JP2000054071A - Hot rolled thin steel sheet and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a thin, wide steel sheet substitutable for a cold rolled steel sheet by specifying respective contents of C, Si, Mn, P, S, Al, and N in a steel composition, forming a part of N into a state of solid solution, and also controlling hot rolling conditions. SOLUTION: A steel, having a chemical composition consisting of, by weight, <=0.10% C, <=0.10% Si, <=0.8% Mn, <=0.04% P, <=0.02% S, <=0.150% Al, 0.0050-0.0200% N, and the balance Fe with inevitable impurities, is used, and further, N in a state of solid solution is allowed to exist by >=0.0030%. A slab of steel with this composition is heated to >=1000 deg.C and hot rolled to <=1.4 mm sheet thickness under the condition of >=800 deg.C finish rolling delivery side temperature. Cooling is started within 2 sec after the completion of finish rolling, and the resultant steel sheet is coiled at >=400 deg.C. By this method, the hot rolled steel sheet having >=340 MPa tensile strength TS can be obtained. As to the inevitable impurities in the steel composition, it is preferable to control the total content of Cu, Ni, Cr, and Mo and also the total content of Nb, Ti, V, and B to <=0.2% and to <=0.01%, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a thin hot-rolled steel sheet and a method for producing the same, and more particularly to a cold-rolled steel sheet which has conventionally been applied because of its difficulty in hot-rolling.
It is intended to make it possible to use a thin wide member of about mm or less as a substitute for a cold-rolled steel sheet. The hot rolled steel sheet of the present invention is suitable for use in relatively light working such as forming into a pipe by roll forming.

[0002]

2. Description of the Related Art Thin steel sheets are used for various purposes and are in great demand. For example, furniture, building materials, water pipes and cylinders. Although these materials are not particularly strict in terms of material, they are used as parts having the meaning of structural members, so they require a tensile strength (TS) of approximately 340 MPa or more. Accordingly, a C-Mn steel containing an appropriate amount of Cu, Ni, Cr, Mo, Nb, Ti, V, B, or the like is used.

Conventionally, most of hot-rolled steel sheets have a thickness of more than 1.4 mm, and those having a thickness of about 1.4 mm can be manufactured only in a narrow width of 900 mm or less. Cold-rolled steel sheets have been widely used in various fields. However, in the case of cold-rolled steel sheets, pickling, cold rolling, and annealing are inevitable as manufacturing processes after hot rolling, so that an increase in product price was inevitable. Therefore, development of a hot-rolled steel sheet in place of such a cold-rolled steel sheet has been desired.

[0004] In order to manufacture a hot-rolled steel sheet that can be substituted for a cold-rolled steel sheet, a technique for controlling the cross-sectional shape and dimensions with high precision and a material control technique for imparting a uniform material in the longitudinal and width directions of the steel sheet are indispensable. . In recent hot rolling technology,
For the shape and size control, high precision control can be performed by employing a pair cross mill. Also, the uniformity of the material can be dealt with to some extent by improving the technology of controlling the temperature of the steel sheet during rolling. Furthermore, in order to stably control the shape / dimensions and the temperature of the steel sheet from the front end to the tail end of the material to be rolled, for example, as described in Japanese Patent Application Laid-Open No. 9-296252, an input of a finishing rolling mill is required. By joining the preceding sheet bar and the succeeding sheet bar on the side and continuously finish rolling, a technology has been developed that can perform stable rolling while applying tension to the steel plate at the leading and trailing ends. Have been. With these techniques, it is becoming possible to control the shape / dimensions and the material control with high precision even for hot-rolled steel sheets.

[0005] However, the thickness of the conventional C-Mn steel is
Hot rolling of a thin material having a width of 1.4 mm or less and a sheet width exceeding 900 mm causes an increase in rolling load during rolling. Therefore, when the rolling load exceeds the capacity of the rolling mill, it is impossible to manufacture.
In addition, even if rolling can be performed within the capability range of the rolling mill, when the rolling load is large, the control output in the above-described shape / dimension control becomes large, so that a product having good shape / dimension accuracy should be manufactured. Becomes difficult. In order to reduce the rolling load, it is conceivable to reduce the diameter of the work roll. However, in this case, an increase in the unit consumption of the roll and the accompanying increase in cost are new problems.

[0006] It is conceivable to increase the purity of the steel to reduce the hot deformation resistance and reduce the rolling load. However, if the purity of the steel is increased unnecessarily, the required strength level (tensile strength TS is 340). (MPa or more) cannot be obtained, and there is a limit to the method of reducing the deformation resistance during hot rolling. For the above reasons, plate thickness: 1.4 mm or less, plate width: 90
It was difficult to produce a hot-rolled steel sheet having a tensile strength TS of 340 MPa or more with a thickness of 0 mm or more at a level of shape and dimensional accuracy that can be replaced with a cold-rolled steel sheet.

[0007] In the above-mentioned steel sheet temperature control,
It is important to cool the steel sheet between the finishing mill and the downstream winder (generally called a hot run table).
When a steel sheet having an uneven shape is cooled, the cooling rate cannot be made uniform, and as a result, the uniformity of the material deteriorates. For this reason, despite the relatively low level of required materials, a technology that can produce thin, wide, hot-rolled steel sheets at low cost and in a stable manner has been developed. Not.

[0008]

SUMMARY OF THE INVENTION The present invention has been developed in view of the above circumstances, and is intended for thin (1.4 mm or less) and wide (900 mm or more) hot-rolled steel sheets for general use. This makes it possible to manufacture it, and it is possible to replace it with a hot-rolled steel sheet in applications where cold-rolled steel sheets are currently used.

[0009]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have manufactured steel sheets with variously changed component compositions and manufacturing conditions, and conducted many material evaluation tests.
As a result, conventionally, nitrogen, which was not actively used in the field where such workability is required, was utilized as a strengthening element, and Cu, N, which had been conventionally used as a strengthening element, was used.
By reducing the hot deformation resistance by not containing i, Cr, Mo, Nb, Ti, V, and B, etc., it becomes possible to manufacture thin and wide hot-rolled steel sheets, and as a product after rolling. It has been found that a thin wide-width hot-rolled steel sheet having sufficient workability and strength can be advantageously produced. In other words, it has been found that by adding N, the strength of the product at room temperature can be effectively improved without increasing the hot deformation resistance of the steel sheet.

[0010] When N is used as a strengthening element, the amount of hardening at the time of baking of a product is larger than that of a conventional C-Mn steel, and an effective increase in strength level can be achieved. It has also been found that it is advantageous in maintaining. The present invention is based on the above findings.

That is, according to the present invention, C: 0.10 wt% or less;
Si: 0.10 wt% or less, Mn: 0.8 wt% or less, P: 0.04 wt% or less, S: 0.02 wt% or less, Al: 0.150 wt% or less, N: 0.00
50-0.0200wt% and N as solid solution 0.0030wt%
%, The balance being Fe and unavoidable impurities, and having a thickness of 1.4 mm or less and a TS of 340 MPa or more.

Further, the present invention provides a method for producing a semiconductor device comprising: C: 0.10% by weight or less;
0.10 wt% or less, Mn: 0.8 wt% or less, P: 0.04 wt% or less,
S: 0.02 wt% or less, Al: 0.150 wt% or less, N: 0.0050-
A steel slab containing 0.0200wt%, with the balance being Fe and unavoidable impurities, heated to 1000 ° C or more, and then heated to a finish thickness of 800mm or more and heated to a sheet thickness of 1.4mm or less under the condition of 800 ° C or more. This is a method for producing a thin hot-rolled steel sheet, characterized in that cold rolling is started within two seconds after the finish rolling, and then rolled at a temperature of 400 ° C. or more.

In the present invention, among the unavoidable impurity elements, Cu, Ni, Cr, and Mo are 0.2% by weight or less in total, and Nb, Ti,
It is preferable to suppress V and B to 0.01 wt% or less in total.

In the present invention, the material preferably has an elongation of 30% or more and a bake hardenability of 30 MPa or more.

Furthermore, in order to improve the shape and dimensional accuracy of the steel sheet in the production of the steel of the present invention, it is necessary to improve the shape and dimensional accuracy of the steel sheet by using a sheet bar, which is currently practically used in some cases, at the entrance side of the finishing rolling mill. It is extremely effective to apply a so-called continuous rolling technique of continuously finishing and rolling by joining the sheet bars.
Here, as the shape of the steel sheet, the sheet crown (width direction 25mm
The thickness difference between the position and the center position) is preferably 30 μm or less.

On the entry side of the finishing mill, the use of an edge heater for heating the width-wise end of the material to be rolled (sheet bar) to make the temperature of the material to be rolled uniform in the width direction is equivalent to material uniformity. It is advantageous in terms of. Further, since the temperature also tends to decrease at the longitudinal end of the material to be rolled, a heating device (hereinafter, referred to as a sheet bar heater) capable of heating over the entire width of the material to be rolled on the entry side of the finishing rolling mill is used. It is preferable to heat the portion where the temperature is lowered to uniform the temperature distribution in the longitudinal direction of the material to be rolled. In addition, when rolling after performing the above-described bonding, the sheet bar may be wound in a coil shape on the entry side of the bonding apparatus. In this case, particularly, the outermost winding portion and the innermost winding portion of the coil are sometimes used. It is preferable to use such a sheet bar heater because the temperature is easily lowered in the portion.

In addition, in the cooling after rolling, a technique of masking the cooling water at both ends in the width direction to prevent overcooling of the edge is also one of the important techniques from the viewpoint of uniformity of the material. The thin hot-rolled steel sheet of the present invention may be used as a pickling plate, or may be used as a so-called black scale with an iron oxide film present on the surface.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. First, the reason why the composition of the steel material is limited to the above range in the present invention will be described. C: 0.10 wt% or less If the C content exceeds 0.10 wt%, the ductility of the steel sheet deteriorates due to the increase in the amount of carbide in the steel, which is not preferable from the viewpoint of formability. And more importantly,
When the C content exceeds 0.10 wt%, a phenomenon in which the rolling load is significantly increased during hot rolling was confirmed. In the production of a thin hot rolled steel sheet intended in the present invention, since an increase in deformation resistance is the most harmful phenomenon, it is necessary to avoid such adverse effects as much as possible. In addition, weldability and the like also deteriorate. Therefore, the C content is set to 0.10 wt% or less, but from the viewpoint of improving the moldability, it is more preferable to set the C content to 0.08 wt% or less.

Si: 0.10 wt% or less Si is an element that significantly increases the hot deformation resistance of steel, and it is desirable to reduce it as much as possible. It is also desirable to reduce the amount from the viewpoint of improving the surface properties. From the above viewpoint, the amount of Si is set to 0.10 wt% or less. More preferably, it is 0.050% by weight or less.

Mn: 0.8 wt% or less Mn is an element effective for preventing hot cracking due to S, and must be contained in accordance with the amount of S contained. Also
Mn has an effect of making crystal grains fine, and is also useful for improving the material. However, the addition of a large amount of Mn is not preferable because the detailed mechanism is unknown, but the hot deformation resistance of the steel sheet is increased. In addition, the weldability and the formability of the weld tend to deteriorate. From the above, the upper limit of Mn was set to 0.8 wt%. In applications where better corrosion resistance and moldability are required, the content is desirably 0.50 wt% or less.

P: not more than 0.04 wt% When P is contained in a large amount, it significantly hardens the steel, and particularly deteriorates the stretch flangeability of the steel sheet. Further, since the steel has a strong tendency to segregate in the steel, embrittlement of the weld occurs due to the strong tendency. From the above, the upper limit of P is set to 0.04 wt%. Particularly, when these characteristics are regarded as important, the content is preferably set to 0.02 wt% or less.

S: not more than 0.02 wt% S is present as an inclusion and not only reduces the ductility of the steel sheet but also deteriorates the corrosion resistance. Therefore, it is desirable to reduce S as much as possible. Acceptable in the range. 0.0 for applications requiring particularly good workability
It is desirable that the content be 15 wt% or less.

Al: 0.150 wt% or less Al is an element added as a deoxidizing element for steel, which not only contributes effectively to improving the cleanliness of the steel, but is also useful for refining the structure of the steel. It is. In the present invention, N in a solid solution state is used as a strengthening element. However, aluminum-killed steel to which Al is added in an appropriate range has better mechanical properties than conventional rimmed steel to which Al is not added. On the other hand, an increase in the Al content leads to deterioration of the surface properties and remarkable reduction of the solute N, and it is difficult to secure a necessary minimum tensile strength of 340 MPa. In consideration of these points, the upper limit of the Al content is set to 0.150 wt%. From the viewpoint of material stability
About 0.010-0.080 wt% is desirable.

N: 0.0050 to 0.0200 wt% and N in a solid solution state of 0.0030 wt% or more N is the most important additive element in the present invention. That is, by optimizing the manufacturing conditions after adding an appropriate amount of N, it is possible to secure an appropriate amount of solid solution N, and the solid solution strengthening effect of N can be used to achieve a target TS of 340 MPa or more. Can be obtained stably. Further, N also has an effect of lowering the transformation point of steel, and its addition is more effective in a situation in which it is not desired to perform rolling with a thin material that greatly reduces the transformation point. Further, when considering the application range of the steel of the present invention, it is desirable that the strength of the steel sheet tends to increase by painting and baking after forming or by simple aging. An effect of increasing the strength (which is considered as a kind of strain age hardening) can be obtained.

Such effects can be stably obtained by adding the content of about 50 ppm or more. However, when added in excess of 0.0200 wt%, not only the incidence of internal defects or surface defects in the steel sheet increases, but also the occurrence of slab cracks and the like during continuous casting becomes remarkable, so the upper limit is 0.0200 wt%. %. From the viewpoint of improving material stability and yield considering the entire manufacturing process,
A range of 0.0070 to 0.0170 wt% is more preferred. In addition,
Even if N is added, spot welding within the scope of the present invention,
It does not adversely affect weldability such as arc welding and laser welding.

Further, in order to ensure sufficient strength of the steel sheet and effectively exhibit strain age hardening due to N, the N in the solid solution state must be 0.0030 wt% or more. Here, the solid solution N amount is a value obtained by subtracting precipitated N (determined by a dissolution method using a bromine ester) from the total N amount in the steel. This is based on the fact that, as a result of examining various methods for analyzing the precipitated N, the method utilizing the above-described dissolution by bromine ester was the best and corresponded appropriately to the change in the material. It was also confirmed that the value obtained by this analysis method was consistent with the value measured by the internal friction method or the like.

Further, among the impurity elements unavoidably mixed into the steel, it is advantageous to suppress particularly the following elements to a predetermined level or less. (Cu + Ni + Cr + Mo): 0.2 wt% or less, (Nb + Ti + V +
B): 0.01 wt% or less When these elements are contained, the strength of the steel sheet as a product is increased, but at the same time, the deformation resistance when hot rolling the steel sheet is remarkably increased. In addition, the chemical conversion property and the surface treatment characteristics in a broader sense are remarkably deteriorated, and further, the deterioration of the weldability due to the hardening of the weld is also remarkable. Therefore,
It is preferable that these elements be suppressed to the above range.
Since they exhibit the same behavior when they are contained alone or in combination, they are classified into two groups based on the empirical formula affecting the hot deformation resistance, and the upper limit is set for the total amount of each.

Next, the reason why the thickness of the steel sheet is limited to the above range in the present invention and the mechanical properties required for the steel sheet will be described. Steel plate thickness: 1.4 mm or less The effects of the present invention can be exerted even when the steel plate is thick, but when the steel plate exceeds 1.4 mm, plastic working (rolling) is performed.
However, regulations regarding deformation resistance are not so strict. That is, even if elements such as C and Mn are added in a normal range,
It can be manufactured without any problems, and the contribution of the present invention is small. Therefore, in the present invention, the plate thickness is set to 1.4 mm or less.

Tensile strength (TS): 340 MPa or more Unless the tensile strength is about 340 MPa or more, it is difficult to apply it to a wide range including replacement of cold-rolled steel sheets. This is because the components to be applied have some structural member-like surface. For even more coverage, 370
It is desirable to have a tensile strength of MPa or more.

Further, in the present invention, good characteristics can be obtained for elongation and bake hardenability as described below. Elongation: 30% or more The elongation of a steel sheet shall be measured with a normal JIS No. 5 tensile test piece. Can be converted. If the elongation of the steel sheet is not about 30%, it is difficult to achieve the target item of replacing the cold-rolled steel sheet. Therefore, the elongation is preferably at least 30%, more preferably at least 35%. Note that these materials are preferably uniform in the longitudinal direction and the width direction of the steel sheet, and the standard deviation σ of the tensile strength and elongation at a plurality of points in the longitudinal direction and the width direction is 20 MPa or less, respectively. It is desirable to make it 2% or less.

Bake hardening amount: 30 MPa or more In the steel of the present invention, the bake hardening amount is determined at 170 ° C. after applying 5% tensile strain from no strain (zero applied strain).
It is defined as the increase in yield stress after aging for 20 minutes. If this value is approximately 30 MPa or more, it will appear as an effective increase in strength when actually used as a part, and a thinner steel plate can be achieved. A more desirable curing amount is 40 MPa or more. In addition, unlike the conventional low carbon steel, the steel of the present invention is expected to increase the strength even if it is not subjected to the accelerated aging treatment by heating (artificial aging) or left at room temperature after forming. Can be almost 70% of full aging
The degree can be expected at a minimum.

The crown of the steel sheet (25 mm from the end of the steel sheet)
The thickness difference between the position and the central portion) is preferably 30 μm or less. This is because, as an alternative to a cold-rolled steel sheet, in order to widen its applicability range, it is necessary to set the sheet crown to about 30 μm or less even if it depends on the sheet thickness. More preferably, it is 20 μm or less.

Further, if the shape of the steel plate is not flat, when performing various processes such as pipe making, for example, it may cause troubles such as sticking on a production line, or abrasion may occur. Therefore, it is preferable that the shape of the steel sheet be in a range of not more than 20 mm in wave height of the steel sheet. The measurement of the wave height is performed in accordance with the Japan Iron and Steel Federation standard, and the wave height is determined on a surface plate.

Next, the production method of the present invention will be specifically described. The slab is desirably manufactured by a continuous casting method in order to prevent macro segregation of components, but can also be manufactured by an ingot making method or a thin slab casting method. In addition, after the slab is manufactured, it is once cooled to room temperature and then heated again.Otherwise, without cooling, immediately put it into a heating furnace as a flake or hold a small amount of heat, and then immediately Energy saving processes such as direct rolling and direct rolling can be applied without any problem. In particular, direct rolling is one of useful techniques for effectively securing N in a solid solution state.

The hot rolling conditions are defined as follows. Slab heating temperature: 1000 ° C. or higher The lower limit of the slab heating temperature is defined from the viewpoint of securing N in a solid solution state as an initial state. The upper limit is not particularly limited, but considering the loss accompanying the increase in oxidation weight, 12
It is desirable that the temperature be 80 ° C or lower.

Finish rolling temperature during hot rolling: 800 ° C. or higher By setting the finish rolling temperature to 800 ° C. or higher and performing cooling after hot rolling described below, a uniform and fine hot rolled sheet structure can be obtained. In use, it can be used without any problem. However, when the finish rolling temperature is lower than 800 ° C., the structure of the steel sheet becomes non-uniform, a part of the processed structure remains, and the risk of various problems during press forming increases. Further, even if a higher winding temperature is used to avoid the retention of the processed structure at a lower finish rolling temperature, in this case, the strength is significantly reduced due to the generation of coarse grains and the solid solution N 340 MPa, it is difficult to obtain the target tensile strength of 340 MPa. Therefore, the finish rolling temperature was set to 800 ° C or higher. In particular,
In order to improve the mechanical properties, the temperature is preferably set to 820 ° C. or higher.

Winding temperature: 400 ° C. or more When the winding temperature is lowered, the strength tends to increase. However, when the temperature is lower than 400 ° C., the shape of the steel sheet is remarkably disturbed, and the risk of causing a problem in actual use increases. Further, the uniformity of the material tends to decrease, which is not desirable. Therefore, the hot rolling temperature was set to 400 ° C or higher. 450 when higher material uniformity is required
It is desirable that the temperature be equal to or higher than ° C.

Cooling after completion of hot rolling Water cooling is started immediately after hot rolling, and so-called slow cooling with a lower heat transfer coefficient than usual is applied to maintain a flat shape and uniformity of material. It is effective in ensuring the performance. Further, it is necessary to start water cooling within 2 seconds after the finish rolling in order to obtain the strength and bake hardenability of the final product. This is because the rolling distortion tends to promote the precipitation of aluminum nitride after rolling, but this phenomenon can be prevented by cooling as quickly as possible after rolling is completed and shortening the time in the high-temperature region. This is because N in a solid solution state can be secured. Further, the fineness of the hot-rolled sheet can also achieve the finer structure of the final product.

Regarding the heat transfer coefficient during cooling, 4
It is preferable to be in the range of 50 to 800 W / m ² · K. Since the securing of the heat transfer coefficient is 450 W / m ² · less than K when the deposition amount of aluminum nitride is most becoming in solid solution N is difficult, whereas, the heat transfer coefficient of 800 W / m ² · If the temperature exceeds K, the difference in cooling rate in the width direction of the steel sheet becomes large, and not only the shape of the steel sheet deteriorates, but also local overcooling occurs, so that it is difficult to ensure the uniformity of the material.

Temper rolling reduction: 5% or less Temper rolling (skin pass rolling) of a hot-rolled steel sheet is preferably performed to suppress or reduce yield point elongation and to adjust surface roughness and the like. Although the detailed mechanism is unknown, it is also useful for improving the shape of the original plate, which is one of the important characteristics. However, when processing with a strength exceeding 5% results in deterioration of ductility and the applicable application is limited, the rolling reduction is preferably 5% or less.

Next, a description will be given of manufacturing conditions that are desirably applied supplementarily. First, it is desirable to perform continuous finish rolling in which the preceding material and the following material are joined on the entry side of the finish rolling mill and finish rolling is performed continuously. By continuously rolling the preceding material and the succeeding material in this manner, so-called unsteady rolling portions at the leading and trailing ends of the material to be rolled are eliminated, so that stable hot rolling is performed on the material to be rolled. It can be achieved over the entire length and width. Such rolling is also extremely effective in improving the cross-sectional shape and dimensions of the steel sheet. Therefore, it is possible to flatten the shape of the steel sheet over the entire length, and it is easy to obtain uniform cooling conditions in the longitudinal direction and the width direction when cooling the steel sheet after rolling on the hot run table. Therefore, it is advantageous in obtaining a uniform material.

There are no particular restrictions on the joining method on the entry side of the finishing mill, and the welding method, laser welding, electron beam welding, and other joining methods can be applied in exactly the same manner. . In addition, by performing continuous rolling, the leading end of the material to be rolled can be stably threaded. Therefore, it is difficult to apply heat with a low coefficient of friction, which is difficult to apply in ordinary batch rolling from the viewpoint of threadability and biting properties. Cold rolling, that is, hot rolling using a large amount of lubricant, can be performed, and the rolling load can be reduced, and at the same time, the surface pressure of the roll can be reduced, so that the life of the roll can be extended. From the above, in a thin hot-rolled steel sheet, it is extremely effective to continuously finish-roll a preceding material and a following material.

Further, the use of an edge heater for heating the widthwise end of the material to be rolled on the entry side of the finishing mill is as follows.
This is advantageous in making the temperature of the material to be rolled uniform in the width direction.
In the present invention, since the uniformity of the steel sheet temperature during rolling and cooling on a hot run table is important, the width direction end, where the temperature is particularly liable to decrease, is heated on the entry side of the finishing mill, and the steel sheet is heated. It is preferable to make the temperature distribution uniform in the width direction.

Further, since the temperature at the longitudinal end of the material to be rolled is also liable to decrease, the portion at the longitudinal end where the temperature is lowered is heated by a sheet bar heater on the entry side of the finishing rolling mill, and the material to be rolled is heated. It is preferable to make the temperature distribution in the longitudinal direction uniform. When rolling after performing the above-described bonding, the sheet bar may be wound in a coil shape on the entry side of the bonding apparatus. In this case, the outermost winding portion and the innermost winding portion of the coil are particularly used. It is particularly advantageous to use such a sheet bar heater because the temperature is likely to drop in the area.

It is recommended that the heating amount when the material to be rolled is heated using the edge heater or the sheet bar heater as described above is such that the temperature difference in the final finish rolling is 20 ° C. or less. However, this temperature difference slightly changes depending on the steel composition and the like. In addition, the uniformity of the material can be further improved by using a continuous rolling technique in which the preceding material and the subsequent material are joined and finish rolling is performed, or by using a sheet bar heating by an edge heater and a sheet bar heater.

[0046]

EXAMPLES Example 1 A steel slab having the composition shown in Table 1 was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 1.2 mm and a width of 1000 mm. Table 3 shows the results of investigation on the tensile properties and bake hardenability of the obtained hot-rolled steel sheet. At this time, a JIS No. 5 test piece was used for tensile properties. The baking hardening amount was set to a standard aging condition of 20 minutes at 170 ° C. with 2% prestrain.

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

[Table 3]

As is clear from Table 3, all of the hot-rolled steel sheets obtained according to the present invention have TS ≧ 357 MPa and El ≧ 37.
%, BH ≧ 51 MPa. Also,
It was confirmed that such a characteristic value hardly changed in the longitudinal direction and the width direction of the steel sheet, the material and the shape were uniform, and the sheet crown could be controlled to 30 μm or less. In addition, the total amount of Cu, Ni, Cr and Mo was as large as 0.3 wt% (No. 11) and the total amount of Nb, Ti, V and B was 0.02 wt%.
(No. 12), the rolling load is increased by about 10% compared to No. 1, so the sheet crown tends to be about 10 μm larger than No. 1-3 and No. 7-9. It was observed. In addition,
Rolling load for No.2, 3, 7 ~ 9 is ± 5% of No.1
Was within.

On the other hand, in the case of adding steel sheets by adding Cu, Ni and Nb and Ti (Nos. 4 and 5), although the strength was increased, the rolling load was no. 15 ~
By 20%, not only the shape was disturbed, but also the rate at which the plate thickness deviated from the target value increased sharply. Further, since the rolling load increased, the control ability of the sheet crown control means (specifically, the work roll bender pressure control means and the roll cross angle control means) of the rolling mill could not keep up, and the sheet crown increased to about 50 μm. Therefore, it could not be put to practical use. In addition, No. 10 in which the N content exceeds the upper limit of the present invention.
In the steel sheet, a defect called blowhole occurred on the surface of the steel sheet. The tensile properties of the same steel sheet were examined even under the condition of not pickling, but no remarkable difference was recognized as compared with the pickled steel sheet.

Example 2 C: 0.041 wt%, Si: 0.005 wt%, Mn: 0.15 wt%, P:
It contains 0.009 wt%, S: 0.005 wt%, Al: 0.039 wt% and N: 0.0116 wt%, and the remainder is made of a steel slab having a substantially Fe composition. The manufacturing conditions are various as shown in Table 4. And finally, a hot-rolled steel sheet having a thickness of 1.0 mm and a width of 1000 mm was manufactured. During hot rolling, No. 1 to 6 and No. 9 to 12 were heated and melt-welded by heating the tail end of the preceding material and the tip of the succeeding material for the sheet bar roughly rolled to a thickness of 25 mm. Joining and continuous tandem finish rolling were performed. On the other hand, No. 1-4, No.
6, Nos. 9 to 10 and Nos. 12 to 13 were subjected to finish rolling after heating the width end using an edge heater on the entrance side of the finishing mill. No. 1-5, No. 9-10, N
For o.12 to 13, the sheet bar heater was used to heat the temperature-lowering portion at the leading and trailing end of the sheet bar on the inlet side of the edge heater. Table 4 also shows the results of investigations on the tensile properties, sheet crown, shape, and uniformity of the material of the obtained hot-rolled steel sheet.

Here, the sheet crown and the shape were measured by taking a sample from the central position in the longitudinal direction of the steel sheet. The thickness of the strip crown was measured by measuring the difference in strip thickness at the center of the width with the 25 mm yarn from the width end, and the shape was evaluated by measuring the wave height described above. Regarding the uniformity of the material, three points in the longitudinal direction of the steel sheet (15 m from the front end, central position in the longitudinal direction, and 15 m from the tail end), width direction 5
From the points (25 mm and 100 mm positions from both ends, respectively, and the center of the width), a total of 15 tensile test pieces were sampled, and the standard deviation σ of the tensile strength values of these 15 pieces was obtained and evaluated.

[0054]

[Table 4]

As shown in Table 4, all of the hot-rolled steel sheets (Nos. 1 to 7 and Nos. 12 to 13) obtained according to the present invention had TS ≧
It exhibited excellent tensile properties of 345 MPa and El ≧ 35%. In addition, each of these steel plates has a plate crown of 22 to
It was in the range of 28 μm, and the shape and material uniformity were also good. On the other hand, the comparative examples (Nos. 8 to 11) in which the production conditions deviated from the proper range of the present invention were inferior to any of the inventive examples in any of tensile strength, sheet crown, shape, and material uniformity. . Then, in order to compare the characteristics in actual use, a small-diameter ERW pipe was manufactured together with the cold-rolled annealed material having a thickness of 1.0 mm, but the hot-rolled steel sheets of Nos. 1 to 7 were compared with the cold-rolled annealed material. Similarly, it could be manufactured without any problems.

[0056]

Thus, according to the present invention, it is possible to stably provide a thin and wide hot-rolled steel sheet which can be used as a substitute for a cold-rolled steel sheet. That is, the hot-rolled steel sheet according to the present invention is:
By optimizing the chemical composition and hot rolling conditions and securing a sufficient amount of solid solution N at the final product stage, the strength is increased, and sufficient strength is secured by utilizing strain age hardening due to N. Also, at this time, since the addition of N does not increase the hot deformation resistance unlike other alloying elements, it can sufficiently cope with the hot rolling of a thin material having a large deformation resistance. It is extremely important industrially. The property of not increasing the deformation resistance is extremely important from the viewpoint of guaranteeing the shape and dimensions of the steel sheet with high accuracy. Further, the present invention provides
There is also an advantage that it can be applied without problems to the use of the scale phase on the surface without performing pickling.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Takao Uchiyama 1-term Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term (reference) 4K037 EA01 EA04 EA05 EA15 EA18 EA23 EA25 EA27 FA02 FA03 FC03 FC04 FE01 FE02 FE03

Claims (3)

[Claims] C: 0.10 wt% or less, Si: 0.10 wt% or less, Mn: 0.8 wt% or less, P: 0.04 wt% or less, S: 0.02 wt% or less, Al: 0.150 wt% or less, N: 0.0050 -0.0200wt% and N as solid solution
It contains 0.0030wt% or more, and the balance is composed of Fe and unavoidable impurities. The plate thickness is 1.4mm or less and TS is 340MPa.
A thin hot-rolled steel sheet characterized by the above. 2. The method according to claim 1, wherein, among the unavoidable impurity elements, Cu, Ni, Cr and Mo are suppressed to a total of 0.2 wt% or less and Nb, Ti, V and B are controlled to a total of 0.01 wt% or less. Features Thin hot-rolled steel sheet. 3. C: 0.10 wt% or less, Si: 0.10 wt% or less, Mn: 0.8 wt% or less, P: 0.04 wt% or less, S: 0.02 wt% or less, Al: 0.150 wt% or less, N: 0.0050 After heating a steel slab with a composition of Fe and unavoidable impurities to 1000 ° C or higher, the final rolling exit temperature: 800 ° C or higher, and the sheet thickness: 1.4 mm or less A method for producing a thin hot-rolled steel sheet, comprising: hot rolling, starting cooling within 2 seconds after finishing rolling, and winding at a temperature of 400 ° C. or more.