BRPI1008592B1 - production method of acid-resistant heavy steel sheet for use in line piping - Google Patents

Abstract

METHOD OF PRODUCTION OF HEAVY STEEL SHEET FOR ACID-RESISTANT LINE PIPING The present invention relates to the steel plate for use in line pipe of a plate thickness of 25 mm or more with excellent toughness and acid resistance which is produced using a steel plate containing C, Si, Mn, Nb and Ti, restricting AI, P and N and, in addition, containing Ca: 0.001 to 0.004%, restricting S: 0.0008% or less and 0: 0 , 0030% or less, and which has Ca, O and S contents satisfying [Ca] (1- 124 [0]) / 1, 25 [S]> 3.0, heating to a heating temperature Ti of 1000 to 1150 ° C in the range satisfying T1 ~ -7970 / (log ([Nb] x [C]) - 3.31) -170, coarse lamination and final lamination, then performing accelerated cooling. At that time, the final lamination is carried out so that the thickness of the sheet becomes 25 mm or more if the finishing temperature is 800 ° C or more and the reduction ratio is 950 ° C or less 3 or more. By obtaining the cooling rate of the accelerated cooling from 1 O to 30 ° C / s and by obtaining the interruption temperature of 200 to 500 ° C, it becomes possible to achieve both a good acid resistance characteristic and a drop in temperature. (...)

Description

Technical Field

[001] The present invention relates to the use of a steel plate production method for line piping with excellent resistance to cracking induced by hydrogen in an environment that includes hydrogen sulfide (H2S), that is, the "resistance acid "and which also has excellent toughness.

Background Technique

[002] The steel pipe used for pipe lines that carry acid oil and acid gas that include hydrogen sulfide and the steel plate used for auxiliary installations of pipe lines require acid resistance. It is observed that, "acid resistance" is the resistance to cracking induced by hydrogen (HIC resistance) in corrosive environments containing hydrogen sulfide.

[003] It is known that acid resistance worsens due to the formation of MnS that elongates in the lamination direction or the formation of agglomerate-like inclusions. In addition, to improve acid resistance in an extremely rigorous corrosive environment, the method of controlled rolling and water cooling of a steel material has been proposed, from which the levels of P, S, O and N have been reduced, to which Ca is added and it is controlled in the MnS state (for example, PLT 1).

[004] In addition, from the point of view of improving the transport efficiency of the pipeline lines and reducing costs by thicker etc., higher resistance of the steel plate is being demanded for the use of the pipeline. To satisfy this demand, for example, it was proposed the method of production of steel plate that has a resistance to X70 or similar, that has a uniform microstructure of the metal in the direction of the thickness of the plate and comprised of fine bainite and that has excellent acid resistance (eg PLT 2).

[005] In addition, when laying line piping in arctic regions, it is necessary to improve the low temperature toughness of the steel plate for use in the line piping. In order to face such problems, the method of producing high strength steel plate has been proposed, which is improved in low temperature toughness and acid resistance (for example, PLT's 3 to 5).

[006] These objectives aim to reduce the amount of C in order to suppress an increase in hardness, reducing the amount of S and adding Ca in order to control the state of MnS, and reducing the amount of Al in order to control the state of oxides in order to achieve both acid resistance and low temperature toughness.

Citation List Patent Literature

[007] PLT 1: Japanese Patent Publication (A) No. 62-112722

[008] PLT 2: Japanese Patent Publication (A) No. 61-165207

[009] PLT 3: Japanese Patent Publication (A) No. 03-236420

[0010] PLT 4: Japanese Patent Publication (A) No. 05-295434

[0011] PLT 5: Japanese Patent Publication (A) No. 07-242944

Summary of the Invention Technical Problem

[0012] To guarantee the acid resistance characteristic, it is necessary to heat the steel sheet to a high temperature and cause the NbC and other coarse precipitates that precipitated and grew at the time of casting to dissolve. However, if the steel plate is heated to a high temperature, the size of the crystal grain becomes coarse.

[0013] In particular, when producing a steel sheet with a sheet thickness of 25 mm or more, the reduction in the recrystallization region and in the non-recrystallization region becomes insufficient and the toughness, in particular, a good characteristic of weight loss breakdown (also called DWTT characteristic) which is assessed by a weight loss breakdown test (DWTT), cannot be guaranteed.

[0014] The present invention was obtained to solve this problem and aims to provide a method of producing steel sheet for use in line piping with excellent acid resistance and toughness that allows to achieve both a good acid resistance characteristic as the DWTT feature for steel sheet with a sheet thickness of 25 mm or more.

Solution to the Problem

[0015] The present invention was carried out based on the discovery that by strictly limiting the contents of S and O, addition of Ca, control of the value of ESSP, which is expressed by the following formula, [Ca] (1-124 [O] ) / 1.25 [S] is high to control the state of sulfides, further limiting the amount of C so that it is low, the control of the heating temperature of the steel plate according to the levels of Nb and C, and also the temperature control and the proportion of reduction of the hot rolling, making larger the particles of Nb carbides and other precipitates are suppressed, the size of the crystal grain becomes thinner, and it is possible to produce a steel plate in which both excellent acid resistance and high toughness are achieved. The rationale is as follows:

[0016] 1) An acid-resistant heavy steel plate production method for use in line pipe with excellent toughness characterized by the processing of a steel plate that contains, in mass%, C: 0.01 up to 0.08%, Si: 0.1 to 0.5%, Mn: 1.0 to 1.5%, Nb: 0.010 to 0.040%, Ca: 0.001 to 0.004% and Ti: 0.005 to 0.030%, whose Al limits: 0.08% or less, P: 0.015% or less, S: 0.0008% or less, O: 0.0030% or less, and N: 0.0050% or less, which has Ca contents , O and S satisfying [Ca] (1-124 [O]) / 1.25 [S]> 3.0 by heating so that a heating temperature Ti in the range of 1000 to 1150 ° C and contents of Nb and C satisfy T1 £ -7970 / (log ([Nb] x [C]) - 3.31) -170, also performing a final lamination at a finishing temperature of 800 ° C or more and a reduction ratio at 950 ° C or less than 3 or more in order to provide a sheet thickness of 25 mm or more, performing accelerated cooling with a cooling rate of 10 to 30 ° C / s, and interrupting ace cooling read at 200 to 500 ° C.

[0017] 2) An acid-resistant heavy steel plate production method for use in line pipe with excellent toughness as shown in 1 above characterized by the fact that the steel plate also contains, in mass%, a or more of Ni: 0.5% or less, Cu: 0.5% or less, Cr: 0.5% or less, and Mo: 0.3% or less.

[0018] 3) A method of producing acid resistant heavy steel plate for use in line pipe with excellent toughness as presented in 1 or 2 above characterized by the fact that the steel plate also contains, in mass% , V: 0.06% or less.

[0019] 4) An acid-resistant heavy steel plate production method for use in line pipe with excellent toughness as shown in 1 or 2 above characterized by the fact that the steel plate also contains, in mass% , B: 0.0020% or less.

[0020] 5) An acid-resistant heavy steel plate production method for use in line pipe with excellent toughness as shown in 1 or 2 above characterized by the fact that the steel plate also contains, in mass% , Mg: 0.01% or less. Advantageous Effects of the Invention

[0021] In accordance with the present invention, in particular, it is possible to supply heavy steel sheet for use in line piping which has a sheet thickness of 25 mm or more and which has excellent toughness, in particular, DWTT characteristic and strength to acid. The contribution in the industry is extremely notable.

Description of Modalities

[0022] The inventors used steels with which the levels of Ca, S and O were varied and the controlled ESSP value found by the following formula, [Ca] (1-124 [O]) / 1.25 [ S] to produce steel sheets with sheet thicknesses of 25 mm or more and evaluated their acid resistance and toughness. Note that [Ca], [O] and [S] are the contents of the respective elements expressed by mass%. In addition, the same applies to other formulas.

[0023] Acid resistance was assessed by performing a test based on the NACE (National Association of Corrosion and Engineer) TM0284 and examining the occurrence of HIC (hydrogen-induced cracking). If the HIC area ratio is approximately 5% or less, the acid resistance characteristics are considered to be good. In addition, toughness was assessed by performing a DWTT test at -40 ° C, finding the percentage of ductile fracture, and using 85% as the criteria for passing judgment, etc.

[0024] The NACE test is a test method that investigates whether the crack occurs after 96 hours in a solution of 5% NaCl + 0.5% acetic acid with a pH 2.7 with hydrogen sulfide gas.

[0025] The inventors investigated the microstructure and precipitates of the samples in which HIC occurred and found that the steel plate on which the acid resistance was impaired had coarse NbC precipitate. Next, the inventors found that in the steel plate on which the DWTT characteristic had decreased, the size of the crystal grain became more coarse.

[0026] Furthermore, the inventors analyzed the relationship between the state of precipitation and the grain size of the steel sheet and the production conditions and as a result found that the steel sheet on which the coarse NbC precipitated had a low temperature heating, whereas the steel plate on which the crystal grains became thick had a high heating temperature.

[0027] In addition, the steel plate which has excellent resistance to acid and tenacity in which the increase in grain size was suppressed by a certain decrease in the heating temperature and causing the NbC to form a solid solution for controlling the amounts of C and Nb up to appropriate ranges.

[0028] The inventors adjusted the reduction ratio to 800 to 950 ° C up to 3, produced the steel sheet 25 mm or more and investigated the relationship between the heating temperature and the DWTT characteristic.

[0029] As a result, they realized that if the heating temperature is above 1150 ° C, the size of the crystal grain increases and the DWTT characteristic decreases. On the other hand, they realized that if the heating temperature becomes lower than 1000 ° C, the coarse NbC causes the DWTT characteristic to decrease.

[0030] Next, the inventors adjusted the heating temperature to 1000 to 1150 ° C in the range and studied the relationship between the appropriate levels of Nb and C for Nb and C to form solid solutions in the steel and the heating temperature. As a result, they realized that if Nb and C form solid solutions in steel or precipitate as NbC in steel this is affected by the product of solubility, so the value of log ([Nb] x [C]) is important.

[0031] The inventions engaged in an additional study and found that the fact of bringing the heating temperature T1 to the range of 1000 to 1150 ° C and using the heating temperature T1 and the levels of Nb and C is satisfied T1 £ -7970 / (log ([Nb] x [C]) - 3.31) -170 by heating the steel sheet are extremely important to achieve both acid resistance and toughness.

[0032] The heating temperature T1 that satisfies this condition corresponds to the temperature at which the NbC dissolves in steady state. Therefore, if the above relationship is satisfied, it is believed that the dissolution of NbC that precipitates the steel plate is promoted, no coarse NbC remains and the occurrence of ICH can be suppressed.

[0033] In the following, the present invention will be explained in detail. First, the composition of the steel used in the present invention will be explained. Note that, "%" means% by mass. C: 0.01 to 0.08%

[0034] C is an element that improves the resistance to steel. As an effective amount, 0.01% or more needs to be added. On the other hand, if the amount of C exceeds 0.08%, the formation of carbides is promoted and the resistance to HIC is impaired, so the upper limit becomes 0.08%. In addition, to eliminate the drop in HIC resistance, weldability, toughness etc., the amount of C is preferably made 0.06% or less. Si: 0.1 to 0.5%

[0035] Si is an element of deoxygenation. 0.1% or more must be added. On the other hand, if the amount of Si exceeds 0.5%, the toughness of the zone affected by the heat of the weld (HAZ) drops, thus the upper limit is made 0.5%. The preferable range is 0.15 to 0.35%. Mn: 1.0 to 1.5%

[0036] Mn is an element that improves the resistance to and the toughness. It is necessary to add 1.0% or more. On the other hand, Mn is an element that forms MnS to degrade acid resistance, in order to suppress HIC, the upper limit needs to be 1.5%. The preferable range is 1.1 to 1.4%. Nb: 0.010 to 0.040%

[0037] Nb is an element that increases the non-recrystallization temperature region to make the size of the crystal grain thinner, forms carbides and nitrides, and contributes to the improvement of resistance. You need to add 0.010% or more. On the other hand, in the present invention, the prevention of thick carbide formation is extremely important. The upper limit must be 0.040%. The preferred range is 0.011 to 0.025%, while the most preferable range is 0.012 to 0.020%. Ca: 0.001 to 0.004%

[0038] Ca is an element that forms CaS sulfides, prevents the formation of MnS that elongates in the lamination direction, and contributes notably to the improvement of HIC resistance. If the amount of Ca addition is less than 0.001%, the effect is not obtained, so that the lower limit is made 0.001%. On the other hand, if the amount of Ca addition is above 0.004%, the oxide clusters are formed and thus the resistance to HIC is impaired, so that the upper limit is made 0.004%. The preferable range is 0.0025 to 0.0035%. Ti: 0.005 to 0.030%

[0039] Ti is an element that is used as a deoxygenating agent or as a nitride forming element needs to be added to increase the fineness of the crystal grains by 0.005% or more. On the other hand, if an excessive amount of Ti is added, the toughness drops due to the formation of coarse nitrides, so that the upper limit is made 0.030%. The preferred range is 0.010 to 0.020%. Al: 0.08% or less

[0040] Al is an element of deoxygenation. If the amount of addition exceeds 0.08%, aggregate agglomerates of Al oxides are formed and the acid resistance is impaired, so that the content is limited to 0.08% or less. In addition, toughness is required, the upper limit on the amount of Al is preferably made 0.03%. The most preferable upper limit of Al is 0.01%. The lower limit on the amount of Al is not particularly limited, however, to reduce the amount of oxygen in the molten steel, it is preferable to add Al by 0.0005% or more. P: 0.015% or less

[0041] P is an impurity. If the content is greater than 0.015%, resistance to HIC is impaired. Therefore, the upper limit of the P content is made 0.015%. S: 0.0008% or less

[0042] S is an element that forms MnS that stretches in the lamination direction at the time of hot lamination and thus causes the resistance to HIC to decrease. Therefore, in the present invention, it is necessary to reduce the amount of S. The upper limit is made 0.0008%. The smaller the amount of S, the more preferable, however, it is difficult to make it less than 0.0001%. From the point of view of production costs, 0.0001% or more is preferable. O: 0.0030% or less

[0043] O is an impurity. To suppress aggregation of oxides and improve resistance to HIC, the upper limit needs to be limited to 0.0030%. To suppress the formation of oxides in order to improve toughness, the amount of O is preferably made 0.0020% or less. N: 0.0050% or less

[0044] N is an impurity. If the N content exceeds 0.0050%, the Ti and Nb carbonitrides are easily added and the resistance to HIC is impaired. Therefore, the upper limit on the amount of N is made 0.0050%. Note that, when toughness etc. is required, to suppress the increase in the TiN grain, the upper limit on the amount of N is preferably made 0.0035%. When using TiN, NbN and other nitrides etc. and trying to increase the fineness of austenite grains at the time of heating, it is preferably included 0.0010% or more of N. [Ca] (1-124 [O]) / 1.25 [S]> 3.0

[0045] In the present invention, the value of [Ca] (1-124 [O]) / 1.25 [S], that is, the value of ESSP, needs to be increased. The ESSP value is the ratio of the amount of Ca to the amount of S that is necessary to cause the formation of CaS considering the fact that Ca forms oxides. To add Ca to form CaS and set S, the ESSP value needs to be made above 3.0.

[0046] Note that if the amount of S becomes 0, the value of ESSP becomes infinitely large, however in this case, the MnS cannot be formed. Therefore, if the amount of Ca is in the above range, the upper limit of the ESSP value does not need to be prescribed. Note that, in the present invention, as elements that improve strength and toughness, it is preferable to add one or more elements between Ni, Cu, Cr, Mo, V, B and Mg. Ni: 0.5% or less

[0047] Ni is an effective element to improve toughness and strength and contributes in the same way to improve corrosion resistance, thus 0.01% or more is preferably added. On the other hand, Ni is a costly element, in order to reduce production costs, the upper limit is preferably limited to 0.5%. Cu: 0.5% or less

[0048] Cu is an effective element to increase the resistance and contributes in the same way to the improvement of the resistance to corrosion, thus 0.01% or more is preferably added. On the other hand, Cu is also an expensive element, in order to reduce production costs, the upper limit is preferably limited to 0.5%. Cr: 0.5% or less

[0049] Cr is an effective element to increase resistance. 0.01% or more is preferably added. On the other hand, if a large amount is added, the hardening capacity becomes greater and the toughness sometimes decreases, so the upper limit is preferably made of 0.5% Mo: 0.3% or less

[0050] Mo is an element that improves the hardening capacity and simultaneously forms carbonitrides and improves strength. To achieve this effect, 0.01% or more is preferably added. On the other hand, Mo is a costly element, in order to reduce production costs, the upper limit is preferably limited to 0.3%. If the strength of the steel is high, the resistance to HIC and the toughness sometimes decrease, so the upper limit is preferably 0.2%. V: 0.06% or less

[0051] V is an element that forms carbides and nitrides and contributes to the improvement of resistance. To achieve this effect, 0.01% or more is preferably added. On the other hand, if the addition is above 0.06% V, a drop in toughness is sometimes invited, so the upper limit is preferably made 0.06%. B: 0.0020% or less

[0052] B is an element that is segregated at the grain boundaries to contribute notably to the hardening capacity. To achieve this effect, 0.0001% or more is preferably added. On the other hand, if B is added excessively, segregation at the grain boundaries becomes excessive and a drop in toughness is sometimes invited, so the upper limit is preferably made 0.0020%. Mg: 0.01% or less

[0053] Mg is an element that acts as a deoxygenation agent and as a desulfurization agent. In particular, it forms fine oxides and eliminates grain-size thickening, so it is effective for improving toughness. To achieve this effect, 0.0001% or more is preferably added. On the other hand, if more than 0.01% Mg is added, the oxides aggregate easily and become thicker causing a drop in HIC resistance and toughness. Therefore, the upper limit on the amount of Mg is preferably made 0.01%.

[0054] The production conditions will be explained below.

[0055] The steel containing the above composition is cast in a steelmaking method, then continuously molded into a steel plate. The steel plate is heated and laminated by heavy plate rolling which consists of coarse rolling and final rolling to become steel plate. In the present invention, the conditions of the heating temperature of the steel plate, the temperature and the proportion of reduction of the final lamination and the accelerated cooling after the final lamination are important. Heating temperature T1: 1000 to 1150 ° C

[0056] First, in the present invention, the heating temperature range of the steel plate is important. If the heating temperature is less than 1000 ° C, the NbC that precipitated on the steel sheet will not form a solid solution, the coarse NbC will remain on the steel sheet and the acid resistance will decrease.

[0057] On the other hand, if the heating temperature is above 1150 ° C, the size of the crystal grain of the steel sheet becomes larger, the recrystallization in the coarse rolling region becomes insufficient in the steel sheet of a thickness above 25 mm and the size of the crystal grain of the final steel plate is greatly influenced by the size of the crystal grain at the time of heating. In particular, the DWTT characteristic is easily affected by the size of the crystal grain, so that if the size of the crystal grain increases, the DWTT characteristic decreases. Therefore, the heating temperature T1 of the steel plate will be in the range of 1000 to 1150 ° C. T1 £ -7970 / (log ([Nb] x [C]) - 3.31) -170

[0058] In addition, in order to make the NbC that precipitated on the steel sheet form a solid solution, it is important to adequately control the heating temperature T1 of the steel sheet and its relationship with the levels of Nb and C. This is because, as explained above, the dissolution and precipitation of NbC are affected by the solubility of the product, so the value of log ([Nb] x [C]) becomes important.

[0059] In the present invention, it is necessary that the heating temperature be in the range of 1000 to 1150 ° C and, in addition, make the heating temperature T1 of the steel plate and the contents of Nb and C satisfy T1 £ -7970 / (log ([Nb] x [C]) - 3.31) -170.

[0060] Due to this fact, thick NbC does not remain on the steel plate and the acid resistance can be improved.

[0061] Finishing Temperature: 800 ° C or more

[0062] After heating the steel sheet to satisfy the above conditions, the sheet is rolled raw and additionally subjected to a final rolling. On this occasion, the final lamination temperature is made 800 ° C or more in order to make the microstructure uniform.

[0063] This is because if it is less than 800 ° C, the forms of ferrite and the microstructure of the steel sheet after lamination become associated. If the microstructure becomes associated, the acid resistance sometimes decreases. In addition, depending on the conditions of the final cast, sometimes the worked ferrite remains on the steel sheet and the toughness is impaired.

[0064] Reduction Ratio at 950 ° C or lower: 3 or more

[0065] In the final lamination, to make the size of the crystal grain thinner, it is necessary to control the lamination temperature and the proportion of the reduction. In particular, by increasing the proportion of the reduction at a low temperature to carry out the final lamination, it is possible to thin the microstructure of the steel sheet. If the lamination temperature exceeds 950 ° C, recrystallization occurs, so the proportion of the reduction to 950 ° C or less is important.

[0066] If the reduction ratio at 950 ° C or less is less than 3, the grain size will become thinner, it will be difficult to obtain a uniform microstructure and the toughness and acid resistance will decrease. Therefore, the proportion of the reduction of 950 ° C or less until the end of the final lamination is made 3 or more. The ratio of the reduction of 950 ° C or less until the end of the final lamination is the ratio of the thickness of the sheet after lamination to the thickness of the sheet at 950 ° C. Plate Thickness: 25 mm or more

[0067] The present invention makes the steel sheet, with a sheet thickness of 25 mm or more, thin and uniform in microstructure. That is, if the sheet thickness becomes 25 mm or more, it becomes difficult to guarantee a proportion of the final lamination reduction. To make the microstructure thin, the heating temperature needs to be lowered. Therefore, in the production method of the present invention, the thickness of the sheet after the final lamination is obtained with 25 mm or more. Accelerated Cooling Rate: 10 to 30 ° C / s

[0068] After the final lamination, accelerated cooling is performed. The accelerated cooling is carried out immediately after the final cleaning, however due to the configuration of the production line, the temperature can also decrease a little. However, in order to eliminate the formation of polygonal ferrite and avoid the formation of a layered ferrite structure and of the phases transformed at low temperature (bainite or martensite) if the temperature drops, the accelerated cooling start temperature is preferably made 700 ° C or more.

[0069] Accelerated cooling is carried out to make the microstructure of the ferrite or bainitic ferrite steel sheet thin acicular. In order to eliminate the transformation of polygonal ferrite and avoid the formation of pearlite in order to obtain such a microstructure, it is necessary to become the cooling rate of 10 ° C / s or more.

[0070] On the other hand, the cooling rate of accelerated cooling is above 30 ° C / s, it will be formed excessively martensite, the hardness will become non-uniform and the resistance to acid and toughness will decrease. Therefore, the cooling rate of accelerated cooling is adjusted to 10 to 30 ° C / s. Note that the cooling rate is the cooling rate in the center of the sheet thickness of the steel sheet.

[0071] Accelerated Cooling interruption temperature: 200 to 500 ° C

[0072] The temperature of interruption of the accelerated cooling was made in the range of 200 to 500 ° C in order to suppress the formation of martensite. To eliminate the transformation of polygonal ferrite and prevent the formation of pearlite, the accelerated cooling interruption temperature must be 500 ° C or lower.

[0073] On the other hand, if the accelerated cooling interruption temperature becomes 200 ° C or lower, excess martensite will be formed, the hardness will become non-uniform and the acid resistance and toughness will decrease.

EXAMPLES

[0074] Steel that has each of the chemical composition shown in table 1, was cast in a converter by secondary refining and was continuously molded to produce a 250 mm thick steel plate. The steel plate obtained was hot rolled under the conditions presented in table 2 to obtain steel plate. The HIC resistance of the steel plate after production was assessed by a NACE test.

[0075] The conditions of the NACE test were saturation of a 5% NaCl solution + 0.5% acetic acid, pH 2.7 by gaseous hydrogen sulfide, an immersion time of 96 hours, examination for existence cracking and measurement of the HIC fracture rate (CAR).

[0076] Tenacity was assessed by a DWTT test. With a steel plate, based on API 5L3 and ASTM E436, notched specimens were manufactured using the direction of the plate width as the longitudinal direction and making the notches parallel to the direction of the plate width. DWTT was performed at - 35 ° C and the percentage of ductile fracture (SA) was discovered.

[0077] The results are shown in table 2.

[0078] Note that Table 1 shows the value of the formula [Ca] (1-124 [O]) / 1.25 [S] as ESSP. Tables 1 and 2 show the value of the formula -7970 / (log ([Nb] x [C]) - 3.31) -170 as T1.

[0079] Nos. 1 to 9 have the composition of steel sheets and production conditions within the range of the present invention, have CARs of 5% or less and have excellent resistance to acids.

[0080] On the other hand, N ° 10 is an example in which the ESSP value is less than the range of the present invention and the acid resistance decreases.

[0081] In addition, N ° 11 is an example in which the amount of C is large and the heating temperature is low and unsatisfactory, so that the resistance to HIC decreases. N ° 12 is an example where the heating temperature is low and the toughness is impaired.

Industrial Applicability

[0082] As explained above, in accordance with the present invention, in particular, it becomes possible to provide heavy steel sheet for use in line piping with a sheet thickness of 25 mm or more and with excellent toughness, in particular , DWTT characteristic and acid resistance. Therefore, the present invention is extremely notable in contribution to the industry and great in applicability in the industry.

Claims (5)

1. Production method of acid-resistant heavy steel plate for use in line piping, characterized by the processing of a steel plate containing, in mass%, C: 0.01 to 0.08%, Si: 0 , 1 to 0.5%, Mn: 1.0 to 1.5%, Nb: 0.010 to 0.040%, Ca: 0.001 to 0.004%, and Ti: 0.005 to 0.030%, Al: 0.08% or less, P: 0.015% or less, S: 0.0008% or less, O: 0.0030% or less, and N: 0.0050% or less, which has Ca, O and S content satisfying [Ca] (1 -124 [O]) / 1.25 [S]> 3.0 by heating so that a heating temperature T1 in the range of 1000 to 1150 ° C and contents of Nb and C satisfy T1 £ -7970 / ( log ([Nb] x [C]) - 3.31) -170, also performing a final lamination at a finishing temperature of 800 ° C or more and a reduction ratio at 950 ° C or less than 3 or more in order to provide a sheet thickness of 25 mm or more, performing accelerated cooling with a cooling rate of 10 to 30 ° C / s and interrupting said accelerated cooling between 200 and 500 ° C. 2. Production method of acid-resistant heavy steel plate for use in line piping according to claim 1, characterized by the fact that said steel plate also contains, in mass%, one or more of Ni: 0.5% or less, Cu: 0.5% or less, Cr: 0.5% or less, and Mo: 0.3% or less. 3. Production method of acid-resistant heavy steel sheet for use in line piping according to claim 1 or 2, characterized by the fact that said steel sheet also contains, in mass%, V: 0, 06% or less. 4. Production method of acid-resistant heavy steel plate for use in line piping according to claim 1 or 2, characterized by the fact that said steel plate also contains, in mass%, B: 0, 0020% or less. 5. Production method of acid-resistant heavy steel sheet for use in line piping according to claim 1 or 2, characterized by the fact that said steel sheet also contains, in mass%, Mg: 0, 01% or less.