KR101330299B1 - Method for manufacturing anti-seasoning hot-rolled steel having excellent corrosion resistance property and the hot-rolled steel by the same method - Google Patents

Abstract

It introduces medium-carbon hot-rolled steel sheet and its manufacturing method which are excellent in aging resistance to prevent processing defects such as fluting by optimizing steel components, manufacturing process and shot blasting conditions, as well as processability and paintability. do.
The method for producing hot rolled steel sheet having excellent workability according to the present invention, in weight%, carbon (C): 0.10 to 0.25%, manganese (Mn): 0.1 to 0.3%, aluminum (Al): 0.02 to 0.07%, nitrogen (N) : 0.001 ~ 0.004%, the rest of the hot rolled steel sheet made of steel containing iron (Fe) and other unavoidable impurities, by spraying a shot ball having a diameter of 0.2 ~ 0.35mm at a speed of 50 ~ 60m / sec Short blasting is characterized in that.

Description

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

The present invention relates to a medium-carbon hot-rolled steel sheet excellent in aging resistance and a method for manufacturing the same, and more particularly, to a method for producing a medium-carbon-based hot rolled steel sheet excellent in corrosion resistance by optimizing the steel component and manufacturing process.

In order to be used as a steel sheet for household appliances and automobiles, excellent workability, endurance (flutting resistance) and paintability are required.

Fluting refers to a phenomenon in which a machining portion breaks during machining. When fluting occurs, it is difficult to maintain the shape of the machining portion, so 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 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. .

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 .

It should be understood that the foregoing description of the background art is merely for the purpose of promoting an understanding of the background of the present invention and is not to be construed as adhering to the prior art already known to those skilled in the art.

Japanese Laid-Open Patent Application No. 1989-282420 Korean Patent Publication No. 1996-23130

The present invention has been proposed to solve such a conventional problem, by adding aluminum and nitrogen in the steel components in the medium-carbon steel, by optimizing the cooling conditions and shot blasting conditions, such as processing defects such as fluting of steel sheet It is an object of the present invention to provide a medium-carbon hot rolled steel sheet having excellent aging resistance to be suppressed and a method of manufacturing the same.

Medium carbon hot rolled steel sheet manufacturing method excellent in aging resistance according to the present invention for achieving this object, by weight, carbon (C): 0.10 ~ 0.25%, manganese (Mn): 0.1 ~ 0.3%, aluminum (Al) : 0.02 ~ 0.07%, Nitrogen (N): 0.001 ~ 0.004%, the remainder is a short ball of 0.2 ~ 0.35mm diameter on the surface of the hot-rolled steel sheet made of steel containing iron (Fe) and other unavoidable impurities, It is characterized in that the shot blasting by spraying at a speed of 50 ~ 60m / sec.
The steel component is characterized in that 5≤ [(Al (%) / 27) / (N (%) / 14)] ≤15.

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The hot rolled steel sheet is to finish rolling the steel of the composition at 850 ~ 900 ℃, to cool the finish rolled steel at a cooling rate of 60 ~ 100 ℃ per second, and to wind up at a temperature of 550 ~ 650 ℃ after the cooling It features.

Surface roughness index ratio (Rmax / Ra) of the steel is characterized in that 30 to 35.

The medium-carbon hot-rolled steel sheet excellent in aging resistance according to the present invention for achieving this object, by weight, carbon (C): 0.10 ~ 0.25%, manganese (Mn): 0.1 ~ 0.3%, aluminum (Al): 0.02 ~ 0.07%, nitrogen (N): 0.001 ~ 0.004%, the remainder is made of steel containing iron (Fe) and other unavoidable impurities, characterized in that the surface roughness index ratio (Rmax / Ra) is 30 ~ 35 .

The steel component is characterized in that 5≤ [(Al (%) / 27) / (N (%) / 14)] ≤15.

Deformed ferrite structure produced by the shot blasting process is characterized in that occupies 5 to 15% in the cross section of all the thickness direction of the hot-rolled steel sheet.

The present invention has the advantage that it is possible to manufacture high value-added steel sheet used in home appliances, automobiles, etc. by improving the machinability and at the same time to ensure the bending resistance through proper component control and manufacturing process optimization.

In addition, the descalability is also excellent to increase the efficiency of pickling operations, there is an advantage that can be obtained to prevent environmental pollution and shorten the process.

1 is a hot rolled steel sheet structure photograph according to the present invention,
2 is a characteristic evaluation graph different from the thermal roughness surface roughness index ratio according to the present invention.

Hereinafter, with reference to the accompanying drawings will be described a medium-carbon hot rolled steel sheet excellent in aging resistance according to a preferred embodiment of the present invention and a method of manufacturing the same.

As described above, the hot rolled steel sheet of the present invention relates to a high value-added hot rolled steel sheet that can be used for home appliances and automobiles by satisfying bending resistance, workability, and paintability at the same time. As completed, the steel component according to the present invention is limited 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 reduced, so the upper limit is limited to 0.25%.

On the other hand, if the carbon content is less than 0.1%, additional operation time is required for decarburization during steelmaking, and there is a problem in that it is difficult to secure a stable material because a sudden change of material occurs. The lower limit is limited to 0.1%.

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 improves, but it is difficult to secure the strength. Therefore, in order to secure the target strength, 0.1% or more should be added.

On the other hand, if manganese is added excessively, the upper limit is limited to 0.3% because it causes economic deterioration and central segregation caused by the addition of a large amount of alloying elements.

Aluminum (Al) is generally an element added for molten steel deoxidation, but in combination with solid solution in steel, it has an aspect of improving aging characteristics. Therefore, the lower limit thereof is limited to 0.02%. Not only does it cause defects, but there is a problem of degrading workability, so the upper limit is limited to 0.07% and the management range is limited to 0.02 to 0.07%.

Nitrogen (N) is an element useful for reinforcing materials while existing in solid state in steel. However, since it is the main element causing aging phenomenon, 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%.

On the other hand, in order to secure appropriate processing characteristics, it is also important to properly manage the fractions of precipitates and solid solution.

In order to secure the characteristics of the present invention, it is necessary to maintain the value of the atomic ratio [(Al (%) / 27) / (N (%) / 14)] of aluminum (Al) and nitrogen (N) to 5 to 15. have. When the atomic ratio of Al / N is less than 5, there is a problem in that the breakdown resistance at room temperature is degraded due to excessive solid solution elements in steel, and the grain restraining effect is insufficient at high temperatures. On the other hand, when the atomic ratio value of Al / N exceeds 15, there is a problem that the ductility is lowered and the workability decreases. Therefore, the atomic ratio value of Al / N is preferably limited to 5-15.

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, the hot rolled steel sheet according to the present invention is preferably finish-rolled steel made of the composition component at 850 ~ 900 ℃.

When the finish rolling temperature is lower than 850 ° C, as the hot rolling is finished in the low temperature region, the grains are rapidly blended, resulting in deterioration of rollability and workability.

On the other hand, when the finish rolling temperature is higher than 900 ℃, uniform hot rolling is not performed throughout the thickness, the grain refinement is insufficient, resulting in a drop in impact toughness due to grain coarsening, the finish rolling temperature range is 850 It is preferable to manage at -900 degreeC.

In addition, the hot rolled steel sheet is preferably cooled to the finish-rolled steel at a cooling rate of 60 ~ 100 ℃ per second in a 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 100 ° C / sec or more, it acts as a factor of the occurrence of material deviation due to the widthwise cooling unevenness, so the range of the cooling rate is preferably managed at 60 to 100 ° C / sec.

Finally, the hot rolled steel sheet is preferably wound at a temperature of 550 ~ 650 ℃ 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 650 ℃, there is a problem 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 coiling temperature to 550 ~ 650 ℃.

As described above, nitrogen (N) 0.001 to 0.004%, balance iron (Fe), which is 0.1% to 0.25% of carbon (C), 0.1 to 0.3% of manganese (Mn), and 0.02 to 0.07% of aluminum (Al). And other unavoidable impurities, the hot-rolled steel sheet is manufactured according to the usual hot-rolling process, and shot blasting at the surface of the hot-rolled steel sheet with a shot ball diameter of 0.2 to 0.35 mm and a blasting speed of 50 to 60 m / sec. Is carried out.

This shot blasting process is one of the most distinctive technical constructions of the present invention for securing endurance. The short blasting process is performed by introducing a suitable compressive stress to the surface of the hot-rolled steel sheet to obtain a dislocation density, a modified ferrite lid grain) is newly generated, thereby reducing the phenomenon of dislocation of the dislocation by the hiring element which is a main cause of the aging phenomenon, thereby improving the endurance.

In order to achieve this effect, the diameter of the shot ball used for shot blasting is preferably controlled to 0.2 to 0.35 mm.

If the diameter of the shot ball is less than 0.20 mm, the effect of imparting mechanical impact on the surface layer is small, so that it is difficult to obtain an adequate residual stress. If the diameter of the shot ball exceeds 0.35 mm, the maximum roughness of the surface rises sharply, so that This is because it acts as a factor of crack occurrence.

On the other hand, the injection speed of the shot blasting is preferably controlled to 50 ~ 60m / sec.

If the injection speed is less than 50m / sec, the impact pressure of the shot ball acting on the surface layer is low, it is difficult to secure the desired age resistance, if it exceeds 60m / sec, the depth of the surface hardened layer is 15% 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 30 to 35.

Here, Rmax denotes the maximum point height in the surface roughness curve of the hot-rolled steel sheet, and Ra denotes the center line average surface roughness.

As the surface roughness index ratio increases, the density of the working potential increases as the fixation potential in the surface layer of the steel sheet leaves the Gottrell atmosphere with carbon and nitrogen dissolved in the steel.

If the surface roughness index ratio is less than 30, the appropriate ratio of rough acid and bone is not secured, so that the adsorptivity of organic matters is deteriorated, and the density distribution of the movable potential for fixing solid solution elements 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 30 to 35.

On the other hand, when the surface roughness index ratio exceeds 25, processing cracking starts to occur while reaching the saturation state in terms of securing the adsorptivity, so that the management range is preferably 15 to 25.

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

If the deformed ferrite structure is less than 3% in all cross-sections, the target fermentation resistance cannot be obtained because the deformed ferrite structure does not exhibit the organizational characteristics that can sufficiently fix solid elements in the steel. If it exceeds 10% of the cross-sectional area, it will act as a hardening factor of the material in the processing and post-treatment process, thereby degrading the workability, so it is preferable to set the management range to 3 to 10%.

FIG. 1 (a) shows a steady state ferrite grains having a low dislocation density. FIG. 1 (b) shows a transformed structure having a dislocation structure having a high potential density by the short blasting process according to the present invention. Show ferrite lips. Deformed ferrite structure produced by shot blasting is preferably controlled to occupy 5 to 15% of the cross section in all thickness directions of the hot rolled steel sheet in order to ensure appropriate age resistance. If the deformed ferrite structure is less than 5% in the cross section of all thickness directions, the target aging resistance cannot be obtained because the organizational properties that can sufficiently fix solid elements in the steel cannot be obtained. If it exceeds 15% of the cross-sectional area, it acts as a hardening factor of the material in the processing and post-treatment process, thereby degrading the workability, so the management range is preferably 5 to 15%.

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.

According to the present invention described above, in order to find out the technical effects of the method of manufacturing a medium-temperature hot-rolled steel sheet excellent in aging resistance was performed as follows.

First, the respective steels (three invention steels and three comparative steels) having the composition shown in [Table 1] below were prepared, and after reheating in a heating furnace at 1250 ° C. for 2 hours, according to the hot rolling conditions disclosed in [Table 2]. Hot rolling was performed.

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

Steel grade Chemical composition (% by weight) Inter-element ratio C Mn P Al N Al / N Inventive Steel 1 0.12 0.12 0.012 0.045 0.0021 11.1 Invention river 2 0.21 0.21 0.009 0.034 0.0029 6.1 Invention steel 3 0.18 0.28 0.004 0.061 0.0034 9.3 Comparative River 1 0.06 0.24 0.009 0.113 0.0024 24.4 Comparative River 2 0.15 0.86 0.021 0.042 0.0122 1.8 Comparative Steel 3 0.45 0.59 0.010 0.036 0.0030 6.2

※ Weight ratio between elements: [(Al (%) / 27) / (N (%) / 14)]

division Used steel grade Reheating temperature (℃) Finishing temperature (℃) Cooling rate (° C / s) Coiling temperature (캜) Shotball diameter (mm) Blasting speed (m / s) Inventory 1 Inventive Steel 1

1250 890 60 600 0.25 50 Inventive Example 2 1250 890 70 650 0.30 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 (%)
Processability
Bending property Inventory 1 30.2 Not occurring 33.1 Good Good Inventive Example 2 33.6 Not occurring 34.8 Good Good Inventory 3 32.9 Not occurring 32.4 Good Good Honorable 4 31.8 Not occurring 36.8 Good Good Inventory 5 34.2 Not occurring 35.4 Good Good Inventory 6 32.0 Not occurring 37.1 Good Good Honorable 7 30.8 Not occurring 30.4 usually Good Inventive Example 8 32.6 Not occurring 28.9 usually Good Proposition 9 32.5 Occur 26.7 usually Good Comparative Example 1 39.6 Not occurring 16.4 Bad usually Comparative Example 2 37.4 Occur 38.9 Bad Bad Comparative Example 3 38.6 Occur 23.9 Bad Bad Comparative Example 4 24.5 Occur 18.8 Bad Bad

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

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 If the number of microscopic cracks on the surface is 4 or more or the sum of the total crack length is within 10 mm Step 5 Cracks occur, and the total length of the cracks is 10 mm or more

Inventive Examples 1 to 6 are cases in which both the steel component control, hot rolling process and shot blasting process conditions according to the present invention are satisfied.

In all the invention examples, no yield point stretching occurred, and the surface roughness index ratio (Rmax / Ra) was also in the range of 30 to 35, which satisfies the management range and no bending occurs 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 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, the shot ball diameter is 0.91 mm larger than the control range under shot blasting conditions, and the shot blasting speed is also performed at 90 m / sec larger than the control range. In this case, the elongation at yield point does not occur, which is good in terms of durability. However, both workability and paintability are reduced due to increase in surface roughness and internal hardening layer, which is unsuitable for use as a hot-rolled steel sheet requiring high quality.

Comparative Examples 2 to 4 are steel sheets (Comparative Steels 1 to 3) 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. In most cases, it was difficult to suppress the yield point stretching phenomenon, and the aging resistance was lowered, and the bending and phenomena at the time of processing did not satisfy the processing and coating properties.

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 30 to 35, the fluting index and cracking sensitivity are excellent. In contrast, both the fluting index and the machinability index exceed the threshold in the region below the above control range, causing problems such as bending and processing cracks. In the region above the above management range, the aging resistance indicates the saturation value. The problem that the processing cracks could be confirmed.,

While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims It will be apparent to those of ordinary skill in the art.

Claims (7)

By weight%, carbon (C): 0.10 to 0.25%, manganese (Mn): 0.1 to 0.3%, aluminum (Al): 0.02 to 0.07%, nitrogen (N): 0.001 to 0.004%, the rest iron (Fe) And shot blasting by spraying a shot ball having a diameter of 0.2 to 0.35 mm at a speed of 50 to 60 m / sec to a surface of a hot rolled steel sheet made of steel containing other unavoidable impurities. Excellent medium carbon hot rolled steel sheet manufacturing method.
The method according to claim 1,
The steel component is 5≤ [(Al (%) / 27) / (N (%) / 14)] ≤15, characterized in that the medium-carbon hot rolled steel sheet excellent in age resistance.
The method according to any one of claims 1 to 3,
The hot rolled steel sheet is a finish rolled steel of the composition at 850 ~ 900 ℃,
Cooling the finish-rolled steel at a cooling rate of 60 ~ 100 ℃ per second,
Method for producing a medium-carbon hot rolled steel sheet excellent in aging resistance, characterized in that the winding after the cooling at a temperature of 550 ~ 650 ℃.
The method according to claim 1,
The surface roughness index ratio (Rmax / Ra) of the steel, characterized in that 30 to 35, the method of producing a medium-carbon hot rolled steel sheet excellent in aging resistance.
By weight%, carbon (C): 0.10 to 0.25%, manganese (Mn): 0.1 to 0.3%, aluminum (Al): 0.02 to 0.07%, nitrogen (N): 0.001 to 0.004%, the rest iron (Fe) And it is made of steel containing other unavoidable impurities, characterized in that the surface roughness index ratio (Rmax / Ra) of 30 to 35, excellent aging resistance medium carbon hot rolled steel sheet.
The method according to claim 5,
The component of the steel is 5≤ [(Al (%) / 27) / (N (%) / 14)] ≤15, characterized in that the medium-carbon hot rolled steel sheet excellent in aging resistance.
In claim 5,
The deformed ferrite structure produced by shot blasting treatment occupies 5 to 15% of the cross section in all thickness directions of the hot rolled steel sheet.

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