JPH09296252A - Thin hot rolled steel sheet excellent in formability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a thin hot rolled steel sheet showing good formability even in the tip part in the width direction by specifying the steel componental compsn., hot rolling conditions, the quantity of crowns and the relation between the same and the sheet thickness. SOLUTION: A steel slab contg., by weight, 0.02 to 0.20% C, <=1.00% Si, 0.05 to 1.50% Mn, <=0.04% P, <=0.015% S, 0.005 to 0.150% Al, <=0.020% N, and the balance Fe is heated at <=1200 deg.C and is subjected to rough rolling, and the obtd. sheet bar is coiled and is held hot. This is recoiled and is joined with the article preceding thereto, the sheet width edge part is heated in such a manner that its temp. is made higher than that of the sheet width center part by >=50 deg.C, and finish continuous rolling using pair cross rolling in one or more stages is executed in the poststage. Next, it is air-cooled for >=2sec, is thereafter water-cooled in such a manner that cooling water from one or more directions of the upper and lower directions does not directly hit the edge part of the steel sheet and is coiled. Then, the steel sheet in which the sheet thickness is regulated to <=1.2mm, the quantity of crowns is regulated to <=30μm, and the ratio of the crown quantity/the sheet thickness is regulated to <0.030, having a ferritic structure free from strains over the whole width direction, and in which the surface hardness in the center part in the width direction does not fall below the surface hardness at a position of 5mm sheet edge in the width direction can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a thin hot-rolled steel sheet that is suitable for forming such as press forming and that is used as hot-rolled.

[0002]

2. Description of the Related Art Thin steel sheets include hot-rolled steel sheets and cold-rolled steel sheets. Conventionally, cold-rolled steel sheets have been exclusively used for applications requiring formability, particularly deep drawing formability. . The reason why the cold-rolled steel sheet is used for such an application is that, in the cold-rolled steel sheet, it is possible to perform a higher degree of texture control, so that an excellent deep drawability of r value (Rankford value) of 1.4 or more is achieved. It is possible to achieve However, since this cold-rolled steel sheet is manufactured through more steps, it is necessarily disadvantageous in terms of cost. On the other hand, the hot rolled steel sheet is advantageous in cost, but is generally inferior in formability. Therefore, efforts have been continued to improve the formability of hot-rolled steel sheets, and in addition to the reduction of non-metallic inclusions, the improvement of elongation to compensate for the low r value and the lubrication during forming. Attempts have been made to expand the range of application by techniques such as characteristic improvement (eg, Japanese Patent Publication No. 6-78568).

[0003]

However, it has been the actual situation that the conventional hot-rolled steel sheet manufacturing techniques described above have not yet been able to achieve sufficient formability or expansion of the scope of application. As described above, the following items can be listed as the main factors that hinder the molding processability or the expansion of the applicable range. 1) A so-called edge drop in which the plate thickness at the end portion in the width direction of the steel plate is extremely thinner than the plate thickness at the central portion, and the cross-sectional shape is not uniform. 2) The hardness of the end portion in the width direction of the steel sheet is higher than the hardness of the central portion, and the material is non-uniform. 3) Uneven structure (including crystal grain size) in the width direction of the steel sheet. These items did not cause much problems up to a plate thickness of 3 to 4 mm. However, due to the recent trend of thinner plate thickness, it is 2 mm or less, more specifically 1.2 m.
When trying to manufacture a thin hot-rolled steel sheet of m or less,
The above factors become apparent. For this reason, when manufacturing a thin steel sheet by the conventional technology, it is necessary to largely trim the end in the width direction before commercialization, which leads to a decrease in the yield of materials and products or an increase in cost. There was a problem.

Therefore, an object of the present invention is to provide a thin hot-rolled steel sheet which suppresses edge drop of the steel sheet, has a uniform hardness and structure in the width direction of the steel sheet, can withstand severe forming, and can be replaced with a cold-rolled steel sheet. It is to provide the manufacturing method. Another object of the present invention is to provide a method for manufacturing a thin hot-rolled steel sheet having good formability even in the widthwise end portion by making the strip temperature uniform in the hot rolling step and optimizing the rolling method. To provide.

[0005]

In order to achieve the above-mentioned object, the inventors conducted a metallurgical investigation on the mechanical properties of the current steel sheet and made a detailed study on the steel composition and hot rolling conditions. Study was carried out. As a result, among the hot-rolled steel sheets, particularly in the case of a thin hot-rolled steel sheet having a sheet thickness of 1.2 mm or less, in addition to regulating the absolute value of the crown amount of the steel sheet, the relationship between this and the sheet thickness is fixed. It has been found that the formability is dramatically improved by restricting to a range, eliminating the structural difference in the plate width direction, and controlling the hardness difference between the center part and the end part of the plate width within a certain range. . The present invention has been completed based on the above findings, and its gist configuration is as follows.

(1) C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.01
5 wt% or less, Al: 0.005-0.150 wt%, N: 0.020 wt%
Contains the following, the balance consisting of Fe and unavoidable impurities,
The plate thickness is 1.2 mm or less, the crown amount is 30 μm or less, the crown amount / plate thickness is less than 0.030, and it consists of a ferrite structure with no distortion over the entire width direction, and the surface hardness of the width direction central part is 5 mm in the width direction plate edge. A thin hot-rolled steel sheet with excellent formability, which is characterized by not lowering the surface hardness at the position.

(2) C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.
015 wt% or less, Al: 0.005-0.150 wt%, N: 0.020
wt% or less, and Nb: 0.0030 to 0.0400 wt%, Ti: 0.
It contains one or more selected from B: 0.0005 to 0.0020 wt% and the balance is Fe and unavoidable impurities, the plate thickness is 1.2 mm or less, and the crown amount is 30.
μm or less, crown amount / plate thickness is less than 0.030, and consists of a ferrite structure without distortion over the entire width direction, and the surface hardness at the widthwise central portion does not fall below the surface hardness at the widthwise plate edge 5 mm position. A thin hot-rolled steel sheet with excellent formability.

(3) C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.0.
015 wt% or less, Al: 0.005-0.150 wt%, N: 0.020
Includes wt% or less, Cu: 0.50 wt% or less, Ni: 0.50 wt
% Or less, Cr: 0.50 wt% or less, and one or more selected from the rest, the balance being Fe and inevitable impurities,
The plate thickness is 1.2 mm or less, the crown amount is 30 μm or less, the crown amount / plate thickness is less than 0.030, and it consists of a ferrite structure without distortion over the entire width direction, and the surface hardness of the width direction center part is 5 mm in the width direction plate edge. A thin hot-rolled steel sheet with excellent formability, which is characterized by not lowering the surface hardness at the position.

(4) C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.
015 wt% or less, Al: 0.005-0.150 wt%, N: 0.020
wt% or less, and Nb: 0.0030 to 0.0400 wt%, Ti: 0.
It contains one or more selected from B: 0.0005 to 0.0020 wt% and further contains Cu: 0.50 wt% or less, Ni: 0.50 wt% or less, Cr: 0.50 wt% or less. Contains 1 or 2 or more kinds, the balance is Fe and unavoidable impurities, and has a plate thickness of 1.2 mm or less and a crown amount of 30.
μm or less, the amount of crown / plate thickness is less than 0.030, and consists of a ferrite structure without distortion over the entire width direction, and the surface hardness of the widthwise central part does not fall below the surface hardness at the widthwise plate edge 5 mm position. A thin hot-rolled steel sheet with excellent formability.

(5) C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.
015 wt% or less, Al: 0.005-0.150 wt%, N: 0.020
A steel slab containing less than wt% is heated to a temperature of 1200 ° C or less and rough-rolled, the obtained sheet bar is wound and kept heat, then this sheet bar is rewound and joined with the preceding sheet bar. Then, the strip width edge portion of the sheet bar is heated so that the strip width edge portion is higher than the strip width center portion by 50 ° C. or more, and at least one stage is subjected to finish continuous rolling employing pair cross rolling, and rolling is completed. After air-cooling for 2 seconds or more, the water is cooled while being masked so that at least one of the upper and lower cooling water does not hit the edge portion of the steel sheet directly, and is wound up. (1) to (4) The method for manufacturing a thin hot-rolled steel sheet according to any one of 1.

(6) C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.
015 wt% or less, Al: 0.005-0.150 wt%, N: 0.020
A steel slab containing less than wt% is heated to a temperature of 1200 ° C or less and rough-rolled, the obtained sheet bar is wound and kept heat, then this sheet bar is rewound and joined with the preceding sheet bar. In addition, the plate width edge part of the sheet bar is heated so that the plate width edge part is higher than the plate width center part by 50 ° C. or more, and at least one stage adopts pair cross rolling,
In addition, at least one of the subsequent stages is subjected to finish continuous rolling while lubricating, and after air cooling for 2 seconds or more after completion of rolling, masking is performed so that at least one of the upper and lower cooling water does not directly hit the edge part of the steel sheet. The method for producing a thin hot-rolled steel sheet according to any one of the above (1) to (4), characterized by cooling with water while winding.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. (1) Steel composition; C: 0.02 to 0.20 wt% When the amount of C exceeds 0.20 wt%, the hardenability increases, and as a result, it becomes difficult to stabilize the material by cooling control on the hot run table. However, it causes deterioration of toughness and makes deep drawing difficult. On the other hand, when the content is less than 0.02 wt%, the detailed mechanism is not always clear, but it becomes difficult to control the amount of dissolved C throughout the entire processing heat treatment process.
Therefore, the C content needs to be in the range of 0.02 to 0.20 wt%. Further, in order to further stabilize the material and improve the workability, it is desirable to set it in the range of 0.03 to 0.15 wt%.

Si: 1.00 wt% or less Si is a useful element for reducing the amount of oxides in steel, but if added in excess of 1.00 wt%, the strength is remarkably increased and the ductility is lowered, resulting in a press. Molding becomes difficult. Therefore, the Si content is 1.00 wt% or less, preferably 0.50 wt%.

Mn: 0.050 to 1.50 wt% Mn is a useful element that suppresses surface cracking due to hot embrittlement due to S and brings about the effect of making the structure uniform and fine. These effects are obtained by adding at least 0.05 wt%, but if added over 1.50 wt%, the mechanism is not necessarily clear, but the uniformity of the material tends to decrease. Therefore, the amount of Mn added is 0.05 to 1.50 wt.
%. If workability is particularly important, 0.60
It is desirable to set it to wt% or less.

P: 0.04 wt% or less P is an element that reduces workability and corrosion resistance. P
When the amount exceeds 0.04 wt%, the effect becomes remarkable, so the content is made 0.04 wt% or less, preferably 0.02 wt% or less. However, excessive reduction of the amount of P leads to an increase in manufacturing cost and is not desirable.

S: 0.015 wt% or less S is an element that adversely affects workability. S amount 0.01
By setting the content to 5 wt% or less, press workability (especially stretch flange property) can be significantly improved. Note that when particularly high local ductility is required, it is desirable to reduce it to 0.007 wt% or less.

Al: 0.005 to 0.150 wt% Al acts as a deoxidizer and is an essential element for improving cleanliness, and it is necessary to add approximately 0.005 wt% or more. On the other hand, if it is added in excess of 0.150 wt%, the cleanliness improvement effect will be saturated, and there will be problems such as an increase in manufacturing cost and an increase in the tendency of surface defects to occur. Therefore, the amount of Al added is 0.005 to 0.150 wt%, preferably 0.020 to 0.08.
0 wt%

N: 0.020 wt% or less N causes deterioration of the internal material, so N must be limited to 0.020 wt% or less. When importance is attached to aging resistance, it is desirable to reduce it to 0.0050 wt% or less, and to further lower it to 0.0030 wt% or more when a good r-value level and stable bake hardenability are required.

In addition to the above basic components, Nb: 0.0030 to 0.04
00wt%, Ti: 0.0030-0.1000wt% and B: 0.0005-0.
Any one or more selected from the group of Cu: 0.50 wt% or less, Cu: 0.50 wt% or less, and Cr: 0.50 wt% or less can be added.

Nb: 0.0030 to 0.0400 wt% Nb has the effect of refining the steel structure, and it is desirable to add Nb when a beautiful appearance after forming is required.
Such an effect is obtained by adding 0.0030 wt% or more. Although the mechanism is not always clear when added in excess of 0.0400 wt%, it is not preferable because it increases the hardness of the steel plate end portion. For this reason, when adding Nb, 0.
0030-0.0400wt%. From the viewpoint of material stability, the range is preferably 0.020 wt% or less.

Ti: 0.0030 to 0.1000 wt% Similar to Nb, Ti is an element effective for refining the steel structure. It is also useful for increasing the strength of steel by precipitation strengthening. These effects are exhibited by addition of 0.0030 wt% or more, but addition of more than 0.1000 wt% not only saturates the effect but also increases the risk of surface defects. Therefore,
The amount of Ti added is 0.0030 to 0.1000 wt%.

B: 0.0005 to 0.0020 wt% B is an element effective for making the structure fine, and the effect can be obtained by adding 0.0005 wt% or more. However, adding more than 0.0020 wt% is not preferable because it significantly increases the hardness of the steel plate edge.

Cu: 0.50 wt% or less Cu is an element useful for improving the uniformity of the material of steel, but if added in excess of 0.50 wt%, the effect will be saturated and the cost will increase. , 0.50 wt% or less.

Ni: 0.50 wt% or less Ni, like Cu, is an element useful for improving the uniformity of the material quality of steel, but even if added in excess of 0.50 wt%, the effect saturates. Add cost in the range of 0.50wt% or less because it will increase the cost.

Cr: 0.50 wt% or less Cr, like Cu, is an element useful for improving the uniformity of the material quality of steel, but even if added in excess of 0.50 wt%, the effect saturates. Add cost in the range of 0.50wt% or less because it will increase the cost.

(2) Plate Thickness, Crown, Hardness, and Structure of Steel Plate; Conventional hot-rolled steel plates having a plate thickness of about 4.0 to 2.3 mm have been used for advanced forming.
When the plate thickness changes, the molding characteristics also change, but the change is not monotonous, especially when the plate thickness is 1.2 mm or less, a higher level of quality, specifically in the plate width direction. It became clear that homogeneity such as thickness and hardness is required.

That is, in the case of a comparatively thick hot-rolled steel sheet having a thickness of more than 1.2 mm, (the thickness of the central portion in the width direction)-
Even if the crown amount defined by (the plate thickness of the plate end portion in the plate width direction of 25 mm) is about 70 to 80 μm, which is the conventional level, it did not hinder the molding process. However, when the plate thickness is reduced to 1.2 mm or less, with the conventional level of crown amount, troubles frequently occur during press forming, especially with the material taken from the end portion in the width direction. This is because the wrinkle suppressing force cannot be applied during press molding, especially deep drawing, because the plate thickness at the width end is too thin, resulting in uneven flow of material. It seems that In order to prevent such troubles in press molding and enable deep drawing, it is even more strict than the above definition: (thickness of the central portion in the width direction)-(plate end portion 5 mm in the width direction). It is necessary to limit the amount of crown defined by (plate thickness of) to 30 μm or less, and set (crown amount) / (plate thickness) to 0.030 or less.

The structure is an important factor for preventing troubles in press molding. That is, it is necessary that the ferrite grains be formed without distortion over the entire plate width direction of the product. This is because the hardness in the width direction becomes uneven if there is a structure in which distortion remains in the width direction of the steel sheet.
The residual strain described here can be determined by the width spread of the X-ray diffraction peak as is well known. In the steel of the present invention, the full width at half maximum of the (200) peak corresponds to 0.20 ° or less. In addition, it is possible to make a determination by the presence or absence of a strained pattern even by observing the structure with an ordinary optical microscope. Although related to the above structure, the hardness difference in the width direction of the steel sheet is also an important requirement for preventing troubles in press forming. That is, the surface hardness at the widthwise plate edge 5 mm position is smaller than the surface hardness at the widthwise central portion, that is, the widthwise plate edge 5
When the surface hardness at the mm position is He and the surface hardness at the center portion in the width direction is Hc, it is necessary that Hc−He ≧ 0 is satisfied, that is, He does not fall below Hc. If this condition is not satisfied, when the material including the edge portion is press-molded, for example, the flange portion is caused to flow in unevenly, and it is determined that the molding is defective. About HR30T hardness, He exceeds Hc by 1 point or more, which is advantageous and desirable from the viewpoint of stability of quality, but 10
Exceeding more than the point is not preferable from the viewpoint of stable press moldability.

(3) Manufacturing conditions: It is desirable that the slab as a rolling material is manufactured by a continuous casting method in order to minimize the deviation of the components. Then, hot rolling including rough rolling and finish rolling is performed. In the hot rolling, the slab after casting may be once cooled and then reheated as usual, or may be inserted into the heating furnace as a hot piece. The heating temperature of the slab at this time must be 1200 ° C or lower. By heating at a low temperature of 1200 ° C or less, it is possible to reduce material variation in the plate width direction. This is probably because the initial structure is uniformly refined.

The high temperature slab is generally subjected to hot rough rolling.
Use a sheet bar with a thickness of 20 to 70 mm. In the following steps, a sheet bar manufactured by a sheet bar caster or the like can be treated in the same manner. In order to improve the material level and the homogeneity of the material, these sheet bars need to be wound around a coil and kept heat before the finish rolling. Although the detailed mechanism is not always clear, homogenization of the material is promoted by the heat-treatment and a slight bending strain applied at the time of winding. This coil is rewound to reverse the front end and the rear end, and the front end of the seat bar after reversing is joined to the rear end of the preceding sheet bar not only in the width direction of the seat bar but also in the longitudinal direction. Contributes to uniform rolling temperature. Next, the width end portion (about 25 to 100 mm from the plate end) of this sheet bar is at a temperature higher than the center portion of the plate width by 50 ° C. or more on the entry side of the finishing rolling mill (the upper limit is not specified, but is 150 ° C. or less). It is better to). In the normal rolling method, a remarkable uneven temperature distribution occurs in the width direction of the steel sheet by the rough rolling process, and the temperature at the edge of the sheet bar is 50% higher than at the center in the same width direction.
The temperature is as low as ℃ or more. In the present invention, in order to compensate for this temperature, the material is made uniform over the entire width by heating with an edge heater. The heating means is not particularly limited, but examples thereof include high frequency induction heating and gas heating.

It is to be noted that joining the sheet bars on the entry side of the finish rolling mill and continuously performing the finish rolling not only makes the finish rolling temperature uniform but also applies tension to the leading and trailing edges to make the steel sheet longitudinal direction. It greatly contributes to the homogenization of the structure, the stabilization of the material, and the stabilization of the rolling shape. The joining method of the sheet bars is not particularly specified, and a plurality of sheet bars may be continuously supplied to finish rolling, and any means may be used.

For finish rolling, a tandem rolling mill having 6 to 7 stages is usually used. It is most desirable to apply the pair cross rolling mechanism to all of the 6 to 7 stages, but it is necessary to apply the pair cross rolling mechanism to at least one of the subsequent stages. The application of the pair cross rolling mechanism is extremely effective in reducing edge drops and crowns, and is essential for achieving the crown amount of 30 μm or less defined in the present invention.

Further, as means for improving the uniformity of the sectional shape of the steel sheet and the uniformity of the structure in the thickness direction, it is effective to apply lubrication rolling and finish rolling. At this time, the type of oil used for lubrication, the method of applying oil, etc. are not specified, but the friction coefficient estimated from the load data of the rolling mill, etc., will be remarkable if a condition of about 0.15 or less is realized. It is recommended to use a lubricating oil that satisfies this friction coefficient.

The finish rolling finish temperature may be determined according to the material level. If the temperature is too high, the structure becomes non-uniform, while if it is too low, the generated ferrite remains strained and the rolling load increases. In addition, the ductility of the product will be reduced. Therefore, the preferred rolling end temperature is
The range is (Ar ₃ transformation point −150 ° C.) to (Ar ₃ transformation point + 150 ° C.).

After finishing rolling, at least 2 se
It is necessary to provide an air cooling time (water cooling start delay time) or more to improve the material uniformity in the width direction of the hot rolled coil. The mechanism of improvement is not necessarily clear, but if water cooling is started immediately after finishing rolling is completed, the strain of ferrite strained by rolling in the low temperature region will not be sufficiently released, but at a certain elapsed time. If this is the case, it is considered that the strain in the ferrite structure is released by the self-annealing effect in a sense, and the material is made uniform. Therefore, it is necessary to secure an air cooling time of 2 seconds or more after the hot rolling and before the water cooling. The upper limit time of air cooling is not particularly specified, but is naturally determined by the constraint on the hot rolling equipment that can secure the winding temperature described later.

As described above, when the steel sheet air-cooled for a predetermined time is water-cooled on the hot run, it is necessary to realize uniform cooling in the width direction of the steel sheet. Cooling water gushes from the nozzle,
It was confirmed that although the steel plate collides with the steel plate, the position where it is directly exposed to water has the greatest cooling efficiency, and that the width end has a higher cooling efficiency than the center part. As a means for solving such non-uniform cooling, an equipment study was conducted so that the cooling water does not directly hit the width end of the steel sheet. As a result, good results were obtained when masking the cooling water in the range of 50 to 150 mm at the width end. It is most desirable to perform this masking on both the upper and lower parts, but it is effective if it is performed on at least one.

The coiling temperature of the steel strip after finish rolling and cooling is changed according to the target material level, but it is 600 to 750 ° C. from the viewpoint of material stability and operation stability.
It is preferable to set it in the range. The hot-rolled coil manufactured by the above-mentioned method becomes a product by removing the scale by a usual method.

[0038]

Example 1 Various steels shown in Table 1 were melted into a continuous cast slab,
The slab was heated in the temperature range of 1100-1150 ° C. After that, a sheet bar having a thickness of 35 mm was roughly rolled, wound once, then rewound, induction heated, and joined to the trailing end of the preceding sheet bar. The range of 120 mm from the width edge of the joined sheet bars is heated to a temperature 50 to 100 ° C higher than the center of the sheet width using an edge heater, and this is feedback-controlled to optimize the rolling end temperature. It was The finish rolling machine used was a 7-stage tandem mill and had a pair cross function in all stages, and the pair cross function was applied to 5 to 7 stages among them. The rolling end temperature was from the Ar ₃ transformation point to (Ar ₃ transformation point −50 ° C.). After finishing rolling, water cooling was started after 2-3 seconds of air cooling. At this time, in the range of 100 mm in the plate width direction, masking was performed with a gutter so that the cooling water from the upper cooling nozzle of the hot run did not directly collide with the steel plate. Then, it is wound at 680 ° C and subjected to normal pickling, and the plate width is 1520m.
A hot rolled steel sheet having a thickness of 1.2 mm and a thickness of 1.2 mm was manufactured.

Using the obtained hot-rolled steel sheet as a test material, the amount of crown, microstructure, and Rockwell hardness HR30T were measured, and a cylindrical deep drawing test was conducted in order to investigate the deep drawing characteristics at the edges in the plate width direction. went. In the cylindrical deep-drawing test, blanking was performed so that the end of the blank was located 3 mm from the width edge of the steel plate, blank diameter: 180 mm, punch diameter: 95 mm,
Wrinkle suppression: 700 kgf, Lubricant: Beef tallow, Dice Shoulder radius:
It was carried out under the condition of 5 mm. After drawing into a cylinder, the height of the cylinder was measured in the circumferential direction, and the difference between the maximum height and the minimum height was measured. If this difference is large, it means that the material flow is not uniform. Table 2 shows the above measurement and test results.
Are shown together. It should be noted that the deep drawing characteristics of all the samples taken from positions other than the widthwise end could be molded without any problem.

[0040]

[Table 1]

[0041]

[Table 2]

As is clear from the results obtained, the thin hot-rolled steel sheet produced by the method of the present invention has the following features: crown amount, crown / sheet thickness, difference in hardness between width end portion and central portion, and structure. It can be seen that the range is satisfied, and even the width end portion has good deep drawing characteristics.

Example 2 Steel A having the composition shown in Table 1 was hot-rolled under the conditions shown in Table 3 and subjected to normal pickling, and a thin hot-rolled steel sheet having a sheet width of 1250 mm and various sheet thicknesses. Was manufactured. The properties of these thin hot-rolled steel sheets were investigated in the same manner as in Example 1, and a cylindrical deep drawing test was conducted. However, the clearance of the mold in the cylindrical deep drawing test was adjusted for each plate thickness such that the plate thickness x (1 + 0.2). The results of these tests are also shown in Table 4.

[0044]

[Table 3]

[0045]

[Table 4]

From the above results, in the thin hot-rolled steel sheet produced by the method of the present invention, the amount of crown, the crown / sheet thickness, the difference in hardness between the width end portion and the central portion, and the structure satisfy the range of the invention,
It is clear that the material in the width direction and the plate thickness are uniform, and that even at the width end portion, good deep drawing characteristics are obtained.

[0047]

As described above, according to the present invention, when low carbon steel is used as the raw material and is manufactured by strictly controlling the hot rolling process and the cooling process, the edge drop of the steel plate is eliminated and the steel plate width is reduced. Uniformity of structure and hardness can be realized. Therefore, according to the present invention, even in the widthwise end portion, the same press forming as in the central portion can be obtained, troubles in the press forming step can be avoided, and the trimming allowance accompanying homogenization in the steel sheet width direction can be obtained. It is possible to reduce the yield (improve the yield). Further, when the hot rolled steel sheet according to the present invention is further subjected to cold rolling to obtain a cold rolled steel sheet, it is uniform in the width direction, and thus a good cold rolling material can be provided,
It contributes to the improvement of the shape of the cold rolled steel sheet.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/54 C22C 38/54

Claims (6)

[Claims] 1. C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.015 wt% or less, Al: 0.005 to 0.150 wt%, N: 0.020 wt% or less, the balance consisting of Fe and unavoidable impurities, the plate thickness is 1.2 mm or less, the crown amount is 30 μm or less, and the crown amount / plate thickness is less than 0.030, and there is no distortion in the entire width direction. It consists of a ferrite structure, and the surface hardness at the widthwise center does not fall below the surface hardness at the widthwise plate edge 5 mm position.
A thin hot-rolled steel sheet having excellent formability, which is characterized in that 2. C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.015 wt% or less, Al: 0.005 to 0.150 wt%, N: 0.020 wt% or less, Nb: 0.0030 to 0.0400 wt%, Ti: 0.0030 to 0.1000 wt%, B: 0.0005 to 0.0020 wt%, and one or more selected from the rest, and the balance Fe and It consists of unavoidable impurities and has a plate thickness of 1.2 mm or less, a crown amount of 30 μm or less, a crown amount / plate thickness of less than 0.030, and a ferrite structure with no distortion over the entire width direction, and the surface hardness at the center in the width direction. Is the width direction plate edge 5m
A thin hot-rolled steel sheet having excellent formability, which is characterized by not lowering the surface hardness at the m position. 3. C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.015 wt% or less, Al: 0.005 to 0.150 wt%, N: 0.020 wt% or less, Cu: 0.50 wt% or less, Ni: 0.50 wt% or less, Cr: 0.50 wt% or less, and one or more types selected, with the balance being Fe and inevitable impurities. With a thickness of 1.2 mm or less, a crown amount of 30 μm or less, and a crown amount / plate thickness of less than 0.030, consisting of a ferrite structure with no distortion over the entire width direction, and having a surface hardness in the width direction central portion in the width direction. Board edge 5m
A thin hot-rolled steel sheet having excellent formability, which is characterized by not lowering the surface hardness at the m position. 4. C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.015 wt% or less, Al: 0.005 to 0.150 wt%, N: 0.020 wt% or less, and one or more selected from Nb: 0.0030 to 0.0400 wt%, Ti: 0.0030 to 0.1000 wt%, B: 0.0005 to 0.0020 wt%, and Cu: 0.50. wt% or less, Ni: 0.50 wt% or less, Cr: 0.50 wt% or less, and the balance consists of Fe and inevitable impurities. The plate thickness is 1.2 mm or less and the crown amount is 30 μm or less, crown amount / plate thickness of less than 0.030, consisting of a ferrite structure with no strain over the entire width direction, and the surface hardness of the widthwise central part is 5 m in the widthwise plate edge.
A thin hot-rolled steel sheet having excellent formability, which is characterized by not lowering the surface hardness at the m position. 5. C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.015 wt% or less, Al: 0.005 to 0.150 wt%, N: 0.020 wt% or less of a steel slab is heated to a temperature of 1200 ° C. or less and roughly rolled, and the obtained sheet bar is wound to retain heat, and then this sheet bar is rewound to remove the preceding sheet. Joined with the bar, and further heating the plate width edge part so that the plate width edge part is higher than the center part of the plate width by 50 ° C. or more,
At least one subsequent stage is subjected to finish continuous rolling that employs pair cross rolling, and after air cooling for 2 seconds or more after rolling is finished, masking is performed so that at least one of the upper and lower cooling water does not directly hit the edge portion of the steel sheet. While cooling with water and winding up, any one of claims 1 to 4 is characterized.
Item 8. A method for manufacturing a thin hot-rolled steel sheet according to item. 6. C: 0.02 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 0.05 to 1.50 wt%, P: 0.04 wt% or less, S: 0.015 wt% or less, Al: 0.005 to 0.150 wt%, N: 0.020 wt% or less of a steel slab is heated to a temperature of 1200 ° C. or less and roughly rolled, and the obtained sheet bar is wound to retain heat, and then this sheet bar is rewound to remove the preceding sheet. Joined with the bar, and further heating the plate width edge part so that the plate width edge part is higher than the center part of the plate width by 50 ° C. or more,
At least one of the subsequent stages employs pair cross rolling, and at least one of the subsequent stages is subjected to finish continuous rolling while being lubricated, and after air cooling for 2 seconds or more after completion of rolling, the upper part,
5. The thin hot-rolled steel sheet according to claim 1, wherein at least one of the lower cooling water is water-cooled while being masked so as not to hit the edge portion of the steel sheet directly and wound up. Production method.