JP3169293B2 - Automotive thin steel sheet excellent in impact resistance and method for producing the same - Google Patents

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 for an automobile which is mainly used as an automobile part after being subjected to processing such as press forming, and has excellent characteristics especially in the event of a collision while the automobile is running. That is, it is a proposal regarding a thin steel sheet suitably used as a material of a part where impact resistance is required and a method for manufacturing the same. Recently, as the momentum for global environmental protection has increased, there has been a demand for a reduction in CO ₂ emissions from automobiles. For this reason, the weight of automobile bodies has been reduced, which means that the thickness of the steel sheet is reduced by increasing the strength of the steel sheet. It has been demanded.
Furthermore, focusing on the design philosophy of the car body, not only the strength of the steel sheet was simply increased, but more importantly, the steel sheet was excellent in impact resistance in the event of a collision during traveling, that is, deformed at a high strain rate. In this case, it is necessary to develop a steel plate having a large deformation resistance and a thin steel plate. Only by realizing this, it is possible to reduce the weight of the vehicle body while improving the safety of the vehicle and to provide a more desirable steel plate for a vehicle .

[0002]

2. Description of the Related Art Conventionally, the method of strengthening the quality of steel sheets for automobiles is mainly performed in ferritic single-phase structure steels by solid solution strengthening by adding substitutional elements such as Si, Mn and P, or carbon / nitride forming elements such as Nb and Ti. Is generally used to enhance precipitation. For example, JP-A-56-13
As described in No. 9654, etc., workability, in order to improve the aging, ultra-low carbon steel contains Ti, Nb, and further contains a strengthening component such as P in a range that does not impair workability. Many steel sheets with high strength have been proposed. In addition, for example, Japanese Unexamined Patent Publication No. Sho 59-193221 proposes a method for further increasing the strength by adding Si.

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, these proposals suggest that the yield strength or tensile strength, which is an indicator of the strength of a steel sheet, should be set at a strain rate of 10%.
Judgment is based on an extremely slow static evaluation method of ^-3 to 10 ^-2 (s ^-1 ). However, in the actual automobile body design, involving than such "static" strength, considering safety upon collision, the impact deformation at strain rate 10~10 ⁴ (s ^-1) Since “dynamic” strength is more important,
Such conventional proposals cannot be said to provide truly effective means for reducing the weight of an automobile body.

[0004]

Conventionally, the static strength and the dynamic strength described above are uniquely treated as having the same tendency, and mainly the static strength alone is used as a reference. Was determined. However, according to the research of the inventors, dynamic strength does not always correspond to static strength, and therefore, improvement in static strength of various improved materials does not directly lead to improvement in dynamic strength. It turns out. And this tendency was remarkable especially about the high-tensile steel plate.

[0005] Fig. 1 shows the effect of softening on deformation speed and strength.
It shows the effect of steel and high tensile steel. This figure clearly shows
Like, deformation speed 10^-3~Ten^-2(s^-1) Static strength and 10
~Ten ^Four(s^-1The relationship between the dynamic strength of
In particular, dynamic strength may not be as high as mild steel.
I understand. This means that the thickness of automotive high-strength steel sheets
When the thickness is reduced based on the degree value, the dynamic strength, that is,
Impact strength means that it will be insufficient
are doing. And this also means that only the static strength values
Conventional thinking of reducing the thickness of high-strength steel sheets by reference
Suggests that you have to look back. Departure
The purpose of Ming is to overcome the above-mentioned problems of the prior art.
The static strength of high-strength steel sheets
Value of dynamic strength with respect to the value of
To provide automotive thin steel sheets with high impact resistance
is there.

[0006]

As a result of intensive research aiming at solving the above-mentioned problems, not only the strength under a low strain rate like mild steel but also the strength under a high strain rate, that is, impact resistance It has been found that it is not enough for a high-strength steel sheet having excellent strength to simply exhibit a high static strength. This also does not mean that it is sufficient to simply develop a material exhibiting a high value only at a high strain rate, that is, a dynamic strength (which is uneconomical). It has been found that it is necessary that the dynamic strength be well balanced. That is, a thin steel sheet having excellent press formability and excellent impact strength under a high strain rate has a static-dynamic ratio = (yield stress at a strain rate of 10 ² (s ^-1 )) / (strain rate
The static-dynamic ratio is 1.6, defined as the yield stress at 10 ^-3 (s ^-1 ).
If the above thin steel sheet is used as a material for automobiles, high strength and strain rate dependence equivalent to that of mild steel sheet can be obtained even at high strain rates, so the safety improvement of automobile body is reduced. It was found that it could be achieved according to the realization of.

Based on these findings, the inventors further studied in detail the effects of the chemical composition and the manufacturing conditions on the static-dynamic ratio, and obtained a thin steel sheet for an automobile according to the present invention having the following gist configuration. The manufacturing method of it was developed. That is, the present invention relates to (1) C: 0.01 to 0.30 wt%, Si: 0.50 to
3.00wt%, Mn: 0.50 ~ 3.00wt%, Ni: 0.02 ~ 1.00wt%,
P: 0.01 to 0.15 wt%, the balance consisting of Fe and unavoidable impurities, the structure of which has an austenite phase of 10% or more by volume and a ferrite phase containing 0.0020 wt% or less of C. (2) or C: 0.01 to 0.30 wt%, Si: 0.50 to
3.00wt%, Mn: 0.50 ~ 3.00wt%, Ni: 0.02 ~ 1.00wt%,
P: Hot finish rolling of a steel slab containing 0.01 to 0.15 wt%, the balance being Fe and unavoidable impurities, is completed at a temperature of 800 ° C. or more, and after completion of the hot finishing rolling, 4
When winding in the temperature range of 00 to 200 ° C, the cooling rate from the end of the hot finish rolling to 500 ° C is set to 40 to 100 ° C.
C./sec, and a cooling rate from 500 ° C. to winding up at a rate of 10 to 50 ° C./sec to produce hot-rolled steel sheets. Method for producing thin steel sheet for automobiles with excellent properties, (3) C: 0.01 to 0.30
wt%, Si: 0.50 to 3.00 wt%, Mn: 0.50 to 3.00 wt%, Ni:
0.02 to 1.00 wt%, P: 0.01 to 0.15 wt%, with the balance being
The steel slab consisting of Fe and unavoidable impurities is subjected to hot rolling and cold rolling, and in the subsequent process, finish annealing is first performed at a temperature of 800 ° C or more, and in subsequent cooling, up to 500 ° C, 40 ° C Cool at a cooling rate of ~ 100 ° C / sec and at a temperature range of 400 ~ 200 ° C for 1.5 seconds ~ 300
A method of manufacturing a thin steel sheet for automobiles having excellent impact resistance, characterized in that a cold-rolled steel sheet is manufactured by holding the steel sheet for a second.

[0008]

The present inventors have repeatedly studied the chemical composition, the steel structure, and the manufacturing conditions in order to improve the static-dynamic ratio of the above-mentioned thin steel sheet for automobiles. As a result, the amount of retained austenite in the steel and the amount of C in the ferrite phase have a large influence on the static-dynamic ratio, and it is necessary to optimize the component composition to keep these amounts in a suitable range. It has been found that it is extremely effective in improving the static-dynamic ratio. Further, in order to make the static-dynamic ratio of the thin steel sheet to be manufactured to show the static-dynamic ratio of the mild steel sheet: 1.6 or more, in addition to adjusting the “component composition” and the “structure”, various other factors in the manufacturing process are required. By controlling the final `` cooling condition '' of the heat history, that is, the cooling condition after hot rolling for a hot-rolled steel sheet and the cooling condition after cold-rolling annealing for a cold-rolled steel sheet, the amount of retained austenite and It has been found that controlling the amount of C in the target range has a particularly effective effect on the improvement of each strength under both high and low strain rates.

First, the respective constituent elements constituting the present invention and the contents thereof are limited for the following reasons in order to improve the formability as well as the static-dynamic ratio. C: 0.01 to 0.30 wt% C is an element that needs to be added in order to form an austenite phase, but on the other hand, it must be suppressed from the viewpoint of improving elongation and r-value, which are indicators of press formability. Must. That is, if this C is less than 0.01 wt%, sufficient retained austenite is not generated, and the static-dynamic ratio cannot be improved. On the other hand, if the C content exceeds 0.30 wt%, the C% in the ferrite phase increases and the static-dynamic ratio tends to decrease, so the C content was limited to the range of 0.01 to 0.30 wt%.

Si: 0.50% to 3.00% by weight Basically, it is necessary to add 0.50% by weight or more of Si in order to obtain a necessary strength level as a steel sheet for automobiles. However, when the content exceeds 3.00 wt%, the formation of the residual austenite phase is hindered, and the amount of solid solution C in the steel sheet is increased, thereby lowering the dislocation speed during deformation and reducing the static This leads to a decrease in dynamic ratio. Therefore, the Si content was limited to the range of 0.50 to 3.00 wt%.

Mn: 0.50 to 2.5 wt% Mn is an element necessary for forming a retained austenite phase, but must be suppressed from the viewpoint of improving elongation and r-value which are indicators of press formability. is there. So Mn
Is added at least 0.50 wt% in order to generate retained austenite and improve the static-dynamic ratio. On the other hand, 2.5 wt
%, The C content in the ferrite is increased to lower the static-dynamic ratio, and the steel sheet is significantly hardened. For this reason, the content of Mn was limited to the range of 0.50 to 2.5 wt%.

Ni: 0.02 to 1.00 wt% Ni has not conventionally been a particularly added component in this type of steel. However, in the present invention, the action of producing residual austenite to improve the static-dynamic ratio. Therefore, 0.02% by weight or more is added. However, this Ni is 1.
If added in excess of 20 wt%, the steel sheet will be hardened, and the C content in the ferrite phase will also increase, impairing the improvement in the static-dynamic ratio. Therefore, the Ni content was limited to the range of 0.02 to 1.00 wt%.

P: 0.01 to 0.15 wt% P is contained at a lower limit of 0.01 wt% in order to obtain a sufficient reinforcing effect as a material for automobiles. Basically, P may be adjusted in accordance with the target strength level at 0.01 wt% or more, but when it is contained in excess of 0.15 wt%, the hot-rolled base sheet is hardened remarkably, and In addition to the deterioration of the cold rolling property, the surface treatment property also remarkably deteriorates. Therefore, the upper limit of the P content is set to 0.15 wt%.

Next, in order to make the static-dynamic ratio equal to or higher than that of mild steel in the present invention, the structure of the thin steel sheet must be such that an austenite phase of 10% or more by volume remains.
This is generally known that the strengthening of the steel sheet is performed by the strain-induced transformation of retained austenite.
This induced transformation differs depending on the speed at which the strain is introduced even with the same strain amount. That is, as the amount of retained austenite increases, the strain rate (dislocation rate) increases,
Transformation is promoted, thereby improving the static-dynamic ratio. This amount must be at least 10%, and if it is less than this, the required strain rate cannot be obtained and the improvement of the static-dynamic ratio cannot be expected.

In the present invention, in order for the static-dynamic ratio to be comparable to that of mild steel, the steel structure must be such that the C content in the ferrite phase is 0.0020 wt% or less. The reason for this is
This is because, when the amount of deformation is relatively small, the deformation behavior of the ferrite phase becomes dominant with respect to the static-dynamic ratio, and the static-dynamic ratio can be improved by lowering the amount of C in the ferrite phase.

Next, the features of the manufacturing method of the present invention will be described. In the present invention, a steel slab obtained by melting and casting a steel material having the above-described composition is subjected to hot rolling or cold rolling according to a conventional method. a. Hot Rolling Step Treatment The above steel slab is subjected to hot finish rolling at a temperature of 800 ° C. or more, and 400 to 200 ° C. after completion of the hot finish rolling.
When winding in the temperature range of, the cooling rate from the end of the hot finish rolling until reaching 500 ℃ 40 ℃ / sec ~ 100 ℃ / s
The hot rolled steel sheet is manufactured by performing the cooling at a speed of ec and further performing the cooling speed from 500 ° C. to winding at a speed of 10 ° C./sec to 50 ° C./sec. b. Cold rolling process treatment The above steel slab is subjected to hot rolling and cold rolling, and in the subsequent process, finish annealing is first performed at a temperature of 800 ° C or higher, and then cooling is performed at 40 ° C up to 500 ° C. / sec ~
A cold-rolled steel sheet is manufactured by cooling at a cooling rate of 100 ° C./sec and holding at a temperature range of 400 to 200 ° C. for 1.5 to 300 seconds.

Hereinafter, the reasons for limiting the processing conditions in each of the above steps will be described. The reason why the hot-rolling finish rolling is performed at 800 ° C. or higher is that if the temperature is lower than this temperature, the structure becomes mixed and the formability deteriorates. Also, after hot rolling is completed, 500
The reason why cooling to 40 ° C is performed in the range of 40 ° C / sec to 100 ° C / sec is that if the temperature is lower than 40 ° C / sec, a coarse bainite phase precipitates and the static-dynamic ratio decreases. This is because the number of precipitation sites required for generation of the γ phase is insufficient. And
Cooling from 500 ° C to winding temperature from 10 ° C / sec to 50 ° C / sec
At a speed lower than 10 ° C / sec, a coarse bainite phase precipitates and the static-dynamic ratio decreases, while at a speed higher than 50 ° C, the precipitation site required for the formation of the residual γ phase is formed. Is insufficient. Furthermore, take-up temperature is 200 ℃
The reason for limiting to the range of ~ 400 ° C is that if the temperature exceeds 400 ° C, the austenite phase is insufficiently precipitated and the static-dynamic ratio decreases, and
If the temperature is lower than 200 ° C, the austenite phase is similarly insufficiently precipitated and the static-dynamic ratio decreases, so the range is limited to the range of 200 ° C to 400 ° C.

Next, the reason why the finish annealing after the cold rolling is performed at 800 ° C. or more is that if the temperature is lower than this temperature, recrystallization is insufficient and formability is deteriorated. Cooling down to 500 ℃ 40 ℃ / sec ~
In the range of 100 ° C / sec, coarse bainite phase precipitates at a temperature lower than 40 ° C / sec and the static-dynamic ratio decreases. Is insufficient. Cooling from 500 ° C at 10 ° C / sec to 50
At a rate of less than 10 ° C / sec, a coarse bainite phase precipitates and the static-dynamic ratio decreases at a rate of less than 10 ° C / sec. This is because precipitation becomes insufficient. The reason for limiting the range from 200 ° C to 400 ° C for 1.5 seconds or more and 300 seconds or less is that if the temperature exceeds 400 ° C, the austenite phase is insufficiently precipitated and the static-dynamic ratio decreases. If the precipitation of the phase is insufficient, the static-dynamic ratio decreases.If it is less than 1.5 seconds, the precipitation of the austenite phase is insufficient, and the static-dynamic ratio decreases. Since there is no such material, each is limited to the above range.

According to the present invention, not only hot rolled steel sheets and cold rolled steel sheets but also surface-treated steel sheets made of these materials can be similarly provided with the effect of improving the static-dynamic ratio. In addition, for example, for a hot-dip galvanized steel sheet in which surface treatment and finish annealing are performed simultaneously on a cold-rolled steel sheet, the cooling rate control after the adhesion of the plating or after the alloying treatment is controlled by the cooling control after the annealing in the present invention. What should be done in the same way. Further, the steel of the present invention and the method of the present invention are directed to steel plates for automobiles, but needless to say, they are similarly effective for applications requiring strength at high strain rates.

[0020]

EXAMPLE Steels having various chemical compositions as shown in Table 1 are melted in a converter, and a steel slab obtained by continuous casting is corrected to the following conditions. Each test piece was obtained by variously changing the ratio of the C content in the ferrite to the γ phase by the treatment. That is, the above-mentioned steel slab was first hot-rolled into hot-rolled steel sheets of 35 mmt, and then these hot-rolled steel sheets were cold-rolled into cold-rolled steel sheets of 0.7 mmt. The strain rates of these hot-rolled steel sheets and cold-rolled steel sheets obtained were determined to be 10 ^-3 (S ^-1 ) and 10
The yield strength at ² (S ^-1 ) was measured to determine the static-dynamic ratio. The manufacturing conditions and characteristics are summarized in Table 1.

[0021]

[Table 1]

[0022]

[Table 2]

As is evident from the results shown in Table 1, the steel sheets (Nos. 1 to 3) conforming to the present invention are hot rolled steel sheets having an excellent static-dynamic ratio with respect to the comparative examples (Nos. 4 to 8). Has been obtained. Further, as is clear from the results shown in Table 2, the steel sheets (Nos. 9 to 11), which are cold-rolled steel sheets in which the component composition and production conditions were optimized according to the requirements of the present invention, All have high static-dynamic ratios.

[0024]

As explained above, according to the present invention,
By optimizing the component composition of the steel sheet and controlling the steel structure, and further controlling the cooling rate after hot rolling and annealing,
It is possible to manufacture thin steel sheets for automobiles having a much higher static-dynamic ratio than in the past, and furthermore, by using them for automobile parts, it is possible to reduce the weight and improve the safety of automobile bodies.

[Brief description of the drawings]

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

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/08 C22C 38/08 (72) Inventor Makoto Imanaka 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Pref. Headquarters (72) Inventor Takaaki Hira 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Headquarters (56) References JP-A-7-3326 (JP, A) JP-A 7-137979 JP, A) JP-A-7-18375 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/46-9/48 C21D 8 / 02-8/04

Claims (3)

(57) [Claims] 1. C: 0.01 to 0.30 wt%, Si: 0.50 to 3.00 wt%
%, Mn: 0.50 to 3.00 wt%, Ni: 0.02 to 1.00 wt%, P: 0.01 to 0.15 wt%, the balance being Fe and unavoidable impurities, the structure of which is 10% or more by volume of austenite Having a phase,
A thin steel sheet for automobiles with excellent impact resistance, which contains 0.0020 wt% or less of C in the ferrite phase. 2. C: 0.01 to 0.30 wt%, Si: 0.50 to 3.00 wt%
%, Mn: 0.50 to 3.00 wt%, Ni: 0.02 to 1.00 wt%, P: 0.01 to 0.15 wt%, the balance being Fe and unavoidable impurities. Finish finishing rolling,
Then, when winding is performed in a temperature range of 400 to 200 ° C. after completion of the hot finish rolling, 500 ° C. after completion of the hot finish rolling.
The cooling rate is 40 to 100 ° C / sec, and the cooling rate from 500 ° C to winding is 10 to 50 ° C / s.
A method for manufacturing a thin steel sheet for automobiles having excellent impact resistance, wherein a hot-rolled steel sheet is manufactured by performing the process at a speed of ec. 3. C: 0.01 to 0.30 wt%, Si: 0.50 to 3.00 wt%
%, Mn: 0.50 to 3.00 wt%, Ni: 0.02 to 1.00 wt%, P: 0.01 to 0.15 wt%, the balance being Fe and unavoidable impurities. In the subsequent process, first, the finish annealing is performed at a temperature of 800 ° C or higher, and in the subsequent cooling, cooling is performed at a cooling rate of 40 to 100 ° C / sec up to 500 ° C, and the temperature of 400 to 200 ° C. A method for manufacturing a thin steel sheet for automobiles having excellent impact resistance, characterized in that a cold-rolled steel sheet is manufactured by holding the steel sheet for 1.5 seconds to 300 seconds in an area.