KR101433442B1 - Method for manufacturing formable hot-rolled steel having excellent corrosion resistance and the hot-rolled steel by the same method - Google Patents

Abstract

A hot-rolled steel sheet for processing which is excellent in corrosion resistance, which is used in household appliances, automobiles, and the manufacturing method thereof.
A method for manufacturing a hot-rolled steel sheet excellent in corrosion resistance of the present invention comprises 0.001 to 0.005% of carbon (C), 0.05 to 0.3% of manganese (Mn), 0.001 to 0.020% of phosphorus (P) (Al), 0.001 to 0.004% of nitrogen (N), 0.05 to 0.25% of tin (Sn), 0.01 to 0.05% of vanadium (V) Characterized in that the hot-rolled steel sheet made of steel containing unavoidable impurities is subjected to surface treatment such that the deformed ferrite structure occupies 3 to 10% in the thickness direction.

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 having excellent corrosion resistance and a method of manufacturing the same, and more particularly, to a steel sheet for ultra-low carbon steel having excellent low-breakability, paintability, workability and corrosion resistance, 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 is a cause of the fluting phenomenon. As a result, it is difficult to carry out a high purification treatment in 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 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, and then hot-rolled at a temperature of Ar ₃ transformation point or higher to obtain a material having a thickness of 1.4 to 2.3 mm. The steel slab is wound at a temperature of 600 to 700 ° C. to improve endurance and uniform stretchability 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

DISCLOSURE 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 vanadium (V) and tin (Sn) in a steel component and optimizing cooling conditions and shot blasting conditions, And a method of manufacturing the hot-rolled steel sheet. The present invention also provides a hot-rolled steel sheet excellent in creep resistance and corrosion resistance by suppressing machining defects such as fluting and a manufacturing method thereof.

In order to achieve the above object, the present invention provides a method for manufacturing a hot-rolled steel sheet having excellent corrosion resistance, which comprises 0.001 to 0.005% carbon, 0.05 to 0.3% manganese, 0.001 to 0.020% phosphorus, , 0.01 to 0.07% of aluminum (Al), 0.001 to 0.004% of nitrogen (N), 0.05 to 0.25% of tin (Sn), 0.01 to 0.05% of vanadium (V) Characterized in that the hot-rolled steel sheet made of steel containing residual iron (Fe) and other unavoidable impurities is subjected to surface treatment such that the deformed ferrite structure occupies 3 to 10% in the thickness direction.

And 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 the hot-rolled steel sheet.

The steel component is characterized by having an atomic ratio of 1.0? [V (%) / 51] / [C (%) / 12]? 3.0.

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.

The hot-rolled steel sheet is characterized by having a surface roughness index ratio (R _max / R _a ) of 10 to 20.

In order to achieve the above object, according to the present invention, there is provided a hot-dip galvanized steel sheet having excellent corrosion resistance, comprising 0.001 to 0.005% carbon, 0.05 to 0.3% manganese, 0.001 to 0.020% phosphorus, (S) of 0.02% or less (excluding 0%), aluminum (Al) of 0.01 to 0.07%, nitrogen (N) of 0.001 to 0.004%, tin (Sn) of 0.05 to 0.25%, vanadium (V) And other unavoidable impurities, and has 3 to 10% of the deformed ferrite structure in the thickness direction.

And a surface roughness index ratio (Rmax / Ra) of 10 to 20 by short blasting treatment.

The steel component is characterized by having an atomic ratio of 1.0? [V (%) / 51] / [C (%) / 12]? 3.0.

The present invention is capable of securing endurance through appropriate component control and optimization of manufacturing process, and at the same time improving workability and paintability to produce high value-added steel sheets for use in household appliances, automobiles, etc. There is an advantage.

In addition, it has an excellent descaling property, which can increase the efficiency of pickling work, and also has advantages of prevention of environmental pollution and shortening of process.

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, a hot-dip galvanized steel sheet having excellent corrosion resistance according to a preferred embodiment of 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 0.005%.

As the content of carbon (C) increases, the tensile and yield strength increase. However, when the content exceeds 0.005%, excessive workability of the material deteriorates the bending property of the material and also causes a disadvantage .

When 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. Therefore, the carbon content is limited to 0.001% to 0.005%.

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. , The content thereof is limited to 0.02% or less. 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%.

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%

Vanadium (V) not only exhibits an effect of delaying the recrystallization of ferrite when added in large amounts, but also binds with C and so forth in the steel to increase the strength of the steel sheet. In order to attain the target strength, 0.01% or more of addition is required, while when it exceeds 0.05%, the manufacturing cost is increased and the hot-rolling workability is deteriorated. Therefore, the content is limited to 0.01 to 0.05%.

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 desirable to maintain the atomic ratio between the alloy elements within a management range of 1.0? [V (%) / 51] / [C (%) / 12]? 3.0. The variables in the above relational expressions are expressed by the atomic ratio of each element. When the V / C atomic ratio is lower than 1.0, it is difficult to secure breakability and workability by the expression of the solid element. On the other hand, when the V / C atomic ratio is larger than 3.0, the workability can be secured, (V (%) / 51] / [C (%) / 12] ≤ 3.0 in terms of the inter-element atomic ratio

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

In manufacturing the hot-rolled steel sheet having excellent surface characteristics of the present invention, the surface treatment is performed such that the deformed ferrite structure occupies 3 to 10% in the thickness direction of the hot-rolled steel sheet. Various methods can be applied to the surface treatment method, The rights can not be limitedly interpreted only by the shot blasting method which is one of the surface treatment methods, and various methods equivalent thereto are also within the scope of the present invention.

First, 0.001 to 0.005% of carbon (C), 0.05 to 0.3% of manganese (Mn), 0.001 to 0.020% of phosphorus (P), 0.02% To 0.07% nitrogen, 0.001-0.004% nitrogen, 0.05-0.25% tin, 0.01-0.05% vanadium (V), the balance iron (Fe) and other unavoidable impurities A hot-rolled steel sheet is produced, and shot blasting is performed on the hot-rolled steel sheet surface at a shot diameter of 0.1 to 0.4 mm and a blasting rate of 40 to 65 m / sec.

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 ensure 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 shot blasting to be 10 to 20.

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 10, 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 20, the density of the movable potential reaches the saturation state, but the tip crack starts to occur during processing, and the range is preferably 10 to 20.

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 ° C, the structure of the final product is coarsened and a cementite phase is formed, resulting in deterioration of workability and corrosion resistance.

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, paintability was evaluated as good when both paint adhesion and surface characteristics were excellent among evaluation items of paints. In case where only one of the two characteristics was excellent, it was indicated as bad.

Corrosion resistance was evaluated as good when corrosion loss was less than 0.05 g / cm < ² > after salt water spray test for 240 hours with 5% NaCl,

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 Sn V V / C Inventive Steel 1 0.0021 0.19 0.012 0.007 0.041 0.0025 0.21 0.023 2.58 Invention river 2 0.0029 0.25 0.009 0.009 0.035 0.0031 0.17 0.017 1.38 Invention steel 3 0.0042 0.14 0.008 0.009 0.052 0.0019 0.09 0.043 2.41 Comparative River 1 0.0024 0.22 0.006 0.009 0.043 0.0033 - 0.005 0.49 Comparative River 2 0.0043 0.24 0.037 0.002 0.038 0.0062 0.03 0.074 4.05 Comparative Steel 3 0.0305 0.54 0.006 0.005 0.034 0.0039 0.58 0.035 0.27

※ Elemental composition ratio is calculated as [V (wt.%) / 51] / [C (wt.%) / 12] as atomic ratio, respectively.

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
Paintability
Corrosion resistance
Processability
Bending property Inventory 1 13.6 Not occurring Good Good Good Good Inventory 2 15.1 Not occurring Good Good Good Good Inventory 3 17.4 Not occurring Good Good Good Good Honorable 4 11.9 Not occurring Good Good Good Good Inventory 5 14.5 Not occurring Good Good Good Good Inventory 6 12.8 Not occurring Good Good Good Good Honorable 7 14.8 Not occurring Good Good usually Good Honors 8 18.2 Not occurring Good Good usually Good Proposition 9 11.8 Occur Good Good usually Good Comparative Example 1 25.4 Not occurring Bad Bad Bad usually Comparative Example 2 23.8 Occur Bad Bad Bad Bad Comparative Example 3 10.8 Not occurring usually Bad Bad usually Comparative Example 4 8.4 Occur Bad Good 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 10 to 20, satisfying the control range, and no bending phenomenon occurred during processing, there was.

In addition, no cracks were generated during the bending process, and thus, a high workability and paintability were exhibited, and it was possible to manufacture hot-rolled steel sheets and original plates for plating, which are excellent in workability and surface characteristics.

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.

That is, in the case of Comparative Examples 2 and 3 in which the addition amount of tin (Sn) deviates from the composition range of the steel of the invention, the corrosion resistance can not be fundamentally secured, and therefore, the object of the present invention can not be achieved. Also, in the case of Comparative Example 2 in which the vanadium (V) content was low and the V / C atom ratio was not satisfied even when the manufacturing process and the shot blasting condition according to the present invention were applied, the yield point elongation phenomenon occurred, There was no. On the other hand, in Comparative Example 3 where the vanadium (V) content was higher than the V / C atomic ratio, the yield point stretching phenomenon did not occur. However, as nitrogen or the like deviated from the control range, not only the workability was poor, ) Could not solve the problem of cost increase due to over-adding.

On the other hand, when the content of tin (Sn) satisfies the range of the present invention, a steel in which the atomic ratio of vanadium (V), carbon (C) and V / C is out of the range of the present invention is applied under the manufacturing process and shot blasting In the case of Comparative Example 4, the elongation at the yield point and the bending phenomenon occurred, so that the workability could not be secured.

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 10 to 20, the fluting index and the 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 (8)

(P) of 0.001 to 0.020%, sulfur (S) of 0.02% or less (excluding 0%), aluminum (Al) of 0.001 to 0.005%, manganese (Mn) (Fe), and other unavoidable impurities, in the range of from 0.001 to 0.07%, nitrogen (N), from 0.001 to 0.004%, tin (Sn) from 0.05 to 0.25%, vanadium (V)
In an atomic ratio, 1.0≤ [V (%) / 51] / [C (%) / 12] The surface roughness index by a shot blasting treatment to the surface of the hot-rolled steel sheet made of steel which satisfies the ≤3.0 ratio (R _max / R _a ) of 10 to 20 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 a short 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 the hot-rolled steel sheet.
delete The method according to claim 1 or 2,
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 rolled at 580 to 700 占 폚.
delete (P) of 0.001 to 0.020%, sulfur (S) of 0.02% or less (excluding 0%), aluminum (Al) of 0.001 to 0.005%, manganese (Mn) 0.001 to 0.004% of nitrogen (N), 0.05 to 0.25% of tin (Sn), 0.01 to 0.05% of vanadium (V), the balance Fe and other unavoidable impurities,
And the surface roughness index ratio (Rmax / Ra) is 10 to 20% by shot blasting. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet is a steel having an atomic ratio of 1.0? [V (%) / 51] / [C (%) / 12]? 3.0. 20 to form a deformed ferrite structure, and the deformed ferrite structure occupies 3 to 10% in the thickness direction.
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