KR101889186B1 - High-strength hot-rolled steel plate having excellent hydrogen induced cracking resistance and dwtt toughness at low temperature, and method for manufacturing the same - Google Patents

Abstract

One aspect of the present invention relates to a steel sheet comprising, by weight%, 0.03 to 0.04% of C, 0.05 to 0.3% of Si, 1.2 to 1.4% of Mn, 0.008% Ti: 0.002 to 0.015%, N: 0.002 to 0.006%, Cu: 0.1 to 0.3%, Ni: 0.1 to 0.3% 0.2 to 0.35% of Cr, 0.05 to 0.35% of Mo, 0.0005 to 0.003% of Ca, the balance of Fe and unavoidable impurities,
The present invention relates to a high-strength hot-rolled steel sheet excellent in hydrogen organic cracking resistance and DWTT low-temperature toughness having an acicular shape ferrite of not less than 90% and not more than 2% of MA in an area fraction and an average effective grain size of the acicular ferrite of not more than 15 탆 will be.
(Cu / Ni) * (Ti / N)? 9
(In the above relational expression 1, the symbol of each element represents the content of each element in weight%.)

Description

HIGH STRENGTH HOT-ROLLED STEEL PLATE HAVING EXCELLENT HYDROGEN INDUCED CRACKING RESISTANCE AND DWTT TOUGHNESS AT LOW TEMPERATURE AND METHOD FOR MANUFACTURING THE SAME,

The present invention relates to a hot-rolled steel sheet having excellent hydrogen-organic crack resistance and DWTT low-temperature toughness, and a method for producing the same.

Recently, demand for energy such as China and India is increasing. Recently, however, the mining and transporting environment is getting worse due to the increase of H ₂ S gas content or the expansion of mining area in cold regions where low temperature toughness is required such as Siberia and Alaska.

Hydrogen sulfide (H ₂ S) gas is known to be a main cause of hydrogen embrittlement phenomenon such as HIC (Hydrogen induced crack) in steel, and it is required to have a steel with excellent resistance to breakage of steel in the environment containing H ₂ S , A steel material excellent in low temperature toughness is demanded in order to withstand external impacts in extreme environments and to minimize economic and environmental losses in case of an accident.

As a method for effectively controlling the hydrogen organic cracking, there have been proposed a means for controlling the length of the nonmetallic inclusions and the hardness of the segregation portion, a method for improving the hydrogen-organic cracking resistance by controlling the composition of the nonmetallic inclusions, A method of refining the effective crystal grains in order to secure the low-temperature toughness, and a method of improving the low-temperature toughness by controlling the amount of non-metallic inclusions. For example, Patent Document 1 discloses a method for improving the low temperature toughness by controlling the amount of non-metallic inclusions.

However, the above-mentioned prior arts do not guarantee two properties at the same time due to different components and manufacturing methods which are satisfied with each of them, and it is more difficult in high-strength afterglow steels in which hydrogen organic cracking occurs well. The reason for this is that the higher the strength, the more the amount of diffusion water flowing into the steel, the more segregated at the central part and the coarse grains grow well.

Therefore, there is a demand for development of high strength after-hot-rolled steel sheet excellent in hydrogen organic cracking resistance and DWTT low-temperature toughness and a manufacturing method thereof.

Korean Patent Publication No. 2008-0042296

An aspect of the present invention is to provide a high strength hot rolled steel sheet excellent in hydrogen organic crack resistance and DWTT low temperature toughness which can be preferably applied to a corrosive environment including cyanide and H ₂ S, and a method of manufacturing the same.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

One aspect of the present invention relates to a steel sheet comprising, by weight%, 0.03 to 0.04% of C, 0.05 to 0.3% of Si, 1.2 to 1.4% of Mn, 0.008% Ti: 0.002 to 0.015%, N: 0.002 to 0.006%, Cu: 0.1 to 0.3%, Ni: 0.1 to 0.3% 0.2 to 0.35% of Cr, 0.05 to 0.35% of Mo, 0.0005 to 0.003% of Ca, the balance of Fe and unavoidable impurities,

The present invention relates to a high-strength hot-rolled steel sheet excellent in hydrogen organic cracking resistance and DWTT low-temperature toughness having an acicular shape ferrite of not less than 90% and not more than 2% of MA in an area fraction and an average effective grain size of the acicular ferrite of not more than 15 탆 will be.

In another aspect of the present invention, there is provided a ferritic stainless steel comprising 0.03 to 0.04% of C, 0.05 to 0.3% of Si, 1.2 to 1.4% of Mn, 0.008% or less of P (exclusive of 0% (Excluding 0%), Al: 0.02 to 0.05%, Nb: 0.04 to 0.07%, V: 0.03 to 0.06%, Ti: 0.005 to 0.015%, N: 0.002 to 0.006% 0.1 to 0.3% of Cr, 0.2 to 0.35% of Cr, 0.05 to 0.35% of Mo, 0.0005 to 0.003% of Ca, the balance of Fe and unavoidable impurities,

Heating the slab to 1150 to 1230 캜;

Hot rolling the heated slab so that the final hot rolling temperature is in the range of Ar 3 to 900 ° C to obtain a hot rolled steel sheet; And

And cooling the hot-rolled steel sheet in a temperature range of Ar 3 to 800 ° C to finish cooling in a temperature range of 450 to 520 ° C and winding up the hot-rolled steel sheet, wherein the cold rolled steel sheet has excellent resistance to hydrogen organic cracking and DWTT low temperature toughness And a method for producing the same.

(Cu / Ni) * (Ti / N)? 9

(In the above relational expression 1, the symbol of each element represents the content of each element in weight%.)

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, it is possible to provide a hot rolled steel sheet having a high yield strength, excellent hydrogen organic cracking resistance, a low DWTT transition temperature, and a high DWTT ductile wavefront ratio at -30 캜, and a method for producing the same.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a photograph of a microstructure of Inventive Example 1 taken by an optical microscope. FIG.
2 is a photograph of the microstructure of Comparative Example 1 taken by an optical microscope.
3 is a photograph of the microstructure of Comparative Example 7 taken by an optical microscope.
4 is a photograph of a wavefront after the DWTT test at -30 ° C in Inventive Example 1 and Comparative Example 1. FIG.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

Hydrogen organic crack  Resistant and DWTT  High strength with excellent low temperature toughness Fleece  Hot-rolled steel sheet

Hereinafter, a high strength afterglow hot-rolled steel sheet excellent in hydrogen organic cracking resistance and DWTT low temperature toughness according to one aspect of the present invention will be described in detail.

A high strength afterglow hot-rolled steel sheet excellent in hydrogen organic cracking resistance and DWTT low temperature toughness according to one aspect of the present invention comprises 0.03 to 0.04% of C, 0.05 to 0.3% of Si, 1.2 to 1.4% of Mn, 0.008 0.001% or less (excluding 0%), S: 0.001% or less (excluding 0%), Al: 0.02 to 0.05%, Nb: 0.04 to 0.07%, V: 0.03 to 0.06% 0.1 to 0.3% of Cu, 0.1 to 0.3% of Ni, 0.2 to 0.35% of Cr, 0.05 to 0.35% of Mo, 0.0005 to 0.003% of Ca, the balance of Fe and unavoidable impurities, 1,

The microstructure includes acicular ferrite of 90% or more and MA of 2% or less in area fraction, and the average effective grain size of the acicular ferrite is 15 탆 or less.

(Cu / Ni) * (Ti / N)? 9

(In the above relational expression 1, the symbol of each element represents the content of each element in weight%.)

First, the alloy composition of the present invention will be described in detail. Hereinafter, the unit of each element content means weight% unless otherwise specified.

C: 0.03 to 0.04%

C is the most economical and effective alloying element to strengthen the steel.

When the C content is less than 0.03%, the strengthening effect is small and it is difficult to satisfy the high strength yield strength and tensile strength to be achieved in the present invention. On the other hand, when the C content is more than 0.04%, there is a problem that the center segregation which decreases the hydrogen organic cracking resistance is increased. Therefore, the C content is preferably 0.03 to 0.04%.

Si: 0.05 to 0.3%

Si is an effective component for deoxidation and solid solution strengthening, and is preferably added in an amount of 0.05 wt% or more for the above effect. However, when the Si content is more than 0.3%, not only the weldability and the brittleness are lowered but also the inclusions in the steel are increased and the hydrogen-organic cracking property is lowered. Therefore, the Si content is preferably 0.05 to 0.3%.

Mn: 1.2 to 1.4%

Mn is an essential component for securing strength and toughness.

When the Mn content is less than 1.2%, it is difficult to secure the strength. When the Mn content is more than 1.4%, the center segregation is promoted at the performance, thereby reducing the hydrogen-organic cracking property and the cryogenic DWTT toughness. Therefore, the Mn content is preferably 1.2 to 1.4%.

P: 0.008% or less (excluding 0%)

P is an impurity. When the P content is more than 0.008%, central segregation is promoted along with Mn at the time of playing to deteriorate impact toughness and hydrogen organic cracking resistance as well as weldability. Therefore, the P content is preferably 0.008% or less, and more preferably 0.008% or less.

S: 0.001% or less (excluding 0%)

S is an impurity which reacts with Mn in the steel to form MnS, thereby greatly reducing the brittleness. When the S content is more than 0.001%, the hydrogen organic cracking resistance may be greatly lowered. Therefore, the S content is preferably 0.001% or less.

Al: 0.02 to 0.05%

Al is a component that deoxidizes with Si.

When the Al content is less than 0.02%, it is difficult to obtain a deoxidation effect. When the Al content exceeds 0.05%, the alumina aggregate is increased to lower the hydrogen hydrogen organic cracking property and the low temperature DWTT property. Therefore, the Al content is preferably 0.02 to 0.05%.

Nb: 0.04 to 0.07%

Nb is a component exhibiting precipitation strengthening effect by the addition of a small amount, and is preferably added in an amount of 0.04% or more for the above effect.

However, when the Nb content is more than 0.07%, there is a fear that the weldability is lowered due to a large amount of precipitate. Therefore, the Nb content is preferably 0.04 to 0.07%.

V: 0.03 to 0.06%

V, like Nb, is a component which induces a precipitation strengthening effect by the addition of a small amount, and is preferably added at 0.03% or more for the above effect.

However, in the carbon range of the present invention, when the Nb content is more than 0.06%, there is a fear that the weldability is lowered due to a large amount of precipitates. Therefore, the V content is preferably 0.03 to 0.06%.

Ti: 0.005 to 0.015%

Ti is precipitated as TiN in the steel and inhibits the growth of austenite grains during reheating, thereby obtaining high strength and excellent impact toughness, and also precipitates TiC and strengthens the steel.

In order to obtain the above effect in the range of the present invention, the Ti content should be 0.005% or more. On the other hand, when the Ti content is more than 0.015%, the above effect is saturated and coarse TiN may be generated, which may lower the hydrogen-organic cracking property and the low-temperature DWTT characteristics. Therefore, the Ti content is preferably 0.005 to 0.015%.

N: 0.002 to 0.006%

N is an element for solubility strengthening, but it is an element that inhibits the growth of austenite grains by precipitation of Ti and TiN in the steel.

When the N content is less than 0.002%, the above-mentioned effect is insufficient. When the N content is more than 0.006%, coarse TiN precipitates and the low temperature DWTT characteristics may be deteriorated. Therefore, the N content is preferably 0.002 to 0.006%.

Cu: 0.1 to 0.3%

Cu is an element added to improve corrosion resistance and toughness with increasing strength through solid solution strengthening.

When the Cu content is less than 0.1%, the above-mentioned effect is insufficient, and when the Cu content is more than 0.3%, the toughness may be lowered due to the formation of precipitates. Therefore, the Cu content is preferably 0.1 to 0.3%.

Ni: 0.1 to 0.3%

Ni, like Cu, is an element added to improve toughness with strength enhancement through solution strengthening.

When the Ni content is less than 0.1%, the above-mentioned effect is insufficient, and when the Ni content is more than 0.3%, the toughness may be lowered due to the formation of precipitates. Therefore, the Ni content is preferably 0.1 to 0.3%.

Cr: 0.2 to 0.35%

Cr is an element with high hardenability and is added for strength enhancement through transformation strengthening.

When the Cr content is less than 0.2%, the above-mentioned effect is insufficient. When the Cr content is more than 0.35%, the texture such as the upper bainite is formed, and the toughness is lowered as the microstructure is totally uneven. Therefore, the Cr content is preferably 0.2 to 0.35%.

Mo: 0.05 to 0.35%

Mo is an element added to increase the strength through strengthening of the transformation, which is an element having hardenability higher than that of Cr.

When the Mo content is less than 0.05%, the above-mentioned effect is insufficient. When the Mo content is more than 0.35%, the secondary phase such as MA is formed in a large amount, and toughness may be lowered. Therefore, the Mo content is preferably 0.05 to 0.35%.

Ca: 0.0005 to 0.003%

Ca is an element that plays a role in suppressing the formation of inclusions that lower the toughness by spheroidizing the shape of the emulsion inclusion.

When the Ca content is less than 0.0005%, the above-mentioned effect is insufficient, and when the Ca content is more than 0.003%, the amount of non-metallic inclusion is rather increased to lower the hydrogen-organic cracking resistance and low temperature toughness. Therefore, the Ca content is preferably 0.0005 to 0.003%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

Not only the ranges of the above-described respective element contents are satisfied, but also Cr, Mo, Cu, Ni, Ti and N are added so as to satisfy the following relational expression 1.

(Cu / Ni) * (Ti / N)? 9

(In the above relational expression 1, the symbol of each element represents the content of each element in weight%.)

The relational expression 1 is a formula designed considering the influence of Cr, Mo, Cu, Ni, Ti, and N on transformation strengthening, precipitation strengthening, uniformity of microstructure, resistance to hydrogen organic cracking, and DWTT characteristics.

When the value of the relational expression 1 is less than 3, it is difficult to secure a homogeneous microstructure because the transformation strengthening and precipitation strengthening effect is not optimized, and it is difficult to secure the hydrogen hydrogen organic cracking property by the increase of the MA phase. On the other hand, when the value of the relational expression 1 is more than 9, the amount of the non-uniform low temperature transformation phase such as the upper bainite is increased and it is difficult to secure the hydrogen hydrogen organic cracking property and the DWTT low temperature toughness due to the formation of coarse TiN.

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

The microstructure of the present invention contains not less than 90% of needle-shaped ferrite and not more than 2% of MA (martensite-austenite) at an areal fraction, and the average effective grain size is not more than 15 탆. Here, the average effective grain size means an average size of effective grains having a grain boundary with high hardness (15 degrees) by performing Electron Backscatter Diffraction (EBSD) analysis.

If the acicular type ferrite is less than 90% or MA is more than 2%, the hydrogen-organic cracking property may be lowered. When the average effective grain size of the acicular ferrite is less than 15 mu m, it is difficult to secure the low temperature toughness of DWTT.

At this time, the yield strength of the hot-rolled steel sheet of the present invention is 500 to 600 MPa and the crack area ratio (CAR) may be 5% or less. At this time, CAR is after observing cracks extent by the 1 atm H ₂ S gas of 5% NaCl + a back, an ultrasonic flaw detection method and soaked for 0.5% CH specimen for 96 hours in ₃ COOH solution saturated in accordance with the NACE TM0284, and hydrogen The total area of the specimen cracks generated by the steel plate is divided by the total area of the specimen.

The DWTT (Drop Weight Tearing Test) transition temperature of the hot-rolled steel sheet of the present invention is -25 ° C or less, and the DWTT ductile wavefront ratio at -30 ° C may be 85% or more. The DWTT ductile wave fracture rate is known to be a property of propagation resistance of cracks when cracks occur due to external damage to line pipes carrying crude oil or gas. .

On the other hand, the thickness of the hot-rolled steel sheet of the present invention may be 20 mm or less. If the thickness exceeds 20 mm, winding may be difficult. In particular, the lower limit of the thickness is not limited, but the lower limit of the thickness may be 15 mm since it can be more effectively applied to articles of 15 mm or more.

Hydrogen organic crack  Resistant and DWTT  High strength with excellent low temperature toughness Fleece  Manufacturing method of hot-rolled steel sheet

Hereinafter, a method for manufacturing a high strength post hot-rolled steel sheet excellent in hydrogen organic cracking resistance and DWTT low temperature toughness, which is another aspect of the present invention, will be described in detail.

According to another aspect of the present invention, there is provided a method of manufacturing a high strength hot rolled steel sheet excellent in hydrogen organic cracking resistance and DWTT low temperature toughness, comprising the steps of: preparing a slab satisfying the alloy composition of the present invention; Heating the slab to 1150 to 1230 캜; Hot rolling the heated slab so that the final hot rolling temperature is in the range of Ar 3 to 900 ° C to obtain a hot rolled steel sheet; And cooling the hot-rolled steel sheet at a temperature in the range of Ar 3 to 800 ° C to finish cooling in a temperature range of 450 to 520 ° C and winding the hot-rolled steel sheet.

Each step will be described in detail below.

Slab preparation step

A slab satisfying the above alloy composition is prepared.

Since the control of the non-metallic inclusions according to the present invention can be obtained through control of the process conditions in the ordinary secondary refining process, the step of preparing the slab is not particularly limited. For example, the secondary refining process is performed by Ar bubbling in a degassing process such as Ar bubbling and VTD (Vacuum Tank Degasser) or RH (vacuum degassing facility) in an LF (Ladle Furnace) Can be controlled. 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. After refining the molten steel, molten steel may be continuously cast to produce a slab.

Slab heating step

The slab is heated to 1150 to 1230 DEG C in a heating furnace.

The heating temperature of the slab is determined by the solid solution temperature of the Nb-based precipitate, and in the component range of the present invention, the solid solution of Nb is available at a temperature higher than 1150 ° C.

When the slab heating temperature is heated to less than 1150 占 폚, it is difficult to secure sufficient strength. When the slab is heated to more than 1,230 占 폚, the grain size of the steel sheet becomes very large and toughness may be deteriorated. Therefore, the slab heating temperature is preferably 1150 to 1230 ° C.

Hot rolling step

The hot slab is hot-rolled to obtain a hot-rolled steel sheet so that the final hot-rolling temperature is in the range of Ar 3 to 900 ° C.

If the finish rolling temperature is higher than 900 ° C., there is a high possibility that uneven and coarse crystal grains can be formed, so that the low temperature DWTT property pursued by the present invention can not be ensured. If the final rolling temperature is lower than Ar 3, The hydrogen-organic cracking property and the low-temperature DWTT toughness can be very low.

At this time, the hot rolling can be performed so that the rolling reduction at Tnr or lower is 70% or more.

The amount of reduction at a temperature below Tnr (temperature at which the recrystallization temperature of austenite is stopped) has a great influence on the grain size and uniformity of the microstructure of the hot-rolled steel sheet. The crystal grain size and uniformity are highly correlated with the low temperature toughness. Therefore, in order to control the grain size and the uniformity, it is desirable that the reduction rate is 70% or more at the time of rolling. If the reduction rate is less than 70%, the homogeneity of the crystal grain size is lowered and the low temperature toughness may be lowered. The upper limit of the reduction rate is not particularly limited, and may be the maximum reduction rate of the thickness.

Cooling and Coiling  step

The hot-rolled steel sheet is cooled at a temperature in the range of Ar 3 to 800 ° C, cooled at a temperature in the range of 450 to 520 ° C, and then wound.

When the cooling start temperature is lower than Ar 3, coarse ferrite may be formed before cooling to lower the toughness, and brittle fracture texture which lowers the hydrogen hydrogen organic cracking property and the low temperature DWTT toughness can be developed.

The reason for carrying out the winding is to ensure the self-tempering effect and the residual stress relaxation effect due to the post-winding (temperature rise).

When the cooling is terminated and the coiling temperature is more than 520 ° C, the transformation is unstable and coarse pearlite structure can be formed, and the MA phase can be formed in a large amount by increasing the carbon partitioning, thereby securing the hydrogen organic cracking property and low temperature DWTT property There is a difficulty in. On the other hand, when it is less than 450 DEG C, the rigidity of the steel sheet becomes large, and it is very difficult to normally wind it.

At this time, the cooling can be performed at a cooling rate of 10 to 30 DEG C / sec.

If the cooling rate is less than 10 ° C / sec, a coarse pearlite structure that lowers the toughness can be easily formed. If the cooling rate is more than 30 ° C / sec, the generation of a mild secondary phase such as MA is promoted, Low-temperature DWTT characteristics.

On the other hand, a track spraying step of spraying cooling water for 20 seconds or more on the outer side of the hot-rolled steel sheet during winding can be further included.

When the cooling water injection time is less than 20 seconds, the structure at the central portion in the width direction becomes uneven and it is difficult to stably secure low temperature DWTT physical properties.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

The slabs were prepared by refining molten steel having the composition shown in the following Table 1 to prepare slabs. The slabs were then heated under the conditions described in Table 2, and then subjected to finish hot rolling at the finish hot rolling temperature described in Table 2, , And then cooling water was sprayed for 30 seconds on the outer side of the hot-rolled steel sheet during winding to produce a hot-rolled steel sheet having a thickness of 19 mm.

The yield strength, microstructure, DWTT characteristics, and crack area ratio (CAR) of the produced hot-rolled steel sheet were measured and evaluated.

The MA fraction was measured by color etching method using two step etching process and pre-treatment by post-processing and post-pearlite fraction image analysis. The tissues except MA were observed as acicular ferrite and were not described separately.

The effective grain size was calculated by means of EBSD (Electron Back Scattered Diffraction).

In addition, the yield strength (YS) was measured by a room temperature tensile test. The low temperature DWTT property was measured by using a DWTT tester while lowering the temperature by using liquid nitrogen. The ductile fracture ratio was measured, Transition temperature was evaluated based on 85%. The ductile wavefront ratios at -30 캜 were measured and are shown in Table 3 below.

Car was immersed in 5% NaCl + 0.5% CH ₃ COOH solution saturated with 1 atm H ₂ S gas for 96 hours according to NACE TM0284, and the degree of cracking was observed by ultrasonic flaw detection method. And the total area of cracks of the steel plate was divided by the total area of the specimen.

In the following Table 1, relational expression 1 is (Cr / Mo) * (Cu / Ni) * (Ti / N) where each element symbol represents the content of each element in weight%.

Steel grade Alloy composition (wt%, P *, S *, N *, and Ca * are ppm by weight) relation
Equation 1 C Si Mn P * S * Al Cr Mo Cu Ni Ti Nb V N * Ca * Inventive Steel 1 0.035 0.25 1.3 42 8 0.03 0.25 0.15 0.21 0.2 0.01 0.06 0.049 48 18 3.65 Invention river 2 0.038 0.22 1.25 51 7 0.027 0.3 0.3 0.15 0.16 0.012 0.05 0.045 36 22 3.1 Invention steel 3 0.034 0.23 1.28 48 9 0.025 0.3 0.1 0.25 0.26 0.014 0.046 0.055 48 25 8.41 Comparative River 1 0.065 0.18 1.6 56 9 0.027 0.23 0.1 0.21 0.12 0.01 0.05 0.047 44 21 9.15 Comparative River 2 0.036 0.25 1.34 113 18 0.029 0.26 0.13 0.23 0.22 0.012 0.054 0.053 43 17 5.84 Comparative Steel 3 0.036 0.26 1.3 61 8 0.028 0.42 0.1 0.23 0.19 0.013 0.056 0.053 45 43 14.7 Comparative Steel 4 0.037 0.24 1.35 53 7 0.026 0.32 0.07 0.25 0.11 0.01 0.048 0.047 49 20 21.2 Comparative Steel 5 0.035 0.22 1.29 45 8 0.025 0.23 0.33 0.25 0.21 0.01 0.057 0.041 56 21 1.48 Comparative Steel 6 0.035 0.25 1.28 56 8 0.027 0.3 0.15 0.23 0.19 0.013 0.056 0.053 45 56 6.99

division Steel grade Slab
Heating temperature (℃) Finishing hot rolling
Temperature (℃) Coiling temperature
(° C) Inventory 1 Inventive Steel 1 1225 795 488 Inventory 2 Invention river 2 1217 788 501 Inventory 3 Invention steel 3 1228 810 495 Comparative Example 1 Comparative River 1 1226 795 502 Comparative Example 2 Comparative River 2 1225 786 512 Comparative Example 3 Comparative Steel 3 1221 791 496 Comparative Example 4 Comparative Steel 4 1223 806 498 Comparative Example 5 Comparative Steel 5 1199 786 488 Comparative Example 6 Comparative Steel 6 1221 801 502 Comparative Example 7 Inventive Steel 1 1258 812 511 Comparative Example 8 Invention river 2 1228 804 589

division Steel grade Yield strength
(MPa) Effective grain
Size (㎛) MA
(area%) DWTT
Transition temperature (캜) -30 ° C DWTT
Ductile fracture ratio (%) CAR (%) Inventory 1 Inventive Steel 1 541 11.8 1.2 -35 96 0.8 Inventory 2 Invention river 2 556 12.6 1.6 -30 89 2.1 Inventory 3 Invention steel 3 510 13.4 0.7 -35 97 0 Comparative Example 1 Comparative River 1 576 14.1 4.3 -10 24 23.2 Comparative Example 2 Comparative River 2 542 12.9 0.9 -15 66 7.5 Comparative Example 3 Comparative Steel 3 551 13.7 1.3 -20 73 13.6 Comparative Example 4 Comparative Steel 4 523 13.6 0.6 -20 77 7.8 Comparative Example 5 Comparative Steel 5 551 11.6 3.6 -30 87 6.6 Comparative Example 6 Comparative Steel 6 536 12.6 0.8 -25 81 6.4 Comparative Example 7 Inventive Steel 1 548 18.9 0.9 -10 41 2.4 Comparative Example 8 Invention river 2 545 15.6 3.8 -20 65 8.9

The inventive examples satisfying the alloy composition and the manufacturing conditions of the present invention have a yield strength of 500 to 600 MPa, a DWTT transition temperature of -25 캜 or less, a DWTT ductile waveguide ratio of 85% or more at -30 캜, a CAR of 5% ≪ / RTI > FIG. 1 is a photograph of the microstructure of Inventive Example 1 taken by an optical microscope, showing that the phase fraction of MA is small and the effective grain size is small.

In Comparative Examples 1 to 6, the production conditions proposed in the present invention were satisfied, but the hydrogen organic cracking resistance or DWTT characteristics were poor due to the unsatisfied alloy composition.

Comparative Example 1 was a case where the C and Mn contents were exceeded, and a large amount of MA was formed, and the hydrogen organic cracking resistance and DWTT characteristics were weakened. FIG. 2 is a photograph of the microstructure of Comparative Example 1 taken by an optical microscope, confirming that a large amount of MA was formed.

In Comparative Example 2, the P and S contents were exceeded. In Comparative Example 3, the Cr and Ca contents were exceeded, and the hydrogen organic cracking resistance and the DWTT characteristics were inferior.

In Comparative Example 4, the content of each element was satisfied, but the value of the relation 1 was more than 9, and the hydrogen organic cracking resistance and DWTT characteristics were inferior.

In Comparative Example 5, the content of each element was satisfactory, but the value of the relation 1 was less than 3, and a large amount of MA was formed and the hydrogen organic cracking resistance was weakened.

In Comparative Example 6, the Ca content was exceeded, and the hydrogen organic cracking resistance and DWTT characteristics were weakened.

In Comparative Examples 7 and 8, the alloy composition proposed in the present invention was satisfied, but in Comparative Example 7, the slab heating temperature was exceeded, and in Comparative Example 8, the coiling temperature was exceeded, . As a result, the crystal grain size became coarse and the hydrogen organic cracking resistance or DWTT characteristic was weakened.

FIG. 3 is a photograph of the microstructure of Comparative Example 7 taken by an optical microscope, and it can be confirmed that the grain size is large.

FIG. 4 is a photograph of a wavefront after DWTT test at -30 ° C in Inventive Example 1 and Comparative Example 1, demonstrating that Inventive Example 1 has excellent DWTT characteristics.

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 in the appended claims. It will be possible.

Claims (8)

0.001% or less (excluding 0%), S: 0.001% or less (excluding 0%), Al: 0.02% or less, C: 0.03 to 0.04%, Si: 0.05 to 0.3%, Mn: Ti: 0.005-0.015%, N: 0.002-0.006%, Cu: 0.1-0.3%, Ni: 0.1-0.3%, Cr: 0.2-0.3%, Nb: 0.04-0.07% 0.35%, Mo: 0.05 to 0.35%, Ca: 0.0005 to 0.003%, the balance Fe and unavoidable impurities,
Wherein the microstructure includes an acicular ferrite having an acicular ferrite content of 90% or more and a MA of 2% or less, and an average effective grain size of the acicular ferrite is 15 탆 or less, and having excellent hydrogen organic cracking resistance and DWTT low temperature toughness.
Relation 1: 3.1? (Cr / Mo) * (Cu / Ni) * (Ti / N)? 8.41
(In the above relational expression 1, the symbol of each element represents the content of each element in weight%.)
The method according to claim 1,
Wherein the yield strength of the hot-rolled steel sheet is 500 to 600 MPa and the hydrogen crack crack resistance and DWTT low temperature toughness with a crack area ratio (CAR) of 5% or less are excellent.
The method according to claim 1,
Wherein the DWTT transition temperature of the hot-rolled steel sheet is -30 占 폚 or lower and the DWTT ductile waveguide ratio at -30 占 폚 is 85% or higher, and the DWTT low-temperature toughness is excellent.
The method according to claim 1,
Wherein the thickness of the hot-rolled steel sheet is 15 to 20 mm and the hydrogen-organic cracking resistance and the DWTT low-temperature toughness are excellent.
0.001% or less (excluding 0%), S: 0.001% or less (excluding 0%), Al: 0.02% or less, C: 0.03 to 0.04%, Si: 0.05 to 0.3%, Mn: Ti: 0.005-0.015%, N: 0.002-0.006%, Cu: 0.1-0.3%, Ni: 0.1-0.3%, Cr: 0.2-0.3%, Nb: 0.04-0.07% 0.35%, Mo: 0.05 to 0.35%, Ca: 0.0005 to 0.003%, the balance Fe and unavoidable impurities, and satisfying the following relational expression 1:
Heating the slab to 1150 to 1230 캜;
Hot rolling the heated slab so that the final hot rolling temperature is in the range of Ar 3 to 900 ° C to obtain a hot rolled steel sheet; And
And cooling the hot-rolled steel sheet in a temperature range of Ar 3 to 800 ° C to finish cooling in a temperature range of 450 to 520 ° C and winding up the hot-rolled steel sheet, wherein the cold rolled steel sheet has excellent resistance to hydrogen organic cracking and DWTT low temperature toughness ≪ / RTI >
Relation 1: 3.1? (Cr / Mo) * (Cu / Ni) * (Ti / N)? 8.41
(In the above relational expression 1, the symbol of each element represents the content of each element in weight%.)
6. The method of claim 5,
And a track spraying step of spraying cooling water for at least 20 seconds outside the hot rolled steel sheet during the winding.
6. The method of claim 5,
Wherein said hot rolling has a hydrogen organic cracking resistance and a DWTT low temperature toughness which are carried out such that the reduction amount at Tnr or lower is 70% or more.
6. The method of claim 5,
Wherein said cooling is excellent in hydrogen organic cracking resistance and DWTT low temperature toughness at a cooling rate of 10 to 30 DEG C / sec.

Images