KR101410104B1 - Method for manufacturing formable hot-rolled steel having excellent anti-fluting, surface quality and the hot-rolled steel by the same method - Google Patents

Abstract

A hot-rolled steel sheet for processing which is excellent in bending resistance and surface characteristics used for household appliances and automobiles, and a manufacturing method thereof.
The method for manufacturing a hot-rolled steel sheet having excellent creep resistance and surface characteristics according to the present invention is characterized by comprising 0.001 to 0.003% of carbon (C), 0.05 to 0.3% of manganese (Mn), 0.001 to 0.020% of phosphorus (P) (S) of 0.020% or less (excluding 0%), aluminum (Al) of 0.01 to 0.07%, nitrogen (N) of 0.001 to 0.004%, boron (B) of 0.001 to 0.003% A shot ball having a diameter of 0.10 to 0.40 mm is shot-blasted at a speed of 40 to 65 m / sec on the surface of a hot-rolled steel sheet made of steel containing iron (Fe) and other unavoidable impurities.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot-rolled steel sheet for machining,

The present invention relates to a hot-rolled steel sheet for machining, which has excellent creep resistance and surface characteristics, and a method of manufacturing the same, and more particularly, to a hot-rolled steel sheet having excellent low- And a hot-rolled steel sheet produced by the method.

In steel sheets used in household appliances and automobiles, it is necessary to prevent occurrence of machining defects due to aging. Conventionally, materials such as ultra low carbon steel to which carbonitride-forming elements excellent in endurance have been added have been used for suppressing aging processing defects.

In particular, in automobiles, it is necessary to prevent the steel sheet from being oxidized by the external atmosphere condition, and it is also necessary to prevent organic substances such as paint As the coating process is carried out on the surface, the surface property management is becoming a very important factor.

The bending property is also referred to as fluting. Fluting means a phenomenon in which a machined portion is bent into a diamond shape when machining. When fluting is generated, it is difficult to maintain the shape of the formed portion. Should be limited.

However, in general, it is difficult to substantially limit the aging phenomenon due to the solid element which causes the fluting phenomenon. As a result, it is difficult to carry out a high-refining process at the steelmaking process step or a carbonitride formation such as titanium (Ti), niobium Adding elements, and fixing the hiring elements.

However, the addition of these carbonitride-forming elements contributes to the suppression of machining defects such as fluting, but it also reduces the productivity by increasing the steelmaking process steps for high-cleanliness and increases the manufacturing cost by adding expensive alloying elements There are disadvantages.

In addition, it is known that it is extremely difficult to suppress fluting in the low-carbon carbon steel fundamentally. In the case where rigidity and machinability such as home appliances and automobiles are required, establishment of a plan for suppressing the fluting phenomenon is urgently required .

On the other hand, various processing characteristics such as elongation flangeability, bending property and drawing ability are required not only to prevent flutting caused by the above-mentioned endurance, but also to improve the shape dynamics of the final product and improve the production process and improve the productivity. In addition, since these structures are exposed to the external environment, the surface coating of the steel sheet is performed in order to improve the weatherability. Therefore, in order to secure such processing characteristics, development of a steel sheet for plating,

Japanese Patent Laid-Open No. 1989-282420 discloses "a method for manufacturing a hot-rolled steel sheet for processing and a heat-treating method for a hot-rolled steel sheet ".

This is a method for producing a steel sheet suitable for a high strength member for automobiles or industrial equipment, which comprises adding a titanium (Ti), niobium (Ni) and a rare earth element to an ultra low carbon steel base to produce a hot- And a method for manufacturing the same.

However, as described above, this method is effective for suppressing machining defects such as fluting by addition of carbonitride-forming elements, but it is difficult to reduce the productivity and to add expensive alloying elements by increasing the steelmaking process time for high- Thereby raising the cost of production.

As another example, Korean Patent Laid-Open No. 2001-60648 (a method of producing a hot-rolled steel sheet excellent in endurance and uniform stretching properties) contains 0.02 to 0.05% carbon (C), manganese (Mn) (P), 0.015% or less of sulfur (S), 0.01 to 0.04% or less of aluminum (Al), or 0.004% or less of nitrogen (N) ) And other unavoidable impurities is reheated and then subjected to finish hot rolling at a temperature higher than Ar ₃ transformation point to produce a material having a thickness of 1.4 to 2.3 mm and winding at a temperature of 600 to 700 ° C to improve endurance and uniform stretching properties A method is disclosed.

However, there is a problem that it is difficult to expect high anti-aging property because the aging phenomenon by the solid element such as carbon and nitrogen contained in the steel can not be completely prevented by simply controlling the coiling temperature as in this method.

Japanese Patent Laid-Open No. 2008-190008 discloses a "method for producing a hot-rolled steel sheet excellent in endurance."

(P), 0.09% or less of sulfur (P), 0.015% or less of sulfur (S), 0.015% or less of aluminum (Si) (Al) and 0.0005 to 0.015% nitrogen (N), and the remainder is formed by hot-rolling the slab composed of the iron (Fe) and other unavoidable impurities at an average cooling rate of 60 DEG C / , And then subjected to skin pass rolling at an elongation of 0.1 to 1.0% using a small-diameter roll satisfying t / R? 0.0055 (t is the sheet thickness and R is the roll diameter).

However, since this method lowers the coiling temperature to less than 400 ° C, the generation of the low-temperature precipitates differs due to the non-uniformity of the widthwise temperature, and by causing the material deviation, the shape defects, . Further, since the roll diameter is controlled individually according to the thickness of the plate for inducing the surface movable potential, it is difficult to apply it to production of various sizes of materials such as a commercial operation line.

It should be understood that the foregoing description of the background art is merely for the purpose of promoting an understanding of the background of the present invention and is not to be construed as adhering to the prior art already known to those skilled in the art.

Japanese Laid-Open Patent Application No. 1989-282420 Korean Patent Publication No. 2001-60648 Japanese Patent Laid-Open No. 2008-190008

SUMMARY OF THE INVENTION The present invention has been proposed in order to solve such conventional problems, and it is an object of the present invention to provide a method of coating a steel sheet by adding boron (B) and tin (Sn) in a steel component and optimizing cooling conditions and shot blasting conditions, And a method for producing the same, which is excellent in the creep resistance and surface characteristics by suppressing processing defects such as fluting.

In order to achieve the above object, according to the present invention, there is provided a method for manufacturing a hot rolled steel sheet having excellent creep resistance and surface properties, comprising 0.001 to 0.003% of carbon (C), 0.05 to 0.3% 0.001 to 0.003% of boron (B), 0.05 to 0.003% of boron (B), 0.001 to 0.020% of sulfur, 0.020% or less of sulfur (excluding 0%), 0.01 to 0.07% To 0.25%, and the remainder is shot blasted at a speed of 40 to 65 m / sec on a surface of a hot-rolled steel sheet made of steel containing iron (Fe) and other unavoidable impurities with a diameter of 0.10 to 0.40 mm. ,

The steel component is characterized by satisfying 0.5 ≦ [(B (%) / 11) / (N (%) / 14)] 1.5.

The steel having the above composition is finish-rolled at 900 to 950 占 폚, and the finish-rolled steel is cooled at a speed of 20 to 80 占 폚 / s, and the cooled steel is wound at 580 to 700 占 폚.

And the surface roughness index ratio (R _max / R _a ) is set to 7 to 16 by the shot blasting.

In order to achieve the above object, according to the present invention, there is provided a hot-rolled steel sheet excellent in creep resistance and surface properties, comprising 0.001 to 0.003% of carbon (C), 0.05 to 0.3% of manganese (Mn) 0.001 to 0.003% of boron (B), 0.05 to 0.25% of tin (Sn), 0.01 to 0.07% of aluminum (Al) %, And the remainder is made of steel containing iron (Fe) and other unavoidable impurities, and has a surface roughness index ratio (Rmax / Ra) of 7 to 16 by a shot blasting treatment.

The steel component is characterized by satisfying 0.5 ≦ [(B (%) / 11) / (N (%) / 14)] 1.5.

And the deformed ferrite structure produced by the shot blasting treatment occupies 3 to 10% in the thickness direction of the hot-rolled steel sheet.

Fig. 1 is a photograph of the texture of the hot-rolled steel sheet according to the present invention,
2 is a characteristic evaluation graph according to the surface roughness index ratio of the hot-rolled steel sheet according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described with reference to the accompanying drawings.

The present invention has been completed by repeatedly conducting research and experiments on a high-value-added hot-rolled steel sheet which can simultaneously be used for home appliances and automobiles by satisfying surface properties such as breaking property and workability as well as paintability. As well.

Carbon (C) is an element added to improve the strength of a steel sheet, and its content is limited to 0.001% to 0.003%.

As the carbon (C) content increases, the tensile and yield strengths increase. However, when the content exceeds 0.003%, excessive workability of the material deteriorates the toughness of the material and causes a sudden material deviation .

If the carbon content is less than 0.001%, it is advantageous from the viewpoint of workability, but it causes softening of the material due to abnormal growth of the structure, which makes it difficult to secure a stable material.

Manganese (Mn) is widely used as a solid solution strengthening element. It is an important element which increases the strength of steel and improves hot workability, but also inhibits the ductility and workability of the material due to MnS formation.

If Mn content is low, processability is improved but it is difficult to secure strength. Therefore, in order to secure the target strength, 0.05% or more of manganese (Mn) content should be added.

On the other hand, if manganese is added excessively, the addition of a large amount of alloying element lowers the economical efficiency and causes not only a cause of center segregation but also deteriorates the weldability, so the upper limit is limited to 0.3%.

Phosphorus (P) is an element which plays a role of improving the strength of steel and improving corrosion resistance. It is preferably added in a large amount in order to secure these properties, but it is an element that causes center segregation during casting. , Its content is limited to 0.02% or less (excluding 0%). On the other hand, if the content of phosphorus is less than 0.001%, it is difficult to secure strength and corrosion resistance.

Sulfur (S) is combined with manganese (Mn) in the steel to form nonmetallic inclusions serving as a corrosion starting point, and also acts as a factor of the embrittlement of brittleness. Therefore, the content of sulfur is limited to 0.02% or less (exclusive of 0%), preferably 0.01% or less.

Aluminum (Al) is an element added for the deoxidation of molten steel, but it has a function of improving the aging property by bonding with the solid element in steel. Therefore, the lower limit is limited to 0.01%, and if excess is added, Thereby causing surface defects as well as deteriorating processability. Therefore, the upper limit is preferably 0.07%.

Boron (B) is an element which not only improves the aging property by combining with the solid element in the steel but also has an effect of increasing the strength of the material by addition of a small amount as an element for improving hardenability. In order to obtain the desired material properties, at least 0.0010% of addition is required. However, if it exceeds 0.0030%, the material deteriorates and causes grain boundary cracking at the time of performance, and there is a problem of roughening the surface of the hot-rolled steel sheet, so that the content thereof is limited to 0.0010 to 0.0030%.

Tin (Sn) is an element which exists in a solid state in the steel to improve high-temperature characteristics and corrosion resistance. That is, it is heated and oxidized by heat treatment to form an Sn-based oxide such as Sn ₂ O ₃ on the surface of the steel sheet, thereby suppressing the formation of an alloy layer on the surface of the steel sheet, thereby improving the corrosion resistance and discoloration resistance at high temperatures. In order to obtain such an effect, it is preferable to add 0.05% or more. However, if the content exceeds 0.25%, the effect of contributing to improvement in corrosion resistance and discoloration resistance is lowered, 0.25%

Nitrogen (N) is an element which is useful for reinforcing materials while existing in solid state in steel, but it is a main element causing aging phenomenon. Therefore, it is necessary to control below a certain amount in order to secure processability. If it is less than 0.001%, sufficient stiffness can not be obtained, and sites for formation of precipitates are reduced. Therefore, the lower limit is limited to 0.001%.

On the other hand, in order to ensure proper processing characteristics, it is also necessary to appropriately manage the fraction of the precipitate and the employment phase.

In order to secure the characteristics of the present invention, it is necessary to maintain the atomic ratio of B and N, [(B (%) / 11) / (N (%) / 14)] at 0.5 to 1.5. If the B / N value is less than 0.5, the bending resistance at room temperature deteriorates excessively at the room temperature, and the effect of suppressing the crystal grains at high temperature is insufficient. When the B / N value is more than 1.5, , The value of B / N is limited to 0.5 to 1.5.

Hereinafter, a method for manufacturing a hot-rolled steel sheet having excellent processability and surface characteristics according to the present invention will be described in detail using component-controlled steels as described above.

First, 0.001 to 0.003% of carbon (C), 0.05 to 0.3% of manganese (Mn), 0.001 to 0.020% of phosphorus (P), 0.020% (Fe) and other unavoidable impurities in a range of 0.01 to 0.07% nitrogen, 0.001-0.004% nitrogen, 0.001-0.003% boron (B), 0.05-0.25% tin And a shot blasting is carried out at a spraying speed of 40 to 65 m / sec using a short ball having a diameter of 0.1 to 0.4 mm on the surface of the hot-rolled steel sheet.

This shot blasting process is one of the most distinctive technical constructions of the present invention for securing the creep resistance (endurance), and introduces a suitable compressive stress to the surface of the hot-rolled steel sheet to increase the dislocation density, By newly producing modified ferrite grains, it is possible to reduce the dislocation of dislocation caused by the hiring element which is a main cause of aging, thereby improving the anti-aging effect.

It is preferable to control the diameter of the shot ball used for shot blasting to 0.10 to 0.40 mm.

If the diameter of the shot ball is less than 0.10 mm, the mechanical peeling effect of the surface layer is small and it is difficult to obtain an appropriate dislocation density increasing effect. If the diameter of the shot ball exceeds 0.40 mm, the surface roughness increases sharply, This is because it acts as a factor of.

On the other hand, the injection speed of the shot blasting is preferably controlled to 40 to 65 m / sec.

If the injection speed is less than 40 m / sec, the impact pressure of the short ball acting on the surface layer is low and it is difficult to secure the desired endurance. If it exceeds 65 m / sec, the depth of the surface hardened layer exceeds 10% This is because it causes non-uniform machining.

In order to secure the mechanical properties according to the present invention, it is preferable to control the surface roughness index ratio (Rmax / Ra) of the hot-rolled steel sheet after the shot blasting to be 7 to 16.

Here, Rmax denotes the maximum point height in the surface roughness curve of the hot-rolled steel sheet, and Ra denotes the center line average surface roughness.

As the surface roughness index ratio increases, the density of the movable potential increases as the fixing potential at the surface layer of the steel sheet deviates from the Cottrell atmosphere with the carbon and nitrogen dissolved in the steel.

When the surface roughness index ratio is less than 7, the ratio of the acidity and the bone is not ensured appropriately. As a result, the adsorbability of the organic matter is lowered and the density distribution of the movable potential, which can fix the solid element, is insufficient. On the other hand, when the surface roughness index ratio exceeds 16, the density of the movable potential reaches the saturation state, but the tip crack starts to occur during processing, and the range is preferably 7 to 16.

FIG. 1 (a) shows a steady state ferrite grains having a low dislocation density. FIG. 1 (b) shows a transformed structure having a dislocation structure having a high potential density by the short blasting process according to the present invention. Show ferrite lips. It is preferable that the deformed ferrite structure produced by shot blasting is controlled to occupy 3 to 10% in the thickness direction of the hot-rolled steel sheet so as to ensure proper endurance.

If the deformed ferrite structure is less than 3% in the thickness direction, the structural characteristics capable of sufficiently fixing the solid element in the steel can not be obtained. Therefore, the desired antioxidant property can not be secured and the deformed ferrite structure exceeds 10% It acts as a factor of hardening of the material in the lower surface treatment and the post-treatment process, which causes deterioration of the workability.

As described above, a hot-rolled steel sheet excellent in endurance and machinability can be produced by applying a shot blasting process under a predetermined condition to the surface of a steel sheet produced through a conventional hot-rolling process. Furthermore, by optimizing the hot rolling process, it is possible to manufacture hot rolled steel sheets having more excellent endurance, workability, paintability and the like, and the optimized hot rolling process will be described in detail below.

First, in the hot-rolled steel sheet according to the present invention, the steel composed of the above-mentioned composition components is preferably subjected to finish rolling at 900 to 950 ° C.

When the finish rolling temperature is lower than 900 캜, the hot rolling is completed in the low temperature region, the blending of the crystal grains proceeds rapidly, and the rolling property and workability are lowered. On the other hand, if the finish rolling temperature is higher than 950 占 폚, uniform hot rolling is not carried out over the entire thickness, so that grain refinement becomes insufficient and impact toughness due to crystal grain coarsening is lowered.

Further, the hot-rolled steel sheet is preferably cooled in a run-out-table (ROT) at a cooling rate of 20 to 80 ° C per second.

If the cooling rate in the ROT is less than 20 DEG C / s, relatively coarse crystal grains are formed due to dynamic grain growth, which causes deterioration of strength and workability. On the other hand, if the cooling rate is 80 DEG C / s or more, the cooling in the width direction becomes uneven and a material deviation occurs.

Finally, the hot-rolled steel sheet is preferably rolled at a temperature of 580 to 700 DEG C after cooling in the ROT step.

If the coiling temperature is less than 580 DEG C, the generation of low-temperature precipitates is caused by the non-uniformity in the width direction during cooling and holding, resulting in material variation, which adversely affects the workability. On the other hand, if the coiling temperature exceeds 700 캜, the structure of the final product becomes coarse, and the workability and corrosion resistance are deteriorated.

In order to investigate the technical effects of the method of manufacturing a hot-rolled steel sheet having excellent processability and surface characteristics according to the present invention, the following experiment was conducted.

First, respective steels (three kinds of inventive steel and three kinds of comparative steels) having compositions as shown in the following Table 1 were prepared and reheated in a heating furnace at 1250 ° C for 2 hours, and then subjected to the hot rolling conditions described in [Table 2] Followed by hot rolling.

The physical properties and mechanical properties of each steel type were measured and shown in Table 4. In Table 4, the corrosion resistance was evaluated as good when the corrosion loss was less than 0.05 g / cm 2 after salt water spray test with 5% NaCl for 240 hours, and when the corrosion loss was more than 0.05 g / cm 2,

The paintability was evaluated as good when both the coating adhesion and the surface property were excellent in evaluation items of the coating material.

Crack lengths of the bending test specimens were measured and classified into five stages as shown in Table 3 below. The first stage was evaluated as good, the second to third stages were evaluated as normal, and the fourth to fifth stages were evaluated as defective.

Endurance Hyosung is classified according to the degree of occurrence of surface breakage after steel plate processing. Breaking index which expresses this is divided into 5 steps, and the first step with a relatively small tilting is good, and the second to third steps , And the fourth to fifth steps in which tilting occurred to such an extent that visual observation was possible was judged as defective.

Steel grade

Chemical composition (% by weight) Elemental
Composition ratio
C Mn P S Al N B Sn B / N Inventive Steel 1 0.0026 0.16 0.012 0.007 0.041 0.0025 0.0021 0.21 1.069 Invention river 2 0.0018 0.21 0.009 0.009 0.035 0.0031 0.0016 0.17 0.657 Invention steel 3 0.0021 0.24 0.008 0.009 0.052 0.0019 0.0019 0.09 1.273 Comparative River 1 0.0024 0.22 0.006 0.009 0.043 0.0033 0.0005 - 0.193 Comparative River 2 0.0083 0.24 0.037 0.002 0.038 0.0062 - 0.03 0.000 Comparative Steel 3 0.0305 0.54 0.006 0.005 0.034 0.0025 0.0037 0.581 1.884

※ Elemental composition ratio is calculated as atomic ratio [(B (%) / 11) / (N (%) / 14)].

division Used steel grade Reheating
Temperature
(° C) Wrap-up
Temperature
(° C) Cooling
speed
(° C / s) Coiling
Temperature
(° C) Short ball
diameter
(mm) Blasting
speed
(m / sec) Inventory 1
Inventive Steel 1
1250 890 50 600 0.25 50 Inventory 2 1250 890 60 640 0.32 60 Inventory 3 1250 890 70 600 0.32 50 Honorable 4 Invention river 2
1250 910 70 600 0.19 50 Inventory 5 1250 910 70 640 0.27 60 Inventory 6 Invention steel 3 1250 890 60 640 0.31 50 Honorable 7 Inventive Steel 1
1250 750 50 600 0.25 50 Honors 8 1250 890 10 600 0.32 50 Proposition 9 Invention river 2 1250 890 40 400 0.31 50 Comparative Example 1 Inventive Steel 1 1250 910 60 650 0.91 90 Comparative Example 2 Comparative River 1 1250 910 60 640 0.16 50 Comparative Example 3 Comparative River 2 1250 910 60 640 0.25 60 Comparative Example 4 Comparative Steel 3 1250 890 60 640 0.25 60

step Specific criteria Stage 1 No cracks in the bending process area Step 2 The cracks in the bending process area did not occur, but the surface of the process area was rough Step 3 The number of microcracks on the surface is about 1 to 3 Step 4 When the number of microcracks on the surface is 4 or more, or the sum of the total crack length is within 10 mm Step 5 Cracks occur, and the total length of the cracks is 10 mm or more

The experimental results shown in Table 4 according to the above criteria are summarized as follows.

division Surface roughness
Exponent ratio Yield point stretching phenomenon
Corrosion resistance
Paintability
Processability
Bending property Inventory 1 14.1 Not occurring Good Good Good Good Inventory 2 11.9 Not occurring Good Good Good Good Inventory 3 12.4 Not occurring Good Good Good Good Honorable 4 9.7 Not occurring Good Good Good Good Inventory 5 13.4 Not occurring Good Good Good Good Inventory 6 8.5 Not occurring Good Good Good Good Honorable 7 10.4 Not occurring Good Good usually Good Honors 8 14.9 Not occurring Good Good usually Good Proposition 9 13.9 Occur Good Good usually Good Comparative Example 1 24.1 Not occurring Bad Bad Bad usually Comparative Example 2 22.4 Occur Bad Bad Bad usually Comparative Example 3 5.6 Occur Bad usually Bad Bad Comparative Example 4 10.4 Occur Good Bad Bad Bad

Inventive Examples 1 to 6 are cases in which both the steel component control, hot rolling process and shot blasting process conditions according to the present invention are satisfied.

In all of the examples, no yield point elongation phenomenon occurred at all, and the surface roughness index ratio (Rmax / Ra) was in the range of 7 to 16, satisfying the control range, and no bending phenomenon occurred during processing, there was.

Furthermore, it was possible to manufacture a hot-rolled steel sheet and a plating plate excellent in workability and corrosion resistance by satisfying the standard value of corrosion loss in NaCl atmosphere.

Examples 7 to 9 are cases in which the steel component control (invention steel 1, invention steel 2) and shot blasting process conditions according to the present invention were satisfied, but the hot rolling process was not satisfied.

More specifically, in Inventive Example 7, the finishing rolling temperature is set to 750 DEG C lower than the control range. In Example 8, the cooling rate in the ROT process is 10 DEG C / s, which is lower than the control range. And the coiling temperature is 400 DEG C lower than the control range.

In Examples 7 to 9, some yield point elongation phenomena occurred but still exhibited good endurance. Although the processability was slightly lowered due to crystal grain precipitation, precipitation of solid elements and material variations, It can be used as a high value added steel sheet for home appliances, automobiles and the like.

Comparative Example 1 satisfies the steel component control (inventive steel 1) and hot rolling process conditions according to the present invention, but does not satisfy the shot blasting condition.

More specifically, shot blasting is carried out at a speed of 90 m / sec, which is 0.91 mm larger than the control range, and the shot blasting speed is larger than the control range in shot blasting.

In this case, the elongation at yield point does not occur, which is good in terms of durability, but the paintability and workability are also lowered due to an increase in surface roughness and an increase in internal hardening layer, which is unsuitable for use as a hot-rolled steel sheet requiring high quality.

Comparative Examples 2 to 4 are cases in which the steel materials (comparative steels 1 to 3) satisfying the hot-rolling process condition and the shot blasting condition according to the present invention but not satisfying the steel component condition are used.

In most cases, it is difficult to inhibit the draw point stretching phenomenon, so that the endurance property is lowered and the processing and coating properties according to the present invention can not be satisfied due to the occurrence of the bending phenomenon at the time of processing.

Further, in Comparative Examples 1 and 2 in which the content of Sn was low, it was not possible to obtain the desired surface characteristics due to the inability to secure surface characteristics. In Comparative Example 3 in which the Sn content was too high, And cracking at the tip end of the steel sheet causes the rolling workability to deteriorate remarkably. Also, in Comparative Examples 1 to 3, even if B and the like are out of the patent range, even when the manufacturing process and the shot blasting conditions according to the present invention are applied, since the atomic ratio between the elements is not satisfied, the desired workability such as bending resistance I could not.

Finally, FIG. 2 is a graph showing changes in endurance (expressed as a fluting index) and workability (expressed as cracking sensitivity during processing) according to the surface roughness index ratio using Invention steel 1.

In order to satisfy the desired endurance and processability, it is preferable to manage the fluting index to 2 or less and the crack generation sensitivity to 0.5 or less.

When the control range of the surface roughness index ratio according to the present invention is adjusted to 7 to 16, the fluting index and crack generation sensitivity exhibit excellent behavior. On the other hand, in the area lower than the upper management range, both the fluting index and the workability index exceed the reference value, and there is a problem that the bending phenomenon and the working crack occur. In the region above the upper management range, the anti- It is possible to confirm the problem that processing cracks occur.

While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims It will be apparent to those of ordinary skill in the art.

Claims (7)

(P) of 0.001 to 0.020%, sulfur (S) of 0.020% or less (excluding 0%), aluminum (Al) of 0.001 to 0.003%, manganese (Mn) 0.001 to 0.003% of boron (B), 0.05 to 0.25% of tin (Sn) and the balance of iron (Fe) and other unavoidable impurities,
The surface roughness index ratio (R (%) / (N (%) / 14)) of the hot-rolled steel sheet prepared by the steel satisfying 0.5? _max / R _a ) of 7 to 16 to form a modified ferrite structure,
Wherein the modified ferrite structure occupies 3 to 10% in the thickness direction of the hot-rolled steel sheet.
The method according to claim 1,
Wherein the shot blasting is performed by shot blasting a shot ball having a diameter of 0.10 to 0.40 mm at a speed of 40 to 65 m / sec, and having excellent creep resistance and surface characteristics.
The method according to any one of claims 1 to 3,
The steel having the above composition is finely rolled at 900 to 950 占 폚,
The finish rolled steel is cooled at a rate of 20 to 80 DEG C / s,
Wherein the cooled steel is wound at 580 to 700 占 폚.
delete (P) of 0.001 to 0.020%, sulfur (S) of 0.020% or less (excluding 0%), aluminum (Al) of 0.001 to 0.003%, manganese (Mn) 0.001 to 0.003% of boron (B), 0.05 to 0.25% of tin (Sn) and the balance of iron (Fe) and other unavoidable impurities,
The surface roughness index ratio ( _Rmax / Rmax) was measured by a shot blasting treatment on a hot-rolled steel sheet made of steel satisfying 0.5? [(B (%) / 11) / R _a ) of from 7 to 16,
Wherein the modified ferrite structure occupies 3 to 10% in the thickness direction of the hot-rolled steel sheet. delete delete

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