KR101354931B1 - Method for manufacturing hot-rolled steel having excellent corrosion resistance property and the hot-rolled steel by the same method - Google Patents

Abstract

The present invention relates to a method for manufacturing a hot rolled steel sheet used in home appliances, automobiles, and the like, and to control the addition amount of boron (B), copper (Cu), chromium (Cr), etc. and the ratio of alloying elements among the steel components of the medium-low carbon base. The main object of the present invention is to provide a method for producing a hot rolled steel sheet having excellent workability and corrosion resistance by introducing a shot blasting method in a descaling process of a hot rolled steel sheet.
Process for producing a hot rolled steel sheet excellent in workability and corrosion resistance according to the present invention for achieving the above object, by weight, carbon (C) 0.01 ~ 0.10%, manganese (Mn) 0.05 ~ 0.3%, phosphorus (P) 0.001 ~ 0.020%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 ~ 0.07%, nitrogen (N) 0.001 ~ 0.004%, boron (B) 0.001 ~ 0.003%, copper (Cu) 0.05 ~ 0.1-0.4 mm diameter shot ball on the surface of hot-rolled steel sheet made of steel containing 0.5%, chromium (Cr) 0.1-0.5%, balance iron (Fe) and other unavoidable impurities. Short blasting in sec.

Description

METHOD FOR MANUFACTURING HOT-ROLLED STEEL HAVING EXCELLENT CORROSION RESISTANCE PROPERTY AND THE HOT-ROLLED STEEL BY THE SAME METHOD}

The present invention relates to a method for producing a hot rolled steel sheet having excellent workability and corrosion resistance, and a hot rolled steel sheet manufactured by the same. It is about a method.

Conventionally, in order to suppress processing defects by aging of home appliances, automobiles, etc., materials such as ultra low carbon steel added with a carbon nitride forming element having excellent aging resistance have been used. Characteristics required for such products include workability such as bending resistance. In addition, in the case of automobiles, organic materials such as paint are coated on the surface to prevent oxidation of the steel sheet due to outdoor conditions and to obtain a desired surface color, so the management of the surface characteristics becomes an important management factor in terms of paintability.

Flucting refers to a phenomenon in which a processed part is bent into a diamond shape during processing, and when such fluting occurs, it is difficult to maintain the shape of the molded part, so it must be strictly limited in the actual process.

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. Therefore, when strict shape freezeability and processability are required, such as home appliances and automobiles, there is a demand for the establishment of a method capable of suppressing the bending phenomenon during processing.

On the other hand, in order to improve the shape freezing properties and improve the manufacturing process to increase productivity, not only the fluting prevention by the above-described aging resistance but also various processing characteristics such as elongation flangeability, bending property, and drawing property are required. . 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 Laid-Open Patent Publication No. 1989-282420 (name of the invention: a manufacturing method of a hot rolled steel sheet for processing and a processing heat treatment method of a hot rolled steel sheet) is a method for producing a steel sheet suitable for a high strength member for automobile or industrial equipment. A method for producing a hot rolled steel sheet excellent in workability and aging is disclosed by adding titanium (Ti), niobium (Nb) and some rare earth elements to a low carbon steel base. 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 (name of the invention: a method for manufacturing a high-temperature hot rolled steel sheet having excellent aging resistance) adds a trace amount of zirconium, a carbonitride-forming element, to ultra-low carbon aluminum-kilted steel, A method of improving the aging resistance by hot rolling in the temperature range directly above the Ar ₃ transformation point to coarsen the ferrite grains is disclosed. However, according to this method, it is necessary to add special elements such as zirconium in order to increase the aging, which not only causes deterioration of steelmaking workability and cost increase, but also deteriorates shape freezeability due to low material strength. .

As another example, Korean Patent Laid-Open Publication No. 2001-60648 (name of the invention: a method of manufacturing a hot rolled steel sheet having excellent resistance to ageing and uniform stretching) has a weight% of carbon (C): 0.02 to 0.05%, manganese (Mn) : 0.10 to 0.30%, boron (B): 10 to 30 ppm, phosphorus (P): 0.020% or less, sulfur (S): 0.015% or less, aluminum (Al): 0.01 to 0.04%, nitrogen (N): 40 ppm or less After reheating the steel slab composed of residual iron (Fe) and other unavoidable impurities, it is hot rolled to a finish rolling temperature of more than Ar ₃ transformation point to a thickness of 1.4 to 2.3 mm and wound at a temperature of 600 to 700 ° C. And a method in which uniform stretching characteristics can be improved. However, it is difficult to expect high aging resistance because it is not possible to completely prevent aging due to solid elements such as carbon and nitrogen contained in steel only by adding a very small amount of boron and controlling the coiling temperature as in this method. There was this.

As another example, Japanese Laid-Open Patent Publication No. 2008-190008 (name of the invention: a method for manufacturing a hot rolled steel sheet having excellent aging resistance) in weight%, carbon (C): 0.04-0.25%, silicon (Si): 0.001- 0.5%, manganese (Mn): 0.05-1.5%, phosphorus (P): 0.09% or less, sulfur (S): 0.015% or less, aluminum (Al): 0.01-0.08%, nitrogen (N): 0.0005-0.015% And slab of a component consisting of residual iron (Fe) and other unavoidable impurities, hot rolled, cooled to less than 400 ° C. at an average cooling rate of 60 ° C./s or more, and then wound by t / R ≧ 0.0055 (t The method of performing skin pass rolling of 0.1-1.0% of elongation rate using the small diameter roll which satisfy | fills R is a roll diameter) is disclosed. 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. In addition, since roll diameters must be controlled in accordance with the thickness of the plate to induce surface operating potential, it is difficult to apply when producing various sizes of materials such as commercial operation lines.

The present invention has been proposed to solve such a conventional problem, and in addition to the addition of boron (B), copper (Cu) and chromium (Cr) among 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 method for producing a hot rolled steel sheet having excellent bending resistance and corrosion resistance while securing coating property of the steel sheet and suppressing processing defects such as fluting.

Process for producing a hot rolled steel sheet excellent in workability and corrosion resistance according to the present invention for achieving the above object, by weight, carbon (C) 0.01 ~ 0.10%, manganese (Mn) 0.05 ~ 0.3%, phosphorus (P) 0.001 ~ 0.020%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 ~ 0.07%, nitrogen (N) 0.001 ~ 0.004%, boron (B) 0.001 ~ 0.003%, copper (Cu) 0.05 ~ Shotballs with a diameter of 0.1 to 0.4 mm are placed on the surface of a hot rolled steel sheet made of steel containing 0.5%, chromium (Cr) 0.1 to 0.5%, balance iron (Fe) and other unavoidable impurities. Short blasting in sec.

In addition, the component of the steel is 0.5 ≤ [(B (%) / 11) / (N (%) / 14)] ≤ 1.5, 7 ≤ [(Cu (%) / 64) / (S (%) / 32) ] It is desirable to satisfy ≤ 25.

In addition, the hot-rolled steel sheet is the finish rolling of the steel made of the composition at 860 ~ 950 ℃, and cooling the finished rolled steel at a cooling rate of 30 ~ 100 ℃ per second, after winding the winding at a temperature of 550 ~ 700 ℃ desirable.

In addition, it is preferable that the surface roughness index ratio (Rmax / Ra) is set to 27 to 35 by the shot blasting.

On the other hand, the hot rolled steel sheet excellent in workability and corrosion resistance prepared according to the present invention, by weight%, carbon (C) 0.01 ~ 0.10%, manganese (Mn) 0.05 ~ 0.3%, phosphorus (P) 0.001 ~ 0.020%, sulfur ( S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 to 0.07%, nitrogen (N) 0.001 to 0.004%, boron (B) 0.001 to 0.003%, copper (Cu) 0.05 to 0.5%, chromium (Cr ) Is made of steel containing 0.1 to 0.5%, balance iron (Fe) and other unavoidable impurities, and the surface roughness index ratio (Rmax / Ra) is 27 to 35 by shot blasting treatment.

In addition, the component of the steel is 0.5 ≤ [(B (%) / 11) / (N (%) / 14)] ≤ 1.5, 7 ≤ [(Cu (%) / 64) / (S (%) / 32) ] It is desirable to satisfy ≤ 25.

In addition, the deformed ferrite structure produced by the shot blasting treatment is preferably occupied by 3 to 10% of the phase fraction 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 It allows to manufacture.

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 hot-rolled steel sheet tissue photograph 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, the technical configuration of the present invention will be described in more detail.

As described above, the hot-rolled steel sheet of the present invention is the high value-added hot-rolled steel sheet that can be used for home appliances and automobiles by satisfying the bending resistance, workability and paintability at the same time, and completed the present invention. The steel component according to the invention is preferably controlled as follows.

Carbon (C) is an element added to improve the strength of the steel sheet, the tensile strength and yield strength increases as the content is increased, but if excessively added, the workability of the material is lowered and the bending resistance is also lowered, so the upper limit is limited to 0.10%. On the other hand, if the carbon content is less than 0.01%, not only additional operating time is required for decarburization during steelmaking, but also it is difficult to secure stable materials such as target strength. Therefore, the content of carbon is preferably managed to 0.01 to 0.10%.

Manganese (Mn) is an element widely used as a solid solution strengthening element and is an important element for increasing the strength of steel and improving hot workability, but is an element that inhibits the ductility and workability of a material due to MnS formation. If the content of manganese is small, the workability is improved, but it is difficult to secure the strength. Therefore, it is necessary to add 0.05% or more to secure the target strength. On the other hand, when the manganese is added excessively, the upper limit is limited to 0.3% because there is a problem of economic deterioration and central segregation caused by the addition of a large amount of alloying elements, and also deteriorates weldability.

Phosphorus (P) is an element that improves the strength and corrosion resistance of steel, and it is preferable to add a large amount in order to secure these properties. Its content is limited to 0.02% or less. On the other hand, if the phosphorus content is less than 0.001%, it is difficult to secure strength and corrosion resistance, so it is better to manage it with 0.001 to 0.02%.

Sulfur (S) combines with 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, 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 generally an element added for deoxidation of molten steel, but it is combined with solid solution in steel to improve aging characteristics. Therefore, the lower limit thereof is limited to 0.01%. In addition to causing surface defects, there is a problem of lowering the workability, so the upper limit thereof is 0.07%, and the management range is limited to 0.01 to 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 below a certain amount in order to secure workability, so the upper limit is limited to 0.004%. In addition, if less than 0.001% is not obtained sufficient stiffness and the site for precipitate formation is reduced, the lower limit is limited to 0.001%, the management range is limited to 0.001 ~ 0.004%.

Boron (B) is an element that combines with solid solution elements in steel to improve the aging properties and enhances the strength of the material by adding a small amount as a hardening element. Therefore, at least 0.001% The above addition is required. However, if the content exceeds 0.003%, there is a problem of deterioration of material and grain boundary cracking during playing, as well as a problem of roughening the surface of the hot rolled steel sheet. Therefore, the content is limited to 0.001% to 0.003%.

Copper (Cu) is an effective element for improving the corrosion resistance of steel. It forms a Cu ₂ S -based sulphide on the surface of the steel sheet, thereby exhibiting a mechanism for suppressing the generation of rust. In order to obtain such an effect, addition of 0.05% or more is required. On the other hand, when the addition exceeds 0.5%, the upper limit is limited to 0.5% because it acts as a cause of cracking due to grain boundary embrittlement in the playing step.

Chromium (Cr) is an element that serves to form a stable rust prevention layer similar to Cu and is preferably added at least 0.1% to secure corrosion resistance. On the other hand, if the amount of Cr added is more than 0.5%, rather than acting as a factor inducing the hole corrosion rather than the effect of improving the corrosion resistance, it sharply increases the manufacturing cost, the content is limited to 0.1 to 0.5%.

On the other hand, in order to secure appropriate processing characteristics, it was also necessary to properly manage the fraction of precipitates and solid solution. In order to secure the characteristics of the present invention, it is necessary to keep the value of the atomic ratio [(B (%) / 11) / (N (%) / 14)] between B and N at 0.5 to 1.5. When the B / N value is less than 0.5, the bending resistance at room temperature is deteriorated due to excessive solid solution in steel, and the effect of suppressing grains at low temperature is insufficient. Therefore, the value of B / N is limited to 0.5 to 1.5.

In addition, it is preferable to limit the atomic ratio [(Cu (%) / 64) / (S (%) / 32)]] value of Cu / S within a certain range. If the atomic ratio of Cu / S exceeds 25, the frequency of occurrence of surface defects due to solid solution Cu increases. On the other hand, below 7.0, the formation of Cu sulfide, which contributes to the improvement of corrosion resistance, is insufficient. To 7 to 25.

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, in weight percent, carbon (C) 0.01 to 0.10%, manganese (Mn) 0.05 to 0.3%, phosphorus (P) 0.001 to 0.020%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 to 0.07%, nitrogen (N) 0.001 to 0.004%, boron (B) 0.001 to 0.003%, copper (Cu) 0.05 to 0.5%, chromium (Cr) 0.1 to 0.5%, balance iron (Fe) and other unavoidable impurities A hot rolled steel sheet is manufactured according to a conventional hot rolling process using steel, and shot blasting is performed on the surface of the hot rolled steel sheet at a spraying speed of 40 to 65 m / sec.

This short blasting process is one of the most characteristic technical configurations of the present invention for ensuring the bending resistance (aging resistance), by introducing an appropriate compressive stress on the surface of the hot-rolled steel sheet, the dislocation density, among which the operation potential density is greatly increased By newly producing modified ferrite grains, it is possible to reduce the seizure of dislocations due to the solid solution element, which is the main cause of the aging phenomenon, thereby improving the aging resistance.

In order to achieve this effect, it is preferable to control the diameter of the shot ball used for shot blasting to 0.10 to 0.40 mm. If the diameter of the shot ball is less than 0.10 mm, the mechanical peeling effect of the surface layer is small, so that it is difficult to obtain an appropriate dislocation density increasing effect. 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 ~ 65m / sec. If the injection speed is less than 40m / sec, the impact pressure of the shot ball acting on the surface layer is low, it is difficult to secure the desired aging resistance, if it exceeds 65m / sec, the depth of the surface hardened layer is 10% in the thickness direction of the material This is because it causes the non-uniform processing beyond.

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 27 to 35. Here, Rmax means the maximum height in the surface roughness curve of the hot-rolled steel sheet, Ra means the average surface roughness of the center line. Increasing the surface roughness ratio increases the density of the working potential as the fixation potential in the surface layer of the steel sheet leaves the Cottrell atmosphere with carbon and nitrogen dissolved in the steel. If the surface roughness index ratio is less than 27, 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 35, 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 27 to 35.

(A) of FIG. 1 shows a ferrite grain in a steady state with a low dislocation density, and FIG. 1 (b) shows a strain having a network structure with a high density of dislocations in which dislocations are integrated by a shot blasting process according to the present invention. Shows ferrite lip. Deformed ferrite structure produced by shot blasting is preferably controlled so that the phase fraction in the thickness direction of the hot-rolled steel sheet occupies 3 to 10% 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 to act as a factor to deteriorate the workability, it is preferable to make the management range of 3 to 10%.

As described above, if the shot blasting process of a predetermined condition is applied to the surface of the steel sheet produced through the usual 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 more excellent endurance, workability, and paintability can 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 made of the above composition components at 860 ~ 950 ℃. 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. On the other hand, when the finish rolling temperature is higher than 950 ° C, uniform hot rolling is not performed throughout the thickness, so that grain refinement is insufficient, resulting in a drop in impact toughness due to grain coarsening, and thus the finish rolling temperature range is 860. It is preferable to manage at -950 degreeC.

In addition, the hot rolled steel sheet is preferably cooled to the finish-rolled steel at a cooling rate of 30 ~ 100 ℃ per second in a run-out (ROT, Run-out-table). If the cooling rate in the ROT is less than 30 ° C./sec, 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 100 ° C / sec or more, it acts as a factor of material variation due to the widthwise cooling unevenness, and therefore, the range of the cooling rate is preferably managed at 30 to 100 ° C / sec.

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 lower than 550 ° C, the formation behavior of low-temperature precipitates is different due to the width-wise temperature unevenness during cooling and holding, thereby causing material variation, which adversely affects workability. On the other hand, when the coiling temperature exceeds 700 ℃, the problem occurs that the processability and corrosion resistance is reduced as the structure of the final product is coarse, it is preferable to manage the range of the winding temperature to 550 ~ 700 ℃.

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

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

Steel grade
Chemical composition (wt%) Inter-element ratio C Mn P S Al N B Cu Cr B / N Cu / S Inventive Steel 1 0.023 0.21 0.007 0.008 0.054 0.0025 0.0021 0.36 0.26 1.07 22.5 Invention river 2 0.041 0.19 0.012 0.009 0.045 0.0031 0.0016 0.17 0.41 0.66 9.4 Invention steel 3 0.056 0.27 0.009 0.009 0.051 0.0030 0.0019 0.14 0.19 0.81 7.8 Comparative River 1 0.003 0.28 0.007 0.006 0.035 0.0027 0.0006 - 0.18 0.28 0.0 Comparative River 2 0.031 0.18 0.008 0.005 0.029 0.0065 - 0.02 0.05 0.00 2.0 Comparative Steel 3 0.114 0.78 0.009 0.006 0.034 0.0039 0.0007 0.94 0.98 0.23 78.3

※ Component ratio between elements: [(B (%) / 11) / (N (%) / 14)], [(Cu (%) / 64) / (S (%) / 32)]

division Used steel grade Reheat temperature
(℃) Finishing temperature
(℃) Cooling rate
(° C / s) Coiling temperature
(℃) Short ball
Diameter (mm) Blasting
Speed (m / sec) Inventory 1
Inventive Steel 1
1250 890 40 600 0.25 50 Inventive Example 2 1250 890 70 650 0.32 60 Inventory 3 1250 890 90 600 0.32 50 Honorable 4 Invention river 2
1250 910 70 600 0.19 50 Inventory 5 1250 910 70 650 0.27 60 Inventory 6 Invention steel 3 1250 890 70 600 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 60 650 0.91 90 Comparative Example 2 Comparative River 1 1250 910 60 650 0.16 50 Comparative Example 3 Comparative River 2 1250 910 60 650 0.25 60 Comparative Example 4 Comparative Steel 3 1250 890 60 650 0.25 60

division Surface roughness
Exponent ratio Yield point extension phenomenon Elongation
(%) Corrosion resistance Paintability Processability Bending property Inventory 1 28.4 Not occurring 34.1 Good Good Good Good Inventive Example 2 30.2 Not occurring 33.8 Good Good Good Good Inventory 3 31.4 Not occurring 36.4 Good Good Good Good Honorable 4 29.4 Not occurring 35.8 Good Good Good Good Inventory 5 30.1 Not occurring 37.4 Good Good Good Good Inventory 6 31.5 Not occurring 38.1 Good Good Good Good Honorable 7 27.6 Not occurring 31.4 Good Good usually Good Inventive Example 8 30.4 Not occurring 30.9 Good Good usually Good Proposition 9 27.1 Occur 19.7 Good Good usually Good Comparative Example 1 40.9 Not occurring 20.4 Bad Bad Bad usually Comparative Example 2 35.4 Not occurring 40.9 Bad usually Bad Good Comparative Example 3 28.6 Occur 24.9 Bad usually Bad Bad Comparative Example 4 19.7 Occur 19.8 Good Good Bad Bad

In Table 3, corrosion resistance after saline spray test with 5% NaCl for 240 hours was determined to be less than 0.05 g / cm ^{2, and} worse than that. The paintability was good when the coating adhesiveness and surface characteristics were excellent among the evaluation items of the coating material, and when only one of the two characteristics was excellent, the case where both characteristics were not good was indicated as defective.

The workability was measured by crack length for bending test specimens and divided into five stages as shown in [Table 4] below. One stage was good, two to three stages were normal, and four to five stages were marked 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 4 or more microcrack on point or sum of total crack length within 10mm Step 5 Cracks occur, and the total length of the cracks is 10 mm or more

The bending resistance (aging resistance) was classified according to the degree of surface bending after steel sheet processing. The bending index representing this is divided into five stages, and the first stage with relatively small bending is good. Three stages were determined as normal, and four to five stages where the bending phenomenon occurred to the extent that the naked eye could be observed were judged as defective.

According to the above criteria, the experimental results disclosed in Table 3 above 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. Yield point stretching did not occur in all the invention examples, and the surface roughness index ratio (Rmax / Ra) was also within 27 to 35, satisfies the management range, and no bending occurred during processing, thereby ensuring 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. 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. 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 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 short blasting conditions are not satisfied. More specifically, in the case of shot blasting conditions, the shot ball is 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, no yield point stretching phenomenon occurs, which is good in terms of aging resistance, but due to the increase in surface roughness and the increase of internal hardening layer, both coating properties and workability are reduced, and thus, they are not suitable for use as hot rolled steel sheets 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 were used. In most cases, it is difficult to inhibit the draw point stretching phenomenon, so that the endurance property is lowered and the processing and coating properties according to the present invention can not be satisfied due to the occurrence of the bending phenomenon at the time of processing.

Furthermore, in Comparative Examples 2 and 3, in which Cu or the like was not added, the target corrosion resistance was not obtained because the surface properties could not be secured. There also appeared a problem that the rolling workability was significantly reduced, such as cracks. In addition, even when B and the like deviate from the patent scope, even if the manufacturing process and shot blasting conditions according to the present invention are applied, the processability such as bending resistance, which is a fundamental target, cannot be secured because the atomic ratio between elements is not satisfied. .

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 27 to 35, the fluting index and cracking sensitivity are excellent. 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 (9)

By weight%, carbon (C) 0.01 to 0.10%, manganese (Mn) 0.05 to 0.3%, phosphorus (P) 0.001 to 0.020%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 to 0.07%, nitrogen (N) 0.001 to 0.004%, boron (B) 0.001 to 0.003%, copper (Cu) 0.05 to 0.5%, chromium (Cr) 0.1 to 0.5%, balance iron (Fe) and other unavoidable impurities Method for producing a hot rolled steel sheet excellent in workability and corrosion resistance characterized in that the shot blasting shot ball having a diameter of 0.1 ~ 0.4mm on the surface of the hot-rolled steel sheet made of steel to a blasting injection speed of 40 ~ 65m / sec. The method according to claim 1,
The composition of the steel is 0.5 ≤ [(B (%) / 11) / (N (%) / 14)] ≤ 1.5, 7 ≤ [(Cu (%) / 64) / (S (%) / 32)] ≤ Process for producing hot rolled steel sheet excellent in workability and corrosion resistance characterized in that to satisfy 25. The method according to claim 1 or 2,
The hot rolled steel sheet is a method of manufacturing a hot rolled steel sheet excellent in workability and corrosion resistance, characterized in that the finish rolling the steel consisting of the composition at 860 ~ 950 ℃. The method according to claim 3,
The hot rolled steel sheet is a method of manufacturing a hot rolled steel sheet excellent in workability and corrosion resistance, characterized in that for cooling the finish-rolled steel at a cooling rate of 30 ~ 100 ℃ per second. The method of claim 4,
The hot rolled steel sheet is a method of producing a hot rolled steel sheet excellent in workability and corrosion resistance, characterized in that the winding after the cooling at a temperature of 550 ~ 700 ℃. The method according to claim 1,
Surface roughness ratio (Rmax / Ra) of 27 to 35 by the shot blasting method of producing a hot rolled steel sheet excellent in workability and corrosion resistance. By weight%, carbon (C) 0.01 to 0.10%, manganese (Mn) 0.05 to 0.3%, phosphorus (P) 0.001 to 0.020%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 to 0.07%, nitrogen (N) 0.001 to 0.004%, boron (B) 0.001 to 0.003%, copper (Cu) 0.05 to 0.5%, chromium (Cr) 0.1 to 0.5%, balance iron (Fe) and other unavoidable impurities A hot rolled steel sheet having excellent workability and corrosion resistance, which is made of steel and has a surface roughness index ratio (Rmax / Ra) of 27 to 35. The method of claim 7,
The composition of the steel is 0.5 ≤ [(B (%) / 11) / (N (%) / 14)] ≤ 1.5, 7 ≤ [(Cu (%) / 64) / (S (%) / 32)] ≤ Hot rolled steel sheet excellent in workability and corrosion resistance, characterized by satisfying 25. The method of claim 7,
Includes deformed ferrite tissue produced by shot blasting treatment,
The deformed ferrite structure is a hot rolled steel sheet excellent in workability and corrosion resistance, characterized in that the phase fraction in the thickness direction of the hot rolled steel sheet occupies 3 to 10%.

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