JP4725437B2 - Continuous cast slab for thick steel plate, method for producing the same, and thick steel plate - Google Patents

Description

  The present invention relates to a continuous cast slab for a thick steel plate, a method for producing the same, and a thick steel plate. Specifically, high-strength steel plates with excellent resistance to hydrogen-induced cracking (hereinafter referred to as “HIC resistance”) used in line pipes, offshore structures, pressure vessels, etc. The present invention relates to a cast slab and a manufacturing method thereof.

  As used herein, “thick steel plate” refers to a steel plate having a thickness of 12.7 to 38.1 mm (0.5 to 1.5 inches).

  In the present specification, the “continuous cast slab” may be simply referred to as “slab”.

  In recent years, due to energy depletion, the development of natural gas wells and oil wells under harsh environments containing a large amount of hydrogen sulfide has been actively conducted. For line pipes, oil wells, marine structures and pressure vessels used for the extraction, refining, storage and transportation of natural gas and crude oil containing a large amount of hydrogen sulfide, the sampling and transportation High-strength materials are required for the purpose of increasing efficiency and reducing laying costs.

  As a result, oil leaks, destruction, and explosions have occurred in line pipes, oil well pipes, offshore structures and pressure vessels used for the extraction, refining, storage, and transportation of natural gas and crude oil containing a large amount of hydrogen sulfide. It often occurs.

  One of the causes of such accidents is that hydrogen gas generated by the corrosion reaction enters and diffuses into the steel material and accumulates around MnS and oxide inclusions in the steel material, which is molecularized and cracked by its internal pressure. It is known that there are hydrogen-induced cracks (hereinafter referred to as “HIC”) that cause

  Therefore, various techniques for suppressing HIC in line pipes, offshore structures and pressure vessels, especially line pipes have been studied, and for example, proposed in Patent Documents 1-7.

  That is, in the case of melting a pipe pipe steel in Patent Document 1, a single addition of Ca or a combined addition of Ca and Ce was adjusted so as to satisfy a specific formula “for a line pipe with excellent resistance to hydrogen-induced cracking” Steel manufacturing method "is disclosed.

  Patent Document 2 contains Ni in the range of more than 0.20% and less than 3.0%, and includes one or two of 5.0% or less of Cr and 2.0% or less of Mo in total Cr + Mo. By including the amount in the range of 0.5% or more, “steel for line pipes excellent in sour resistance” is disclosed, in which both HIC resistance and sulfide stress corrosion cracking resistance are improved.

  Patent Document 3 discloses a phenomenon in which molten steel components such as C, S, P, and Mn are positively segregated in the center portion (final solidified portion) in the thickness direction of a steel slab that causes toughness reduction and HIC, particularly in thick plate materials. The method of preventing center segregation, specifically, by increasing the interval in the slab thickness direction of the guide rolls arranged in the drawing direction from directly under the mold, the solid fraction of the center part of the slab (Fs) causes bulging at a position of 0.1 or less, the maximum thickness of the slab is 20 to 100 mm thicker than the short side length of the mold, and at least one pair of rolling rolls immediately before the solidification completion point A “continuous casting method” of steel is disclosed in which a reduction of 20 mm or more per pair is applied and the amount corresponding to the bulging amount is reduced.

  Patent Document 4 discloses a continuous casting method and slab that can efficiently reduce unsolidified slabs and reduce segregation at the center of the slab in the thickness direction, specifically, bulging slabs including unsolidified parts. A continuous casting method in which the rolling is performed by using a pair of rolling rolls, and the lower roll of the pair of rolling rolls protrudes from the lower pass line of the slab in the continuous casting machine. And the above-described “continuous casting method” in which the center segregation ratio C / Co of Mn in the thickness direction center portion of the slab satisfies the relationship expressed by the specific formula over the width W expressed by the specific formula A manufactured “slab” is disclosed.

  Patent Document 5 discloses a high-strength steel excellent in HIC resistance and a method for producing the same, specifically, a ratio M / M of the segregation portion M to the average Mn content M0 in the steel while having a specific chemical composition. In a method for producing a high-strength steel sheet comprising continuous casting of molten steel having a specific chemical composition and hot rolling after “high-tensile steel sheet having excellent HIC resistance” with M0 of 1.20 or less. In the continuous casting of the molten steel, the ratio M / M0 of the Mn content M at the center of the slab and the average Mn content M0 of the molten steel is 1.20 in the vicinity of the crater end where the internal molten steel of the slab completes solidification. A “manufacturing method of high-strength steel excellent in HIC resistance” that performs the following forging work is disclosed.

In Patent Document 6, a steel slab containing a specific chemical component and limiting the Cu content to 0.05% or less is heated to 1000 to 1250 ° C., and rolling is finished in a temperature range of Ar ₃ temperature or higher. In addition, “a method for producing a steel sheet for high-strength line pipe excellent in HIC resistance” that performs accelerated cooling to a temperature range of 600 ° C. or less and 450 ° C. or more at a cooling rate of 5 to 20 ° C./sec is disclosed. .

  Patent Document 7 is a high strength steel of API X65 grade or higher, and exhibits excellent HIC resistance against HIC in the central segregation part and HIC generated from the vicinity of the surface and inclusions, and excellent toughness in the welded part. A method of manufacturing a strength steel plate, specifically, steel containing a specific chemical component is hot-rolled under conditions of a heating temperature of 1000 to 1250 ° C. and a rolling end temperature of 750 to 950 ° C., and then 2 ° C./s. Cool to 600-700 ° C. at the above cooling rate, then heat at least once to a temperature of 600-700 ° C., and the time for the average temperature of the steel sheet to be 600-700 ° C. is 3 minutes or more. A method for producing a high-strength steel sheet having excellent properties ”is disclosed.

Japanese Patent Laid-Open No. 54-110119 JP 61-60866 A JP-A-9-57410 JP 2004-1079 A Japanese Patent Laid-Open No. 6-220577 Japanese Patent Laid-Open No. 9-209037 JP 2003-226922 A

  In order to increase the strength of thick steel plates used for line pipes, offshore structures and pressure vessels, especially line pipes, Nb and Ti are often contained in combination.

  However, when such a thick steel plate containing a composite of Nb and Ti is used for a line pipe, an offshore structure or a pressure vessel, especially a line pipe, the techniques disclosed in the aforementioned Patent Documents 1 to 7 are not necessarily used. It was not that good HIC resistance was obtained.

  Accordingly, an object of the present invention is to provide a thick steel plate excellent in HIC resistance having a crack area ratio CAR of 4% or less in the HIC test even when Nb and Ti are contained in combination, and a continuous steel plate suitable as a material thereof. It is to provide a cast slab and a manufacturing method thereof.

  The “crack area ratio CAR” in the above HIC test refers to the crack area ratio (%) with respect to the observation surface.

  In order to solve the above-mentioned problems, the present inventor made various studies, and as a result, first, the following knowledge (a) was obtained.

  (A) In the case where Nb and Ti are contained in combination, even if the conventional measures proposed for MnS and macrosegregation for suppressing the generation of HIC are implemented, coarse inclusions in the steel are the starting point. HIC may occur.

  Thus, using Nb and Ti composite-containing steel containing various component elements, coarse inclusions in the steel were investigated, and as a result, the following knowledge (b) was obtained.

  (B) Ti, Nb, C, N, and Si play a major role in the formation of coarse inclusions in the steel when Nb and Ti are contained in combination.

  Then, the present inventor is based on the idea that the generation of the coarse inclusions can be prevented if the component design is performed in consideration of the interaction of various component elements and the segregation behavior during solidification. Then, a specimen is taken from a thick steel plate hot rolled to a thickness of 20 mm, and a HIC test is performed by a method specified by NACE TM-02-84. We conducted a survey.

  As a result, the following items (c) and (d) became clear.

  (C) Coarse inclusions (carbides, nitrides, or composite carbonitrides thereof) having a major axis of 10 μm or more may be generated in the central portion in the thickness direction of the thick steel plate.

  (D) When analyzed by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray detector (EDS), the content of coarse inclusions from which cracks originate may change in various ways. And Nb-containing (Ti, Nb) (C, N) carbonitride (hereinafter referred to as “(Ti, Nb) (C, N) -based inclusions”).

  Therefore, when a micro sample was taken from the continuous cast slab before rolling and the central segregation portion was observed, the same coarse (Ti, Nb) (C, N) as the thick steel plate hot-rolled to the thickness of 20 mm was observed. ) It was found that system inclusions might be generated, and the following important findings (e) were obtained.

  (E) In order to reduce the generation of HIC when Nb and Ti are contained in combination, it is necessary to prevent the formation of (Ti, Nb) (C, N) inclusions at the steelmaking stage.

  From the knowledge of (e) above, the present inventor will examine the mechanism by which the coarse (Ti, Nb) (C, N) inclusions are generated in the steelmaking stage. A sample was taken and observed with an optical microscope to examine the state of formation of (Ti, Nb) (C, N) inclusions in the continuous cast slab.

  As a result, the following item (f) became clear.

  (F) In the continuous cast slab, (Ti, Nb) (C, N) inclusions are generated over the entire region in the thickness direction, and the number thereof is remarkably increased particularly in the central segregation portion. At the same time, coarse particles having a major axis of 10 μm or more are also produced.

  Then, next, the formation mechanism of (Ti, Nb) (C, N) inclusions was examined using a microsegregation model.

  Below, the examination method and the knowledge obtained are explained.

  [1] FIG. 1 schematically shows a distribution state of a target component element content Cs in a solid phase (that is, a solidified portion) and a target component element content Cl in a liquid phase (that is, in molten steel) in a solidification process. FIG. In FIG. 1, the vertical axis indicating the content of component elements is represented as “solute concentration”. Hereinafter, a description will be given with reference to FIG.

  Assuming that solidification proceeds one-dimensionally, local equilibrium is established at the solid-liquid interface, and the solute (component element of interest) is distributed according to the equilibrium distribution coefficient k. Further, since the liquid phase has a sufficient diffusion rate, it has a uniform composition, and in the solid phase, it diffuses from the solid-liquid interface toward the center of solidification according to the diffusion coefficient.

Next, when solidification proceeds, the solid-liquid interface proceeds to the right while maintaining the distribution ratio according to the equilibrium distribution coefficient k at the solid-liquid interface, that is, maintaining the relationship of “Cs = k × Cl”. For this reason, the solute (target component element) is discharged to the liquid phase side at the solid-liquid interface, and the content of the solute (target component element) in the residual molten steel increases. This is the cause of micro-segregation. When the content of Ti and N in the residual molten steel in the solidification process exceeds the activity product of TiN represented by the following formula (2), TiN is also present in the residual molten steel in the solidification process. Assuming that NbC crystallizes when the content of Nb and C in NbC exceeds the activity product of NbC represented by the following formula (3), TiN and NbC are dissolved in each other (Ti, Nb ) (C, N) inclusions are formed, and the composition of (Ti, Nb) (C, N) inclusions changes depending on the timing at which the respective solubility products are exceeded and the content at that time. It is done.
Log [Ti] [N] = − 19800 / T + 7.78 (2) Formula
Log [Nb] [C] = − 4530 / T + 2.39 (3).

  [2] Hereinafter, an example of a specific calculation result of the micro-segregation situation will be described. In the calculation, TiN and NbC were crystallized by using the component system shown in Table 1 in consideration of C, Si, Mn, P and S which are main elements of steel and Ti, Nb and N which are constituent elements of TiN and NbC. In order to determine the start, the interaction of each component element was considered.

  The temperature distribution in the continuous cast slab during continuous casting was calculated by heat transfer and solidification analysis, and solidification proceeded according to the model described in [1] above in accordance with this temperature history.

  In FIG. 2, the result about C, P, Ti, and N is shown as an example of the calculation result of the microsegregation situation performed about the component system of Table 1. FIG. In FIG. 2, the vertical axis indicating the component element content is represented as “solute concentration (mass%)”.

  From FIG. 2, it is recognized that the content of the component elements increases as the solid fraction fs increases (that is, the progress of solidification). In addition, it is recognized that the progress of segregation is different because the solid-liquid equilibrium distribution coefficient and diffusion rate are different for each component element.

  At the end of solidification, the contents of Ti, N, Nb, and C in the residual molten steel are the activity product of TiN represented by the formula (2) and the activity of NbC represented by the formula (3), respectively. The product is exceeded. In the calculation of this microsegregation model, TiN starts to crystallize when the solid fraction fs is 0.91, whereas NbC does not crystallize until the solid fraction fs is 0.99.

  In addition, as already described as matter (f), (Ti, Nb) (C, N) inclusions are generated in the continuous cast slab over the entire region in the thickness direction. In the segregation part, the number thereof is remarkably increased, and a coarse part having a major axis of 10 μm or more is also generated.

  From the above, the following item (g) became clear.

  (G) Regarding the generation of (Ti, Nb) (C, N) inclusions in the steelmaking stage when Nb and Ti are contained in combination, it is necessary to consider macro segregation.

  Macro segregation in continuous casting is a phenomenon in which molten steel in which solutes (component elements) between dendritic trees are concentrated due to bulging, reduction or solidification shrinkage of continuous cast slabs moves and accumulates in the center. That is, macrosegregation is so-called “center segregation”, in which molten steel having a component element content corresponding to a certain solid phase ratio fs is accumulated and solidified.

  Then, this inventor investigated the component element content of the center segregation part about various continuous cast slabs. As a result, the following knowledge (h) was obtained.

  (H) The central segregation portion corresponds to a solidified molten steel having a solid phase ratio fs of 0.6.

  Therefore, the present inventor decided to calculate macro segregation and micro segregation behavior again.

  Below, the examination method and the knowledge obtained are explained.

  [3] Using the component system shown in Table 1 as an example in the case of containing Nb and Ti in combination, each in the residual molten steel when solidified to a solid fraction fs of 0.6 with respect to the initial component system The content of the solute (component element) was calculated, and the microsegregation behavior was calculated using the content thus obtained as the initial content.

  As a result, in the central segregation part, TiN started to crystallize at a solid fraction fs of 0.68, and NbC crystallized at a solid fraction fs of 0.94.

  As already mentioned, TiN and NbC are dissolved in each other. In general, it is known that when a compound is crystallized during the solidification process, a coarser compound is generated as the solid phase ratio fs at the start of crystallization is lower.

  For this reason, the following matters (i) and (j) were clarified from the crystallization calculation results of TiN and NbC.

  (I) NbC crystallization is induced by using the previously crystallized TiN as a nucleus.

  (J) Formation of coarse (Ti, Nb) (C, N) inclusions is related to TiN crystallization at an early stage of the solidification process.

  [4] Then, using the chemical composition of the continuous cast slab produced by continuous casting as the initial component system, the microsegregation behavior is calculated by the method of [3] above, and the test piece is taken from the thick steel plate rolled to 20 mm. The HIC test was conducted by the method defined by NACE TM-02-84 and the state of occurrence of HIC was investigated.

  As a result, first, it was found that TiN started to crystallize before NbC in all the continuous cast slabs used in the calculation, and the following conclusion (k) was reached.

  (K) The generation of coarse (Ti, Nb) (C, N) inclusions may be arranged by the solid phase rate fs at which TiN starts to crystallize.

  Therefore, in FIG. 3, for each continuous cast slab, the TiN crystallization start solid phase ratio fs obtained by calculation as described above and the crack area ratio CAR (%) in the HIC test, that is, the crack of the observation surface The relationship of area ratio was arranged.

  In FIG. 3, the vertical axis indicating the crack area ratio CAR in the HIC test is simply represented as “CAR”, and the horizontal axis indicating the TiN crystallization starting solid fraction fs obtained from the calculation is “fscal. ".

  From FIG. 3, a clear correlation was found between the calculated value of the TiN crystallization initiation solid fraction fs in the central segregation part and the state of HIC generation, and as a result, the following knowledge (1) was obtained.

  (L) The crack area ratio CAR in the HIC test exceeds 4% when the calculated value of the TiN crystallization initiation solid phase ratio fs in the central segregation part is 0.72 or less.

  In addition, when this inventor investigated the HIC generation | occurrence | production part in detail using SEM equipped with an optical microscope and EDS about each continuous cast slab, it is coarse (Ti, Nb) (C , N) Formation of system inclusions was observed.

  In general, the limit of flow of molten steel between dendrite trees in the solidification process of steel is said to be about a solid phase rate of about fs 0.7, and when TiN starts to crystallize at a solid phase rate of less than this, TiN agglomerates and coalesces by the flow of the residual molten steel to generate coarse TiN, which causes crystallization of NbC to be induced, and in the central segregation portion, coarse (Ti, Nb) (C, N) It is considered that system inclusions were generated.

  Next, the present inventor, in steel containing a composite of Nb and Ti, C, Si, Mn, P, S, Nb, Ti, Al, Ca, N, Cu, Ni, Cr, Mo and V By varying the content, the microsegregation behavior was calculated by the method of [3] above, and each of the effects on the crystallization start solid fraction fs of TiN in the central segregation part of the continuous cast slab described in [4] above. The influence of elements was examined.

  As a result, the following findings (m) and (n) were obtained.

  (M) C: 0.03-0.10%, Si: 0.05-0.4%, Mn: 0.8-1.8%, P: 0.010% or less, S: 0.002% Hereinafter, Nb: 0.01-0.10%, Ti: 0.005-0.03%, Al: 0.005-0.06%, Ca: 0.0005-0.0060%, N: 0.00. In the ranges of 0015 to 0.007%, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.5% or less, Mo: 1.0% or less, and V: 0.2% or less, Among the above elements, only Ti and N, which are constituent elements of Si and TiN, affect the crystallization initiation solid phase ratio fs of TiN, and the influence of Ti and N is large.

  (N) Of the above Ti, N and Si, Si is added as a deoxidizer and can be replaced with other deoxidizers, so the content range is larger than Ti and N . For this reason, Si has a non-negligible effect on the formation of coarse TiN that causes HIC generation, and hence coarse (Ti, Nb) (C, N) inclusions.

  FIG. 4 shows the relationship between the decrease in the content of N, Ti and Si in mass% and the degree of increase in the crystallization starting solid fraction fs of TiN. In FIG. 4, the vertical axis indicating the degree of increase in the crystallization start solid phase fraction fs of TiN (calculated from calculation) is simply expressed as “TiN crystallization start fs increase degree”, and the mass of each element. The horizontal axis indicating the amount of decrease in content in% was expressed as “component decrease (mass%)”.

  Although Si is not a constituent element of TiN, Si has an effect on the solidification rate fs of TiN crystallization in the central segregation part of the continuous cast slab. This is considered to be caused by the fact that it is extremely large compared with that between the constituent elements of.

  Based on the above findings (m) and (n), the present inventor has linearity in the correlation between the content of Ti, N, and Si and the crystallization start solid fraction fs of TiN in the central segregation part. As a result, the following equation (1) was derived and the following conclusion (o) was reached.

If the element symbol in the formula (o) is the content in steel in mass% of the element and the formula (1) is satisfied, theoretically coarse (Ti, Nb) (C, N) inclusions Can be prevented.
1.48-1.5 * Si-14.3 * Ti-73 * N> 0.72 ... (1) Formula.

  The above formula (1) is a theoretical calculation formula. For this reason, it is assumed that it is difficult to completely remove coarse (Ti, Nb) (C, N) inclusions in the continuously cast slab actually produced.

  However, even when coarse (Ti, Nb) (C, N) -based inclusions are present in the test piece collected from the center segregation part of the continuously cast slab that was actually produced by further study by the present inventors, It became clear that the occurrence of HIC can be reduced when a specific condition is satisfied, and the following important knowledge (p) was obtained.

(P) In the central segregation portion of the continuous cast slab, the surface density of (Ti, Nb) (C, N) -based inclusions having a major axis exceeding 10 μm is 2 pieces / mm ² or less, and the major axis is 1 to 10 μm (Ti, If the surface density of Nb) (C, N) inclusions is 2 to 200 / mm ² , the occurrence of HIC in the thick steel plate obtained by rolling this continuous cast slab can be reduced. When the above conditions are satisfied, the occurrence of HIC can be reduced because there are sufficiently few coarse (Ti, Nb) (C, N) inclusions having a major axis exceeding 10 μm in the continuous cast slab and the major axis is 1 to 10 μm. (Ti, Nb) (C, N) -based inclusions are present in an area density of 2 to 200 / mm ² , and it is assumed that the stress applied to the inclusions is dispersed.

The surface density of the (Ti, Nb) (C, N) inclusions is that in a continuous cast slab, and when the continuous cast slab is rolled into a thick steel plate, its center segregation. in part, greater than the major diameter 10μm (Ti, Nb) (C , N) based surface density of inclusions with two / mm ² or less, the diameter 1~10μm (Ti, Nb) (C , N) type inclusions The surface density is 0.5 to 20 pieces / mm ² .

  The present inventor further modeled the actual continuous casting process, calculated the growth of the solidified shell and the three-dimensional molten steel flow in the continuous cast slab, and compared it with the occurrence of HIC.

  Below, the examination method and the knowledge obtained are explained.

  [5] First, a two-dimensional heat transfer solidification calculation is performed with the heat removal conditions in the mold and in the secondary cooling as boundary conditions, taking into account the deformation and solidification shrinkage of the solidified shell accompanying the change of the roll cavity in the roller apron, The growth of the solidified shell was studied.

  Next, based on the above examination results, the flow behavior of the molten steel injected into the mold from the immersion nozzle in the solidified shell was calculated. In the solid-liquid coexistence range, the flow resistance increased with an increase in the solid phase ratio fs, and the calculation was performed assuming that the solid phase ratio fs stopped flowing when the solid phase ratio fs exceeded 0.8.

  In the above calculation model, the molten steel flow at the end of solidification causes macro segregation, leading to the formation of coarse (Ti, Nb) (C, N) inclusions, and HIC is generated.

  Therefore, the maximum flow velocity during the solidification process at the center of the continuous cast slab was investigated, and the relationship between the crack area ratio CAR (%) in the HIC test, that is, the crack area ratio with respect to the observation surface was arranged.

  That is, the steel of the same component system in the component range shown in Table 2 is continuously cast under various conditions shown in Table 3, and the calculated maximum flow velocity in the solidification process of the central portion of the continuous cast slab under the above assumptions The relationship with the crack area ratio CAR (%) in the HIC test using a test piece taken from a thick steel plate rolled to 20 mm was investigated.

  Table 3 also shows the calculation result of the maximum flow velocity in the solidification process of the center part of the continuous cast slab and the crack area ratio CAR in the HIC test.

  As shown in Table 3, in the solidification process at the center of the slab in Test No. 1 where a slab pressure gradient of 1.2 mm / m was applied in the solidification process and the secondary cooling specific water amount was 1.47 L / kg. The calculated maximum flow rate is 1.2 cm / s. In the case of this test number 1, the crack area ratio CAR in the HIC test was 0%, and no HIC was generated.

  On the other hand, even if the slab pressure gradient is 1.2 mm / m, which is the same as test number 1, in the case of test number 2 in which the secondary cooling specific water amount is reduced to 0.67 L / kg, The calculated maximum flow rate during the solidification process increased to 1.6 cm / s. And in the case of this test number 2, the crack area ratio CAR in the HIC test increased to 2.3%. In general, it is known that if the crack area ratio CAR in the HIC test is 4% or less, there is no problem in actual operation. Therefore, the value of 2.3% of the crack area ratio CAR is acceptable. A numeric value within the range.

  Moreover, even if the secondary cooling specific water amount is 1.47 L / kg which is the same as the test number, in the case of test number 3 where the slab pressure gradient is not set, calculation of the maximum flow velocity in the solidification process of the slab center is calculated. The value increased to 6.3 cm / s. The crack area ratio CAR in the HIC test increased greatly to 14.2%.

  On the other hand, in the case of Test No. 4 and Test No. 5 using different continuous casting equipment, if the calculated value of the maximum flow velocity in the solidification process of the slab center is large from Table 3, the crack area in the HIC test It was observed that the rate CAR increased.

  Therefore, as a result of the same examination, it has been found that the following item (q) holds regardless of the continuous casting equipment used.

  (Q) If the calculated value of the maximum flow velocity in the solidification process of the slab center is less than 3 cm / s, the crack area ratio CAR in the HIC test is a low value of 4% or less, causing a problem in actual operation. There is nothing.

Therefore, steel containing Nb and Ti in combination is continuously cast under various conditions, and in the same manner as described above, the calculated value of the maximum flow velocity in the solidification process of the slab center and the continuous cast slab In addition to investigating the relationship with the crack area ratio CAR in the HIC test using specimens taken from the center, various factors during continuous casting are the calculated values of the maximum flow velocity during the solidification process in the center of the slab. As a result, if the calculated value of the maximum flow velocity in the solidification process at the center of the slab shown in (q) is less than 3 cm / s, the crack area ratio CAR in the HIC test Was found to be a low value of 4% or less, and the following findings (r) were obtained.

  (R) The slab surface temperature (° C) and roll pitch (m) have a large effect on the calculated maximum flow velocity during the solidification process at the center of the slab, and the rolling gradient corresponding to the solidification shrinkage on the roller apron. As shown in FIG. 5, if the product of both is less than 400, the calculated value of the maximum flow velocity is less than 3 cm / s.

  In FIG. 5, the vertical axis indicating the calculated value of the maximum flow velocity in the solidification process of the slab center is simply expressed as “center flow velocity (cm / s)”, and the slab surface temperature (° C.). The horizontal axis indicating the product of the roll pitch (m) is expressed as “roll pitch × surface temperature”.

  As already mentioned, macrosegregation in continuous casting (that is, center segregation) is the movement of molten steel in which the solute (component element) between dendritic trees is concentrated due to bulging, reduction or solidification shrinkage of continuous cast slabs, It is a phenomenon that accumulates in the center, that is, a phenomenon that occurs when concentrated molten steel flows by microsegregation. And even if the flow of molten steel occurs in the early stage of solidification, it does not affect the center segregation. On the other hand, when solidification progresses and the solid phase ratio fs at the center of the continuous cast slab increases, the molten steel does not flow.

  For this reason, it reached the conclusion that the solid phase rate fs at the center of the slab should be controlled in a specific region, and as a result of further studies, the following knowledge (s) was obtained.

  (S) Continuous casting with the product of the slab surface temperature (° C.) and the roll pitch (m) being less than 400 at a position where the solid phase ratio fs in the center of the slab is 0.4 to 0.8. By this, the calculated value of the maximum flow velocity in the solidification process of the slab center can be controlled to be less than 3 cm / s.

  The present invention has been completed based on the above findings, and the gist of the present invention is the continuous cast slab for thick steel plates shown in the following (1) and (2), and the thick steel plates shown in (3) and (4). And a thick steel plate shown in (5) and (6).

(1) By mass%, C: 0.03-0.10%, Si: 0.05-0.4%, Mn: 0.8-1.8%, P: 0.010% or less, S: 0.002% or less, Nb: 0.01-0.10%, Ti: 0.005-0.03%, Al: 0.005-0.06%, Ca: 0.0005-0.0060% and N: 0.0015 to 0.007% is contained, the balance is composed of Fe and impurities, and has a chemical composition satisfying the following formula (1). Further, in the central segregation part, the major axis exceeds 10 μm (Ti, Nb ) The surface density of (C, N) -based inclusions is 2 pieces / mm ² or less, and the surface density of (Ti, Nb) (C, N) -based inclusions having a major axis of 1 to 10 μm is 2 to 200 pieces / mm ^2. A continuous cast slab for thick steel plate, characterized in that
1.48-1.5 * Si-14.3 * Ti-73 * N> 0.72 ... (1) Formula.
However, the element symbol in the formula (1) represents the steel content in mass% of the element.

  (2) In place of a part of Fe, the chemical composition is Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.5% or less, Mo: 1.0% or less, and V: 0 The continuous cast slab for thick steel plates according to (1) above, containing one or more of 2% or less.

(3) By mass%, C: 0.03-0.10%, Si: 0.05-0.4%, Mn: 0.8-1.8%, P: 0.010% or less, S: 0.002% or less, Nb: 0.01-0.10%, Ti: 0.005-0.03%, Al: 0.005-0.06%, Ca: 0.0005-0.0060% and N: 0.0015 to 0.007% is contained, and the balance is a continuous casting method of steel having a chemical composition satisfying the following formula (1) while being composed of Fe and impurities, For thick steel sheets, characterized in that the product of continuous cast slab surface temperature (° C.) and roll pitch (m) is less than 400 at a position where the solid phase ratio fs is 0.4 to 0.8. Manufacturing method of continuous cast slab.
1.48-1.5 * Si-14.3 * Ti-73 * N> 0.72 ... (1) Formula.
However, the element symbol in the formula (1) represents the steel content in mass% of the element.

  (4) The chemical composition of the steel is replaced with a part of Fe, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.5% or less, Mo: 1.0% or less, and V : The manufacturing method of the continuous cast slab for thick steel plates as described in said (3) containing 1 type or 2 types or more of 0.2% or less.

(5) By mass%, C: 0.03-0.10%, Si: 0.05-0.4%, Mn: 0.8-1.8%, P: 0.010% or less, S: 0.002% or less, Nb: 0.01-0.10%, Ti: 0.005-0.03%, Al: 0.005-0.06%, Ca: 0.0005-0.0060% and N: 0.0015 to 0.007% is contained, the balance is composed of Fe and impurities, and has a chemical composition satisfying the following formula (1). Further, in the central segregation part, the major axis exceeds 10 μm (Ti, Nb ) The surface density of (Ti, Nb) (C, N) inclusions having a surface density of (C, N) -based inclusions of 2 pieces / mm ² or less and a major axis of 1 to 10 μm is 0.5 to 20 pieces / mm ^2. Thick steel plate characterized by being mm ² .
1.48-1.5 * Si-14.3 * Ti-73 * N> 0.72 ... (1) Formula.
However, the element symbol in the formula (1) represents the steel content in mass% of the element.

  (6) Instead of a part of Fe, the chemical composition is Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.5% or less, Mo: 1.0% or less, and V: 0 The thick steel plate according to (5) above, containing one or more of 2% or less.

  Hereinafter, the inventions related to the continuous cast slabs for thick steel plates (1) and (2) above, the inventions related to the continuous cast slab for thick steel plates (3) and (4), and (5) and ( The inventions related to the thick steel plate of 6) are referred to as “present invention (1)” to “present invention (6)”, respectively. Also, it may be collectively referred to as “the present invention”.

  The “center segregation part” in the present invention (1) and the present invention (2) refers to a range of ± 1 mm in the thickness direction of the continuous cast slab from the center line of macro segregation. Further, the “center segregation portion” in the present invention (5) and the present invention (6) refers to a range of ± 1 mm from the center line of macro segregation in the thickness direction of the thick steel plate.

  The “roll” in the present invention (3) and the present invention (4) is, for example, a guide roll for supporting the slab while suppressing bulging, a pinch roll for pulling out the slab, and for rolling down the slab. Refers to the rolling roll.

  The thick steel plate of the present invention has excellent HIC resistance with a crack area ratio CAR of 4% or less in the HIC test, even though it contains Nb and Ti combined for high strength. So it can be used for line pipes, offshore structures and pressure vessels. The continuous cast slab for thick steel plate of the present invention is suitable as a material for the thick steel plate, and the continuous cast slab for thick steel plate can be easily manufactured by the method of the present invention.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.

(A) Chemical composition of continuous cast slab for thick steel plate and thick steel plate C: 0.03 to 0.10%
C has the effect | action which raises the intensity | strength of steel. However, if the content is less than 0.03%, it is difficult to obtain a predetermined strength for applications such as line pipes. On the other hand, if the content of C exceeds 0.10%, a macro segregation part is formed at the center of the thickness of the slab during continuous casting, which causes generation of HIC, and further contains 1.0% or more of Mn. In some cases, solidification starts with the δ phase as the primary crystal, and a peritectic reaction occurs during the solidification process, resulting in a subperitectic steel in which the δ phase remains even when solidification is completed. In the case of continuous cast slabs, hypoperitectic steel may cause non-uniform solidification and vertical cracking due to stress associated with the peritectic reaction, so it is necessary to lower the casting speed compared to other steel types. Productivity is hindered. Therefore, the C content is defined as 0.03 to 0.10%.

Si: 0.05-0.4%
Si is one of the elements effective for reducing the oxygen content in steel as a deoxidizing element, and also has an effect of strengthening steel. If the molten steel is continuously cast in a state where it has not been sufficiently deoxidized, bubbles are generated in the steel, resulting in product defects, and sometimes breakout is induced and operation becomes impossible. However, if the Si content is less than 0.05%, it is difficult to obtain the above effects. On the other hand, when the content exceeds 0.4%, striped martensite is generated and the weld heat affected zone toughness is lowered. In addition, since it has a strong interaction with Ti, it produces coarse TiN, and thus coarse (Ti, Nb) (C, N) inclusions although it is not a TiN constituent element. Therefore, the Si content is specified to be 0.05 to 0.4%.

Mn: 0.8 to 1.8%
Mn has the effect of increasing the strength of the steel. However, if the content is less than 0.8%, it is difficult to obtain sufficient strength. On the other hand, if the content of Mn exceeds 1.8%, it concentrates at the center segregation part and lowers the HIC resistance. Therefore, the Mn content is defined as 0.8 to 1.8%.

P: 0.010% or less P is one of impurity elements in steel. P has a large tendency to segregate due to a small distribution coefficient at the solid-liquid interface at the time of solidification. Particularly, when the content increases and exceeds 0.010%, HIC resistance is increased due to remarkable concentration in the central segregation part. It causes a great decline in sex. Therefore, the content of P is set to 0.010% or less. In order to prevent deterioration of the HIC resistance at the center segregation part, the P content is preferably less than 0.008%. From the viewpoint of ensuring sufficient HIC resistance, the P content should be as low as possible.

S: 0.002% or less S is one of the impurity elements in steel. Since S has a small distribution coefficient at the solid-liquid interface at the time of solidification, it tends to segregate. In addition, MnS is generated at the segregation part and becomes the starting point of HIC generation. In particular, when the S content is increased and exceeds 0.002%, the HIC resistance is greatly reduced. Therefore, the content of S is set to 0.002% or less. In order to stably obtain HIC resistance under conditions that require a higher level than high-strength steel and the like, the S content is preferably 0.001% or less. From the viewpoint of ensuring sufficient HIC resistance, the S content should be as low as possible.

Nb: 0.01 to 0.10%
Nb has the effect of forming carbonitrides in the steel, increasing the strength of the steel and improving the toughness. Nb also has the effect of controlling the microstructure of the steel sheet through solid solution and precipitation, particularly in the thermal processing control (TMCP) method. In order to obtain these effects, the Nb content needs to be 0.01% or more. However, if its content exceeds 0.10%, the structure cannot be controlled because it does not dissolve at the time of heating. Therefore, the Nb content is defined as 0.01 to 0.10%. Note that the Nb content is preferably 0.015 to 0.075%.

Ti: 0.005 to 0.03%
Ti has the effect | action which improves the intensity | strength of steel. In Ti, N in the steel is fixed as TiN, and the amount of precipitation of NbN or AlN combined with Nb or Al is reduced. Therefore, when bending and straightening the slab accompanying continuous casting, austenite grains It also has an effect of preventing surface cracks of the slab due to the dynamic precipitation of NbN and AlN at the boundary. In order to obtain such an effect, the Ti content needs to be 0.005% or more. However, if its content exceeds 0.03%, a large number of carbides are produced, leading to a reduction in the toughness of the weld heat affected zone, and also causing coarse TiN to be produced. Therefore, the Ti content is specified to be 0.005 to 0.03%. The Ti content is preferably 0.010 to 0.025%.

Al: 0.005-0.06%
Al is one of the elements effective for reducing the oxygen content in steel as a deoxidizing element, and the Al content necessary for this purpose is 0.005% or more. If the Al content is less than 0.005%, desulfurization becomes insufficient in addition to deoxidation, and the yield of Ca to be added decreases, so the effect of Ca described later cannot be sufficiently obtained. , HIC occurs due to segregation of sulfides and S in the steel. However, when the Al content increases, the alumina produced by deoxidation causes HIC. In particular, when the Al content exceeds 0.06%, the generation of HIC due to alumina becomes remarkable. Therefore, the Al content is specified to be 0.005 to 0.06%.

Ca: 0.0005 to 0.0060%
Ca acts as a desulfurization element to reduce the S content in steel to prevent the formation of MnS and to control the form of sulfide. In order to obtain such an effect, the Ca content needs to be 0.0005% or more. However, even if the Ca content exceeds 0.0060%, the effect is saturated and the production cost is increased. Therefore, the content of Ca is defined as 0.0005 to 0.0060%.

N: 0.0015 to 0.007%
N, together with Nb and C, forms carbonitrides in the steel and has the effect of increasing the strength of the steel and improving the toughness. N also has a function of forming a nitride with Al, Ti, etc., miniaturizing the microstructure, and improving mechanical properties. In order to acquire the said effect, it is necessary to make content of N 0.0015% or more. However, if the N content increases, especially exceeding 0.007%, during bending and straightening of the slab accompanying continuous casting, AlN and TiN dynamically precipitate at the austenite grain boundaries, and the surface of the slab Cracks will occur. Therefore, the N content is set to 0.0015 to 0.007%.

Value of “1.48-1.5 × Si-14.3 × Ti-73 × N”: more than 0.72 The element symbol in the formula is the content in steel in mass% of the element, and “1 .48-1.5 × Si-14.3 × Ti-73 × N ”is 0.72 or less, the theoretically large (Ti, Nb) (C, N) inclusions Generation | occurrence | production can be blocked | prevented and generation | occurrence | production of HIC resulting from the production | generation of a coarse (Ti, Nb) (C, N) type inclusion can be suppressed. Therefore, it is defined that the value of “1.48−1.5 × Si−14.3 × Ti−73 × N” exceeds 0.72, that is, the expression (1) is satisfied.

  For the above reason, the continuous cast slab for thick steel plate according to the present invention (1) and the thick steel plate according to the present invention (5) are C, Si, Mn, P, S, Nb, Ti, Al, Ca and N. In the above-mentioned range, and the balance is defined as having a chemical composition satisfying the above formula (1) while being composed of Fe and impurities.

  The continuous cast slab for thick steel plate and the thick steel plate according to the present invention may be selected from Cu, Ni, Cr, Mo and V, which will be described later, instead of a part of Fe, if necessary. Two or more elements may be added and added as optional additional elements.

  Hereinafter, the above optional additive elements will be described.

Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.5% or less, Mo: 1.0% or less and V: 0.2% or less Cu, Ni, Cr, Mo and V are All have the effect | action which raises the intensity | strength of steel. For this reason, when it is desired to further improve the strength, it may be contained in the following range.

Cu: 0.5% or less Cu has the effect of improving the hardenability of steel and thereby increasing the strength. In order to obtain this effect, the Cu content is preferably 0.1% or more. On the other hand, when it contains exceeding 0.5%, the hot workability and machinability of steel will fall. Therefore, when Cu is included, the content of Cu is set to 0.5% or less.

  Since Cu induces surface cracks called “copper cracks” during continuous casting, when Cu is contained in an amount of 0.2% or more, Ni is contained in a composite amount of 1/3 or more. It is preferable.

Ni: 1.0% or less Ni has an effect of improving the strength of steel by solid solution strengthening. Ni also has the effect of improving toughness. In order to obtain these effects, the Ni content is preferably 0.1% or more. On the other hand, even if it contains exceeding 1.0%, the effect will be saturated and cost will increase, and weldability will also fall. Therefore, the Ni content when contained is set to 1.0% or less.

Cr: 1.5% or less Cr has an effect of increasing the strength of steel. Cr also has the effect of increasing toughness. In order to obtain these effects, the Cr content is preferably 0.05% or more. On the other hand, if it exceeds 1.5%, welding cracks are caused. Therefore, the Cr content when contained is set to 1.5% or less.

Mo: 1.0% or less Mo has an effect of improving hardenability and thereby increasing strength. In addition, since it is an element that hardly segregates microscopically, it also has an effect of suppressing the generation of HIC due to center segregation. In order to obtain these effects, the Mo content is preferably 0.02% or more. On the other hand, since Mo is an expensive element, its addition in large amounts leads to an increase in cost. In particular, when it exceeds 1.0%, a hardened phase such as bainite and martensite is generated, resulting in a decrease in HIC resistance. Bring Therefore, the Mo content in the case of inclusion is set to 1.0% or less.

V: 0.2% or less V has the effect of increasing the strength by forming a carbonitride and forming a solid solution in ferrite. In order to obtain this effect, the V content is preferably 0.01% or more. On the other hand, if the content exceeds 0.2%, the precipitation state in the weld heat affected zone changes and the toughness decreases. Therefore, when V is included, the content of V is set to 0.2% or less.

  Said Cu, Ni, Cr, Mo, and V can be contained only in 1 type, or 2 or more types of composites.

  For the reasons described above, the chemical composition of the continuous cast slab for thick steel plate according to the present invention (2) is replaced with a part of Fe of the continuous cast slab for thick steel plate according to the present invention (1), Cu: 0 0.5% or less, Ni: 1.0% or less, Cr: 1.5% or less, Mo: 1.0% or less, and V: containing one or more of 0.2% or less Stipulated.

  Further, the chemical composition of the thick steel plate according to the present invention (6) is replaced with a part of Fe of the thick steel plate according to the present invention (5), Cu: 0.5% or less, Ni: 1.0% or less, It was specified to contain one or more of Cr: 1.5% or less, Mo: 1.0% or less, and V: 0.2% or less.

(B) Surface density of (Ti, Nb) (C, N) inclusions in each central segregation part of continuous cast slabs for thick steel plates and thick steel plates This is the case of having the chemical composition described in the above section (A). However, in the center segregation part of the continuous cast slab for thick steel plate, the surface density of (Ti, Nb) (C, N) inclusions having a major axis exceeding 10 μm is 2 pieces / mm ² or less, and the major axis 1 to If the 10 μm (Ti, Nb) (C, N) inclusions are not in the area density of 2 to 200 / mm ² , the crack area ratio in the HIC test of the thick steel plate obtained by rolling the continuous cast slab CAR exceeds 4%, and good HIC resistance cannot be ensured.

Therefore, the continuous cast slab for thick steel plates according to the present invention (1) and the present invention (2) has a surface density of (Ti, Nb) (C, N) inclusions having a major axis exceeding 10 μm at the center segregation portion. The surface density of (Ti, Nb) (C, N) inclusions having a major axis of 1 to 10 μm at 2 pieces / mm ² or less was defined as ² to 200 pieces / mm ² .

  In addition, as already stated, the "center segregation part" of the continuous cast slab for thick steel plates according to the present invention (1) and the present invention (2) is the thickness of the continuous cast slab from the center line of macro segregation. It indicates the range of ± 1mm in the vertical direction.

Even in the case of having the chemical composition described in the above section (A), the surface density of (Ti, Nb) (C, N) inclusions having a major axis exceeding 10 μm is 2 at the center segregation portion of the thick steel plate. pieces / mm ² or less, moreover, the major axis 1~10μm (Ti, Nb) (C , N) type inclusions are 0.5 to 20 pieces at a surface density / mm ² Otherwise, HIC test of the steel plate The crack area ratio CAR at 4 exceeds 4%, and good HIC resistance cannot be ensured.

Therefore, in the thick steel plates according to the present invention (5) and the present invention (6), the surface density of (Ti, Nb) (C, N) -based inclusions having a major axis exceeding 10 μm at the center segregation portion is 2 / mm ^2. In the following, it was specified that the surface density of (Ti, Nb) (C, N) inclusions having a major axis of 1 to 10 μm was 0.5 to 20 pieces / mm ² .

  In addition, as already stated, the “center segregation part” of the thick steel plate according to the present invention (5) and the present invention (6) is a range of ± 1 mm from the center line of macro segregation in the thickness direction of the thick steel plate. Point to.

  In order to allow (Ti, Nb) (C, N) inclusions to exist in the center segregation portion of the continuous cast slab for thick steel plate with the above dimensions and surface density, for example, the present invention (3) Alternatively, the steel may be continuously cast by the method defined in the present invention (4).

  In order to allow (Ti, Nb) (C, N) inclusions to exist in the center segregation part of the thick steel plate with the above dimensions and surface density, for example, the present invention (3) and the present invention (4 The continuous cast slab for thick steel plate obtained by continuous casting of steel by the method specified in (3) may be subjected to treatment by ordinary hot rolling or TMCP method.

(C) Continuous casting method The steel having the chemical composition described in the above section (A) is subjected to continuous casting slab surface at a position where the solid phase ratio fs at the center of the continuous casting slab is 0.4 to 0.8. Thick steel plates obtained by rolling continuous cast slabs obtained by continuous casting with the product of temperature (° C.) and roll pitch (m) less than 400 have a crack area ratio CAR of 4% or less in the HIC test. Excellent HIC resistance.

  Therefore, in the method for producing a continuous cast slab for a thick steel plate according to the present invention (3) and the present invention (4), the steel having the chemical composition described in the above section (A) is used in the central portion of the continuous cast slab. When the solid phase ratio fs is 0.4 to 0.8, the product of the continuous cast slab surface temperature (° C.) and the pitch (m) of the roll is set to be less than 400, and it is defined that continuous casting is performed.

  As already described, the “roll” in the method for producing a continuous cast slab for a thick steel plate according to the present invention (3) and the present invention (4) is, for example, supported while suppressing bulging of the slab. A guide roll for pulling out, a pinch roll for pulling out the slab, and a reduction roll for reducing the slab.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  The steel 1-20 which has the chemical composition shown in Table 4 was continuously cast, and the slab was manufactured. The continuous casting was performed at a casting speed of 0.7 to 0.8 m / min using a vertical bending slab continuous casting facility having a thickness of 300 mm and a width of 2300 mm.

  In Table 5, as other conditions at the time of continuous casting, the secondary cooling specific water amount, the slab pressure gradient, and the slab surface temperature at the position where the solid phase ratio fs at the center of the slab is 0.4 to 0.8 ( (° C.) and roll pitch (m) (in Table 5, “surface temperature (° C.) × roll pitch (m)”).

  In addition, the steel 1-12 in Table 4, the steel 14, and the steel 15 are steel which has a chemical composition in the range prescribed | regulated by this invention. On the other hand, steel 13 and steels 16 to 20 in Table 4 are steels of comparative examples whose chemical compositions deviate from the conditions specified in the present invention.

  A sample was collected from the continuous cast slab thus obtained, and the precipitates in the central segregation part were investigated.

  Moreover, the slab obtained by continuous casting was heated to 1100-1200 degreeC, the finish rolling temperature was 850-750 degreeC, and it hot-rolled to the thick steel plate of thickness 15-25mm. Immediately after hot rolling, water cooling was performed, cooling was stopped at 470 to 420 ° C., and then the mixture was allowed to cool in the air.

  A sample was collected from the thick steel plate thus obtained, and the precipitate in the central segregation portion was investigated. In addition, a tensile test piece and an HIC test piece were collected and subjected to a tensile test and an HIC test at room temperature.

  The specific contents will be described below.

1. Investigation of precipitates in continuous cast slabs After taking a cross-sectional sample of the continuous cast slab on the outlet side of the continuous casting facility, making a micro sample to include the center segregation part of the sample, mechanically polishing, 10% The state of macrosegregation was investigated by corroding with an aqueous hydrochloric acid solution.

  Next, the above sample was filled with resin, mirror-polished, and observed with an optical microscope at a magnification of 1000 to investigate the state of formation of (Ti, Nb) (C, N) inclusions.

  In order to investigate the central segregation portion, the center line of macro segregation is determined on the corroded surface, and within the range of ± 1 mm from the center line to the thickness direction of the continuous cast slab (Ti, Nb) ( C, N) The size (major axis) and the number of generations (surface density) of the inclusions were measured.

2. Thick steel plate survey 2.1. Precipitation investigation of thick steel plate Take a cross-section sample from the thick steel plate, make a micro sample to include the center segregation part of the sample, mechanically polish it, then corrode with 10% hydrochloric acid aqueous solution, macro segregation situation investigated.

  Next, the above sample was filled with resin, mirror-polished, and observed with an optical microscope at a magnification of 1000 to investigate the state of formation of (Ti, Nb) (C, N) inclusions.

  In order to investigate the center segregation portion, the center line of macro segregation is determined on the corroded surface, and within the range of ± 1 mm from the center line to the thickness direction of the thick steel plate (Ti, Nb) (C, N) The size (major axis) and the number of generations (surface density) of the system inclusions were measured.

2.2. Tensile test Tensile test specimens were taken from a thick steel plate perpendicular to the rolling direction and subjected to a tensile test at room temperature to measure the yield strength (YP).

2.3. HIC Test A test piece was taken from a position where 1 mm was removed from both skins of a thick steel plate, and an HIC test was performed by a method defined by NACE TM-02-84 to measure a crack area ratio CAR.

  Table 6 shows the results of the above tests and investigations.

  From Table 6, it is clear that the test numbers E1 to E12 that satisfy the conditions of the present invention are excellent in strength and HIC resistance.

  Among the above, the test numbers E3 to E12 are particularly high in proof stress, and for example, it is apparent that they are suitable for high-grade line pipe applications of X70 (YP: 482 MPa class (70 ksi class)) or higher as defined by API. is there.

  On the other hand, in the case of test numbers E13 to E20 of comparative examples that deviate from the conditions specified in the present invention, the HIC resistance is low, and the characteristics are not sufficient when used for a line pipe or the like. .

  As mentioned above, although the present invention was concretely explained with the example, the present invention is not limited to these examples. Those not disclosed as examples are also included in the present invention as long as they satisfy the requirements of the present invention.

  The present invention does not conflict with the prevention of MnS generation that causes cracking, the reduction of the content of C, Mn, and P that causes central segregation or the mitigation of central segregation. Needless to say, this is effective in preventing the occurrence of HIC.

  In addition, as a method of reducing the center segregation, when the central part of the slab solidifies, a reduction gradient is applied to the slab to a degree corresponding to the solidification shrinkage amount or slightly higher than that. . In recent years, continuous casting equipment equipped with a roller apron that can be position controlled by hydraulic pressure has been used, and it is quickly aligned according to the casting conditions such as changing the casting speed according to the steel type or casting. The center segregation can be reduced stably over the entire length.

  The thick steel plate of the present invention has excellent HIC resistance with a crack area ratio CAR of 4% or less in the HIC test, even though it contains Nb and Ti combined to increase the strength. Can be used for line pipes, offshore structures and pressure vessels. The continuous cast slab for thick steel plate of the present invention is suitable as a material for the thick steel plate, and the continuous cast slab for thick steel plate can be easily manufactured by the method of the present invention.

It is a figure which shows typically the distribution condition of the target component element content Cs in the solid phase (namely, solidification part) in the solidification process, and the target component element content Cl in the liquid phase (namely, in molten steel). It is a figure which shows the calculation result about the change of content of C, P, Ti, and N in the residual molten steel of the component system of Table 1. It is a figure which shows the relationship between the calculated value of the crystallization start solid phase fraction fs of the continuous cast slab manufactured by continuous casting, and the crack area ratio CAR (%) in a HIC test. It is a figure which shows the relationship between the decreasing amount of content of N, Ti, and Si, and the increase degree of the calculated value of the crystallization start solid phase ratio fs of TiN. It is a figure which shows the influence of the product of continuous cast slab surface temperature (degreeC) and roll pitch (m) on the calculated value of the maximum flow velocity in the solidification process of a continuous cast slab center part.

Claims (6)

In mass%, C: 0.03-0.10%, Si: 0.05-0.4%, Mn: 0.8-1.8%, P: 0.010% or less, S: 0.002 %, Nb: 0.01-0.10%, Ti: 0.005-0.03%, Al: 0.005-0.06%, Ca: 0.0005-0.0060% and N: 0 .0015-0.007%, the balance is composed of Fe and impurities, and has a chemical composition satisfying the following formula (1). Further, in the central segregation part, the major axis exceeds 10 μm (Ti, Nb) (C , N) The surface density of the inclusions is 2 pieces / mm ² or less, and the surface density of the (Ti, Nb) (C, N) inclusions having a major axis of 1 to 10 μm is 2 to 200 pieces / mm ^2. A continuous cast slab for thick steel plates.
1.48-1.5 × Si-14.3 × Ti-73 × N> 0.72 (1) where the element symbol in the formula (1) is steel in mass% of the element. Represents the medium content.   In place of a part of Fe, the chemical composition is Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.5% or less, Mo: 1.0% or less, and V: 0.2% The continuous cast slab for thick steel plates according to claim 1, comprising one or more of the following. In mass%, C: 0.03-0.10%, Si: 0.05-0.4%, Mn: 0.8-1.8%, P: 0.010% or less, S: 0.002 %, Nb: 0.01-0.10%, Ti: 0.005-0.03%, Al: 0.005-0.06%, Ca: 0.0005-0.0060% and N: 0 .0015 to 0.007%, and the balance is a continuous casting method of steel having a chemical composition satisfying the following formula (1), which consists of Fe and impurities, and has a solid phase ratio at the center of the continuous cast slab Continuous casting for thick steel plate, characterized in that, at a position where fs is 0.4 to 0.8, continuous casting is performed by setting the product of continuous cast slab surface temperature (° C.) and roll pitch (m) to less than 400. A manufacturing method of a piece.
1.48-1.5 × Si-14.3 × Ti-73 × N> 0.72 (1) where the element symbol in the formula (1) is steel in mass% of the element. Represents the medium content.   Instead of a part of Fe, the chemical composition of steel is Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.5% or less, Mo: 1.0% or less, and V: 0.00. The manufacturing method of the continuous cast slab for thick steel plates of Claim 3 containing 1 type or 2 types or more of 2% or less. In mass%, C: 0.03-0.10%, Si: 0.05-0.4%, Mn: 0.8-1.8%, P: 0.010% or less, S: 0.002 %, Nb: 0.01-0.10%, Ti: 0.005-0.03%, Al: 0.005-0.06%, Ca: 0.0005-0.0060% and N: 0 .0015-0.007%, the balance is composed of Fe and impurities, and has a chemical composition satisfying the following formula (1). Further, in the central segregation part, the major axis exceeds 10 μm (Ti, Nb) (C , N) The surface density of the inclusions is 2 pieces / mm ² or less, and the surface density of the (Ti, Nb) (C, N) inclusions having a major axis of 1 to 10 μm is 0.5 to 20 pieces / mm ² . Thick steel plate characterized by being.
1.48-1.5 × Si-14.3 × Ti-73 × N> 0.72 (1) where the element symbol in the formula (1) is steel in mass% of the element. Represents the medium content. In place of a part of Fe, the chemical composition is Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.5% or less, Mo: 1.0% or less, and V: 0.2% The thick steel plate according to claim 5 containing one or more of the following.

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