JP5796369B2 - 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 CH ₃ COOH + 5% NaCl, which is a problem in the present invention, With 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 was 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, by setting the quenching temperature to (Ac ₁ + Ac ₃ ) / 2 or more and Ac ₃ points or less and the quenching cooling rate to 15 ° C./s or more, the portion austenitized by reverse transformation becomes hard bainite or martensite by quenching. Thus, it was found that the yield ratio can be lowered without deteriorating the sour-resistant performance by making the remainder that has not undergone reverse transformation become equiaxed tempered polygonal ferrite and pseudopolygonal ferrite.

The yield ratio decreases as the difference in hardness between polygonal ferrite and pseudopolygonal ferrite (hereinafter referred to as soft ferrite) and hard bainite or martensite (hereinafter referred to as hard second phase) decreases. It was also found that if the difference in thickness becomes too large, the HIC resistance and toughness deteriorate. On the other hand, by setting the tempering temperature to Ac ₁ to (Ac ₁ + Ac ₃ ) / 4, C is concentrated, and the hard second phase having higher hardness can be dispersed in the soft ferrite, and the yield ratio is Although it was also found that the temperature dropped significantly, it was impossible to ensure sour resistance performance within this quenching temperature range.

  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 containing 10 to 60 vol% of polygonal ferrite and pseudopolygonal ferrite having an average aspect ratio of 2.0 or less and an average particle size of 40 μm or less, and the hardness (Hv) difference from the hard second phase is 20 to 100. It is a tempered low-yield specific thickness steel plate with excellent sour-resistant performance.

The second invention is, after re-heating and hot rolling continuously cast slab having the component composition according to the first aspect of the present invention, heated from room temperature to (Ac1 + Ac3) / 2 or more Ac ₃ point or less of the temperature was held Then, it is water-cooled at a cooling rate of 800 to 500 ° C. at 15 ° C./s or more and 80 ° C./s or less, reheated from room temperature to a temperature of 550 ° C. or more and Ac ₁ point or less, and then air cooled. This is a method for producing a tempered low-yield-thickness steel plate with excellent sour resistance.

  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.

  Further, since Nb present in a precipitated state at the quenching temperature hardly gives an increase in strength, when it is necessary to improve the toughness of the welded portion, Nb is preferably less than 0.010%. More preferably, the quenching condition of the present invention is 0.004% or more and less than 0.010%.

  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.

Volume fraction of polygonal ferrite and pseudopolygonal ferrite: 10 to 60 vol%
The yield ratio was lowest when the volume fraction was 40 vol%, and when the volume fraction exceeded 60 vol%, the strength decreased significantly, so the upper limit was set to 60 vol%. In addition, since the yield ratio increases greatly when it is less than 10 vol%, the volume fraction of polygonal ferrite and pseudopolygonal ferrite is set in the range of 10 to 60 vol%. More preferably, it is the range of 20-50 vol%.

Average grain size of polygonal ferrite and pseudo-polygonal ferrite: 40 μm or less The larger the average grain size of polygonal ferrite and pseudo-polygonal ferrite, the lower the yield ratio. However, if the average grain size exceeds 40 μm, the toughness deteriorates. , 40 μm or less. More preferably, it is 25 μm or less. On the other hand, the finer the average particle size, the better the toughness, but the increase in the yield ratio is not so large, so the lower limit of the average particle size is not particularly specified.

Polygonal ferrite, pseudopolygonal ferrite, and hard second phase average aspect ratio: 2.0 or less Polygonal ferrite and pseudopolygonal ferrite are more preferable as they are equiaxed because propagation resistance to HIC cracking increases. When the value exceeds 2.0, the propagation resistance becomes weak and the HIC characteristics deteriorate, so the average aspect ratio of polygonal ferrite and pseudopolygonal ferrite is 2.0 or less.

Structure of the second phase If the second phase is hard, strain concentration occurs due to the difference in hardness (Hv) from soft ferrite (polygonal ferrite and pseudopolygonal ferrite), and the yield ratio decreases. In order to give a hardness difference of, the structure of the second phase is preferably tempered bainite or tempered martensite and a mixed structure thereof. This is because the yield ratio can be reduced as the hardness difference increases. On the other hand, as-quenched bainite and martensite deteriorate the toughness of the base metal, and the hardness difference from the soft ferrite becomes too large and becomes a propagation path of HIC cracks. Therefore, the structure of the second phase is tempered bainite or tempered martensite and a mixed structure thereof.

Hardness (Hv) difference between polygonal ferrite and pseudo-polygonal ferrite and the second phase:
20-100
The yield ratio decreases as the hardness (Hv) difference between the polygonal ferrite and pseudopolygonal ferrite and the second phase increases, but if the hardness (Hv) difference is too large, the toughness of the base metal deteriorates and the propagation path of the HIC crack As a result, the HIC characteristics deteriorate. When the hardness (Hv) difference exceeds 100, the effect is significant, so the upper limit is set to 100. On the other hand, since the yield ratio does not decrease when the hardness (Hv) difference is less than 20, the hardness (Hv) difference between polygonal ferrite and pseudopolygonal ferrite and the hard second phase is set in the range of 20-100. More preferably, it is the range of 20-60.

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 ₁ + Ac ₃ ) / 2 or more and Ac ₃ points or less When the quenching temperature is Ac ₃ or more, untransformed polygonal ferrite and pseudopolygonal ferrite do not remain and the yield ratio cannot be lowered. . In addition, when the quenching temperature is lower than (Ac ₁ + Ac ₃ ) / 2, the concentration of C into the reverse transformed austenite becomes remarkable, and very hard martensite and island martensite (MA) are generated during quenching. Therefore, the quenching temperature is set to (Ac ₁ + Ac ₃ ) / 2 or more and Ac ₃ points or less in order to become a starting point of HIC cracking. The Ac ₁ and Ac ₃ points are preferably obtained by a four master test or the like, but may be obtained by the equations (4) and (5).

Quenching cooling rate: 15 ° C./s or more and 80 ° C./s or less The cooling rate from 800 ° C. to 500 ° C. during quenching is 15 ° C./s or more. The faster the quenching cooling rate, the harder the structure produced when the reverse transformed austenite transforms again, and the strength can be increased, and the yield ratio can be lowered. When the quenching cooling rate is less than 15 ° C./s, the second phase having sufficient hardness cannot be obtained, so that the desired strength and yield ratio cannot be obtained. Since a large amount of sites are generated, becoming the starting point of HIC cracking and toughness is deteriorated, the quenching cooling rate is set to 15 ° C./s or more and 80 ° C./s or less.

Tempering temperature: 550 ° C. or more and Ac ₁ point or less By performing the tempering treatment, 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 temperatures lower than 550 ° C., and reverse transformation occurs when Ac exceeds ₁ point, and the reverse transformation structure of high C deteriorates toughness. Therefore, the tempering temperature is 550 ° C. or more and Ac ₁ point or less. .

  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. When the plate thickness was less than 12.7 mm, a 6.4 mmφ round bar tensile test piece was used. 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. The difference in hardness between the polygonal ferrite and pseudo-polygonal ferrite and the second phase was measured by 20 points each at 10 g in the center of the structure with micro Vickers, and the average value was used.

  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 16 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. No. 7 has a high quenching holding temperature, so that soft ferrite cannot be obtained and the yield ratio is high. No. In No. 8, the quenching temperature is low, so that the ferrite fraction is too high, and thus the hard second phase is dispersed in the ferrite and the HIC characteristics are deteriorated. No. No. 9 has a low quenching cooling rate and the hardness of the hard second phase is low, so that the desired strength is not obtained. No. Since No. 10 has not been tempered, the hardness difference from the second phase is large, and cracking occurs in HIC. No. In No. 11, since Mn and PHIC exceed the upper limit, many cracks are observed in HIC. No. Since No. 12 has a low ACR, many cracks are observed in the HIC. No. In No. 13, since C exceeds the upper limit, many cracks are observed in HIC, and cracks are also generated in SSC. No. No. 14 has no addition of Ti, so the ferrite grains are coarsened and the toughness is deteriorated. No. No. 15 is cracked by SSC because B is added. No. No. 16 is manufactured by TMPC and has a large aspect ratio of soft ferrite and second phase, so that 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 The structure of the central part contains 10-60 vol% of soft ferrite containing polygonal ferrite and pseudopolygonal ferrite having an average aspect ratio of 2.0 or less and an average particle diameter of 40 μm or less, and hardness (Hv) with the hard second phase A tempered low-yield specific thickness steel plate excellent in sour resistance, characterized in that the difference is 20 to 100.

A method for producing a tempered type low yield ratio steel plate according to claim 1, after the reheating and hot rolling the continuous cast slab, from room temperature (Ac 1 + _Ac 3) / ₂ or more Ac following _three After heating and holding to temperature, cooling at 800 to 500 ° C. with water at a cooling rate of 15 ° C./s to 80 ° C./s, reheating from room temperature to a temperature of 550 ° C. to Ac ₁ point, and then air cooling A method for producing a tempered low-yield-thickness steel sheet having excellent sour resistance, characterized by