WO2015022729A1

WO2015022729A1 - Steel plate

Info

Publication number: WO2015022729A1
Application number: PCT/JP2013/071864
Authority: WO
Inventors: 斎藤　直樹; 勝己榑林; 康哲 ▲高▼橋
Original assignee: 新日鐵住金株式会社
Priority date: 2013-08-13
Filing date: 2013-08-13
Publication date: 2015-02-19
Also published as: EP2860276B1; KR101542709B1; EP2860276A4; JP5510620B1; KR20150029615A; CN104520463B; JPWO2015022729A1; EP2860276A1; CN104520463A

Abstract

This steel plate has an A value of 4.5% or less; has a Pcm value of 0.25% or less; has a yield strength of 460 - 580 N/mm² and a tensile strength of 550 - 670 N/mm²; has a difference between the hardness of a 1/8 t part positioned at 1/8 of the plate thickness along the direction of thickness from the surface and the hardness of a 1/2 t part positioned at 1/2 of that plate thickness along the direction of plate thickness from that surface of 20 or less by Vickers hardness; and an average crystal grain diameter of 35 µm or less at the 1/8 t part.

Description

steel sheet

The present invention has a yield strength of 460 N / mm ² to 580 N / mm ² , a tensile strength that is suitable for application to welded structures such as buildings, construction machines, offshore structures, large cranes for ships, and civil engineering structures. is has a strength of ^{550N / mm 2 ~ 670N / mm} 2, further comprising a uniform properties in the thickness direction, excellent weldability and base material toughness and HAZ toughness, than the plate thickness 80mm The present invention relates to a suitable thick high strength steel sheet.

2. Description of the Related Art In recent years, thick high-tensile steel sheets have been applied to welded structures such as buildings, construction machines, offshore structures, large-scale cranes for ships, and civil engineering structures as the size of the structures increases.
When thick high-tensile steel plates are applied to large structures, the difference in strength and toughness in the plate thickness direction is highly advanced by predicting their deformation behavior and fracture behavior when constructing and designing complex welded structures. It is not preferable when building reasonable safety. Therefore, a thick high-tensile steel sheet having uniform characteristics in the sheet thickness direction is required.
Thick high-tensile steel plates are often used in parts requiring high safety in large offshore structures and large cranes. What is most concerned about the destruction of structures is the occurrence of brittle fracture from welded joints such as weld defects. Therefore, in the welded portion, excellent weldability is required to prevent the occurrence of defects, and high weld heat affected zone toughness (hereinafter referred to as HAZ toughness) is often required for brittle fracture.

In particular, thick high-tensile steel sheets with a plate thickness of 80 mm or more are usually alloys such as C, Mn, Cr, Mo, and V that improve the hardenability for the purpose of imparting a predetermined strength to the center of the plate thickness. Appropriate amounts of elements are added and manufactured by quenching and tempering. During quenching, it is well known that strength and toughness change depending on the depth in the thickness direction from the surface layer to the thickness center due to the difference in cooling rate in the thickness direction. Further, as the plate thickness increases, not only the difference in cooling rate during the quenching process, but also the difference in heating rate between the surface layer and the plate thickness center portion during heating during the quenching process increases. In the surface layer portion of the steel plate, the time for holding at a high temperature is longer than that in the center portion of the plate thickness, and the crystal grains tend to be coarser than in the center portion of the plate thickness. If there is a difference in crystal grains between the vicinity of the surface layer and the central portion of the plate thickness, there may be a difference in materials such as strength.
In general, in many steel standards, the position of 1/4 of the plate thickness along the plate thickness direction from the surface of the steel plate, in other words, 1 / of the plate thickness from the surface of the steel plate to the center of the plate thickness in the plate thickness direction. The characteristics of the position advanced by 4 (hereinafter referred to as 1/4 t portion) are defined. However, when the plate thickness is increased in an offshore structure or the like and a high level of safety against destruction is required, the thickness of the plate is ½ of the plate thickness from the surface of the steel plate toward the center of the plate thickness. Even at the position (hereinafter referred to as 1 / 2t portion), stable and high characteristics are required.

In view of the above, as a thick high-tensile steel plate to be applied to future large structures, not only has excellent weldability but also high base metal toughness and weld heat-affected zone toughness, a plate unique to thick high-tensile steel plates It is important to eliminate the non-uniformity in the thickness direction. It is clear that the weldability is determined by the alloy composition from many studies, and can be evaluated by an index such as a Pcm value. In many cases, good weldability that does not require preheating can be achieved by limiting the content of alloy elements having high hardenability such as Cr and Mo and setting the Pcm value to, for example, 0.25% or less. Therefore, in order to ensure excellent weldability, as described above, it is important to ensure strength without adding an element that increases hardenability as much as possible. As one of such prior arts, an invention of a high-tensile steel plate containing a large amount of Cu has been conventionally disclosed.

For example, Patent Document 1 and Patent Document 2 disclose inventions relating to a method for producing a high-tensile steel sheet containing 0.6% to 1.5% and 0.5% to 2.0% of Cu, respectively. These inventions are premised on the application of thermal processing control that performs controlled rolling during hot rolling and, as a rule, includes accelerated cooling after rolling. For this reason, the manufacturing methods disclosed in

Patent Documents

1 and 2 are not suitable for manufacturing thick high-tensile steel sheets targeting 80 mm or more. In addition, when these manufacturing methods are used, the microstructure in the vicinity of the surface thickness part of the sheet thickness and the center part is greatly increased due to the effect of controlled rolling, etc. There is a concern.

Patent Document 3 discloses a method for producing a high-toughness high-strength steel (high-strength steel sheet) containing 0.5% to 4.0% Cu and having excellent elongation characteristics and a tensile strength of 686 MPa or more. ing. The object of Patent Document 3 is a high strength steel having a tensile strength of 686 MPa or more that exceeds the assumption of the present invention, and has high hardenability in which addition of alloy elements such as Cr, Mo, and V is allowed. High strength steel. Therefore, the manufacturing method described in Patent Document 3 cannot be adopted as a means for solving the problem intended by the present invention because of concern about material uniformity in the thickness direction.

Patent Document 4 discloses a high-tensile steel sheet containing 0.8% to 1.5% Cu and having excellent weld toughness. Although this high-tensile steel plate is added with Cu and Ni, the assumption of the plate thickness is 77 mm as can be seen from the examples in Reference 4, and the intention is different from the present invention suitable for a plate thickness of 80 mm or more. . In Patent Document 4, it is specified that in the production of a high-strength steel sheet, rolling is performed while restricting the total amount of rolling at 900 ° C. or less, and direct water cooling is performed after rolling. Therefore, there is a great concern for the material uniformity in the thickness direction. Further, although the N / Al ratio is defined to be in the range of 0.3 to 3.0, the Al content is 0.013% or less as disclosed in the examples. As a result, deoxidation with normal Al cannot be performed, and there is a concern that the conventional general production method is slightly deviated, lacks in stability, and costs increase.

Patent Document 5, Patent Document 6 and Patent Document 7 all disclose a method for producing a steel for high heat input welding containing 0.2% to 2.0% Cu and excellent in low temperature toughness. A feature of these steel sheets is that the S content is controlled to be 0.003% to 0.008%. By adding S and keeping the S content in the above range, fine MnS is precipitated in the steel, and excellent HAZ toughness is obtained for high heat input welding. Although these techniques have a certain effect on high heat input welding, the target plate thickness is a thin material of about 32 mm, which is greatly different from the intention of the present invention. Furthermore, the addition of S promotes the generation of MnS inclusions that are highly likely to adversely affect toughness, particularly in thick high-tensile steel sheets. For this reason, the techniques disclosed in Patent Documents 5 to 7 are not excellent methods on the premise of producing thick high-tensile steel sheets.

Patent Document 8 discloses a high-strength thick steel plate excellent in CTOD characteristics and containing 0.70% to 1.75% Cu. However, the strength level of these steel plates is 780 MPa class (tensile strength of 780 MPa or more), which is significantly different from the strength intended by the present invention. Further, since these steel sheets contain 0.005% to 0.0015% of B, the increase in hardness in the vicinity of the plate thickness surface layer portion becomes extremely large. Therefore, it is estimated that the strength difference in the thickness direction is large in the steel sheet disclosed in Patent Document 8. Further, these steel sheets have an Al content of 0.01% or less, and cannot be deoxidized by ordinary Al. Therefore, it is not suitable for solving the problems of the present invention, such as a slight deviation from the conventional general manufacturing method and a high cost for stability.

As described above, Cu addition is a technique that has been applied to many inventions. However, for example, there is no prior art that can ensure material uniformity in the thickness direction of a thick high-tensile steel plate exceeding 80 mm without substantially containing an alloying element such as Cr, Mo, or V.

Japanese Patent Publication No. 7-81164 Japanese Laid-Open Patent Publication No. 5-179344 Japanese Patent Laid-Open No. 5-186820 Japanese Patent No. 4432905 Japanese Laid-Open Patent Publication No. 2-254118 Japanese Unexamined Patent Publication No. 2-250917 Japanese Unexamined Patent Publication No. 3-264614 Japanese Laid-Open Patent Publication No. 2001-335884

The present invention is, in the conventional invention could not be achieved, the yield strength ^{460N / mm 2 ~ 580N / mm} 2, with a tensile strength of ^{550N / mm 2 ~ 670N / mm} 2, for example, there a thick high tensile steel plate or 80mm Thus, a thick high-tensile steel sheet having uniform characteristics in the thickness direction and excellent in weldability, base metal toughness and HAZ toughness is provided.

The inventors of the present invention repeated many experiments on a method for producing a thick high-tensile steel plate. As a result, in order to ensure high weldability and HAZ toughness of the base material, the Pcm value is controlled to be in the range of 0.25% or less and substantially contains Cr, Mo, V and B with high hardenability. It was found that it is important not to let them. In addition, in this invention, high weldability shows that a weld crack does not generate | occur | produce in 0 degreeC in actual welding. In this case, preheating is unnecessary during welding.
Furthermore, it has been found that it is effective to contain a high concentration of Ni simultaneously with a high concentration of Cu in order to ensure the characteristics after stress relief annealing and the HAZ toughness. Furthermore, in order to obtain a thick high-tensile steel sheet having material uniformity in the sheet thickness direction, the content of Cu and Ni is limited to a specific high concentration range, and then TMCP which has been the mainstream of conventional Cu-added steels. It has been found effective to apply a quenching and tempering treatment rather than a treatment (Thermo Mechanical Control Process).

FIG. 1 shows a cross-sectional hardness distribution in the thickness direction after quenching and tempering treatment in two types of steel plates having a thickness of 110 mm containing 1.15% Cu and 1.81% or 3.22% Ni. FIG. In general, the cross-sectional hardness in the thickness direction of thick high-tensile steel sheets tends to increase in hardness from the inside to the vicinity of the surface layer portion, and the degree is more conspicuous as the content of alloying elements that improve hardenability increases. Become. As can be seen from FIG. 1, in the case of 3.22% Ni-containing steel compared to 1.81% Ni-containing steel (steel having a Ni content of 1.81%), it extends from the surface layer portion to the inside of the plate thickness. The range of high hardness has spread, and the Vickers hardness at 1 / 8th of the plate thickness (hereinafter referred to as 1 / 8t part) and the Vickers hardness at 1 / 2t part along the plate thickness direction from the surface of the steel plate. The difference (ΔHv) is 38. The ΔHv of the 3.22% Ni-containing steel is significantly higher than that of the 1.81% Ni steel. Here, the surface of the steel sheet does not mean a specific surface during rolling, but simply means one surface of the steel sheet.

As described above, ΔHv depends on the alloy element content. FIG. 2 shows the results of experimental determination of the relationship between ΔHv and the content of alloy elements. FIG. 2 shows ΔHv, which is the difference between the hardness at 1/8 t portion and the hardness at 1/2 t portion of a steel plate having a thickness of 100 mm with varying contents of Cu and Ni. . The number in the circle in the figure is ΔHv. When measuring the hardness of a cross section in the plate thickness direction of a steel plate, a region having a high hardness due to center segregation may appear near the center of the plate thickness depending on the state of the slab. Such locally hard areas (local hardened areas) are extremely small areas with respect to the overall thickness of the thick high-tensile steel sheet, and are considered to have little effect on the strength of the steel. . Therefore, when measuring the hardness distribution of the cross section of a steel plate, it is desirable to exclude the data of the above-mentioned local hardening part. As can be seen from FIG. 2, there is a correlation between the A value (A = Cu + Ni), which is the sum of the Cu content and the Ni content, and ΔHv, and the A value exceeds 4.5%. , ΔHv was found to exceed 20. Further, it was found that even if the Cu content is a low value of 1.5% or less, ΔHv may also exceed 20 if the Ni content exceeds 3.0%. On the other hand, the lower limit value of the A value is not particularly limited. However, from the viewpoint of securing HAZ toughness and strength, which will be described later, the lower limit is 1.2% and 0.7% for the Ni content and the Cu content, respectively. Therefore, the lower limit value of the A value is preferably 1.9%, which is the sum of the lower limit values of the Cu content and the Ni content.

Furthermore, the present inventors simulated a weld heat affected zone at −40 ° C. in order to investigate the effects of Cu content and Ni content on HAZ toughness (vE (HAZ)), which is a major element of the present invention. An impact test was also conducted. The result is shown in FIG. In an ordinary large structure, if Charpy absorbed energy at −40 ° C. is 42 J or more, the occurrence of brittle fracture can be prevented. Therefore, whether or not the Charpy absorbed energy at −40 ° C. is 42 J or more was determined as a pass / fail criterion. The numerical value in the circle in FIG. 3 is the Charpy absorbed energy at −40 ° C. As can be seen from FIG. 3, toughness of the steel material is greatly improved by increasing the Ni content, and in order to secure 42 J or more with an impact test value as described later, a Ni content of 1.2% or more is necessary. I found out. However, it has also been found that if the Cu content exceeds 2.5%, the toughness decreases even if the Ni content is 1.2% or more.

As described above, the HAZ toughness is strongly influenced by the alloy composition (content of alloy components). On the other hand, the toughness of the base material needs to be examined in consideration of the microstructure, specifically the crystal grain size, in addition to the alloy composition. In particular, it is necessary to examine what the crystal grain size will be for each plate thickness position of a thick high-tensile steel plate exceeding 80 mm. The tensile strength of ^{550N / mm 2 ~ 670N / mm} 2 steel as contemplated by the present invention, in general, the microstructure becomes ferrite and bainite mixed tissue. Therefore, it is not easy to evaluate the crystal grain size from observation of a microstructure using an optical microscope that has been conventionally performed. Therefore, in the present invention, an EBSD method (electron beam backscatter diffraction pattern analysis method) often used for crystal orientation analysis is used, and a region surrounded by a grain boundary having an angle of 30 ° or more is defined as a crystal grain. The circle equivalent grain size of the crystal grain was defined as the crystal grain size. The frequency distribution of the measured crystal grain size was calculated, and the crystal grain size at which the cumulative frequency from the fine grain side was 70% was defined as the average crystal grain size. An example of actual measurement is shown in FIG. FIG. 4 shows 0.08% C-0.15% Si-1.51% Mn-0.008% P-0.0010% S-1.15% Cu-1.23% Ni-0.012% The cumulative frequency (%) with respect to the crystal grain size of steel having Ti-0.012% Nb-0.035% Al-0.0039% N as a component is shown. In obtaining the cumulative frequency, first, the steel melted in the above components was hot-rolled so that the plate thickness was 140 mm, and quenched and tempered after hot rolling. And the crystal grain in each plate | board thickness position of the surface layer part (namely, surface part or outermost layer), 1 / 8t part, 2 / 8t part (1 / 4t part), and 3 / 8t part of the steel plate after quenching and tempering The diameter was obtained, and the cumulative frequency (%) with respect to the crystal grain size was obtained. The crystal grain size corresponding to a cumulative frequency of 70% is the average crystal grain size. As can be seen from FIG. 4, in this experimental result, the average crystal grain size at each plate thickness position varies depending on the sampling position in the plate thickness direction of the steel plate, and is generally 20 μm or more at the outermost layer and 1 / 8t portion. On the other hand, it was 15 μm or less in the 2 / 8t part and the 3 / 8t part.

Furthermore, the present inventors investigated how toughness changes with respect to the crystal grain defined above. FIG. 5 shows 0.08% C-0.15% Si-1.51% Mn-0.008% P-0.0010% S-1.15% Cu-1.23% Ni shown above. The crystal grain size and test temperature change at 20 ° C intervals in a quenched and tempered steel with a plate thickness of 140 mm containing -0.012% Ti-0.012% Nb-0.035% Al-0.0039% N The relationship with the toughness obtained by the Charpy test carried out is shown. The fracture surface transition temperature (vTrs) obtained by the Charpy test was used as an index of toughness. Here, vTrs identifies the ductile fracture surface and the brittle fracture surface from the fracture surface characteristics of the test piece, measures the area ratio of the brittle fracture surface to the total fracture surface area, and determines the area ratio of the brittle fracture surface. Is the temperature at which the area ratio of the brittle fracture surface is 50%. vTrs indicates that the smaller the value, the better the toughness. In addition, the sampling position of the Charpy test piece is the same position as the site where the crystal grain size was measured, and the sampling direction is a direction perpendicular to the rolling direction.
In FIG. 5, the vertical axis represents vTrs (toughness), and the horizontal axis d ^−1/2 represents the reciprocal of the square root of the average crystal grain size. In this figure, the larger the value of d ^−1/2 × 100 on the horizontal axis, the finer the crystal grain size.
As is apparent from FIG. 5, there is a substantially linear correlation between vTrs and d ^−1/2 . This corresponds to a relationship conventionally called a Hall-Petch relationship. The vTrs on the vertical axis is also affected by the component system, and it is known that the toughness improves particularly when the Ni content increases. FIG. 5 shows a case where the Ni content is 1.23%, and this Ni amount is close to 1.2%, which is the lower limit of the Ni amount necessary for improving the HAZ toughness. Therefore, when this FIG. 5 is used, when the content of Ni, which is the alloy component having the greatest influence on toughness, is close to the lower limit of the range of the present invention, it is possible to predict how much crystal grain size is necessary. It becomes possible. Details will be described below.

Since fracture in a normal large structure occurs from a welded joint, HAZ toughness is important as a steel material. However, in order to further improve the safety of the structure, high toughness is required not only for HAZ toughness but also for the base material (portion not affected by welding heat). In general, brittle fracture is assumed to occur due to welding defects, etc., but most of these defects are not defects that exist on the surface that are easy to find, but are brittle fractures that occur inside the steel sheet. It has the greatest impact. This is because it is assumed that defects inside the steel sheet are likely to be the most severe stress state with respect to the crack growth, although the possibility that defects in the steel plate are found is low, and depending on the acting stress state.
Assuming a fracture from a defect in the welded part, even if a brittle crack occurs, the toughness of the base material in the vicinity of the defect must be high in order to prevent it from occurring. It is assumed that such a severe stress state is mainly in the region of 1 / 8t to 7 / 8t, which is the inner side of the steel sheet. Therefore, the toughness required for the base metal should be defined on the inner side closer to the center of the steel plate than the 1/8 t portion rather than the vicinity of the surface layer of the plate thickness.
For the above reasons, an energy value of 42 J or more required as the Charpy absorbed energy (vE-40) at −40 ° C., which is generally required, is required on the inner side of the steel plate from 1/8 t part from the steel plate surface. Therefore, in the present invention, the crystal grain size on the inner side from the 1/8 t part is defined.
Considering the transition curve obtained with a conventional steel plate, it is necessary that vTrs be −10 ° C. or lower in order to satisfy the absorbed energy 42J at −40 ° C.
From FIG. 5, the average crystal grain size corresponding to vTrs of −10 ° C. (broken line in the figure) is 35 μm. Therefore, it was found that if the average crystal grain size is 35 μm or less, vTrs ≦ −10 ° C. can be satisfied. Each point in FIG. 5 is taken from the plate thickness position indicated by the display in (). As described above, it is considered that the steel sheet surface layer portion does not significantly affect the destruction of the actual structure. Therefore, in the present invention, the average crystal grain at the position excluding the region from the outermost layer portion to 1/8 t portion. Define the diameter. Since the thick steel plate is held in the heat treatment furnace for a long time, the crystal grain size tends to be coarser on the steel plate surface layer side than on the plate thickness central portion. Therefore, it is particularly important that the average crystal grain size at 1/8 t part of the plate thickness is 35 μm or less. Furthermore, by setting the average crystal grain size of the 3 / 8t part of the plate thickness to 35 μm or less, the average crystal grain size of both the 1 / 8t part and the 3 / 8t part of the plate thickness may be set to 35 μm or less.
As described above, the finer the average crystal grain size, the better the toughness, but it is not easy to make it fine. Therefore, the lower limit value of the average crystal grain size may be 5 μm, 10 μm, or 15 μm.
In order to improve the safety of steel structures, there is a concept that requires higher toughness for the base material in consideration of strain aging and the like. In particular, in the case of strain aging, according to the study by the inventors, when a strain of about 5% is applied cold, and then aging treatment is performed at 250 ° C. (held for 2 hours), the Charpy transition temperature is − It has been found that the temperature rises by about 15 ° C. Therefore, when higher toughness is required in consideration of strain aging, vTrs is desirably 15 ° C. lower and −25 ° C. or lower. For this purpose, it was found from FIG. 5 that the average crystal grain size at 1/8 t part of the plate thickness should be 25 μm or less. That is, for the same reason as described above, the average crystal grain size of the 3 / 8t portion of the plate thickness may be 25 μm or less.
In addition, since the cooling rate at the time of quenching is higher in the vicinity of the surface layer of the steel plate than in the steel plate, there is a tendency that a sufficient quenching structure can be obtained, but the strength is increased. Therefore, the toughness in the vicinity of the surface layer is not necessarily high compared to the inside of the steel plate (for example, 1/4 t portion). However, as mentioned above, when considering the safety against brittle fracture as a structure, it is easy to find a potential crack such as a weld defect under conditions where extreme bending deformation does not occur, and The inside of the plate thickness (inside of 1 / 8t) tends to be more severe with respect to the occurrence of brittle cracks than in the vicinity of the surface layer where the binding force is low. For this reason, in the present invention, if the internal toughness is considered from 1 / 8t, it is considered sufficient for ensuring the safety of the structure, and the internal average crystal grain size is specified from 1 / 8t. It was decided.
The steel plate manufactured based on the above techniques exhibits excellent weldability, base metal toughness, and weld heat affected zone toughness while ensuring uniformity in the thickness direction. In particular, the effect is large in a steel plate having a plate thickness of 80 mm or more. However, in a steel sheet having a plate thickness exceeding 200 mm, the cooling rate at the central portion of the plate thickness is remarkably lowered, leading to the coarsening of the microstructure, so that there is a high possibility that the predetermined strength and toughness cannot be satisfied. Therefore, the thickness of the steel plate manufactured according to the present invention may be 200 mm or less. If necessary, the upper limit of the plate thickness may be 175 mm, 150 mm, or 125 mm. The lower limit of the plate thickness may be 90 mm or 100 mm.

As described above, the present invention is substantially free of these elements, for example, with respect to a thick high-tensile steel plate of 80 mm or more, which contains a large amount of alloy elements such as Cr and Mo, and the contents of Cu and Ni. This is based on the fact that the conditions under which steel having excellent weldability, base metal toughness and HAZ toughness can be manufactured are specified by appropriately controlling the thickness.

(1) That is, the steel sheet according to one embodiment of the present invention has a chemical composition of mass%, C: 0.03% to 0.12%, Si: 0.05% to 0.30%, Mn: 1 20% to 1.65%, Cu: 0.7% to 2.5%, Ni: 1.2% to 3.0%, Nb: 0.005% to 0.030%, Ti: 0.005 % To 0.030%, Al: 0.015% to 0.065%, N: 0.0020% to 0.0060%, Mo: 0% to 0.04%, Cr: 0% to 0.08% , V: 0% to 0.01%, B: 0% to 0.0005%, P: 0.010% or less, S: 0.002% or less, Ca: 0% to 0.0030%, Mg: 0 % To 0.0030%, REM: 0% to 0.0030%, balance: Fe and impurities; A value represented by the following formula (a) is 4.5% or less; Indicated Pcm value be 0.25%; yield strength ^{460N / mm 2 ~ 580N / mm} 2, and a tensile strength be ^{550N / mm 2 ~ 670N / mm} 2; from the surface along the thickness direction The difference between the hardness of the 1 / 8t portion, which is 1/8 of the plate thickness, and the hardness of the 1 / 2t portion, which is 1/2 the plate thickness along the plate thickness direction from the surface, A crystal orientation analysis using an electron beam backscatter diffraction pattern analysis method and defining a region surrounded by a grain boundary having a crystal orientation difference of 30 ° or more as a crystal grain; When the crystal grain size is defined as the crystal grain size and the crystal grain size at which the cumulative frequency when calculating the frequency distribution of the crystal grain size is 70% from the fine grain side is defined as the average crystal grain size The average crystal grain size in the 1 / 8t part is 35 μm or less.
A = Cu + Ni (a)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (b)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are the contents of each element, and the unit is mass%.

(2) In the steel plate according to (1), the average crystal grain size in a 3 / 8t portion that is a position of 3/8 of the plate thickness along the plate thickness direction from the surface of the steel plate. It may be 35 μm or less.

(3) In the steel plate described in (1) above, the average crystal grain size in the 1 / 8t portion may be 25 μm or less.

(4) In the steel plate according to (3) above, the average crystal grain size in a 3 / 8t portion that is a position of 3/8 of the plate thickness along the plate thickness direction from the surface of the steel plate. It may be 25 μm or less.

(5) In the steel plate according to any one of (1) to (4), the plate thickness of the steel plate may be 80 mm or more.

According to the present invention, it is possible to provide a thick high-tensile steel plate that is excellent in the uniformity of the base material in the plate thickness direction and excellent in weldability, base material toughness, and HAZ toughness.

Based on 0.12% C-0.08% Si-1.45% Mn-1.15% Cu, Ni content is 1.81% or 3.22%, all other components are It is the figure which showed the result of having measured the hardness distribution in the plate | board thickness direction cross section of the steel plate after hardening and tempering about two types of steel plates with a thickness of 110 mm which are in the range of embodiment. Using 0.13% C-0.12% Si-1.55% Mn as the basic component system, the Cu content is 0.3% to 3.6%, and the Ni content is 0.57% to 3%. About a steel plate having a thickness of 100 mm that has been hardened and tempered after hot rolling, and has a hardness of 12.5 mm (1/8 t part) from the surface layer, It is the figure which calculated | required the difference (Hv: 98N) with the hardness of 2t part, and showed the influence of Cu and Ni amount. 0.13% C-0.12% Si-1.55% Mn-0.012% Ti-0.013% Nb as a basic component system, Cu content is 0.3% to 3.6%, Ni Heat input (3.5 kJ) received at the time of welding with respect to a 1/2 t part of a quenched and tempered steel having a thickness of 100 mm and a content of 0.57% to 3.5% and other components within the range of the present embodiment. FIG. 6 is a graph showing the relationship between Charpy absorbed energy (J) and Cu and Ni contents when an impact test at −40 ° C. is performed after applying a welding heat cycle corresponding to / mm). 0.08% C-0.15% Si-1.51% Mn-0.008% P-0.0010% S-1.15% Cu-1.23% Ni-0.012% Ti-0. Thickness surface layer portion after quenching and tempering of a steel plate having a thickness of 140 mm having a component of 012% Nb-0.035% Al-0.0039% N, each thickness position of 1/8 t part to 3/8 t part It is the figure which showed the relationship between the crystal grain diameter and cumulative frequency (%). 0.08% C-0.15% Si-1.51% Mn-0.008% P-0.0010% S-1.15% Cu-1.23% Ni-0.012% Ti-0. The outermost layer portion of the plate thickness (6 mm below is the center of the test piece) after quenching and tempering of a steel plate having a thickness of 140 mm having a component of 012% Nb-0.035% Al-0.0039% N and VTrs obtained from a test using test pieces taken at right angles to the rolling direction from each plate thickness position of 1 / 8t to 3 / 8t, and the reciprocal of the square root of the average grain size at each plate thickness position. It is the figure which showed the relationship (relationship of a hall petch). Mo content is 0.06% C-0.18% Si-1.35% Mn-1.05% Cu-1.35% Ni-0.013% Ti-0.015% Nb After performing multilayer welding at a heat input of 25 kJ / mm on a steel plate with a thickness of 100 mm with the thickness changed to 0.12%, a full-thick CTOD test piece was taken in the direction perpendicular to the weld line and used as the notch position. The average value of the critical CTOD value (δc) when the CTOD test was performed three times at a test temperature of −10 ° C. at a fusion line (FL) between the weld metal and the base metal (FL) and a position 3 mm from the FL (FL + 3 mm). It is the figure which showed the relationship with Mo content. 0.05% to 0.06% C-0.15% to 0.18% Si-1.30% to 1.35% Mn-1.05% to 1.10% Cu-1.30% to 1 .35% Ni-0.012% -0.013% Ti-0.012% -0.015% Nb as the basic component system, with Cr content of 0.05% -0.14% In a steel plate having a thickness of 100 mm, which has been subjected to quenching at 900 ° C. and tempering at 580 ° C. and multilayer welding with a heat input of 25 kJ / mm, the sampling direction is perpendicular to the weld line and the notch position is the weld metal and the mother metal. The critical CTOD value (δc) obtained in the CTOD test performed three times at a test temperature of −10 ° C. using a full-thickness CTOD test piece located 3 mm from the fusion line (FL) with the material (FL + 3 mm) It is the figure which showed the relationship between the average value of Cr and the content of Cr. 0.05% to 0.06% C-0.15% to 0.18% Si-1.30% to 1.35% Mn-1.05% to 1.10% Cu-1.30% to 1 .35% Ni-0.012% -0.013% Ti-0.012% -0.015% Nb as the basic component system, with V content of 0.005% -0.05% In a steel plate having a thickness of 100 mm, which has been subjected to quenching at 900 ° C. and tempering at 580 ° C. and multilayer welding with a heat input of 25 kJ / mm, the sampling direction is perpendicular to the weld line and the notch position is the weld metal and the mother metal. The critical CTOD value (δc) obtained in the CTOD test performed three times at a test temperature of −10 ° C. using a full-thickness CTOD test piece located 3 mm from the fusion line (FL) with the material (FL + 3 mm) It is the figure which showed the relationship between the average value of V and the content of V. 0.08% C-0.15% Si-1.51% Mn-0.008% P-0.0010% S-1.15% Cu-1.23% Ni-0.012% Ti-0. It has a component of 012% Nb-0.035% Al-0.0039% N. Rolling, holding temperatures of 450 ° C. and 550 ° C., preliminary heat treatment with changing holding time, holding at 920 ° C. for 120 minutes The relationship between the holding temperature of the preliminary heat treatment and the average crystal grain size of 1/8 t part is shown in a steel plate having a thickness of 140 mm that has been subjected to quenching treatment that is subsequently water-cooled and tempering treatment that is held at 590 ° C. for 100 minutes and then air-cooled It is a figure.

Hereinafter, the steel plate (steel plate concerning this embodiment) concerning one embodiment of the present invention is explained in detail.
First, the reasons for limiting the chemical composition of the steel sheet according to this embodiment will be described.

C: 0.03% to 0.12%
C is an element that improves the strength of the base material. In order to obtain the effect, the C content needs to be 0.03% or more. In order to improve the strength, the lower limit of the C content may be 0.04%, 0.05%, 0.06%, or 0.07%. On the other hand, if the C content exceeds 0.12%, the material uniformity in the thickness direction is impaired due to the increase in hardenability. Also, the HAZ toughness decreases at the same time as the hardness of the weld increases. Therefore, the upper limit of C content is 0.12%. In order to improve the HAZ toughness, the upper limit of the C content may be 0.11%, 0.10%, 0.09%, or 0.08%.

Si: 0.05% to 0.30%
Si is an element effective for deoxidation and an element for improving strength. In order to acquire the effect, it is necessary to make Si content 0.05% or more. In order to improve the strength, the lower limit of the Si content may be 0.06%, 0.08%, 0.10%, or 0.13%. On the other hand, if the Si content exceeds 0.30%, the HAZ toughness decreases, so the upper limit of the Si content is set to 0.30%. In order to improve HAZ toughness, the upper limit of the Si content may be 0.25%, 0.22%, 0.20%, or 0.18%.

Mn: 1.20% to 1.65%
Mn is an element effective for deoxidation and an element for improving strength. In order to acquire the effect, it is necessary to make Mn content 1.20% or more. In order to improve the strength, the lower limit of the Mn content may be 1.25%, 1.28%, 1.30%, 1.33%, 1.35%, or 1.37%. On the other hand, if the Mn content exceeds 1.65%, the material uniformity in the plate thickness direction is impaired due to the increase in hardenability, and segregation in the slab becomes prominent, thereby reducing the HAZ toughness. Therefore, the upper limit of the Mn content is 1.65%. In order to improve HAZ toughness, the upper limit of the Mn content may be 1.60%, 1.58%, 1.55%, 1.52%, 1.50%, or 1.47%.

Cu: 0.7% to 2.5%
Cu is a main alloy element for the steel sheet according to the present embodiment, and is a few elements that improve the strength of the base material without impairing the weldability and the HAZ toughness. By making Cu content 0.7% or more, there is a remarkable effect in increasing the strength. For this reason, the minimum of Cu content is made into 0.7%. In order to improve the strength, the lower limit of the Cu content is 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1.0%, 1.05% or 1.1. % May be used. On the other hand, when the Cu content exceeds 2.5%, the hardenability is increased, and there is a concern that the HAZ toughness is lowered as shown in FIG. Therefore, the upper limit of the Cu content is set to 2.5%. In order to improve HAZ toughness, the upper limit of Cu content may be 2.3%, 2.1%, 1.9%, 1.7%, 1.6%, 1.5%, or 1.4%. .

Ni: 1.2% to 3.0%
Ni is also a major alloying element for the steel sheet according to the present embodiment, and is an effective element for improving the base material strength and toughness and for improving the HAZ toughness. From the viewpoint of HAZ toughness, the Ni content needs to be 1.2% or more as shown in FIG. In order to improve the above characteristics, the lower limit of the Ni content is 1.25%, 1.3%, 1.35%, 1.4%, 1.45%, 1.5%, 1.55% or 1 It may be 6%. On the other hand, when the Ni content exceeds 3.0%, as shown in FIG. 2, a material difference in the plate thickness direction occurs. Therefore, the upper limit of Ni content is limited to 3.0%. In order to further reduce the material difference in the plate thickness direction, the upper limit of the Ni content is 2.8%, 2.6%, 2.4%, 2.2%, 2.0%, 1.9% or 1. It may be 8%.

Nb: 0.005% to 0.030%
Nb is an element that improves the strength and is effective in making the base material crystal grains fine. In order to acquire the effect, it is necessary to make Nb content 0.005% or more. The lower limit of the Nb content may be 0.007%, 0.010%, 0.012%, 0.013%, or 0.015% in order to improve the strength and refine the crystal grains. On the other hand, if the Nb content exceeds 0.030%, the HAZ toughness decreases, so the upper limit of the Nb content is 0.030%. In order to improve the HAZ toughness, the upper limit of the Nb content may be 0.027%, 0.025%, 0.022%, or 0.020%.

Ti: 0.005% to 0.030%
Ti is an element that forms a nitride and contributes to the refinement of crystal grains in the weld heat affected zone. In order to acquire the effect, it is necessary to make Ti content 0.005% or more. In order to improve HAZ toughness, the lower limit of the Ti content may be 0.007%, 0.010%, or 0.012%. On the other hand, if the Ti content exceeds 0.030%, the nitride is coarsened, and there is a concern that the HAZ toughness is reduced. Therefore, the upper limit of Ti content is 0.030%. In order to prevent reduction in HAZ toughness, the upper limit of Ti content may be 0.025%, 0.020%, or 0.018%.

Al: 0.015% to 0.065% or less Al is an element that is effective for deoxidation, and at the same time, forms nitrides and is effective for refining the base material and HAZ crystal grains. In order to acquire the effect, it is necessary to make Al content 0.015% or more. The lower limit of the Al content may be set to 0.020%, 0.025%, 0.028%, 0.031%, or 0.035% for refining the base material and the HAZ crystal grains. On the other hand, when the Al content exceeds 0.065%, coarse nitrides are formed and the toughness tends to be lowered. Therefore, the upper limit of the Al content is 0.065%. In order to prevent a decrease in toughness, the upper limit of the Al content may be 0.060%, 0.055%, 0.052%, 0.050%, or 0.048%.

N: 0.0020% to 0.0060%
N is an element that combines with elements such as Ti and Al to form nitrides. From the viewpoint of forming nitrides, the N content needs to be 0.0020% or more. In order to form nitride more reliably, the lower limit of the N content may be 0.0024% or 0.0028%. On the other hand, if the N content exceeds 0.0060%, the HAZ toughness decreases, so the upper limit of the N content is set to 0.0060%. In order to prevent a decrease in HAZ toughness, the upper limit of the N content may be 0.055%, 0.050%, or 0.045%.

Cr: 0% to 0.08%
Mo: 0% to 0.04%
V: 0% to 0.01%
Cr, Mo and V are elements that increase the hardenability and increase the difference in hardness between the surface layer portion and the thickness center portion in the thick high-tensile steel plate. Moreover, when Cr, Mo and V are contained, there is a concern that the HAZ toughness is lowered. Therefore, it is necessary to reduce these elements in the steel plate according to the present embodiment.
As described above, in order to evaluate the HAZ toughness, a Charpy test is often used. Recently, a CTOD test for obtaining a CTOD value that can be reflected in a design considering more fracture mechanics is also performed. The CTOD value is the crack opening displacement, and is the opening amount at the crack tip when a brittle fracture occurs from the fatigue crack tip. A method for obtaining this CTOD value experimentally is the CTOD test. The CTOD test is usually performed at a design temperature at which the structure is actually operated. The CTOD value is affected by the microstructure of the steel plate at the tip of the fatigue crack, that is, hardness, crystal grain size, carbide state, presence of brittle structure, etc., so it is more sensitive to these metallurgical factors than the Charpy test. It is said that it is. In many cases, if the CTOD value is 0.1 mm or more, it is determined that the steel sheet has sufficient resistance to brittle fracture.

The present inventors verified the influence of the contents of Cr, Mo, V, which are elements with particularly high hardenability, on the CTOD value. FIG. 6 is a diagram showing the results of conducting a CTOD test on actual welded joints of a plurality of steel plates with varying amounts of Mo and evaluating the influence of the amount of Mo. In this test, first, 0.06% C-0.18% Si-1.35% Mn-1.05% Cu-1.25% Ni-0.013% Ti was used as a basic component system, and it contained Mo. Steel whose amount was changed from no addition (content contained as impurities) to 0.12% was melted, and a steel plate having a thickness of 100 mm was manufactured by hot rolling. Thereafter, the steel plate was quenched at 900 ° C. and tempered at 580 ° C., and then subjected to multilayer welding at a heat input of 25 kJ / mm. From the welded steel plate, a full thickness CTOD specimen was taken in a direction perpendicular to the weld line. The notch position of the CTOD test was a fusion line (FL) between the weld metal and the base metal and a position 3 mm from the FL (FL + 3 mm). Then, three CTOD tests were performed on the collected specimens at a test temperature of -10 ° C.
In FIG. 6, the vertical axis represents the average value of three critical CTOD values δc (may be described as δc-10 ° C.) at −10 ° C., and the horizontal axis represents the Mo content. It can be seen from FIG. 6 that Mo reduces the CTOD characteristics of the welded joint, and in particular, the CTOD characteristics at the FL + 3 mm position and the FL position. In addition, when δc ≧ 0.1 mm is used as a criterion for acceptance, it is understood that the Mo content needs to be 0.04% or less.
As for the Mo content, it is desirable that the content is small. However, it is not desirable to prevent the Mo content from being completely contained because the cost increases. Further, considering the case where it is contained as an impurity or intentionally, the upper limit of the Mo content is set to 0.04%. A more preferable upper limit of the content is 0.03%, 0.02% or 0.01%.

Similarly, the effect of Cr content and V content on HAZ toughness was investigated. The results are shown in FIGS. In both figures, a welded joint was prepared in the same manner as in FIG. 6 and a CTOD test was conducted at a test temperature of −10 ° C. with a notch at a position of FL + 3 mm. The obtained δc, Cr and V contents were obtained. FIG. When the content of both Cr and V increases, δc falls below 0.1 mm at a certain content. When the upper limit values of the contents of both of which δc is not less than 0.1 mm are determined from FIGS. 7 and 8, the upper limit of the Cr content is 0.08% and the upper limit of the V content is 0.01%. Therefore, the upper limit of Cr content is 0.08% regardless of impurities or intentional inclusion. In order to improve HAZ toughness, the upper limit of the Cr content may be 0.06%, 0.05%, 0.04%, or 0.03%. Further, the upper limit of the V content is 0.01% regardless of impurities or intentional inclusion. In order to improve the HAZ toughness, the upper limit of the V content may be 0.008%, 0.005%, 0.003%, or 0.001%.
In addition, Cr, Mo, and V may be mixed as impurities from scrap or the like during the production of molten steel, but the lower limit is not particularly limited, and the lower limit is 0%.

B is also an element effective for improving the hardenability by increasing the hardness after quenching treatment with a small amount of content like Cr, Mo and V. However, in the case of a thick high-tensile steel plate, the difference in the quenching hardness between the surface layer portion and the plate thickness center portion increases due to the inclusion of B. Therefore, B is not preferable from the viewpoint of uniformity in the thickness direction. However, it is technically difficult not to completely contain them. Therefore, considering the case where it is contained as an impurity, the upper limit of the B content is set to 0.0005%. Even when it is intentionally contained, the upper limit is 0.0005%. The upper limit of the B content may be 0.0004%, 0.0003%, 0.0002%, or 0.0001% for further uniformity in the thickness direction. Although B may be mixed as an impurity from scraps or the like during the production of molten steel, the lower limit is not particularly limited, and the lower limit is 0%.

P and S are impurity elements contained in the steel and lower the base metal toughness and HAZ toughness. Therefore, the smaller the content, the better. In the present invention, the upper limit of P is 0.010% or less, preferably 0.007%, 0.005% or less or 0.003%, and the upper limit of S is limited to 0.002% or less. The upper limit of S may be limited to 0.001% or 0.0008%. There is no need to particularly limit the lower limit of the P amount and the S amount, and the lower limit is 0%.

Ca has the effect of reducing the influence of MnS, which is harmful to toughness, by spheroidizing the sulfide of the steel sheet. In order to acquire this effect, you may contain 0.0001% or more. However, since the weldability is impaired when the Ca content is excessive, the Ca content is limited to 0.0050% or less. In order to improve weldability, the upper limit of the Ca content may be 0.0040%, 0.0035%, or 0.0030%. Ca may be mixed as an impurity from scrap, refractory, or the like during manufacturing of molten steel, but the lower limit is not particularly limited, and the lower limit is 0%.

Mg and REM are elements that improve the HAZ toughness by forming an oxide in the steel sheet. In order to acquire this effect, you may contain 0.0001% or more. However, if the contents of Mg and REM are excessive, coarse oxides are generated, leading to a decrease in toughness. Therefore, the Mg content and the REM content are limited to 0.0030% or less, respectively. If necessary, the upper limit of these contents may be 0.0025% or 0.0020%. Mg and REM may be mixed as impurities from scraps, refractories, etc. during the production of molten steel, but the lower limit is not particularly limited, and the lower limit is 0%.
Here, REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. Note that the content of REM means the total content of these elements.

Even if an alloy element (for example, Mo, Cr, V, B, P, S, Ca, Mg, REM) with no lower limit is intentionally added or mixed as an impurity, If the content is within the claims, the steel sheet is interpreted as being within the claims of the present invention.
The steel sheet according to the present embodiment contains the above components, with the balance being iron and impurities. However, in the steel plate according to the present embodiment, in addition to the above components, for the purpose of further improving the strength, toughness, etc. of the steel material itself, or as impurities from auxiliary materials such as scrap, Sb, As, Sn, Pb, Zr, Zn, W, and Co may be contained. However, the upper limit of the content is preferably as follows.
Since Sb impairs the toughness of HAZ, the upper limit of the Sb content may be 0.02%. In order to improve HAZ toughness, the upper limit of the Sb content may be 0.01%, 0.005%, or 0.002%.
Since As and Sn impair the toughness of HAZ, the upper limit of the content of As and Sn may be 0.02%. As needed, it is good also considering the upper limit of content of As and Sn as 0.01%, 0.005%, or 0.002%.
In order to improve strength and toughness, the contents of Pb, Zr, Zn, and W may be 0.1% or less, 0.01%, or 0.005% or less, respectively. There is no particular need to determine these lower limits, and it is 0%.
Co may be contained as an impurity in Ni. Since Co impairs the HAZ toughness, the upper limit of the Co content may be 0.3%, 0.1%, or 0.05%. There is no particular need to determine the lower limit, and the lower limit is 0%.

A value (= Cu + Ni): 4.5% or less In the present embodiment, it is necessary to control ΔHv, which is an index mainly showing the uniformity of strength, in the thickness direction of the base material. As can be seen from FIG. 2, Cu + Ni, that is, the total of Cu content and Ni content, and when the A value represented by the following formula (1) exceeds 4.5%, ΔHv, which is the difference between the Vickers hardness and the Vickers hardness at the 1/2 t portion, exceeds 20, and the characteristics in the thickness direction are not uniform. From this result, in addition to the limitation of the range of each element described above, the upper limit of the A value is set to 4.5%. In order to further reduce the difference in hardness in the thickness direction, the upper limit of the A value is set to 4.2%, 4.0%, 3.8%, 3.5%, 3.3% or 3 as necessary. It may be 0%. The lower limit of the A value is not particularly limited, but 1.9% of the total of the lower limits of the Cu content and the Ni content is a substantial lower limit.
A = Cu + Ni (1)
Here, Cu and Ni in the above formula (1) are the contents of each element, and the unit is mass%.

Furthermore, in order to ensure weldability in the steel sheet according to the present embodiment, in addition to limiting the range of individual elements, the chemical composition is such that the Pcm value obtained by the following formula (2) is 0.25% or less. Limit. The Pcm value is often applied as an index representing the weld cracking sensitivity similarly to the carbon equivalent (Ceq), and is calculated from the content of the alloy contained in the steel. Formula (2) includes elements such as Cr, Mo, V, and B that are not substantially contained in the present invention. However, since these elements may be mixed as impurities from various alloy raw materials in the process of industrial production, the inclusion of alloy elements including such impurities is necessary when evaluating weldability. The amount needs to be evaluated. If each alloy element is not contained (not detected), the calculation may be performed with the term set to 0.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (2)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are the contents of each element, and the unit is mass%.
In the steel sheet according to the present embodiment, when the Pcm value exceeds 0.25%, low temperature cracking is likely to occur when welding at 0 ° C., so the upper limit of the Pcm value is set to 0.25%. The lower limit of the Pcm value does not need to be specified, but the lower limit may be 0.15% or 0.18%.

Next, the steel plate according to the present embodiment can be manufactured by the following manufacturing method.
First, a molten steel having a steel component (chemical composition) adjusted to the above-described range is made into a slab by continuous casting or an ingot-making method (casting step: S1). Thereafter, the obtained slab is heated (heating step: S2). The lower limit of the target heating temperature in the heating step is preferably 950 ° C. for the purpose of sufficiently reducing the thickness of the thick high-tensile steel plate to the center of the plate thickness. On the other hand, if the heating temperature exceeds 1250 ° C., the scale of the steel sheet cannot be peeled off and the steel sheet surface flaws may occur, so the upper limit is desirably set to 1250 ° C.

After the heating step, the heated slab is hot rolled to form a steel plate (hot rolling step: S3). After the hot rolling process, the steel sheet is cooled as it is to 350 ° C. or less (cooling process: S4). In order to reheat beyond the Ac3 transformation point after the cooling step, accelerated cooling may be performed as necessary if there is a restriction on the cooling location. If the cooling stop temperature in the cooling process exceeds 350 ° C., it is not desirable because embrittlement may occur due to coarse precipitates such as aluminum nitride.
The Ac1 transformation point mentioned here refers to a temperature at which austenite starts to occur locally when the steel is heated from the ferrite phase at room temperature. Further, when the temperature is further increased, the two-phase state of ferrite and austenite becomes an austenite single phase. The temperature at which this austenite single phase is obtained is called the Ac3 transformation point. These transformation points can usually be obtained experimentally by utilizing the difference in thermal expansion coefficient between ferrite and austenite. That is, an expansion-temperature curve obtained by heating steel at a constant heating rate (for example, 2.5 ° C./min) can be measured and experimentally obtained from the change point of thermal expansion.
After the cooling process, a quenching process in which the temperature is higher than the Ac3 transformation point and water-cooled and a tempering process in which the temperature is lower than the Ac1 transformation point and air-cooled are performed (quenching and tempering process: S5).
When the heating temperature at the time of quenching is less than the Ac3 transformation point, a sufficient quenched structure cannot be obtained, so that the strength or toughness is lowered. On the other hand, it is preferable that the heating temperature at the time of quenching is low from the viewpoint of preventing coarsening of crystal grains. For this reason, it is good also considering the upper limit of heating temperature as 930 degreeC, 910 degreeC, or 890 degreeC. Further, if the heating temperature at the time of tempering exceeds the Ac1 transformation point, the strength or toughness may be remarkably lowered.
In recent years, not the method of reheating and quenching and tempering a steel plate cooled after rolling as in this embodiment, but directly cooling after rolling and tempering it (direct quenching + tempering treatment) There is an example applied also in manufacture of a high-tensile steel plate. However, this method is not suitable for the steel plate according to this embodiment. The reason is as follows.
The crystal grain size of the steel sheet that has been directly quenched after rolling depends on the heating and rolling temperature. When low-temperature heating is performed or rolling is performed at a low temperature in order to reduce the crystal grain size, the rolling temperature is lowered on the steel sheet surface side that is easily cooled. As a result, immediately after rolling, the sheet thickness surface side becomes a flat fine-grained austenite structure by hot rolling, and the central side is hardly affected by rolling, and isotropic and slightly coarse-grained austenite structure generated by recrystallization. In many cases. When the steel sheet having these austenite structures is directly quenched, the region from the surface layer affected by rolling to the vicinity of 1 / 8t part is a microstructure mainly composed of fine ferrite and bainite structures transformed from the processed austenite. On the contrary, on the inner side of the 2 / 8t portion, a coarse ferrite and bainite structure is formed. As a result, the average crystal grain size of 3 / 8t part becomes 35 μm or more.
That is, the crystal grains of the steel sheet (directly quenched steel) that has been directly quenched in this way are finer on the 1/8 t side than the surface layer than the center of the sheet thickness, and are completely opposite to the steel sheet according to the present embodiment. It has a microstructure. That is, in the directly quenched steel, even if the crystal grain size of the base material in the 1 / 8t part is defined, the crystal grain size inside the steel plate is coarser than the crystal grain size in the 1 / 8t part. The toughness of the base material cannot be defined due to restrictions within the scope of the present invention. Furthermore, since the crystal grains on the surface layer side are fine, the surface layer side tends to be hardened even in the hardness distribution in the thickness direction, and ΔHv ≦ 20 cannot be satisfied.
As described above, the direct quenching + tempering method is not suitable as a means for securing material uniformity in the thickness direction and imparting excellent toughness in the thick high-tensile steel plate. In order to ensure material uniformity in the plate thickness direction and to make the average crystal grain size of 1 / 8t part 35 μm or less, it is necessary to perform quenching and tempering after cooling once.

Furthermore, in the present embodiment, the temperature of the steel sheet is 550 ° C. or higher and the Ac1 transformation is performed for the purpose of uniformizing the crystal grain size in the thickness direction during quenching between the hot rolling process and the quenching and tempering process. It is preferable to further include a step (preliminary heat treatment step: S6) of performing a preheat treatment so that the holding time in this temperature range is 5 hours or more and 500 hours or less. By performing this preliminary heat treatment step, the difference in crystal grain size in the plate thickness direction as shown in FIG. 4 can be reduced. In other words, this pre-heat treatment prevents the coarsening of crystal grains that occur when the heating time of the surface layer to 1/8 t part takes a long time in the heating process during the quenching treatment of thick high-tensile steel sheets as described above. Therefore, it is a process performed prior to quenching. The metallurgical meaning of this pre-heat treatment is to cause Ti and Nb carbonitrides or aluminum nitride precipitates finely precipitated after hot rolling to act as pinning particles during quenching by Ostwald growth. The purpose is to coarsen to an appropriate size. FIG. 9 shows 0.08% C-0.15% Si-1.51% Mn-0.008% P-0.0010% S-1.15% Cu-1.23% Ni-0.012% After rolling steel having a component of Ti-0.012% Nb-0.035% Al-0.0039% N to 140 mm, preliminary heat treatment was performed at different temperatures of 450 ° C. and 550 ° C. Then, it is a figure which shows the change of the average crystal grain diameter of 1 / 8t part of the steel plate which performed the quenching process which water-cools after hold | maintaining at 920 degreeC for 120 minutes, and the tempering process hold | maintained at 590 degreeC for 100 minutes and air-cooling.
As can be seen from FIG. 9, when the temperature of the preliminary heat treatment is 450 ° C., the average crystal grain size tends to gradually decrease as the holding time becomes long, but to make the average crystal grain size 25 μm or less. , A very long holding time of 100 hours or more is required. On the other hand, when the temperature of the preliminary heat treatment was 550 ° C., the retention time was 5 hours or more, the average crystal grain size was 25 μm or less, and clear grain refinement was observed. In view of the above, in order to refine the average crystal grain size in the vicinity of 1 / 8t part of the plate thickness that is likely to be coarsened, it is desirable to hold at 550 ° C. or higher for 5 hours or longer as a preliminary heat treatment. I understood. The toughness was further improved by reducing the average crystal grain size. Furthermore, the grain refinement effect by the pre-heat treatment described above is more effective for the crystal grains on the surface layer side, so the difference in toughness with the center of the plate thickness becomes smaller and the toughness in the plate thickness direction is also improved. It tends to be uniform. However, when the holding time in the pre-heat treatment is 500 hours or more, the coarsening of the precipitated particles proceeds remarkably during the pre-heat treatment, and the number density of the particles is reduced accordingly, so that the pinning effect is reduced. Therefore, it is desirable that the upper limit of the holding time be 500 hours. Note that when the preliminary heat treatment temperature exceeds the Ac1 transformation point, austenite transformation partially occurs in the steel sheet. In this case, since the growth rate of precipitates is different between ferrite and austenite, uniform precipitate growth cannot be expected in the steel sheet. Therefore, it is preferable that the heating temperature (holding temperature) at the time of the preliminary heat treatment be equal to or lower than the Ac1 transformation point.

After the preliminary heat treatment step, the steel sheet is cooled to 350 ° C. or lower and then quenched. The quenching process is a process of cooling the steel sheet heated to a temperature exceeding the Ac3 transformation point. From the viewpoint of preventing the coarsening of crystal grains, it is preferable that the heating temperature during quenching is low. For this reason, it is good also considering the upper limit of heating temperature as 930 degreeC, 910 degreeC, or 890 degreeC.

A tempering process is carried out following the quenching process. The tempering process is an important process for the purpose of controlling the strength and toughness within a predetermined range. In the present embodiment, the tempering process is performed at a temperature equal to or lower than the Ac1 transformation point for the purpose of ensuring the uniformity of the material in the thickness direction. The temperature range is preferably 500 ° C. to 650 ° C., more preferably 550 ° C. to 610 ° C. In order to make the difference ΔHv between Vickers hardness between 1 / 8t part and 1 / 2t part of the plate thickness from the surface in the hardness distribution in the plate thickness direction, it is effective to perform tempering treatment at the above temperature. It is.

Steel strips obtained by melting steels A1 to A10 and B1 to B29 having the composition shown in Tables 1 and 2 were made into steel plates having a thickness of 80 to 200 mm according to the manufacturing conditions shown in Tables 3 and 4. It was.

In the production, the heating temperature was 950 ° C. to 1250 ° C., followed by hot rolling, followed by air cooling or water cooling. Thereafter, for

test numbers

5, 10, 15 and 26, a preliminary heat treatment was performed before the quenching treatment. Quenching and tempering treatments were performed on the steel plates with test numbers 1 to 51 except for test number 18. Test No. 18 was water-cooled to 100 ° C. immediately after rolling, and only tempered without quenching. Thereafter, in order to evaluate the strength characteristics of the base material, a No. 14 tensile test piece specified in JIS Z 2201 was collected, and a tensile test specified in JIS Z 2241 was performed. As a result of the test, the yield strength ^{460N / mm 2 ~ 580N / mm} 2, and a tensile strength of ^{550N / mm 2 ~ 670N / mm} 2 was judged to be acceptable. Furthermore, the impact test piece was extract | collected based on JISZ2242, and the test was implemented. In the impact test conducted as an evaluation of the base metal toughness, the average value of the three absorbed energy at −40 ° C. was described as vE-40 (base material), and 42 J or more was accepted. In addition, about the tensile test piece, it extract | collected from the 1 / 4t part of the board thickness which is often prescribed | regulated also by the normal steel material specification. The impact test specimens were collected from three places of 1 / 8t part, 1 / 4t part, and 1 / 2t part. Tables 3 and 4 show 1 / 2t part (thickness center part) with the lowest toughness. Only the test results of) are listed. The sampling direction was a direction perpendicular to the rolling direction. For the Ac1 and Ac3 transformation points, a cylindrical test piece having a diameter of 3 mmφ and a length of 10 mm was sampled by machining from a 1/4 t part of the plate thickness, and a thermocouple was attached to the end of the test piece. It was read from the change in the amount of thermal expansion in the longitudinal direction of the test piece when heated from room temperature to 950 ° C. at a heating rate of 2.5 ° C./min by high frequency induction heating.

In addition, after taking a microstructure test piece in the direction perpendicular to the rolling direction from 1 / 8t part and 3 / 8t part of the plate thickness, mirror polishing, and using EBSD method, the crystal orientation is an angle of 30 ° or more A region surrounded by a grain boundary having a grain size was defined as a crystal grain, and the equivalent circle diameter of the crystal grain was defined as a crystal grain size. Then, the frequency distribution with respect to the crystal grain size of each sample was measured, and the crystal grain size at which the cumulative frequency calculated from the fine grain side was 70% was defined as the average crystal grain size.

Further, the Vickers hardness distribution (load 98N) in the cross section in the thickness direction was measured, and the difference in hardness between the 1/8 t portion and the 1/2 t portion of the thickness was described as ΔHv as an index of material uniformity. Moreover, the case where ΔHv was 20 or less was regarded as acceptable. Here, the 1 / 8t part of the plate thickness is present in two locations in the steel sheet (that is, the position that becomes the 1 / 8t part and the 7 / 8t part when viewed from one surface). Or the larger of the difference in hardness between the 1 / 8t part and the 1 / 2t part.

As an evaluation of weldability, an evaluation was performed in a y-type weld crack test specified in JIS Z 3158. Welding was performed by CO ₂ welding at a heat input of 1.5 kJ / mm, and a steel plate whose front and back surfaces were cut so as to have a thickness of 50 mm centered on the central portion of the plate thickness was used as a test steel plate. As a result of the test, a test temperature at which the root cracking rate was 0% was obtained, and if it was 0 ° C., it was regarded as acceptable.

On the other hand, for the purpose of evaluating the HAZ toughness, a butt joint having a groove shape of K type with a heat input of 3.5 kJ / mm to 4.5 kJ / mm was prepared by submerged arc welding. From this butt joint, three impact test pieces based on JIS Z 3128 were taken with the notch position as a fusion line, and an impact test was conducted at a test temperature of −40 ° C. The average value of the three test pieces is shown in Tables 3 and 4 as vE-40 (HAZ).
Also, from the same butt joint, a full-thickness CTOD test piece (B × B type) conforming to BS7448 was taken as a fusion line called CGHAZ (Coarse grain HAZ) at the notch position, and API was tested at a test temperature of −10 ° C. Three CTOD tests based on (American Petroleum Institute) standard RP 2Z and BS (British Standards) standard 7448 were performed. These minimum values are shown in Tables 3 and 4 as δc−10 ° C. In the impact test, 42 J or more was evaluated as acceptable, and in the CTOD test (δc), 0.1 mm or more was evaluated as acceptable.
It should be noted that the impact test result and the CTOD test result have a rough correlation, but one may be good but the other may be low. Therefore, it is necessary to satisfy both of the HAZ toughness in a structure that is severely demanded for fracture.

The steel components underlined in Table 1 and Table 2, the A value (Cu + Ni) and the Pcm value indicate that the values are outside of the present invention, and numerical values underlined in Tables 3 and 4 Indicates that the characteristics are insufficient. The balance in Tables 1 and 2 is Fe and impurities.

In Test Nos. 1 to 17 in Table 3, all steel components and production conditions are within the scope of the present invention. All of these steels satisfy the target values in terms of tensile properties and toughness (impact properties) of the base metal and ΔHv, which is an index of uniformity in the thickness direction. Further, no cracks were observed in all weldability at 0 ° C., and in HAZ toughness, both the absorbed energy (vE-40) and the CTOD value (δc−10 ° C.) satisfied the target values.

Of these, the

test grain numbers

5, 10 and 15 in which the preliminary heat treatment within the scope of the present invention was performed, the average crystal grain size was 1/8 t part and 3/8 t of the plate thickness as compared with the others. The average grain size of all the parts is 25 μm or less. As a result, the

test numbers

5, 10 and 15 have better toughness of the base material than other steels.

On the other hand, in test numbers 18 to 22 in Table 4, the components are within the scope of the present invention, but the production conditions are not desirable, and the base material characteristics and / or uniformity in the thickness direction satisfy the target values. Not. Test numbers 23 to 51 are steel plates manufactured using steel whose chemical composition deviates from the scope of the present invention. Test numbers 23 to 51 do not satisfy the target values for at least one of the base material strength and toughness, ΔHv, crack stop temperature, vE-40 (HAZ) and δc-10 ° C., as shown in Table 4. As a result.

Test No. 18 was obtained by performing only a tempering process on a steel sheet that had been subjected to a water cooling process (direct quenching) immediately after rolling, and was manufactured without the quenching process. In this steel plate, the base metal toughness is as low as 29 J and ΔHv is as high as 29. Test number 19 is an example in which the tensile properties of the base material do not satisfy the target value as a result of the quenching temperature being a two-phase quenching process. Test No. 20 is an example in which the tempering temperature is 705 ° C., the yield strength is low as a result of exceeding the Ac1 transformation point, and ΔHv does not satisfy the target value. Test No. 21 is an example in which the cooling stop temperature after rolling is as high as 395 ° C., and heating for quenching is started therefrom. In this example, since the cooling stop temperature is high, the coarsening of the precipitate occurs in the heating stage of the next quenching process, and the toughness of the base material is lowered.
Furthermore, the test number 22 is an example in which the quenching temperature is 950 ° C. and deviates from a desirable range. In Test No. 22, the crystal grain size is coarse and the base material toughness is low.
Test Nos. 23, 25 and 27 are examples in which the contents of C, Si and Mn deviate from the scope of the present invention. In these materials, the tensile strength of the base material does not satisfy the target value, and the test numbers 23 and 25 have low yield strength.
On the contrary, the test number 24 is an example in which C is 0.14%, which is out of the range of the present invention, and the Pcm value is out of 0.27%. As a result, the base metal toughness is low, ΔHv is inferior to 32 in the thickness direction uniformity, the crack stop temperature is as high as 25 ° C., and the absorbed energy and δc of the weld are low. Similarly, test number 28 is 1.89% for Mn, 0.11% for Cr for test number 46, and 0.0006% for B for test number 49, both deviating from the scope of the present invention. Since these elements are all elements that increase the hardenability of the base material, the test numbers 28, 46, and 49 all have values of ΔHv exceeding 20, and the yield strength and base material toughness do not satisfy the scope of the present invention. There is also.
On the other hand, test number 26 is 0.37% for Si, test number 29 is 0.012% for P, test number 30 is 0.004% for S, test number 40 is 0.038% for Nb, and test number 42 is Ti is 0.036%, test number 44 is 0.077% Al, test number 45 is N 0.0075%, test number 47 is Mo 0.05%, test number 48 is V 0.012% Both deviate from the scope of the present invention. When these elements are contained beyond the scope of the present invention, the HAZ toughness decreases. Therefore, either vE-40 (HAZ) or δc-10 ° C or both do not satisfy the target value.
Next, effects of Cu and Ni, which are main elements for the steel of the present invention, will be described. As for test number 31, test number 37, and test number 38, Cu has shifted | deviated from the range of this invention low. Therefore, in the test number 31, the tensile strength is low, and in the test numbers 37 and 38, the tensile strength and the yield strength are low. Furthermore, in test No. 37, Ni is 3.29%, which is out of the range of the present invention, so ΔHv is also high as 55, and in test No. 38, Ni is off as low as 1.11. The toughness of the welds is low.
Furthermore, Test Nos. 32, 35 and 36 are examples in which Cu deviates from the range of the present invention and the Pcm value exceeds 0.25%. As a result, the crack stop temperature of all of these is 25 ° C., which does not satisfy the target, and the HAZ toughness is also low. Among them, in Test No. 35, Ni is 1.05%, which deviates from the scope of the present invention, and vE-40 (HAZ) and δc-10 ° C. are low. Test number 36, on the other hand, is an example where Ni deviates from the scope of the present invention, so Cu + Ni is 6.00%, which is outside the scope of the invention 4.5%. As a result, ΔHv is 59. Not satisfied with the goal.
Test No. 33 and Test No. 34 are examples in which Cu is contained within the scope of the present invention, but Ni deviates from the scope of the present invention. That is, in test number 33, Ni is 0.92%, which is out of the range of the present invention, and as a result, the toughness of the base material and the welded part does not satisfy the target. Test number 34, on the other hand, is an example in which Ni is 3.15%, which is out of the range of the present invention. At the same time, Cu + Ni is 4.63%, which deviates from 4.5% of the present invention. , ΔHv is as high as 45.
Test No. 39 is an example in which Nb deviates slightly, and the base material has low yield strength and tensile strength. Test No. 41 is an example in which Ti is shifted to a low level of 0.003%, and vE-40 (HAZ) is low. Test No. 43 is an example in which Al is shifted to a low level of 0.014%, the crystal grains of the base material are not sufficiently refined, and the toughness of the base material is low. Test Nos. 50 and 51 are examples in which the individual component ranges are within the scope of the present invention, but the A value or the Pcm value is independently shifted. Test number 50 is an example in which the A value is 4.60% and deviates from 4.5%, which is the scope of the present invention. In this case, ΔHv is 31, which does not satisfy the scope of the present invention. Test No. 51 has a Pcm value of 0.27%, which is outside the scope of the present invention. As a result, the crack stop temperature is as high as 25 ° C. and does not satisfy the target value.

According to the present invention, it is possible to provide a thick high-tensile steel plate that is excellent in the uniformity of the base material in the thickness direction and excellent in the toughness, weldability and HAZ toughness of the base material.

Claims

Chemical composition is mass%,
C: 0.03% to 0.12%,
Si: 0.05% to 0.30%,
Mn: 1.20% to 1.65%
Cu: 0.7% to 2.5%,
Ni: 1.2% to 3.0%,
Nb: 0.005% to 0.030%,
Ti: 0.005% to 0.030%,
Al: 0.015% to 0.065%,
N: 0.0020% to 0.0060%,
Mo: 0% to 0.04%,
Cr: 0% to 0.08%,
V: 0% to 0.01%
B: 0% to 0.0005%
P: 0.010% or less,
S: 0.002% or less,
Ca: 0% to 0.0030%,
Mg: 0% to 0.0030%,
REM: 0% to 0.0030%,
Balance: Fe and impurities;
A value shown by following formula (1) is 4.5% or less;
The Pcm value represented by the following formula (2) is 0.25% or less;
Yield strength 460N / mm 2 ~ 580N / mm 2, and a tensile strength be 550N / mm 2 ~ 670N / mm 2;
The hardness of the 1 / 8t portion that is 1/8 of the plate thickness along the plate thickness direction from the surface, and 1 / 2t that is the position of 1/2 of the plate thickness along the plate thickness direction from the surface The difference from the hardness of the part is 20 or less in terms of Vickers hardness;
Crystal orientation analysis using an electron beam backscattering diffraction pattern analysis method is performed, a region surrounded by a grain boundary having a crystal orientation difference of 30 ° or more is defined as a crystal grain, and a circle equivalent grain size of the crystal grain is defined as a crystal grain. When the crystal grain size at which the cumulative frequency when calculating the frequency distribution of the crystal grain size is 70% from the fine grain side is defined as the average crystal grain size, An average crystal grain size of 35 μm or less;
A steel sheet characterized by that.
A = Cu + Ni (1)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (2)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are the contents of each element, and the unit is mass%.
Further, the average crystal grain size in a 3 / 8t portion which is a position of 3/8 of the plate thickness along the plate thickness direction from the surface of the steel plate is 35 μm or less. The described steel sheet.
Furthermore, the said average crystal grain diameter in the said 1 / 8t part is 25 micrometers or less, The steel plate of Claim 1 characterized by the above-mentioned.
Further, the average crystal grain size in a 3 / 8t portion which is a position of 3/8 of the plate thickness along the plate thickness direction from the surface of the steel plate is 25 μm or less. The described steel sheet.
The steel sheet according to any one of claims 1 to 4, wherein the thickness of the steel sheet is 80 mm or more.