KR20090110384A - Thick high-strength steel plate and process for producing the same - Google Patents

Abstract

A thick high-strength steel plate with the following characteristics. The steel plate consists of a Ni-containing steel, having a bainite main structure and exhibiting a pearlite fraction of 5% or below. In front and backside surface layer regions of 5% plate thickness, the fraction of coarse ferrite of over 25 μm equivalent circle diameter is 10% or below, and the average equivalent circle diameter of cementite is 0.5 μm or less. When the interior region excluding the above surface layer regions in a cross section perpendicular to the rolling direction of the plate is divided into isotropic areas and when measurement lines according to a sectioning method are drawn in a T-direction parallel to the plate thickness and the isotropic area wherein on the measurement lines, excluding isotropic areas of less than 8 μm equivalent circle diameter, the angle made by axes closest to the T-direction among the axes of multiple isotropic areas continuously adjacent to each other is less than 20° is regarded as one equal crack propagation resistance area, the average equivalent circle diameter (d) of the equal crack propagation resistance area is defined by the formula d=(7.11 x [Ni]+11) x (1.2-t/300) (μm).

Description

THICK HIGH-STRENGTH STEEL PLATE AND PROCESS FOR PRODUCING THE SAME

The present invention relates to a thick high strength steel sheet (hereinafter also referred to as a thick high strength high arrest steel sheet or a high arrest steel sheet) having excellent brittle crack propagation stopping characteristics (hereinafter also referred to as an arrestability), and a method of manufacturing the same.

In particular, the present invention is a thick material (hereinafter referred to as a thick material) having a sheet thickness of 50 mm or more, and a temperature at which Kca = 6000 N / mm ^1.5 is obtained even if the yield strength is 390 to 460 MPa [hereinafter, the index of _{arrestability} (T _Kca-6000 ). It is related with the thick high strength steel plate excellent in the brittle crack propagation stop characteristic which is also referred to as -10 degrees C or less, and its manufacturing method.

Moreover, the steel plate to which this invention is applied is applied to welded structures, such as shipbuilding, a building, a bridge, a tank, and a marine structure. In addition, the steel sheet of this invention may be distributed as a secondary processed product processed into a steel pipe, a column, etc. in some cases.

An application claims priority on March 5, 2007 based on Japanese Patent Application No. 2007-54279 for which it applied to Japan, and uses the content here.

In recent years, along with the increase in the thick strength of steel materials used with the increase in the size of steel structures, the demand for brittle crack propagation stop characteristics (restorability) is demanded from the viewpoint of securing safety. By the way, when strength and plate | board thickness become large generally, securing of an arrest property rapidly increases difficulty, and becomes a factor which inhibits application of the thick high strength steel plate to steel structures. At the same time, the demand for short delivery times of consumers is increasing year by year, and there is a strong demand for productivity improvement in steel sheet manufacturing processes.

As a metallurgical factor which improves the arrestability of steel materials, (i) grain refinement, (ii) Ni addition, (iv) embrittlement second phase control, (iv) aggregate structure control, etc. are known.

As a method for miniaturizing the crystal grains of (i), there is a technique described in Patent Document 1 (Japanese Patent Application Laid-Open No. 02-129318). This is a method of rolling a 30 to 50% two-phase region in a range of 700 to 750 ° C. after rolling at a rolling reduction of 50% or more in an unrecrystallized region of at least ₃ Ar. In addition, as a special method for refining the grain size of the steel sheet, before the rolling or after the end of the rough rolling, the surface of the steel piece is cooled, and rolling is started in a state in which a temperature difference with the inside is generated and reheated to generate fine ferrite in the surface layer portion. The method is described in Patent Document 2 (Japanese Patent Application Laid-Open No. 06-004903) and Patent Document 3 (Japanese Patent Application Laid-Open No. 2003-221619).

Ni addition of (ii) suppresses the propagation of brittle cracks by promoting cross slip in a low temperature region (Non Patent Literature 1: Damuraimao, "Steel Material Strength Theory", Daily Industrial Newspaper) (Published July 5, 1969, p. 125). It is known to improve the arrestability of the matrix (Non-Patent Document 2: Hasebe, Kawaguchi, `` Breakage fracture propagation of Ni-added steel sheet by tapered DCB test) For static properties, "iron and steel, vol. 61 (1975), p. 875).

As a method of controlling the embrittlement 2nd phase of (iii), there exists a technique described in patent document 4 (Unexamined-Japanese-Patent No. 59-047323). This is a technique for finely dispersing martensite as a brittle phase in the mother ferrite.

Regarding the control of the collective structure of (iii), a method of developing the (211) surface in parallel with the rolling surface by performing rolling under low temperature and high pressure with ultra low carbon bainite steel is disclosed in Patent Document 5 (Japanese Patent Application Laid-Open No. 2002-241891). Korean Patent Publication No.

However, since the microstructure is a ferrite main body, the method described in Patent Document 1 targets low-temperature steel having a relatively low strength and a sheet thickness of about 20 mm. Therefore, when applied to the thick material of 50 mm or more of plate | board thickness which this invention makes object, it is difficult to ensure a rolling reduction rate from a slab thickness viewpoint. Moreover, there exists a problem that temperature waiting time becomes long and productivity falls remarkably.

In the method described in the same document, it is also difficult to secure the yield strength of 390 MPa or more.

When trying to apply the invention described in patent document 2, 3 to the thick material which this invention makes object, even if the structure form is the same, securing an arrest property becomes difficult, and the effect by refinement | miniaturization of surface ferrite becomes relatively small. There is a problem. Moreover, also as a manufacturing process, since temperature control of a plate | board thickness direction becomes more difficult, and the rolling reduction rate in a reheating process must be enlarged, there exists a problem of greatly inhibiting productivity.

Moreover, there exists a problem that alloy cost is too high in order to manufacture the steel plate which has desired arrestability only by Ni addition like the said (ii). Therefore, even when attempting to secure the arrestability by using Ni addition and structure refinement in order to reduce the amount of Ni addition, no attempt has been made to separate and quantify the effect on the arrestability of other factors used in combination with Ni addition. It is difficult to say that the manufacturing instructions of the arrest steel sheet are clear.

In thick materials, it is difficult to finely disperse martensite as in the invention described in Patent Document 4. In addition, in a thick high strength steel sheet, this kind of embrittlement phase may lower the brittle fracture generation characteristics.

In addition, when the invention described in Patent Document 5 is applied to a thick material, the rolling efficiency is extremely lowered, and there is a problem that it is not suitable for industrial production.

As described above, the present invention is a thick material having a sheet thickness of 50 mm or more, and even if the yield strength is 390 to 460 MPa, the arrestability index (T _{Kca = 6000} ) becomes -10 ° C or less, and is applied to a large structure. As far as possible, a technique for producing a high-rest steel sheet stably and efficiently has not yet been established.

This invention is made | formed in view of the said situation, and provides the thick high strength steel plate excellent in the brittle crack propagation stop characteristic which has sufficient arrestability as a large structural steel, and is industrially stable and can be manufactured efficiently, and its manufacturing method For the purpose of

This invention is a thick high strength steel plate excellent in the brittle crack propagation stop characteristic which can solve the said subject, and its manufacturing method, The summary is as follows.

[1] In mass%, C: 0.01 to 0.14%, Si: 0.03 to 0.5%, Mn: 0.3 to 2.0%, P: 0.020% or less, S: 0.010% or less, Ni: 0.5 to 4.0%, Nb: 0.005 To 0.050%, Ti: 0.005 to 0.050%, Al: 0.002 to 0.10%, N: 0.0010 to 0.0080%, and the remainder is made of Fe and unavoidable impurities, and Ceq, which is defined by Equation 1 below, is 0.30: To 0.50%,

As a microstructure mainly composed of 60% or more of bainite as a volume fraction, the pearlite fraction is 5% or less, and the microstructure in the surface layer region from the front and back surfaces to a depth of 5% of the sheet thickness, respectively, The fraction of coarse ferrite whose equivalent diameter is more than 25 µm is 10% or less, and the average circular equivalent diameter of cementite is 0.5 µm or less,

Electron Back Scattering for an internal region excluding the surface layer region in the T cross section when a cross section perpendicular to the sheet rolling direction is a T cross section and a direction parallel to the plate surface within the T cross section is a T direction. Pattern: Hereafter, crystal orientation analysis using EBSP) is performed, and the T cross-sectional structure is divided into regions where crystal orientations are equal (hereinafter, referred to as isotropic regions), and the T cross-sectional structure divided into the isotropic regions is defined. Applying a cutting method in accordance with JIS G 0551, drawing an arbitrary measurement line in the T direction, and a plurality of circular equivalent diameters successively neighboring, except for an isotropic region whose original equivalent diameter is less than 8 μm on the measurement line The angle (hereinafter, referred to as crack propagation deflection angle) between the <001> axes closest to the T direction among each of the three <001> axes in the isotropic region of 8 μm or more is referred to as 20 ° A plurality of isotropic regions that are successively neighboring on the measurement line, which are less than, may be regarded as an area (hereinafter, isocratic propagation resistance region) as well as an isotropic region having an adjacent circular equivalent diameter of less than 8 μm on the measurement line. In this case, an average circular equivalent diameter (hereinafter referred to as an effective grain diameter) calculated by the cutting method of the isocratic propagation resistance region is 8 μm or more and d (μm) or less in the following formula (2), Thick high strength steel plate with excellent brittle crack propagation stopping characteristics.

Here, [X] is content (mass%) of element X, t is plate | board thickness (mm).

[2] Further, in mass%, one kind or two of Cu: 0.05 to 1.5%, Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0%, V: 0.005 to 0.10%, and B: 0.0002 to 0.0000%. The thick high strength steel plate excellent in the brittle crack propagation stop characteristic as described in said [1] characterized by containing more than 1 type.

[3] The above-mentioned [1] or [], wherein the mass% contains one or two or more of Mg: 0.0003 to 0.0050%, Ca: 0.0005 to 0.0030%, and REM: 0.0005 to 0.010%. The thick high strength steel plate excellent in the brittle crack propagation stop characteristic as described in 2].

[4] Ar ₃ or more after heating the steel strip having the composition according to any one of [1] to [3] at 950 to 1150 ° C and performing rough rolling at a cumulative reduction rate of 30% or more at a temperature of 900 ° C or higher. And finish rolling at a temperature below T (° C.) of the following equation (3) and at a cumulative reduction ratio of 40% or more, and then 50 ° C. or less at a cooling rate of 8 ° C./s or more on average from a thickness of Ar ₃ to a sheet thickness of 8 ° C./s or more. Accelerated cooling to temperature, The manufacturing method of the thick high strength steel plate excellent in the brittle crack propagation stop characteristic.

Here, [Ni] represents Ni content (mass%) and t represents plate thickness (mm).

[5] A method for producing a thick high strength steel sheet having excellent brittle crack propagation stop characteristics according to the above [4], wherein the tempering treatment is performed at a temperature of 300 to 600 ° C after completion of the accelerated cooling.

By the application of the present invention, a high-arrest steel sheet that is a thick material having a sheet thickness of 50 mm or more, and which has an earthing index (T _{Kca = 6000} ) of -10 ° C or less even when the yield strength is 390 to 460 _MPa. The industrial effect is very large in that it can be provided by a stable and efficient manufacturing method.

1 is an example of the result of analyzing the boundary between the crystal orientation map and the equal crack propagation resistance region by EBSP.

2 is a graph showing a change in the arrestability with the amount of Ni added.

3 is a graph showing the effect of the amount of Ni and the effective grain diameter on the arrestability.

4 is a graph showing the relationship between the ferrite fraction and the arrestability.

5 is a graph showing the relationship between the average equivalent circular diameter of cementite and the arrestability.

Fig. 6 is a graph showing the relationship between the fraction of coarse ferrite having a diameter of more than 25 µm and a restorability in a 5% region of sheet thickness from the front and back surfaces.

Fig. 7 is a graph showing the relationship between the amount of Ni required to provide predetermined arrestability and the effective grain size.

8 is a graph showing the plate thickness dependence of the effective grain size necessary to impart a predetermined arrestability.

9 is a graph showing the relationship between the amount of Ni necessary to impart predetermined arrestability and the finish rolling temperature.

10 is a graph showing the plate thickness dependence of the finish rolling temperature necessary to impart a predetermined arrestability.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described in detail.

The present inventors conducted an experimental examination of the arrestability control factor of the yield strength of 390 to 460 MPa grade steel for the steel having the microstructure of the bainite main body (60% or more by volume fraction), and a thick material having a sheet thickness of 50 mm or more. Rado has been found a means to ensure a stable stability. An important point in this invention is the new viewpoint of following (1)-(5).

(1) The fracture unit when the brittle crack propagates corresponds very well to the grain size obtained by crystal orientation analysis using EBSP, not the apparent grain size. Specifically, among the particles surrounded by grain boundaries (except for particles having a diameter equivalent to less than 8 μm) having an angle (crack propagation deflection angle) necessary to coincide the <001> axis closest to the T direction perpendicular to the rolling direction to 20 ° or more. The correlation between the average circular equivalent diameter (effective grain diameter) (including the particle equivalent diameter less than 8 µm) and the arrestability of the isocracker propagation resistance region is good.

(2) When Ni is added 0.5% or more, the effect of improving the arrestability is apparent. The effect of Ni and the effect of atomization are independent, and the law of addition almost holds. That is, by adding Ni, even if the structure is rough, the equivalent restoring property can be ensured, and manufacturing load such as high temperature of the finish rolling temperature can be reduced.

(3) Even if the effective grain size is minute, when the pearlite fraction is more than 5%, coarse pearlite tends to be a starting point of brittle fracture, and the arrestability is also reduced. In order to avoid this, it is necessary to control the cooling rate and the stop temperature of the accelerated cooling process.

(4) Fine cementite having an average circular equivalent diameter of 0.5 µm or less contributes to improving the arrestability. In order to keep the size of cementite fine, it is necessary to control the accelerated cooling after rolling and the heat processing conditions performed continuously.

(5) When the coarse ferrite fraction generated in the surface layer portion exceeds 10%, the arrestability is deteriorated even though the effective grain size of the plate thickness average is fine. In order to avoid this, it is necessary to control so that finishing rolling temperature and cooling start temperature may not fall too much.

Hereinafter, each requirement which comprises this invention is demonstrated in detail.

In general, the basic organizational unit governing the toughness of bainite steel is not the former austenite grain size, but a region called a packet or a block (for a "packet and block", a non-patent document: Matsuda, Inoue, Mimura, Okamura, "Low Alloy" Toughness and Effective Grain Diameter of Tempered High Tensile Steel ”, Proc. of Int. sympo. on Toward Improved Ductility and Toughness, Climax Molybdeum Co., Kyoto (1971), p. 47].

By the way, the structure observation by a normal optical microscope makes it difficult to measure the size of a packet and a block, and it is very difficult to objectively define a basic tissue unit when ferrite mixes.

Therefore, the present inventors first manufactured a steel plate having a plate thickness of 50 mm under various conditions using a steel sheet containing no Ni, and based on the method described in WES 3003 for evaluating the arrestability, a 29 mm depth was applied to a 500 mm square specimen. The temperature gradient type ESSO test was done using the test piece which processed the notch of. After that, the wavefront of the test piece was observed with a scanning electron microscope, and the unit (waveform unit) of the cleaved surface surrounded by the soft fracture portion called a tear ridge was measured, and it was confirmed that there was a good correlation between this and the arrestability. .

Next, EBSP measurement was performed in the cross section perpendicular to the wavefront, and the conditions of the wavefront unit boundary were examined in detail by comparing the crystal orientation analysis result and the wavefront photograph in the particles immediately below the wavefront. An example thereof is shown in FIG. 1. Based on the EBSP azimuth map, the representative point in the isotropic region is analyzed and a cube composed of the {100} plane, which is considered a cleavage plane, and a direction assumed when the crack propagates along the {100} plane is shown in FIG. 1. In the city.

The number in Fig. 1 is an angle (crack propagation deflection angle) required to coincide the closest <001> axis, and is an angle required to allow alignment of the {100} plane perpendicular to the T direction. From this, the crack propagation direction clearly changes when the crack propagation deflection angle is 20 ° or more, as shown in (a), (b), (c) and (f), and from the wavefront observation result to the actual wavefront unit boundary. It was confirmed that it was done. However, even if the angle is 20 degrees or less, the propagation direction may not be changed in a region having a small size as shown in (d) to (e). It is estimated that this corresponds to the occurrence of cracks or the soft wavefront partially present. Such an example was seen in the area | region below 8 micrometers in circular equivalent diameter, and it was confirmed that it does not form a clear boundary also in wavefront observation. In the case where an area of less than 8 mu m exists when determining the effective grain size, the boundary between the equal crack propagation resistance area may be determined by incorporating any adjacent area and investigating the deflection angles between the areas on both sides. As described above, when the boundary having a crack propagation deflection angle of 20 ° or more is determined from the EBSP analysis result except for particles smaller than 8 μm, and the average circular equivalent diameter of the area surrounded by the boundary is calculated, the effective grain diameter is estimated. can do.

The relationship between the effective grain diameter and the arrestability measured in this way was examined in detail. In order to give the restorability of a level applicable to large structural steels, it is necessary to perform finish rolling at a low temperature of 800 ° C. or lower, and to yield yield strength. In order to set it as 390 Mpa or more, it is necessary to ensure cooling start temperature, and it turned out that it is very difficult to manufacture efficiently and stably.

Therefore, the effect by Ni addition was examined in detail as a means of solving the said subject. Steel sheets 50 mm thick and 80 mm thick were manufactured under the same manufacturing conditions using steel sheets produced by varying the balance of Ni and Mn in various ways so that the microstructures and strengths were almost equal. Investigated. As a result, although there was almost no change in the effective grain size, it was confirmed that as the amount of Ni increased, the arrestability improved. This state is shown in FIG.

Here, the _{arrestability} was evaluated at a temperature (T _{Kca = 6000} ) at which the brittle crack propagation stop characteristic _{Kca = 6000 N} / mm ^1.5 . 2 shows that when the amount of Ni is 0.5% or more, the arrestability is obviously improved. When the wavefront of the ESSO test piece was observed, it was confirmed that the three-dimensional unevenness became remarkable with increasing Ni amount. This is considered to be because the cross slip is encouraged by the solid solution Ni and the crack propagation direction becomes more random.

Next, the arrestability of the steel sheet which rolled the said Ni containing steel piece on various conditions for the purpose of isolate | separating and quantifying the effect of Ni addition and the effective grain size refinement was investigated. As a result, it was found that the effect of improving the restorability due to the atomization can be almost added without depending on the amount of Ni. This state is shown in FIG. In other words, by utilizing an appropriate amount of Ni, the arrestability can be secured even without making the effective grain size small. Therefore, when steel material production efficiency is required rather than Ni alloy cost, finishing rolling temperature can be made high by Ni addition, and temperature waiting time is shortened, and it becomes possible to raise productivity of a thick material remarkably.

The present inventors also examined the effect of tissue factors other than the effective grain diameter on the arrestability. This is because it is confirmed that the arrestability is not sufficient even though the effective grain size is fine.

One is pearlite mixed in the structure of the bainite subject. If the fraction of the pearlite structure is increased, the large pearlite is increased and this is the starting point of brittle fracture, and the arrestability tends to be deteriorated. Therefore, as shown in FIG. 4, the pearlite fraction needs to be 5% or less.

In addition, it was confirmed that the size of cementite contained in bainite also affects the arrestability. As shown in FIG. 5, when the average circular equivalent diameter of cementite exceeds 0.5 micrometers, an arrestability falls. This is presumably because fine cementite relieves the stress state at the crack tip by generating microcracks at the interface with the matrix prior to the main crack propagation. On the other hand, when cementite becomes coarse, it becomes a factor which causes brittle fracture similarly to pearlite, and the arrestability also falls.

In addition, the coarse ferrite produced in the surface layer portion was found to lower the arrestability. This surface coarse ferrite is as relatively Ken chingseong rolling to a temperature lower stronger than Ar _3, or may complete the rolling at the Ar ₃ or more start of accelerated cooling is shown in Figure 6 is produced when the fall below the Ar ₃ If the fraction of the ferrite having a circular equivalent diameter of more than 25 µm in the region of 5% of the plate thickness from the front and back surface is 10% or less, the remarkable arrestability can be avoided.

In order to clarify the manufacturing guideline of the thick high strength high arrest steel sheet while considering the above-described structure factors, the effective grain diameter and the amount of Ni on the restoring property are used by using a steel sheet that satisfies the conditions for the above-described pearlite, cementite and surface ferrite. , The influence of the plate thickness was analyzed in more detail. As a result, it was found that the following d or less was required as a condition of the effective grain size.

d = (7.11 × [Ni%] + 11) × (1.2-t / 300)

Here, [Ni] represents Ni content (mass%) and t represents plate thickness (mm).

D is based on the effect of the effective grain diameter and Ni on the restraint of the sheet thickness of 50 mm, and the first and second formulas derived from FIG. 7 and the front and back surfaces of the plate thickness of 80 mm containing 2% of Ni are ground. Based on the test results at the time of changing, the formula of the plate thickness effect derived from FIG. 8 is combined. If the effective grain size is larger than d, the frequency of the ridge formed when brittle cracks propagate from one particle to another is not sufficient, so that the effect of suppressing crack propagation becomes small and the arrestability is lowered. .

Then, the reason for limitation of the manufacturing conditions in this invention is demonstrated.

In this invention, the heating temperature of the steel piece was 950-1150 degreeC. If the reheating temperature is less than 950 ° C, the solution element is insufficient in solution solution, and if the reheating temperature is higher than 1150 ° C, the heated austenite grain size becomes coarse, resulting in difficulty in final microstructure.

The next rough rolling needs to be performed at a temperature of 900 ° C or higher and a cumulative reduction ratio of 30% or higher. If these conditions are not satisfied, recrystallization of the austenite particles does not proceed sufficiently, resulting in a mixed structure, which may cause material unevenness.

The finishing rolling performed subsequently is the most important process from the viewpoint of the refinement of the effective grain diameter which governs the arrestability, and is equal to or more than Ar ₃ (temperature at which ferrite starts to precipitate from austenite during cooling) and below T (° C). The temperature is carried out at a cumulative reduction ratio of 40% or more.

T = (37 × [Ni] +810) × (1.1-t / 500)

Here, [Ni] represents Ni content (mass%) and t represents plate thickness (mm).

T is based on the above experimental results, and the first formula obtained from FIG. 9 showing the relationship between the amount of Ni required to satisfy T _{Kca = 6000} ≤ -10 ° C and the finish rolling temperature, and using a steel piece containing 2% Ni The formula of the plate | board thickness effect derived from FIG. 10 is combined based on the test result when the plate | board thickness and finish rolling temperature were variously changed. If the temperature is less than Ar ₃ , coarse processed ferrite having a circular equivalent diameter of more than 25 μm is formed in the surface layer portion, and the arrestability, strength, toughness, and ductility decrease. On the other hand, if the temperature exceeds T or the cumulative reduction ratio is less than 40%, the effective grain size is not sufficiently miniaturized, so that the arrestability decreases. By selecting a temperature slightly lower than T in accordance with the amount of added Ni, the temperature waiting time before finish rolling is reduced, and the thick high strength steel sheet can be efficiently produced.

After completion of the finish rolling, accelerated cooling is performed from a temperature of Ar ₃ or higher to a temperature of 500 ° C. or lower at a cooling rate of 8 ° C./s or more in a sheet thickness average. If the cooling start temperature is less than Ar _3, the coarse ferrite fraction of the surface layer portion is more than 10%, and the arrestability is reduced. If the cooling rate is less than 8 ° C / s or the cooling stop temperature is higher than 500 ° C, not only the strength is insufficient, but the refinement of the effective grain diameter is insufficient, and the cementite contributing to the improvement of the arrest is coarse or pearlite is 5%. It is excessively generated and the arrestability is reduced.

After accelerated cooling, in order to adjust strength and toughness, you may perform tempering process at the temperature of 300-600 degreeC. If the tempering treatment temperature is less than 300 ° C., the ductility and toughness are not sufficiently improved. If the tempering treatment temperature is more than 600 ° C., cementite becomes coarse and the arrestability is lowered.

Next, the reason for component limitation of this invention is demonstrated.

C (carbon) is an element that contributes to the generation of cementite and prevention of structure coarsening, and is added to 0.01% or more because it is an indispensable element for increasing strength at low cost. On the other hand, when the addition amount is increased, it is difficult to secure the heat input heat HAZ (Heat Affected Zone) toughness, and cementite is also easily coarsened, so the upper limit is 0.14%.

Si (silicon) is added at 0.03% or more in order to solidify the matrix as an inexpensive deoxidation element, but if it exceeds 0.5%, the upper limit is made 0.5% because it degrades the weldability and the HAZ toughness.

Since Mn (manganese) is effective as an element which improves the strength and toughness of a base material, 0.3% or more is added, but excessive addition deteriorates HAZ toughness and weld cracking property, and makes 2.0% an upper limit.

Although P (phosphorus) and S (sulfur) are so preferable that there is little content, since it costs enormous cost in order to reduce this industrially, P makes 0.02% and S makes 0.01% an upper limit.

Ni (nickel) is added in an amount of 0.5% or more because it is effective for securing strength, improving the arrestability and HAZ toughness, but the increase in the amount of Ni is limited to 4.0% or less because it increases the cost of the steel sheet.

Nb (niobium) contributes to microstructure, transformation, and precipitation strengthening by the addition of trace amounts, and is added at 0.005% or more because it is an effective element for securing the base metal strength, but when excessively added, it hardens the toughness by significantly hardening HAZ. The upper limit is made 0.050%.

Ti (titanium) is effective in improving the strength, toughness, and HAZ toughness of the base material by microstructure, precipitation strengthening, and fine TiN formation by the addition of a small amount, but is added in an amount of 0.005% or more. In order to deteriorate, 0.050% is made into an upper limit.

Al (aluminum) is added to 0.002% or more because it is an important deoxidation element. However, when excessively added, the surface quality of the steel piece is damaged and an inclusion harmful to toughness is formed.

N (nitrogen) is added at 0.0010% or more because it forms nitride with Ti to improve HAZ toughness. However, N (nitrogen) is limited to 0.0080% or less because embrittlement by solid solution N occurs when excessively added.

Selective addition elements are defined for the following reasons.

Cu (copper), Cr (chromium), and Mo (molybdenum) all improve the hardenability and are effective in increasing the strength, and therefore, 0.05% or more are added. On the other hand, excessive addition decreases the HAZ toughness, so Cu is 1.5% or less, and Cr and Mo are limited to 1.0% or less.

V (vanadium) is added in an amount of 0.005% or more because it contributes to the increase in strength due to precipitation strengthening.

B (boron) is an element that improves the hardenability, and is effective for increasing the strength of steel by adding an appropriate amount, but excessive addition is limited to 0.0002 to 0.0030% because it impairs weldability.

Mg (magnesium), Ca (calcium), and REM form fine oxides or sulfides that contribute to the improvement of HAZ toughness, but excessive addition decreases toughness by coarsening inclusions, so Mg is 0.0003 to 0.0050% and Ca is 0.0005 to 0.0030% and REM are limited to the range of 0.0005 to 0.010%. In addition, REM means rare earth elements, such as La and Ce.

Moreover, in order to make a base material strength and joint property compatible, it is necessary to restrict Ceq represented by a following formula to the range of 0.30 to 0.50%. If Ceq is less than 0.30%, it is difficult to secure the base material yield strength of 390 MPa or more of a thick material having a plate thickness of 50 mm or more, and if it is more than 0.50%, it is difficult to secure weldability and joint toughness, and the strength is too high to the arrestability. There is a possibility of deterioration.

Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15

+ ([Cr] + [Mo] + [V]) / 5

Here, an element symbol shows content (mass%) of an element. That is, when the element symbol is X, [X] represents content (mass%) of the element X.

Example

Hereinafter, the effects of the present invention will be more obvious by Examples. In addition, this invention is not limited to a following example, It can change and implement suitably in the range which does not change the summary.

Using the steel piece which has the chemical component of Table 1, the steel plate of 50-80 mm of plate | board thickness was started by the manufacturing conditions of Table 2 and Table 3. Table 4 and Table 5 show the structure, base material strength, and arrestability.

The surface coarse ferrite fraction (surface coarse alpha fraction) was measured by image analysis from the optical micrograph of the T cross section of the steel plate outermost layer part.

The pearlite fraction was measured from the optical micrograph of the T cross section of 5 mm below the surface of a steel plate, 1/4 equivalent part of plate | board thickness, and plate | board thickness center part.

The cementite particle diameter (theta particle size) produced the extraction replica from the three plate | board thickness positions similar to the above, and calculated the average circular equivalent diameter from the photograph | photographed using the transmission electron microscope.

The effective grain diameter is obtained by taking a sample for EBSP from the same three plate thickness positions so that the T section becomes the measurement surface, and measuring an area of 500 × 500 µm with a pitch of 1 µm. The grain boundary was determined by carrying out azimuth analysis over the range of 2 mm in total length by -5 micrometer pitch, and it calculated by the cutting method based on JISG0555.

The yield strength (YP) and tensile strength (TS) were evaluated using the No. 4 tensile test piece of JIS Z 2201 taken from the sheet thickness center part in the T direction.

The arrestability was evaluated at the temperature which shows Kca = 6000 N / mm ^1.5 by performing the temperature gradient type ESSO test.

Nos. 1 to 22, which are examples of the present invention, had chemical components in a predetermined range and were manufactured under predetermined conditions, and therefore, all had sufficient strength as YP: 390 to 460 MPa grade steel, and the arrestability was also good.

On the other hand, since No. 23-45 which is a comparative example has any deviation of the range of this invention from a chemical component and manufacturing conditions, an arrestability fell.

In Nos. 23 and 41, the temperature of the finish rolling was lower than that of Ar ₃ , and a large amount of coarse ferrite was produced in the surface layer portion, so that the arrestability decreased.

Nos. 28 and 42 had a rolling end temperature of Ar ₃ or more, but since the accelerated cooling start temperature was less than Ar ₃ , the surface-coarse ferrite fraction also increased, and the arrestability decreased.

Nos. 24 and 37 had small cooling rates of accelerated cooling.

Nos. 33 and 40 had a cooling stop temperature higher than 500 ° C.

Since No. 26 and 38 had heat processing temperature exceeding 600 degreeC, both cementite diameter became large and sufficient arrestability was not obtained.

Since No. 34 was air-cooled without performing accelerated cooling, the effective grain size did not become fine, and the arrestability fell.

Nos. 27 and 35 had a small cumulative reduction ratio of finish rolling.

Since No.25, 30, and 36 had high finish rolling temperature, effective grain size coarsened and the arrestability fell.

No. 29 had high heating temperature.

Nos. 31 and 39 had small cumulative reduction rates of rough rolling.

No. 32 had a high heating temperature and had a low crude cumulative reduction ratio, so that the effective crystal grain diameters all increased, and the arrestability decreased.

Since No. 43 had many C content, cementite became large and the arrestability fell and HAZ toughness also fell.

No. 44 had little Ni, resulting in insufficient arrestability.

Since No. 45 had a high Ceq, the strength was excessively increased and the arrestability was deteriorated.

By the application of the present invention, a high-arrest steel sheet that is a thick material having a sheet thickness of 50 mm or more, and which has an earthing index (T _{Kca = 6000} ) of -10 ° C or less even when the yield strength is 390 to 460 _MPa. In the point that it becomes possible to provide by the stable and efficient manufacturing method, industrial availability is very large.

Claims (6)

In mass%, C: 0.01 to 0.14%, Si: 0.03 to 0.5%, Mn: 0.3 to 2.0%, P: 0.020% or less, S: 0.010% or less, Ni: 0.5 to 4.0%, Nb: 0.005 to 0.050% , Ti: 0.005 to 0.050%, Al: 0.002 to 0.10%, N: 0.0010 to 0.0080%, and the remainder is made of Fe and unavoidable impurities, and Ceq, which is defined by Equation 1 below, is 0.30 to 0.50% , As a microstructure mainly composed of 60% or more of bainite as a volume fraction, the pearlite fraction is 5% or less, and the microstructure in the surface layer region from the front and back surfaces to a depth of 5% of the sheet thickness, respectively, The fraction of coarse ferrite whose equivalent diameter is more than 25 µm is 10% or less, and the average circular equivalent diameter of cementite is 0.5 µm or less, Electron Back Scattering for an internal region excluding the surface layer region in the T cross section when a cross section perpendicular to the sheet rolling direction is a T cross section and a direction parallel to the plate surface within the T cross section is a T direction. Crystal orientation analysis using a pattern) is carried out, and the T-section structure is divided into regions (equivalent orientation regions) in which the crystal orientations are equal, and the cutting method according to JIS G 0551 is applied to the T-section structure divided into the isotropic regions. Apply an arbitrary measurement line in the T direction, and except for an isotropic region having a circular equivalent diameter of less than 8 μm on the measurement line, each of three consecutive isotropic regions having a plurality of circular equivalent diameters of at least 8 μm. Continuously adjacent on the measurement line, the angle (crack propagation deflection angle) between the <001> axes closest to the T direction among the <001> axes is less than 20 °. When the number of isotropic regions are regarded as one region (equal crack propagation resistance region) as well as an isotropic region with an adjacent equivalent circular diameter of less than 8 μm on the measurement line, by the cutting method of the isocratic propagation resistance region The calculated average round equivalent diameter (effective grain diameter) is 8 micrometers or more and d (micrometer) or less of following formula (2), The thick high strength steel plate characterized by the above-mentioned. <Equation 1>

<Equation 2>

Here, [X] is content (mass%) of element X, t is plate | board thickness (mm). The method of claim 1, in mass%, Cu: 0.05 to 1.5%, Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0%, V: 0.005 to 0.10%, B: 0.0002 to 0.0030% It further contains 2 or more types, The thick high strength steel plate. The thick high strength steel sheet according to claim 1, further comprising one or two or more of Mg: 0.0003 to 0.0050%, Ca: 0.0005 to 0.0030%, and REM: 0.0005 to 0.010%. The thick high strength steel sheet according to claim 2, further comprising one or two or more of Mg: 0.0003 to 0.0050%, Ca: 0.0005 to 0.0030%, and REM: 0.0005 to 0.010%. Any one of claims 1 to 4, after carrying out any one of the preceding composition is heated to have the slabs, with 950 to 1150 ℃, and more than 30% of the cumulative reduction ratio at least 900 ℃ temperature rough rolling according to one wherein, Ar ₃ to more, equation Finish rolling is carried out at a temperature of T (° C) of 3 or less and at a cumulative reduction ratio of 40% or more, and then accelerates from a temperature of Ar ₃ or more to a temperature of 500 ° C or less at a cooling rate of 8 ° C / s or more as a sheet thickness average. Cooling is performed, The manufacturing method of the thick high strength steel plate. <Equation 3>

Here, [Ni] represents Ni content (mass%) and t represents plate thickness (mm). The manufacturing method of the thick high strength steel plate of Claim 5 characterized by tempering-processing at the temperature of 300-600 degreeC after completion | finish of the said accelerated cooling.

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