KR101330285B1 - Formable hot-rolled steel sheet having excellent surface properties and manufacturing method thereof - Google Patents

Abstract

Hot rolled steel sheet excellent in processability and surface properties used in home appliances, automobiles, and the like, and a manufacturing method thereof are introduced.
Hot rolled steel sheet manufacturing method excellent in workability and surface properties of the present invention, by weight%, carbon (C) 0.01 ~ 0.10%, manganese (Mn) 0.5 ~ 1.5%, phosphorus (P) 0.01 ~ 0.06%, sulfur (S) 0.020% or less (except 0%), aluminum (Al) 0.01 ~ 0.07%, nitrogen (N) 0.001 ~ 0.004%, titanium (Ti) 0.02 ~ 0.06%, cobalt (Co) 0.01 ~ 0.05%, the rest iron (Fe) On the surface of the hot-rolled steel sheet made of steel containing and other unavoidable impurities, a shot ball of 0.1 to 0.4 mm in diameter shot blasting at a speed of 40 ~ 65m / sec.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet having excellent workability and surface properties,

The present invention relates to a hot rolled steel sheet having excellent workability and surface properties and a method for manufacturing the same, and more particularly, to a steel sheet and a low to medium carbon steel sheet having excellent low tolerability, processability and corrosion resistance by optimizing steel components and manufacturing processes, and the preparation thereof. It is about a 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 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.

Korean Unexamined Patent Publication No. 1996-23130 discloses a method for producing a hot rolled steel sheet having high aging resistance.

A small amount of zirconium, which is a carbonitride-forming element, is added to the ultra-low carbon aluminum-kilted steel, and Ar ₃ By hot rolling in the temperature range directly above the transformation point to coarsen the ferrite grains. It relates to a method of improving the aging resistance.

However, according to this method, it is necessary to add a special element such as zirconium in order to increase the aging, steelmaking workability is deteriorated, the cost is increased, and the shape freezeability deteriorates as the material strength is lowered. have.

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. 1996-23130 Japanese Patent Laid-Open No. 2008-190008

The present invention has been proposed to solve such a conventional problem, by adding manganese (Mn), titanium (Ti) and cobalt (Co) of the steel components in the low-low carbon steel, by optimizing the cooling conditions and shot blasting conditions, etc. It is an object of the present invention to provide a method for producing a hot rolled steel sheet excellent in bending resistance, workability, corrosion resistance, and surface properties that ensures coating property of a steel sheet and suppresses processing defects such as fluting. .

Hot-rolled steel sheet manufacturing method excellent in workability and surface properties according to the present invention for achieving this object, by weight, carbon (C) 0.01 ~ 0.10%, manganese (Mn) 0.5 ~ 1.5%, phosphorus (P) 0.01 ~ 0.06%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 ~ 0.07%, nitrogen (N) 0.001 ~ 0.004%, titanium (Ti) 0.02 ~ 0.06%, cobalt (Co) 0.01 ~ 0.05 %, The remainder is characterized in that the shot blasting at a speed of 40 ~ 65m / sec shot on the surface of the hot-rolled steel sheet made of steel containing iron (Fe) and other unavoidable impurities at a speed of 40 ~ 65m / sec.

The steel component is characterized by satisfying 0.015 ≦ [(Ti × Co) / C] weight ratio ≦ 0.06.

Steel composed of the composition is finish rolled at 860 ~ 950 ℃, the finish rolled steel is cooled at a rate of 60 ~ 120 ℃ / s, characterized in that the cooled steel is wound at 550 ~ 700 ℃.

The surface roughness index ratio (R _max / R _a ) is 13 to 23 by the shot blasting.

Hot-rolled steel sheet excellent in workability and surface properties according to the present invention for achieving this object, by weight%, carbon (C) 0.01 ~ 0.10%, manganese (Mn) 0.5 ~ 1.5%, phosphorus (P) 0.01 ~ 0.06% , Sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 ~ 0.07%, nitrogen (N) 0.001 ~ 0.004%, titanium (Ti) 0.02 ~ 0.06%, cobalt (Co) 0.01 ~ 0.05%, The remainder is made of steel containing iron (Fe) and other unavoidable impurities, characterized in that the surface roughness index ratio (Rmax / Ra) is formed by 13 ~ 23 by the shot blasting treatment.

The steel component is characterized by satisfying 0.015 ≦ [(Ti × Co) / C] weight ratio ≦ 0.06.

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.

According to the method of manufacturing the hot rolled steel sheet of the present invention configured as described above, high value-added steel sheet used in home appliances, automobiles, etc. by improving bending resistance through proper control of components and optimization of the manufacturing process and improving workability, corrosion resistance and paintability There is an advantage that can be prepared.

In addition, there is an advantage that the descaling property is also excellent, the efficiency of the pickling work can be improved, the environmental pollution can be prevented, and the process can be shortened.

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.

Hereinafter, a hot-rolled steel sheet having excellent processability and surface characteristics according to a preferred embodiment of the present invention and a method of manufacturing the same 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 the steel sheet and its content is limited to 0.01% to 0.10%.

As the carbon (C) content increases, the tensile and yield strengths increase, but when the content is excessively added in excess of 0.10%, the workability of the material decreases and the bending resistance also decreases.

In addition, if the content is less than 0.01%, additional operating time for decarburization during steelmaking operations is reduced.

It is necessary, and there is a disadvantage that it is difficult to secure a stable material such as the target strength.

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 the Mn content is low, the workability is improved but it is difficult to secure the strength. Therefore, the Mn content should be 0.5% or more in order to secure the target strength.

On the other hand, if manganese is added excessively, addition of a large amount of alloying element lowers the economic efficiency and causes not only a cause of center segregation but also deteriorates weldability, so the upper limit is limited to 1.5%

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 0.06% or less. On the other hand, if the content of phosphorus is less than 0.01%, it is difficult to secure strength and corrosion resistance.

Sulfur (S) combines with manganese (Mn) in the steel to form non-metallic inclusions that serve as a starting point for corrosion, as well as acting as a factor of red brittleness, 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.

Aluminum (Al) is an element added for the deoxidation of molten steel, but it has a function of improving the aging characteristics by binding with the solid element in the 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%.

Nitrogen (N) exists in solid solution in steel and is useful for reinforcing materials, but since it is the main element causing aging phenomenon, it is necessary to manage it to a certain amount or less to secure workability. The upper limit is limited to 0.004%. In addition, if it is less than 0.001%, sufficient rigidity cannot be obtained and the site for precipitate formation will decrease, so the lower limit is limited to 0.001%.

Titanium (Ti) delays the recrystallization of ferrite and is bonded to C, N, etc. in steel, and thus, increases the strength of the steel sheet. Titanium (Ti) of 0.02% or more is added in order to secure the target strength, but the amount of the addition is limited to 0.06% or less in order to prevent manufacturing cost increase and deterioration of hot rolling workability.

Cobalt (Co) is an element that promotes the formation of precipitates in steel, and at least 0.01% or more is required to obtain such an effect. However, if the content exceeds 0.05%, the problem due to the increase in manufacturing cost due to the addition of a large amount of expensive alloying elements is greater than the contribution to the promotion of precipitation, the content is limited to 0.01 ~ 0.05%.

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

In order to secure the characteristics of the present invention, it is preferable to maintain a management range between alloy elements at 0.015 ≦ [(Ti x Co) / C] weight ratio ≦ 0.06.

The variable in the above relation is the weight ratio of each element. When the [(Ti × Co) / C] weight ratio was lower than 0.015, it was difficult to secure the bending resistance and workability due to the expression of the solid solution element, whereas when the [(Ti × Co) / C] weight ratio was larger than 0.06 Processability can be ensured, but there are problems that surface defects occur due to excessive addition elements, and productivity is lowered. Therefore, it is preferable to control the weight ratio between elements to 0.015≤ [(Ti x Co) / C] weight ratio ≤0.06. Do.

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, carbon (C) 0.01 ~ 0.10%, manganese (Mn) 0.5 ~ 1.5%, phosphorus (P) 0.01 ~ 0.06%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 ~ 0.07% Hot rolled steel sheet following conventional hot rolling process, steel containing 0.001 ~ 0.004% of nitrogen (N), 0.02 ~ 0.06% of titanium, 0.01 ~ 0.05% of cobalt (Co), balance iron (Fe) and other unavoidable impurities To prepare, using a shot ball having a diameter of 0.1 ~ 0.4mm on the surface of the hot rolled steel sheet, shot blasting at a spraying speed of 40 ~ 65m / 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 short 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 process to be 13 to 23.

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.

If the surface roughness index ratio is less than 13, the appropriate ratio of rough acid and bone is not secured, so that the adsorptivity of organic matters is lowered and the density distribution of the working potential for fixing solid solution is insufficient, making it difficult to secure aging resistance. On the other hand, when the surface roughness index ratio exceeds 23, the density of the operating potential reaches saturation, but the tip crack occurs during processing, so the range is preferably 13 to 23.

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, the hot rolled steel sheet according to the present invention is preferably finish-rolled steel made of the composition component at 860 ~ 950 ℃.

When the finish rolling temperature is less than 860 ° C., as the hot rolling is finished in the low temperature region, the grains are rapidly mixed, and the rolling property and the workability are deteriorated. 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.

In addition, it is preferable that the hot rolled steel sheet is cooled at a cooling rate of 60 to 120 ° C. per second in a run-out-table (ROT).

If the cooling rate in the ROT is less than 60 ° C / s, relatively coarse grains are formed by dynamic grain growth, which causes a decrease in strength and workability. On the other hand, if the cooling rate is 120 ° C / s or more, the cooling in the width direction becomes nonuniform, resulting in material variation.

Finally, the hot rolled steel sheet is preferably wound at a temperature of 550 ~ 700 ℃ after cooling in the ROT step.

If the coiling temperature is less than 550 ° C. during cooling and holding, a difference in low temperature precipitate generation behavior occurs due to the nonuniformity in the width direction, causing material variation, which adversely affects 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, 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.

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.

The bendability was classified according to the degree of surface breakage after the steel plate processing, and the bendability index expressing the bendability index was divided into five steps, and the first step with a relatively small tilting was 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 (wt%) Elemental
Composition ratio C Mn P S Al Co N Ti Inventive Steel 1 0.049 0.78 0.040 0.008 0.048 0.031 0.0021 0.038 0.024 Invention river 2 0.038 0.95 0.052 0.007 0.051 0.029 0.0018 0.045 0.034 Invention steel 3 0.031 1.21 0.038 0.009 0.055 0.041 0.0030 0.029 0.038 Comparative River 1 0.003 0.18 0.007 0.009 0.034 - 0.0029 0.040 0.000 Comparative River 2 0.071 1.24 0.051 0.002 0.028 0.006 0.0074 - 0.000 Comparative Steel 3 0.035 1.90 0.043 0.003 0.036 0.014 0.0031 0.008 0.003

Inter-element component ratio: Weight ratio of [Ti (%) x Co (%) / C (%)] component

division Used steel grade Reheating
Temperature
(℃) Wrap-up
Temperature
(℃) Cooling
speed
(° C / s) Coiling
Temperature
(℃) Short ball
diameter
(mm) Blasting
speed
(m / sec) Inventory 1
Inventive Steel 1
1250 890 70 600 0.25 50 Inventive Example 2 1250 890 90 650 0.32 60 Inventory 3 1250 890 110 600 0.32 50 Honorable 4 Invention river 2
1250 910 90 600 0.19 50 Inventory 5 1250 910 90 650 0.27 60 Inventory 6 Invention steel 3 1250 890 90 600 0.31 50 Honorable 7 Inventive Steel 1
1250 750 70 600 0.25 50 Inventive Example 8 1250 890 15 600 0.32 50 Proposition 9 Invention river 2 1250 890 70 400 0.31 50 Comparative Example 1 Inventive Steel 1 1250 910 90 650 0.91 90 Comparative Example 2 Comparative River 1 1250 910 70 650 0.16 50 Comparative Example 3 Comparative River 2 1250 910 70 650 0.25 60 Comparative Example 4 Comparative Steel 3 1250 890 70 650 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 The tensile strength
(kgf / mm ² ) Elongation
(%)
Paintability
Processability
Anti-aging Inventory 1 15.1 Not occurring 51.8 27.6 Good Good Good Inventive Example 2 19.0 Not occurring 53.4 26.1 Good Good Good Inventory 3 21.7 Not occurring 54.9 28.3 Good Good Good Honorable 4 14.5 Not occurring 54.4 26.8 Good Good Good Inventory 5 17.3 Not occurring 56.1 27.4 Good Good Good Inventory 6 18.4 Not occurring 52.9 25.3 Good Good Good Honorable 7 14.1 Not occurring 57.1 20.4 Good usually Good Inventive Example 8 17.4 Not occurring 50.6 24.6 Good usually Good Proposition 9 12.1 Occur 66.3 12.4 Good usually Good Comparative Example 1 28.2 Not occurring 60.8 17.0 Bad Bad Good Comparative Example 2 22.8 Not occurring 31.6 39.2 Bad Bad Good Comparative Example 3 9.1 Occur 43.9 25.7 Good Bad Bad Comparative Example 4 8.1 Occur 49.3 23.6 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 the invention examples, the tensile strength of 50kgf / mm2 or more was secured, no yield point elongation occurred at all, the surface roughness index ratio (Rmax / Ra) is also within 13 ~ 23, satisfies the management range and bending during processing It did not occur, it was able to secure excellent aging resistance.

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 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 Inventive Example 8, the cooling rate in the ROT process is 15 deg. C / s lower than the control range. And the coiling temperature is 400 DEG C lower than the control range. In Example 9, some yield point elongation phenomenon occurred but still exhibited good endurance. Although the processability was slightly lowered due to crystal grain precipitation, precipitation of solid elements and material variation, it satisfied the high quality requirements, It can be used as a high value added steel sheet for household appliances and automobiles.

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, and the endurance property is lowered, and the processing and coating characteristics 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 3, in which the component weight ratio between elements, [(Ti × Co) / C] is 0.00 and does not contain Ti, a carbonitride-forming element, the aging phenomenon occurs as a whole, which makes it difficult to secure workability and also has low strength. There was a problem of deterioration of shape freezeability at the time. On the other hand, Comparative Example 4, in which the weight ratio of the inter-element component, [(Ti × Co) / C] is 0.003, can secure the strength, but the target aging resistance and workability can be achieved even if the manufacturing process and shot blasting conditions according to the present invention are applied. 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 13 to 23, the fluting index and the crack generation sensitivity show 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)

By weight%, carbon (C) 0.01-0.10%, manganese (Mn) 0.5-1.5%, phosphorus (P) 0.01-0.06%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01- 0.07%, nitrogen (N) 0.001-0.004%, titanium (Ti) 0.02-0.06%, cobalt (Co) 0.01-0.05%, the rest contains iron (Fe) and other unavoidable impurities,
On the surface of the hot-rolled steel sheet made of steel satisfying 0.015≤ [(Ti x Co) / C] weight ratio ≤0.06, a shot ball having a diameter of 0.1 to 0.4 mm was shot blasted at a speed of 40 to 65 m / sec. A method for producing a hot rolled steel sheet having excellent workability and surface properties, characterized by forming increased strained ferrite grains.
The method according to claim 1,
The modified ferrite grain (Ferrite grain) is characterized in that occupies 3 to 10% in the thickness direction of the hot rolled steel sheet, a method of producing a hot rolled steel sheet excellent in workability and surface properties.
The method according to claim 1,
The steel having the above composition is finishing rolled at 860 to 950 캜,
The finish rolled steel is cooled at a rate of 60 ~ 120 ℃ / s,
The cooled steel is wound at 550 ~ 700 ℃, characterized in that the hot rolled steel sheet manufacturing method and excellent surface properties.
The method according to claim 1,
The surface roughness index ratio (R _max / R _a ) by 13 ~ 23 by the shot blasting, characterized in that the hot rolled steel sheet manufacturing method excellent in workability and surface properties.
By weight%, carbon (C) 0.01-0.10%, manganese (Mn) 0.5-1.5%, phosphorus (P) 0.01-0.06%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01- 0.07%, nitrogen (N) 0.001-0.004%, titanium (Ti) 0.02-0.06%, cobalt (Co) 0.01-0.05%, the rest contains iron (Fe) and other unavoidable impurities,
It is made of steel satisfying 0.015≤ [(Ti x Co) / C] weight ratio ≤0.06, and the processability and surface are characterized by the formation of modified ferrite grains having an increased working potential density by shot blasting treatment. Hot rolled steel with excellent characteristics.
The method according to claim 5,
Hot-rolled steel sheet having excellent workability and surface properties, wherein the modified ferrite grain of the steel occupies 3 to 10% in the thickness direction of the hot-rolled steel sheet.
The method according to claim 5,
Surface roughness index ratio (R _max / R _a ) of the hot rolled steel sheet is a hot rolled steel sheet excellent in workability and surface characteristics, characterized in that 13 ~ 23.

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