KR101889173B1 - High strength fine spheroidal graphite steel sheet having low yield ratio and manufacturing method thereof - Google Patents

Abstract

0.001 to 0.02% of P, 0.01% or less of S, 0.01 to 0.1% of Al, 0.01 to 0.1% of Cr, 0.1 to 1.5% of Cr, 0.10 to 0.40% of C, , Residual ferrite and spheroidized carbide containing 10 to 30% by area of the microstructure and containing the remainder Fe and unavoidable impurities, in a proportion of 0.01 to 0.5%, Mo: 0.01 to 0.5%, Cu: 0.01 to 0.4%, Ni: 0.01 to 0.4% And a method for producing the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength multi-spheroidal steel sheet and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

More particularly, the present invention relates to a high strength and low abrasion type micro spheroidal steel sheet which can be suitably used for manufacturing a tubular tube, and a method of manufacturing the same.

Generally, high strength, toughness, and delayed fracture resistance are required for well pipe steel pipes. Among them, when used at a depth of 500 m or more, low resistance and high tensile strength are required. In particular, fine spheroidal steels used as fluid tube supports, joints, and other components require high moldability and processability, and therefore should have high strength resistance and moldability.

In relation to this, Patent Document 1 discloses a steel material for a high-strength and low-refractory steel pipe, but has a too high yield strength and tensile strength and a somewhat higher yielding ratio to be applied to the same parts such as a fluid tube support. In Patent Document 2, a ferrite fraction of about 80% is secured and a yield ratio of 0.7 or less is realized. However, in order to obtain wear resistance and corrosion resistance, improvement in terms of composition and microstructure is required.

Korean Patent Laid-Open Publication No. 2013-0105008 Korean Patent Laid-Open Publication No. 2015-0004430

One of the objects of the present invention is to provide a high strength and low abrasion resistance microspheroidal steel sheet and a method of manufacturing the same.

An aspect of the present invention is a steel sheet comprising, by weight, 0.10 to 0.40% of C, 0.1 to 1.5% of Mn, 0.005 to 0.3% of Si, 0.005 to 0.02% of P, 0.01% 0.01 to 0.4% of Ni, 0.01 to 0.4% of Ni, 0.01 to 0.4% of Cr, 0.01 to 1.5% of Cr, 0.01 to 0.5% of Mo, 0.01 to 0.4% of Cu, 0.01 to 0.4% of Cu, There is provided a fine spheroidized steel sheet comprising a remainder ferrite and a spheroidized carbide.

Another aspect of the present invention is to provide a method of manufacturing a semiconductor device, comprising: 0.10 to 0.40% of C, 0.1 to 1.5% of C, 0.005 to 0.3% of Si, 0.005 to 0.02% 0.01 to 0.4% of Cu, 0.01 to 0.4% of Ni, 0.01 to 0.4% of Cr, 0.01 to 1.5% of Cr, 0.01 to 0.5% of Mo, 0.01 to 0.4% of Cu, 0.01 to 0.4% of Cu, 0.01 to 0.4% of Ni, and a balance of Fe and unavoidable impurities. Preparing a hot rolled steel sheet comprising the steel sheet and spheroidizing the hot rolled steel sheet at 650 to 780 DEG C to obtain a micro-spheroidized steel sheet containing 10 to 30% by area of pearlite, residual ferrite and spheroidized carbide in a microstructure And a method for manufacturing a micro-spheroidized steel sheet.

As one of various effects of the present invention, the steel sheet according to the present invention has high strength and low resistance, and is excellent in moldability, abrasion resistance and corrosion resistance, and thus can be suitably used in the manufacture of a tubulation.

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.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph showing microstructure of Inventive Example 1. FIG.
2 is a photograph of the microstructure observed in Comparative Example 6. Fig.
3 is a photograph of the microstructure observed in Comparative Example 7. Fig.

Hereinafter, a high-strength and low-resistance micro-spheroidal steel sheet according to one aspect of the present invention will be described in detail.

First, the alloy component and the preferable content range of the fine spheroidized steel sheet will be described in detail. It is to be noted that the content of each component described below is based on weight unless otherwise specified.

C: 0.10 to 0.40%

Carbon is an element that affects strength and toughness. When the carbon content is less than 0.10%, it is difficult to secure the target strength. On the other hand, when the carbon content exceeds 0.40%, the toughness and formability are deteriorated due to excessive strength increase and cementite formation. Therefore, the carbon content is preferably limited to 0.10 to 0.40%.

Mn: 0.1 to 1.5%

Manganese is added as a solid solution strengthening element to increase strength and to prevent embrittlement of embrittlement by formation of FeS. In order to obtain such an effect in the present invention, at least 0.1% should be added. On the other hand, if it is contained in excess of 1.5%, center segregation and micro segregation become severe and the final carbide becomes coarse. This may hinder moldability and weldability, so the manganese content is limited to 0.1 to 1.5%.

Si: 0.005 to 0.3%

Silicon has an effect of improving strength by solid solution strengthening. If it is less than 0.005%, the effect of improving the strength is insufficient, and when it is added in excess of 0.3%, the increase of the red scale defects may adversely affect the surface quality. Therefore, the content of silicon is preferably 0.005 to 0.3%.

P: 0.005 to 0.02%

Phosphorus is a strong element of employment enhancement. In order to secure the strength, 0.005% or more of phosphorus should be added. On the other hand, if it exceeds 0.02%, there is a problem that the impact characteristic is disadvantageously limited so that the lower limit and the upper limit are limited to 0.005% and 0.02%, respectively.

S: not more than 0.01%

Sulfur is an impurity inevitably contained in the steel, and it is necessary to control the content as low as possible to increase the amount of the precipitate as an element which is apt to form a nonmetallic inclusion. In the present invention, the upper limit is controlled to 0.01%.

Al: 0.01 to 0.1%

Aluminum is mainly added to deoxidize and to trap nitrogen with AlN. If the aluminum content is less than 0.01% and the above-mentioned purpose can not be achieved, and if the content is more than 0.1%, excessive strength increase and slab defect may occur during performance, so the content is limited to 0.01 to 0.1%.

Cr: 0.01 to 1.5%

Chromium contributes to spheroidization and fine dispersion of carbide, improving the ingotability of steel and enhancing solubility. In order to obtain such effects in the present invention, 0.01% or more should be added. On the other hand, if it is added in excess of 1.5%, center segregation and unnecessary inclusions can be formed, so it is preferable to limit the upper limit to 1.5%.

Mo: 0.01 to 0.5%

Molybdenum has the effect of improving strength and improving corrosion resistance. In order to obtain such effects in the present invention, 0.01% or more should be added. On the other hand, if it is added excessively, an unnecessary secondary phase may be formed to weaken the toughness, and the cost may be increased by expensive elements, which may adversely affect the cost, so that the upper limit is limited to 0.5%.

Cu: 0.01 to 0.4%

Copper has a great effect on improving corrosion resistance. In order to obtain such effects in the present invention, 0.01% or more should be added. On the other hand, if it is added excessively, it will have an adverse effect on toughness, and it will limit the upper limit to 0.4% because it can impose an increase in cost due to expensive elements.

Ni: 0.01 to 0.4%,

Nickel has a great effect on improving the toughness of steel. In order to obtain such an effect in the present invention, 0.01% or more of nickel should be added. On the other hand, when nickel is added excessively, nickel is an expensive element.

The rest of the composition is Fe. However, it is not possible to exclude inevitable impurities that are not intended from the raw material or the surrounding environment in a conventional manufacturing process, since they may be inevitably incorporated. These impurities are not specifically referred to in this specification, as they are known to one of ordinary skill in the art.

Hereinafter, the microstructure of the micro-spheroidized steel sheet of the present invention will be described in detail.

The microspheroidal steel sheet of the present invention contains 10 to 30 area% of pearlite, residual ferrite and spheroidized carbide in its microstructure. When pearlite is not transformed into 100% spheroidized structure and pearlite of 10 to 30 area% remains, not only the tensile strength is secured but also the yield strength is lowered compared with the tensile strength. It is advantageous to realize a low resistance. A more preferable range of the pearlite area ratio is 15 to 28 area%.

According to one example, the average grain size of the ferrite may be between 5 and 30 mu m. If the ferrite grain size is less than 5 탆, it may be disadvantageous to the low-resistance ferrite. If the ferrite grain size is more than 30 탆, a decrease in tensile strength may be caused.

According to one example, the average grain size of the spheroidized carbide may be from 0.2 to 0.8 mu m, more preferably from 0.5 to 0.8 mu m. The average grain size of the spheroidized carbide depends on the annealing time of the spheroidizing annealing. If the average grain size of the spheroidized carbide is too small, the ductility may be lowered. On the other hand, if the average grain size is too large, the tensile strength may be lowered.

Here, the average grain size means an equivalent circular diameter of the particles detected by observing one end face of the micro-spheroidized steel sheet.

In the present invention, the thickness of the steel sheet is not particularly limited, but may be 3.0 to 6.0 mm, for example.

The micro-spheroidized steel sheet of the present invention has high strength and low resistance properties, and according to one non-limiting example, the micro-spheroidized steel sheet of the present invention has a tensile strength of 500 to 600 MPa (more preferably, a tensile strength of more than 550 MPa and 600 MPa or less) A yield strength of 250 to 480 MPa and a yield ratio of 0.6 to 0.8.

The micro-spheroidized steel sheet of the present invention has an advantage of excellent formability. According to one non-limiting example, the micro-spheroidized sheet of the present invention can have a hardness of 179 HV or less and an elongation of 25% or more.

When the micro-spheroidized steel sheet of the present invention is immersed in a 50 vol% sulfuric acid aqueous solution for 1 hour according to ASTM-G31, the micro-spheroidized steel sheet of the present invention has an advantage of abrasion resistance and corrosion resistance. The corrosion reduction rate may be 20 mg / cm ² / hr or less.

The above-described fine spheroidized steel sheet of the present invention can be produced by various methods, and the production method thereof is not particularly limited. However, as a preferable example, it can be produced by the following method.

Hereinafter, a method of manufacturing a high strength and low-resistance micro-spheroidal steel sheet according to another aspect of the present invention will be described in detail.

Slab reheat step

First, a slab having the above-mentioned component system is prepared and then reheated to a temperature suitable for the subsequent hot rolling process. In this case, the slab reheating temperature may be set at a typical level of 1100 to 1300 ° C. If the slab reheating temperature is less than 1100 ° C, it may be difficult to obtain a sufficient temperature of the slab plate necessary for the plate. On the other hand, when the temperature is higher than 1300 ° C, abnormal austenite growth and surface defects due to scale may occur.

Hot rolling step

Next, the reheated slab is rough-rolled, and then subjected to finish rolling at a temperature of Ar3 or higher to obtain a hot-rolled steel sheet. This is because, if 2-phase rolling is performed, it is difficult to obtain a desired homogeneous carbide according to the present invention, as the pre-ferrite having no carbide is produced.

The finishing rolling temperature is more preferably 800 to 950 占 폚. If the finishing rolling temperature is less than 800 ° C, the rolling load may be too great to carry out the subsequent process. On the other hand, when the finish rolling temperature is higher than 950 ° C, there is a fear of causing a scaling defect on the surface. On the other hand, in the present invention, the rough rolling temperature is not particularly limited, and a typical level of 1000 to 1100 DEG C can be applied.

Cooling step

Next, the hot-rolled steel sheet is cooled. At this time, the cooling rate is limited to 50 DEG C / sec or more and 300 DEG C / sec or less. And is cooled at the above-mentioned rapid cooling rate and held slightly on the run-out table (ROT), thereby maximizing the pearlite transformation. It is difficult to secure a pearlite fraction of 60% by area or more at a cooling rate of less than 50 DEG C / sec, and uniform cooling due to unevenness in the width direction at a cooling rate exceeding 300 DEG C / The coil shape becomes very poor. Therefore, the cooling rate was limited to 50 to 300 ° C / sec.

Coiling  step

Next, the cooled hot-rolled steel sheet is wound at a temperature of 500 to 700 ° C. The reason why the coiling temperature is limited to 500 to 700 ° C is that the temperature interval is a period in which the pearlite structure can be maximally obtained. When the coiling temperature is controlled within the above range, a microstructure composed of 60% or more of area% of pearlite and the remaining ferrite can be secured. If the coiling temperature is lower than 500 占 폚, bainite or martensite structure which is a low-temperature transformation structure is formed, so that uniform pearlite can not be obtained. On the other hand, when the coiling temperature is higher than 700 ° C, a very coarse pearlite comes out and takes a very long time to spheroidize. The coiling temperature is preferably 500 to 700 占 폚.

Pickling phase

Next, optionally, the wound hot rolled steel sheet can be pickled. The pickling temperature is naturally cooled to a range of room temperature (about 25 ° C) to 200 ° C, then pickled to remove the surface layer scale. At this time, if the pickling temperature of the hot-rolled steel sheet exceeds 200 ° C, there is a problem that the surface layer portion of the hot-rolled steel sheet is over-pickled and the surface roughness of the surface layer is deteriorated, so the pickling temperature is limited to room temperature to 200 ° C.

Sphericalization Annealing  step

The wound hot-rolled steel sheet is spheroidized and annealed in a BAF (Batch Annealing Furnace) annealing furnace. At this time, the annealing temperature is preferably 650 to 780 ° C. A temperature of less than 650 ° C. is a temperature at which the driving force for spheroidizing the lamellar cementite is so low that it is difficult to substantially spheronize the cementite and the size of the spheroidized carbide becomes very large at a temperature exceeding 780 ° C.

In the present invention, the spheroidization annealing time is not particularly limited. However, the pearlite area ratio, the grain size of the spheroidized carbide, and the spheroidization fraction of the micro spheroidized steel sheet depend on the spheroidization annealing time, so that the pearlite is appropriately annealed It is necessary to give time, and according to one non-limiting example, the spheroidizing annealing time can be from 4 hours to 16 hours.

According to one example, when spheroidizing annealing is performed, a magnetic field of 5 to 15 T can be applied to the hot-rolled steel sheet. When a magnetic field in the above range is applied to the hot-rolled steel sheet, the carbon diffusion in the pearlite structure is promoted, and the interfacial energy between the ferrite cementite is increased to promote spheroidization of the pearlite. As a result, the sintering time for spheroidizing can be further shortened and the process cost can be saved. In addition, the average grain size of the spheroidized carbide is relatively coarsened, thereby improving the ductility of the steel.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the description of these embodiments is intended only to illustrate the practice of the present invention, but the present invention is not limited thereto. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

( Example )

The steel slab having the component system shown in the following Table 1 was reheated at 1200 ° C for 2 hours and then hot rolled and wound under the conditions shown in Table 2 below. In each example, the rough rolling temperature was set to 1000 占 폚, and the cooling rate after finish rolling was set to 100 占 폚 / sec. Thereafter, the rolled hot-rolled steel sheet was pickled and subjected to spheroidizing annealing under the conditions shown in Table 2 below. In Table 2, FDT represents the finish rolling temperature, CT represents the coiling temperature, BAT temperature represents the spheroidizing annealing temperature, and BAT time represents the spheroidizing annealing time. On the other hand, a magnetic field of 10 T was applied to each hot rolled steel sheet wound around the spheroidizing annealing.

Then, microstructures of each micro-spherical steel sheet were observed and the spheroidization fraction of spheroidized carbide was measured. The results are shown in Table 3 below.

Tensile tests and corrosion resistance tests were carried out on the respective micro-spherical steel sheets produced, and tensile strength and the like were measured. The results are shown in Table 3 below. The tensile test was carried out in accordance with JIS No. 5 based on the rolling direction. The corrosion resistance test was carried out according to ASTM-G31, and the corrosion loss ratio was measured by immersing the specimen in a 50 vol.% Sulfuric acid aqueous solution for 1 hour. In Table 3, YS, TS, YR and El mean yield strength, tensile strength, yield ratio (yield strength / tensile strength), and elongation, respectively.

Steel grade Alloy composition (% by weight) C Mn Si P S Al Cr Mo Cu Ni A 0.45 1.10 0.20 0.010 0.004 0.019 0.91 0.20 0.20 0.21 B 0.07 0.50 0.20 0.011 0.003 0.021 1.12 0.19 0.20 0.20 C 0.25 0.71 0.20 0.012 0.004 0.022 1.03 0.20 0.20 0.19 D 0.30 0.80 0.21 0.011 0.004 0.019 1.05 0.19 0.21 0.21 E 0.24 0.03 0.20 0.010 0.003 0.020 1.13 0.19 0.19 0.20 F 0.26 2.15 0.19 0.009 0.004 0.021 1.01 0.20 0.19 0.19 G 0.20 0.95 0.18 0.010 0.004 0.020 - 0.18 0.22 0.19 H 0.23 0.77 0.20 0.012 0.003 0.019 1.98 0.19 0.21 0.20 I 0.25 0.80 0.21 0.010 0.004 0.021 1.10 - 0.20 0.19 J 0.28 0.72 0.19 0.009 0.004 0.019 0.93 0.71 0.019 0.21 K 0.24 0.83 0.20 0.011 0.003 0.020 1.02 0.22 - - L 0.27 0.79 0.18 0.012 0.003 0.021 1.10 0.19 0.22 - M 0.29 0.87 0.21 0.010 0.004 0.019 1.09 0.21 - 0.19 N 0.28 0.71 0.19 0.011 0.004 0.022 1.12 0.19 0.51 0.21

Steel grade Thickness (mm) FDT (占 폚) CT (° C) BAT temperature (캜) BAT Time (hr) Remarks A 4.0 900 620 680 12 Comparative Example 1 900 620 730 15 Comparative Example 2 B 4.5 890 640 680 10 Comparative Example 3 890 640 730 12 Comparative Example 4 C 4.0 890 620 700 8 Inventory 1 890 620 710 10 Inventory 2 890 620 600 12 Comparative Example 5 890 620 700 20 Comparative Example 6 D 4.5 900 640 710 7 Inventory 3 900 640 790 12 Comparative Example 7 900 640 720 25 Comparative Example 8 900 460 710 12 Comparative Example 9 E 4.5 890 620 700 8 Comparative Example 10 F 4.5 890 620 700 8 Comparative Example 11 G 4.5 890 620 700 8 Comparative Example 12 H 4.5 890 620 700 8 Comparative Example 13 I 4.5 890 620 700 8 Comparative Example 14 J 4.5 890 620 700 8 Comparative Example 15 K 4.5 890 620 700 8 Comparative Example 16 L 4.5 890 620 700 8 Comparative Example 17 M 4.5 890 620 700 8 Comparative Example 18 N 4.5 890 620 700 8 Comparative Example 19

Steel grade Perla
Area ratio
(%) Bera
Average grain size
(μm) Spheroidal carbide average grain size
(μm) YS
(MPa) TS
(MPa) YR Hand
(%) Hardness
(HV) Corrosion reduction rate
(mg / cm ² / hr) Remarks A 10 16 0.7 500 631 0.79 26 201 16.71 Comparative Example 1 12 18 0.8 522 663 0.79 27 209 15.46 Comparative Example 2 B 2 18 0.6 287 390 0.74 32 121 14.55 Comparative Example 3 3 17 0.8 293 408 0.72 33 125 15.94 Comparative Example 4 C 25 19 0.7 355 565 0.63 31 166 13.67 Inventory 1 20 20 0.6 380 573 0.66 30 169 12.89 Inventory 2 60 14 0.3 538 670 0.8 25 211 13.76 Comparative Example 5 0 22 1.2 393 481 0.82 31 146 15.32 Comparative Example 6 D 22 19 0.7 370 562 0.66 32 168 16.44 Inventory 3 3 21 - 440 690 0.64 19 214 15.02 Comparative Example 7 0 23 1.3 388 473 0.82 32 144 16.74 Comparative Example 8 15 18 0.9 536 680 0.79 22 215 15.60 Comparative Example 9 E 23 17 0.7 290 410 0.71 31 126 18.68 Comparative Example 10 F 26 21 0.8 511 670 0.76 23 212 19.20 Comparative Example 11 G 52 19 0.4 533 584 0.91 29 172 19.53 Comparative Example 12 H 16 24 0.8 520 646 0.80 24 201 15.37 Comparative Example 13 I 19 23 0.7 330 477 0.74 32 143 18.76 Comparative Example 14 J 24 21 0.7 530 709 0.75 19 233 14.39 Comparative Example 15 K 27 18 0.8 355 521 0.68 32 155 24.35 Comparative Example 16 L 21 23 0.6 381 556 0.69 31 158 22.19 Comparative Example 17 M 17 26 0.7 395 570 0.69 32 159 21.97 Comparative Example 18 N 22 25 0.7 441 620 0.71 24 189 13.04 Comparative Example 19

Referring to FIG. 3, Inventive Examples 1 to 3, which satisfy the alloy composition and microstructure proposed in the present invention, show that all of the material requirements of the present invention are satisfied.

In contrast, Comparative Examples 1 and 2 exceeded the allowable range of the present invention of 500 to 600 MPa at a tensile strength of 631 MPa and 663 MPa, respectively, because the carbon content exceeded the range of the present invention. In addition, the hardnesses were 201HV and 209HV, respectively, and deviated from the allowable range of the present invention of 179HV or less.

In Comparative Examples 3 and 4, the carbon content was below the range of the present invention, and the pearlite fraction had a value of less than 10%, and the tensile strengths were 390 MPa and 408 MPa, respectively, which were outside the allowable range of 500-600 MPa.

In Comparative Example 5, the alloy composition satisfied the range proposed in the present invention, but the spheroidization annealing temperature was below the range suggested by the present invention, and the spheroidization hardly proceeded, and the pearlite area ratio was 60%. The final material has a tensile strength of 670 MPa and a hardness of 211 HV.

In Comparative Examples 6 and 8, the alloy composition satisfied the range proposed in the present invention, but the residual pearlite was not present due to excessive spheroidization annealing, and the average grain size of the carbide was excessively large, High yield ratio.

In Comparative Example 7, the alloy composition satisfied the range suggested by the present invention, but the sintering temperature exceeded the range suggested by the present invention, and the sintering process was reversed in the spheroidizing annealing process. Accordingly, Martensite was formed. So that strength and hardness are out of the allowable range of the present invention.

In Comparative Example 9, since the coiling temperature was low, bainite was formed before the spheroidizing annealing heat treatment, so that bainite was formed as the final structure, and the strength and hardness were out of the allowable range of the present invention.

In Comparative Examples 10 and 11, the manganese content was out of the range of the present invention, and the tensile strength was out of the allowable range of the present invention.

Comparative Example 12 is an example in which chromium is not added. As a result, quartzite has not sufficiently progressed due to its small particle size and sphericity, resulting in a pearlite area ratio of 52% higher than 30%, and the yield strength is out of the scope of the present invention. Further, in Comparative Example 13, the chromium content exceeded the range suggested by the present invention, and the tensile strength was out of the allowable range of the present invention.

In Comparative Examples 14 and 15, the molybdenum content was out of the range of the present invention, and the tensile strength was out of the allowable range of the present invention.

Comparative Examples 16 to 18 are examples in which copper and / or nickel is not added, and the corrosion reduction rate is out of the scope of the present invention.

In Comparative Example 19, the copper content exceeded the range proposed by the present invention, and the tensile strength was out of the allowable range of the present invention.

On the other hand, FIG. 1 is a photograph of the microstructure observed in Inventive Example 1, which visually confirms that 25% of pearlite structure is contained in layered form.

Fig. 2 is a photograph of the microstructure observed in Comparative Example 6, and it can be visually confirmed that the residual pearlite is not present and that the microstructure is composed only of ferrite and spheroidized carbide.

3 is a photograph of the microstructure observed in Comparative Example 7, and it can be visually confirmed that bainite and martensite structure were formed by reverse transformation.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the scope of the present invention is not limited to the disclosed exemplary embodiments, but various modifications and changes may be made without departing from the scope of the invention. To those of ordinary skill in the art.

Claims (13)

0.001 to 0.02% of P, 0.01% or less of S, 0.01 to 0.1% of Al, 0.01 to 0.1% of Cr, 0.1 to 1.5% of Cr, 0.10 to 0.40% of C, 0.01 to 0.5% of Mo, 0.01 to 0.4% of Cu, 0.01 to 0.4% of Ni, the balance of Fe and unavoidable impurities,
10 to 30 area percent of pearlite, residual ferrite and spheroidized carbide in microstructure,
The average grain size of the ferrite is 5 to 20 mu m,
Wherein the average grain size of the spheroidized carbide is 0.2 to 0.8 占 퐉.
delete delete The method according to claim 1,
A fine spherical steel sheet having a thickness of 3.0 to 6.0 mm.
The method according to claim 1,
A micro-spherical steel sheet having a tensile strength of 500 to 600 MPa and a yield ratio of 0.6 to 0.8.
The method according to claim 1,
A micro-spherical steel sheet having a hardness of 179 HV or less and an elongation of 25% or more.
The method according to claim 1,
A micro-spherical steel sheet having a corrosion reduction rate of 20 mg / cm ² / hr or less when immersed in an aqueous 50 vol.% Sulfuric acid solution for 1 hour in accordance with ASTM-G31.
0.001 to 0.02% of P, 0.01% or less of S, 0.01 to 0.1% of Al, 0.01 to 0.1% of Cr, 0.1 to 1.5% of Cr, 0.10 to 0.40% of C, A hot-rolled steel sheet comprising 0.01 to 0.5% of Mo, 0.01 to 0.4% of Cu, 0.01 to 0.4% of Ni, 0.01 to 0.4% of Ni, balance Fe and unavoidable impurities, step; And
Spheroidizing the hot-rolled steel sheet at 650 to 780 ° C for 4 to 16 hours to obtain a micro-spheroidized steel sheet containing 10 to 30% by area of pearlite, residual ferrite and spheroidized carbide in a microstructure;
Wherein the method comprises the steps of:
9. The method of claim 8,
And applying a magnetic field of 5 to 15 T to the hot-rolled steel sheet during the spheroidizing annealing.
9. The method of claim 8,
The step of preparing the hot-
0.001 to 0.02% of P, 0.01% or less of S, 0.01 to 0.1% of Al, 0.01 to 0.1% of Cr, 0.1 to 1.5% of Cr, 0.10 to 0.40% of C, %, Mo: 0.01 to 0.5%, Cu: 0.01 to 0.4%, Ni: 0.01 to 0.4%, the remainder Fe and unavoidable impurities;
Subjecting the reheated slab to rough rolling, and finishing rolling at a temperature equal to or higher than Ar3 to obtain a hot-rolled steel sheet; And
Cooling the hot-rolled steel sheet at a speed of 50 to 300 ° C / sec and winding at 500 to 700 ° C;
Wherein the method comprises the steps of:
11. The method of claim 10,
Wherein the reheating temperature is 1100 to 1300 DEG C when the slab is reheated.
11. The method of claim 10,
Wherein the rough rolling temperature of the reheated slab is from 1000 to 1100 ° C.
11. The method of claim 10,
Wherein the finishing rolling temperature of the rough rolling slab is 800 to 950 占 폚.

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