KR101760518B1 - Method for manufacturing hot-rolled steel having excellent anti-aging property and the hot-rolled steel by the same method - Google Patents

Abstract

The present invention relates to a method of manufacturing a high strength hot-rolled steel sheet for use in electric appliances, automobiles, etc., and is characterized by controlling the ratio of the amount of cobalt (Co), tungsten (W) and niobium (Nb) And a method of manufacturing a high strength hot-rolled steel sheet excellent in bending resistance by introducing a shot blasting method in a descaling process of a hot-rolled steel sheet.
In order to achieve the above object, according to the present invention, there is provided a method of manufacturing a high strength hot-rolled steel sheet excellent in creep resistance, which comprises 0.01 to 0.10% of carbon (C), 0.5 to 1.5% of manganese (Mn) 0.01 to 0.04% of sulfur (S), 0.020% or less of sulfur (excluding 0%), 0.01 to 0.07% of aluminum (Al), 0.001 to 0.004% of nitrogen (N), 0.0005 to 0.0020% of boron (B) Of the deformed ferrite structure in the thickness direction of the hot-rolled steel sheet made of steel containing 0.05 to 0.05% of tungsten (W), 0.01 to 0.05% of niobium (Nb), 0.03 to 0.06% of niobium (Nb) Is 3 to 10%.

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a high-strength hot-rolled steel sheet excellent in bending resistance, and a hot-rolled steel sheet produced by the method and a method for manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a method for manufacturing a hot-rolled steel sheet excellent in bending resistance and a hot-rolled steel sheet produced by the method. More particularly, the present invention relates to a method for manufacturing a low- To a method of manufacturing a high-strength hot-rolled steel sheet.

Conventionally, materials such as ultra-low carbon steels in which a carbonitride-forming element excellent in endurance has been added have been used in order to suppress machining defects caused by aging of household appliances and automobiles. Properties required for such products are workability such as creep resistance. Also, in the case of automobiles and the like, the coating of organic substances such as paint to obtain the desired surface color and prevention of oxidation of the steel sheet due to the ambient conditions is a very important management factor in terms of paintability

Fluting refers to a phenomenon in which a machining portion is bent into a diamond shape at the time of machining. If such fluting occurs, it is difficult to maintain the shape of the formed portion, and therefore, 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 Publication 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% by weight of carbon (C) (P): 0.020% or less, sulfur (S): 0.015% or less, aluminum (Al): 0.01-0.04%, nitrogen (N): 40 ppm or less , The remainder iron (Fe) and other unavoidable impurities is reheated and then hot rolled at a finishing rolling temperature equal to or higher than the Ar ₃ transformation point to a thickness of 1.4 to 2.3 mm and rolled at a temperature of 600 to 700 ° C, And a method capable of improving the uniform stretching property. 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 merely adding a very small amount of boron and controlling the coiling temperature as in this method, it is difficult to expect high endurance .

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 individually according to the thickness of the plate to induce the surface movable potential, it is difficult to apply to a case of producing materials of various sizes such as a commercial operation line.

SUMMARY OF THE INVENTION The present invention has been proposed in order to solve such conventional problems, and it is an object of the present invention to provide a method of optimizing cooling conditions and shot blasting conditions by adding cobalt (Co), tungsten (W) and niobium (Nb) A method of manufacturing a high-strength hot-rolled steel sheet excellent in rolling resistance, workability, corrosion resistance, and surface characteristics, which secures paintability of a steel sheet and suppresses processing defects such as fluting, and a hot- have.

In order to achieve the above object, according to the present invention, there is provided a method of manufacturing a high strength hot-rolled steel sheet excellent in creep resistance, which comprises 0.01 to 0.10% of carbon (C), 0.5 to 1.5% of manganese (Mn) 0.01 to 0.04% of sulfur (S), 0.020% or less of sulfur (excluding 0%), 0.01 to 0.07% of aluminum (Al), 0.001 to 0.004% of nitrogen (N), 0.0005 to 0.0020% of boron (B) Of the deformed ferrite structure in the thickness direction of the hot-rolled steel sheet made of steel containing 0.05 to 0.05% of tungsten (W), 0.01 to 0.05% of niobium (Nb), 0.03 to 0.06% of niobium (Nb) Shot blasting is performed on the surface of the hot-rolled steel sheet so that the short ball diameter is 0.1 to 0.4 mm and the blasting speed is 40 to 65 m / sec.

Further, the steel component is constituted to satisfy 3? [B (ppm) x Al (%) x Co (%)} / N (%)? 12.

The hot-rolled steel sheet is obtained by finishing the steel having the composition described above at 860 to 950 캜, cooling the finely-rolled steel at a cooling rate of 60 to 120 캜, cooling the steel at a temperature of 500 to 680 캜 desirable.

It is preferable that the surface roughness index ratio (Rmax / Ra) is 10 to 20 by the shot blasting.

On the other hand, the high-strength hot-rolled steel sheet excellent in in-creep resistance produced according to the present invention contains 0.01 to 0.10% of carbon (C), 0.5 to 1.5% of manganese (Mn), 0.01 to 0.06% of phosphorus (P) (S) of 0.020% or less (excluding 0%), aluminum (Al) of 0.01 to 0.07%, nitrogen (N) of 0.001 to 0.004%, niobium (Nb) of 0.03 to 0.06%, cobalt (Co) (Rmax / Ra) of 10 to 20, which is made of steel containing 0.0005 to 0.0020% of tungsten (B), 0.01 to 0.05% of tungsten (W), residual iron (Fe) and other unavoidable impurities, And the phase fraction in the thickness direction of the deformed ferrite structure produced by the treatment is 3 to 10%.

It is also preferable that the steel component satisfies 3? [B (ppm) x Al (%) x Co (%)} / N (%)? 12.

delete

According to the method for manufacturing a high-strength hot-rolled steel sheet of the present invention having the above-described structure, it is possible to secure the creep resistance through appropriate component control and optimization of the manufacturing process, and to improve workability, corrosion resistance and paintability, Which makes it possible to manufacture high value-added steel sheets.

In addition, it is also excellent in descaling property, so that the efficiency of the pickling work can be enhanced, and the prevention of environmental pollution and the shortening of the process can be simultaneously obtained.

1 is a photograph of a hot-rolled steel sheet according to the present invention.
FIG. 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, the technical composition of the present invention will be described in more detail.

As described above, the high-strength hot-rolled steel sheet of the present invention has been studied and experimented on high-value-added hot-rolled steel sheets that can be used for home appliances and automobiles simultaneously satisfying surface properties such as creep resistance, workability, As a completed invention, it is preferable to control the steel component according to the present invention as follows.

Carbon (C) is an element added to improve the strength of a steel sheet. As the content increases, tensile and yield strength increase. However, when the amount is excessively added, the workability of the material decreases and the bending resistance decreases. On the other hand, when the carbon content is less than 0.01%, additional working time is required for decarburization at the time of steel making, and it is difficult to obtain a stable material such as a desired strength. Therefore, it is preferable to control the carbon content to 0.01 to 0.10%.

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 is improved, but it is difficult to secure the strength. Therefore, in order to secure the target strength, 0.5% or more should be added. On the other hand, when manganese is added excessively, the addition of a large amount of alloying element causes economical deterioration and center segregation as well as deterioration of 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. However, since it is an element that causes center segregation during casting, Its 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, so it is preferable to control the content to 0.01 to 0.04%.

Sulfur (S) not only forms a nonmetallic inclusion which acts as a corrosion starting point by binding with Mn in the steel, but also acts as a factor of the embrittlement 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.

Aluminum (Al) is an element added for the deoxidation of molten steel. However, since it has an aspect of improving aging characteristics by bonding with solid elements in steel, its lower limit is limited to 0.01%, and when it is added excess, There is a problem that not only causes surface defects but also deteriorates workability, the upper limit is set to 0.07% and the management range is limited to 0.01 to 0.07%.

Nitrogen (N) is an element that is useful for strengthening materials while existing in solid state in steel, but it is a main element causing aging phenomenon. Therefore, 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%.

Boron (B) is an element which not only improves the aging property by combining with the solid element in the steel but also has an effect of increasing the strength of the material by addition of a small amount as an element for improving hardenability. At least 0.0005% addition is necessary to obtain the desired material properties. However, when the content exceeds 0.0020%, the material deteriorates and causes intergranular cracks at the time of performance, and there is a problem of roughening the surface of the hot-rolled steel sheet, and therefore the content thereof is limited to 0.0005 to 0.0020%.

Cobalt (Co) is an element promoting the formation of an intragranular precipitate. In order to obtain such an effect, at least 0.01% or more of addition is required. However, if it exceeds 0.05%, the effect of the increase in the manufacturing cost due to the addition of a large amount of the expensive alloying element rather than the effect of contributing to the precipitation promotion largely acts, so the content is limited to 0.01 to 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 rolling property. Therefore, the content thereof is limited to 0.01 to 0.05%.

Niobium (Nb) is an element that not only exhibits an effect of retarding the recrystallization of ferrite but also an effect of increasing the strength of the steel sheet by precipitation in combination with C, N, or the like in the steel. In order to secure the desired strength, 0.03% or more of addition is required, while when it exceeds 0.06%, the manufacturing cost is increased and the hot rolling workability is deteriorated. Therefore, the content is limited to 0.03 to 0.06%.

On the other hand, in order to secure proper processing characteristics, it was also necessary to appropriately manage the fraction of the precipitate and the employment phase. In order to secure the characteristics of the present invention, it is desirable to maintain the management ranges between the alloy elements mutually between 3 ≤ [B (ppm) × Al (%) × Co (%)} / N (% On the other hand, when the ratio is larger than 12, it is possible to secure the workability. However, when the ratio of the elements is less than 3, it is difficult to secure the bending property and the workability by the expression of the solid element. It is preferable to control the inter-element composition ratio as 3? [B (ppm) x Al (%) x Co (%)} / N (%)? 12.

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.

First, 0.01 to 0.10% of carbon (C), 0.5 to 1.5% of manganese (Mn), 0.01 to 0.04% of phosphorus (P), 0.020% 0.01 to 0.05% of tungsten (W), 0.03 to 0.06% of niobium (Nb), 0.01 to 0.07% of nitrogen, 0.001 to 0.004% of nitrogen, 0.0005 to 0.0020% A hot-rolled steel sheet is produced by a conventional hot-rolling process with steel containing residual iron (Fe) and other unavoidable impurities, and a shot ball having a diameter of 0.1 to 0.4 mm and a blasting rate of 40 to 65 m / sec Shot blasting is performed.

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 a modified ferrite grain, it is possible to improve the durability by decreasing the dislocation phenomenon by the hiring element which is the main cause of the aging phenomenon.

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. 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 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 ensure the mechanical properties according to the present invention, it is preferable to control the surface roughness index ratio (Rmax / Ra) of the hot-rolled steel sheet after shot blasting to be 10 to 20. Here, Rmax denotes the maximum point height in the surface roughness curve of the hot-rolled steel sheet, and Ra denotes the center line average surface roughness. As the surface roughness index ratio increases, the density of the movable potential increases as the fixing potential at the surface layer of the steel sheet deviates from the Cottrell atmosphere with the carbon and nitrogen dissolved in the steel. When the surface roughness index ratio is less than 10, the ratio of the acidity and the bone is not ensured appropriately. As a result, the adsorbability of the organic matter is lowered and the density distribution of the movable potential, which can fix the solid element, is insufficient. On the other hand, when the surface roughness index ratio exceeds 20, the density of the movable potential reaches the saturation state, but the tip crack starts to occur during processing, so that the management range is preferably 10 to 20.

FIG. 1 (a) shows a steady state ferrite grains having a low dislocation density. FIG. 1 (b) shows a transformed structure having a dislocation structure having a high potential density by the short blasting process according to the present invention. Show ferrite lips. It is preferable that the deformed ferrite structure produced by shot blasting is controlled so that the phase fraction in the thickness direction of the hot-rolled steel sheet occupies 3 to 10% in order to secure adequate 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, it is possible to manufacture hot rolled steel sheets having more excellent endurance, workability, paintability and the like, and the optimized hot rolling process will be described in detail below.

First, in the hot-rolled steel sheet according to the present invention, it is preferable to finish-finish the steel composed of the above-mentioned composition components at 860 to 950 ° C. When the finish rolling temperature is lower than 860 占 폚, the hot rolling is completed in the low temperature region, so that the blending of the crystal grains proceeds rapidly, resulting in a deterioration of 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 is preferably cooled in a run-out-table (ROT) at a cooling rate of 60 to 120 ° C per second. If the cooling rate in the ROT is less than 60 DEG C / second, relatively coarse crystal grains are formed due to the dynamic crystal grain growth, which causes deterioration of strength and workability. On the other hand, when the cooling rate is 120 deg. C / second or more, it causes the material deviation due to the nonuniform cooling in the width direction. Therefore, the range of the cooling rate is preferably controlled to 60 to 120 deg.

Finally, the hot-rolled steel sheet is preferably rolled at a temperature of 500 to 680 DEG C after cooling in the ROT step. When the coiling temperature is less than 500 캜, 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 variation, which adversely affects the workability. On the other hand, if the coiling temperature exceeds 680 DEG C, there is a problem that the workability and corrosion resistance are lowered as the texture of the final product is coarsened. Therefore, the coiling temperature is preferably controlled at 500 to 680 DEG C.

In order to investigate the technical effect of the method of manufacturing a high strength hot-rolled steel sheet excellent in endurance according to the present invention, the following experiment was conducted.

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. The physical properties and mechanical properties of each steel grade were measured and shown in Table 3.

Steel grade
Chemical composition (% by weight) Elemental
Composition ratio C Mn P S Al N B Co W Nb Inventive Steel 1 0.032 0.87 0.031 0.008 0.027 0.0026 0.0009 0.038 0.021 0.051 3.55 Invention river 2 0.061 0.59 0.027 0.008 0.049 0.0028 0.0015 0.021 0.038 0.045 5.51 Invention steel 3 0.089 1.34 0.018 0.006 0.058 0.0035 0.0014 0.046 0.044 0.037 10.67 Comparative River 1 0.007 0.19 0.015 0.008 0.006 0.0042 0.0014 - - - 0.00 Comparative River 2 0.003 0.58 0.008 0.009 0.035 0.0032 - - - 0.065 0.00 Comparative Steel 3 0.071 1.31 0.010 0.002 0.031 0.0056 - 0.042 0.067 0.005 0.00 Comparative Steel 4 0.134 0.29 0.009 0.004 0.112 0.0049 0.0016 0.036 - - 13.17 Comparative Steel 5 0.051 0.82 0.069 0.025 0.032 0.0018 0.0021 0.005 0.041 - 1.87 Comparative Steel 6 0.031 1.89 0.043 0.003 0.034 0.006 - - - 0.027 0.00

* Elemental composition ratio between elements: 3? [{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 / sec) Inventory 1
Inventive Steel 1
1250 890 70 600 0.25 50 Inventory 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 Honors 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 Comparison 6 1250 890 70 640 0.25 60

division Surface roughness
Exponent ratio Yield point extension
Presence or absence The tensile strength
level Paintability Processability Bending property Inventory 1 14.6 Not occurring Good Good Good Good Inventory 2 18.1 Not occurring Good Good Good Good Inventory 3 17.8 Not occurring Good Good Good Good Honorable 4 13.7 Not occurring Good Good Good Good Inventory 5 14.2 Not occurring Good Good Good Good Inventory 6 15.9 Not occurring Good Good Good Good Honorable 7 7.9 Not occurring Good Good usually Good Honors 8 25.4 Not occurring Good Good usually Good Proposition 9 5.6 Occur Good usually usually Good Comparative Example 1 29.4 Not occurring Good usually Bad usually Comparative Example 2 12.8 Occur Bad Good Bad Bad Comparative Example 3 16.7 Not occurring Bad Good Good Good Comparative Example 4 8.1 Occur Good Bad Bad Bad Comparative Example 5 4.6 Occur Bad Bad Bad Bad Comparative Example 6 9.2 Occur Bad Bad Bad Bad Comparative Example 7 14.8 Occur Good Good Bad Bad

In Table 3, paintability was evaluated as good when both coating adhesion and surface characteristics were excellent among evaluation items of paints, and when both of the two properties were unsatisfactory, it was indicated as defective.

The cracking length of the bending test specimens was measured and classified into five steps as shown in Table 4 below. The first step was evaluated as good, the second to third steps were evaluated as normal, and the fourth to fifth steps were evaluated as defective.

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 bending resistance (endurance test) is classified according to the degree of surface bending after the steel plate processing. The bending index that expresses the bending strength is divided into five steps, and the first step with a relatively small bending phenomenon is good. The third stage was judged to be normal, and the fourth to fifth stage where the tilting phenomenon occurred was judged to be defective.

The experimental results shown in Table 3 according to the above criteria are summarized as follows.

Inventive Examples 1 to 6 are cases in which both the steel component control, hot rolling process and shot blasting process conditions according to the present invention are satisfied. In all of the examples, the tensile strength was more than 490 MPa and the target strength level was secured. Also, the elongation at yield point did not occur at all, and the surface roughness index ratio (Rmax / Ra) So that excellent endurance can be secured. 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. Describing in more detail, in the case of Inventive Example 7, the finishing rolling temperature is set to 750 deg. C, which is lower than the control range. In Inventive Example 8, the cooling rate in the ROT process is 15 deg. And the coiling temperature is 400 DEG C lower than the control range. In Examples 7 to 9, some yield point elongation phenomena occurred but still exhibited good endurance. Although the processability was slightly lowered due to crystal grain precipitation, precipitation of solid elements and material variations, it 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, in the short blast condition, the short ball is 0.91 mm larger than the control range and the shot blasting speed is 90 m / sec, which is larger than the control 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 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.

Further, Comparative Examples 2 and 3, in which the C content is low and the composition ratio of [{B (ppm) x Al (%) x Co (%)} / N (%)) is 0.0, Could not be secured. In Comparative Examples 4 to 7 in which the amount of addition of Nb, B, W, Co, etc. in the steel component is out of the range of the present invention method, as shown in Table 3, the yield point elongation phenomenon appears in most cases, It was not possible to secure endurance and workability.

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 desirable to manage the fluting index to 2 or less and the crack generation sensitivity to 0.5 or less. When the control range of the surface roughness index ratio according to the present invention is adjusted to 10 to 20, the fluting index and the crack generation sensitivity exhibit excellent behavior. On the other hand, in the area lower than the upper management range, both the fluting index and the workability index exceed the reference value, and there is a problem that the bending phenomenon and the working crack occur. In the region above the upper management range, the anti- It is possible to confirm the problem that processing cracks occur.

Claims (10)

(P) of 0.01 to 0.04%, sulfur (S) of 0.020% or less (excluding 0%), aluminum (Al) of 0.01 to 0.10% (B), 0.01 to 0.05% of cobalt (Co), 0.01 to 0.05% of tungsten (W), 0.03 to 0.06% of niobium (Nb), 0.03 to 0.06% of niobium (Fe) and other unavoidable impurities in the thickness direction of the hot-rolled steel sheet is 3 to 10% in the thickness direction of the hot-rolled steel sheet, the diameter of the short ball is 0.1 to 0.4 mm, Shot blast at a speed of 40 to 65 m / sec. delete The method according to claim 1,
Wherein the composition of the steel satisfies 3 ≦ [{B (ppm) × Al (%) × Co (%)} / N (%)] ≦ 12 . Claim 1
The method according to claim 1 or 3,
Wherein the hot-rolled steel sheet is subjected to finish rolling at a temperature of 860 to 950 캜 in which the steel having the composition described above is subjected to finish rolling. The method of claim 4,
Wherein the hot-rolled steel sheet is cooled at a cooling rate of 60 to 120 占 폚 per second of the finish-rolled steel. The method of claim 5,
Wherein the hot-rolled steel sheet is rolled at a temperature of 500 to 680 캜 after the cooling. The method according to claim 1,
Wherein the surface roughness index ratio (Rmax / Ra) is 10 to 20 by the shot blasting. (P) of 0.01 to 0.06%, sulfur (S) of 0.020% or less (excluding 0%), aluminum (Al) of 0.01 to 0.10% (B), 0.01 to 0.05% of tungsten (W), 0.01 to 0.05% of tungsten (W), 0.03 to 0.07% of nitrogen, 0.001 to 0.004% of nitrogen, 0.03 to 0.06% of niobium, 0.01 to 0.05% of cobalt (Rmax / Ra) of 10 to 20, and the phase fraction in the thickness direction of the deformed ferrite structure produced by the shot blasting treatment is in the range of 3 - 10% by weight based on the total weight of the high-strength hot-rolled steel sheet. The method of claim 8,
Wherein the composition of the steel satisfies 3? [B (ppm) x Al (%) x Co (%)} / N (%)? 12. delete

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