crystal grains become finer, and solute C easily precipitates at grain boundaries instead of within the crystal grains. Therefore, as in the case of less than 0.01 wt%, the required supersaturation degree cannot be obtained, and the amount of solid solution C increases and the static-dynamic ratio cannot be improved. Therefore, in the present invention, the amount of C is limited to the range of 0.01 to 0.10 wt%.

Si: 1.5 wt% or less Si is a solid solution strengthening element and is an element effective for producing a high strength steel sheet. Therefore, 0.005 wt% or more is preferably added. However, if the content exceeds 1.5 wt%, the center segregation becomes excessive and the press formability deteriorates, so the upper limit of the Si content was limited to 1.5 wt%.

Mn: 3.0 wt% or less Mn fixes S that causes red hot embrittlement as MnS, and is a solid solution strengthening element similar to Si. Therefore, Mn is an effective element for producing high strength steel sheet. Therefore, addition of 0.05 wt% or more is desirable. However, if the content exceeds 3.0 wt%, the cost will increase and the center segregation will increase excessively, degrading the press formability. Therefore, the upper limit of the Mn content is 3.0 wt%.
Limited to%.

P: 1.00 wt% or less P is an element effective in producing a high strength steel sheet, so 0.005 wt% or more is preferably added. However, since the content of more than 1.0 wt% deteriorates the secondary work embrittlement resistance, the upper limit of the P content is limited to 1.0 wt%.

S: 0.10 wt% or less S is an effective element for improving chemical conversion treatability, and therefore 0.005 wt% or more is preferably added. However,
Since the content of more than 0.10 wt% deteriorates the press formability, the upper limit of S is limited to 0.10 wt%.

Al: 0.10 wt% or less Al is an element that is added as a deoxidizing agent in the steelmaking stage and is effective for fixing solid solution N as AlN that deteriorates press formability and aging resistance. Therefore, at least 0.
Addition of 010 wt% is desirable. However, the addition of more than 0.10 wt% will increase the cost, so the upper limit of Al is 0.10 wt%.
Limited to.

N: 0.0050 wt% or less N is an interstitial solid solution element, and when N is a solid solution in steel, deterioration of press formability, aging resistance and improvement of static-dynamic ratio cannot be expected, so N is as much as possible. Need to reduce. Further, since it is necessary to add a large amount of expensive Al for fixing the solid solution N, the upper limit of N is limited to 0.0050 wt% in the present invention.

The thin steel sheet according to the present invention is required to be at least a steel having the above-described composition, and further, it is necessary to have a structure as described below. Ferrite crystal grain size (Fd): 10 to 50 (μm) The reason why the ferrite crystal grain size in steel must be 10 to 50 μm is that if it is less than 10 μm, fine cementite is hard to precipitate in the crystal grains, and it is statically moving. This is because the ratio will not improve. Further, if the ferrite crystal grain size is larger than 50 μm, the surface of the steel sheet will be roughened during press working. Relationship between the number of cementites (Cn) in one crystal grain and the average area (Ca) per cementite and Fd, Cn × Ca / Fd: 0.10 to 2.00 When Cn × Ca / Fd is less than 0.10. Since the content of C is large and the static-dynamic ratio is not improved, and if it exceeds 2.00, the press formability is deteriorated due to excessive precipitation of cementite, so in the present invention, the range is set to 0.10 to 2.00.

The thin steel sheet according to the present invention has the above-described composition and steel structure. In manufacturing such a thin steel sheet, it is necessary to select at least the following manufacturing conditions. Slab heating temperature: 1250 ° C or less The reason for setting the slab heating temperature to 1250 ° C or less is that by heating the slab at a low temperature, re-dissolution of AlN precipitated during cooling after continuous casting is suppressed, and finally cold rolling is performed. This is because it is necessary to reduce the solid solution N in the annealed plate.

High-temperature holding and bending / bending back treatment of the sheet bar before hot finish rolling: In the manufacturing method according to the present invention, the sheet bar is heated to 950 to before hot finish rolling.
It is necessary to keep the sheet bar at a high temperature of 1100 ° C and bend the sheet bar in this temperature range to perform the bending back treatment. By carrying out such a treatment, precipitates are likely to be precipitated and a steel having a predetermined structure can be obtained. In addition, the size of the bending in this bending / bending process is
The radius of curvature is preferably 200 to 1000 mm. In addition, it is desirable to keep the temperature at a high temperature within 10 minutes in view of the problem of temperature drop during operation.

Cooling and Winding Conditions after Finish Rolling When the final product is a hot rolled steel sheet, after finishing rolling, it is cooled to a temperature range of 300 to 500 ° C. at a cooling rate of 10 ° C./sec. The reason for this is that by cooling at 10 ° C / sec or more, solid solution C in steel
Can be supersaturated, and 1 to 300-500 ℃
This is because cementite is likely to precipitate in the crystal grains by allowing the cementite to stay for more than a minute. On the other hand, if the final product is a cold rolled steel sheet, there are no particular restrictions, but in order to improve the material after cold rolling and annealing, especially the static-dynamic ratio, a coiling temperature of 600 ° C or higher may be used. Desirably, it is particularly desirable to sufficiently precipitate AlN at the stage of hot rolling.

Continuous annealing conditions for producing a cold-rolled steel sheet: This continuous annealing is performed by heating in the temperature range from the recrystallization temperature to the Ac ₃ transformation point. The reason for this is that it is impossible to perform press working due to the rolling structure below the recrystallization temperature, while,
This is because when the Ac ₃ transformation point is exceeded, the texture becomes random and the press workability is significantly deteriorated. In this process, the cooling after heating is 10-1000 ℃ / up to the temperature range of 300-500 ℃.
Perform at a rate of sec and let it stay at this temperature for more than 20 seconds. The holding time is 20 seconds to 10 minutes. The reason for this limitation is that the solid solution C in the steel can be made into a supersaturated state by cooling at 10 ° C / sec or more.
This is because by maintaining the temperature at ˜500 ° C., cementite can be easily precipitated in the crystal grains. Ie, 30
This is because precipitation of cementite is less likely to occur in a temperature range of less than 0 ° C or more than 500 ° C, and a cooling rate of more than 1000 ° C / sec and a holding time of more than 10 minutes are not possible in terms of equipment.
The reason why the holding time is 20 seconds or more is that cementite precipitates in the crystal grains and the amount of solute C can be reduced.

[0022]

【Example】

Example (1): Example of hot-rolled steel sheet A hot-rolled steel strip was produced by treating a continuously cast slab having the composition shown in Table 1 under the production conditions shown in Table 2.
That is, the continuous cast slab is reheated at 1300 ° C. or lower, and after three-pass rough rolling, before and after finish rolling, the coil box is used to reverse the front and rear of the sheet bar to perform hot rolling, The grain size and the morphology of cementite in the grain were changed by forming a hot rolled sheet with a thickness of 2.5 mm and then winding it in the temperature range of 250 to 470 ℃ by changing the cooling rate. Samples were taken from these hot-rolled sheets, processed into JIS No. 5 tensile test pieces, and then subjected to static tensile test (strain rate: 10 ^-3 ) and dynamic tensile test (strain rate: 10 ² ). The yield strength (YP) was measured, and the static-dynamic ratio (YP in the dynamic tensile test / YP in the static tensile test) of each steel plate was obtained. The results are shown in Table 3. As shown in this table, it was found that all of the thin steel sheets (No. 1 to 3, 8 to 11) satisfying the steel composition and microstructure conditions of the present invention had a large static-dynamic ratio and excellent impact resistance.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

Example (2): Example of Cold Rolled Steel Sheet A cold rolled steel strip was produced by treating a continuously cast slab having the composition shown in Table 4 under the production conditions shown in Table 5.
That is, first, the continuous cast slab is reheated at a temperature of 1300 ° C. or lower, rough-rolled in three passes, and then, before finish rolling, a coil box is used to reverse the front and rear of the sheet bar to perform hot rolling. As a hot-rolled sheet with a thickness of 3.2 mm, 550 to 650 ℃
It was wound in the temperature range of. This hot-rolled sheet was then pickled and cold-rolled to give a cold-rolled sheet having a plate thickness of 0.8 mm. Then, recrystallization annealing was performed under the continuous annealing conditions as shown in Table 5 to change the crystal grain size and the morphology of cementite in the crystal grains. After temper rolling with a rolling reduction of 1.0%, it was processed into JIS No. 5 tensile test pieces and subjected to a static tensile test (strain rate: 10 ^-3 ) and a dynamic tensile test (strain rate: 10 ² ). Each yield strength (YP) was measured, and the static-dynamic ratio (YP in the dynamic tensile test / YP in the static tensile test) of each steel plate was calculated.
The results are shown in Table 6. As a result, it was found that all of the thin steel sheets satisfying the steel composition and microstructure conditions of the present invention have a large static-dynamic ratio and excellent impact resistance.

[0027]

[Table 4]

[0028]

[Table 5]

[0029]

[Table 6]

[0030]

As described above, according to the present invention,
It is possible to obtain a high-strength thin steel sheet for automobiles which has excellent formability and excellent impact resistance represented by a static-dynamic ratio.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the effect of mild steel and high-strength steel on the relationship between deformation rate and strength.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuya Miura 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Iron & Steel Co., Ltd. Technical Research Division (72) Inventor Makoto Imanaka 1 Kawasaki-cho, Chuo-ku, Chiba-shi Address Kawasaki Iron & Steel Co., Ltd. Technical Research Headquarters (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Iron & Steel Co., Ltd. Technical Research Headquarters

Claims (3)

[Claims] 1. C: 0.01 to 0.10 wt%, Si: 1.5 wt% or less, Mn: 3.0 wt% or less, P: 1.00 wt% or less, S: 0.10 wt% or less, Al: 0.10 wt% or less, N: The content is 0.0050 wt% or less, the balance is Fe and unavoidable impurities, and the ferrite crystal grain size Fd (μm) in an arbitrary cross section is 10 to 50 μm and exists in one ferrite crystal grain. Cn of cementite
A thin steel sheet having excellent impact resistance, characterized in that the relationship between (pieces) and the area Ca (μm ² ) per cementite and Fd is composed of a structure satisfying the following formula. (Cn × Ca) /Fd=0.10 to 2.00 2. C: 0.01 to 0.10 wt%, Si: 1.5 wt% or less, Mn: 3.0 wt% or less, P: 1.00 wt% or less, S: 0.10 wt% or less, Al: 0.10 wt% or less, N: A steel slab containing 0.0050wt% or less and the balance of Fe and unavoidable impurities is heated to a temperature of 1250 ° C or less to perform hot rough rolling, and then kept in the temperature range of 950 to 1100 ° C and bent. In addition, after performing the bending back treatment, hot finish rolling is performed, and thereafter, it is cooled at a cooling rate of 10 ° C / sec or more and retained in the temperature range of 300 to 500 ° C for 1 minute or more, so that hot rolled steel A method for producing a thin steel sheet having excellent impact resistance, which is characterized by using a belt. 3. C: 0.01 to 0.10 wt%, Si: 1.5 wt% or less, Mn: 3.0 wt% or less, P: 1.00 wt% or less, S: 0.10 wt% or less, Al: 0.10 wt% or less, N: A steel slab containing 0.0050 wt% or less and the balance of Fe and unavoidable impurities is heated to a temperature of 1250 ° C or less to perform hot rough rolling, and then is held in the temperature range of 950 to 1100 ° C for bending and bending. After the bending back treatment, hot finishing rolling is performed, cold rolling is performed, and then the recrystallization temperature to the Ac ₃ transformation point temperature range is heated for 30 seconds or more, and cooled to 10 to 1000 ° C / second. A method for producing a thin steel sheet having excellent impact resistance, which comprises cooling at a speed and then subjecting it to a cold-rolled steel strip by continuous annealing in which it is retained in a temperature range of 300 to 500 ° C for 20 seconds to 10 minutes.