JP5147276B2 - High-tensile steel plate with excellent brittle cracking suppression / stop properties and low temperature toughness of weld heat affected zone - Google Patents

Description

  The present invention mainly relates to the low temperature toughness of a portion affected by heat (hereinafter, sometimes referred to as “welding heat affected zone” or “HAZ”), and brittle cracks when welding steel plates as materials for ships, bridges and the like. The present invention relates to a high-strength steel sheet with improved properties for suppressing the generation or stopping the propagation of the generated brittle cracks. In particular, even when used in applications exposed to low temperatures, the present invention relates to a high-strength steel sheet that has excellent HAZ toughness and base metal (steel) toughness, and also exhibits excellent brittle cracking suppression and stopping characteristics. . The present invention is not limited to the above steel material welding method, and can be applied to submerged arc welding, electrogas arc welding, and the like. However, in the following, it is said that it is difficult to ensure the toughness of HAZ. A case where single-sided submerged arc welding is performed will be described as an example.

  The properties required for steel plates used in bridges and ships are becoming increasingly severe in recent years, and particularly good toughness is required. In general, these steel plates are often joined by welding. In particular, HAZ is affected by heat during welding and has a problem that toughness tends to deteriorate. This deterioration in toughness becomes more noticeable as the heat input during welding increases, and the cause is that the larger the heat input during welding, the slower the cooling rate of the HAZ, the lower the hardenability and the formation of coarse island martensite. It is thought that there is to do. Therefore, in order to improve the toughness of HAZ, it is considered that the heat input during welding should be suppressed as much as possible. However, in order to increase the welding work efficiency, for example, large heat input such as electrogas welding, electroslag welding, submerged welding, etc. Adoption of a welding method is desired.

  In recent years, single-sided submerged arc welding with large heat input, for example, with a heat input of 50 to 200 kJ / cm, has been widely used to produce marine structures and low-temperature tanks for storing liquefied gas such as LPG in a short period of time. It has been adopted. However, while this welding can realize high efficiency in construction, it is difficult to stably secure the toughness of the HAZ formed by welding, and it must be manufactured by applying multilayer welding with low heat input. There are many. Accordingly, a steel sheet having high HAZ toughness is required for manufacturing the low-temperature tank and the like, in which the high heat input welding method capable of high-efficiency construction is adopted, and even at a low temperature of about −60 ° C. ing.

  Until now, various methods have been proposed to improve the low temperature toughness of the HAZ. For example, Patent Document 1 and Patent Document 2 propose a method for improving HAZ toughness by suppressing the austenite grain coarsening by pinning particles such as TiN and Al oxide. Patent Document 3 and Patent Document 4 disclose a technique for miniaturizing crystal grains by making many ferrite transformation nuclei exist in austenite grains. Specifically, the use of TiN, MnS, BN, Ti oxide or the like as ferrite transformation nuclei achieves refinement of crystal grains and improves the low temperature toughness of HAZ.

  However, in any of the above methods, when performing single-sided submerged arc welding with high heat input, precipitates such as TiN are considerably dissolved, and it is difficult to suppress subsequent grain coarsening, In order to secure excellent HAZ toughness (hereinafter sometimes referred to as “HAZ toughness” or simply “HAZ toughness”) at a low temperature of about −60 ° C., further improvement is considered necessary. .

  On the other hand, in order to ensure safety as a structure, it is necessary to suppress the occurrence of cracks due to brittle fracture in steel [hereinafter referred to as “brittle crack suppression characteristics” or CTOD (Crac-Tip Opening Displacement) characteristics. Is desired]. This is because if a brittle crack occurs, the structure itself is destroyed. However, a high-strength steel sheet that has improved the HAZ toughness during high heat input welding while suppressing the occurrence of brittle cracks (hereinafter, these characteristics are sometimes referred to as “brittle crack occurrence suppression characteristics”) has not been known so far. .

  In addition, even if a brittle crack occurs, it is also an important requirement to stop the propagation of the brittle crack and to minimize the propagation region of the brittle crack (hereinafter, sometimes referred to as “brittle crack stop characteristic”). This is because if the generated brittle crack propagates over a wide range, the structure itself is destroyed. However, a high-tensile steel sheet that has improved the HAZ toughness during high heat input welding while suppressing the propagation of the brittle cracks that have occurred is not known.

For these reasons, there is a demand for the realization of a steel sheet having excellent HAZ toughness after high heat input welding and brittle crack initiation suppressing characteristics or brittle crack stopping characteristics.
Japanese Patent Publication No. 55-026164 Japanese Patent No. 2950076 Japanese Patent Publication No. 07-068577 Japanese Patent Publication No. 05-017300

  The present invention has been made in view of such circumstances, and the purpose thereof is excellent in low temperature toughness of HAZ and base metal even when welding is performed with high heat input, and brittleness of base material (steel plate). An object of the present invention is to provide a high-strength steel sheet that is excellent in crack generation suppressing characteristics or brittle crack stopping characteristics.

The high-strength steel sheet of the present invention that has achieved the above object is C: 0.03 to 0.09% (meaning “mass%”, the same shall apply hereinafter), Si: 0.01 to 0.25%, Mn : 1.20 to 1.60%, P: 0.010% or less (not including 0%), S: 0.003% or less (not including 0%), Al: 0.02 to 0.04% Nb: 0.005-0.016%, B: 0.0006-0.0020%, N: 0.0045-0.0090%, Ti: 0.008-0.020%, When the following microstructure (1) is satisfied, the balance is iron and inevitable impurities, and the metal structure of the plane perpendicular to the steel sheet surface is observed in parallel with the rolling direction of the steel sheet having a thickness t (mm), It has a gist in the point of satisfying (a) to (c), and welding heat is satisfied by satisfying these requirements. High tensile steel brittle cracking inhibiting properties were also excellent with low temperature toughness of the sounding portion can be realized.
(A) The ferrite area ratio is 75% or more,
(B) The average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less,
(C) The average aspect ratio of ferrite grains at the t / 4 position is 1.6 or less.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)
{In formula, B shows B content (mass ppm).
NT is
The relationship between N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) is
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
(N-Ti / 3.4) <0 indicates NT = 0}

  Another structure of the high-strength steel sheet of the present invention that can achieve the above object satisfies the above-mentioned chemical composition (which satisfies the requirement of the above formula (1)) and has a thickness t (mm). When observing the metal structure, it has a gist in that the average grain size of the ferrite grains in the region from the steel sheet surface to the t / 100 position is 25 μm or less. It is possible to realize a high-tensile steel sheet having excellent low-temperature toughness and brittle crack stopping characteristics.

In the steel material of the present invention, if necessary, Cu: 0.5% or less (not including 0%), Ni: 0.8% or less (not including 0%), and V: 0.05% or less One or more selected from the group consisting of (not including 0%) may be included so as to satisfy the following formula (2), and further Ca: 0.003% or less (not including 0%) May be included.
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (2)
{Wherein Cu, Ni, Nb, and V represent the content (% by mass) of each element}

  According to the present invention, even when high heat input welding is performed on a steel plate, HAZ exhibits excellent toughness at about −60 ° C., and therefore, a low temperature tank for storing liquefied gas such as offshore structures and LPG, etc. For example, a single-sided submerged arc welding method with high heat input can be used for the manufacture of the above, and the above-mentioned offshore structures and the like can be manufactured in a shorter period of time. Can increase the sex.

The present inventor has intensively studied to obtain a high-strength steel sheet that is particularly excellent in low temperature toughness of HAZ when welding with high heat input is performed.
as a result,
(I) The balance of C, 0.09% or less and Si, 0.25% or less are set relatively low, the balance between the specified amount of B, N and Ti is optimized, and a certain amount of Nb is added. For example, the generation of coarse ferrite from the austenite grain boundary (hereinafter sometimes simply referred to as “grain boundary ferrite”) is sufficiently suppressed, and the grain refinement within the austenite grain can be achieved.
(Ii) Furthermore, when Cu, Ni, and V are added to further increase the strength, if the contents of the Cu, Ni, V, and Nb are comprehensively controlled, deterioration of the HAZ toughness can be suppressed.
The specific method was found based on the idea.

  First, in the present invention, by optimizing the balance of individual prescribed amounts of B, N, and Ti and rigorously optimizing the amount of dissolved B, the crystal grains in the austenite grains can be refined, and as a result, HAZ It is characterized in that the low-temperature toughness of can be greatly improved.

FIG. 1 shows that 0.06% C-0.20% Si-1.4% Mn-0.03% Al-0.010% Nb is a basic component, and B, N, and Ti are within the specified ranges described later. (B-NT / 1.3) {B is B content (mass ppm), NT is N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) Relationship
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
When (N-Ti / 3.4) <0, NT = 0 is indicated.
The same applies to equation (1) below}
A heat cycle test was performed using steel sheets having various values of and the low temperature toughness (vE- ₆₀ ) of HAZ was measured as described in the examples described later, and these results were organized. In the thermal cycle test, welding heat input: 60 kJ / cm (plate thickness 12 mm) was assumed, and after heating and holding at 1400 ° C. × 5 seconds, cooling from 800 ° C. to 500 ° C. was performed in 150 seconds.

From FIG. 1, in order to achieve a low temperature toughness of HAZ of vE- ₆₀ : 100 J or more, as shown in the following formula (1), the value of (B-NT / 1.3) is -20 ppm or more and 10 ppm or less. It is understood that it is necessary to make it fall within the range.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)

  As shown in the above formula (1), by optimizing the balance of B, N and Ti, the solid solution B existing at the grain boundary in the austenite grain suppresses the coarsening of the grain boundary ferrite, and from the grain boundary. It is considered that the effect of suppressing the formation of the ferrite side plate and the effect of BN as the ferrite transformation nucleus could be maximized.

  As described above, in order to optimize the balance of B, N, and Ti to reliably increase the low temperature toughness of HAZ and to ensure the strength of the base material (steel plate), the contents of B, N, and Ti are respectively specified. Must be within the range.

  In the steel sheet of the present invention, in order to satisfy the basic characteristics as the steel material, in addition to basic components such as C, Si, Mn, P, S, and Al, B, which is a component related to the formula (1), N, Ti, etc. need to be adjusted appropriately. The reasons for limiting the range of B, N, Ti, etc. are as follows.

[B: 0.0006 to 0.0020%]
B fixes solute N which is harmful to HAZ toughness by generating BN, and also has an effect of promoting the formation of intragranular ferrite. Solid solution B also has the effect of suppressing the coarsening of grain boundary ferrite and the formation of ferrite side plates, and making the crystal grains in the austenite grains finer. In order to fully exhibit this effect, it is necessary to contain B 0.0006% or more. On the other hand, when there is too much B, a crystal | crystallization is formed in a fixed direction by the effect | action of excess solute B, and HAZ toughness deteriorates on the contrary. Therefore, the B content is limited to 0.0020% or less. In addition, the minimum with preferable B content is 0.0008%, and a preferable upper limit is 0.0018%.

[N: 0.0045 to 0.0090%]
N is an element that forms nitrides with elements such as Ti and Al to improve the HAZ toughness, and therefore may be contained in an amount of 0.0045% or more (preferably 0.0060% or more). In addition, solid solution N becomes a cause of degrading HAZ toughness. By increasing the total nitrogen amount, the above-mentioned nitride increases, but solid solution N also becomes excessive. Therefore, in the present invention, the N content is suppressed to 0.0090% or less.

[Ti: 0.008 to 0.020%]
Ti is an element that generates TiN-based precipitates and promotes the formation of intragranular ferrite, and is also effective in suppressing austenite grain coarsening. It is also an element contributing to high strength. In order to exert such an action effectively, it is necessary to contain Ti by 0.008% or more, and preferably 0.012% or more. However, if Ti is excessively contained, the HAZ toughness is reduced instead, so the content is made 0.020% or less.

  In the present invention, as described above, the balance of individual prescribed amounts of B, Ti, and N is optimized, and a certain amount of Nb is added. Nb is an element useful for sufficiently suppressing the formation of coarse grain boundary ferrite and achieving crystal grain refinement in the austenite grains. In the present invention, Nb is contained in an amount of 0.005% or more in order to sufficiently exhibit such an effect. However, if contained excessively, hard phase MA (Martensite-Austenite constituent) is easily generated, crystals are formed in a certain direction, and the HAZ toughness is deteriorated, so that it is suppressed to 0.016% or less.

  In order to increase the low temperature toughness of HAZ more reliably, it is effective to further reduce C and Si. In this invention, in order to suppress the production | generation in HAZ of MA which is a hard phase, and to ensure the HAZ toughness at about -60 degreeC, C amount is suppressed to 0.09% or less. On the other hand, C is an element essential for securing the strength of the steel sheet, so 0.03% or more is contained.

  Furthermore, by reducing Si to 0.25% or less, the formation of MA can be sufficiently suppressed, and the low temperature toughness of HAZ can be easily ensured. On the other hand, since Si is an element that is used for deoxidation of molten steel and effectively acts to improve the strength, it may be contained in an amount of 0.01% or more, preferably 0.05% or more.

  In addition, as described above, in order to reliably increase the HAZ toughness and to have other properties such as strength and toughness of the base material (steel plate), it is necessary to set the content of components other than the above within the following range.

[Mn: 1.20 to 1.60%]
Mn is an element useful for capturing S as MnS and suppressing degradation of HAZ toughness due to S. It is also an element that contributes to increasing the hardenability (high tensile strength TS and high yield strength YS) by increasing the hardenability. In order to exhibit such an action effectively, it is necessary to contain 1.20% or more of Mn. However, if the Mn content is excessive, the HAZ toughness deteriorates on the contrary, so it is suppressed to 1.60% or less.

[P: 0.010% or less (excluding 0%)]
P is an element that deteriorates the HAZ toughness, so it is necessary to reduce it as much as possible. In the present invention, P is suppressed to 0.010% or less.

[S: 0.003% or less (excluding 0%)]
S is an element that generates coarse sulfides and degrades the HAZ toughness. Therefore, it is necessary to reduce as much as possible, and in the present invention, it is suppressed to 0.003% or less.

[Al: 0.02-0.04%]
Al is an element that is used as a deoxidizer and that also generates AlN-based precipitates to improve the HAZ toughness during high heat input welding. In the present invention, Al is contained in an amount of 0.02% or more. However, when the Al content is excessive, oxide inclusions such as alumina increase and MA formation is promoted to deteriorate the HAZ toughness. Therefore, the Al content is suppressed to 0.04% or less.

  The contained elements specified in the present invention are as described above, and the balance is iron and unavoidable impurities, and as the unavoidable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed. Further, it is possible to further contain the following elements.

[From the group consisting of Cu: 0.5% or less (not including 0%), Ni: 0.8% or less (not including 0%), and V: 0.05% or less (not including 0%) 1 or more types selected (however, within the range of the following formula (2))]
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (2)
{Wherein Cu, Ni, Nb and V represent the content (mass%) of each element}

  Cu, Ni and V are all useful elements for securing the strength of the steel sheet. Cu is an element effective for increasing the strength (tensile strength TS and yield strength YS) by solid solution strengthening and precipitation strengthening. However, if excessively contained, the hot workability is inhibited, so the content is suppressed to 0.5% or less.

  Ni is an element that simultaneously improves the strength and toughness of the steel sheet. In order to exhibit such an action effectively, it is preferable to contain 0.2% or more. However, if it is excessively contained, the cost is increased, so the content is suppressed to 0.8% or less.

  V is an element useful for enhancing the hardenability of the steel sheet to ensure high strength and for increasing the temper softening resistance. However, if excessively contained, the HAZ toughness deteriorates, so the content is limited to 0.05% or less.

  Further, in the present invention, as described above, Nb is suppressed to 0.016% or less, and the contents of Cu, Ni, Nb and V are restricted as shown in the following formula (2), thereby comprising Cu, Ni and V. Even when one or more selected from the group is contained, excellent HAZ toughness can be ensured.

FIG. 2 shows that 0.06% C-0.20% Si-1.4% Mn-0.03% Al-0.014Ti-0.0014% B-0.0065% N as a basic component, Cu: One or more selected from the group consisting of 0.5% or less, Ni: 0.8% or less, and V: 0.05% or less, and a specified amount of Nb, including (Cu + Ni + 60Nb + 20V) having various values. A heat cycle test was conducted using a steel plate, and the low temperature toughness (vE- ₆₀ ) of HAZ was measured as described in Examples below, and these results were arranged. The heat cycle test was conducted by heating at 1400 ° C. × 5 seconds and then cooling from 800 ° C. to 500 ° C. in 150 seconds assuming welding heat input: 60 kJ / cm (plate thickness 12 mm).

From FIG. 2, when one or more selected from the group consisting of Cu: 0.5% or less, Ni: 0.8% or less, and V: 0.05% or less is contained, vE ₋ as the low temperature toughness of HAZ. It can be seen that in order to achieve ₆₀ : 100 J or more, the value of (Cu + Ni + 60Nb + 20V) needs to be 1.4% or less as shown in the following formula (2). While suppressing Nb to 0.016% or less and comprehensively limiting the contents of Cu, Ni, Nb, and V as described above, the formation of MA, which is a hard phase, is suppressed and excellent HAZ toughness is achieved. Can be secured.
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (%) (2)
{Wherein Cu, Ni, Nb, and V represent the content (% by mass) of each element}

[Ca: 0.003% or less (excluding 0%)]
Ca is an element effective for fixing S, which adversely affects HAZ toughness, as CaS, and for improving the toughness by controlling the form of nonmetallic inclusions in a granular form. In order to exert such effects sufficiently, it is preferable to contain 0.0010% or more of Ca, but even if Ca is contained excessively, these effects are saturated and the HAZ toughness deteriorates. Therefore, the Ca content is preferably 0.003% or less.

  As described above, the steel sheet satisfying the chemical composition has good HAZ toughness. However, the requirements for improving the characteristics for suppressing the occurrence of brittle cracks without degrading the HAZ toughness were also examined. . As a result, for a steel material having a thickness t (mm), when the metal structure of the plane parallel to the rolling direction and perpendicular to the steel material surface was observed, (a) the ferrite area ratio was 75% or more, (b) If the average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less and (c) the average aspect ratio of the ferrite grains at the t / 4 position is 2.0 or less, the occurrence of brittle cracks in the steel sheet is suppressed. It has been clarified that the above-described properties can be improved and the HAZ toughness is not deteriorated. Hereinafter, the reason for this definition will be described in detail.

  The metal structure of the high-tensile steel plate according to the present invention is mainly composed of ferrite in order to ensure the strength of the steel plate. “Ferrite main body” means that the ferrite fraction in the steel sheet is 75% by area or more, and when the metal structure of the cross section of the steel sheet is observed, the ferrite area ratio may be 75% or more. The area ratio of ferrite is preferably 80% or more, and more preferably 85% or more.

  The remainder of the metal structure is not particularly limited as long as pearlite, bainite, martensite, or the like is generated as the second phase. The area ratio of the second phase may be less than 25%, preferably less than 20%, more preferably less than 15%.

  The metal structure of the steel sheet is mainly composed of ferrite, and in order to improve the CTOD characteristics, it is important to appropriately adjust both the equivalent circle diameter and the aspect ratio of the ferrite grains. That is, as a result of repeating various experiments by the present inventors, the average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less, and the average aspect ratio of the ferrite grains at the t / 4 position is 2.0 or less. It turned out to be necessary.

This is clear from the examples described later, and FIG. 3 shows the influence of the average equivalent circle diameter and aspect ratio of the ferrite grains on the CTOD characteristics at the t / 2 position of the steel sheet. In FIG. 3, the X axis shows the average equivalent circle diameter of the ferrite grains at the t / 2 position, the Y axis shows the CTOD characteristics (δc- _{60 ° C.} ), and □ shows the average aspect ratio of the ferrite grains at the t / 2 position. 1.6 or less, ○ indicates that the average aspect ratio of the ferrite grains at the t / 2 position exceeds 1.6 and 2.0 or less, and Δ indicates the results when the average aspect ratio exceeds 2.0, respectively. Yes.

FIG. 4 shows the influence of the average equivalent circle diameter and aspect ratio of ferrite grains on the CTOD characteristics at the t / 4 position of the steel sheet. In FIG. 4, the X-axis shows the average equivalent circle diameter of the ferrite grains at the t / 4 position, the Y-axis shows the CTOD characteristics (δc- _{60 ° C.} ), and ◯ shows the average aspect ratio of the ferrite grains at the t / 4 position. 2.0 or less and ● indicate the results when the average aspect ratio of the ferrite grains at the t / 4 position exceeds 2.0, respectively.

FIG. 5 shows the relationship between the relative position from the center of the steel plate (t / 2 position) and the CTOD characteristics (δc _{-60 ° C} ). In FIG. 5, the X-axis indicates the relative position when the steel plate center portion (t / 2 position) is 0%. For example, a relative position of 25% indicates the t / 4 position.

As is clear from these results, (1) CTOD characteristics tend to be improved (the value of δc- _{60 ° C.} tends to increase) as the average equivalent circle diameter of the ferrite grains at the t / 2 position decreases. (2) If the average equivalent circle diameter of the ferrite grains at the t / 4 position is 20.0 μm or less and the average aspect ratio is 2.0 or less, δc _{−60 ° C.} becomes 0.20 mm or more, and the CTOD characteristic is It can be seen that it can be reliably improved, and (3) the CTOD characteristics tend to be low at the center of the steel material, so that the CTOD characteristics should be managed at the center of the steel sheet.

  The reason why this phenomenon occurs is considered as follows. That is, in brittle fracture, the boundary between crystal grains (crystal grain boundary) serves as resistance to crack propagation. Therefore, if the grain boundaries exist densely, brittle fracture itself is less likely to occur, Even if a brittle fracture occurs, the propagation of cracks can be prevented if the grain boundaries are densely present in the direction in which the cracks propagate. However, since the ferrite grains extend in the rolling direction in the rolling process, the aspect ratio of the ferrite grains increases. Therefore, the major axis of the ferrite grains is aligned in the rolling direction, and the minor axis is easily aligned in the plate thickness direction.

  Therefore, although the crystal grain boundaries are densely present in the plate thickness direction, the crystal grain boundaries in the rolling direction are sparse, so the density of the crystal grain boundaries is likely to vary, and brittle fracture is likely to occur. . Further, once brittle fracture occurs, cracks easily propagate along the grain boundary in the rolling direction. On the other hand, if the average equivalent circle diameter of the ferrite grains is reduced and the average aspect ratio is reduced, there is almost no variation in the density of the crystal grain boundaries, so that brittle fracture hardly occurs. The boundary acts as a resistance and can prevent crack propagation.

  In the steel sheet of the present invention, the average equivalent circle diameter of the ferrite grains is 20.0 μm or less, and the average aspect ratio of the ferrite grains is 2.0 or less. The position where the average equivalent circle diameter is controlled is the t / 2 position where the thickness of the steel material is t. Since it is generally known that brittle fracture occurs near the center of the plate thickness (see FIG. 5), the occurrence of brittle fracture can be suppressed by appropriately controlling the structure at the t / 2 position. Further, the position where the average aspect ratio is controlled is the t / 4 position where the thickness of the steel material is t. This is selected as a position showing the average characteristics of the steel sheet.

  As the plate thickness increases, the difference between the temperature at the t / 2 position of the steel sheet and the strain introduced at the t / 2 position and the temperature near the surface of the steel material (for example, the t / 4 position) and the introduced strain increase. By controlling the temperature at the t / 2 position and appropriately controlling the structure at the t / 2 position, the occurrence of brittle cracks can be suppressed.

  The average equivalent circle diameter of the ferrite grains at the t / 2 position is preferably 17.5 μm or less, and more preferably 16 μm or less. The lower limit of the average equivalent circle diameter of the ferrite grains is not particularly specified and is preferably as small as possible, but is usually about 7 μm or more (particularly 10 μm or more) because there is a limit to the reduction. The equivalent circle diameter means the diameter of a circle when ferrite grains are converted into a circle having the same area.

  On the other hand, the average aspect ratio of the ferrite grains at the t / 4 position is preferably 1.9 or less, more preferably 1.8 or less. The aspect ratio of the ferrite grain means the ratio (Dl / Dt) of the grain diameter (Dl) in the rolling direction of the ferrite grain to the grain diameter (Dt) in the plate thickness direction.

  The average equivalent circle diameter and average aspect ratio of the ferrite grains can be calculated, for example, by the following procedure. First, a sample is cut out so that a surface that includes the front and back surfaces of the steel material is parallel to the rolling direction and is perpendicular to the steel surface (the front surface of the steel material), and this exposed surface is polished. And mirror finish.

  The method for polishing the exposed surface is not particularly limited. For example, polishing may be performed using a wet emery polishing paper of # 150 to # 1000 or a polishing method having an equivalent function. In addition, when performing mirror finish, an abrasive such as diamond slurry may be used.

  The mirror-finished sample is corroded using a 3% nital solution to reveal the grain boundary of the ferrite structure, and then photographed at a magnification of 100 or 400 and taken into an image analyzer. The image is captured so that the area corresponds to 1 mm × 1 mm or more at any magnification.

  Next, in the image analysis device, the ferrite grain region (area) surrounded by the grain boundary is converted into a circle having an equivalent area, and the diameter of the converted circle is defined as the equivalent circle diameter of the ferrite grain. Measure the equivalent diameter. This is measured for all observation visual fields, and the average is calculated by averaging the results.

  On the other hand, as for the aspect ratio of the ferrite grains, for the ferrite grains surrounded by the grain boundaries, the grain size Dl in the rolling direction and the grain size Dt in the plate thickness direction are measured, and the ratio of Dl to Dt (Dl / Dt) is determined. Calculate as aspect ratio. This is performed for all observation fields, and the average aspect ratio is calculated by averaging the results.

  The metal structure of the steel sheet of the present invention is mainly composed of ferrite, the average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less, and the average aspect ratio of the ferrite grains at the t / 4 position is 2.0 or less. For this purpose, the slab obtained by casting is heated to 1000 to 1200 ° C., then roughly rolled, and then finish rolled in the austenite non-recrystallization temperature range. In the following, description will be given in order.

  It is preferable that the temperature which heats a slab shall be 1000-1200 degreeC. In order to refine the ferrite structure obtained after rough rolling and subsequent cooling (natural cooling or forced water cooling), it is effective to roll and recrystallize the austenite structure. Although the lower limit of the recrystallization temperature of austenite depends on the chemical composition of the steel material, it is usually 850 to 900 ° C. Therefore, in order to roll and recrystallize the austenite structure above this lower temperature, the heating temperature is 1000 ° C. or higher. It is good to do. However, if the temperature exceeds 1200 ° C., the initial austenite structure becomes too coarse, and it is difficult to sufficiently refine the austenite structure even if the austenite structure is rolled and recrystallized. Therefore, the heating temperature is preferably 1200 ° C. or lower.

  The heated slab may be roughly rolled at a cumulative reduction ratio of 40% or more in the austenite recrystallization temperature range. By rough rolling at a cumulative reduction ratio of 40% or more in the recrystallization temperature range of austenite, the ferrite structure can be refined by recrystallization and reduction to an austenite structure close to uniform grains. A ferrite structure close to grains can be formed. If the cumulative reduction ratio in the recrystallization temperature range is less than 40%, refinement by reduction in the recrystallization temperature range becomes insufficient, and therefore coarse austenite grains are mixed after rolling. Therefore, the finally obtained metal structure tends to be in a mixed grain state in which coarse ferrite grains and fine ferrite grains are mixed. As described above, when the metal structure is in a mixed grain state, there is a tendency that variations in brittle crack suppression characteristics are likely to occur. Therefore, in order to sufficiently refine the austenite structure in the austenite recrystallization temperature range, it is recommended that the cumulative rolling reduction in the austenite recrystallization temperature range be 40% or more. The austenite recrystallization temperature range varies somewhat depending on the chemical composition, but is set to about 900 to 1000 ° C. in the present invention.

  The cumulative rolling reduction is preferably as large as possible, and as the cumulative rolling reduction increases, the equivalent circle diameter of the ferrite grains can be reduced to about 25 to 30 μm. However, even if the cumulative reduction rate in the recrystallization temperature range of austenite exceeds 70%, the effect is almost saturated, so the cumulative reduction rate may be about 70% or less.

The cumulative reduction rate, t ₀ the thickness at the temperature (calculated value) is 1000 ° C. at t / 2 position of the steel material, the thickness at a temperature at t / 2 position of the steel material (calculated value) of 900 ° C. t _{When set to 1} , it can be calculated by the following equation (3).
Cumulative rolling reduction (%) = [(t ₀ −t ₁ ) / t ₀ ] × 100 (3)

However, when the rough rolling start temperature is lower than 1000 ° C., the steel thickness at the start of the rough rolling is t _0, and when the rough rolling start temperature exceeds 1000 ° C., the temperature at the t / 2 position of the steel is 1000. The cumulative rolling reduction is calculated with the steel material thickness at ₀ ° C. as t ₀ . On the other hand, when the rough rolling end temperature does not reach 900 ° C. (when it exceeds 900 ° C.), the steel material thickness at the end of rough rolling is t _1, and when the rough rolling end temperature is lower than 900 ° C., 900 ° C. The cumulative rolling reduction is calculated with the steel material thickness at ₁ as t ₁ .

  The temperature at the time of rough rolling is preferably based on the temperature calculated by calculating the temperature at the t / 2 position using a process computer. This is for appropriately controlling the metal structure at the t / 2 position. In addition, compared with the temperature of t / 2 position (calculated value), the temperature of the steel sheet surface (actually measured value) is lower by about 50 to 70 ° C. when the steel material thickness is 150 mm, and when the steel material thickness is 100 mm. Is about 40-50 ° C. lower. Therefore, the temperature at which the rough rolling is performed may be controlled using the temperature of the steel sheet surface (actually measured value) as a reference in consideration of such a temperature difference.

  After rough rolling with a cumulative reduction ratio of 40% or more in the recrystallization temperature range of austenite, cooling to the austenite non-recrystallization temperature range, and finish rolling to a true strain of 0.5 or more in the austenite non-recrystallization temperature range. It is recommended. This is because the ferrite grains can be further refined by finish rolling in the austenite non-recrystallization temperature range. That is, since the metal structure obtained by rolling in the austenite recrystallization temperature range is an austenite structure having an average particle size of about 25 to 30 μm, ferrite grains obtained by air cooling the steel material as it is or by forced cooling. The average equivalent-circle particle diameter of is no more than about 25 μm. Therefore, CTOD characteristics cannot be improved sufficiently. On the other hand, if the finish rolling is performed in the austenite non-recrystallization temperature range, strain is introduced into the ferrite grains, so that the ferrite grains can be further refined.

  In this finish rolling, the true strain is preferably 0.5 or more. If the true strain amount is less than 0.5, the ferrite grains may be insufficiently refined, and the CTOD characteristics may not be sufficiently improved. The larger the true strain amount, the better. The larger the amount, the smaller the ferrite grains.

The austenite non-recrystallization temperature range is a temperature range where the austenite structure does not recrystallize even when the steel material is rolled. Although this temperature range varies somewhat depending on the chemical composition of the steel material, in the present invention, finish rolling is performed by setting the true strain amount to be 0.5 or more in the region where the temperature at the t / 2 position of the steel material is 850 ° C. or less. However, if the temperature range of finish rolling becomes too low, the flatness (ie, aspect ratio) of the ferrite grains tends to be extremely large, and the CTOD characteristics tend to deteriorate. Therefore, the finish rolling end temperature is preferably “Ar ₃ transformation point + 10 ° C.” or higher. The temperature of the Ar ₃ transformation point can be calculated by the following formula (4) based on the content of chemical components contained in the steel material. However, [] has shown content (mass%) of each element.
Ar ₃ transformation point (° C.) = 868−369 × [C] + 24.6 × [Si] −68.1 × [Mn] −36.1 × [Ni] −20.7 × [Cu] −24.8 × [Cr] + 190 × [V]
... (4)

The true strain amount is, t ₂ thickness when the temperature (calculated) of 850 ° C. at t / 2 position of the steel material, the thickness of the temperature (calculated value) of the finish rolling end temperature of t / 2 position of the steel material when the t _3, can be calculated by the following equation (5) below.
True strain = ln (t ₂ / t ₃ ) (5)

However, when the finish rolling start temperature is lower than 850 ° C., the thickness of the steel material at the start of finish rolling is t _2, and when the finish rolling start temperature exceeds 850 ° C., the temperature at the t / 2 position of the steel material is 850. The true strain is calculated with the steel material thickness at ° C. being t ₂ . On the other hand, when the finish rolling end temperature is not reached "Ar ₃ transformation point + 10 ° C." (if more than "Ar ₃ transformation point + 10 ° C."), the steel thickness during the finish rolling end and t _3, the finish rolling end If the temperature falls below "Ar ₃ transformation point + 10 ° C." calculates the true strain steel thickness in the "Ar ₃ transformation point + 10 ° C." as t _3.

  The temperature at which the finish rolling is performed is based on the temperature calculated by calculating the temperature at the t / 2 position using a process computer.

The temperature at the time of finish rolling is preferably based on the temperature calculated by calculating the temperature at the t / 2 position using a process computer when the thickness of the steel material is t (mm). This is for appropriately controlling the metal structure at the t / 2 position. When the thickness of the steel material is about 40 to 80 mm, the temperature difference between the temperature inside the steel plate (temperature at the t / 2 position) and the surface temperature of the steel plate is at most about 10 to 40 ° C. In view of the above, the surface temperature (actually measured value) of the steel sheet may be used as a reference (for example, “850 ° C.−temperature difference”, “Ar ₃ transformation point + 10 ° C.−temperature difference”).

  What is necessary is just to cool in accordance with a conventional method after completion | finish of finish rolling. The cooling method is not particularly limited, and air cooling or forced cooling may be used. The cooling rate at this time is not particularly limited, but the present inventors have confirmed that the size of the ferrite grains is not affected as long as it is about 4 ° C./second or less.

  The present inventors have also studied the requirements for improving the brittle crack stopping characteristics without degrading the HAZ toughness. As a result, when observing the metal structure of a steel sheet having a thickness t (mm), if the average grain size of ferrite grains in the region from the steel sheet surface to the t / 100 position is 25 μm or less, the brittle crack stopping characteristics of the steel material It can be improved and the HAZ toughness is not deteriorated.

  This is clear from the examples described later. In particular, FIG. 6 shows the average grain size of ferrite grains and brittle crack stopping characteristics (Kca value at −60 ° C.) in the region from the steel sheet surface to the t / 100 position. Shows the relationship. According to FIG. 6, it can be seen that the smaller the average grain size of the ferrite grains in the region from the steel surface to the t / 100 position, the more the brittle crack stopping characteristics are improved (the Kca value at −60 ° C. is increased). .

  The reason why the brittle crack stopping characteristic can be improved by reducing the average grain size of the ferrite grains on the surface of the steel sheet is considered as follows. In other words, the propagation of brittle cracks is because the boundary between crystal grains (grain boundaries) becomes resistance to crack propagation, so if the grain boundaries exist densely, brittle fracture itself is less likely to occur. Even if a minute brittle crack is generated, it is considered that the propagation of the crack is stopped. Therefore, if the ferrite grains are refined, the propagation of the generated brittle cracks can be stopped.

In the present invention, by setting the average particle size to 25 μm or less, it is possible to ensure Kca: 5900 N / mm ^1.5 or more at −60 ° C. in a brittle fracture propagation stop test (refer to Examples for details), and brittleness The crack stop characteristics can be improved. The average particle size is preferably 20 μm or less.

  The average particle diameter of the ferrite grains can be calculated according to the method described above (method for measuring the average equivalent circle diameter of ferrite). The average grain size of the ferrite grains is observed in a region from the steel surface to the t / 100 position. If the ferrite grain size in the region from the steel sheet surface to the t / 100 position is appropriately controlled, the present inventors consider that the brittle crack stopping characteristics of the entire steel sheet as well as the steel sheet surface part are improved. This is because it became clear.

  The metal structure in the region from the steel surface to the t / 100 position is mainly composed of ferrite. The term “ferrite main body” means basically the same as the steel sheet described above, but the ferrite fraction may be 50 area% or more. The remainder of the metal structure is not particularly limited as long as pearlite, bainite, martensite, or the like is generated as the second phase. The area ratio of the second phase may be less than 50%, preferably less than 45%, more preferably less than 40%.

  To adjust the average grain size of the ferrite grains in the region from the steel sheet surface to the t / 100 position to 25 μm or less, to adjust the temperature range of finish rolling after heating and rough rolling the slab obtained by casting. May be air-cooled or forcibly cooled, and then finish-rolled in an austenite recrystallization temperature range, an austenite non-recrystallization temperature range, or a two-phase temperature range with a true strain amount of 0.5 or more. This is because the ferrite grains can be refined by setting the temperature range of the finish rolling to an appropriate temperature range. That is, the metal structure obtained by air cooling or forced cooling after rolling according to a conventional method without temperature control is a ferrite structure having an average grain size of about 35 μm or more, and therefore the brittle crack stopping property cannot be sufficiently improved. . On the other hand, if the finish rolling is performed in an appropriate temperature range, the ferrite grains can be further refined. In particular, if the finish rolling is performed in a two-phase temperature range, the ferrite grains can be directly deformed, so that a large true strain is introduced and the ferrite grains can be further refined.

  In order to make the true strain amount 0.5 or more in finish rolling, if the true strain amount is less than 0.5, the ferrite grains may not be sufficiently refined, and the brittle crack stopping property cannot be sufficiently improved. Because there is. The larger the true strain amount, the better. The larger the amount, the smaller the ferrite grains.

The temperature range at which the finish rolling is performed varies somewhat depending on the chemical composition of the steel material. Therefore, in the present invention, it is preferable to finish-roll with the true strain introduced in the region where the surface temperature of the steel sheet is 900 ° C. or less being 0.5 or more. However, if the temperature range of finish rolling becomes too low, the work embrittlement of the ferrite structure becomes remarkable, and the brittle crack stopping property tends to be lowered. Therefore, the finish rolling finish temperature is preferably “Ar ₃ transformation point −40 ° C.” or higher. The temperature of the Ar ₃ transformation point can be calculated by the above formula (4) based on the content of chemical components contained in the steel material. The said temperature should just control the temperature from a steel material surface to t / 100 position in the said range.

The true strain amount can be calculated by the following equation (6), where t _{4 is} the thickness of a steel slab when the surface temperature of the steel is 900 ° C., and t ₅ is the thickness of the steel slab at the finish rolling finish temperature.
True strain amount = ln (t ₄ / t ₅ ) (6)

However, when the finish rolling start temperature is lower than 900 ° C., the true strain is calculated with the steel material thickness at the start of finish rolling as t ₄ . When the finish rolling start temperature exceeds 900 ° C., the thickness of the steel slab when the surface temperature of the steel material is 900 ° C. is t ₄ .

  What is necessary is just to cool in accordance with a conventional method after completion | finish of finish rolling. The cooling method is not particularly limited, and air cooling or forced cooling may be used.

  The various steel plates of the present invention obtained as described above can be used as materials for structures such as bridges, high-rise buildings, ships, etc., and can be used not only for small to medium heat input welding but also for large heat input welding (for example, 40 kJ / mm). Also in the above, it is possible to prevent toughness deterioration of the weld heat affected zone and to have excellent brittle crack suppression characteristics or brittle crack stop characteristics. Moreover, the steel plate of the present invention can be advantageously applied to so-called thick steel plates. The plate thickness at this time is about 7 mm or more, and the upper limit is not particularly limited, but is usually about 40 mm or less.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

[Example 1]
Using various steel slabs having the chemical composition shown in the following Tables 1 and 2 [Ar ₃ transformation point was calculated based on the above formula (4)], the manufacturing conditions shown in Tables 3 and 4 (slab heating temperature, coarse Various steel plates were produced under the rolling conditions and finish rolling conditions. The temperature at this time is managed by the temperature at the t / 2 position and the t / 4 position (t is the plate thickness), and the detailed temperature management procedure is as follows.

[Temperature management procedure]
1. Using the process computer, the heating temperature at the position (t / 4 or t / 2 position) from the front surface to the back surface of the steel slab is calculated based on the atmospheric temperature from the start of heating to the end of heating and the in-furnace time.
2. Using the calculated heating temperature, based on the rolling pass schedule during rolling and the data of the cooling method (water cooling or air cooling) between passes, a method suitable for calculation such as the difference method is used to calculate the rolling temperature at any position in the plate thickness direction. The rolling is carried out while using the calculation.
3. The surface temperature of the steel sheet is measured using a radiation type thermometer installed on the rolling line. However, the theoretical value is also calculated in the process computer.
4). The surface temperature of the steel sheet measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is collated with a calculated temperature calculated from a process computer.
5. If the difference between the calculated temperature and the measured temperature is ± 30 ° C or more, recalculate the calculated surface temperature so that it matches the measured temperature to obtain the calculated temperature on the process computer. The calculated temperature is used as it is.
6). Using the calculated temperature calculated above, the rolling temperature in the region to be controlled is managed.

  About each steel plate obtained as mentioned above, the steel plate (base material) and the toughness evaluation of HAZ were implemented in the following manner, respectively.

[Evaluation of base metal toughness]
A V-notch test piece of JIS Z 2202 is taken in the rolling direction from a part cut from the surface side of each steel plate, and subjected to a Charpy impact test according to the procedure of JIS Z 2242, and the absorbed energy at a test temperature of −60 ° C. (VE- ₆₀ ) was measured. And it was evaluated that the absorbed energy (vE- ₆₀ ) is 100 J or more and has excellent base material toughness.

[Evaluation of HAZ toughness]
Single-sided submerged arc welding using the steel plate was performed by the FCB method. The FCB method is a method of laying a backing flux on a copper plate, pressing it against the back of the groove, and completing the welding while forming a back bead from one side of the surface. Yes. The groove shape is shown in FIG. 7 [(a) when the plate thickness is 12 mm, (b) when the plate thickness is 30 mm]. As the welding material, the following low-temperature steel welding material (manufactured by Kobe Steel) was used, and welded joints were produced under the welding conditions shown in FIG. 8 and Table 5.

[Welding material]
・ Wire; US-255
・ Front flux; PFI-50LT
・ Backing flux; MF-1R

Then, a V-notch test piece of JIS Z 2202 with a notch perpendicular to the surface of the plate at a position of HAZ (bond portion, bond portion + 1 mm (location of 1 mm on the base metal side) [HAZ1 mm]) cut from the surface side. Three of each were collected and subjected to a Charpy impact test in accordance with JIS Z 2242. And the thing whose average value of the absorbed energy (vE- ₆₀ ) in _-60 degreeC was 100 J or more was evaluated that it was excellent in the low temperature toughness of HAZ.

  In addition, the observation of the metal structure (equivalent circle diameter and aspect ratio of ferrite grains) and brittle crack suppression characteristics of each steel plate were measured by the following procedure.

[Observation of metal structure (measurement of equivalent circle diameter and aspect ratio)]
A sample is cut out so that a surface including the front surface and the back surface of the steel plate is parallel to the rolling direction and perpendicular to the steel surface (the front surface of the steel material), and the exposed surface is polished. Mirror finish. The exposed surface was polished using # 150 to # 1000 wet emery polishing paper and then mirror-finished using diamond slurry as an abrasive.

  The mirror-finished sample was corroded using a 3% nital solution to reveal the grain boundary of the ferrite structure, and then photographed at a magnification of 400 × to obtain a photograph of 6 cm × 8 cm (that is, 150 μm × 400 ×) Equivalent to 200 μm). The 6 cm side of the photo corresponds to the plate thickness direction, and the 8 cm side corresponds to the rolling direction. This was taken into the image analysis apparatus so that the area corresponds to 1 mm × 1 mm or more at any magnification.

  Next, in the image analysis device, the ferrite grain region (area) surrounded by the grain boundary is converted into a circle having an equivalent area, and the diameter of the converted circle is defined as the equivalent circle diameter of the ferrite grain. The equivalent diameter was measured. This was measured for all observation visual fields, and the average equivalent circle diameter was calculated by averaging the results.

  On the other hand, as for the aspect ratio of the ferrite grains, for the ferrite grains surrounded by the grain boundaries, the grain size Dl in the rolling direction and the grain size Dt in the plate thickness direction are measured, and the ratio of Dl to Dt (Dl / Dt) is determined. Calculated as aspect ratio. This was performed for all observation visual fields, and the average aspect ratio was calculated by averaging the results.

  The measurement positions of the equivalent circle diameter and aspect ratio of the ferrite grains were t / 2 position and t / 4 position when the thickness of the steel material was t (mm). The number of observation fields was 35. When calculating the average equivalent circle diameter and aspect ratio of the ferrite grains, the ferrite area ratio in the metal structure was also measured.

[Evaluation of brittle crack suppression properties]
The brittle fracture initiation characteristics are determined based on the crack tip opening displacement test based on the “crack tip opening displacement test (CTOD test)” defined by WES1108 (established on February 1, 1995) issued by the Japan Welding Association (WES). The opening displacement (δc) at the start of unstable fracture was measured and evaluated based on this result. In conducting the crack tip opening displacement test, the “Guidelines on the Welding Heat Affected Zone CTOD Test Method” defined by WES1109 (established on April 1, 1995) was also taken into consideration.

The test piece is a P.S. of WES1108 (established on February 1, 1995). 6 “standard three-point bending test piece” shown in FIG. 6 was used. The test temperature was −60 ° C., and δc− _{60 ° C.} (mm) was measured. In this invention, the case where (delta) c- ₆₀ degreeC is 0.20 mm or more is set as a pass.

Table 6 shows the structure [average equivalent circular diameter and aspect ratio of ferrite (α)] and ferrite fraction at the t / 2 position or t / 4 position in each steel sheet, and the matrix characteristics (plate thickness, vE _-60 And δc _{−60 ° C.} ) and HAZ toughness are shown in Tables 7 and 8 below together with actual welding conditions (construction method and heat input).

  From these results, it can be considered as follows (note that the following No. indicates the experiment No. in the table).

  No. satisfying the requirements defined in the present invention. Steel plates of 3, 5, 6, 12, 14, 15, 18, 20, 21 are steel plates with excellent low temperature toughness of HAZ and excellent base material properties (toughness, brittle cracking suppression properties), The steel sheet is welded by a high heat input single-sided submerged arc welding method, and exhibits excellent characteristics even when used for low temperature conditions.

  On the other hand, No. which does not satisfy the provisions of the present invention. 1, 2, 4, 7 to 11, 13, 16, 17, 19, 22 to 41 do not satisfy any of the requirements defined in the present invention, and have HAZ toughness and brittle cracking suppression characteristics. At least one of the characteristics is inferior.

[Example 2]
Each steel material having the chemical composition shown in Tables 1 and 2 was melted in a converter, and various steel slabs (steel types No. 1 to 38) manufactured by continuous casting were used. After heating, roughly rolled, air-cooled or After forced cooling, finish rolling was performed to produce various steel plates. Tables 9 and 10 below show finish rolling finishing temperature (surface temperature), cooling conditions after rolling (cooling method, cooling rate), and true strain amount at 900 ° C. or less. The cooling rates shown in Tables 9 and 10 are average values (average cooling rate) from the start of cooling to 500 ° C.

  Each steel plate obtained as described above was evaluated for HAZ toughness in the same manner as in Example 1, and the brittle crack stopping property was evaluated by the following method.

[Evaluation of brittle crack stopping properties]
The brittle crack stop property was performed according to the “brittle fracture propagation stop test” defined by the steel type qualification test method (established on March 31, 2003) published by the Japan Welding Association (WES). In the test, a test piece having the shape shown in FIG. 7.2 of the brittle fracture propagation stop test method is used, and a temperature gradient is applied to the test piece in an arbitrary temperature range selected from the range of −190 ° C. to + 60 ° C. A total of 4 specimens were used. The Kca value was calculated by the following formula (7). In the following formula (7), c is the length from the propagation portion entrance to the brittle crack tip, T is the temperature at the brittle crack tip (unit is K), σ is the gross stress of the propagation portion, and W is the propagation portion width. Yes. A graph showing the correlation between 1 / T and Kca value is created using 1 / T on the X axis and Kca value on the Y axis, and the intersection of 4 approximate curves and 213K is expressed as the Kca value at −60 ° C. did. In the present invention, a case where Kca at −60 ° C. is 5900 N / mm ^1.5 or more is regarded as acceptable (excellent in brittle crack stopping characteristics).

  In addition, the observation of the metal structure in each steel sheet (the average particle diameter of ferrite at the position t / 100 from the surface layer was also measured in accordance with the procedure shown in Example 1. When calculating the average particle diameter of the ferrite grains, The metal structure in the region up to the t / 100 position was observed and the ferrite area ratio was also measured at the same time, and as a result, the area ratio of ferrite occupying the metal structure was 50% or more. The number of observation fields is at least 6, and in the case of 400 times, the number of observation fields is at least 35.

  These results are shown in Tables 11 and 12 below. Note that an example satisfying both the HAZ toughness and the brittle crack stopping characteristics was determined as an example of the present invention (◯), and an example not satisfying at least one of the characteristics was comprehensively determined as a comparative example (x).

  These results can be considered as follows. No. 42 to 46, 48 to 50, 52, 54, 56 to 58, 60 to 66, 68, and 70 to 73 satisfy the requirements defined in the present invention, and are excellent in the low temperature toughness of HAZ. The steel sheet is also excellent in material characteristics (brittle crack stopping characteristics), and exhibits excellent characteristics even when the steel sheet is welded by a high heat input single-sided submerged arc welding method and used for low temperature conditions.

  On the other hand, No. which does not satisfy the requirements defined in the present invention. 47, 51, 53, 55, 59, 67, 69, and 74 to 93 do not satisfy any of the requirements defined in the present invention, and are at least one of HAZ toughness and brittle crack stopping properties. It is inferior to.

  FIG. 9 shows the relationship between the average grain size of the ferrite grains and the brittle crack stopping characteristics in the region from the steel sheet surface to the t / 100 position. If the true strain amount is controlled to 0.5 or more, t / It can be seen that the average grain size of ferrite grains in the region up to 100 positions can be controlled to 25 μm or less.

It is a graph which shows the relationship between (B-NT / 1.3) and vE- _{60 of} HAZ. It is a graph which shows the relationship between (Cu + Ni + 60Nb + 20V) and vE- _{60 of} HAZ. It is a graph which shows the influence which the average equivalent circular diameter and aspect ratio of a ferrite grain have on CTOD characteristics (δc _{-60 ° C} ) at the t / 2 position of a steel plate. It is a graph which shows the influence which the average equivalent circle diameter and aspect ratio of a ferrite grain have on CTOD characteristics (δc _{-60 ° C} ) at the t / 4 position of a steel plate. It is a graph which shows the relationship between the relative position from a steel plate center part (t / 2 position), and CTOD characteristic ((delta) c- ₆₀ degreeC). It is a graph about the relationship between the average particle diameter of the ferrite grain in a t / 100 position of a steel plate, and a brittle crack stop characteristic (Kca value in -60 degreeC). Sectional drawing of the groove shape in the welding in an Example is shown. The schematic diagram of the electrode arrangement | positioning at the time of FCB welding is shown. It is a graph which shows the relationship between the true strain amount and the average particle diameter of a ferrite grain in the area | region from a steel material surface to t / 100 position.

Claims (4)

C: 0.03 to 0.09% (meaning “mass%”, the same applies hereinafter), Si: 0.01 to 0.25%, Mn: 1.20 to 1.60%, P: 0.010% Or less (excluding 0%), S: 0.003% or less (not including 0%), Al: 0.02 to 0.04%, Nb: 0.005 to 0.016%, B: 0.00. 0006 to 0.0020%, N: 0.0045 to 0.0090%, Ti: 0.008 to 0.020%, respectively, satisfy the following formula (1), the balance is iron and inevitable impurities And the brittleness characterized by satisfying the following (a) to (c) when observing the metal structure of the plane parallel to the rolling direction of the steel sheet of thickness t (mm) and perpendicular to the steel sheet surface: High-tensile steel plate with excellent crack-inhibiting properties and low-temperature toughness of weld heat affected zone.
(A) The ferrite area ratio is 75% or more,
(B) The average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less,
(C) The average aspect ratio of ferrite grains at the t / 4 position is 2.0 or less.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)
{In formula, B shows B content (mass ppm).
NT is
The relationship between N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) is
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
(N-Ti / 3.4) <0 indicates NT = 0} C: 0.03 to 0.09%, Si: 0.01 to 0.25%, Mn: 1.20 to 1.60%, P: 0.010% or less (excluding 0%), S: 0.003% or less (excluding 0%), Al: 0.02 to 0.04%, Nb: 0.005 to 0.016%, B: 0.0006 to 0.0020%, N: 0.00. 0045-0.0090%, Ti: 0.008-0.020% respectively, satisfying the following formula (1), the balance being iron and inevitable impurities, and the metal of the steel sheet having a thickness t (mm) When the structure is observed, the average grain size of ferrite grains in the region from the steel sheet surface to the t / 100 position is 25 μm or less, which is excellent in brittle crack stopping characteristics and low temperature toughness of the heat affected zone of welding Tensile steel plate.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)
{In formula, B shows B content (mass ppm).
NT is
The relationship between N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) is
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
(N-Ti / 3.4) <0 indicates NT = 0} Further, Cu: 0.5% or less (not including 0%), Ni: 0.8% or less (not including 0%), and V: 0.05% or less (not including 0%) The high-tensile steel sheet according to claim 1 or 2, comprising at least one selected from the group consisting of:
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (2)
{Wherein Cu, Ni, Nb, and V represent the content (% by mass) of each element}   Furthermore, the high-tensile steel plate according to any one of claims 1 to 3, further comprising Ca: 0.003% or less (not including 0%).

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