JPH09184045A - Extremely thin hot rolled steel sheet excellent in impact resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively produce an extremely thin hot rolled steel sheet with <1.2mm sheet thickness excellent in impact resistance at a high strain rate at high yield without deteriorating its bore expandability. SOLUTION: A steel stock contg., by weight, 0.0001 to 0.02% C, 0.01 to 2.0% Si, 0.1 to 3.0% Mn, 0.01 to 0.15% P, <=0.010% S, 0.001 to 0.05% Al, and the balance Fe with inevitable impurities is heated at 900 to 1250 deg.C and is subjected to rough rolling to form into a sheet bar. Next, this sheet bar is joined with the preceding sheet bar, and, while lubrication is executed under the conditions of >=900m/min rolling rate, the Ar3 transformation point to (the Ar3 transformation point + 50 deg.C) rolling finishing temp. and >=95% draft, it is subjected to finish rolling into <1.2mm sheet thickness. Within 0.5sec after that, it is cooled to <=450 deg.C at a cooling rate of >=50 deg.C/sec and is coiled round a coil to obtain the extremely thin hot rolled steel sheet in which the metallic structure is composed of a ferrite single phase one having >=12 ferrite grain size number and the thickness of the oxidized layer on the surface is regulated to <=3μm.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention is mainly used for automobile parts and the like, and is particularly suitable for use as a material for a portion which is required to have excellent impact resistance in the event of a collision during traveling of the automobile. The present invention relates to an ultrathin hot-rolled steel sheet having a plate thickness of less than 1.2 mm and a method for manufacturing 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
As part of this, it is required to reduce the weight of automobile bodies. This
One way to reduce the weight is to increase the strength of the steel plate.
It is believed that reducing the thickness is effective. Furthermore, recently
Based on the car body design concept of
Not only in the case of graduation, but in the event of a collision during travel,
Steel sheet with excellent impact resistance, that is, when deformed at a high strain rate
The development of steel sheets with high deformation resistance in
It is possible to improve the vehicle's performance and reduce the weight of the vehicle body.
It is attracting attention as one that contributes to efficacy. On the other hand, in recent years
Conventionally used due to cost reduction of materials used
Instead of the cold-rolled steel sheet that was used, the hot-rolled steel sheet especially 1.2 mm thick
The tendency to adopt ultra-thin hot-rolled steel sheet of less than
There are two. From such a situation, it is
Ultra-thin hot-rolled steel sheet with excellent impact resistance from the standpoint of strikedown
Is eager to develop.

[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.

However, even if the strength of the steel sheet is increased by such a method, the thickness of the automobile body can be reduced to some extent, but the above-mentioned impact resistance is not essentially improved. This is because these proposals use yield strength or tensile strength, which is an index of steel plate strength, at strain rates of 10 ⁻³ to 10 ^−3.
-2 (s ^-1 ), which is extremely slow, is calculated based only on the so-called static evaluation method, but in actual car body design, safety at the time of collision rather than such static strength is used. This is because the strength based on the so-called dynamic evaluation method, which is accompanied by shock deformation with a strain rate of 10 to 10 ⁴ (s ^-1 ), 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. In addition, Japanese Unexamined Patent Publication No. 7-90482 proposes a steel sheet having a two-phase structure of martensite and ferrite from the viewpoint of improving impact resistance. However, this technique has a problem in that the hole expandability is poor in the duplex structure steel, and it is difficult to apply it to a portion where stretch flange forming is required.

On the other hand, a general hot rolling method which has been conventionally used for producing a hot rolled steel sheet has a thickness of 100 mm
A 300 mm slab was roughly rolled to a thickness of 20 mm to 60 mm and then finish-rolled to a thickness of 10 mm or less. In order to roll to a final strip thickness of less than 1.2 mm by this hot rolling method, the following factors not only make the operation difficult, but also lead to deterioration of product quality and commercialization. Didn't. That is, when the plate thickness becomes extremely thin, the temperature decrease particularly at the surface layer portion and the end portion of the steel sheet during rolling becomes large, and finish rolling is performed within a predetermined temperature range (Ar ₃ transformation point or more) over the entire length and width of the steel sheet. Becomes very difficult. In order to secure this finish rolling temperature, it is necessary to increase the rolling speed, but it becomes difficult to control the shape and the rolling load becomes large. Therefore, a method for controlling the shape of the ultra-thin hot-rolled steel sheet has been proposed, for example, in JP-A-63-260604. In this method, the front end of the material to be rolled is rolled to a predetermined length thicker than the finished plate thickness, and then the running gauge is changed to reduce the thickness to the finished plate thickness and the succeeding part is rolled.

[0006]

However, the method disclosed in the above-mentioned Japanese Patent Laid-Open No. 63-260604 has a problem that the yield loss due to off-gauge is as high as 6%, and an increase in cost cannot be avoided. Further, in the rolling of the ultra-thin hot-rolled steel sheet, the rolling reduction of the finish rolling becomes large, so that it becomes difficult to make the rolling roll bite. In order to avoid this bite failure, if the plate thickness after rough rolling is made thin, the temperature drop of the steel plate during finish rolling becomes even greater,
It becomes difficult to secure the finish rolling temperature. Further, if the reduction ratio of the leading portion of the steel sheet is reduced and the sheet thickness is increased during finish rolling, the yield of the product will be reduced. Incidentally, there are some proposals from the past regarding the technique of making the ferrite grain size fine in a hot-rolled plate having a general plate thickness.
Japanese Patent No. 8796 discloses a method of suppressing the surface layer of 100 μm from the surface to have a particle size of 10 μm or less by starting quenching from the final stage of finish hot rolling. However, the rapid cooling during hot rolling is disadvantageous in stably controlling the hot rolling end temperature within a predetermined temperature range, and also has a difficulty in equipment arrangement. Moreover, even if such a method is applied to ultra-thin steel sheets, the grain size number 12
There has been a problem that it is impossible to make the particles finer. As explained above, with the conventional technology, the plate thickness is 1.2 mm.
There was a problem that the ultra-thin hot-rolled steel sheet of less than 10% could not be manufactured at low cost without lowering the yield. Furthermore, there is a problem that the ultra-thin hot-rolled steel sheet having impact resistance as described above cannot be manufactured at low cost by such a conventional technique.

Therefore, an object of the present invention is to provide a plate thickness excellent in impact resistance at a high strain rate without deteriorating hole expandability 1.
It is to develop ultra-thin hot-rolled steel sheets with a thickness of less than 2 mm. Another object of the present invention is to produce an ultrathin hot-rolled steel sheet having a thickness of less than 1.2 mm, which is excellent in impact resistance under high strain rate without lowering the hole expandability, with good yield and at low cost. Establishing manufacturing technology. The specific object of the present invention is to expand the hole.
75% or more, static-dynamic ratio = dynamic yield stress (strain rate 10 ³ (s
^-1 ) yield stress / static yield stress (strain rate 10 ^-3 (s ^-1 )
When the static-dynamic ratio is 1.6 or more, the absorbed energy up to 30% strain at high strain rate deformation (strain rate 10 ³ (s ^-1 )) is 20.
Absorbed energy of 0 MJ / m ³ or more or up to fracture is 32
It is to propose an ultra-thin hot-rolled steel sheet having a sheet thickness of 0 MJ / m ³ or more and a sheet thickness of less than 1.2 mm and its manufacturing technology.

[0008]

Means for Solving the Problems As a result of intensive studies aimed at realizing the above-mentioned objects, the present inventors have found that the chemical composition, hot rolling method and its conditions, cooling conditions after rolling, winding conditions, etc. are appropriate. By controlling the thickness of the ultrathin hot-rolled steel sheet with a thickness of less than 1.2 mm to be a fine-grained ferrite single phase with a ferrite grain size number of 12 or more, it becomes possible to obtain a surface texture suitable for forming. It was found that the problem of can be solved. That is, the present invention has the following content as a gist configuration.

(1) C: 0.0001 to 0.02 wt%, Si: 0.01 to 2.
0 wt%, Mn: 0.1 to 3.0 wt%, P: 0.01 to 0.15 wt%,
S: 0.010 wt% or less, Al: 0.001 to 0.05 wt%, the balance Fe and unavoidable impurities, the metallic structure is a ferrite single-phase structure with a ferrite grain size number of 12 or more, and the thickness of the oxide layer on the surface. Is 3 μm or less and the plate thickness is 1.2
Ultra-thin hot-rolled steel sheet with less than mm excellent in impact resistance.

(2) C: 0.0001 to 0.02 wt%, Si: 0.01 to 2.
0 wt%, Mn: 0.1 to 3.0 wt%, P: 0.01 to 0.15 wt%,
S: 0.010 wt% or less, including Al: 0.001 to 0.05 wt%,
And B: 0.0001 to 0.01 wt%, Ti: 0.001 to 2.0 wt%, N
b: contains any one or more selected from 0.0005 to 1.0 wt%, the balance consists of Fe and unavoidable impurities, and the metal structure is a ferrite single-phase structure with a ferrite grain size number of 12 or more. The thickness of the oxide layer is 3 μm or less,
Ultra-thin hot-rolled steel sheet with a thickness of less than 1.2 mm and excellent in impact resistance.

(3) C: 0.0001 to 0.02 wt%, Si: 0.01 to 2.
0 wt%, Mn: 0.1 to 3.0 wt%, P: 0.01 to 0.15 wt%,
S: 0.010 wt% or less, including Al: 0.001 to 0.05 wt%,
And Cr: 0.001-2.0 wt%, Ni: 0.001-2.0 wt%, M
o: 0.001 to 2.0 wt%, Cu: 0.001 to 2.0 wt%, and one or more selected from the rest, the balance consisting of Fe and unavoidable impurities, and the metallographic structure of ferrite grain size number 12 or more. An ultra-thin hot-rolled steel sheet with a ferrite single-phase structure, an oxide layer thickness of 3 μm or less, and a sheet thickness of less than 1.2 mm, with excellent impact resistance.

(4) C: 0.0001 to 0.02 wt%, Si: 0.01 to 2.
0 wt%, Mn: 0.1 to 3.0 wt%, P: 0.01 to 0.15 wt%,
S: 0.010 wt% or less, including Al: 0.001 to 0.05 wt%,
And B: 0.0001 to 0.01 wt%, Ti: 0.001 to 2.0 wt%, N
b: contains any one or more selected from 0.0005 to 1.0 wt%, Cr: 0.001 to 2.0 wt%, Ni:
0.001 to 2.0 wt%, Mo: 0.001 to 2.0 wt%, Cu: 0.001
~ 2.0 wt% selected from the group consisting of one or more selected from the group consisting of Fe and unavoidable impurities, the metal structure of which is a ferrite single-phase structure with a ferrite grain size number of 12 or more, and a surface oxide layer The thickness is 3 μm or less, and the plate thickness is
Ultra-thin hot-rolled steel sheet with an impact resistance of less than 1.2 mm.

(5) In producing the ultrathin hot-rolled steel sheet according to any one of the above (1) to (4), the steel material having the composition described in each is heated to 900 to 1250 ° C. The sheet bar is formed by rough rolling, and then this sheet bar is joined to the preceding sheet bar, and the rolling speed is 900 m / min or more,
Rolling finish temperature Ar ₃ transformation point to (Ar ₃ transformation point + 50 ° C) and finish rolling to a sheet thickness of less than 1.2 mm while lubricating under conditions of 95% or more rolling reduction, and then cooling rate of 50 ° C / sec within 0.5 seconds.
A method for producing an ultra-thin hot-rolled steel sheet, which comprises cooling to 450 ° C. or less and winding it into a coil.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION

(1) In the present invention, the reason why the chemical composition of steel is limited as described above will be explained below. C: 0.0001 to 0.02 wt% C is preferably reduced from the viewpoint of improving elongation and r value, which are indicators of press formability, but the content is 0.00
If the amount is less than 01 wt%, the secondary work brittleness resistance is deteriorated and the strength of the welded portion is lowered, which is not preferable. On the other hand, C content is 0.
When it exceeds 02wt%, C to make the ferrite single phase
Ti and Nb for immobilization must be added excessively, which is not preferable. Therefore, the C content is 0.0001 to 0.02 wt.
%, Preferably 0.0003 to 0.008 wt%.

Si: 0.01 to 2.0 wt% Si may be added in a necessary amount according to the target strength level, but if added in excess of 2.0 wt%, the steel sheet hardens and the formability deteriorates. The surface treatability also deteriorates significantly.
Therefore, the upper limit of the Si content is 2.0 wt%. Also, 0.
If it is less than 01 wt%, the manufacturing cost will increase, so the lower limit is made 0.01 wt%.

Mn: 0.1 to 3.0 wt% Mn is preferably reduced from the viewpoint of improving the elongation and r value, which are indicators of press formability, but when it is less than 0.1 wt%, it is used as an automobile material. You cannot get a sufficient strengthening effect. On the other hand, if it is added in excess of 3.0 wt%, the steel sheet is significantly hardened and the formability is lowered. Therefore, the Mn content is in the range of 0.1 to 3.0 wt%, preferably 0.3 to 1.5 wt%.

P: 0.01 to 0.15 wt% P is an element useful for strengthening steel, and at least 0.01
Addition of wt% is required. On the other hand, if added in excess of 0.15 wt%, the hot-rolled base plate is significantly hardened, resulting in deterioration of formability and also remarkable deterioration of surface treatment property. Therefore, the upper limit of the P content is 0.15 wt%.

S: 0.010 wt% or less By reducing the content of S, the precipitates in the steel are reduced and the workability is improved. Such an effect can be obtained by setting the S content to 0.010 wt% or less. 0.0001
Since it takes a very high manufacturing cost to make it less than wt%, it is preferable to set the lower limit to about 0.0001 wt%.

Al: 0.001 to 0.05 wt% Al improves workability by adding 0.05 wt% or less.
If it is less than 1 wt%, inclusions increase, and the workability decreases accordingly. Therefore, the Al content should be 0.001-0.05wt.
% Range.

B: 0.0001 to 0.01 wt% B is an element useful for improving the secondary work embrittlement resistance, and its effect appears when 0.0001 wt% or more is added. on the other hand,
If added over 0.01 wt%, the effect will be saturated, so
The range is 0.0001 to 0.01 wt%, preferably 0.0002 to 0.0020.

Ti: 0.001 to 2.0 wt% Ti is an element useful for improving formability, and its effect is 0.
Although it appears with the addition of 01 wt% or more, even if it exceeds 2.0 wt%, the effect is saturated and the manufacturing cost rises.
The range is 0.01 to 2.0 wt%, preferably 0.01 to 1.5.

Nb: 0.0005 to 1.0 wt% Nb is an element that raises the recrystallization temperature of austenite, facilitates the accumulation of rolling strain in austenite, and reduces the ferrite grain size. The effect appears when 0.0001 wt% or more is added, but even if added over 1.0 wt%, the effect saturates, so 0.0001 to 1.0 wt%, preferably 0.001
The range is to 0.1.

Cr: 0.001 to 2.0 wt% Cr is an element useful for strengthening steel, and it may be added in a necessary amount according to the target strength level. The effect is 0.001
Although it appears when more than 2.0 wt% is added, it is saturated even if added over 2.0 wt% and the manufacturing cost increases. Therefore, the Cr content is set to 0.001 to 2.0 wt%, preferably 0.01 to 1.0.

Ni: 0.001 to 2.0 wt%, Ni is an element useful for strengthening steel, and it may be added in a necessary amount according to the target strength level. The effect is 0.001
Although it appears when more than 2.0 wt% is added, it is saturated even if added over 2.0 wt% and the manufacturing cost increases. Therefore, the Ni content is set to 0.001 to 2.0 wt%, preferably 0.01 to 1.0.

Mo: 0.001 to 2.0 wt% Mo is an element useful for strengthening steel, and may be added in a necessary amount according to the target strength level. The effect is 0.001
Although it appears when more than 2.0 wt% is added, it is saturated even if added over 2.0 wt% and the manufacturing cost increases. Therefore, the Mo content is set to 0.001 to 2.0 wt%, preferably 0.01 to 1.0.

Cu: 0.001 to 2.0 wt% Cu is an element useful for strengthening steel, and may be added in a necessary amount according to a target strength level. The effect is 0.001
Although it appears when more than 2.0 wt% is added, it is saturated even if added over 2.0 wt% and the manufacturing cost increases. Therefore, the Cu content is set in the range of 0.001 to 2.0 wt%, preferably 0.01 to 1.0.

(2) In the ultra-thin hot-rolled steel sheet according to the present invention, as described above, the metal structure is a ferrite single-phase structure having a ferrite grain size number of 12 or more, and the thickness of the oxide layer on the surface is 3 μm or less. There is a need to. The reason is that the metal structure must first be a ferrite single-phase structure in order to secure excellent hole expandability. Further, the reason why the ferrite grain size number is 12 or more is that coarse grains of less than 12 lead to a decrease in dynamic strength and sufficient impact resistance cannot be obtained. FIG. 1 shows the effect of the ferrite grain size number on the absorbed energy obtained from the elongation-stress curve at ^high strain rate (strain rate 10 ³ (s ⁻¹ )) tensile deformation used as an index of impact resistance. Here, the absorbed energy is a value defined in FIG. The manufacturing conditions of FIG. 1 will be described later in Examples. Furthermore, the thickness of the oxide layer on the steel plate surface is 3 μm
What follows is that the productivity of ultra-thin steel sheet is determined by the pickling speed for removing the oxide layer on the surface of the steel sheet, and in order to achieve high productivity, the thickness of the oxide layer on the surface must be 3 μm or less. In addition, when processed without pickling, the oxide layer hardly peels off, which is advantageous in impact resistance after molding.

(3) Next, the ultrathin hot rolled steel sheet according to the present invention is
A steel slab is heated to 900 to 1250 ° C and rough-rolled to form a sheet bar, and then this sheet bar is joined to a preceding sheet bar. Rolling speed is 900 m / min or more, rolling end temperature Ar ₃ transformation point ~ (Ar ₃ transformation point + 50 ℃) and rolling reduction under conditions of 95% or more, finish rolling to a plate thickness of less than 1.2mm, and then within 0.5 seconds at a cooling rate of 50 ℃ / sec or more 450
It is manufactured by cooling to below ℃ and winding into a coil. Each manufacturing condition will be described below.

Rolling heating temperature If the rolling heating temperature is lower than 900 ° C., the finish rolling cannot be carried out at a temperature higher than the Ar ₃ transformation point, and it becomes ferrite region rolling and becomes coarse crystal grains, and the ferrite grain size number is 12
You can't do any more. On the other hand, if it exceeds 1250 ° C, the crystal grain size of ferrite becomes large, and similarly, it becomes impossible to set the ferrite crystal grain size number to 12 or more. Therefore, the heating temperature is in the range of 900 to 1250 ° C.

Finish rolling The slab heated to the above temperature range is roughly rolled into a sheet bar, and this sheet bar is finish rolled. At this time, the reason why the sheet bar is joined to the preceding sheet bar and finish rolling is performed while lubricating is as follows. First, lubrication is performed to reduce the increase in the load on the roll that accompanies an increase in the finishing reduction rate. Also, the sheet bar is to be joined only if the finished plate thickness is less than 1.2 mm, if the tension is not applied on the rolling roll exit side, the plate will wavy on the hot run table.
This is to prevent the shape from being deteriorated and to prevent the plate from slipping due to lubrication and efficient rolling. By rolling in this way, it becomes possible to improve the yield at the front and rear ends of the plate, which had a poor yield in the past. The rolling speed of 900 m / min or more is necessary to suppress the temperature drop during rolling and maintain the finish rolling end temperature at the Ar ₃ transformation point to (Ar ₃ + 50 ° C.). Is.

The finish rolling finish temperature is set to the Ar ₃ transformation point to (Ar ₃
(+ 50 ° C.) is because rolling at a rolling end temperature lower than the Ar ₃ transformation point results in rolling in the ferrite region, and coarse crystal grains do not become fine grains of ferrite grain size number 12 or more, (Ar ₃ + 50 ° C.) If it exceeds, the accumulation of strain in the austenite grains before transformation into ferrite will be insufficient, and the ferrite grain size number after transformation will likewise not be 12 or more. Also, when the finishing reduction rate is less than 95%, the ferrite grain size number does not exceed 12 for the same reason. Therefore, the rolling end temperature of finish rolling is set to the Ar ₃ transformation point to (Ar ₃ +50).
℃) and reduction rate of 95% or more.

Cooling and winding after rolling In order to control the ferrite crystal grain size in an ultra-thin steel sheet having a sheet thickness of less than 1.2 mm, it is extremely important to quench immediately after finish rolling to the above sheet thickness. After finishing rolling,
Cooling to 450 ° C or less at a cooling rate of 50 ° C / sec or more within 0.5 seconds is to allow the strain accumulated in austenite to be released when the material is allowed to cool for more than 0.5 seconds after the completion of rolling to obtain a predetermined ferrite grain size. If the cooling rate is less than 50 / sec, the specified ferrite grain size cannot be obtained for the same reason. Furthermore, if the cooling stop temperature exceeds 450 ° C, the ferrite grains become coarse. This is because the desired ferrite grain size cannot be obtained. If the cooling stop temperature is too low, uniform cooling will be difficult and the variation in materials will increase.
C is desirable. In addition, in order to suppress the thickness of the oxide layer (scale) on the surface of the steel sheet to 3 μm or less, especially at a cooling rate of 50 ° C / sec within 0.5 seconds after finishing rolling.
Thus, it is effective to cool it to 450 ° C or lower.

The effect of the ultra-thin hot-rolled steel sheet according to the present invention can be similarly given to the surface-treated steel sheet using this material. Further, in the above description, the use for automobiles was exclusively described, but it goes without saying that the technique according to the present invention is similarly effective for other uses requiring strength under a high strain rate. .

[0034]

【Example】

-Example The steel of the chemical composition shown in Table 1 was melted in the converter. The steel slabs of these components are heated and rough rolled under the conditions shown in Table 2 to form a sheet bar of 25 to 35 mm, and this sheet bar is then joined to the preceding sheet bar, and the rolling speed and rolling under each condition are performed. Finishing rolling was performed while lubricating (using a lubricating oil) at the finishing temperature and the rolling reduction, followed by cooling and winding to obtain an ultrathin steel sheet having the thickness shown in Table 3.

[0035]

[Table 1]

[0036]

[Table 2]

These hot-rolled steel sheets thus obtained have a medium thickness direction.
The structure is observed at the center position and the ferrite grain size is
(JIS) and measure the thickness of the oxide layer on the surface of the steel sheet
The texture was measured by a structure photograph. Also these obtained
From hot-rolled steel sheet, pulling parallel part width 5mm, length 10mm
Sampling rate is 10^Three(s^-1) And 10^-3(s^-1)
Tensile test is performed, and the static-dynamic ratio is calculated from each yield stress.
Was. Moreover, according to the definition of FIG.^Three(s ^-1))
Energy up to 30% strain and absorption energy up to fracture
I asked for energy. Furthermore, measure the hole expansion ratio and
It was used as an index of flangeability. These measured characteristic values are
It is shown in Table 3.

[0038]

[Table 3]

By any of the methods of the present invention, an ultrathin steel sheet having a ferrite grain size number of 12 or more and a good shape could be produced with a good yield. As is clear from the results shown in Table 3, the characteristics are as follows: static-dynamic ratio is 1.6 or more, absorbed energy up to 30% strain is 200 MJ / m ³ or more, or absorbed energy up to fracture is 320 MJ / m ³ or more. It has excellent impact resistance and good stretch flangeability with a hole expansion rate of 75% or more.

[0040]

As described above, according to the present invention,
Manufacture of ultra-thin hot-rolled steel sheets that have a better static-dynamic ratio than before and are also excellent in forming processing such as stretch flangeability by having a ferrite structure in which the chemical composition, scale thickness and grain size of the steel sheet are properly controlled. It becomes possible to do. Moreover, according to the method of the present invention, since this steel sheet can be stably manufactured with high yield, productivity can be improved and an inexpensive hot-rolled steel sheet can be manufactured. Therefore, by applying the ultrathin hot-rolled steel sheet according to the present invention for automobiles, it becomes possible to economically achieve the weight reduction and the improvement of safety of the automobile body without impairing the press formability. .

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between absorbed energy at the time of high strain rate deformation and a ferrite crystal grain size number.

FIG. 2 is a diagram that defines absorbed energy during high strain rate deformation.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location C22C 38/58 C22C 38/58 (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Address Kawasaki Iron & Steel Co., Ltd. Technical Research Laboratory (72) Inventor Takashi Obara 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel & Industrial Co., Ltd. Technical Research Institute

Claims (5)

[Claims] 1. C: 0.0001 to 0.02 wt%, Si: 0.01 to 2.0 wt%, Mn: 0.1 to 3.0 wt%, P: 0.01 to 0.15 wt%, S: 0.010 wt% or less, Al: 0.001 to 0.05 wt% %, The balance consisting of Fe and unavoidable impurities, the metallic structure is a ferrite single-phase structure with a ferrite grain size of 12 or more, and the thickness of the oxide layer on the surface is 3 μm or less and the plate thickness is less than 1.2 mm. An ultra-thin hot rolled steel sheet with excellent impact resistance. 2. C: 0.0001 to 0.02 wt%, Si: 0.01 to 2.0 wt%, Mn: 0.1 to 3.0 wt%, P: 0.01 to 0.15 wt%, S: 0.010 wt% or less, Al: 0.001 to 0.05 wt% %, And contains any one or more selected from B: 0.0001 to 0.01 wt%, Ti: 0.001 to 2.0 wt%, Nb: 0.0005 to 1.0 wt%, and the balance Fe and unavoidable impurities. An ultra-thin hot-rolled steel sheet consisting of a ferrite single-phase structure with a ferrite grain size number of 12 or more, a surface oxide layer thickness of 3 μm or less, and a sheet thickness of less than 1.2 mm, which is excellent in impact resistance. 3. C: 0.0001 to 0.02 wt%, Si: 0.01 to 2.0 wt%, Mn: 0.1 to 3.0 wt%, P: 0.01 to 0.15 wt%, S: 0.010 wt% or less, Al: 0.001 to 0.05 wt% %, And contains any one or more selected from Cr: 0.001 to 2.0 wt%, Ni: 0.001 to 2.0 wt%, Mo: 0.001 to 2.0 wt%, Cu: 0.001 to 2.0 wt%. The balance consists of Fe and unavoidable impurities, the metal structure is ferrite single-phase structure with ferrite grain size number 12 or more, the thickness of the oxide layer on the surface is 3 μm or less, and the plate thickness is less than 1.2 mm Impact resistance Ultra-thin hot rolled steel sheet with excellent heat resistance. 4. C: 0.0001 to 0.02 wt%, Si: 0.01 to 2.0 wt%, Mn: 0.1 to 3.0 wt%, P: 0.01 to 0.15 wt%, S: 0.010 wt% or less, Al: 0.001 to 0.05 wt% % And B: 0.0001 to 0.01 wt%, Ti: 0.001 to 2.0 wt%, Nb: 0.0005 to 1.0 wt%, and any one or more selected from Cr: 0.001 to 2.0 wt%. %, Ni: 0.001 to 2.0 wt%, Mo: 0.001 to 2.0 wt%, Cu: 0.001 to 2.0 wt%, and the balance contains Fe and unavoidable impurities. Ultra-thin hot-rolled steel sheet with excellent impact resistance with a ferrite single-phase structure with a ferrite grain size number of 12 or more, a surface oxide layer thickness of 3 μm or less, and a sheet thickness of less than 1.2 mm. 5. When manufacturing the ultra-thin hot-rolled steel sheet according to any one of claims 1 to 4, a steel material having the composition of the composition described in each paragraph is heated to 900 to 1250 ° C. A sheet bar is formed by rolling, then this sheet bar is joined to the preceding sheet bar, and the rolling speed is 900 m / min or more, the rolling end temperature Ar ₃ transformation point to (Ar ₃ transformation point + 50 ° C) and the rolling reduction of 95% or more. An ultra-thin hot-rolled steel sheet characterized by being finish-rolled to a thickness of less than 1.2 mm while being lubricated under the conditions described above, then cooled to 450 ° C or less at a cooling rate of 50 ° C / sec or more within 0.5 seconds and then wound into a coil. Manufacturing method.