JP5796368B2 - Tempered low-yield-thickness steel plate with excellent sour resistance and manufacturing method thereof - Google Patents

Description

  The present invention relates to a thick steel plate used for a pressure vessel of an oil refinery plant in a wet hydrogen sulfide corrosive environment and a method for producing the same, and in particular, a 350 to 550 MPa class thick steel plate having both a sour resistance performance and a low yield ratio, and its production method. It relates to a manufacturing method.

  The quality of crude oil is decreasing year by year, and the hydrogen sulfide concentration is increasing. Therefore, the pressure vessel of an oil refinery plant is also resistant to wet hydrogen sulfide corrosion stress, that is, excellent resistance to hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSC). Called sex). In addition, since pressure vessels are generally manufactured by cold and hot working, the thick steel plate used for them has a low yield ratio (yield stress / tensile strength x 100) in order to suppress springback during processing. Furthermore, when the yield stress is extremely high with respect to the surrounding members to be joined by welding when used as a structural member, it promotes plasticization of the surrounding members when subjected to vibration such as an earthquake. Therefore, a low yield ratio is desirable in order to ensure fracture safety. Generally, a thick steel plate having a yield ratio of 85% or less is considered to be excellent in these characteristics.

  Many studies have been made to ensure sour-resistant performance mainly in the line pipe field. To improve HIC characteristics, the amount of second-phase structure is reduced by lowering C, and Mn-S is lower. Generally, techniques such as reduction of elongation MnS due to reduction, reduction of central segregation due to low Mn-P reduction, spheroidization of MnS by optimization of Ca addition amount, and suppression of Ca cluster formation are generally used. On the other hand, in order to improve SSC characteristics, it is effective to reduce the surface layer hardness of the base metal, HAZ, and weld metal, and the hardenability is reduced due to the reduction of alloy elements and the optimum cooling and quenching conditions are optimized. Generally, reduction of surface hardness by tempering and tempering is performed.

  Various studies have also been conducted in the field of pressure vessels. For example, in Patent Documents 1 and 2, a low CB-free system is selected as a component system that effectively reduces the HAZ hardness while reducing a decrease in the strength of the base material, and Nb is added to accelerate immediately after controlled rolling. A method is disclosed in which the lack of strength is compensated for by precipitation strengthening by performing cooling or direct quenching + tempering.

  Patent Documents 3 and 4 disclose a method for achieving both sour resistance performance and hot workability by performing normalization on a thick steel plate whose structure is uniformly refined by controlled rolling. Patent Document 5 discloses a method of improving the SSC resistance by reducing the surface hardness after quenching and tempering by optimizing the rolling heating temperature of the B-added steel. Moreover, in patent document 6, the outstanding toughness and HIC-proof performance are ensured by performing 2 phase area hardening + tempering process after control rolling.

Japanese Patent Laid-Open No. 2-8322 JP-A-2-263918 Japanese Patent Laid-Open No. 8-283839 JP-A-8-283840 JP 59-74219 JP 56-90921 A

  However, the thick steel plate produced by direct quenching or TMCP disclosed in Patent Documents 1 and 2 undergoes hot transformation because the microstructure refined by rolling is retransformed when hot working is performed. Even if heat treatment is performed after processing, the originally obtained strength-toughness balance cannot be obtained, and the stretch structure formed by controlled rolling serves as a crack propagation path of HIC, so that HIC characteristics can be secured. There is a problem that it is difficult.

  In Patent Documents 3 and 4, the HIC resistance performance and the hot working performance are compatible by making a fine uniform structure over the entire thickness by the normalization process, but the manufacturing method is difficult to ensure the strength. In order to ensure, a large amount of alloy is required, which not only increases the cost, but also increases the HAZ hardness, which causes a problem of deterioration in SSC resistance.

  In Patent Document 5, the surface layer hardness of the base material can be effectively reduced, but since the optimum amount of C is high and B addition is essential, it is difficult to reduce the HAZ hardness in this range. Yes, the generation of SSC starting from HAZ cannot be avoided.

In the production method and component range of Patent Document 6, it is said that desired characteristics can be obtained with artificial seawater and 100% H ₂ S (commonly referred to as BP sour conditions), but the CH ₃ COOH + 5% NaCl solution that is the subject of the present invention. In 100% H ₂ S (commonly known as NACE sour conditions), cracking cannot be prevented unless the components are controlled more strictly and the second phase fraction and the amount and form of inclusions are controlled.

  In addition, many steel plates having a low yield ratio have been disclosed mainly in studies aimed at the construction field, but these studies do not disclose measures for ensuring sour-resistant performance.

  As described above, in the inventions so far, it has been difficult to produce a thick steel plate having a low yield ratio and excellent cold-to-hot workability without reducing sour resistance performance.

  Accordingly, an object of the present invention is to provide a thick steel plate having a low yield ratio and excellent workability between cold and hot without decreasing the sour resistance performance and a method for producing the same.

  In order to solve the above-mentioned problems, the inventors diligently studied the chemical composition, manufacturing method, and structure of steel materials for thick steel plates that are subjected to normalization and quenching and tempering treatment, which are considered to have excellent hot workability. The following findings were obtained.

  First, in order to obtain excellent HIC resistance performance, it has been confirmed that it is effective to optimize the addition amount of low C-low Mn-low S-low P-Ca as is conventionally known. In particular, with regard to the optimum addition amount of Ca, by controlling the ACR defined by the formula (3) to 1.0 to 4.0, the central portion of the plate thickness due to the spheroidization of the elongated MnS (hereinafter referred to as 1 / 2t) It was found that the HIC cracking at ¼ and the HIC cracking at 1/4 part of the plate thickness (hereinafter referred to as ¼t) due to the generation of Ca clusters can be suppressed.

  It was also found that even when the elongation MnS almost disappeared, HIC cracks occurred starting from fine precipitates such as NbC and defects such as air bubbles when the hardness of the central segregation part was high at 1/2 t.

  In the present invention, by using PHIC defined by the formula (2) as an index for rationally evaluating the influence of the component on the hardness of the center segregation, the alloy component affecting the 1/2 t HIC crack caused by the center segregation part. Made it possible to evaluate the impact of As the PHIC increases, the hardness of the central segregation part increases and cracking is likely to occur. However, by performing quenching and tempering treatment, (1) suppression of propagation of HIC cracks by equiaxed organization, (2) firing It has been found that the hardness of the central segregation part can be reduced by the reversion process, and the upper limit value of the HIC crack limit becomes larger than that of a thick steel plate produced by TMCP such as a line pipe.

  Further, it was found that the quenching and tempering treatment can secure the base material strength with a lower component than the normalizing treatment, so that the HAZ hardness can be reduced and the SSC resistance is excellent. Moreover, as the influence of a component, it confirmed again that low CB freeness would be desirable as said conventionally.

  On the other hand, it is known that the yield ratio of quenched and tempered steel increases as C decreases, and it is difficult to achieve both sour resistance performance. Therefore, in the present invention, the relationship between the form of the structure obtained by quenching and tempering and the yield ratio was investigated. As a result, the main structure is polygonal ferrite or pseudo-polygonal ferrite, and the yield ratio is low in the case of the structure form including a hard second phase between the grains. It was found that the yield ratio becomes smaller as the average particle size is larger and the fraction of the hard second phase is larger. It was also found that it is important to make both structures equiaxed in order to achieve both HIC resistance, and both characteristics can be achieved by setting the average aspect ratio to 2 or less.

  The present invention has been made by further studying the above knowledge, and the gist of the present invention is as follows.

  In the first invention, C: 0.03% or more and less than 0.08%, Si: 0.5% or less, Mn: 0.5 to 1.5%, P: 0.010% or less, S: 0.00. 0030% or less, Al: 0.005 to 0.050%, Ti: 0.005 to 0.025%, B: 0.0003% or less, Ca: 0.0005 to 0.0050%, O: 0.0030 In addition, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.10% or less, V: 0 1 or 2 or more types selected from 10% or less, Ceq defined by formula (1) is 0.28 or more, PHIC defined by formula (2) is 1.00 or less, formula The ACR defined in (3) is 1.0 to 4.0, and is a thick steel plate composed of the remaining Fe and inevitable impurities. It is a structure mainly composed of lignal ferrite and pseudo-polygonal ferrite, the average particle diameter thereof is 10 to 40 μm, the average aspect ratio is 2.0 or less, the volume fraction of hard second phase is 20 vol% or less, the average It is a tempered low-yield-thickness steel plate excellent in sour resistance performance, characterized in that the aspect ratio is 2.0 or less.

In the second invention, the continuous cast slab having the component composition described in the first invention is reheated and hot-rolled, and then heated and held from room temperature to a temperature of Ac ₃ or higher, and then 800 to 500 ° C. Excellent cooling performance with water cooling at a cooling rate of 1 ° C / s to 25 ° C / s, reheating from room temperature to a temperature of 550 ° C to Ac ₁ point, and then air cooling. It is a tempered low yield ratio steel plate.

  The present invention relates to a thick steel plate used for a pressure vessel of an oil refinery plant in a wet hydrogen sulfide corrosive environment and a method for producing the same, and in particular, production of a 350 to 550 MPa class thick steel plate having both a sour resistance performance and a low yield ratio. Is possible and is extremely effective in the industry.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. About component composition First, the reason which prescribed | regulated the component composition of the steel of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.03% or more and less than 0.08% C is the most effective element for increasing the hardenability during the quenching process and increasing the strength of the base material. If C is less than 0.03%, sufficient strength cannot be ensured, and if it is 0.08% or more, the fraction and hardness of the second phase structure increase and the HIC performance deteriorates. Further, since the HAZ hardness also increases, the C content is set to a range of 0.03% or more and less than 0.08%. Preferably, it is 0.03% or more and less than 0.05% of range.

Si: 0.5% or less Si is added for deoxidation, but if the Si content exceeds 0.5%, toughness and weldability deteriorate, so the Si content is 0.5% or less. Preferably it is 0.3% or less.

Mn: 0.5 to 1.5%
Mn is added to improve the strength and toughness of the base metal. However, if less than 0.5%, the effect is not sufficient, and if added over 1.5%, the hardness of the central segregation part increases and MnS is generated. Because of this, the HIC performance deteriorates, so the Mn content is made 0.5 to 1.5%. Preferably, it is 1.0 to 1.4% in range.

P: 0.010% or less P is an unavoidable impurity and remarkably increases the hardness of the central segregation part, and as a result, deteriorates the HIC performance. Since this tendency becomes remarkable when it exceeds 0.010%, the amount of P is made 0.010% or less. Preferably, it is 0.008% or less.

S: 0.0030% or less S is generally an MnS inclusion in steel, but the form is controlled from MnS to a spherical Ca (O, S) inclusion by addition of Ca. However, if the amount of S is large, the total amount of Ca (O, S) inclusions increases and becomes the starting point of HIC cracking, so the amount of S is made 0.0030% or less. Preferably, it is 0.0010% or less.

Al: 0.005 to 0.050%
Al is added as a deoxidizer and needs to contain 0.005% or more in order to fix the oxide. However, if it exceeds 0.050%, the cleanliness decreases and the ductility decreases. , 0.005 to 0.050% of range.

Ti: 0.005-0.025%
Ti is an essential element for forming TiN and suppressing coarsening of γ grains during heating and holding before quenching to ensure the toughness of the base material. Further, since TiN is stable even at high temperatures, CGHAZ formed when welding is refined to improve toughness and reduce HAZ hardness. In order to obtain these effects, addition of 0.005% or more is necessary, but addition of 0.025% causes TiN to become coarse and the pinning force is saturated, and during hot working and SR processing. Therefore, the Ti content is in the range of 0.005 to 0.025%. Preferably, it is 0.005 to 0.015% of range.

B: 0.0003% or less B is an element harmful to the SSC resistance. In the present invention, in order to suppress the mixing of B as much as possible, the steelmaking raw materials are examined and the content is made 0.0003% or less.

Ca: 0.0005 to 0.0050%
Ca is an element effective for controlling ductility and improving HIC resistance by controlling the form of oxysulfide inclusions. However, if it is less than 0.0005%, the effect is small, and the addition exceeds 0.0050%. In this case, since the generation of Ca clusters serves as the starting point of HIC cracking and the starting point of ductile cracks during deformation, the Ca content is in the range of 0.0005 to 0.0050%.

O: 0.0030% or less O forms an oxide with Al, Ca, etc., and exists as an inevitable inclusion in the steel. If an oxide with an O content exceeding 0.0030% is generated, it becomes the starting point of cracking of HIC and the starting point of ductile cracks, so the amount of O is made 0.0030% or less.

  Although the above is the basic component of the present invention, in order to obtain desired strength and toughness, one or more selected from Cu, Ni, Cr, Mo, Nb, and V shown below may be contained. Good.

Cu: 0.5% or less Cu is an effective element for improving toughness and increasing strength. However, when Cu is added in excess of 0.5%, weldability deteriorates. The amount of Cu is preferably 0.5% or less.

Ni: 0.5% or less Ni is an effective element for improving toughness and increasing strength. However, when Ni is added in excess of 0.5%, weldability deteriorates. The amount of Ni is preferably 0.5% or less.

Cr: 0.5% or less Cr increases the hardenability and improves the resistance to temper softening. In order to secure the strength of the quenched and tempered steel, both reduce the strength drop after tempering. Although it is an effective element, weldability deteriorates when added over 0.5%. Therefore, when Cr is added, the Cr content is preferably 0.5% or less.

Mo: 0.5% or less Mo increases both hardenability and improves temper softening resistance, and thus has both the effects of reducing strength reduction after tempering. Although it is the most effective element for securing the strength of the return-treated steel, the weldability deteriorates due to the addition exceeding 0.5%. Therefore, when adding Mo, the amount of Mo should be 0.5% or less. It is preferable.

Nb: 0.10% or less Nb is effective in increasing the strength due to both the effect of increasing hardenability and the effect of precipitation of NbC during tempering treatment, but the addition of over 0.10% causes precipitation embrittlement. Therefore, when Nb is added, the Nb content is preferably 0.10% or less in order to cause deterioration in toughness and deterioration in SSC performance by increasing HAZ hardness.

V: 0.10% or less V is effective in increasing the strength due to both the effect of improving hardenability and the effect of precipitation of VC during tempering treatment, but the addition of over 0.10% causes precipitation embrittlement. When V is added to cause the toughness to deteriorate and the weldability to deteriorate, the V content is preferably 0.10% or less.

  In the present invention, the ranges of Ceq, PHIC and ACR defined in the formulas (1) to (3) are further defined.

Ceq: 0.28 or higher The higher the value of Ceq, the higher the hardenability and the higher the strength. Ceq is set to 0.28 or more in order to obtain the strength of 350 to 550 MPa class targeted in the present invention.

PHIC: 1.00 or less PHIC is an equation devised to estimate the material of the central segregation part from the content of each alloy element. The higher the PHIC, the higher the concentration of the central segregation part, and the central segregation part hardness becomes higher. To rise. In the present invention, the hardness of the center segregation part is reduced by performing the tempering treatment. However, if the PHIC exceeds 1.00, HIC cracking due to the hardening of the center segregation part occurs. 00 or less.

ACR: 1.0-4.0
Ca has a high affinity with O. First, CaO is generated, and the remaining Ca binds to S to form CaS. ACR is an index representing the existence form of O, S and Ca in these steels. When ACR is less than 1.0, O and S exist excessively with respect to Ca, so S becomes MnS. Contributes to 1 / 2t HIC cracking. On the other hand, if it exceeds 4.0, excessively added Ca becomes a cluster and promotes HIC cracking of ¼ t. Therefore, ACR is set in the range of 1.0 to 4.0. Preferably, it is the range of 1.5-3.5.

2. About structure | tissue In this invention, the volume fraction of a metal structure in 1 / 2t of a thick steel plate, a particle size, and an aspect ratio are prescribed | regulated.

Average particle diameter of polygonal ferrite and pseudopolygonal ferrite: 10 to 40 μm
The average grain size of polygonal ferrite and pseudo-polygonal ferrite, which are the main structure of the steel sheet of the present invention, decreases as the soft ferrite has a larger grain size. As a result, the yield ratio is increased, and as a result, the toughness deteriorates if the yield ratio exceeds 40 μm. Therefore, the range of the average particle diameter is set to 10 to 40 μm.
Preferably, it is 10-30 micrometers.

  The main component means that it contains 80 vol% or more of polygonal ferrite and pseudopolygonal ferrite, and is preferably 95 vol% at the maximum. If polygonal ferrite and pseudo-polygonal ferrite exceed 95 vol%, it is difficult to ensure strength and the yield ratio becomes high. Moreover, if it is less than 80 vol%, when the HIC test is performed, cracks propagate through the hard second phase, and excellent HIC performance cannot be ensured.

  Polygonal ferrite is a massive ferrite and is a structure observed white by an optical microscope when it is subjected to nital etching. On the other hand, pseudo-polygonal ferrite has a slightly square shape compared to polygonal ferrite, and refers to a slightly baked structure than polygonal ferrite. In addition, the fact that both the structures do not contain lath-like carbides in the grains can also be mentioned as a structure discrimination criterion.

Hard second phase volume fraction: 20 vol% or less Hard second phase consisting of low temperature transformation structure such as bainite and high C structure such as island martensite (MA) and cementite is pointed in soft ferrite matrix. By being present, strain is concentrated at the interface of the parent phase, and a low yield ratio is realized due to a decrease in yield stress. However, if the hard second phase is included in excess of 20 vol%, the sour resistance is lowered, so the volume fraction of the hard second phase is 20 vol% or less. In addition, since the effect of a low yield ratio cannot be obtained if it is less than 5 vol%, the lower limit is preferably 5 vol%.

  The hard second phase refers to a low-temperature transformation structure such as bainite or martensite having a higher hardness than the soft ferrite, and a high C structure such as island martensite (MA), cementite, or pearlite.

Average aspect ratio of polygonal ferrite, pseudopolygonal ferrite, and hard second phase: 2.0 or less Because the equiaxed polygonal ferrite, pseudopolygonal ferrite, and hard second phase have higher propagation resistance against HIC cracking However, if the aspect ratio exceeds 2.0, the propagation resistance becomes weak and the HIC characteristics deteriorate, so the average aspect ratio of polygonal ferrite, pseudopolygonal ferrite and the second phase is set to 2.0 or less.

3. Production Method Continuous Casting ACR defined in the present invention is an optimum range for continuous casting. Since the ingot forming method cannot appropriately suppress the formation of MnS and Ca clusters, it is limited to continuous casting.

Quenching temperature: Ac ₃ points or more The quenching temperature is Ac ₃ points or more. The lower the quenching temperature, the finer the structure and the higher the yield ratio. In addition, since the strength is remarkably reduced when Ac becomes ₃ points or less and the desired strength cannot be obtained, the Ac ₃ point is set as the lower limit. Preferably, Ac is ₃ points to 950 ° C. Although it is desirable to obtain Ac ₃ points by a four master test or the like, it may be obtained by equation (5).

Quenching cooling rate: 1 ° C / s or more and 25 ° C / s or less The cooling rate from 800 ° C to 500 ° C during quenching is 1 ° C / s or more and 25 ° C / s or less. The slower the cooling rate during quenching, the coarser and softer the ferrite is obtained. On the other hand, when the cooling rate is 25 ° C./s or more, a structure mainly composed of bainite is obtained, and the yield ratio is increased. 1 ° C./s or more and 25 ° C./s or less. Preferably, it is 3 ° C./s or more and 20 ° C./s or less.

Tempering temperature: 550 ° C. or higher and Ac ₁ point or lower Tempering temperature is 550 ° C. or higher and Ac ₁ point or lower. By performing the tempering process, the hardness of the central segregation part is lowered and the HIC performance is improved. Also, the surface layer hardness is reduced, and the SSC characteristics are improved. This effect cannot be obtained at a temperature lower than 550 ° C., and if the temperature exceeds ₁ Ac, the reverse transformation occurs, and the high C reverse transformation structure deteriorates toughness. Therefore, the upper limit is set to Ac ₁ point. Incidentally, Ac ₁ point, it is desirable to determine the like formastor test, no problem even by Equation (4).

  Steels (A to J) having chemical components shown in Table 1 are made into slabs by a continuous casting method, re-heated and hot-rolled at a high temperature (950 ° C. or higher) so that the plate thickness becomes 15 to 120 mm, and then room temperature. Air-cooled until. Subsequently, after removing the surface scale by shot blasting, a quenching and tempering treatment was performed under the conditions shown in Table 2 to obtain a thick steel plate No. 1-No. 14 was produced. The holding temperature of quenching and tempering was the holding temperature of the furnace, and the holding time was the time from when the furnace temperature reached the target temperature of −20 ° C. until the thick steel plate was taken out from the furnace. The cooling rate at the time of quenching is as follows. 1 is produced, a part sample is taken from a thick steel plate as-quenched (before tempering), and each hardness is compared by comparing the hardness of 1 / 2t with the hardness of a CCT diagram held at 930 ° C. for 10 minutes. The cooling rate for each plate thickness was estimated. The tempering was cooled by air cooling.

  As a comparison, thick steel plate No. No. 15, reheating the continuous cast slab to 1150 ° C., performing rolling reduction at 950 ° C. or less to 50%, finishing the rolling at 760 ° C., and setting the sheet thickness to 30 mm, and then increasing the sheet thickness from 720 ° C. to 400 ° C. It was prepared by cooling with water and cooling to room temperature (referred to as TMCP).

  These steel plates were subjected to characteristic evaluation tests and microstructure quantification by the following methods. The tensile test was performed by collecting a round bar tensile test piece having a diameter of 12.7 mm in accordance with ASTMA370-07a from the direction perpendicular to the rolling at the 1 / 2t position. At this time, those having a tensile strength exceeding 415 MPa (corresponding to the lower limit of A516-Gr.60) and those having a yield ratio (0.5% yield stress / tensile strength × 100) of 85% or less were regarded as acceptable.

  In the Charpy test, three 2 mm V notch specimens taken from the direction perpendicular to the rolling at the 1/2 t position were tested at −50 ° C., and the average value was used.

Based on NACE Standard TM0284-2003, the HIC characteristics were determined by taking three samples each and placing 96 specimens in a 5% NaCl + 0.5% CH ₃ COOH aqueous solution saturated with hydrogen sulfide having a pH of about 3. After immersion for a period of time, the presence or absence of cracks on the entire surface of the test piece was investigated by ultrasonic flaw detection, and evaluated by the crack area ratio (CLR). Here, the maximum value of each steel plate was set as CLR of the steel plate, and CLR ≦ 6% was regarded as acceptable.

Based on NACE Standard TM0177, the SSC characteristics were obtained by taking two round bar tensile samples, applying a load stress of 80% of the base material, and saturating hydrogen sulfide having a pH of about 3 with 5% NaCl + 0. It was evaluated whether or not the test piece was immersed in a 5% CH ₃ COOH aqueous solution for 720 hours to break. At this time, when neither of them was broken, it was evaluated as “No crack”, and when even one piece was broken, it was evaluated as “Crac”.

  The volume fraction and aspect of the microstructure were mirror-polished from an L-plane observation sample taken from the 1 / 2t position of each thick steel plate, 3% nital etched, and three 400x photographs were taken with an optical microscope. And obtained by performing image analysis. At this time, the volume fractions of polygonal ferrite and pseudo-polygonal ferrite were determined by taking the ratio between the entire visual field and the tissue part, assuming that the area ratio was the same as the volume ratio. Further, the average grain diameter of polygonal ferrite and pseudopolygonal ferrite is the average value of the equivalent circle diameter of each crystal grain, and the aspect ratio is the average value of the long side / short side of each crystal grain. As the volume fraction of the hard second phase, the volume fraction of portions other than polygonal ferrite and pseudopolygonal ferrite was adopted, and as the aspect ratio, the average value of the long side / short side of each structure and tissue group was adopted.

    Table 3 shows the test results obtained.

  Thick steel plate No. Since all of Nos. 1 to 6 satisfy the component range, structure form range, and production method range of the present invention, desired strength, toughness, HIC resistance and SSC resistance are obtained. On the other hand, thick steel plate No. 7-No. Since 15 is a comparative example and is outside the scope of the present invention, it does not satisfy any characteristics such as strength and toughness. No. Since No. 7 has a high quenching cooling rate, it has a bainite-based structure and a high yield ratio. No. In No. 8, since the quenching cooling rate is slow, the soft ferrite is too coarse and the strength and toughness are deteriorated. No. 9 is a ferrite-based metal structure, but the ferrite grain size is too fine, so the yield ratio is high. No. In No. 10, since Mn and PHIC exceed the upper limit, many cracks are observed in HIC. No. Since No. 11 has a low ACR, many cracks are observed in the HIC. No. In No. 12, since C exceeds the upper limit, many cracks are observed in HIC, and cracks are also generated in SSC. No. In No. 13, since Ti is not added, ferrite grains are coarsened and toughness is deteriorated. No. No. 14 is cracked by SSC because B is added. No. No. 15 is manufactured by TMPC, and since the aspect ratio of soft ferrite and hard second phase is large, many cracks are observed in HIC.

Claims (2)

By mass%, C: less than 0.03% or more 0.08%, Si: 0.5% or less, Mn: 0.5~1.5%, P: 0.010% or less, S: 0.0030% Hereinafter, Al: 0.005 to 0.050%, Ti: 0.005 to 0.025%, B: 0.0003% or less, Ca: 0.0005 to 0.0050%, O: 0.0030% or less Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.10% or less, V: 0.10 % Or less, Ceq defined by Formula (1) is 0.28 or more, PHIC defined by Formula (2) is 1.00 or less, Formula (3 ) Is defined as 1.0 to 4.0, a thick steel plate made of the remaining Fe and unavoidable impurities, and a plate thickness at a position 1 / 2t of the plate thickness t In tissues the central portion of the tissue mainly composed of soft ferrite comprising polygonal ferrite and quasi-polygonal ferrite, 10 to 40 [mu] m average particle diameter of the soft ferrite, the average aspect ratio is 2.0 or less, the hard second phase A tempered low-yield ratio thick steel plate excellent in sour-resistant performance, characterized by having a volume fraction of 20 vol% or less and an average aspect ratio of 2.0 or less.

It is a manufacturing method of the tempered type low yield specific thickness steel plate of Claim 1 , and after reheating and hot-rolling a continuous casting slab, it heats and hold | maintains from room temperature to the temperature of Ac ₃ or more points, 800 Water cooling at a cooling rate of ˜500 ° C. at 1 ° C./s to 25 ° C./s, reheating from room temperature to a temperature of 550 ° C. to Ac ₁ point, and then air cooling after cooling A method for producing a tempered low yield specific thickness steel plate with excellent performance.