KR101657812B1 - Hot rolled steel plate having excellent pipe expansibility and method for manufacturing the same - Google Patents

Abstract

In one aspect of the present invention, one embodiment of the present invention is characterized in that it comprises 0.02 to 0.05% of C, 0.1 to 0.3% of Si, 0.5 to 1.0% of Mn, 0.02% or less of P (excluding 0) (Excluding 0) of Al, 0.02 to 0.04% of Al, 0.01 to 0.03% of Nb, 0.005 to 0.02% of Ti, 0.01 to 0.3% of Cr, the balance of Fe and other unavoidable impurities, A hot-rolled steel sheet having an effective grain size in the range satisfying the following conditional expression and having excellent drivability.
1.5 < Cr + Si + 2Mn < 2.5
The hot-rolled steel sheet may have a uniform elongation of 15% or more and a work hardening index of 0.15 or more.

Description

TECHNICAL FIELD [0001] The present invention relates to a hot rolled steel sheet having excellent durability and a method for manufacturing the hot rolled steel sheet.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet and a method of manufacturing the same, and more particularly to a hot-dip galvanized steel sheet for a rolling mill having a uniform elongation and a work hardening index.

Recently, major oil companies in the world have attempted a simpler and less costly drilling method by applying the expansion technique of pipe for mining to reduce crude oil and gas development costs. Pipes are partially applied up to 5 km down from the upper part of the oilfield, and require excellent drivability.

It is known that excellent dewaxing ability can be obtained by high uniform elongation and work hardening index, and usually seamless pipes are mainly used. However, the seamless pipe is very expensive compared to ERW (Electric Resistance Welding) steel pipe, has a large thickness deviation, and has a poor roundness. Recently, there is a move to replace steel pipe for expansion with ERW pipe.

Accordingly, the major oil companies are looking for suitable materials to replace steel pipes for expansion with ERW pipes, and the material properties at these centers are high uniform elongation and work hardening index. In addition, the lengthwise material deviation must be minimized because several kilometers of pipe must exhibit a uniform overall ductility.

Conventionally, as a method for obtaining a high uniform elongation and a work hardening index, by increasing the dislocation density of the steel itself, the driving elongation at the time of tensile deformation is increased to increase the uniform elongation or to form an abnormal structure of ferrite, pearlite, ferrite and bainite Thereby causing a large amount of work hardening in the light pearlite and bainite to induce a high work hardening index.

However, the above-mentioned prior arts rarely satisfy a high uniform elongation and a high work hardening index at the same time, and since a strain-based design steel manufacturing technique is usually applied to high-strength steels, There is a possibility that the deviation becomes large.

One of the technical problems of the present invention is to provide a hot dip galvanized steel sheet having excellent uniform elongation and work hardening index and a manufacturing method thereof.

In one aspect of the present invention, one embodiment of the present invention is characterized in that it comprises 0.02 to 0.05% of C, 0.1 to 0.3% of Si, 0.5 to 1.0% of Mn, 0.02% or less of P (excluding 0) (Excluding 0) of Al, 0.02 to 0.04% of Al, 0.01 to 0.03% of Nb, 0.005 to 0.02% of Ti, 0.01 to 0.3% of Cr, the balance of Fe and other unavoidable impurities, A hot-rolled steel sheet having an effective grain size in the range satisfying the following conditional expression and having excellent drivability.

1.5 < Cr + Si + 2Mn < 2.5

The hot-rolled steel sheet may have a uniform elongation of 15% or more and a work hardening index of 0.15 or more.

Another aspect of the present invention is to provide a method for producing a steel sheet, which comprises 0.02 to 0.05% of C, 0.1 to 0.3% of Si, 0.5 to 1.0% of Mn, 0.02% or less of P (excluding 0) Refining the molten steel having a composition including 0.02 to 0.04% of Al, 0.01 to 0.03% of Nb, 0.005 to 0.02% of Ti, 0.01 to 0.3% of Cr, balance Fe and other unavoidable impurities, The slab is reheated at a temperature of 1250 to 1350 DEG C to obtain a hot-rolled steel sheet by finishing hot-rolling the slab reheated at an Ar3 temperature to a non-recrystallization temperature; The method of manufacturing a hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet is cooled at an Ar3 temperature to a non-recrystallization temperature and then rolled after completion at 500 to 600 占 폚. The composition satisfies conditional expressions 1.5 <Cr + Si + 2Mn < The present invention also provides a method of manufacturing a hot-rolled steel sheet having an effective crystal grain size in the range of 20 to 20 mu m.

According to one embodiment of the present invention, it is possible to provide a hot-rolled steel sheet and a manufacturing method which have high uniform elongation and high work hardening index, and which are excellent in heat-expandability. In particular, it is possible to provide an excellent hot dip galvanized steel sheet which can be advantageously used in oil and gas mining sites due to its small size deviation in longitudinal direction, and thus its drivability.

The various and advantageous advantages and effects of the present invention are not limited to the above description, and can be more easily understood in the course of describing a specific embodiment of the present invention.

1 is a flowchart for explaining an example of a method for manufacturing a hot-rolled steel sheet according to the present invention.
FIGS. 2A and 2B are optical micrographs of microstructures of Invention Steel 2 and Comparative Steel 1, respectively, according to an embodiment of the present invention.
3 is a graph showing tensile curves of Inventive Example 1 according to an embodiment of the present invention.
4 is a graph showing the longitudinal yield strength distribution after manufacturing the ERW pipe of Inventive Example 1 according to the embodiment of the present invention.

Hereinafter, various embodiments of the present invention will be described in detail.

The embodiments described below may be modified or combined with other embodiments, and the scope of the present invention is not limited to the specific embodiments described below. Further, the embodiments are provided so that those skilled in the art can more fully understand the present invention. The terms " an embodiment "and &quot; an embodiment &quot; used in the specification do not denote the same embodiments, but are provided to emphasize and describe different features.

The present invention provides, as one embodiment, a ferritic stainless steel comprising 0.02 to 0.05% of C, 0.1 to 0.3% of Si, 0.5 to 1.0% of Mn, 0.02% or less of P (excluding 0) , The balance of Fe and other unavoidable impurities, and is effective in the range of 10 to 20 占 퐉. Provided is a hot-rolled steel sheet having a grain size satisfying the following condition:

1.5 < Cr + Si + 2Mn < 2.5

Hereinafter, the component system of the present invention will be described.

C: 0.02 to 0.05 wt%

C is the most economical and effective alloying element for strengthening the steel. However, when C is added in an amount of 0.02% by weight or less, the effect of strengthening the steel by binding with Nb is very small. When the amount of C is more than 0.05% by weight, the amount of pearlite is increased or a low temperature transformation phase And there is a problem that the sensibility is lowered. Therefore, it is preferable that C is in the range of 0.02 to 0.05 wt%.

Si : 0.1 to 0.3 wt%

The Si is effective for deoxidation and solid solution strengthening, and it is preferably added in an amount of 0.1 wt% or more for the above effect. However, if it exceeds 0.3% by weight, the weldability and the brittleness are lowered. Therefore, the Si content is preferably in the range of 0.1 to 0.3% by weight.

Mn : 0.5 to 1.0 wt%

When Mn is added in an amount of less than 0.5% by weight, it is difficult to secure strength and toughness. When Mn is more than 1.0% by weight, the hardening ability of the Mn is increased. As a result, There is a problem that the dispersibility is poor. Therefore, it is preferable that the Mn is in the range of 0.5 to 1.0 wt%.

P: not more than 0.02 wt% (excluding 0)

When the content of P exceeds 0.02 wt%, the central segregation is promoted along with Mn at the time of playing to lower the toughness as well as the weldability, so that the content of P is preferably controlled to 0.02 wt% or less .

S: 0.003 wt% or less (excluding 0)

When S is more than 0.003% by weight, S reacts with Mn to form MnS, thereby lowering toughness and also lowering the weldability. Therefore, It is preferable to control the content of S to 0.03% by weight or less.

Al : 0.02 to 0.04 wt%

When Al is added as the deoxidizing component together with Si, it is difficult to obtain a deoxidation effect when it is added in an amount of less than 0.02% by weight. When it exceeds 0.04% by weight, the amount of the alumina aggregate is increased to lower the weldability. Is controlled to be in the range of 0.02 to 0.04% by weight.

Nb : 0.01 to 0.03 wt%

Nb is a component exhibiting a precipitation strengthening effect by the addition of a small amount. For the above effect, it is necessary to include at least 0.01 wt%. In the carbon range of the present invention, when Nb is 0.03 wt% or more, the increase in strength due to precipitation strengthening is not large. It is preferable to control the content to 0.03 wt% or less. Therefore, it is preferable that the content of Nb is in the range of 0.01 to 0.03% by weight.

Ti : 0.005 to 0.02 wt%

The Ti is precipitated as TiN in the steel and inhibits the growth of austenite grains during reheating, so that high strength and excellent impact toughness can be obtained, and Ti is precipitated and strengthened the steel. However, in order to obtain the above effect in the carbon range of the present invention, the Ti content needs to be 0.005 wt% or more. On the other hand, when the content of Ti exceeds 0.02% by weight, the effect becomes saturated, so that the content of Ti is preferably controlled to 0.005 to 0.02% by weight.

Cr : 0.01 to 0.3 wt%

The Cr is added to increase strength and ensure corrosion resistance. However, when the Cr content is less than 0.01% by weight, the effect is small. When the Cr content is more than 0.3% by weight, a low temperature transformation phase which is not sought in the present invention is generated. Therefore, it is preferable to control the Cr content to 0.01 to 0.3 wt%.

The hot-rolled steel sheet according to one embodiment of the present invention is composed of Fe and other unavoidable impurities in addition to the above-mentioned composition.

Satisfies the conditional expression 1.5 < Cr + Si + 2Mn < 2.5 in the composition range according to the present embodiment. Here, Cr, Si and Mn mean weight percent of each component. The above equation can be understood as an empirical equation obtained through several experiments.

When the value of the conditional expression is less than 1.5, it is difficult to sufficiently secure the strength, and the effect of work hardening is insufficient. In this case, when the ratio exceeds 2.5, the generation of the low temperature transformation phase is promoted and the strength and work hardening effect are increased But it has a disadvantage in that the uniform elongation becomes small, and it is difficult to secure the drivability pursued by the present invention. The microstructure according to the present embodiment has an effective grain size in the range of 10 mu m to 20 mu m.

Preferably, the main structure of the hot-rolled steel sheet according to the present invention may have a pearlite content of not more than 2% by area in the ferrite base structure.

The hot-rolled steel sheet according to the present embodiment can have a work hardening index of 0.15 or more with a uniform elongation of 15% or more. The hot-rolled steel sheet may have a yield strength of 300 to 450 MPa.

In the hot-rolled steel sheet according to the present embodiment, the longitudinal strength material deviation after the ERW pipe is 20 MPa or less, and the DSF measured through water pressure may be 20% or more.

As another aspect of the present invention, it is possible to provide a method for manufacturing the hot rolled steel sheet described above. Hereinafter, an example of a method of manufacturing a hot-rolled steel sheet according to the present invention will be described in detail.

Referring to FIG. 1, the method of manufacturing a hot-rolled steel sheet according to this embodiment can start with the step (S11) of refining molten steel that satisfies the component system of the hot-rolled steel sheet described above.

As described above, the constituent system of the molten steel includes 0.02 to 0.05% of C, 0.1 to 0.3% of Si, 0.5 to 1.0% of Mn, 0.02% or less of P (exclusive of 0%), S of 0.003 , Ti: 0.001 to 0.02%, Cr: 0.01 to 0.3%, and the balance of Fe and other unavoidable impurities. The composition range satisfies the conditional expression 1.5 < Cr + Si + 2Mn < 2.5.

The control of the nonmetallic inclusions according to the present invention can be obtained through control of the process conditions in a typical secondary refining process, for example, the secondary refining process can be carried out by using Ar bubbling in LF and degassing such as VTD or RH In the process, the inclusions can be controlled by Ar bubbling. Of course, the manufacturing method of the present invention is not necessarily limited to the above process conditions, and non-metallic inclusions can be controlled by various methods.

Then, the refined molten steel is continuously cast to form a slab (S12), and the slab can be reheated (S14).

The reheating temperature applied in this process is determined by the solidus temperature of the Nb-based precipitates. In the range of the present invention, all of them can be used at a temperature higher than 1150 ° C. However, when heated at a temperature lower than 1250 ° C., It is difficult to secure a uniform elongation. When the steel sheet is heated to a temperature exceeding 1350 占 폚, the grain size of the steel sheet becomes excessively larger than necessary, and the toughness and work hardening index rapidly decrease. Therefore, the reheating temperature range is preferably in the range of 1250 to 1350 占 폚 Do.

Next, in order to obtain a hot-rolled steel sheet, the reheated slab can be subjected to finish hot rolling at a temperature from Ar3 to a non-recrystallization temperature (S15).

The amount of reduction at a temperature lower than the non-recrystallization temperature may greatly affect the grain size and uniformity of the hot-rolled steel sheet microstructure. The grain size and uniformity affect the overall material deviation and are highly correlated with the densifying ability. Therefore, in order to control the grain size and the uniformity, it is preferable that the rolling reduction rate is 70% or more. If the reduction rate is less than 70%, the homogeneity of the crystal grain size may be deteriorated and the deviation of the strength material may be large. Therefore, it is preferable that the reduction ratio is in the range of the maximum reduction ratio of the thickness at 70% or more.

On the other hand, the finish hot rolling can be performed in the range of the Ar3 temperature to the non-recrystallization temperature. If rolling at a non-recrystallization temperature, there is a high possibility that uneven and coarse grain growth may occur, adversely affecting the toughness and work hardening index, and in the event that the finish hot rolling is performed in a temperature range of less than Ar3, A low temperature transformation phase may be generated and the drivability may be very low.

Then, the hot-rolled steel sheet is cooled to a temperature of Ar3 or higher and cooled to 500 to 600 ° C (S17).

It is preferable that the hot-rolled steel sheet obtained through the preceding hot-rolling step starts cooling at an Ar3 temperature or higher. If the cooling is started at a temperature lower than Ar3, coarse ferrite may be formed before cooling to lower the bulkability. Therefore, it is preferable to start cooling at the Ar3 temperature or higher.

On the other hand, the cooling rate is preferably in the range of 10 to 30 DEG C / sec. When the cooling rate is less than 10 ° C / sec, the area fraction of the pearlite disclosed in the present invention increases more than necessary. When the cooling rate is more than 30 ° C / sec, .

Thus, the present cooling step can be finished at 500 to 600 DEG C, and then the hot rolled steel sheet cooled in this temperature range can be wound (S19).

If the coiling temperature range is higher than 600 ° C, transformation is unstable and coarse pearlite can be easily generated, which may cause a variation in the strength material and deterioration of the ductility. If the coiling temperature is lower than 500 ° C, It is difficult to secure the sensibility. Therefore, it is preferable that the winding is performed in a temperature range of 500 to 600 캜.

The hot-rolled steel sheet thus obtained contains 2 percent by area or less of perlite in the ferrite matrix structure. The hot-rolled steel sheet may have a yield strength of 300 to 450 MPa. The hot-rolled steel sheet may have a uniform elongation of 15% or more and a work hardening index of 0.15 or more.

In the hot-rolled steel sheet according to the present embodiment, the longitudinal strength material deviation after the ERW pipe is 20 MPa or less, and the DSF measured through water pressure may be 20% or more.

Hereinafter, the operation and effect of the present invention will be described in more detail with reference to specific examples of the present invention. However, the following examples are only illustrative of the present invention in detail and do not limit the scope of the present invention.

( Example )

(Invention steels 1 and 2, Comparative steels 1 to 4) having the composition shown in the following Table 1 were refined to control the nonmetallic inclusions, and then, the thicknesses of 4.8 mm and 7.7 mm Hot-rolled steel sheets (Inventive Examples 1 and 2, Comparative Examples 1 to 7) were produced.

division C Si Mn P S Al Ti Nb Cr Conditional expression value Inventive Steel 1 0.045 0.2 0.9 0.015 0.0015 0.025 0.01 0.02 0.08 2.08 Invention river 2 0.035 0.24 0.9 0.013 0.0021 0.027 0.01 0.03 0.2 2.24 Comparative River 1 0.06 0.23 1.2 0.008 0.0007 0.028 0.01 0.04 0.2 2.83 Comparative River 2 0.05 0.25 1.0 0.007 0.0009 0.03 0.01 0.03 0.3 2.55 Comparative Steel 3 0.02 0.18 0.5 0.011 0.0012 0.027 0.01 0.05 0.03 1.21 Comparative Steel 4 0.04 0.2 0.8 0.025 0.0035 0.025 0.01 0.02 0.07 1.87

Referring to Table 1, inventive steels 1 and 2 satisfy the composition conditions shown in the present invention, while comparative steels 1 to 4 do not satisfy the composition conditions. For example, in the case of Comparative Steel 1, the contents of C, Mn and Nb exceed the conditions of the present invention, and in Comparative Steel 3, the content of Nb is not satisfied. In the case of comparative steel 2, the content of each component satisfied the composition condition of the present invention, but the conditional expression value (2.55) exceeded 2.5, which did not satisfy the conditional expression value according to the present invention.

division Grade Nr. Reheating temperature (℃) Finishing hot rolling temperature (캜) Coiling temperature (캜) Inventory 1 Inventive Steel 1 1305 867 551 Inventory 2 Invention river 2 1278 856 521 Comparative Example 1 Comparative River 1 1288 889 564 Comparative Example 2 Comparative River 2 1311 894 576 Comparative Example 3 Comparative Steel 3 1261 881 586 Comparative Example 4 Comparative Steel 4 1272 861 532 Comparative Example 5 Inventive Steel 1 1313 932 631 Comparative Example 6 Invention river 2 1223 776 447 Comparative Example 7 Invention river 2 1370 951 653

The pearlite area fraction, the effective grain size, the yield strength, the uniform elongation and the work hardening index were measured for the hot-rolled steel sheets thus produced (Examples 1 and 2 and Comparative Examples 1 to 7), and an ERW pipe having a diameter of 5.5 & The results are shown in Table 3 below. [Table 3] &lt; tb &gt; &lt; TABLE &gt;

Specifically, the pearlite area fraction was measured at 500 times magnification using an optical microscope, and the effective grain size was determined at a magnification of 1000 times using EBSD. The yield strength, the uniform elongation, and the work hardening index were measured by a room temperature tensile test. In addition, after ERW pipe is tightened, it is cut to a length of 6 m to cover both holes of the pipe and increase the water pressure in the pipe to measure up to several percent of the pipe (hereinafter referred to as pipe pipe ratio) Respectively.

division surrender
burglar
(MPa) Perlite area fraction
(%) Effective Grain Size (탆) Uniformity
Elongation (%) Work hardening index Longitudinal pipe yield strength
Deviation (MPa) Pipe Flow Rate (%) Inventory 1 389 1.1 13 17.6 0.175 ± 9 26.5 Inventory 2 411 0.8 12 16.4 0.171 ± 15 23.1 Comparative Example 1 472 1.3 8 11.3 0.112 ± 45 12.4 Comparative Example 2 435 1.4 10 12.6 0.134 ± 36 15.4 Comparative Example 3 332 0.4 17 18.4 0.096 ± 23 16.3 Comparative Example 4 402 0.9 12 16.8 0.168 ± 16 19.2 Comparative Example 5 378 0.7 15 15.9 0.134 ± 19 18.1 Comparative Example 6 423 0.3 8 7.4 0.156 ± 34 13.6 Comparative Example 7 356 0.4 24 18.1 0.0102 ± 27 17.3

Referring to Table 3, it was confirmed that some of the hot rolled steel sheets (Comparative Examples 1 to 4) produced using the comparative steels outside the composition conditions according to the present invention did not satisfy desired characteristics. Particularly, in the case of Comparative Example 4, although the other results are shown in a preferable range, the pipe expansion ratio obtained by evaluating actual pipe pipe densities is less than 20%. This is understood to be the result of the P and S contents being excessively contained in the case of Comparative Example 4 and the toughness being lowered due to the reactant with Mn.

On the other hand, in the hot-rolled steel sheets (inventive examples 1 and 2) produced using inventive steels satisfying the composition conditions according to the present invention, it was confirmed that all measurement results satisfied the conditions of the present invention. The microstructures contained less than 2% by area of pearlite in the ferrite matrix and the effective grain size ranged from 10 탆 to 20 탆. For example, referring to FIG. 2A, it can be seen that the microstructure of Inventive Example 2 has a pearlite fraction of 0.8 area% and an effective grain size of 12 μm. On the other hand, in the case of the microstructure of Comparative Example 1, as shown in FIG. 2B, it can be seen that the relatively effective grain size (8 μm) is relatively small.

Fig. 3 shows a tensile curve of the hot-rolled steel sheet according to Inventive Example 1. Fig. As can be seen from Table 3 together with Fig. 3, the hot-rolled steel sheet according to Inventive Examples 1 and 2 had a yield strength in the range of 300 to 450 MPa, a uniform elongation and a work hardening index of 15% or more and 0.15 or more .

Fig. 4 shows the result of measuring the yield strength along the longitudinal direction after the ERW pipe is routed using the hot-rolled steel sheet of Inventive Example 1. Fig. The specimens were measured at the end, at the end, and at the rear end, respectively, and the yield strength along the longitudinal direction was measured. The most significant deviation was about 9Mpa low. Similarly, the longitudinal yield strength material deviation of the ERW pipe obtained from the hot-rolled steel sheet according to Inventive Example 2 was 15 MPa, indicating that there was no large deviation (within 20 MPa). In addition, as shown in Table 3 above, the densities measured by hydraulic pressure for Inventive Examples 1 and 2 were 26.5% and 23.1%, respectively, indicating high densities (20% or more).

On the other hand, Inventive steels 1 and 2 satisfying the composition conditions were used in Comparative Examples 5 to 7. In Comparative Examples 6 and 7, reheating temperatures were set at 1223 ° C and 1370 ° C, respectively. That is, reheating was performed at a temperature condition lower or higher than a preferable temperature condition (1250 to 1350 ° C). Further, in Comparative Examples 5 and 7, the winding step was carried out at a high temperature condition of 631 ° C and 653 ° C, respectively, without cooling sufficiently at a preferable temperature condition (500 to 600 ° C). As a result, in the case of Comparative Example 5, the work hardening index and the pipe expansion ratio were low. In Comparative Example 6, the uniform elongation was low and the yield strength deviation along the length direction was large. In the case of Comparative Example 7, not only the work hardening index and the pipe expansion ratio, but also the yield strength deviation along the length direction were also inferior.

As described above, the reheating temperature is preferably 1250 to 1350 ° C., and it is preferable that the hot-rolled steel sheet is cooled to a temperature in the range of 500 to 600 ° C. to perform the winding step. Further, as shown in Table 2, the finish hot rolling temperature is preferably in the Ar3 temperature range to the non-recrystallization temperature range.

The present invention is not limited by the above-described embodiments and the accompanying drawings, but is intended to be limited only by the appended claims. It will be apparent to 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 appended claims. something to do.

Claims (6)

(Excluding 0), S: not more than 0.003% (excluding 0), Al: 0.02 to less than 0.02%, C: 0.02 to 0.05%, Si: 0.1 to 0.3%, Mn: 0.04% of Nb, 0.01 to 0.03% of Nb, 0.005 to 0.02% of Ti, 0.01 to 0.3% of Cr, the balance of Fe and other unavoidable impurities. The effective grain size is in the range of 10 탆 to 20 탆, A hot-rolled steel sheet having excellent heat-releasing ability, comprising perlite in an amount of 2% or less by area in a ferrite matrix structure, satisfying the following conditional expression, having a uniform elongation of 15% or more and a work hardening index of 0.15 or more.
1.5 < Cr + Si + 2Mn < 2.5
delete The method according to claim 1,
The hot-rolled steel sheet has excellent yield strength with a yield strength of 300 to 450 MPa.
delete (Excluding 0), S: not more than 0.003% (excluding 0), Al: 0.02 to less than 0.02%, C: 0.02 to 0.05%, Si: 0.1 to 0.3%, Mn: 0.04%, Nb: 0.01 to 0.03%, Ti: 0.005 to 0.02%, Cr: 0.01 to 0.3%, the balance Fe and other unavoidable impurities;
Continuously casting the refined molten steel to produce a slab;
Reheating the slab at 1250-1350 占 폚;
Subjecting the reheated slab to finish hot rolling at an Ar3 temperature to a non-recrystallization temperature to obtain a hot-rolled steel sheet; And
Cooling the hot-rolled steel sheet at an Ar3 temperature to a non-recrystallization temperature, winding the hot-rolled steel sheet after completion at 500 to 600 deg. C,
Wherein the composition satisfies the conditional expression 1.5 < Cr + Si + 2Mn < 2.5, and the hot-rolled steel sheet has an effective grain size in the range of 10 m to 20 m.
6. The method of claim 5,
Wherein the cooling process is performed at a rate of 10 to 30 DEG C / sec.

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