KR101639909B1 - Thick hot rolled steel plate having exellent hydrogen induced crack resistance and sulfide stress cracking and method for manufacturing the same - Google Patents

Abstract

The steel sheet according to the present invention is characterized by containing 0.02 to 0.05% of C, 0.05 to 0.5% of Si, 0.5 to 1.4% of Mn, 0.01% or less of P (excluding 0% 0.002 to 0.008% of N, 0.01 to 0.5% of Cr, 0.01 to 0.5% of Mo, 0.0015 to 0.003% of Ca, 0.01 to 0.5% of Cr, 0.01 to 0.07% of Al, 0.01 to 0.07% of Nb, 0.005 to 0.02% Fe and other unavoidable impurities, and satisfies the condition of 1? Cr / Mo? 5, and the microstructure contains at least 2 percent by area of pearlite and 0.5 percent by area or less of MA, The present invention provides a hot-rolled steel sheet excellent in hydrogen-organic cracking resistance and sulfide stress cracking resistance with an inclusion of 50 or less per 100 x 50 mm2.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet having excellent hydrogen organic cracking resistance and sulfide stress cracking resistance,

The present invention relates to a hot rolled steel sheet excellent in organic hydrogen cracking resistance and sulfide stress cracking resistance and a method for producing the same.

With the development of crude oil or natural gas having a high content of H ₂ S gas according to the increase of energy demand, a steel material excellent in resistance to breakage of steel material by hydrogen sulfide (H ₂ S) gas is required. In addition, as the demand for natural gas as a recent energy source has increased, high stress can be applied to the line pipe steel according to high-pressure natural gas transportation, and steel material with excellent sour resistance is required.

Particularly, in the case of a gas containing H ₂ S or steel for transporting crude oil, there is a problem of hydrogen induced crack (HIC) and sulfide stress cracking (SSC) caused by H ₂ S. However, _It is known that the hydrogen generated on the surface of the steel due to the corrosion reaction of the ₂ S atmosphere is cracked due to the pressure of the hydrogen molecules collected on the steel inclusions or the light secondary as the steel penetrates and diffuses into the steel as an atomic state .

As a method for effectively controlling the two cracks (HIC and SSC), there are proposed a method of controlling the length of the non-metallic inclusion and the hardness of the segregation part, and a method of improving the hydrogen cracking resistance by controlling the composition of the nonmetallic inclusions. However, the above-mentioned prior arts have a problem that the non-metallic inclusions present in the steel are inevitably present everywhere and can not simultaneously satisfy the hydrogen-organic cracking resistance and the sulfide stress cracking resistance.

In addition, the probability of occurrence of the two cracks (HIC, SSC) in the H ₂ S environment becomes higher due to the increase of the dislocation density in the steel after the piping, which is not shown until the pipe is manufactured, Which is a phenomenon that can occur in the user.

An object of the present invention is to provide a hot rolled steel sheet having excellent hydrogen-organic cracking resistance and sulfide stress cracking property which can be preferably applied to a corrosive environment, and a method for producing the same.

An embodiment of the present invention is a steel sheet comprising, by weight%, 0.02 to 0.05% of C, 0.05 to 0.5% of Si, 0.5 to 1.4% of Mn, 0.01% or less of P (exclusive of 0%), 0.001% or less of S Nb: 0.002 to 0.008, Cr: 0.01 to 0.5%, Mo: 0.01 to 0.5%, Ca: 0.0015 to 0.003% %, Remainder Fe and other unavoidable impurities and satisfies the condition of 1? Cr / Mo? 5, the microstructure contains 2 percent by area or less of perlite and 0.5 percent by area or less of MA, The present invention provides a hot-rolled steel sheet having excellent hydrogen-organic cracking resistance and sulfide stress cracking resistance, wherein the number of nonmetallic inclusions is 50 or less per 100 x 50 mm2.

Another aspect of the present invention is to provide a method of manufacturing a semiconductor device, comprising: 0.02 to 0.05% of C, 0.05 to 0.5% of Si, 0.5 to 1.4% of Mn, 0.01% 0.002 to 0.008% of N, 0.01 to 0.5% of Cr, 0.01 to 0.5% of Mo, 0.01 to 0.5% of Ca, 0.0015 to 0.5% of Cr, To 0.003%, the balance Fe and other unavoidable impurities, and satisfying the condition of 1? Cr / Mo? 5; Continuously casting the refined molten steel to produce a slab; Reheating the slab at 1150 to 1350 ° C; Subjecting the reheated slab to finish hot-rolling at a temperature of Ar 3 to 950 ° C to obtain a hot-rolled steel sheet; And cooling the hot-rolled steel sheet at a temperature of from Ar 3 to 950 ° C to finish at 450 to 600 ° C and then winding the microstructure, wherein the microstructure contains not more than 2% by area of pearlite and not more than 0.5% by area of MA The present invention also provides a method for manufacturing a hot rolled steel sheet having excellent hydrogen-organic cracking resistance and sulfide stress cracking resistance, wherein non-metallic inclusions having an average size of 20 μm or more are 50 or less per 100 × 50 mm 2.

According to the present invention, it is possible to use the present invention even in an environment where there is high resistance to hydrogen organic cracking, high stress, or corrosion, as well as a posterior material, and hydrogen hydrogen organic cracking resistance and sulfide stress cracking resistance It is possible to provide an excellent after-hot-rolled steel sheet.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a photograph of a microstructure of Invention Steel 1 according to an embodiment of the present invention. Fig.
2A and 2B are optical photographs taken after MA etching of Invention Steel 1 and Comparative Steel 1 according to the embodiment of the present invention, respectively.
3 is a photograph of a fracture surface of a sulfide stress crack in Comparative Example 3 of an embodiment of the present invention.

The present invention relates to a hot rolled steel sheet excellent in hydrogen-organic cracking resistance and sulfide stress cracking resistance and a method for producing the same.

An embodiment of a hot rolled steel sheet having excellent hydrogen-organic cracking resistance and sulfide stress cracking resistance according to the present invention comprises 0.02 to 0.05% of C, 0.05 to 0.5% of Si, 0.5 to 1.4% of Mn, 0.5 to 1.4% of Mn, : 0.01% or less (excluding 0), S: 0.001% or less (excluding 0), Al: 0.02-0.05%, Nb: 0.01-0.07%, Ti: 0.005-0.02% Cr / Mo? 5, 0.01 to 0.5% Mo, 0.01 to 0.5% Ca, 0.0015 to 0.003% Ca and the balance Fe and other unavoidable impurities. The microstructure contains pearlite in an area of 2% , And 0.5% or less of MA, and non-metallic inclusions having an average size of 20 탆 or more provide 50 or less per 100 × 50 mm 2.

Hereinafter, reasons for limiting the above-described composition 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, V, or Ti is very small. When C is added in an amount exceeding 0.05% by weight, There is a problem that segregation increases. Therefore, it is preferable that C is in the range of 0.02 to 0.05% by weight.

Si: 0.05 to 0.5 wt%

The Si is effective 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 it exceeds 0.5% by weight, the weldability and the brittleness are lowered. Therefore, the Si content is preferably in the range of 0.05 to 0.5% by weight.

Mn: 0.5 to 1.4 wt%

When Mn is added in an amount of less than 0.5% by weight, it is difficult to secure strength and toughness. When the Mn content exceeds 1.4% by weight, center segregation is promoted at the performance, It may deteriorate the property and the SSC property. Therefore, it is preferable that the Mn is in the range of 0.5 to 1.4 wt%.

P: 0.01 wt% or less (excluding 0%)

When the content of P exceeds 0.01 wt%, central segregation is promoted along with Mn at the time of playing to deteriorate the impact toughness, HIC resistance and SSC resistance as well as weldability, so that the content of P is 0.01 By weight or less.

S: 0.001 wt% or less (excluding 0%)

The S significantly reacts with Mn in the steel to form MnS, thereby greatly reducing the brittleness. When the content exceeds 0.001 wt%, the HIC property and the SSC property are greatly reduced. Therefore, it is preferable to control the content of S to 0.001 wt% or less.

Al: 0.02 to 0.05 wt%

When Al is added at a content of less than 0.02% by weight, it is difficult to obtain a deoxidation effect. When it exceeds 0.05% by weight, the content of alumina aggregates is increased to decrease the HIC property and the SSC property , It is preferable to control the content of Al to be in the range of 0.02 to 0.05% by weight.

Nb: 0.01 to 0.07 wt%

The 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 exceeding 0.07 wt%, impact toughness and weldability It is preferable to control the content thereof to 0.07 wt% or less. Therefore, the Nb content is preferably 0.01 to 0.07% 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, thereby obtaining a high strength and excellent impact toughness, and also a function of strengthening the steel by precipitation with TiC or the like. 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 above effect becomes saturated, so that the content of Ti is preferably controlled to 0.005 to 0.02% by weight.

N: 0.002 to 0.008 wt%

The N precipitates as Ti and TiN in the steel to inhibit the growth of austenite grains. However, when the N content is less than 0.002% by weight, the effect is small. When the N content is more than 0.008% by weight, coarse TiN precipitates to act as a starting point for hydrogen organic cracking and sulfide stress cracking. To 0.008% by weight.

Cr: 0.01 to 0.5 wt%

The Cr is added to increase strength and ensure corrosion resistance. However, when Cr is added in an amount of less than 0.01% by weight, the effect is small. When Cr is added in an amount of more than 0.5% by weight, the risk of local corrosion increases. Therefore, the content of Cr is preferably controlled to 0.1 to 0.5% by weight.

Mo: 0.01 to 0.5 wt%

The Mo is added for the purpose of increasing the strength and securing the acicular ferrite structure as a low-temperature phase. However, when the Mo content is less than 0.01% by weight, the above effect is small. When the Mo content exceeds 0.5% by weight, a large amount of mild secondary phase such as MA (Martensite austenite) is produced to lower the HIC and SSC resistance. It is preferably controlled to 0.01 to 0.5% by weight.

Ca: 0.0015 to 0.003 wt%

Ca is a component which suppresses the generation point of the hydrogen organic cracking by spheroidizing the emulsion type inclusion. When the content is less than 0.0015% by weight, it is difficult to obtain the above effect. When the content exceeds 0.003% by weight, The amount of inclusions is increased, and the hydrogen organic cracking resistance can be lowered. Therefore, it is preferable that the content of Ca is in the range of 0.0015 to 0.003% by weight.

In the present invention, the ratio of Cr to Mo satisfies the condition of 1? Cr / Mo? 5, and preferably satisfies 1? Cr / Mo? 2. Here, Cr and Mo mean the weight percent value of the component. If the content ratio is less than 1, the total corrosion rate of the steel due to Mo is increased, and the HIC and SSC properties of the steel are lowered by introducing a large amount of hydrogen in the steel, and when the content ratio is more than 5 There is a problem in that the Cr content becomes too large and the SSC property due to local corrosion deteriorates.

The steel sheet provided by the present invention is composed of Fe and other unavoidable impurities in addition to the above-mentioned composition.

The ratio of the contents of Ca to S is preferably in the range of 1.5? Ca / S? 4. Here, Ca and S mean the weight percentage value of the component. When the content ratio is less than 1.5, the MnS formation is easy and the HIC property and the SSC property decrease. When the content ratio exceeds 4, the Ca nonmetal inclusion quantity increases and the HIC property And the SSC property and toughness are deteriorated.

The steel sheet of the present invention is characterized in that not only the above alloy composition and composition but also the microstructure are acicular ferrite having a main structure of not more than 2% by area of pearlite, not more than 0.5% by area of MA as a minor phase, It is preferable that the number of non-metallic inclusions is 50 or less per 100 x 50 mm < 2 >. The control is based on the fact that the sizes of the nonmetallic inclusions that are the origin of the cracks are different depending on the structure and hardness around the cracks. The control can improve the hydrogen-induced organic cracking and the sulfide stress cracking. If the main structure is polygonal ferrite, resistance to stress cracking in the sulfide is reduced. If the pearlite is more than 2 area% or more than 50 non-metal inclusions, it becomes a starting point of hydrogen cracking in the hot rolling step, The HIC property and the SSC property may be lowered. At this time, the non-metallic inclusions causing cracks are oxides mainly containing Al and Ca, and oxides containing Al and Ca in addition to Mg. In addition, if the MA, which is the secondary phase acting as a main hydrogen trap site in the steel, exceeds 0.5% by area, a large amount of diffusible hydrogen may be introduced into the steel, which may reduce the HIC property and the SSC property. The steel sheet according to the present invention has a composite structure including needle-like ferrite, pearlite of 2% by area or less, and MA of 0.5% by area, and all of the above characteristics can be satisfied.

The steel sheet provided by the present invention can have a yield strength of 400 MPa or more and satisfies a CAR (Crack Area Ratio) value of 5% or less, which is a value obtained by dividing the total area of the hydrogen-induced cracked area caused by hydrogen by the steel sheet area, It is preferable that the crack-breaking critical load satisfies 90% or more of the yield strength. By satisfying the above conditions, the present invention can provide a hot-rolled steel sheet having excellent resistance to hydrogen-organic cracking as well as excellent resistance to sulfide stress cracking.

Hereinafter, a method for manufacturing the hot-rolled steel sheet of the present invention will be described.

An embodiment of the method for manufacturing a hot rolled steel sheet having excellent hydrogen-organic cracking resistance and sulfide stress cracking resistance according to the present invention is characterized by containing 0.02 to 0.05% of C, 0.05 to 0.5% of Si, 0.5 to 1.4% of Mn, , P: not more than 0.01% (excluding 0), S: not more than 0.001% (excluding 0), Al: 0.02 to 0.05%, Nb: 0.01 to 0.07%, Ti: 0.005 to 0.02% Refining molten steel having a composition containing 0.01 to 0.5% of Cr, 0.01 to 0.5% of Mo, 0.0015 to 0.003% of Ca, the balance Fe and other unavoidable impurities and satisfying the condition of 1? Cr / Mo? 5 ; Continuously casting the refined molten steel to produce a slab; Reheating the slab at 1150 to 1350 ° C; Subjecting the reheated slab to finish hot-rolling at a temperature of Ar 3 to 950 ° C to obtain a hot-rolled steel sheet; And cooling the hot-rolled steel sheet at a temperature of from Ar 3 to 950 ° C to finish at 450 to 600 ° C and then winding the microstructure, wherein the microstructure contains not more than 2% by area of pearlite and not more than 0.5% by area of MA And non-metallic inclusions having an average size of 20 탆 or more provide 50 or less per 100 횞 50 mm 2.

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. After refining the molten steel, molten steel may be continuously cast to produce a slab.

Reheat temperature

The reheating temperature is determined by the solid-solution temperature of the Nb-based precipitate. In the component range of the present invention, the solid solution can be solidified at a temperature of 1150 ° C or higher. When heated above 1350 ° C, The temperature range is preferably in the range of 1150 to 1350 ° C.

Rolling conditions

The amount of rolling reduction below the non-recrystallization temperature 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 hydrogen organic cracking resistance, low temperature toughness, and yield ratio. Therefore, in order to control the grain size and the uniformity, it is desirable that the rolling reduction 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. 70% to the maximum thickness reduction ratio of the corresponding thickness. On the other hand, the finish hot rolling is preferably performed in a temperature range of Ar 3 to 950 ° C. When rolled at a temperature of 950 ° C or higher, there is a high possibility that rough grain growth may occur unevenly, When the hot rolling is performed in the range, the aggregate structure for generating brittle fracture tends to be generated and the hydrogen organic cracking resistance may be very low.

Cooling and Winding conditions

The cooling of the hot-rolled steel sheet obtained through the hot rolling is preferably started 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 toughness and develop a brittle fracture texture that lowers resistance to hydrogen-induced organic cracking. 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 40 DEG C / sec. If the cooling rate is less than 10 ° C / sec, a coarse pearlite structure that lowers the hydrogen organic cracking resistance can be easily formed. If the cooling rate is more than 40 ° C / sec, the hydrogen organic cracking and sulfide stress A large amount of bainite may be formed to lower the crack resistance, or the martensite (MA) fraction may increase.

Thereafter, the cooling is preferably finished at 450 to 600 ° C, and then the hot-rolled steel sheet is preferably rolled in the temperature range. When the coiling temperature range is higher than 600 ° C, the transformation is unstable and coarse pearlite structure may be formed, which may result in degradation of hydrogen organic cracking resistance. When the temperature is less than 450 DEG C, the steel sheet has a high rigidity, so that normal winding is very difficult. Therefore, it is preferable that the winding is performed in a temperature range of 450 to 600 ° C.

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

[ Example ]

Hot-rolled steel sheets having a thickness of 12 to 18 mm were produced by controlling the non-metallic inclusions by refining molten steel having the composition shown in Table 1 below, and then manufacturing conditions as shown in Table 2 below.

The number of non-metallic inclusions, the pearlite fraction, the yield strength, the MA fraction, the CAR of the hydrogen-induced cracks, and the critical stresses of the fracture failure of the sulfide stress cracks were measured on the thus prepared steel sheet, and the results are shown in Table 3 below. Here, the actual yield strength of Table 3 means "Actual yield strength", which is usually expressed as Ac.YS when SSC is evaluated. In addition, according to NACE TM0284, the hydrogen organic cracking resistance of the steel sheet was immersed in a 5% NaCl + 0.5% CH ₃ COOH solution saturated with 1 atm H ₂ S gas for 96 hours, , And then evaluated by CAR, which is a value obtained by dividing the total sum of hydrogen organic cracking areas of the steel sheet by the total area of the steel sheet.

The sulfide stress cracking resistance was measured by immersing the sample in a 5% NaCl + 0.5% CH ₃ COOH solution saturated with 1 atm H ₂ S gas for 720 hours according to NACE TM0177 while applying a 90% And then evaluated for breakage. The distribution and size of the nonmetallic inclusions in the steel sheet were measured by optical emission spectroscopy. The luminescence analyzer analyzes the nonmetallic inclusions by analyzing the characteristic spectrum of the emitted elements by generating plasma by rapidly heating a region of about 50 μm on the surface of the metal specimen. To obtain statistically meaningful values, several parts were continuously analyzed for the same specimen, and the average value was described. The pearlite fraction was measured by image analysis at 500x magnification using an optical microscope. The MA fraction was analyzed by image analysis at 500x magnification through an optical microscope as well as the pearlite fraction after MA exposure through color etching Image analysis).

division Chemical composition (% by weight) C Si Mn P S Al Cr Mo Ti Nb N Ca Cr / Mo Ca / S Inventive Steel 1 0.035 0.2 1.29 0.008 0.0009 0.03 0.2 0.15 0.01 0.06 0.004 0.002 1.33 2.22 Invention river 2 0.038 0.19 1.3 0.007 0.0008 0.031 0.4 0.3 0.01 0.06 0.0039 0.002 1.33 2.5 Comparative River 1 0.035 0.18 1.31 0.007 0.0008 0.027 0.3 0.01 0.01 0.05 0.0042 0.002 30 2.5 Comparative River 2 0.040 0.19 1.28 0.006 0.0007 0.029 0.2 0.1 0.01 0.06 0.004 0.002 2 2.86 Comparative Steel 3 0.046 0.18 1.29 0.008 0.0007 0.026 0.2 0.2 0.015 0.05 0.0039 0.0042 One 6

division Grade Nr. Reheating temperature (℃) Finishing hot rolling temperature (캜) Coiling temperature (캜) Inventory 1 Inventive Steel 1 1316 798 473 Inventory 2 Invention river 2 1311 811 504 Comparative Example 1 Comparative River 1 1306 856 587 Comparative Example 2 Comparative River 2 1278 889 646 Comparative Example 3 Comparative Steel 3 1296 867 572

division Yield strength (MPa) Perlite fraction (%) 20㎛ or more
Number of nonmetallic inclusions MA fraction (%) CAR (%) Actual yield strength SSC break at 90% load Inventory 1 518 0.1 28 0.04 0 Fracture Inventory 2 541 0.05 39 0.31 1.2 Fracture Comparative Example 1 489 0.9 45 0.8 0.4 Fracture Comparative Example 2 507 2.4 30 2.1 4.3 Fracture Comparative Example 3 533 1.1 85 0.5 6.5 Fracture

As can be seen from Tables 1 to 3, the inventive alloys have excellent yield strengths of 518 MPa or more in Inventive Examples 1 and 2, which satisfy the composition range and composition range and manufacturing conditions. In addition, it can be confirmed that the condition of 0.5 area% or less is satisfied because the fraction of MA structure is very small, and it is confirmed that CAR (crack area ratio) hardly appears. As can be seen from Table 3, by satisfying the conditions of the present invention, it is confirmed that the sulfide stress cracks do not occur even in 90% or more of the yield strength in Examples 1 and 2.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph showing microstructure of Inventive Example 1. FIG. As shown in Fig. 1, in the case of Inventive Example 1, it is shown that the acicular ferrite main body contains very small and small pearlite.

2A and 2B are photographs of microstructure observed after MA etching of Invention Steel 1 and Comparative Steel 1 to observe MA structure. MA etching showed that the MA structure was whitish. When 2a of Invention Steel 1 and 2b of Comparative Steel 1 were compared, almost white did not appear in Figure 2a, and much more white particles in Figure 2b appeared , Indicating that the comparative steel 1 contains a large amount of MA as compared to Inventive Example 1.

3 is a photograph of a fracture surface of a sulfide stress crack in Comparative Example 3 of an embodiment of the present invention. Fig. 3 shows that a large amount of inclusions in the steel resulted in many hydrogen-induced cracks in the rolling direction during the test as in the case of the sulfide stress cracking fracture, and thus fractured.

On the other hand, in Comparative Example 1 in which the condition of 1? Cr / Mo? 5 is satisfied, it is confirmed that the MA fraction is very large, which satisfies the composition of the present invention, It is very important condition to secure organic cracking resistance and sulfide stress cracking at the same time.

In the case of Comparative Example 2, which is in conformity with the component system of the present invention but does not satisfy the production conditions, since the coiling temperature is out of the range of the present invention, the yield strength is low due to insufficient transformation. Furthermore, it is understood that the resistance to hydrogen organic cracking is reduced because a large amount of non-metallic inclusions are contained.

In the case of Comparative Example 3, the condition of 1.5 = Ca / S = 4 is not satisfied even though it corresponds to the component system of the present invention. This is because, when the Ca / S ratio exceeds 4, a large amount of Ca content causes a large amount of inclusions to be oxidized, thereby serving as a starting point for hydrogen organic cracking and sulfide stress cracking.

While the illustrative embodiments of the present invention have been shown and described, various modifications and alternative embodiments may be made by those skilled in the art. Such variations and other embodiments will be considered and included in the appended claims, all without departing from the true spirit and scope of the invention.

Claims (8)

(Excluding 0%), S: 0.001% or less (excluding 0%), Al: 0.02% or less, C: 0.05 to 0.5% 0.001 to 0.08% of N, 0.005 to 0.02% of N, 0.002 to 0.008 of N, 0.001 to 0.5% of Cr, 0.01 to 0.5% of Mo, 0.0015 to 0.003 of Ca, the balance Fe and other unavoidable impurities Wherein the microstructure comprises a needle-shaped ferrite, a pearlite of 2% or less by area and a MA of 0.5% or less by area, and an average size of 20 Wherein the nonmetallic inclusions are 50 or less per 100 x 50 mm < 2 >, and the hydrogen sulfide cracking resistance is excellent.
The hot-rolled steel sheet according to claim 1, which satisfies the condition of 1 Cr Cr / Mo 2 2 and is excellent in resistance to hydrogen-organic cracking and sulfide stress cracking.
The hot-rolled steel sheet according to claim 1, which satisfies the condition of 1.5 < = Ca / S < = 4 and has excellent resistance to hydrogen-organic cracking and sulfide stress cracking.
The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has a yield strength of 400 MPa or more, CAR (Crack Area Ratio) of 5% or less, and a sulfide stress crack cracking critical load of 90% Hot rolled steel sheet with excellent sulfide stress cracking resistance.
(Excluding 0%), S: 0.001% or less (excluding 0%), Al: 0.02% or less, C: 0.05 to 0.5% 0.001 to 0.08% of N, 0.005 to 0.02% of N, 0.002 to 0.008 of N, 0.01 to 0.5% of Cr, 0.01 to 0.5% of Mo, 0.0015 to 0.003% of Ca, the balance Fe and other unavoidable impurities Refining molten steel containing impurities and having a composition satisfying the condition of 1? Cr / Mo? 5;
Continuously casting the refined molten steel to produce a slab;
Reheating the slab at 1150 to 1350 ° C;
Subjecting the reheated slab to finish hot-rolling at a temperature of Ar 3 to 950 ° C to obtain a hot-rolled steel sheet; And
And cooling the hot-rolled steel sheet at a temperature of Ar 3 to 950 ° C to finish at 450 to 600 ° C,
Wherein the microstructure is composed of acicular ferrite having a pearlite content of not more than 2% by area and not more than 0.5% by area of MA, and a nonmetallic inclusion having an average size of not less than 20 탆 not more than 50 per 100 × 50 mm 2 And a method for producing a hot rolled steel sheet having excellent sulfide stress cracking resistance.
6. The method of manufacturing a hot rolled steel sheet according to claim 5, wherein the cooling is performed at a rate of 10 to 40 DEG C / sec, and the hydrogen-organic cracking property and the sulfide stress cracking property are excellent.
6. The method of producing a hot rolled steel sheet according to claim 5, wherein the hot-rolled steel sheet satisfies 1.5? Ca / S? 4, and has excellent hydrogen-organic cracking resistance and sulfide stress cracking resistance.
The steel sheet according to claim 5, wherein the hot-rolled steel sheet has a yield strength of 400 MPa or more, a CAR (Crack Area Ratio) of 5% or less and a sulfide stress crack cracking critical load of 90% A method for producing a hot rolled steel sheet having excellent sulfide stress cracking property.

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