KR20120073633A - Method for manufacturing hot-rolled steel having excellent anti-aging property and the hot-rolled steel by the same method - Google Patents

Abstract

PURPOSE: A method for manufacturing a high-strength hot rolled steel sheet with superior bending resistance and a hot rolled steel sheet manufactured by the same are provided to improve the surface properties of a hot rolled steel sheet by adding manganese, niobium, and cobalt at a controlled ratio. CONSTITUTION: A method for manufacturing a high-strength hot rolled steel sheet with superior bending resistance comprises a step of shot blasting on the surface of a hot rolled steel sheet using shot balls with a diameter of 0.1-0.4mm at a shot speed of 40-65m/sec. The hot rolled steel sheet comprises carbon of 0.01-0.10 weight%, manganese of 0.5-1.5 weight%, phosphorus of 0.01-0.04 weight%, sulfur of 0.020 weight% or less, aluminum of 0.01-0.07 weight%, nitrogen of 0.001-0.004 weight%, boron of 0.0005-0.0020 weight%, niobium of 0.03-0.06 weight%, cobalt of 0.01-0.05 weight%, tungsten of 0.01-0.05 weight%, and iron and inevitable impurities of the remaining amount.

Description

Method for manufacturing high strength hot rolled steel sheet having excellent bending resistance and hot rolled steel sheet manufactured by the same

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

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, in order to prevent oxidation of the steel sheet due to outside conditions and to obtain the surface color desired by the user, organic materials such as paint are coated on the surface, 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, the aging phenomenon due to the employment element that causes the fluting phenomenon is practically difficult to be suppressed to achieve high cleanness at the steelmaking stage and to fix these solid elements with titanium (Ti) and niobium (Nb). Carbonitride-forming elements such as) are added and precipitated. The addition of these carbonitride-forming elements is helpful for suppressing processing defects such as fluting, but it is a factor of lowering productivity due to increased steelmaking time for high clarity and increasing production cost by adding expensive alloying elements.

In addition, it is known that in low and medium carbon steels, it is difficult to suppress the bending phenomenon such as fluting. 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, work is performed to paint organic materials such as paint on the surface of the steel sheet to improve weather resistance. Therefore, development of plating steel sheet capable of securing paintability in terms of materials is required to secure such characteristics. It is becoming.

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, this method helps to suppress processing defects such as fluting by the addition of carbonitride forming elements, as described above, but increases the production cost by increasing the steelmaking time for high clarity and the addition of expensive alloying elements. There was a problem.

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 low temperature precipitates show a difference in the production behavior due to the nonuniformity of the width direction, causing material variation, thereby deteriorating shape defects, winding defects and post-process workability. 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 cobalt (Co), tungsten (W) and niobium (Nb) of the steel components in the medium-low carbon steel, and optimize the cooling conditions and shot blasting conditions, etc. The purpose of the present invention is to provide a method for producing a high strength hot rolled steel sheet excellent in bending resistance, workability, corrosion resistance, and surface properties which ensures coating property of the steel sheet and suppresses processing defects such as fluting. have.

Method for producing a high-strength hot-rolled steel sheet excellent in bending resistance according to the present invention for achieving the above object, in weight%, 0.01 to 0.10% carbon (C), 0.5 to 1.5% manganese (Mn), phosphorus (P) 0.01 to 0.04%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 to 0.07%, nitrogen (N) 0.001 to 0.004%, boron (B) 0.0005 to 0.0020%, cobalt (Co) 0.01 ~ 0.05%, tungsten (W) 0.01 to 0.05%, niobium (Nb) 0.03 to 0.06%, shot ball size 0.1 to 0.4 on the surface of hot-rolled steel sheet made of steel containing residual iron (Fe) and other unavoidable impurities mm, shot blasting consists of shot blasting at 40 ~ 65m / sec.

In addition, the steel component is configured to satisfy 3 ≦ [{B (ppm) × Al (%) × Co (%)} / N (%)] ≦ 12.

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 60 ~ 120 ℃ per second, it is wound up at a temperature of 500 ~ 680 ℃ after cooling desirable.

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

On the other hand, high-strength hot-rolled steel sheet excellent in bending resistance prepared according to the present invention, in weight percent, carbon (C) 0.01 ~ 0.10%, manganese (Mn) 0.5 ~ 1.5%, phosphorus (P) 0.01 ~ 0.06%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 to 0.07%, nitrogen (N) 0.001 to 0.004%, niobium (Nb) 0.03 to 0.06%, cobalt (Co) 0.01 to 0.05%, balance iron It is made of steel containing (Fe) and other unavoidable impurities, and has a surface roughness index ratio (Rmax / Ra) of 10 to 20.

In addition, the steel component preferably satisfies 3 ≦ [{B (ppm) × Al (%) × Co (%)} / N (%)] ≦ 12.

In addition, it is preferable that the deformed ferrite structure generated by the shot blasting treatment occupies 3 to 10% in the thickness direction of the hot rolled steel sheet.

According to the method of manufacturing a high strength hot rolled steel sheet of the present invention configured as described above, while ensuring the bending resistance through proper control of the components and optimization of the manufacturing process, and improve the workability, corrosion resistance and paintability to be used in home appliances, containers, automobiles, etc. Enables manufacturing high value-added steel sheets.

In addition, it has excellent descaling properties, which can increase the efficiency of pickling operations and at the same time prevent environmental pollution and shorten processes.

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 high-strength hot-rolled steel sheet of the present invention satisfies the surface characteristics such as bending resistance, workability, paintability, etc. at the same time, and has repeatedly studied and experimented on high-value hot-rolled steel sheet that can be used for home appliances and automobiles. As completed the 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 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 low, the workability is improved, but it is difficult to secure the strength. Therefore, 0.5% or more must be added to secure the target strength. On the other hand, if the manganese is added excessively, the upper limit is limited to 1.5% because there is a problem of economical deterioration and central segregation caused by the addition of a large amount of alloying elements and a problem of deteriorating 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 characteristics. However, since phosphorus (P) is an element that causes central segregation during casting, it causes a deterioration of workability. Its content is limited to 0.04% or less. On the other hand, if the phosphorus content is less than 0.01% it is difficult to secure the strength and corrosion resistance it is good to manage from 0.01 to 0.04%.

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 (excluding 0%), preferably managed to 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 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 does it cause deterioration of material and grain boundary cracking, but also the roughness of the surface of the hot-rolled steel sheet, the content is limited to 0.0005 to 0.0020%.

Cobalt (Co) is an element that promotes the formation of precipitates in the steel, and at least 0.01% or more must be added 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, so the content is limited to 0.01 to 0.05%.

Tungsten (W) is an element added to secure hardenability and strength characteristics, and more than 0.01% is required to obtain a target strength level in a low temperature region. 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 to 0.05%.

Niobium (Nb) is not only an effect of delaying the recrystallization of ferrite, but also an element showing the effect of increasing the strength of the steel sheet by being precipitated in combination with C, N, etc. in steel. In order to secure the target strength, addition of 0.03% or more is required, while exceeding 0.06% causes the increase of manufacturing cost and the deterioration of hot rolling workability, and therefore the content is limited to 0.03 to 0.06%.

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 preferable to maintain a management range between alloy elements such that 3 ≦ [{B (ppm) × Al (%) × Co (%)} / N (%)] ≦ 12. When the above component ratio is lower than 3, it is difficult to secure the bending resistance and processability by the expression of a solid solution element. On the other hand, when the ratio is higher than 12, it is possible to secure the processability, but the surface defect and the productivity decrease due to the excessive addition element. Since it acts as a factor, it is preferable to manage the component ratio between elements 3 <[{B (ppm) x Al (%) x Co (%)} / N (%)] <12.

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

First, in weight percent, carbon (C) 0.01 to 0.10%, manganese (Mn) 0.5 to 1.5%, phosphorus (P) 0.01 to 0.04%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 to 0.07%, nitrogen (N) 0.001 to 0.004%, boron (B) 0.0005 to 0.0020%, cobalt (Co) 0.01 to 0.05%, tungsten (W) 0.01 to 0.05%, niobium (Nb) 0.03 to 0.06%, Hot-rolled steel sheet is manufactured from steel containing residual iron (Fe) and other unavoidable impurities in accordance with a conventional hot-rolling process, and the shot ball size is 0.1 to 0.4 mm and the blasting spray speed is 40 to 65 m / sec. Shot blasting is carried out.

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 such an effect, it is preferable to control the size of the shot ball used for shot blasting to 0.10 to 0.40 mm. If the size 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. If the size of the shot ball exceeds 0.40 mm, the maximum roughness of the surface is increased rapidly during processing. 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 10 to 20. 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. When the surface roughness index ratio is less than 10, the appropriate ratio of rough acid and bone is not secured, so that the adsorption 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, if the surface roughness index ratio exceeds 20, the density of the movable potential reaches saturation, but since the tip crack occurs during processing, it is preferable to set the management range to 10-20.

(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 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 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 60 ~ 120 ℃ per second in the run-out (ROT, Run-out-table). If the cooling rate in the ROT is less than 60 ° 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 120 ° C / sec 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 / sec.

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 contrary, 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 of manufacturing a high strength hot rolled steel sheet excellent in aging resistance according to the present invention described above was carried out as follows.

First, each steel (inventive steel 3, comparative steel 6) having a composition as shown in [Table 1] below was prepared, and reheated in a heating furnace at 1250 ° C. for 2 hours, followed by the hot rolling conditions disclosed in [Table 2]. Hot rolling was performed accordingly. In addition, the physical properties and mechanical properties of the prepared steels were measured and shown in [Table 3].

Steel grade
Chemical composition (wt%) Interelemental
Ingredient 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 Inventive Steel 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 Steel 1 0.007 0.19 0.015 0.008 0.006 0.0042 0.0014 - - - 0.00 Comparative Steel 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

※ Component ratio between elements: 3 ≤ [{B (ppm) × Al (%) × Co (%)} / N (%)]] ≤ 12

division Steel grade used Reheating temperature
(℃) Finishing temperature
(℃) Cooling rate
(℃ / s) Coiling temperature
(℃) Shortball
Size (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 Inventive Steel 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 Inventive Steel 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 Steel 1 1250 910 70 640 0.16 50 Comparative Example 3 Comparative Steel 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

division Surface roughness
Index ratio Yield point extension
Status The tensile strength
level Paintability Processability Break resistance Inventory 1 14.6 Not occurring Good Good Good Good Inventive Example 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 Inventive Example 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, 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 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 part Step 2 No cracking at the bending part, but the surface of the machining part is rough 3 steps The number of microcracks on the spot occurred 1 to 3 4 steps 4 or more microcrack on point or sum of total crack length within 10mm 5 steps A crack occurs and the length of the total crack sum 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 satisfy the conditions of the steel component control, the hot rolling process and the shot blasting process according to the present invention. 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. No bending occurred in the wire, which ensured excellent aging resistance. Moreover, no cracking occurred during bending, resulting in high processability and paintability, thereby making it possible to manufacture excellent hot rolled steel sheets and original plates for plating.

Inventive 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 are satisfied, but the hot rolling process is 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 was slightly decreased due to grain hybridization, solid element 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 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 7 are cases in which steel grades (comparative steels 1 to 6) that satisfy the hot rolling process conditions and the shot blasting conditions according to the present invention but do not satisfy the steel component conditions are used. In most cases, it was difficult to suppress the yield point stretching phenomenon, and thus the aging resistance was lowered, and the processing and coating properties according to the present invention could not be satisfied due to the occurrence of bending during 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 Nb, 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 a yield point stretching phenomenon and a bending phenomenon occurs. Aging resistance and workability could not be secured.

Finally, Figure 2 is a graph measuring the change in the aging resistance (expressed as fluting index) and the workability (expressed as the crack generation sensitivity during processing) according to the surface roughness index ratio using the invention steel 1. In order to satisfy the target aging resistance and workability, 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 cracking sensitivity are excellent. On the other hand, in the region below the above control range, both the fluting index and the machinability index exceed the threshold, causing the bending and processing cracks to occur. In the region above the above management range, the aging resistance indicates the saturation value while The problem that a process crack generate | occur | produced was confirmed.

Claims (9)

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) 0.020% 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%, niobium (Nb) 0.03 to 0.06%, balance iron Bending resistance characterized by shot blasting on the surface of hot-rolled steel sheet made of steel containing (Fe) and other unavoidable impurities with a shot ball size of 0.1 to 0.4 mm and a blasting speed of 40 to 65 m / sec. Excellent high strength hot rolled steel sheet production method. The method according to claim 1,
Component of the steel is 3 ≤ [{B (ppm) × Al (%) × Co (%)} / N (%)]] ≤ 12 to produce a high strength hot rolled steel sheet having excellent bending resistance, characterized in that . The method according to claim 1 or 2,
The hot rolled steel sheet is a method of producing a high strength hot rolled steel sheet having excellent bending resistance, characterized in that the finish rolling of the steel made of the composition at 860 ~ 950 ℃. The method according to claim 3,
The hot rolled steel sheet is a method of producing high strength hot rolled steel sheet having excellent bending resistance, characterized in that for cooling the finish-rolled steel at a cooling rate of 60 ~ 120 ℃ per second. The method of claim 4,
The hot rolled steel sheet is a method of producing a high strength hot rolled steel sheet excellent in bending resistance, characterized in that the winding after the cooling at a temperature of 500 ~ 680 ℃. The method according to claim 1,
The method of producing a high strength hot rolled steel sheet having excellent bending resistance, characterized in that the surface roughness index ratio (Rmax / Ra) by 10 to 20 by the shot blasting. By weight%, carbon (C) 0.01 to 0.10%, manganese (Mn) 0.5 to 1.5%, phosphorus (P) 0.01 to 0.06%, sulfur (S) 0.020% or less (excluding 0%), aluminum (Al) 0.01 to Surface roughness index made of steel containing 0.07%, nitrogen (N) 0.001 to 0.004%, niobium (Nb) 0.03 to 0.06%, cobalt (Co) 0.01 to 0.05%, balance iron (Fe) and other unavoidable impurities A high strength hot rolled steel sheet having excellent bending resistance, characterized by a ratio (Rmax / Ra) of 10 to 20. The method of claim 7,
High strength hot rolled steel sheet having excellent bending resistance, characterized in that the component of the steel satisfies 3 ≦ [{B (ppm) × Al (%) × Co (%)} / N (%)] ≦ 12. The method of claim 7,
The high-strength hot-rolled steel sheet having excellent bending resistance, characterized in that the modified ferrite structure produced by the shot blasting treatment occupies an amount of 3 to 10% in the thickness direction of the hot-rolled steel sheet.

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