KR20120121362A - Steel plate with excellent low-temperature toughness and welded joint fracture toughness, and its manufacturing method - Google Patents

Abstract

PURPOSE: A steel plate with superior low-temperature toughness and welding joint fracture toughness and a manufacturing method thereof are provided to enable application to a marine structure in a deep-sea region or a cold region. CONSTITUTION: A steel sheet comprises C of 0.02-0.10 mass%, Si of 0.5 mass% or less, Mn of 1.0-2.0 mass%, Ni of 0.10-1 mass%, Nb of 0.005-0.03mass%, Ti of 0.005-0.02mass%, N of 0.0030-0.065 mass%, P of 0.02mass% or less, S of 0.015 mass% or less, Al of 0.01-0.06 mass%, and Fe and inevitable impurities of the remaining amount. The tensile strength of the steel sheet is 470MPa or greater. [Reference numerals] (AA) VTre t/2 C direction(°C); (BB) Acicular ferrite fraction difference(area%)

Description

STEEL PLATE WITH EXCELLENT LOW-TEMPERATURE TOUGHNESS AND WELDED JOINT FRACTURE TOUGHNESS, AND ITS MANUFACTURING METHOD}

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a steel sheet excellent in low-temperature toughness (HAZ) fracture toughness (of base metal) and a method for producing the same, and is also excellent in low-temperature toughness and welded joint failure even when used in an offshore structure of a cold district, for example. The present invention relates to a steel sheet exhibiting toughness and a method of manufacturing the same.

In recent years, excavation and production of resources such as petroleum have shifted to the deep sea areas and cold regions of the ocean. Therefore, in such an area, steel sheets for offshore structures used for digging resources and the like are required to have excellent low temperature toughness and weld joint fracture toughness.

As said weld joint fracture toughness, the Japanese Unexamined Patent Publication No. 2001-247932 is mentioned, for example. This publication discloses that a high CTOD guaranteed low temperature steel excellent in fracture toughness of a weld heat affected zone is obtained by defining the chemical component values of Ti and N, the particle diameter of TiN, and the number of particle diameters thereof. Specifically, it is disclosed that TiN having a particle size of 0.01 to 0.1 µm is present in 5 × 10 ⁵ to 5 × 10 ⁶ particles / mm ^{2 in} the steel before welding, and TiN having a particle diameter of 0.5 µm or more is 10 pieces / cm ² or less. It is.

Japanese Patent Application Laid-Open No. 11-229077 discloses a steel sheet suitable for an offshore structure in an extreme cold and ice region, and includes (a) refinement of effective grain size, (b) reduction of island martensite and trace Nb. It is disclosed that the CTOD characteristics of the multi-layered welds can be significantly improved by simultaneously combining four improvement of grain boundary hardenability, (c) suppression of precipitation hardening, and (d) reduction of HAZ hardness. It is. Moreover, as a manufacturing condition,

(A) The steel slab is heated to 950 to 1300 ° C., rough rolling with a reduction ratio of 10 to 90% is performed in the recrystallization temperature range, and then an unrecrystallized temperature range of at least Ar ₃ point. Finish rolling with a reduction ratio of 10 to 90% at and immediately controlled cooling to 650 to 500 ° C at a cooling rate of 1 to 50 ° C / s, or air-cooling to room temperature, or

(B) The steel slab is heated to 950 to 1300 ° C., rough rolling with a reduction ratio of 10 to 90% in the recrystallization temperature range is performed, and then the reduction ratio is 10 to 90% in the unrecrystallization temperature range of at least Ar ₃ point. Finish-rolling is carried out, and it carries out controlled cooling to 200 degrees C or less at the cooling rate of 1-50 degreeC / s immediately, and tempers at 500 to 650 degreeC after that.

Japanese Patent Publication No. 6-74454 discloses a method for producing a thick high tensile strength steel sheet having excellent low temperature toughness and weldability. The prescribed steel is heated to 900 to 1150 DEG C, rolled to an intermediate stage thickness, and once rolling is stopped. and cooling, or with or without rolling cooling as slabs state, the surface temperature is boyeol (保熱) uniformly to a temperature of the steel before it drops below the Ar ₃ Ar ₃ + 150 ℃ to Ar _3, and then in the Ar ₃ or more Rolling with a reduction ratio of 50 to 70%, and after rolling, cooling to 250 to 600 ° C. at a cooling rate of 1 to 10 ° C./sec, followed by air cooling, or heating the steel to 900 to 1150 ° C. to intermediate thickness even the steel before rolling, and rolled once stopped by cooling, or cooling, without rolling the slab in the state, and the surface temperature drops below the Ar ₃ to a temperature of Ar ₃ + 150 ℃ to Ar ₃ To boyeol, then there has been proposed a method for cooling to perform the rolling of the rolling reduction of 50 to 70% or more at Ar _3, after rolling, from 250 to 600 ℃ at a cooling rate of 1 to 10 ℃ / sec, followed by air cooling.

Japanese Laid-Open Patent Publication No. 2008-266735 discloses a high-resistance-ratio high tensile strength steel sheet which is excellent in low-temperature toughness of HAZ and excellent in low-temperature toughness of a base material even when welding is performed by large heat input. as a manufacturing method, after carrying out a hot rolling, the steel sheet of (Ar ₃ transformation point -40 ℃) from a temperature exceeding by 10 ℃ / sec average cooling rate of at least (Ar ₃ transformation point -40 ℃) and cooled to a temperature below the temperature Once the cooling is performed by stopping the air cooling for 30 to 150 seconds, followed by t / 4 in: from the temperature range of a (t plate thickness), the temperature of the position (Ar ₃ transformation point ℃ -80) to (Ar ₃ transformation point -190 ℃) 350 It is disclosed to cool at an average cooling rate of 10 ° C / sec or more to a temperature range of more than 550 ° C or less.

However, in the above four patent applications, as the low-temperature toughness of the base material, it is difficult to ensure particularly low temperature toughness in the sheet thickness center part (t / 2) C direction (that is, the direction perpendicular to the rolling direction) and weld joint Increasing the fracture toughness (HAZ-CTOD characteristic) has not been examined.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to suitably use steel plates exhibiting low-temperature toughness and weld joint fracture toughness superior to conventional steel sheets (particularly, suitable for offshore structures constructed in large depth areas or cold regions). Steel sheet) and a method for producing the steel sheet.

The steel sheet of the present invention that can solve the above problems,

C: 0.02 to 0.10% (the meaning of "mass%", the same below),

Si: 0.5% or less (not including 0%),

Mn: 1.0 to 2.0%,

Ni: 0.10 to 1%,

Nb: 0.005 to 0.03%,

Ti: 0.005 to 0.02%,

N: 0.0030 to 0.065%,

P: 0.02% or less,

S: 0.015% or less, and

Al: satisfies 0.01 to 0.06%, the balance being iron and inevitable impurities,

All the conditions of the following (A)-(D) are satisfied, and tensile strength is 470 Mpa or more.

(A) Surface part, t / 4 part [t represents plate | board thickness. The same as below, and at t / 2, the minimum value (Amin) of the acyclic ferrite fraction is 50 area% or more, and the maximum value (Amax) and the lowest value of the acyclic ferrite fraction are measured. The difference of (Amin) satisfies Equation 1 below.

(B) In the surface part, t / 4 part, and t / 2 part, when the average crystal grain size of the area | region (diagonal grain) enclosed by the diagonal grain boundary whose orientation difference of two crystals is 15 degrees or more is measured, The maximum value Mmax is 40 micrometers or less, and the difference between the maximum value Mmax and the minimum value Mmin of the average grain size satisfies the following expression (2).

(C) In the surface part, t / 4 part, and t / 2 part, when hardness is measured, the difference of the maximum value of hardness (Hvmax) and the minimum value of hardness (Hvmin) satisfy | fills following formula (3).

(D) When the ultrasonic flaw test prescribed in JIS G 0901 is fully flaw detection with a detection sensitivity of +12 dB, the UT echo height of the internal defect is 50% or less.

The steel sheet is further as another element,

V: 0.5% or less (not including 0%),

B: 0.0005 to 0.003%, and

Ca: It may contain at least one element selected from the group consisting of 0.0005 to 0.003%.

In addition, the steel sheet, as another element,

Cu: 0.3% or less (not including 0%),

Cr: 0.5% or less (not including 0%), and

Mo: may contain one or more elements selected from the group consisting of 0.5% or less (not containing 0%).

The steel sheet can be used for marine structures.

This invention also prescribes the manufacturing method of the said steel plate, This manufacturing method uses the slab which satisfy | fills the said component composition, after heating to 1050 degreeC or more, 1st hot rolling, 1st cooling, 2nd hot rolling , And second cooling are sequentially performed so as to satisfy the following conditions (a) to (d), respectively.

(a) In 1st hot rolling, final pass rolling whose rolling reduction is 10% or more is performed in the state whose temperature of t / 2 part is 950 degreeC or more.

(b) By performing two-step cooling which satisfy | fills the following conditions as 1st cooling, the temperature difference of a surface part and t / 2 part shall be within 70 degreeC.

(First cooling) 0.5T sec or more at a plate thickness direction average cooling rate of 0.6 degrees C / s or more [T represents the starting plate thickness (mm) of 1st cooling. The same as below] After cooling to 1.5T seconds or less, air cooling is performed in 0.5T seconds or more and 1.5T seconds or less.

(Second Cooling) After the first cooling, the cooling is performed at 0.07T sec or more and 1.3T sec or less at a plate thickness direction average cooling rate of 0.6 ° C / s or more, followed by air cooling of 0.07T sec or more and 1.3T sec or less.

(c) In 2nd hot rolling, rolling whose temperature of t / 2 part is the temperature range of less than 950 degreeC is performed so that Formula (4) below may be satisfied.

[Equation 4,

Q: cumulative reduction ratio (%) in the temperature range of t / 2 parts less than 950 ℃,

Ni: Ni content (mass%),

Nb: Nb content (mass%) is shown.

In addition, a reduction ratio is calculated | required by following formula (5).

(d) a second cooling, from the temperature region where the surface temperature portion not less than Ar ₃ transformation point, t / 2 and the cooling by an average cooling rate of the sheet thickness direction satisfying the equation (6) to a temperature of not more than 500 ℃ temperature region portion.

[Equation 6, t represents the final product sheet thickness (mm).

In addition, the plate | board thickness direction average cooling rate is calculated | required from following formula (7).

In Equation (7), θs is the plate thickness direction average temperature (° C) at the start of cooling, θf is the plate thickness direction average temperature (° C) at the cooling stop, and τ represents the cooling time (s).]

According to the present invention, since the structure and the hardness are constant without being influenced by the plate thickness direction, and defects inside the steel sheet are suppressed, it has both low temperature toughness and weld joint fracture toughness superior to conventional steel sheets. The steel sheet of this invention is used suitably especially for offshore structures constructed in a large depth area or a cold region.

FIG. 1 is a diagram showing the relationship between the acubic ferrite fraction difference (Amax-Amin) and the vTrs in the t / 2 part C direction.
Fig. 2 is a diagram showing the relationship between the diagonal grain size difference (Mmax-Mmin) and the vTrs in the t / 2 part C direction.
3 is a graph showing the relationship between the hardness difference (Hvmax-Hvmin) and the limit CTOD value (-10 ° C) of the HAZ part.
It is a figure which shows the relationship between the echo height in the ultrasonic flaw test and the limit CTOD value (-10 degreeC) of a HAZ part.
5 is a view schematically showing a manufacturing process of the present invention.
It is a figure which shows the relationship between the reduction ratio of the final pass rolling in 1st hot rolling, and the echo height in an ultrasonic flaw test.
FIG. 7 is a diagram showing the relationship between the temperature difference and the hardness difference (Hvmax-Hvmin) of the surface portion (after the first cooling) and the t / 2 portion.
8 is a diagram showing the relationship between the first cooling coefficient and the temperature difference between the surface portion (after the first cooling) and the t / 2 portion.
9 is a diagram showing a relationship between the second cooling coefficient and the temperature difference between the surface portion (after the first cooling) and the t / 2 portion.
FIG. 10 is a diagram showing the relationship between the left-side value [Q + (Ni + Nb) × 10] of the equation (4) and the acoustic ferrite fraction difference (Amax-Amin).
Fig. 11 is a graph showing the relationship between the left side value [Q + (Ni + Nb) × 10] and the diagonal grain size difference (Mmax-Mmin) of the expression (4).
It is a figure which shows the sampling position of a test piece in an Example, and the cross section shown by the diagonal part is an optical microscope observation surface and an EBSP observation surface.
13 is a view showing a welding form in the embodiment.

MEANS TO SOLVE THE PROBLEM This inventor earnestly researched so that the low temperature toughness and weld joint fracture toughness, specifically, the steel plate which satisfy | fills following (I) and (II) may be obtained.

(I) As the excellent low-temperature toughness of the base material, vTrs in the sheet thickness center part (t / 2) C direction measured in the below-mentioned Example is -100 degrees C or less (preferably -110 degrees C or less, More preferably, -120 degreeC) Or less, more preferably -130 ° C or less).

(II) As an excellent weld joint fracture toughness, the limit CTOD value (-10 ° C.) of the HAZ portion measured in the Examples described later (hereinafter, this characteristic may be referred to as “HAZ-CTOD characteristic”) is 0.46 mm or more ( Preferably at least 0.6 mm, more preferably at least 0.8 mm, even more preferably at least 1 mm, particularly preferably at least 1.2 mm).

In addition, in this invention, it is important to evaluate the toughness of t / 2 part as a brittle crack generation characteristic that the evaluation position of the low-temperature toughness of said (I) made especially "plate thickness center part (t / 2) C direction". In addition, although it is known that vTrs has a value of + 20 ° C higher in the C direction than the L direction in the t / 2 part, in the present invention, the evaluation is more stringent than the L direction (that is, the safety aspect is evaluated). By evaluating in the C direction, it is for obtaining the steel plate which shows the outstanding low-temperature toughness certainly.

And the present inventor first finds out that it is necessary to make uniform the structure | tissue, hardness, etc. in the plate thickness direction mainly, in order to achieve the both characteristics of said (I), (II), and further studies this in detail. As a result, it was found that all the conditions of the above-mentioned (A) to (D) may be satisfied. Hereinafter, the relationship between the above two characteristics and the conditions of (A) to (D) will be described in detail.

First, in order to satisfy the above (I), it was found that the following (A) and (B) may be satisfied.

(A) At the surface portion, t / 4 portion, and t / 2 portion, when the acyclic ferrite fraction was measured, the minimum value (Amin) of the acyclic ferrite fraction was 50 area% or more, and the highest of the acyclic ferrite fraction. The difference between the value Amax and the minimum value Amin satisfies Equation 1 below.

[Equation 1]

Amax-Amin ≤ 20 area%

(B) In the surface part, t / 4 part, and t / 2 part, when the average crystal grain size of the area | region (diagonal grain) enclosed by the diagonal grain boundary whose orientation difference of two crystals is 15 degrees or more is measured, The maximum value Mmax is 40 micrometers or less, and the difference between the maximum value Mmax and the minimum value Mmin of the average grain size satisfies the following expression (2).

&Quot; (2) "

Mmax-Mmin <40 μm

The above (A) will be described.

In the present invention, in order to attain uniformity of the structure in the entire sheet thickness direction, it is assumed that the acyclic ferrite is likely to be obtained under a wide range of manufacturing conditions. That is, the steel sheet of the present invention has a structure of an acicular ferrite main body without being influenced by the plate thickness direction.

In the conventional steel sheet, even though the structure of the surface portion and the t / 4 portion is an acicular ferrite main body, since the t / 2 portion is a separate structure (that is, the acicular ferrite is less than 50 area%), there is a difference in the structure in the sheet thickness direction. This is considered to have been one of the inhibitors of the low temperature toughness improvement of the base metal. In the present invention, not only the surface portion and the t / 4 portion of the steel sheet, but also the t / 2 portion is an acicular ferrite main body (50 area% or more), which is a structure of the acicular ferrite main body without being influenced by the plate thickness direction (that is, the plate). By reducing the difference in the cyclic ferrite fraction in the thickness direction, the low-temperature toughness in the t / 2 part C direction, which is said to be most difficult to secure, was secured.

FIG. 1 shows the surface portion, t / 4 portion, and t / 2 portion by using the results of the examples described below (hereinafter, FIGS. 2 to 4, 6 to 11 are also summarized using the results of the examples). The relationship between the difference between the maximum value and the minimum value of the acoustic ferrite fraction (Amax-Amin, the difference between the acoustic ferrite fraction) and the vTrs in the t / 2-part C direction is summarized. By setting it as 20 area% or less, it turns out that vTrs <=-100 degreeC can be achieved. (Amax-Amin), Preferably it is 15 area% or less, More preferably, it is 10 area% or less.

On the other hand, the minimum value (Amin) of the acyclic ferrite fraction is 50 area% or more, preferably 55 area% or more, and more preferably 60 area% or more.

Although this invention prescribes that it is an acicular ferrite main body (50 area% or more) also in any position in the plate | board thickness direction, regardless of other structure, structure, such as upper bainite, may exist, for example.

Next, (B) will be described.

In the single-phase structure, which is mainly composed of the above-described acyclic ferrite, the grain boundary is considered to be the resistance of crack propagation. In addition, when the frequency of the grain boundary and the crack collide with each other at the time of crack propagation, it is considered that the crack propagation can be suppressed. That is, it is thought that what is necessary is just to increase the frequency of collision with a crack by making a grain boundary fine. However, in an incineration grain boundary where the two grain boundary orientations forming grain boundaries are small (for example, less than 15 °), the grain boundary energy is small and the effect thereof is small. Therefore, a diagonal grain boundary (large-angle boundary) having a direction difference of 15 ° or more is used. It is necessary to refine the grain size (diagonal grain size) of the crystal grains (diagonal grains) surrounded by the diagonal grain boundaries.

From this point of view, in the present invention, the maximum value Mmax of the diagonal grain sizes of the surface portion, t / 4 portion, and t / 2 portion is 40 µm or less. Mmax becomes like this. Preferably it is 35 micrometers or less, More preferably, it is 30 micrometers or less.

In the present invention, it is also possible to ensure low temperature toughness in the entire sheet thickness direction by ensuring uniform miniaturization of diagonal grains over the plate thickness direction, and to secure excellent low temperature toughness in the t / 2 part C direction. .

Fig. 2 shows the difference between the maximum and minimum values (Mmax-Mmin, diagonal grain size difference) of the average grain size of the diagonal grains at the surface portion, t / 4 portion, and t / 2 portion, and the t / 2 portion C direction. A diagram showing the relationship between vTrs. From this FIG. 2, it can be seen that vTrs ≦ −100 ° C. can be achieved by setting (Mmax-Mmin) to less than 40 μm. (Mmax-Mmin), Preferably it is 35 micrometers or less, More preferably, it is 30 micrometers or less, More preferably, it is 20 micrometers or less.

Next, in order to satisfy said (II), it discovered that what is necessary is to satisfy following (C) and (D).

(C) In the surface part, t / 4 part, and t / 2 part, when hardness is measured, the difference of the maximum value of hardness (Hvmax) and the minimum value of hardness (Hvmin) satisfy | fills following formula (3).

&Quot; (3) &quot;

Hvmax-Hvmin ≤ 50

(D) When the ultrasonic flaw test specified in JlS G 0901 is fully flawed with a detection sensitivity of +12 dB, the UT echo height of the internal defect is 50% or less.

The above (C) will be described first.

If the hardness difference between the surface portion and the inside of the steel sheet is large, stress concentration occurs there, and fracture toughness (HAZ-CTOD characteristic) is deteriorated. Therefore, it is good to make the difference in hardness between a surface part and an inside small.

3 shows the difference between the maximum value Hvmax and the minimum value Hvmin of the hardness of the surface portion, t / 4 portion, and t / 2 portion (hardness difference, Hvmax-Hvmin), and the limit CTOD value of the HAZ portion (-10 ° C). Figure 3 shows the relationship of), but it can be seen from this FIG. 3 that the limit CTOD value (-10 ° C) of the HAZ portion can be at least 0.46 mm by setting the hardness difference to 50 or less. The said hardness difference becomes like this. Preferably it is 40 or less, More preferably, it is 30 or less.

Next, the above (D) will be described.

If the UT echo height of the internal defect is large, that is, if there is a large defect, stress concentration occurs there, and fracture toughness is lowered.

Fig. 4 is a diagram showing the relationship between the echo height in the ultrasonic flaw test and the limit CTOD value (−10 ° C.) of the HAZ part, but from this Fig. 4, when the echo height by the UT is 50% or less, the limit of the HAZ part is shown. It can be seen that the CTOD value (-10 ° C): 0.46 mm or more can be achieved. The echo height by the said UT becomes like this. Preferably it is 45% or less, More preferably, it is 40% or less.

[Manufacturing method]

As described above, the steel sheet of the present invention needs to uniform the structure in the sheet thickness direction such as (Amax-Amin), (Mmax-Mmin), and (Hvmax-Hvmin), but is a thick material. In this case, since the temperature difference in the plate thickness direction at the time of rolling becomes large, the said uniformization becomes difficult. This invention also examined the means for satisfying all of said (A)-(D) and obtaining a steel plate which has the tensile strength of 470 Mpa or more under such a situation.

As a result, in a manufacturing process, after heating to 1050 degreeC or more using the slab which satisfy | fills the component composition mentioned later, especially the conditions which prescribed | regulated 1st hot rolling, 1st cooling, 2nd hot rolling, and 2nd cooling It was found that it was necessary to carry out sequentially so as to satisfy (a) to (d).

Hereinafter, it demonstrates in order of a manufacturing process.

First, as shown in FIG. 5, the slab is heated to 1050 ° C. or higher, but this is for the purpose of completely solidifying Nb effective for the formation of the amorphous ferrite and miniaturization of crystal grains, and for making the structure an austenite single phase. do. Preferably the said heating temperature is 1100 degreeC or more, but an upper limit is about 1200 degreeC.

<(a) First Hot Rolling>

In the first hot rolling (rough rolling), final pass rolling with a reduction ratio of 10% or more is performed in a state where the temperature of the t / 2 part is 950 ° C or higher. 6 is a diagram showing a relationship between the reduction ratio and the echo height by the UT. From this FIG. 6, it can be seen that by setting the reduction ratio to 10% or more, internal defects are compressed to achieve an echo height of UT defined by the above (D): 50% or less. Thus, by achieving the echo height by UT: 50% or less, as shown in FIG. 4, HAZ-CTOD characteristic can be improved. The reduction ratio is preferably 12% or more, and more preferably 15% or more.

The upper limit of the reduction ratio is about 20% from the viewpoint of securing the cumulative reduction ratio during the second hot rolling.

On the other hand, the other conditions in the first hot rolling are not limited, and the pass rolling conditions before the final step are not particularly limited.

<(b) First cooling>

As the first cooling, by performing two-stage cooling that satisfies the following conditions, the temperature difference after the first cooling and the t / 2 part (hereinafter, simply referred to as "the temperature difference between the surface part and t / 2 part" Yes) within 70 ℃. In addition, what is necessary is just to perform this 1st cooling following 1st hot rolling (rough rolling), and although the start temperature of the following 2 step cooling is not specifically asked, it exists in the range of about 900-950 degreeC as surface temperature.

(First cooling) 0.5T or more at a plate thickness direction average cooling rate of 0.6 ° C / s or more [T is the starting plate thickness of the first cooling (mm)] 1.5T or less After cooling, air cooling is 0.5T or more and 1.5T Seconds or less.

(Second Cooling) After the first cooling, the cooling is performed at 0.07T sec or more and 1.3T sec or less at a plate thickness direction average cooling rate of 0.6 ° C / s or more, followed by air cooling of 0.07T sec or more and 1.3T sec or less.

As shown in FIG. 3, in order to sufficiently increase the HAZ-CTOD characteristic, it is necessary to achieve Hvmax-Hvmin ≦ 50. In the present invention, studies have been made to achieve the definition of the hardness difference. First, it has been found that the temperature difference between the surface portion and the t / 2 portion may be within 70 ° C in the first cooling.

FIG. 7 is a diagram showing the relationship between the temperature difference between the surface portion and the t / 2 portion, and the hardness difference (Hvmax-Hvmin). 7 shows that in order to make the hardness difference 50 or less, it is necessary to make the temperature difference between the surface portion and the t / 2 portion within 70 ° C. The temperature difference between the surface portion and the t / 2 portion is preferably within 65 ° C., more preferably within 60 ° C., so that the hardness difference can be made smaller, and as a result, better HAZ-CTOD characteristics can be ensured. have. In addition, in order to make Amin, (Amax-Amin), Mmax, and (Mmax-Mmin) within the prescribed range, it is effective to make the temperature difference of the surface part and t / 2 part within 70 degreeC in this 1st cooling.

In addition, the present inventors also examined specific means for bringing the temperature difference between the surface portion and the t / 2 portion within 70 ° C. As a result, it was found that the cooling (first cooling) after the first hot rolling (rough rolling) may be two-step cooling as described above. In addition, in each of the first cooling and the second cooling in the two-stage cooling, by defining the cooling rate and the cooling time, the surface portion and the t / 2 portion can be shortened without causing coarsening of crystal grains. It was found that the temperature difference can be within 70 ° C.

First, the first cooling is 0.5T or more and 1.5T or less at a plate thickness direction average cooling rate of 0.6 ° C / s or more (hereinafter, the cooling rate at this time may be represented by “C ₁₁ ”), followed by air cooling. 0.5T or more and 1.5T or less. The plate | board thickness direction average cooling rate is a cooling rate calculated | required by the method shown in the Example mentioned later (it is the same below).

If C ₁₁ is less than 0.6 ° C / s, it is difficult to bring the temperature difference between the surface portion and the t / 2 portion within 70 ° C within the prescribed time. Preferably it is 0.7 degreeC / s or more, More preferably, it is 0.8 degreeC / s or more, More preferably, it is 1.0 degreeC / s or more. On the other hand, if the C ₁₁ is too high, the scale on the surface of the steel sheet part is generated, the cooling efficiency is lowered. Therefore, in order to prevent this, the upper limit of C ₁₁ is about 5.0 ° C / s.

C _11: 0.6 ℃ / s cooling time (cooling time ₁₁ C) of at least second or more is not more than 0.5T 1.5T seconds.

Fig. 8 shows the relationship between the value of (C ₁₁ cooling time / starting plate thickness of the first cooling) (this ratio is referred to as the “first cooling coefficient”) and the temperature difference between the surface portion and the t / 2 portion in the first cooling. It is a figure which shows. 8 shows that in order to make the temperature difference of a surface part and a t / 2 part into 70 degreeC, it is necessary to make the 1st cooling coefficient into 0.5 or more. The first cooling coefficient is preferably 0.8 or more, more preferably 1.0 or more.

On the other hand, even if the first cooling coefficient is too large, that is, even if the cooling time is too long, oxides are formed on the surface of the steel sheet and cooling efficiency is lowered, and the temperature difference between the surface portion and the t / 2 portion cannot be within 70 ° C. The upper limit of the cooling coefficient is 1.5.

C _11: after performing the cooling by more than 0.6 ℃ / s performs the air cooling below 1.5T 0.5T seconds or more seconds. By reheating by performing this air cooling, the oxide produced on the steel plate surface can be minimized and it can cool efficiently.

The reason for determining the first cooling coefficient in this air cooling is as described above. On the other hand, in order to make sure that the temperature difference between the surface portion and the t / 2 portion is within 70 ° C in a short time, the air cooling time should be equal to or higher than the cooling time at C _11. Therefore, the first cooling coefficient in air cooling is 0.6 ° C. It is good to make it the same as the 1st cooling coefficient at the time of cooling more than / s, or more (but an upper limit is 1.5).

Following the first cooling, second cooling is performed.

The second cooling is 0.07T seconds or more and 1.3T seconds or less at a plate thickness direction average cooling rate of 0.6 ° C / s or more (hereinafter, the cooling rate at this time may be represented by “C ₁₂ ”), and then air cooling is 0.07 T seconds or more and 1.3T seconds or less.

The reason why C ₁₂ was 0.6 ° C / s or more is the same as that of the first cooling.

_C12 : The cooling time ( _C12 cooling time) in 0.6 degreeC / s or more is 0.07T second or more and 1.3T second or less.

Fig. 9 shows the relationship between the value of (starting plate thickness of C ₁₂ cooling time / first cooling) (this ratio is referred to as the “second cooling coefficient”) and the temperature difference between the surface portion and the t / 2 portion in the second cooling. It is a figure which shows. From this FIG. 9, in order to make the temperature difference of a surface part and t / 2 part into 70 degreeC, it is understood that a 2nd cooling coefficient needs to be 0.07 or more. The second cooling coefficient is preferably 0.1 or more, more preferably 0.2 or more.

On the other hand, even if the second cooling coefficient is excessively large, that is, even if the cooling time is too long, oxides are formed on the surface of the steel sheet and cooling efficiency is lowered, and the temperature difference between the surface portion and the t / 2 portion cannot be within 70 ° C. The upper limit is 1.3.

_C12 : After cooling to 0.6 degreeC / s or more, air cooling is performed 0.07T second or more and 1.3T second or less. By performing this air cooling and regenerating, the oxide produced | generated on the steel plate surface can be minimized and it can cool efficiently.

The reason for determining the second cooling coefficient in this air cooling is as described above. On the other hand, in order to make the temperature difference between a surface part and t / 2 part into within 70 degreeC for a short time certainly, it is good to make air cooling time here also equal or more than cooling time in _C12 . Therefore, the second cooling coefficient in air cooling may be the same as or higher than the second cooling coefficient at the time of cooling above 0.6 ° C / s (but the upper limit is 1.3).

In the present invention, as described above, the focus is on performing the second cooling after the first cooling. By repeatedly performing (cooling + air cooling at 0.6 degrees C / s or more) twice, the oxide film produced | generated on the surface of a steel plate at the time of air cooling (reheating) in 1st cooling can be removed, and cooling efficiency can be improved. As a result, the total cooling time is the same as the one time only (cooling + air cooling at 0.6 ° C / s or more), but since the cooling efficiency is higher, the temperature of the surface portion and the t / 2 portion is not caused without causing coarsening of crystal grains. The difference can be made within 70 degreeC.

On the other hand, in Japanese Patent Laid-Open No. 11-229077 and Japanese Patent Laid-Open No. 2008-266735, the same cooling method is not described, and from the viewpoint of productivity, it is usually only one cooling of 0.6 ° C / s or more. I think. As a result, in these prior arts, it is considered that uniformity of the structure | tissue of a plate | board thickness direction, etc. is not aimed at.

In addition, although the concept of making the temperature difference of t / 2 small from the plate thickness surface layer part is the same as the manufacturing method of Unexamined-Japanese-Patent No. 6-74454, in this manufacturing method, the means which keeps a steel piece warm is taken. However, when heat insulation is performed in this way, it takes time for the temperature difference of t / 2 to become small from a plate thickness surface layer part. In other words, it is assumed that the particles are coarsened at high temperature for a long time, thereby causing coarsening of the crystal grains.

<(c) Second Hot Rolling>

In 2nd hot rolling (finish rolling), rolling in which the temperature of t / 2 part is less than 950 degreeC is performed so that the following formula (4) may be satisfied.

&Quot; (4) &quot;

Q + (Ni + Nb) × 10 ≥ 33

[Equation 4,

Q: cumulative reduction ratio (%) in the temperature range of t / 2 parts less than 950 ℃,

Ni: Ni content (mass%),

Nb: Nb content (mass%) is shown.

In addition, a reduction ratio is calculated | required by following formula (5).

[Equation 5]

Rolling ratio = 100 × (thickness before rolling start-rolling completed thickness) / thickness before rolling start]

By performing 2nd hot rolling (finishing rolling) on the said conditions, the whole sheet thickness direction can form an acyclic ferrite, and the definition of the acyclic ferrite prescribed | regulated by (A) can be achieved. Moreover, the provision of the diagonal grains prescribed | regulated by (B) can be achieved.

FIG. 10 is a diagram showing the relationship between the left side value [Q + (Ni + Nb) × 10] of the equation (4) and the acoustic ferrite fraction difference (Amax-Amin). From this figure, the left side value of the equation (4) is 33 The cumulative reduction in the temperature range of the t / 2 part is less than 950 ° C so that the second hot rolling is carried out so as to be equal to or higher (that is, depending on the amount of Ni and the amount of Nb in the steel, the left side value of Equation 4 is 33 or more). By adjusting the rate, it can be seen that (Amax-Amin) can be 20 area% or less.

11 is a figure which shows the relationship between the left side value of Formula (4), and the difference (Mmax-Mmin, diagonal grain size difference) of the maximum value and minimum value of diagonal grain size, From this figure, the left side value of Formula (4) By performing 2nd hot rolling so that it may become 33 or more, it turns out that (Mmax-Mmin) can be made into less than 40 micrometers.

The left side value of the expression (4) is preferably 40 or more, more preferably 50 or more.

The other conditions in the second hot rolling (finishing rolling) are not particularly limited, and conditions generally performed can be adopted.

In the present invention, after the second hot rolling, the second cooling is further performed to satisfy the following condition (d), whereby a certain strength or more can be secured.

<(d) second cooling>

As the second cooling, the plate thickness direction average cooling rate satisfying the following expression (6) from the temperature range where the temperature of the surface portion is equal to or higher than the Ar ₃ transformation point, and the temperature range where the temperature of the t / 2 portion is 500 ° C or lower (hereinafter, cooling at this time) The speed may be indicated by “C ₂ ”).

&Quot; (6) &quot;

Plate thickness direction average cooling rate ≥ 6420t ^-1.60

[Equation 6, t represents the final product sheet thickness (mm).

In addition, the plate | board thickness direction average cooling rate is calculated | required from following formula (7).

[Equation 7]

Plate thickness direction average cooling rate (℃ / s) = (θs-θf) / τ

In Equation (7), θs is the plate thickness direction average temperature (° C) at the start of cooling, θf is the plate thickness direction average temperature (° C) at the cooling stop, and τ represents the cooling time (s).]

In the present invention, the cooling rate (C ₂ ) from the temperature region where the second hot rolling is performed under the above conditions, and the temperature of the surface portion is equal to or higher than the Ar ₃ transformation point, from the temperature region where the temperature of the t / 2 portion is 500 ° C. or lower, By setting the average cooling rate in the sheet thickness direction that satisfies Equation 6, the hardness value is stable within a wide cooling rate range (the cooling rate is in the range of about 230 ° C / s), and a certain strength or more (470 MPa or more, preferably 510 MPa or more) Tensile strength) can be securely ensured.

After the 2nd cooling (that is, after cooling to the temperature range where the temperature of t / 2 part is 500 degrees C or less on the said conditions), you may cool by air cooling etc., without asking in particular about cooling to room temperature.

The above description is schematically shown in FIG. 5.

Since the steel sheet of this invention exhibits the characteristics of said (I), (II), even if it does not heat-process (quenching, tempering) etc. after that, it can be used as it was cooled.

Although the steel sheet of this invention has the point which ensures the structure | tissue etc. which were specifically manufactured by the said conditions and prescribed | regulated so that both the low-temperature toughness of a base material and the HAZ-CTOD characteristic may be provided, the said structure etc. will be ensured securely, In order to exhibit both characteristics sufficiently and to have weldability, high strength, etc. as a steel plate used, for example, for marine structures etc., it is necessary to satisfy the following component composition.

[C: 0.02 to 0.10%]

C is an element which cannot be neglected in order to secure the strength of steel materials (base materials). In order to exhibit such an effect, it is necessary to contain 0.02% or more. It is preferable to contain C 0.04% or more, More preferably, it is 0.05% or more. However, if C exceeds 0.10%, a large amount of island-like martensite (MA) is generated in the HAZ during welding, which not only causes the toughness of the HAZ but also adversely affects the weldability. Moreover, in addition to making it difficult to secure an acyclic ferrite, the difference in hardness and the diagonal grain size difference become large. Therefore, C is 0.10% or less, preferably 0.08% or less, and more preferably 0.06% or less.

(Si: 0.5% or less (not including 0%)]

Si is an element which contributes to securing the strength of steel by solid solution strengthening. However, if Si exceeds 0.5%, not only does a large amount of phase martensite (MA) be generated in the HAZ during welding, but also causes deterioration of the HAZ toughness and adversely affects weldability. Therefore, Si is made into 0.5% or less. Preferably it is 0.3% or less, More preferably, it is 0.2% or less, More preferably, it is 0.18% or less. On the other hand, in order to ensure the strength of the steel by adding Si, it is preferable to contain 0.02% or more, more preferably 0.05% or more, and still more preferably 0.1% or more.

[Mn: 1.0 to 2.0%]

Mn is an element which contributes to the strength improvement of steel materials (base materials). In order to exhibit these effects effectively, it is necessary to contain 1.0% or more. Mn is preferably 1.2% or more, more preferably 1.4% or more. However, when it exceeds 2.0%, the weldability of steel materials (base material) will deteriorate. Therefore, Mn needs to be suppressed to 2.0% or less. Preferably it is 1.8% or less, More preferably, you may be 1.6% or less.

[Ni: 0.10 to 1%]

Ni is an element that contributes to improving the toughness of the steel itself, as well as increasing the strength of the steel. In addition, there is a function of shifting the transformation start temperature to the low temperature side for a long time, which promotes the acyclic ferriteization of the tissue. Moreover, by containing Ni and increasing hardenability, the influence of the difference of the cooling rate in a plate | board thickness direction can be made small, and as a result, the hardness difference in a plate | board thickness direction can be made small. In order to exhibit such an effect effectively, it is preferable to contain 0.10% or more. More preferably, it is 0.12% or more, More preferably, it is 0.14% or more. Although it is preferable to contain Ni as much as possible, since it is an expensive element, when it contains too much, it becomes expensive. Therefore, for economic reasons, the upper limit is preferably 1%. More preferably, it is 0.8% or less, More preferably, it is 0.6% or less.

[Nb: 0.005 to 0.03%]

Nb suppresses the coarsening of recrystallized grains by the two effects of the solute drag effect by solid solution and the pinning effect by carbonitride precipitation, and contributes to the improvement of base metal toughness. Moreover, like Ni, it has a function of shifting the transformation start temperature to the long-term and low-temperature side, which promotes the cyclic ferrite of the structure. In addition, although Nb is an effective element for forming Nb precipitates to enhance precipitation, when the amount of Nb is small and the amount of Nb precipitates is small, the surface portion has a high cooling rate, thereby ensuring high strength (hardness), but t The / 2 part has a low cooling rate and low Nb precipitates, so the strength (hardness) is low, resulting in a difference in hardness between the surface part and the t / 2 part. From this, Nb is contained 0.005% or more. Preferably it is 0.007% or more, More preferably, it is 0.009% or more. However, when Nb exceeds 0.03%, the base material toughness and the HAZ toughness deteriorate. Therefore, Nb is made into 0.03% or less. Preferably it is 0.025% or less, More preferably, it is 0.02% or less.

[Ti: 0.005 to 0.02%]

Ti is an element that forms nitrides such as TiN and Ti oxides in steel and contributes to the improvement of HAZ toughness. In order to exhibit such an effect, it is necessary to contain Ti 0.005% or more. Preferably it is 0.007% or more, More preferably, you may be 0.010% or more. However, when Ti is excessively contained, the toughness of the steel (base material) deteriorates, and therefore Ti must be suppressed to 0.02% or less. Ti is preferably 0.018% or less, and more preferably 0.016% or less.

(N: 0.0030 to 0.065%)

N is an element that precipitates nitride (for example, TiN, etc.), and the nitride prevents coarsening of austenite particles generated in HAZ during welding, thereby promoting ferrite transformation and improving HAZ toughness. Contribute. In order to exhibit these effects effectively, it is necessary to contain 0.0030% or more. N is preferably at least 0.0035%, more preferably at least 0.004%. As N increases, the Ti-containing nitride is formed to promote the miniaturization of the austenite particles, so that N effectively acts to improve the toughness of the HAZ. However, when N exceeds 0.065%, the amount of solid solution N increases, the toughness of the base material itself deteriorates, and HAZ toughness also falls. Therefore, N needs to be suppressed to 0.065% or less. Preferably it is 0.055% or less, More preferably, you may be 0.045% or less.

(P: 0.02% or less (not including 0%)]

P is an element which is easy to segregate, and in particular, it segregates at grain boundaries in steel materials and deteriorates the toughness of the base material. Therefore, P needs to be suppressed to 0.02% or less. P becomes like this. Preferably it is 0.018% or less, More preferably, you may be 0.015% or less.

(S: 0.015% or less (not including 0%)]

S is a harmful element that combines with Mn to form sulfide (MnS) and deteriorates the toughness of the base material and the ductility in the sheet thickness direction. Therefore, it is necessary to suppress S to 0.015% or less. Preferably it is 0.012% or less, More preferably, it is 0.008% or less, More preferably, it is 0.006% or less.

[Al: 0.01 to 0.06%]

Al is a useful element for deoxidation and is also an effective element for forming AlN and miniaturizing grains. In order to exert such an effect, it is necessary to make Al amount 0.01% or more. Preferably it is 0.02% or more, More preferably, it is 0.03% or more. However, if excessive, the base metal toughness and the HAZ toughness deteriorate, so Al needs to be suppressed to 0.06% or less. Al is preferably 0.04% or less, and more preferably 0.035% or less.

The steel material of this invention contains the said element as an essential component, and remainder consists of iron and an unavoidable impurity. Unavoidable impurities may be contained to such an extent that they do not deteriorate various properties of the steel, and for example, Mg, As, Se, or the like may be contained in total of about 0.1% or less, preferably 0.09% or less.

The steel of the present invention is also effective to further contain, as another element, an element (V, B, Ca) for further improving HAZ toughness or an element (Cu, Cr, Mo) for improving the strength of the steel. The details are as follows.

[V: 0-5% or less (not including 0%), B: 0.0005-0.003%, and Ca: at least one element selected from the group consisting of 0.0005-0.003%]

V, B, and Ca are all elements which improve HAZ toughness.

When it contains V, it is preferable to contain 0.002% or more. More preferably, it is 0.005% or more, More preferably, it is 0.01% or more. On the other hand, when the amount of V exceeds 0.5%, precipitated carbonitrides coarsen and the base metal toughness deteriorates. Therefore, the amount of V is preferably made 0.5% or less. More preferably, it is 0.1% or less, More preferably, it is 0.05% or less.

B is an element which suppresses formation of grain boundary ferrite and improves HAZ toughness. In order to exhibit such an effect, it is preferable to contain 0.0005% or more. More preferably, it is 0.0010% or more. However, when the amount of B exceeds 0.003%, it precipitates as BN in an austenite grain boundary, and causes the fall of HAZ toughness. Therefore, the amount of B is preferably made 0.003% or less. More preferably, it is 0.002% or less.

When Ca is added, the TiN formation temperature is lowered, so that fine TiN is precipitated and HAZ toughness is improved. Further, by Sikkim containing Ca, to suppress the formation of coarse (粗大) a precipitate (crude TiN for the Al ₂ O ₃ as a core), can improve the HAZ toughness.

In order to exhibit such an effect by Ca effectively, it is preferable to contain 0.0005% or more. More preferably, it is 0.0010% or more. However, when the Ca amount exceeds 0.003%, extra inclusions are precipitated, leading to a decrease in HAZ toughness. Therefore, the amount of Ca is preferably made 0.003% or less. More preferably, it is 0.002% or less.

These elements may be contained individually or in plurality.

[Cu: 1% or less selected from the group consisting of 0.3% or less (does not contain 0%), Cr: 0.5% or less (does not contain 0%), and Mo: 0.5% or less (does not contain 0%) More than one element]

Cu, Cr, and Mo are all elements that contribute to increasing the strength of the steel, Cu is an element that enhances the strength of the steel by solid solution strengthening, and Cr, Mo is an element that improves the hardenability to increase the strength of the steel. In order to exhibit these effects, it is good to set it as the following content, respectively.

It is preferable to contain Cu 0.05% or more. More preferably, it is 0.1% or more, More preferably, it is 0.2% or more. However, when it contains exceeding 0.3%, since the toughness of steel materials deteriorates, it is preferable to make Cu 0.3% or less. More preferably, it is 0.28% or less, More preferably, it is 0.25% or less.

It is preferable to contain Cr 0.1% or more. More preferably, it is 0.2% or more, More preferably, it is 0.25% or more. However, when Cr amount exceeds 0.5%, the hardenability of the steel (base material) is significantly increased, and thus the base metal toughness is deteriorated, and the HAZ toughness is also lowered by the production of MA (phase martensite) or the like. Therefore, Cr is preferably 0.5% or less. More preferably, it is 0.4% or less, More preferably, it is 0.3% or less.

It is preferable to contain Mo 0.1% or more. More preferably, it is 0,2% or more, More preferably, it is 0.3% or more. However, when the Mo content exceeds 0.5%, as in the case of Cr, the toughness of the steel (base material) is significantly increased, and thus the base metal toughness deteriorates, and the formation of MA (phase martensite) or the like causes HAZ. Toughness is also lowered. Therefore, it is preferable to make Mo 0.5% or less. More preferably, it is 0.45% or less, More preferably, it is 0.4% or less.

These elements may be contained individually or in plurality.

In particular, when the present invention is a relatively thick steel sheet (a steel sheet having a sheet thickness of, for example, about 30 to 100 mm) that is difficult to homogenize, for example, in the structure of the sheet thickness direction, the effects of the present invention are fully exhibited.

Moreover, since the steel plate of this invention is excellent in low-temperature toughness and weld joint fracture toughness as mentioned above, it is used suitably for the marine structure built, for example in a cold district etc. It can also be used for shipbuilding.

[Example]

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited by the following Example of course, Of course, it carries out by changing suitably to the range which may be suitable for the purpose of the previous and the later. If possible, they are included in the technical scope of the present invention.

Various steel plates of the chemical composition shown in Table 1 obtained by melting the steel and solidifying were used to produce various steel sheets under the production conditions shown in Fig. 5 and Tables 2 and 3 above. In addition, the air cooling time at the time of the 1st cooling in 1st cooling is made the same as each C ₁₁ cooling time shown in Table 2, and the air cooling time at the time of 2nd cooling in 1st cooling is each C shown in Table 2 ₁₂ cooling time was carried out. In addition, the temperature of each part of the plate | board thickness direction in rolling was measured by the following method.

[Method for Measuring Temperature of Rolled Sheet Thickness Direction]

1. Using the process computer, the heating temperature of the position from the surface of a steel piece to the back surface is computed based on the atmospheric temperature and refurbish time from a heating start to a heating end.

2. Using the calculated heating temperature, the rolling temperature at any position in the sheet thickness direction is suitable for the calculation of the difference method, etc., based on the rolling pass schedule during rolling and the data of the cooling method (water cooling or air cooling) between the passes. Rolling is performed while calculating using the method.

3. The surface temperature of the steel sheet is measured using a radial thermometer placed on a rolling line. However, the theoretical value is also calculated by the process computer.

4. Calculation which calculates the surface temperature of the steel plate measured at the time of the start of the 1st hot rolling (rough rolling), the completion of the 1st hot rolling (rough rolling), and the 2nd hot rolling (finishing rolling), respectively from a process computer. Contrast with temperature.

5, when the difference between the calculated temperature and the measured temperature is ± 30 ° C or more, the measured surface temperature is replaced with the calculated surface temperature to be the calculated temperature on the process computer, and if less than ± 30 ° C, the calculated temperature calculated by the process computer Use it as it is.

6. The rolling temperature of the area | region made into a control object is managed using the calculated calculation temperature.

The "plate thickness direction average temperature" in a manufacturing process is an average value of the temperature of the position from the surface of the steel piece calculated | required by the said method to the back surface.

In addition, the plate | board thickness direction average cooling rate is calculated | required from following formula (7).

[Equation 7]

Plate thickness direction average cooling rate (℃ / s) = (θs-θf) / τ

[Equation 7] θs represents the plate thickness direction average temperature (° C.) at the start of cooling, θ f represents the plate thickness direction average temperature (° C.) at the cooling stop, and τ represents the cooling time (s).

About each steel plate obtained as mentioned above, evaluation in the steel plate (base material) and HAZ was performed with the following method, respectively.

[Hardness difference in plate thickness direction (Hvmax-Hvmin)]

The position (surface part) of 1 mm depth from the surface of the cross section (diagonal part) of the test piece enlarged in FIG. 12 (collected from the center part of plate width (C direction), and a test piece upper surface is a steel plate surface, and a test piece lower surface is another steel plate surface)). ), t / 4 parts and t / 2 parts, Vickers hardness test (load 98N) was performed 5 points per each location. And the highest value (Hvmax) and the lowest value (Hvmin) in 3 parts x 5 points = 15 points in total were calculated | required, and (Hvmax-Hvmin) was calculated. The results are shown in Table 4.

[Acoustic Ferrite Fraction Difference (Amax-Amin) in Plate Thickness Direction]

As the measurement of the hardness difference, using a test piece shown in Fig. 12, a nital-corroded optical microscope at five positions per part at a position 1 mm deep from the surface in the cross section, and three parts of t / 4 part and t / 2 part Photographs (magnification: 4OO times) were taken and image analysis was performed. Specifically, the optical micrograph is compared with the structure photograph of the acoustic ferrite (for example, steel bainite photograph collection-I, Nippon Steel Association, 1992, P89), and the acyclic ferrite part in the optical micrograph is painted. The area ratio of the painted portion occupying the optical micrograph was measured by image analysis to obtain an acicular ferrite fraction. 3 parts x 5 places = 15 places in total, and among these 15 places, the highest value (Amax) of the acyclic ferrite fraction and the minimum value (Amin) of the acyclic ferrite fraction are obtained, and then (Amax-Amin) Calculation of the differential ferrite fractions). The results are shown in Table 4.

[Diagonal Crystal Grain Size Difference (Mmax-Mmin) in Plate Thickness Direction]

Using the test piece shown in Fig. 12 as in the measurement of the hardness difference, FE-SEM-EBSP (5 parts per each part at positions of 1 mm depth from the surface in the cross section, three parts of t / 4 part and t / 2 part) ( The diagonal grain diameter (diagonal crystal grain diameter) was measured by Electron Back Scattering Pattern) (electron back scattering diffraction imaging method using an electron emission scanning electron microscope). Specifically, it is as follows.

(i) As shown in the said FIG. 12, as a surface which consists of a rolling direction and a sheet thickness direction, the sample whose plate thickness direction is 0-t (namely, including the front and back surface of a steel plate) is prepared.

(ii) a mirror-finished finish by wet emery polishing papers of # 150 to # 1000, or a polishing method having a function equivalent thereto (polishing methods other than the above-described wet emery polishing papers, polishing methods using an abrasive such as diamond slurry). Conduct.

(iii) Using an EBSP apparatus manufactured by Tex SEM Laboratories, the grain size of the region (diagonal grain) surrounded by the diagonal grain boundary was measured at t / 4 part by setting the boundary having a crystal orientation difference of 15 ° or more as the grain boundary. The measurement conditions at this time were the measurement range: 200x200 micrometers, and the measurement interval: 0.5 micrometer space | interval, and the measurement point whose confidence index which shows the reliability of a measurement orientation is smaller than 0.1 was excluded from the analysis object.

(iv) As an analysis method of data, the thing whose said grain size is 2.5 micrometers or less was considered noise, and was deleted. And the average grain size of the diagonal crystal grain in one observation surface was calculated | required about each of 3 parts x 5 places = 15 places in total.

And (Mmax-Mmin) was calculated | required among the average crystal grain diameters of the said 15 diagonal crystal grains as Mmax and Mmin as the highest value. The results are shown in Table 4.

[Ultrasound flaw test] (UT echo height of internal defect)

+12 dB front flaw detection was performed by the method of JIS G 0901, and the UT echo height of the internal defect was measured. The results are shown in Table 4.

[Low Temperature Toughness of Base Material]

In t / 2 part (center part of plate thickness), NK U4 test piece is extract | collected so that the long direction of a test piece may become C direction, and it carries out the Charpy impact test by the method prescribed | regulated by J1SZ2242, and vTrs (brittleness) from a transition curve. Wavefront transition temperature). And vTrs <= 100 degreeC evaluated the low-temperature toughness of a base material excellent. The results are shown in Table 4.

[Evaluation of HAZ-CTOD Characteristics]

SAW welding was performed on the conditions of the following Table 5, and the welding joint was created. In the CTOD test, the notch was introduced to include 15% or more of the CG-HAZ region according to API-2Z, and the test was conducted according to BS7448. In detail, as shown in FIG. 13, the notch is introduced so that the coarse grain area (CG HAZ) of 0.5 mm is included from the fusion line to the base material side in the area of 2/3 with the center of the specimen sheet thickness as the center measurement. Then, three CTOD tests were carried out at the test temperature of −10 ° C. to obtain a limit CTOD value. And the smallest limit CTOD value of three was evaluated as being excellent in weld joint fracture toughness. The results are shown in Table 4.

[Tension test]

A specimen having a shape of diameter = 12.5 mm and gauge length = 50 mm described in ASTM A370-07a was taken from the plate thickness direction t / 4 C direction, and subjected to a tensile test by the method specified in ASTM A370 to obtain a tensile strength (TS), Yield strength (YP), EL (elongation), and YR (yield ratio) were calculated. The results are shown in Table 4. On the other hand, the steel sheet of the present invention satisfies TS? 470 MPa, YP? 345 MPa, and EL? 23%.

From Tables 1 to 4, the following can be considered.

That is, No. 1-12 satisfy | fills all the requirements of this invention, and it turns out that it exhibits the outstanding low-temperature toughness and weld joint fracture toughness.

On the other hand, the thing which deviates from the requirements prescribed | regulated by this invention is inferior in a characteristic.

No. 13, 26 because of the short first cooling time (C ₁₁ Cooling time, the air-cooling time) in the first cooling, becomes the cooling of the steel sheet is insufficient, the surface part and the t / 2 can not be a temperature difference within 70 ℃ portion, and As a result, hardness difference, Amin, (Amax-Amin), Mmax, (Mmax-Mmin) was out of a prescribed range, and both low-temperature toughness and weld joint fracture toughness became inferior.

No. 15 is the first cooling time (C ₁₁ cooling time, air cooling time) in the first cooling is too long, and no. Since the second cooling time (C ₁₂ cooling time, air cooling time) in the first cooling is too long, the temperature difference between the surface portion and the t / 2 portion cannot be within 70 ° C in any case, and as a result, the hardness difference, Amin, (Amax-Amin), Mmax, and (Mmax-Mmin) are outside the prescribed range, and both low-temperature toughness and weld joint fracture toughness are inferior.

No. Since 14 and 27 have a short second cooling time (C ₁₂ cooling time, air cooling time) in the first cooling, the oxide film on the surface of the steel sheet produced by the first cooling cannot be sufficiently removed, and as a result, the cooling efficiency is lowered. It is assumed that the temperature difference between the part and t / 2 part exceeded 70 degreeC. As a result, hardness difference, Amin, (Amax-Amin), Mmax, (Mmax-Mmin) was out of a prescribed range, and both low-temperature toughness and weld joint fracture toughness became inferior.

No. Since 25, the cooling time of either the first or the second in the first cooling is short, the hardness difference, Amin, (Amax-Amin), Mmax, (Mmax-Mmin) is outside the prescribed range, so that the low temperature toughness and the weld joint break Neither toughness was inferior.

No. 17 is C ₁₁ of the first cooling in the first cooling, and also No. 18, since _C12 of the 2nd cooling in 1st cooling is less than 0.6 degrees C / s, neither can the temperature difference of a surface part and t / 2 part be within 70 degrees C. As a result, (Amax -Amin) or (Mmax-Mmin) exceeds the specified upper limit. In addition, the hardness difference also exceeded the upper limit. As a result, both low-temperature toughness and weld joint fracture toughness are inferior.

No. Since 28 is the first stage cooling instead of the second stage cooling, the oxide film on the surface of the steel sheet produced by the cooling does not sufficiently fall, and the cooling efficiency is lowered. As a result, the temperature of the surface portion and the t / 2 portion is reduced. It is assumed that the difference exceeded 70 ° C. As a result, hardness difference, Amin, (Amax-Amin), Mmax, (Mmax-Mmin) was out of a prescribed range, and both low-temperature toughness and weld joint fracture toughness became inferior.

No. Since 23 and 24 do not perform a 1st hot rolling on a prescribed condition, internal defects are not crimped enough, and the UT echo height of internal defects becomes high, and as a result, HAZ-CTOD characteristic is inferior.

No. 29 satisfies the component composition, but in the second hot rolling, (Amax-Amin) and (Mmax-Mmin) exceed the specified upper limit because the rolling is not performed under the condition (cumulative reduction ratio) that satisfies the expression (4). . As a result, weld joint fracture toughness and low temperature toughness were inferior.

No. Since 19 did not satisfy | fill the prescribed conditions of 2nd cooling, the desired intensity | strength was not able to be secured.

No. 20 to 22 are examples in which the manufacturing conditions are the same as those specified, but the component composition is outside the prescribed range. No. 20 is insufficient Ni and does not satisfy Equation 4, and No. 21 is insufficient in the amount of Nb and satisfies the expression (4). In 22, since the amount of C was excessive, either Amin became remarkably small and (Amax-Amin) and (Mmax-Mmin) exceeded the upper limit specified. In addition, the hardness difference also exceeded the upper limit. As a result, both low-temperature toughness and weld joint fracture toughness are inferior.

Claims (5)

C: 0.02 to 0.10% (the meaning of "mass%", the same below),
Si: 0.5% or less (not including 0%),
Mn: 1.0 to 2.0%,
Ni: 0.10 to 1%,
Nb: 0.005 to 0.03%,
Ti: 0.005 to 0.02%,
N: 0.0030 to 0.065%,
P: 0.02% or less,
S: 0.015% or less, and
Al: satisfies 0.01 to 0.06%,
The balance is iron and inevitable impurities,
Satisfy all of the following conditions (A) to (D),
Steel sheet with a tensile strength of 470 MPa or more.
(A) Surface part, t / 4 part [t represents plate | board thickness. The same as below, and at t / 2, the minimum value Amin of the acyclic ferrite fraction is 50 area% or more, and the highest value Amax and the lowest value of the acyclic ferrite fraction are measured. The difference of (Amin) satisfies Equation 1 below.
[Equation 1]
Amax-Amin ≤ 20 area%
(B) In the surface part, t / 4 part, and t / 2 part, when the average grain size of the area | region (diagonal crystal grain) enclosed by the diagonal grain boundary whose orientation difference of two crystals is 15 degrees or more is measured, The maximum value Mmax is 40 micrometers or less, and the difference between the maximum value Mmax and the minimum value Mmin of the average grain size satisfies the following expression (2).
&Quot; (2) &quot;
Mmax-Mmin <40 μm
(C) In the surface part, t / 4 part, and t / 2 part, when hardness is measured, the difference of the maximum value of hardness (Hvmax) and the minimum value of hardness (Hvmin) satisfy | fills following formula (3).
&Quot; (3) &quot;
Hvmax-Hvmin ≤ 50
(D) When the ultrasonic flaw test prescribed in JIS G 0901 is fully flaw detection with a detection sensitivity of +12 dB, the UT echo height of the internal defect is 50% or less. The method of claim 1,
As another element,
V: 0.5% or less (not including 0%),
B: 0.0005 to 0.003%, and
Ca: A steel sheet containing at least one element selected from the group consisting of 0.0005 to 0.003%. The method of claim 1,
As another element,
Cu: 0.3% or less (not including 0%),
Cr: 0.5% or less (not including 0%), and
Mo: A steel plate containing at least one element selected from the group consisting of 0.5% or less (not containing 0%). The method according to any one of claims 1 to 3,
Steel plate used for offshore structures. As a manufacturing method of the steel plate as described in any one of Claims 1-3,
After heating to 1050 degreeC or more using the slab which satisfy | fills the component composition of any one of Claims 1-3, 1st hot rolling, 1st cooling, 2nd hot rolling, and 2nd cooling are respectively performed. A method for producing a steel sheet excellent in low temperature toughness and weld joint fracture toughness, which is performed sequentially to satisfy the following conditions (a) to (d).
(a) In 1st hot rolling, final pass rolling whose rolling reduction is 10% or more is performed in the state whose temperature of t / 2 part is 950 degreeC or more.
(b) By performing two-step cooling which satisfy | fills the following conditions as 1st cooling, the temperature difference of a surface part and t / 2 part shall be within 70 degreeC.
(First cooling) 0.5T or more at a plate thickness direction average cooling rate of 0.6 ° C / s or more [T represents the starting plate thickness (mm) of the first cooling. The same as below] After cooling to 1.5T seconds or less, air cooling is performed in 0.5T seconds or more and 1.5T seconds or less.
(Second Cooling) After the first cooling, the cooling is performed at 0.07T sec or more and 1.3T sec or less at a plate thickness direction average cooling rate of 0.6 ° C / s or more, followed by air cooling of 0.07T sec or more and 1.3T sec or less.
(c) In 2nd hot rolling, rolling whose temperature of t / 2 part is the temperature range of less than 950 degreeC is performed so that Formula (4) below may be satisfied.
&Quot; (4) &quot;
Q + (Ni + Nb) × 10 ≥ 33
[Equation 4,
Q: cumulative reduction ratio (%) in the temperature range of t / 2 parts less than 950 ℃,
Ni: Ni content (mass%),
Nb: Nb content (mass%) is shown.
In addition, a reduction ratio is calculated | required by following formula (5).
Rolling ratio = 100 × (thickness before rolling start-rolling completed thickness) / thickness before rolling start]
(d) a second cooling, from the temperature region where the surface temperature portion not less than Ar ₃ transformation point, t / 2 and the cooling by an average cooling rate of the sheet thickness direction satisfying the equation (6) to a temperature of not more than 500 ℃ temperature region portion.
&Quot; (6) &quot;
Plate thickness direction average cooling rate ≥ 6420t ^-1.60
[Equation 6, t represents the final product sheet thickness (mm).
In addition, the plate | board thickness direction average cooling rate is calculated | required from following formula (7).
&Quot; (7) &quot;
Plate thickness direction average cooling rate (℃ / s) = (θs-θf) / τ
In Equation (7), θs is the plate thickness direction average temperature (° C) at the start of cooling, θf is the plate thickness direction average temperature (° C) at the cooling stop, and τ represents the cooling time (s).]

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