KR101410095B1 - Method for manufacturing hot-rolled steel having excellent anti-aging and the hot-rolled steel by the same method - Google Patents

Abstract

A hot-rolled steel sheet excellent in endurance and excellent in workability and paintability as well as endurance for suppressing processing defects such as fluting by optimizing steel components, manufacturing processes and shot blasting conditions is disclosed.
The method for producing hot rolled steel sheet excellent in endurance of the present invention comprises 0.001 to 0.003% of carbon (C), 0.1 to 0.5% of manganese (Mn), 0.025% or less of phosphorus (excluding 0% , Co: 0.01 to 0.05%, N (N): 0.02% or less (excluding 0%), Al (Al): 0.03 to 0.09% Shot steel balls having a diameter of 0.10 to 0.40 mm were sprayed at a speed of 40 to 65 m / sec on the surface of a hot-rolled steel sheet made of steel containing 0.001 to 0.004% of Fe and the balance of other unavoidable impurities, .

Description

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

The present invention relates to a hot-rolled steel sheet excellent in anti-aging properties and a method of manufacturing the same, and more particularly, to a hot-rolled steel sheet having excellent workability and paintability as well as anti- will be.

In order to be used as a steel sheet for household appliances and automobiles, excellent workability, endurance (flutting resistance) and paintability are required.

Fluting refers to a phenomenon in which a machining portion breaks during machining. When fluting occurs, it is difficult to maintain the shape of the machining portion, so it must be strictly limited in actual processes.

However, in general, aging phenomenon caused by the solid element which causes the fluting phenomenon is difficult to substantially inhibit, so that it is possible to achieve high purification in the steelmaking step, and at the same time, titanium (Ti), niobium ) And the like are added to precipitate carbonitride.

The addition of these carbonitride-forming elements contributes to suppression of machining defects such as fluting, but it has become a factor of lowering of productivity due to an increase in steelmaking time for high purification and an increase of manufacturing cost by adding expensive alloying elements.

In addition, it is known that it is difficult to suppress the bending phenomenon such as fluting in the basically low-carbon steel.

Accordingly, it has been demanded to establish a method capable of suppressing a tilting phenomenon when machining is required such as home electric appliances and automobiles.

On the other hand, in order for the final product to improve shape mobility and improve the production process by improving the manufacturing process, various processing characteristics such as stretch flangeability, bending property, and drawability are required in addition to fluting by the above- .

In addition, since these structures are exposed to the external environment, coating of organic materials such as paint is performed on the surface of the steel sheet in order to improve weatherability. Therefore, in order to secure such characteristics, development of a steel sheet for plating .

For example, Japanese Patent Application Laid-Open No. 1989-282420 (titled " process for producing hot-rolled steel sheet for processing and heat-treatment for hot-rolled steel sheet ") discloses a method for producing a steel sheet suitable for high- (Ti), niobium (Nb), and a rare earth element or the like are added to a low carbon steel base to produce a hot-rolled steel sheet excellent in workability and aging property.

However, as described above, this method is effective in suppressing machining defects such as fluting by addition of carbonitride-forming elements. However, since the productivity is lowered due to an increase in steelmaking time for high-cleanliness and the manufacturing cost is increased due to the addition of expensive alloying elements .

As another example, Korean Patent Laid-Open Publication No. 1996-23130 (entitled "Method for manufacturing high-temperature hot-rolled steel sheet excellent in durability") discloses a method of adding a trace amount of zirconium or the like as a carbonitride- There is disclosed a method of enhancing endurance by hot-rolling the ferrite grains in the temperature range immediately above the Ar ₃ transformation point to coarsen the ferrite grains.

However, according to this method, since addition of special elements such as zirconium is required to increase the aging property, not only the deterioration of the steelmaking workability and the cost increase but also the shape dynamicness is deteriorated as the material strength is lowered .

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, since the aging phenomenon due to 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, it is difficult to expect a high antioxidant property

As another example, Japanese Patent Laid-Open No. 2008-190008 (a method of manufacturing a hot rolled steel sheet having excellent anti-aging properties) is characterized by containing 0.04 to 0.25% of carbon (C) (P): 0.09% or less; S: not more than 0.015%; aluminum (Al): 0.01 to 0.08%; nitrogen (N): 0.0005 to 0.015% And the remainder iron (Fe) and other unavoidable impurities is hot-rolled and cooled to an average cooling rate of 60 占 폚 / s or less to less than 400 占 폚 and wound, and then t / R? 0.0055 , And R is a roll diameter), a skin pass rolling with an elongation of 0.1 to 1.0% is carried out.

However, since this method lowers the coiling temperature to less than 400 ° C., the non-uniformity of the widthwise temperature causes a difference in the production behavior of the low-temperature precipitates, causing a material deviation, thereby causing defects in shape, poor coiling and post- there was. Further, since the roll diameter is controlled according to the thickness of the plate in order to induce the surface movement potential, there is a problem that it is difficult to apply to the case of producing 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. 1996-23130 Korean Patent Publication No. 2001-60648 Japanese Patent Laid-Open No. 2008-190008

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 cobalt (Co) and boron (B) The present invention also provides a method of manufacturing a hot-rolled steel sheet.

In order to achieve the above object, the present invention provides a method of manufacturing hot rolled steel sheet having excellent anti-aging properties, comprising: 0.001 to 0.003% of carbon (C), 0.1 to 0.5% of manganese (Mn) (Al): 0.03 to 0.09%, boron (B): 0.0005 to 0.0020%, cobalt (Co): 0.01 to 0.05% A short ball having a diameter of 0.10 to 0.40 mm is formed on the surface of a hot-rolled steel sheet made of steel containing 0.001 to 0.004% of nitrogen (N) and the balance of iron (Fe) and other unavoidable impurities, And the shot blasting is performed.

The steel component is characterized by satisfying 4? [B (ppm) x Al (%) x Co (%) / N (%)?

The hot-rolled steel sheet is produced by finishing rolling the steel having the above composition at 890 to 950 캜, cooling the finely-rolled steel at a cooling rate of 20 to 80 캜 per second, and winding the steel at a temperature of 580 to 680 캜 .

And the surface roughness index ratio (Rmax / Ra) is set to 7 to 16 by the shot blasting.

In order to achieve the above object, the hot rolled steel sheet having excellent anti-aging properties according to the present invention comprises 0.001 to 0.003% of carbon (C), 0.1 to 0.5% of manganese (Mn) and 0.025% or less of phosphorus (P) (B): 0.0005 to 0.0020%, cobalt (Co): 0.01 to 0.05%, nitrogen (C): 0.02 to 0.02% (N) of 0.001 to 0.004%, and the balance of Fe and other unavoidable impurities, and has a surface roughness index ratio (Rmax / Ra) of 7 to 16 by shot blasting.

The steel component is characterized by satisfying 4? [B (ppm) x Al (%) x Co (%) / N (%)?

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

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-rolled steel sheet having excellent anti-aging properties according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

As described above, the hot-rolled steel sheet of the present invention is a high-value-added hot-rolled steel sheet that can simultaneously be used for home appliances and automobiles by simultaneously satisfying endurance, workability and paintability. The present inventors have repeatedly conducted research and experiments, As a result, the steel component according to the present invention is restricted as follows.

Carbon (C) is an element added to improve the strength of a steel sheet. As the content increases, the tensile and yield strengths increase. However, if excess is added, the workability of the material decreases, so the upper limit is limited to 0.003%.

On the other hand, when the carbon content is less than 0.001%, additional working time is required for decarburization at the time of steelmaking, and there is a problem that it is difficult to secure a stable material because the material changes rapidly, and the lower limit is limited to 0.001%.

Therefore, it is preferable to control the carbon content to 0.001 to 0.003%.

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

If the manganese content is low, the workability improves, but it is difficult to secure the strength. Therefore, in order to secure the target strength, 0.1% or more should be added.

On the other hand, when manganese is added excessively, the addition of a large amount of alloying element results in a decrease in economy and a cause of center segregation, so the upper limit is limited to 0.5%.

Phosphorus (P) is an element which plays a role of improving the strength of steel and improving the corrosion resistance. It is preferable that a large amount of phosphorus (P) is added in order to secure these properties. However, it is an element that causes center segregation during casting, The content of the bar is limited to 0.025% or less (exclusive of 0%), preferably 0.005 to 0.015%.

Sulfur (S) not only forms a nonmetallic inclusion which acts as a corrosion starting point by binding with manganese in the steel, but also acts as a factor of the embrittlement of brittleness, so it is desirable to reduce the content as much as possible. Therefore, the content of sulfur is limited to 0.02% or less (exclusive of 0%), preferably 0.01% or less.

Although aluminum (Al) is an element added for deoxidation of molten steel, aluminum (Al) is an element to be added for deoxidation of molten steel. However, since it has an aspect of improving aging characteristics by bonding with a solid element in steel, its lower limit is limited to 0.03%, and if excess is added, There is a problem of not only causing defects but also deteriorating processability. Therefore, the upper limit is limited to 0.09% and the management range is limited to 0.03 to 0.09%.

Boron (B) is an element that not only improves the aging property but also increases the hardenability of the material by adding a small amount as an element to improve the aging property. Therefore, in order to obtain the desired material properties, at least 0.0005% Addition is necessary. However, when the content exceeds 0.0020%, the material deteriorates and causes grain boundary cracking during performance, and there is a problem that the surface of the hot-rolled steel sheet is roughened, so it is limited to 0.0005 to 0.0020%.

Cobalt (Co) is an element promoting the formation of precipitates in the steel. In order to obtain such effect, at least 0.01% should be added. However, when cobalt is added in an amount exceeding 0.05%, the effect of increasing the production cost due to the addition of a large amount of an expensive alloying element is more significant than the effect of contributing to the precipitation promotion, so that the range is limited to 0.01 to 0.05% .

Nitrogen (N) is an element useful for reinforcing materials while existing in solid state in steel. However, since it is the main element causing aging phenomenon, it is necessary to control the amount of nitrogen to be below 0.004% in order to secure processability.

Further, when the content is less than 0.001%, sufficient stiffness can not be obtained and sites for forming precipitates are reduced. Therefore, the lower limit is limited to 0.001%, and the management range is limited to 0.001-0.004%.

In order to appropriately control the precipitates and the employment phase fraction to secure further excellent processing characteristics, it is necessary to maintain the content ratio of the alloying elements to 4 [B (ppm) x Al (%) x Co (%) / N .

When [B (ppm) x Al (%) x Co (%) / N (%)] is less than 4, it is difficult to secure high endurance and workability by the expression of solid elements, while [B ) × Al (%) x Co (%) / N (%)] is more than 15, workability can be secured, but some surface defects due to excessive element addition and productivity decrease.

In this respect, the content ratio of the alloying elements is preferably controlled to be 4? [B (ppm) x Al (%) x Co (%) / N (%)?

Hereinafter, a method for manufacturing the hot-rolled steel sheet according to the present invention using the above-described component-controlled steel will be described in detail.

(P) of 0.025% or less (excluding 0%), sulfur (S) of 0.02% or less (excluding 0%), carbon (C) of 0.01 to 0.12%, manganese (Mn) of 0.1 to 0.5% A steel containing 0.03 to 0.09% of aluminum (Al), 0.0005 to 0.0020% of boron (B), 0.01 to 0.05% of cobalt (Co), 0.001 to 0.004% of nitrogen (N), the balance iron (Fe) and other unavoidable impurities A hot-rolled steel sheet is produced by a conventional hot-rolling process, and shot blasting is performed on the surface of the hot-rolled steel sheet at a shot diameter of 0.10 to 0.40 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 endurance. The short blasting process is performed by introducing a suitable compressive stress to the surface of the hot-rolled steel sheet to obtain a dislocation density, a modified ferrite lid grain) is newly generated, thereby reducing the phenomenon of dislocation of the dislocation by the hiring element which is a main cause of the aging phenomenon, thereby improving the endurance.

In order to achieve this effect, it is desirable to control the diameter of the shot ball used for shot blasting to 0.10 to 0.40 mm.

When 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 the effect of proper residual stress. When the diameter of the shot ball exceeds 0.40 mm, the surface roughness increases sharply, This is because it acts as a factor of crack generation.

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 shot ball acting on the surface layer is low, and it is difficult to secure the desired endurance. When the injection speed exceeds 65 m / sec, the depth of the surface hardened layer is 10% Resulting in 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.

When the surface roughness index ratio is less than 7, the ratio of the appropriate roughness to the ridges is not ensured, so that the adsorbability of the organic matter is lowered, and the residual stress distribution in the thickness direction of the material is insufficient.

On the other hand, when the surface roughness index ratio exceeds 16, cracking starts to occur while reaching a saturated state in terms of ensuring adsorption, and therefore, the management 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 strain ferrite structure is less than 3% in the thickness direction, it can not secure the desired antioxidant property because it does not show a structural characteristic capable of sufficiently fixing the solid element in the steel, and the strain ferrite structure is 10% If it exceeds the above range, it acts as a factor of hardening of the material in the processing and post-processing, which causes deterioration of the workability. Therefore, the management range is preferably 3 to 10%.

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, a hot rolled steel sheet having more excellent endurance, workability, and paintability can be manufactured. Hereinafter, the optimized hot rolling process will be described in detail.

First, in the hot-rolled steel sheet according to the present invention, it is preferable that the steel composed of the above-mentioned composition components is subjected to finish rolling at 890 to 950 ° C.

When the finish rolling temperature is less than 890 캜, the hot rolling is completed in the low temperature region, and the blistering of the crystal grains proceeds rapidly, resulting in a reduction in the rolling property and the workability.

On the other hand, if the finish rolling temperature is higher than 950 ° C, uniform hot rolling is not carried out throughout the thickness, and the grain refinement becomes insufficient, and the impact toughness due to crystal grain coarsening is lowered. To 950 < 0 > C.

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

If the cooling rate in the ROT is less than 20 DEG C / second, relatively coarse crystal grains are formed due to the dynamic crystal grain growth, which causes the strength and workability to deteriorate. On the other hand, when the cooling rate is 80 deg. C / sec or more, it causes the material deviation due to the uneven cooling in the width direction. Therefore, the cooling rate is preferably controlled to 20 to 80 deg.

Finally, it is preferable that the hot-rolled steel sheet is rolled at a temperature of 580 to 680 DEG C after cooling in the ROT step.

When the coiling temperature is less than 580 占 폚, the generation behavior of the low-temperature precipitates differs due to the non-uniformity in the width direction during cooling and holding, resulting in a material deviation, which adversely affects the workability. On the other hand, if the coiling temperature exceeds 680 DEG C, a problem arises in that the workability and corrosion resistance are lowered as the texture of the final product forms a coarse cementite phase, so that the range of the coiling temperature is controlled to 580 to 680 DEG C desirable.

In order to investigate the technical effect of the method of manufacturing hot rolled steel sheet having excellent anti-aging properties according to the present invention, the following experiment was conducted.

First, each of the steels (two kinds of inventive steel and six kinds of comparative steels) having the composition as shown in Table 1 below was prepared and reheated for 2 hours in a heating furnace at 1250 DEG C, 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 grade were measured and shown in Table 4.

Steel grade

Chemical composition (% by weight) Elemental composition ratio
C Mn P S Al B Co N Ti Inventive Steel 1 0.0015 0.22 0.009 0.007 0.047 0.0009 0.035 0.0026 - 5.69 Invention river 2 0.0026 0.23 0.011 0.009 0.064 0.0015 0.044 0.0033 - 12.80 Comparative River 1 0.0070 0.19 0.014 0.008 0.039 - - 0.0059 - 0.00 Comparative River 2 0.0021 0.22 0.008 0.009 0.009 - - 0.0036 0.056 0.00 Comparative Steel 3 0.0064 0.47 0.011 0.002 0.041 - 0.032 0.0048 - 0.00 Comparative Steel 4 0.0094 0.26 0.010 0.004 0.048 0.0034 0.104 0.0089 - 19.07 Comparative Steel 5 0.0413 0.45 0.072 0.025 0.042 0.0002 0.083 0.0032 - 2.18 Comparative Steel 6 0.0125 1.21 0.045 0.003 0.034 - - 0.0027 0.041 0.00

* Component ratio between elements: [B (ppm) x Al (%) x Co (%) / N (%)]

division
Used steel grade
Reheat temperature
(° C)
Finishing temperature
(° C)
Cooling rate
(° C / s)
Coiling temperature
(° C)
Short ball diameter
(mm) Blasting
speed
(m / s) Inventory 1 Inventive Steel 1

1250 900 40 600 0.25 50 Inventory 2 1250 900 50 650 0.32 60 Inventory 3 1250 900 70 600 0.32 50 Honorable 4 Invention river 2
1250 910 60 600 0.19 50 Inventory 5 1250 910 70 650 0.27 60 Inventory 6 Inventive Steel 1
1250 750 50 600 0.25 50 Honorable 7 1250 900 15 600 0.32 50 Honors 8 Invention river 2 1250 900 40 400 0.31 20 Comparative Example 1 Inventive Steel 1 1250 910 70 650 0.91 90 Comparative Example 2 Comparative River 1 1250 910 50 650 0.16 50 Comparative Example 3 Comparative River 2 1250 910 50 650 0.25 60 Comparative Example 4 Comparative Steel 3 1250 900 50 650 0.25 60 Comparative Example 5 Comparative Steel 4 1250 900 50 650 0.16 50 Comparative Example 6 Comparative Steel 5 1250 900 50 650 0.25 60 Comparative Example 7 Comparative Steel 6 1250 900 50 650 0.25 60

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.

The cracking length of the bending test specimens was measured and classified into five steps as shown in Table 3 below. The first step was good, the second to third steps were normal, and the fourth to fifth steps were bad.

step Specific criteria Stage 1 No cracks in the bending process area Step 2 Cracks do not occur in the bending portion, but the surface of the portion to be processed is rough Step 3 The number of microcracks on the surface is about 1 to 3 Step 4 If the number of microscopic cracks 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

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 stages, and the first step with a relatively small bending phenomenon is good, and the second to third steps , And 4 to 5 stages in which the bending phenomenon occurred to such an extent that visual observation was possible was judged as defective.

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

division Surface roughness
Exponent ratio Yield point extension
Presence or absence Yield strength
(kgf / mm2) Elongation (%) Paintability Processability Endocytosis Inventory 1 8.6 Not occurring 21.6 39.6 Good Good Good Inventory 2 13.1 Not occurring 23.4 41.4 Good Good Good Inventory 3 12.8 Not occurring 22.8 42.5 Good Good Good Honorable 4 9.5 Not occurring 24.9 40.3 Good Good Good Inventory 5 11.7 Not occurring 25.4 42.6 Good Good Good Inventory 6 12.9 Not occurring 34.1 26.9 Good usually Good Honorable 7 5.2 Not occurring 25.7 40.8 Good usually Good Honors 8 4.9 Occur 28.5 35.4 Good usually Good Comparative Example 1 20.6 Not occurring 26.8 29.6 Bad Bad Good Comparative Example 2 5.4 Occur 26.4 36.1 Good Bad Bad Comparative Example 3 10.8 Not occurring 15.7 43.7 Bad Good Bad Comparative Example 4 4.5 Occur 27.4 21.5 Bad Bad Bad Comparative Example 5 3.6 Occur 30.4 23.4 Bad Bad Bad Comparative Example 6 5.2 Occur 34.8 29.5 Bad Bad Bad Comparative Example 7 9.6 Not occurring 44.5 25.2 Good Bad Bad

Inventive Examples 1 to 5 are cases in which both of the steel component control, hot rolling process and shot blasting process conditions according to the present invention are satisfied. In all of the examples, the yield point elongation did not occur, and the surface roughness index ratio (Rmax / Ra) was in the range of 7 to 16, which satisfied the control range and the bending phenomenon did not occur during processing, . Further, since no work cracks are generated during the bending process, high workability and paintability are exhibited, so that it is possible to manufacture excellent hot-rolled steel sheets and original plates for plating.

Examples 6 to 8 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. Describing in more detail, Inventive Example 6 is performed at a temperature of 750 占 폚 lower than the control range, Example 7 is performed at a cooling rate of 15 占 폚 / s lower than the control range in the ROT process, And the coiling temperature is 400 DEG C lower than the control range. In Examples 6 to 8, 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 satisfied the high quality requirements, It can be used as a high value added steel sheet for household appliances and automobiles because it has paintability.

Comparative Example 1 satisfied the steel component control (Inventive Steel 1) and hot rolling process conditions according to the present invention, but did 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 shot blasting speed is larger than the control range in the short blast condition. In this case, the elongation at yield point does not occur, which is good in terms of durability. However, both workability and paintability are reduced due to increase in surface roughness and internal hardening layer, which is unsuitable for use as a hot-rolled steel sheet requiring high quality.

Comparative Examples 2 to 7 are cases in which the steel materials (comparative steels 1 to 6) satisfying the hot-rolling process condition and the shot blasting condition according to the present invention but not satisfying the steel component condition were 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.

Furthermore, in Comparative Example 2 in which the component ratio [B (ppm) x Al (%) x Co (%) / N (%)] was 0.00 and Ti was not included in the carbonitride forming element, It is not only difficult to ensure workability by the above-mentioned method, but also the strength is low and the shape durability is deteriorated at the time of processing. On the other hand, in Comparative Example 3 in which the component weight ratio, B (ppm) x Al (%) x Co (%) / N (%)) is 0.00 and Ti as the carbonitride forming element is contained, Even if the production process according to the present invention and the shot blasting conditions are applied, the desired antioxidant property and paintability can not be ensured.

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, there is a problem that the fluting index is 2 or more, and there is a problem that the processing is cracked while the anti- I could confirm.

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): not more than 0.025% (excluding 0%), sulfur (S): not more than 0.02% (excluding 0%) ), Aluminum (Al): 0.03 to 0.09%, boron (B): 0.0005 to 0.0020%, cobalt (Co): 0.01 to 0.05%, nitrogen (N): 0.001 to 0.004% Containing an unavoidable impurity,
The surface roughness index ratios of the hot-rolled steel sheets produced by the shot blasting treatment were measured on the surface of the hot-rolled steel sheet made of steel satisfying 4? [B (ppm) x Al (%) x Co (%) / N (% Rmax / Ra) of 7 to 16 to form a modified ferrite structure,
Wherein the deformed 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 spraying a shot ball having a diameter of 0.10 to 0.40 mm at a speed of 40 to 65 m / sec to perform shot blasting.
The method according to any one of claims 1 to 3,
In the hot-rolled steel sheet, the steel having the above composition is subjected to finish rolling at 890 to 950 캜,
The finished steel is cooled at a cooling rate of 20 to 80 DEG C per second,
Wherein the hot rolled steel sheet is rolled up at a temperature of 580 to 680 캜 after the cooling.
delete (P): not more than 0.025% (excluding 0%), sulfur (S): not more than 0.02% (excluding 0%) ), Aluminum (Al): 0.03 to 0.09%, boron (B): 0.0005 to 0.0020%, cobalt (Co): 0.01 to 0.05%, nitrogen (N): 0.001 to 0.004%, the balance being Fe and other unavoidable impurities ≪ / RTI &
(Rmax / Ra (Ra) / Ra (Ra) / Ra (Ra)) of the hot-rolled steel sheet, ) Is formed to 7 to 16 to form a deformed ferrite structure,
Characterized in that the deformed ferrite structure occupies 3 to 10% in the thickness direction of the hot-rolled steel sheet.
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