KR101330286B1 - High strength hot-rolled sheet steel having execellent surface properties and manufacturing method thereof - Google Patents

Abstract

High-strength hot-rolled steel sheet excellent in surface characteristics used for home appliances, automobiles, and the like and a manufacturing method thereof are introduced.
The method for producing a high strength hot rolled steel sheet having excellent surface properties according to the present invention, in weight%, carbon (C): 0.01 to 0.10%, manganese (Mn): 0.5 to 1.5%, phosphorus (P): 0.01 to 0.04%, sulfur ( S): 0.02% or less (excluding 0%), Aluminum (Al): 0.01 ~ 0.07%, Nitrogen (N): 0.001 ~ 0.004%, Boron (B): 0.0005 ~ 0.0020%, Cobalt (Co) 0.01 ~ 0.05% , 0.1-0.4mm diameter shotballs are sprayed at a speed of 40-65 m / s on the surface of hot-rolled steel sheet made of steel containing 0.01 to 0.05% of tungsten (W), the remainder being iron (Fe) and other unavoidable impurities It characterized in that the shot blasting.

Description

High strength hot rolled steel sheet with excellent surface properties and manufacturing method {HIGH STRENGTH HOT-ROLLED SHEET STEEL HAVING EXECELLENT SURFACE PROPERTIES AND MANUFACTURING METHOD THEREOF}

The present invention relates to a high-strength hot rolled steel sheet having excellent surface properties and a method of manufacturing the same, and more particularly, to a high-strength hot rolled steel sheet having excellent surface properties of a medium-low carbon base having excellent bending resistance, workability and corrosion resistance by optimizing steel components and manufacturing processes. And to a method for producing the same.

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.

Korean Laid-Open Patent Publication No. 2001-60648 discloses a method for producing a hot rolled steel sheet having excellent aging resistance and uniform stretching characteristics.

In weight%, carbon (C) 0.02 to 0.05%, manganese (Mn) 0.1 to 0.3%, boron (B) 10 to 30 ppm, phosphorus (P) 0.02% or less, sulfur (S) 0.015% or less, aluminum ( Al) 0.01 ~ 0.04%, nitrogen (N) 40ppm or less, the rest is reheated steel slab composed of iron (Fe) and other unavoidable impurities, and then hot rolled to finish rolling temperature above Ar ₃ transformation point to 1.4 ~ 2.3mm thickness The present invention relates to a method capable of improving the aging resistance and the uniform stretching characteristics by manufacturing at a temperature of 600 to 700 ° C.

However, by adding a very small amount of boron (B) as in this method and controlling the coiling temperature, it is not possible to completely prevent the aging phenomenon caused by solid-solution elements such as carbon and nitrogen contained in the steel. There was a difficult problem to expect.

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

The present invention has been proposed to solve such a conventional problem, by adding cobalt (Co) and tungsten (W) of the steel components in the medium-low carbon steel, and optimize the cooling conditions and shot blasting conditions, etc. It is an object of the present invention to provide a high-strength hot rolled steel sheet excellent in surface characteristics that can secure processing and suppress processing defects such as fluting, and a method of manufacturing the same.

High-strength hot-rolled steel sheet manufacturing method having excellent surface properties according to the present invention for achieving this object, in weight%, carbon (C): 0.01 ~ 0.10%, manganese (Mn): 0.5 ~ 1.5%, phosphorus (P): 0.01 to 0.04%, sulfur (S): 0.02% or less (excluding 0%), aluminum (Al): 0.01 to 0.07%, nitrogen (N): 0.001 to 0.004%, boron (B): 0.0005 to 0.0020%, cobalt (Co) 0.01 to 0.05%, tungsten (W) 0.01 to 0.05%, the remainder on the surface of hot-rolled steel sheet made of steel containing iron (Fe) and other unavoidable impurities, 0.1-0.4 mm shot ball 40-65 It is characterized in that the shot blasting by spraying at a speed of m / s.

The steel component is characterized by satisfying 4 ≦ [{B (ppm) × Al (%) × Co (%)} / N (%)] ≦ 11.

The steel made of the composition is finish rolled at 860 ~ 950 ℃, the finish rolled steel is cooled at a rate of 60 ~ 120 ℃ / s, the cooled steel is characterized in that it is wound at 500 ~ 680 ℃.

And the surface roughness index ratio (R _max / R _a ) is set to 10 to 20 by the shot blasting.

High-strength hot-rolled steel sheet having excellent surface properties according to the present invention for achieving this purpose, in weight%, carbon (C): 0.01 ~ 0.10%, manganese (Mn): 0.5 ~ 1.5%, phosphorus (P): 0.01 ~ 0.04%, sulfur (S): 0.02% or less (excluding 0%), aluminum (Al): 0.01-0.07%, nitrogen (N): 0.001-0.004%, boron (B): 0.0005-0.0020%, cobalt (Co ) 0.01 ~ 0.05%, tungsten (W) 0.01 ~ 0.05%, the remainder is made of steel containing iron (Fe) and other unavoidable impurities, the surface roughness index ratio (R _max / R _a ) by shot blasting treatment Characterized in that formed from 10 to 20.

The steel component is characterized by satisfying 4 ≦ [{B (ppm) × Al (%) × Co (%)} / N (%)] ≦ 11.

Deformed ferrite structure generated by the shot blasting process is characterized in that occupying 3 to 10% in the thickness direction of the hot-rolled steel sheet.

The present invention can manufacture high value-added steel sheet used in home appliances, containers, automobiles, etc. by improving the processability, corrosion resistance, and paintability while at the same time ensuring the bending resistance through proper component control and manufacturing process optimization There is an advantage to that.

In addition, the descalability is also excellent to improve the efficiency of pickling operations, to prevent environmental pollution, as well as to shorten the 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.

Hereinafter, with reference to the accompanying drawings will be described a high-strength hot rolled steel sheet having excellent surface properties and a method of manufacturing the same according to a preferred embodiment of the present invention.

The hot rolled steel sheet for processing having excellent bending resistance and surface properties of the present invention satisfies surface characteristics such as bending resistance and workability as well as paintability at the same time and studies and experiments on high value-added hot rolled steel sheet that can be used for home appliances and automobiles. The steel component of the present invention is limited as follows.

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 operation time is required for decarburization during steelmaking, and it is difficult to secure stable materials such as 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 that serves to improve the strength and corrosion resistance of steel, and it is preferable to add a large amount in order to secure these characteristics. The content is limited to 0.04% 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%.

Boron (B) is an element that combines with a solid solution element in steel to improve the aging properties, as well as to increase the strength of the material by adding a small amount as a hardenability enhancing element. Addition of at least 0.0005% is necessary to ensure the desired material properties. However, if the content exceeds 0.0020%, not only causes deterioration of material and grain boundary cracking during play, but also a problem of roughening the surface of the hot-rolled steel sheet is generated, the content is limited to 0.0005 ~ 0.0020%.

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 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%.

Tungsten (W) is an element which is added to secure the incombustibility and strength characteristics, and it is necessary to add at least 0.01% in order to obtain a target strength level in a low temperature range. On the other hand, if it exceeds 0.05%, there is a problem that it is difficult to secure the rollability, 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 4 ≦ [{B (ppm) × Al (%) × Co (%)} / N (%)] ≦ 11.

When the above component ratio was lower than 4, it was difficult to secure the bending resistance and processability by the expression of a solid solution element. On the other hand, when the ratio of the above component was higher than 11, it was possible to secure the workability but it was possible to reduce the surface defects and the productivity decrease due to the excessive addition element. Since it acts as a factor, it is preferable to manage an element-to-element ratio of 4≤ [{B (ppm) x Al (%) x Co (%)} / N (%)] ≤11.

Hereinafter, a method of manufacturing a high strength hot rolled steel sheet having excellent surface properties according to the present invention using the component controlled steel as described above will be described in detail.

By weight%, carbon (C) 0.01-0.10%, manganese (Mn) 0.5-1.5%, phosphorus (P) 0.01-0.04%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01- 0.07%, nitrogen (N) 0.001 ~ 0.004%, boron (B) 0.0005 ~ 0.0020%, cobalt (Co) 0.01 ~ 0.05%, tungsten (W) 0.01 ~ 0.05%, balance iron (Fe) and other unavoidable impurities In a steel, a hot rolled steel sheet is manufactured according to a conventional hot rolling process, and a shot ball having a diameter of 0.1 to 0.4 mm is sprayed at a speed of 40 to 65 m / sec on the surface of the hot rolled steel sheet.

This shot blasting process is one of the most characteristic technical features of the present invention for ensuring the bending resistance (aging resistance), by introducing a suitable compressive stress on the surface of the hot-rolled steel sheet, the dislocation density, among them the operation potential density is greatly increased By creating new ferrite grains, the resistance to dislocation sticking due to the solid solution element, which is the main cause of the aging phenomenon, is reduced, thereby improving the aging resistance.

It is preferable to control the diameter of the shot ball used for shot blasting to 0.1 ~ 0.4mm.

If the diameter of the shotball is less than 0.10mm, the mechanical peeling effect of the surface layer is small, so that it is difficult to obtain a proper dislocation density increase effect. If the diameter of the shotball exceeds 0.40mm, the maximum roughness of the surface rises rapidly, causing cracks during machining. Because it works.

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

When the spraying speed is less than 40m / sec, the impact pressure of the shotball acting on the surface layer is low, so that it is difficult to secure the desired aging resistance. This is because it causes a process.

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 working potential tends to increase as the settling potential in the steel sheet surface layer leaves the Cottrell atmosphere with carbon and nitrogen dissolved in the steel.

When the surface roughness index ratio is less than 10, 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 to fix solid elements is insufficient, thereby making it difficult to secure aging resistance. On the other hand, when the surface roughness index ratio exceeds 20, the density of the movable potential reaches saturation, but since the tip cracking occurs during processing, it is preferable to set the management range to 10-20.

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.

Deformed ferrite structure produced by shot blasting is preferably controlled to occupy 3 to 10% in the thickness direction of the hot-rolled steel sheet in order to ensure appropriate age resistance.

If the deformed ferrite structure is less than 3% in the thickness direction, the target aging resistance cannot be secured due to the lack of organizational properties that can sufficiently fix the solid solution in the steel. If it exceeds, it acts as a hardening factor of the material in the processing and post-treatment process, thereby acting as a factor degrading workability.

As described above, by applying a shot blasting process of a predetermined condition to the surface of the steel sheet produced through a conventional hot rolling process, it is possible to produce a hot rolled steel sheet excellent in aging resistance and processing characteristics. Furthermore, by optimizing the hot rolling process, a hot rolled steel sheet having better aging resistance, processability, and paintability may be manufactured. Hereinafter, the optimized hot rolling process will be described in detail.

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

If the finish rolling temperature is less than 860 ° C, as the hot rolling is finished in the low temperature region, the crystal grains rapidly progress, resulting in deterioration of the rollability and workability.

In addition, when the finish rolling temperature is higher than 950 ℃, uniform hot rolling is not performed throughout the thickness, the grain refinement is insufficient, the impact toughness due to the grain coarsening appears, so the finish rolling temperature range is 860 ~ It is preferable to manage at 950 degreeC.

Finished rolled steel is preferably cooled in a run-out-table (ROT) at a cooling rate of 60-120 ° C. per second.

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, when the cooling rate is 120 ° C / s or more, it acts as a factor of material variation caused by the widthwise cooling unevenness, so the range of the cooling rate is preferably managed at 60 to 120 ° C / s.

Finally, the hot rolled steel sheet is preferably wound at a temperature of 500 ~ 680 ℃ after cooling in the ROT step. If the coiling temperature is lower than 500 ° C., the formation behavior of low-temperature precipitates is different due to the nonuniformity of the width direction during cooling and holding, thereby causing material variation, which adversely affects the workability. On the other hand, when the coiling temperature exceeds 680 ° C, as the structure of the final product becomes coarse, problems of workability and corrosion resistance decrease, so the range of the coiling temperature is preferably managed at 500 to 680 ° C.

In order to find out the technical effect of the method for producing a high strength hot rolled steel sheet having excellent surface properties according to the present invention described above was performed as follows.

First, respective steels (three kinds of inventive steel and six 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.

In addition, the physical properties and mechanical properties of the prepared steels were measured and shown in [Table 3].

Steel grade Chemical composition (wt%) Elemental
Composition ratio C Mn P S Al N B Co W Inventive Steel 1 0.039 0.79 0.027 0.008 0.036 0.0018 0.0011 0.028 0.026 6.16 Invention river 2 0.058 0.61 0.033 0.008 0.051 0.0026 0.0008 0.034 0.035 5.34 Invention steel 3 0.079 1.24 0.021 0.006 0.049 0.0032 0.0015 0.041 0.043 9.42 Comparative River 1 0.005 0.21 0.014 0.008 0.004 0.0049 0.0013 - - 0.00 Comparative River 2 0.003 0.66 0.006 0.009 0.041 0.0028 - - - 0.00 Comparative Steel 3 0.062 1.41 0.011 0.002 0.038 0.0071 - 0.039 0.074 0.00 Comparative Steel 4 0.127 0.21 0.007 0.004 0.105 0.0036 0.0014 0.031 - 12.66 Comparative Steel 5 0.048 0.88 0.071 0.025 0.036 0.0021 0.0029 0.005 0.039 2.49 Comparative Steel 6 0.036 1.89 0.040 0.003 0.031 0.0059 - - - 0.00

※) Component ratio between elements: expressed as the weight ratio of each element as [{B (ppm) × Al (%) × Co (%)} / N (%)]

division
Used steel grade Reheating
Temperature
(℃) Wrap-up
Temperature
(℃)
Cooling rate
(° 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 80 640 0.32 60 Inventory 3 1250 890 100 600 0.32 50 Honorable 4 Invention river 2
1250 910 100 600 0.19 50 Inventory 5 1250 910 100 640 0.27 60 Inventory 6 Invention steel 3 1250 890 90 640 0.31 50 Honorable 7 Inventive Steel 1
1250 750 50 600 0.25 50 Inventive Example 8 1250 890 15 600 0.32 50 Proposition 9 Invention river 2 1250 890 40 640 0.31 50 Comparative Example 1 Inventive Steel 1 1250 910 90 650 0.91 90 Comparative Example 2 Comparative River 1 1250 910 70 640 0.16 50 Comparative Example 3 Comparative River 2 1250 910 70 640 0.25 60 Comparative Example 4 Comparative Steel 3 1250 890 70 640 0.25 60 Comparative Example 5 Comparative Steel 4 1250 890 70 640 0.16 50 Comparative Example 6 Comparative Steel 5 1250 890 70 640 0.25 60 Comparative Example 7 Comparative 6 1250 890 70 640 0.25 60

In Table 3, the paintability was excellent in both the coating adhesion and the surface properties among the evaluation items of the coating material, and the case in which only one of the two properties was excellent was shown as the case where the two properties were not good. The tensile strength level was good when securing 490 MPa or more, and less than that was indicated as bad. Workability was measured by crack length for the bending test specimen (divided into 5 steps, as shown in Table 4 below), 1 step is good, 2 to 3 steps are normal, 4 to 5 steps are marked as bad.

The bending resistance was classified according to the degree of surface bending after steel sheet processing, and the bending index representing this was divided into five stages, and the first stage, which is relatively insignificant, was good. And, it was determined that the 4 to 5 stages that the bending occurred to the extent that the naked eye can be observed as bad.

According to the above criteria, the experimental results disclosed in Table 3 above are summarized as follows.

division Surface roughness
Exponent ratio Yield point extension
Presence or absence The tensile strength
level Paintability Processability Bending property Inventory 1 13.5 Not occurring Good Good Good Good Inventive Example 2 16.8 Not occurring Good Good Good Good Inventory 3 19.4 Not occurring Good Good Good Good Honorable 4 12.7 Not occurring Good Good Good Good Inventory 5 16.3 Not occurring Good Good Good Good Inventory 6 14.8 Not occurring Good Good Good Good Honorable 7 8.5 Not occurring Good Good usually Good Inventive Example 8 24.2 Not occurring Good Good usually Good Proposition 9 4.9 Occur Good usually usually Good Comparative Example 1 31.1 Not occurring Good usually Bad usually Comparative Example 2 12.5 Occur Bad Good Bad Bad Comparative Example 3 15.9 Not occurring Bad Good Good Good Comparative Example 4 6.9 Occur Good Bad Bad Bad Comparative Example 5 3.1 Occur Bad Bad Bad Bad Comparative Example 6 8.6 Occur Bad Bad Bad Bad Comparative Example 7 13.5 Occur Good Good Bad Bad

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

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 target strength level was secured with a tensile strength of 490 MPa or more, no yield point elongation occurred, and the surface roughness index ratio (Rmax / Ra) was also within the range of 10 to 20, satisfies the management range during processing. No bending occurred in the wafer, thus ensuring excellent aging resistance.

In addition, since no cracking occurs during bending, it exhibits high workability and paintability, thereby making it 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.

In more detail, Example 7 performs the finish rolling temperature at 750 ° C. lower than the control range, and Example 8 performs the cooling rate in the ROT process at 15 ° C./s lower than the control range. It is a case where winding temperature is performed at 400 degreeC lower than a control range.

Inventive Examples 7 to 9 exhibited some yield point stretching but still showed good aging resistance. Although the workability slightly decreased due to grain hybridization, solid solution precipitation, material variation, etc., it satisfies the high quality requirements and is still good. Because of its paintability, it could be used as a high value-added steel sheet for home appliances and automobiles.

Comparative Example 1 is a case where the steel component control (invention steel 1) and the hot rolling process conditions according to the present invention are satisfied, but the shot blasting conditions are not satisfied.

More specifically, in the case of shot blasting conditions, the shot ball is used to be 0.91 mm larger than the management range, and the shot blasting speed is also performed at 90 m / sec larger than the management range.

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 7 are cases in which hot steel process conditions and shot blasting conditions according to the present invention are used, but steel grades (comparative steels 1 to 6) that do not satisfy the steel component conditions 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, Comparative Examples 2 and 3, which have a low C content and a component ratio of [{B (ppm) × Al (%) × Co (%)} / N (%)] 0.0, are essentially strength properties regardless of the shot blasting conditions. Could not be secured. In addition, Comparative Examples 4 to 7 in which the addition amount of B, W, Co, etc., out of the steel components fall outside the scope of the present invention method, as shown in Table 3, most of them show the yield point stretching phenomenon and the bending resistance occurs. And processability 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. In contrast, both the fluting index and the machinability index exceed the threshold in the region below the above control range, causing bending and processing cracks. In the region above the above management range, the aging resistance is still higher than the saturation value. The problem that a process crack generate | occur | produced was confirmed.

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 to 0.10%, manganese (Mn): 0.5 to 1.5%, phosphorus (P): 0.01 to 0.04%, sulfur (S): 0.02% or less (excluding 0%), aluminum ( Al): 0.01 ~ 0.07%, Nitrogen (N): 0.001 ~ 0.004%, Boron (B): 0.0005 ~ 0.0020%, Cobalt (Co) 0.01 ~ 0.05%, Tungsten (W) 0.01 ~ 0.05%, the rest is iron ( Fe) and other unavoidable impurities,
4≤ [{B (ppm) × Al (%) × Co (%)} / N (%)] On the surface of hot-rolled steel sheet made of steel satisfying ≤11, 0.1-0.4mm shotballs 40-65 The method of manufacturing a high strength hot rolled steel sheet having excellent surface properties, characterized in that the shot blasting by spraying at a speed of m / s, to form a modified ferrite grain with increased operating potential density.
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, high strength hot rolled steel sheet manufacturing method having excellent 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 500 ~ 680 ℃, characterized in that the high-strength hot-rolled steel sheet excellent in surface properties.
The method according to claim 1,
The surface roughness index ratio (R _max / R _a ) by 10 to 20 by the shot blasting, characterized in that the high-strength hot-rolled steel sheet manufacturing method having excellent surface properties.
By weight%, carbon (C): 0.01 to 0.10%, manganese (Mn): 0.5 to 1.5%, phosphorus (P): 0.01 to 0.04%, sulfur (S): 0.02% or less (excluding 0%), aluminum ( Al): 0.01 ~ 0.07%, Nitrogen (N): 0.001 ~ 0.004%, Boron (B): 0.0005 ~ 0.0020%, Cobalt (Co) 0.01 ~ 0.05%, Tungsten (W) 0.01 ~ 0.05%, the rest is iron ( Fe) and other unavoidable impurities,
4 ≤ [{B (ppm) × Al (%) × Co (%)} / N (%)]] strain ferrite grains made of steel satisfying ≤ 11 and the operation potential density is increased by shot blasting treatment ( High strength hot rolled steel sheet having excellent surface properties, characterized in that ferrite grains) are formed.
The method according to claim 5,
The modified ferrite grain (Ferrite grain) is characterized in that occupies 3 to 10% in the thickness direction of the hot rolled steel sheet, high strength hot rolled steel sheet having excellent surface properties.
The method according to claim 5,
The surface roughness index ratio (R _max / R _a ) of the hot rolled steel sheet, characterized in that 10 to 20, high strength hot rolled steel sheet having excellent surface properties.

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