KR20080100787A - Steel sheet excellent in brittle crack propagation suspension property and toughness of sheet thickness center, and method for producing the same - Google Patents

Abstract

A steel sheet excellent in brittle crack propagation suspension and a method for producing the same are provided to improve the base material toughness of the board thickness central part by appropriately prescribing each crystallographic orientation relation. A steel sheet comprises next: C: 0.06% through 0.01 ([Mass %] It is meaning. Hereinafter it is identical about the chemical composition) Si: 0.8% through 0.01 Mn: 1.8% through 1.0 Al: 0.08% through 0.01 Nb: with 0.08% through 0.02 Ni: 0.8% through 0.20 is contained.

Description

STEEL SHEET EXCELLENT IN BRITTLE CRACK PROPAGATION SUSPENSION PROPERTY AND TOUGHNESS OF SHEET THICKNESS CENTER, AND METHOD FOR PRODUCING THE SAME}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly relates to steel sheets used as structural materials for ships and bridges. In particular, the present invention relates to a steel sheet which improves the characteristic of stopping propagation of brittle cracks generated and also has excellent base material toughness at the center of sheet thickness. A useful method for producing a steel sheet.

In order to ensure the safety of the structural material, not only the occurrence of cracks due to brittle fracture in the steel sheet is suppressed, but also brittle crack propagation is stopped even when brittle cracks occur, thereby minimizing the propagation region of brittle cracks (hereinafter, (Called brittle crack propagation stop characteristics) is also an important requirement.

Since the above-mentioned brittle fracture occurs in the vicinity of the sheet thickness center part of a steel plate, it is possible to suppress the generation of brittle fracture by appropriately controlling the structure at the sheet thickness center part and improving toughness.

On the other hand, in recent years, container ships have become very large in size, and accordingly, thickening of structural members (eg, hatch coaming, coaming tops, etc.) of ships is in progress. For example, from 2002 maximum load number 6,000TEU, the plan to increase 10,000TEU is currently in progress, and further thickening and high strength of steel plate are needed. However, the thicker the plate thickness, the more difficult the tissue control at the plate thickness center. In addition, with this, it becomes a situation that securing toughness in the center part of the thickness of a steel plate (base material) is difficult. In view of this, in order to secure brittle crack propagation stop characteristics of the steel sheet and the base material toughness at the center of the sheet thickness in the ultra-thick material, the development of a structure-controlled steel sheet over the entire sheet thickness area is required.

After increasing the brittle crack propagation stop characteristics of the steel sheet, it is known that it is effective to refine the equiaxed ferrite grains (α grains) in the surface layer region of the steel sheet. . For example, Japanese Patent Laid-Open No. 1986-235534 discloses recrystallization of ferrite particles due to cooling during rolling and rolling in a subsequent recuperation process, and Ar ₃ A method of miniaturizing ferrite grains by using reverse transformation from ferrite structure to austenite structure due to an elevated temperature above the transformation point has been proposed.

However, in this technique, since recuperation above the Ar ₃ transformation point is essential, a decrease in productivity cannot be avoided and the structure is not controlled to the center of the sheet thickness.

On the other hand, Japanese Patent Laid-Open No. 1992-141517 in order to mitigate the arc publication reduction in productivity, the temperature of the double-row cooling and that after rolling in the temperature increase during the rolling Ar ₃ A technique for recrystallizing ferrite particles by suppressing them below the transformation point and thus miniaturizing the ferrite particles in the surface layer portion region is disclosed. In this technique, not only the steel plate surface layer region, but also the internal structure of the plate thickness is refined by the difference in deformation resistance between the surface layer region and the plate thickness direction.

However, even with such a technique, the brittle crack propagation stop characteristic and the base material toughness of the sheet thickness center portion of the steel sheet to be super-thickness cannot be sufficiently secured, and further improvement is required.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel sheet excellent in brittle crack propagation stop characteristics and toughness in a sheet thickness center by appropriately defining each crystal orientation relationship, and a useful method for producing such a steel sheet. Is in.

The steel sheet of this invention which can achieve the said objective is C: 0.01-0.06% (it means "mass%", and is the same below with respect to a chemical component composition), Si: 0.01-0.8%, Mn: 1.0-1.8 %, Al: 0.01% to 0.08%, Nb: 0.02% to 0.08%, and Ni: 0.20% to 0.8%, respectively, and the ferrite phase is composed of 90% by area or more, and the front and back surfaces of the steel sheet In the region over the entire sheet thickness direction except for the portion corresponding to the sheet thickness of 1%, the average circle-equivalent diameter of the crystal grains surrounded by the diagonal grain boundary at which the orientation difference between the two crystals is 15 ° or more is 8 μm or less, Have an organization that satisfies the relationship.

[Equation 1]

1- (A2-A1) / 100≥ 0.8

However, A1: percentage of the crystal grains having a crystal orientation difference of 55 ° or more in total (area%)

    A2: Percentage of total grains (grain area) with a crystal orientation difference of less than 15 °

In addition, in the present invention, when focusing on any one of the crystal grains having a crystal orientation difference of 55 ° or more, if the crystal orientation difference is 55 ° or more with respect to all of the crystal grains adjacent to the crystal grain, the crystal grain of interest has a crystal orientation difference. Corresponds to a grain of 55 degrees or more. In addition, when "a crystal grain with a crystal orientation difference of less than 15 degrees" focuses on any one grain, if the crystal orientation difference is less than 15 degrees with respect to all the crystal grains which adjoin the crystal grains, the crystal grain which caught the eye is a crystal grain whose crystal orientation difference is less than 15 degrees. Corresponds to

In the steel sheet of the present invention, if necessary, (a) Cu: 0.05 to 0.08%, (b) Cr: 0.05 to 0.5% and / or Mo: 0.05 to 0.5%, (c) Ti: 0.005 to 0.03%, (d) B: 0.00003 to 0.0003%, (e) N: 0.003 to 0.008%, (f) Ca: 0.0005 to 0.0030% and / or rare earth elements: 0.0050 to 0.030%, etc. are also effective. Depending on the type, the characteristics are further improved.

In manufacturing the steel sheet of the present invention as described above, the slab is heated to a temperature of 1,050 to 1,250 ℃, after rolling the cumulative reduction rate of 20% or more in the austenite recrystallization temperature range of the steel sheet surface temperature of 950 ℃ or less, the surface of the steel sheet In the unrecrystallized temperature range where the temperature is 850 ° C or lower, the rolling reduction is terminated at an Ar ₃ transformation point or more with a cumulative reduction ratio of 30% or more, and the average cooling rate immediately after finishing rolling is accelerated to 5 ° C / sec or more. do.

In the steel sheet of the present invention, the crystal orientation relationship and the specific crystal orientation difference in the whole region of the plate thickness direction except for the portion corresponding to the sheet thickness of 1% in the outermost layer of each steel sheet front and back surface together with the chemical composition. By appropriately defining the particle size of the crystal grains having the fine grains, an excellent steel sheet having excellent brittle crack propagation stopping characteristics and toughness at the center of the sheet thickness can be realized, and such steel sheet is useful as a material for various structural materials including shipbuilding and bridge fields.

According to the present invention, by appropriately defining each crystal orientation relationship, the steel sheet excellent in the brittle crack propagation stop characteristic and the toughness of the sheet thickness center portion, and such a steel sheet can be manufactured.

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, the present inventors examined the means for suppressing fatigue crack stop in the steel plate and improving the toughness of the steel plate center part at various angles. As a result, the following knowledge was obtained. That is, in the structure of the steel sheet, it is produced with various orientation relations, but the orientation relation of each crystal lattice selected by the chemical composition of the steel sheet, the formation temperature of the structure, other conditions, etc. changes, so that it has a constant crystal orientation difference. In the grain boundary, in particular, it was found that the brittle crack stop characteristics were improved, and that refinement of the crystal grains having a specific crystal orientation difference resulted in favorable base metal toughness at the center of the steel sheet. EMBODIMENT OF THE INVENTION Hereinafter, the effect of this invention is demonstrated according to the process which completed this invention.

The grain boundary is considered to be a resistance to crack propagation. If the frequency of grain boundary collision with the crack during brittle crack propagation increases, the crack propagation can be stopped. That is, the knowledge that what is necessary is just to increase the frequency of collision with a crack by making a grain boundary small. However, since the grain boundary energy is small and the effect is small at the small angle grain boundary (small diameter angle boundary) where the azimuth difference between both ends forming the grain boundary is small (for example, less than 15 °), the direction difference is 15 °. It is necessary to make the above diagonal boundary (large angle boundary) as much as possible. It is also found that, in the diagonal grain boundary, as the ratio of the orientation difference between two adjacent crystal grains is higher than 55 °, the crack becomes bent, bypassed, or rectified at the grain boundary, so that it is easy to stop, and a good brittle crack propagation stop characteristic is obtained. It was.

Based on the above findings, the inventors of the present application reviewed that when the grains of the diagonal grain boundary (crystal orientation difference is 55 ° or more) and the grain grains of the small grain boundary (crystal orientation difference of less than 15 °) satisfy the relationship of the following formula (1): Good brittle crack propagation stop characteristics are obtained.

[Equation 1]

1- (A2-A1) / 100≥ 0.8

However, A1: percentage of the crystal grains having a crystal orientation difference of 55 ° or more in total (area%)

    A2: Percentage of total grains (grain area) with a crystal orientation difference of less than 15 °

In the steel sheet of the present invention, good grain crack propagation stop characteristics are obtained by increasing the grain ratio of the diagonal grain boundary (crystal orientation difference ≥ 55 °) as compared with the grain ratio of the small grain grain boundary. Also from this point of view, the value [1- (A2-A1) / 100] of the left side of Equation 1 is preferably 0.9 or more. However, there may be a case where the grain size ratio A1 of the diagonal grain boundary is larger than the grain size ratio A2 of the small grain boundary (that is, the value of the left side of Equation 1 is greater than 1.0). Further, in order to increase the grain ratio of the diagonal grain boundary, it is effective to refine the grains, and for this purpose, it is necessary to refine the austenite by recrystallization region rolling, and to introduce the deformation zone by the recrystallization region rolling again. In the steel sheet of the present invention, since Nb and Ni are contained, the effects of the recrystallized region rolling and the unrecrystallized region rolling can be easily obtained.

On the other hand, in order to improve the base material toughness of the sheet thickness center part, the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more, and the crystal grain which made the average value of the diameter (circle equivalent diameter) converted into the circle of the same area into 8 micrometers or less It also turned out to be. The above-mentioned "crystal orientation difference" is also called "deviation angle" or "diagonal angle". In addition, in order to measure such crystal orientation difference, the EBSP method (Electoron Backscattering Pattern Method) may be employed.

By appropriately controlling the crystal orientation relationship as described above, a good brittle crack propagation stop characteristic and a base material toughness can be obtained, but this control is performed in the plate thickness direction except for the portion corresponding to the sheet thickness of 1% at each outermost layer of the steel sheet front and back. It is good to carry out in the whole area | region. The reason why the portion corresponding to "1% of plate thickness" is excluded is that generally "quenching" is excessively performed at the time of water cooling in the steel plate surface portion (1% of plate thickness), and it is difficult to control the structure.

In the steel sheet of the present invention, it is necessary to appropriately control the chemical composition, but the reason for limiting the range of these components is as follows.

[C: 0.01-0.06%]

C is an element necessary for securing the strength of the steel sheet. In order to obtain the lowest strength as the steel sheet, that is, about 490 MPa (also due to the thickness of the steel used), it is necessary to contain 0.01% or more. However, when it contains excessively more than 0.06%, it will become bainite structure easily, and it becomes difficult to make it into the ferrite structure made into the objective by this invention. In this respect, the C content was 0.01 to 0.06%. On the other hand, the minimum with preferable C content is 0.03%, and a preferable upper limit is 0.05%.

[Si: 0.01 to 0.8%]

Si is an element necessary for deoxidation and securing strength, and if it is less than 0.01%, it cannot secure the minimum strength as a structural member. However, when it contains exceeding 0.8% excessively, weldability will deteriorate. On the other hand, the minimum with preferable Si content is 0.05%, and a preferable upper limit is 0.35%.

[Mn: 1.0 to 1.8%]

Mn is an effective element for increasing the strength of the steel sheet, and in order to exhibit such an effect, it is necessary to contain Mn by 1.0% or more. However, when it contains excessively, since weldability is inhibited, it is necessary to set it as 1.8% or less. On the other hand, the minimum with preferable Mn content is 1.40%, and a preferable upper limit is 1.60%.

[Al: 0.01 to 0.08%]

Al is an element that is effective for micronization of crystal grains by generating deoxidation and AlN. In order to exert such an effect, it is necessary to contain Al 0.01% or more. However, since the toughness of the steel sheet becomes coarse when the A1 content is excessive, it is necessary to make it 0.08% or less. On the other hand, the minimum with preferable Al content is 0.02%, and a preferable upper limit is 0.04%.

[Nb: 0.02 to 0.08%]

Nb promotes the miniaturization of austenite grains in the rolling and the introduction of strain bands, thereby remarkably increasing the production sites of ferrite transformation nuclei. As a result, a large number of ferrite particles produced in the austenite to ferrite transformation time have an effect of promoting the formation of ferrite. In addition, by miniaturizing the tissue, the ratio of incineration grain boundaries can be reduced. In order to exert such an effect, it is necessary to contain Nb 0.02% or more. However, when Nb content becomes excess, since the weldability of a steel plate is impaired, it is necessary to set it as 0.08% or less. On the other hand, the minimum with preferable Nb content is 0.04%, and a preferable upper limit is 0.06%.

[Ni: 0.2 to 0.8%]

Since Ni is an austenite stabilizing element, it is possible to roll austenite at low temperatures, thereby introducing a large number of deformation zones, which is effective for miniaturizing the structure after transformation. In addition, by miniaturizing the tissue, the ratio of incineration grain boundaries can be reduced. In order to exhibit such an effect, it is necessary to contain Ni 0.2% or more. However, since the effect is saturated even if Ni content becomes excess, it is necessary to be 0.8% or less. On the other hand, the minimum with preferable Ni content is 0.30%, and a preferable upper limit is 0.60%.

The basic components in the steel sheet of the present invention are as described above, and the balance consists of iron and unavoidable impurities (for example, P, S, O, etc.), but if necessary, (a) Cu: 0.05 to 0.08%, (b ) Cr: 0.05-0.5% and / or Mo: 0.05-0.5%, (c) Ti: 0.005-0.03%, (d) B: 0.00003-0.0003%, (e) N: 0.003-0.008%, (f) It is also effective to contain Ca: 0.0005 to 0.003% and / or rare earth elements: 0.0050 to 0.030%, and the characteristics are further improved depending on the kind of the elements to be contained. The reason for range limitation when containing these elements is as follows.

[Cu: 0.05-0.08%]

Cu is an element effective for raising the strength without degrading the toughness of the weld heat affected zone HAZ. In order to exhibit such an effect, it is preferable to contain Cu 0.05% or more. However, when the Al content is excessive, time cracking tends to occur. Therefore, the Al content is preferably 0.08% or less.

[Cr: 0.05-0.5% and / or Mo: 0.05-0.5%]

Cr and Mo are effective elements for raising the strength of the steel sheet. In order to exhibit such an effect, it is preferable to contain all 0.05% or more. However, when these content becomes excess, since weldability will deteriorate, it is preferable to set it as 0.5% or less.

[Ti: 0.005 to 0.03%]

Ti is an effective element to finely disperse TiN in steel to prevent coarsening of austenite particles, to effectively act as a ferrite transformation nucleus, and to improve the toughness and HAZ toughness of a base material (steel sheet) by miniaturizing ferrite grains. . In order to exhibit such an effect, it is preferable to contain Ti 0.005% or more. However, when the content of Ti becomes excessive, the HAZ toughness is lowered, so it is preferable to be 0.03% or less.

[B: 0.00003 to 0.0003%]

B is an element which exhibits the strength raising effect by accelerated cooling by addition of a trace amount. This effect increases as the content increases, but when it exceeds 0.0003%, the weldability is inhibited, and therefore it is preferable to set it to 0.0003% or less. When exerting the effect by B, the lower limit is preferably 0.00003%.

[N: 0.003-0.008%]

N is an element which forms nitride with elements, such as Al, Nb, and Ti, and exhibits the effect which refines a base material structure. In order to exhibit such an effect, it is preferable to contain N 0.003% or more. However, when the content of N becomes excessive, the solid solution N is increased and the toughness of the weld is degraded. Therefore, the content of N is preferably 0.008% or less.

[Ca: 0.0005 to 0.003% and / or rare earth element (REM): 0.005 to 0.03%]

Ca and REM are effective elements for improving anisotropy such as mechanical strength of steel sheet, improving lamellar tear inhibitory properties, and improving base material toughness. In order to exhibit such an effect, it is preferable to contain Ca 0.0005% or more and REM 0.005% or more. However, since the effect is saturated even if it contains excess Ca, Ca content is preferable to be 0.003% or less. In addition, when REM content becomes excess, large nonmetallic inclusion will produce | generate and deteriorate internal cleanliness, It is preferable to make the content into 0.03% or less.

When manufacturing the steel sheet of the present invention, the slab is heated to a temperature of 1,050 to 1,250 ° C, and after rolling the austenitic recrystallization temperature range of the steel plate surface temperature of 950 ° C or less at a cumulative reduction of 20% or more, the steel plate surface temperature is 850 degrees. ℃ or less push the cumulative rolling reduction in the recrystallization temperature region rolling end the above Ar ₃ transformation point to at least 30%, and the finish rolling, the average cooling rate immediately after the end: if the accelerated cooling is over 5 ℃ / sec. Hereinafter, it demonstrates in order.

It is preferable that the temperature which heats a slab shall be 1,050-1,250 degreeC. In order to solidify Nb (0.02% or more) in the steel to exhibit the effects described above, the heating temperature needs to be 1,050 ° C or more. However, since the initial austenite structure becomes excessively coarse when heated above 1,250 ° C, it is difficult to sufficiently refine the austenite structure even when such austenite structure is rolled and recrystallized. Therefore, it is good that heating temperature shall be 1,250 degreeC or less.

The heated slab is roughly rolled with a cumulative reduction ratio of 20% or more in the austenite recrystallization temperature range where the steel sheet surface temperature is 950 ° C or lower. By rolling at a cumulative reduction ratio of 20% or more in the recrystallization temperature range of austenite, austenite can be refined by recrystallization and reduction, and as a result, the ferrite structure after transformation can be refined. If the cumulative reduction in the recrystallization temperature range is less than 20%, the micronization due to the reduction in the recrystallization temperature range is insufficient, and coarse austenite grains are mixed after rolling. Therefore, the metal structure finally obtained also tends to be in a mixed state in which coarse ferrite grains and fine ferrite grains are mixed. In this manner, when the metal structure is in a mixed state, good base material toughness in the sheet thickness center part cannot be obtained.

Next, rolling is carried out at a cumulative reduction ratio of 30% or more in the unrecrystallized temperature range where the steel plate surface temperature is 850 ° C or lower. By rolling at a cumulative reduction ratio of 30% or more at this time, the austenite can be flattened and a metamorphic nucleation site can be introduced. As a result, the ferrite structure after transformation can be refined. Temperature to terminate the rolling (the finish-rolling temperature), but needs to be not less than Ar ₃ transformation point, which is to avoid incurring the ferrite formation in this step. On the other hand, the unrecrystallized temperature range is a temperature range in which the austenite structure is not recrystallized even when the steel is rolled.

Regarding cooling after rolling, it is necessary to perform accelerated cooling at an average cooling rate of 5 ° C / sec or more. This is to satisfy the relation of Equation 1 by increasing the ratio of the diagonal grain boundary whose crystal orientation difference is 55 degrees or more in the tissue after transformation. On the other hand, about the stop temperature of accelerated cooling, it is set as about 500 degreeC from a viewpoint of preventing the fall of toughness by generation | occurrence | production of island-like martensite MA.

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not restrict | limited by the following example, Of course, it is also possible to change suitably and to implement in the range suitable for the before and a later meaning, and this also It is included in the technical scope of the invention.

EXAMPLE

By using various steel slabs of the chemical composition shown in Table 1 below, at the manufacturing conditions shown in Table 2 (slab heating temperature, uncrystallized area reduction rate, recrystallization area reduction rate, finishing temperature, cooling rate, finish thickness) Steel sheet was prepared. About temperature at this time, it managed by the steel plate surface temperature, and the detailed temperature control procedure is as follows. In addition, Ar ₃ transformation point shown in Table 1 is calculated by the following equation (2).

[Measurement of Temperature During Rolling]

1. Using the process computer, the heating temperature of the steel piece is calculated based on the atmosphere temperature from the start of heating to the end of heating and the time in the furnace.

2. Rolling is performed using the calculated heating temperature based on the rolling pass schedule during rolling or the data of the cooling method between the passes (water cooling or air cooling).

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

4. The surface temperature of the steel sheet actually measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is compared with the calculated temperature calculated from the process computer.

5. If the difference between the calculated temperature and the measured temperature is ± 30 ℃ or more, the calculated surface temperature is recalculated to match the measured temperature to be the calculated temperature on the process computer, and if it is less than ± 30 ℃, the calculated temperature from the process computer is used as it is. I use it.

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

Ar ₃ transformation point (° C.) = 868-369 × [C] + 24.6 × [Si] -68.1 × [Mn] -36.1 × [Ni] -20.7 × [Cu] -24.8 × [Cr]

However, [C], [Si], [Mn], [Ni], [Cu], and [Cr] represent the contents (mass%) of C, Si, Mn, Ni, Cu, and Cr, respectively.

For each steel sheet obtained, A1 (proportion of the crystal grains having a crystal orientation difference of 55 ° or more in total: area%), A2 (proportion of the crystal grains having a crystal orientation difference of less than 15 ° in the whole), and the value of the left side of Equation 1 [1- (A2-A1) / 100] and the average diameter D of the crystal grains enclosed by the diagonal grain boundaries having a crystal orientation difference of 15 ° or more, the brittle crack propagation stop characteristic (Kca value), the base material toughness at the center of the sheet thickness (vTrs) was measured by the following method. These results are collectively shown in Table 3 below.

[Measurement method of A1, A2, 1- (A2-A1) / 100]

(a) The sample containing the front and back surface of plate | board thickness cut parallel to the rolling direction of the steel plate was prepared.

(b) A cross section is polished using a wet emery polishing paper of # 150 to # 1,000 or a polishing method having a function equivalent thereto, and mirror finishing is performed using an abrasive such as a diamond slurry.

(c) The cross section WHEREIN: Measurement area: 200 * 200 (micrometer), measuring pitch: 0.5 micrometer space | interval at each measuring position of plate | board thickness direction using the EBSP apparatus (brand name: "OIM") of Tex SEM Laboratries. It measured and analyzed the data of the crystal orientation difference (what counted the number of crystal grains contained in each division in 1 degree unit division of a crystal orientation difference).

(d) As an analysis method of the text data, it is determined that the crystal orientation difference is 5 ° or less, it is measured noise, and the ratios A1 and A2 that are 55 ° or more and less than 15 ° are determined, and based on this, [1- (A2 -A1) / 100] was calculated. At this time, the average particle diameter (circle equivalent diameter) of the crystal grain with a crystal orientation difference of 15 degrees or more was also measured (the crystal grain diameter 2.0 micrometer or less judges that it is measurement noise).

(e) In the sheet thickness direction, at each position of 1 mm (two locations), t / 4 (t: sheet thickness), 3 t / 4, t / 2 (total five locations) from the front and back surfaces, (c), (d), and in the value of the above formula (1), the lowest value is the total plate thickness value, and in the average particle diameter of the crystal grains having a crystal orientation difference of 15 ° or more, the highest value is the total plate thickness value. 3 is shown.

[Evaluation of Brittle Crack Stop Characteristics]

The brittle crack stop characteristics were carried out in accordance with the brittle fracture propagation stop test defined by the Steel Specified Test Method (established on March 31, 2003) issued by the Japan Welding Society (WES). The test was carried out for four test bodies using a test piece having a shape shown in FIG. 7.2 of the brittle fracture propagation stop test method by taking a temperature gradient at an arbitrary temperature range selected from -190 ° C to + 60 ° C. Kca value was calculated by the following equation. In Equation 3 below, c represents the length from the entrance of the propagation part to the brittle crack tip, T is the temperature (unit: K) of the brittle crack tip, σ is the gross stress of the propagation part, and W is the propagation part width.

A graph showing the correlation between 1 / T and the Kca value was created by using the X axis as the Kca value for which 1 / T and Y axis were calculated, and the intersection between the four-point approximation curve and 273K was set as the Kca value at 0 ° C. Kca values at 0 ° C. are shown in Table 3 below. In the present invention, a pass (excellent brittle crack stop characteristics), if the Kca at 0 ℃ less than 5,900 N / mm ^{1 .5.}

[Evaluation of Base Metal Toughness at Plate Thickness Center]

The base material toughness in the center of the plate thickness is subjected to the V-notch Charpy test (the test method in accordance with JIS Z 2242), the impact test is performed, and the brittle fracture rate (or the ductile fracture rate) is obtained by the JIS method. The brittle wavefront transition temperature vTrs at which the brittle wavefront is 50% was calculated from the curve of (test temperature vs brittle wavefront). At this time, U4 test piece prescribed by NK (Japanese Maritime Association) classification was used for the test piece shape. In ship E grade in NK classification, the impact characteristics of the base metal were evaluated at the test temperature of -40 ° C. Thus, the case where the vTrs was -50 ° C or lower was regarded as pass (good base material toughness at the center of the plate thickness).

As apparent from the results of Table 3, in satisfying the requirements specified in the present invention (Test Nos. 1, 2, 4, 5, 11 to 15, 21, 23), good brittle crack propagation stop characteristics and base material toughness You can see that it was exercised.

On the other hand, in test Nos. 3, 6 to 10, 16 to 20, 22, and 24, any one of the requirements specified in the present invention was missing, and at least one of the characteristics was deteriorated.

Based on these results, the relationship between the value of [1- (A2-A1) / 100] and the brittle crack propagation stop characteristic (Kca) is shown in FIG. 1, and the relationship between the recrystallization temperature range reduction ratio and the plate thickness direction average particle diameter is shown in FIG. In FIG. 2, the relationship between the plate | board thickness direction average particle diameter and plate | board thickness center part vTrs is shown in FIG.

1 is a graph showing the relationship between the value of [1- (A2-A1) / 100] and the brittle crack propagation stop characteristic (Kca).

2 is a graph showing the relationship between the recrystallization temperature area reduction ratio and the plate thickness direction average particle diameter.

3 is a graph showing the relationship between the average thickness in the plate thickness direction and the plate thickness center portion vTrs.

Claims (8)

C: 0.01 to 0.06% (the meaning of "mass%" and the same as for chemical composition), Si: 0.01 to 0.8%, Mn: 1.0 to 1.8%, Al: 0.01 to 0.08%, Nb: 0.02 to 0.08% and Ni: 0.20 to 0.8%, respectively, Ferrite phase is made up of more than 90 area% of the organization, The average circle-equivalent diameter of the crystal grains surrounded by diagonal grain boundaries with the azimuth difference of the two crystals of 15 ° or more in the entire region of the sheet thickness direction except for the portion corresponding to the sheet thickness of 1% at each outermost layer of the steel sheet is 8 μm or less. ego, Steel sheet having a structure that satisfies the relationship of the following formula (1). [Equation 1] 1- (A2-A1) / 100≥ 0.8 However, A1: percentage of the crystal grains having a crystal orientation difference of 55 ° or more in total (area%)     A2: Percentage of total grains (grain area) with a crystal orientation difference of less than 15 ° The method of claim 1, Further a steel sheet containing Cu: 0.05 to 0.08%. The method of claim 1, Further, at least one of Cr: 0.05 to 0.5% and M: 0.05 to 0.5%. The method of claim 1, Further a steel sheet containing Ti: 0.005-0.03%. The method of claim 1, Further steel sheet containing B: 0.00003 to 0.0003%. The method of claim 1, Further, steel sheet containing N: 0.003-0.008%. The method of claim 1, Further, at least one of Ca: 0.0005 to 0.0030% and rare earth element: 0.0050 to 0.030%. As a method of manufacturing the steel sheet according to any one of claims 1 to 7, Heating the slab to a temperature of 1,050-1,250 ° C .; After rolling over 20% cumulative reduction in the austenite recrystallization temperature range where the steel plate surface temperature is 950 ° C or lower, and in the unrecrystallized temperature range where the steel plate surface temperature is 850 ° C or lower, the cumulative reduction ratio is 30% or more, and at the Ar ₃ transformation point or more. Terminating rolling; And Average cooling rate immediately after finishing rolling: The method including the process of accelerated cooling to 5 degrees C / sec or more.

Images