KR100705889B1 - Low yield ratio high tension steel plate having small acoustic anistropy and excellent weldability, and its producing method - Google Patents

Abstract

The present invention provides a high tensile strength steel sheet having a tensile strength of 780 MPa or more, a resistance ratio, small acoustic anisotropy, and excellent weldability, and a method of manufacturing the same.

The steel sheet of the present invention has a mass% of C: 0.010 to 0.080%, Si: 0.02 to 0.50%, Mn: 1.10 to 3.00%, Cu: 1.60% or less, Ni: 0.40 to 2.50%, P: 0.030% or less, S: 0.010% or less, Al: 0.200% or less, N: 0.0100% or less, Cr: 0.30 to 2.00%, Mo: 0.10 to 1.10%, and Ti: 0.002 to 0.030%, and the balance consists of Fe and impurities, and AS and DL in the following formulas are AS≥4.00 and DL≤2.80, and the structure in the 1/4 thickness part mainly consists of bainitic ferrite containing 10 area% or less (excluding 0%) of MA, In addition, the long axis / short axis of the old austenite particles is 1.0 to 3.0 on average, and the hardness ratio of the MA to bainitic ferrite is 1.10 or more.

AS = [Mn] + [Ni] + 2 × [Cu]

DL = 2.5 × [Mo] + 30 × [Nb] + 10 × [V]

(Where [X] represents the content (mass%) of the element X)

Description

LOW YIELD RATIO HIGH TENSION STEEL PLATE HAVING SMALL ACOUSTIC ANISTROPY AND EXCELLENT WELDABILITY, AND ITS PRODUCING METHOD}

BRIEF DESCRIPTION OF THE DRAWINGS The typical CCT figure for demonstrating the relationship between the cooling rate and the structure | tissue in the manufacture of the steel of this invention is shown.

FIG. 2: shows cross-sectional explanatory drawing of the steel plate welded part which shows the sampling part of the test piece for investigating HAZ toughness in an Example.

It is a figure which shows the relationship between the average flatness and acoustic anisotropy of the sphere (gamma) particle in an Example.

It is a figure which shows the relationship of the average circle equivalent diameter of a spherical-gamma particle in an Example, and a base material toughness.

It is a figure which shows the relationship between the hardness ratio with respect to BF of MA, and YR in an Example.

It is a figure which shows the relationship between AS value and tensile strength in an Example.

The present invention relates to a high tensile strength steel sheet having a high tensile strength of 780 MPa or more, a low yield ratio of 85% or less, a low acoustic anisotropy, and excellent base material toughness and weldability.

In the case of bridges or steel sheets for construction (thick steel sheet), if a defect is present in the welded part, this part is likely to be a starting point of fracture. Therefore, if there is a defective part by examining the presence or absence of the defective part by ultrasonic inspection test, Work to repair the site is generally carried out. However, in the steel sheet whose sound velocity changes significantly according to the flaw detection direction, that is, the steel sheet having high acoustic anisotropy, the ultrasonic flaw test cannot detect the exact position of the weld defect, so that the steel sheet applied to the field or the like is required to have small acoustic anisotropy. have.

As described above, in the fields of marine structures, building structures, and the like, in order to simplify welding defect detection due to improvement in welding construction efficiency, it is required that the acoustic anisotropy of the steel sheet is small, and the weldability at cryogenic temperatures of -40 ° C (HAZ toughness) 780MPa class high tensile strength steel sheet with good cracking resistance, crack resistance, and base metal toughness is desired.

Further, in recent years, particularly in the field of high-rise building structures requiring earthquake resistance, steel materials having a low yield ratio YR, which can absorb fracture energy added to the structure during an earthquake and prevent the collapse of the structure, are generally desired. In high tensile steel sheets, YR tends to be high. On the other hand, YR (%) is expressed as YS (0.2% yield strength) / TS (tensile strength) x 100.

Conventionally, although high-temperature toughness is difficult to secure in high tensile strength steel sheets of 780 MPa or more, various techniques have been proposed in recent years to improve base metal toughness. For example, Japanese Patent Laid-Open No. 1999-172365 discloses that the cumulative reduction ratio of unrecrystallized region rolling in hot rolling is set to 50% or more so that the aspect ratio of the old austenite (γ) particles is 3 or more. Japanese Patent Laid-Open No. 2001-220644 discloses performing hot rolling with a rolling finish temperature (FRT) of 850 ° C. or lower so that the flatness of the spherical? Particles is 50% or less on average, and Japanese Patent Laid-Open No. 2001 -200334 discloses reducing the bainite lath width by setting the cumulative reduction ratio of Ar 3 point or more and less than 900 DEG C in hot rolling to 10 to 50%, and further, Japanese Patent Laid-Open No. 1997-3591. It is described to shorten the lath length by hot rolling at a cumulative reduction ratio of 30% or more in the recrystallization temperature range.

On the other hand, in high tensile strength steel of 780 MPa or more, there is a problem that the HAZ toughness deteriorates at the time of high heat input welding. The reason is that when the heat input becomes large, the cooling rate of the HAZ is delayed and the toughness is lowered by generating coarse island-like martensite. This problem occurs in either thick or thin when performing large heat input welding. For this reason, it is common that welding heat input is limited to 5 kJ / mm or less at the time of welding construction, and welding efficiency fell only.

For this problem, various techniques for improving HAZ toughness have been proposed. For example, Japanese Patent Application Laid-Open No. 2000-160281 uses low C, which actively refines the hardenability improving elements Mn, Cr, Mo, or further finely disperses TiN to finely refine spherical γ particles. Japanese Patent Laid-Open Publication No. 194-65680 discloses that the low C, and further refines the spherical γ granules by fine dispersion of Ta ₂ O ₃ , Japanese Patent Laid-Open No. 1993-171341 The publication discloses that Ti and Mg are added as essential components to disperse oxides to refine spherical γ particles and to promote the production of intraparticle ferrite, and Japanese Patent Laid-Open Publication No. 1995-233437 discloses that P _cm ? 0.24, can be and are described, Japanese Patent Laid-Open No. 1990-254120 arc to improve the hardenability and a C _eq ≥0.45 has been described to utilize the precipitation strengthening by the Cu under low carbon member B .

In addition, the high tensile strength steel sheet having excellent weldability and low yield ratio is disclosed in Japanese Patent Application Laid-Open Publication Nos. 1944-248336 and 1994-248337, in which B is added without additive to improve weldability of B-added steel. In order to secure the strength according to the lowering of the hardenability, a method of manufacturing a high tensile strength steel sheet using precipitation hardening of V by hardening and tempering has been described.

The technique of improving the base metal toughness has a limitation in reducing the rolling temperature in the unrecrystallized region of austenite because the amount of addition of Mn, Cu, and Ni having a function of lowering the transformation point is small and the Ar3 point is increased. Since the improvement effect of the base material toughness by rolling is small, the outstanding base material toughness which absorbed energy (vE- ₅₀ ) in the Charpy impact test at _-50 degreeC is 100J or more was not acquired conventionally.

In addition, any of the techniques related to the improvement of the HAZ toughness prevents the hardening of the HAZ at high cooling rate by reducing C, and the reduction of the strength due to the reduction of C is either of Nb, Mo, and V, or It is intended to supplement by adding a combination. However, if these elements are actively added, there is a problem that the bainite block acting as resistance of crack propagation at the time of bainite transformation becomes coarse and the base material toughness and the HAZ toughness deteriorate.

In addition, as a technique for obtaining a hot-rolled steel sheet having a resistance ratio, while ensuring the weldability, a large amount of alloying elements such as C and V must be added to compensate for the decrease in strength due to no addition of B. Therefore, the weldability and the base metal toughness must be added. There is a problem that causes deterioration.

On the other hand, neither technique is considered to reduce acoustic anisotropy, and there is a problem in terms of acoustic anisotropy.

The present invention has been made in view of these problems, and has a high strength with a tensile strength of 780 MPa or more, a resistance ratio, excellent substrate toughness, weldability (welding crack resistance, HAZ toughness), small acoustic anisotropy, and simple defect detection during welding construction. An object of the present invention is to provide a high tensile strength steel sheet and a method of manufacturing the same.

The inventors conducted several experimental studies on the above-mentioned problems, and as a result, Nb, V which adversely affects the base material toughness and HAZ toughness after limiting the component design considering the steel structure mainly composed of bainitic ferrite, that is, C By suppressing the addition of, Mo, and actively adding the hardenability improving elements Mn, Ni, and Cu, the bainitic ferrite (from the high to the low It was found that a structure mainly composed of BF ") can be produced, thereby achieving excellent base material toughness and weldability. In addition, the amount of MA (Martensite-Austenite Constituent) contained in the tissue together with BF, which is the main agent, in the tissue was 10 area% or less and the hardness thereof was increased to increase the hardness of bainitic ferrite of MA. It was found that YR is lowered by setting the ratio (hardness of MA / hardness of BF) to 1.1 or more. In addition, it was found that the acoustic anisotropy can be reduced by controlling the ratio (flatness) of the long axis / short axis of the old austenite particles to a predetermined range. It has also been found that the ratio (flatness) of the old austenite particles can be realized by performing a predetermined amount of hot rolling in a specific temperature range depending on the chemical composition of the steel. This invention is completed based on these knowledge.

That is, the high tensile steel sheet of the present invention is C: 0.010 to 0.080%, Si: 0.02 to 0.50%, Mn: 1.10 to 3.00%, Cu: 1.60% or less, Ni: 0.40 to 2.50%, P: 0.030% S: 0.010% or less, Al: 0.200% or less, N: 0.0100% or less, Cr: 0.30 to 2.00%, Mo: 0.10 to 1.10%, and Ti: 0.002 to 0.030%, with the balance being Fe and impurities And the AS value and the DL value defined by the following formula are AS≥4.00, DL≤2.80, and the tissue in the plate thickness 1/4 site | part contains 10 area% or less of MA (except 0%) Mainly based on nitic ferrite, the average value (also referred to as "average flatness") of the long axis / short axis of the old austenite particles is 1.0 to 3.0, and the hardness ratio of the MA to bainitic ferrite is 1.10 or more. The 1/4 part of the plate thickness refers to a part having a depth of 1/4 of the plate thickness on the plate surface, and the tissue observation surface at the plate thickness 1/4 portion is rolled in the plate thickness direction (vertical direction with respect to the plate surface) and rolling as usual. It is a surface (rolling perpendicular direction cross section, TD surface) containing a direction (length direction).

AS = [Mn] + [Ni] + 2 × [Cu]

DL = 2.5 × [Mo] + 30 × [Nb] + 10 × [V]

([X] shows content (mass%) of the element X)

In the structure of the high tensile steel sheet, in order to further improve the base material toughness, the average value of the equivalent circle diameter (also referred to as the "average circle equivalent diameter") of the old austenite particles is 70 µm or less, preferably 65 µm or less, More preferably, it is preferable to set it as 60 micrometers or less. The circle equivalent diameter refers to a diameter of a circle having an area equal to that of old austenite particles.

Further, as the chemical component, any one or two of (1) B: 0.0050% or less, (2) Nb: 0.010% or less and V: 0.30% or less, (3) Ca: 0.0050% or less and rare earths Element (REM): any one or two of 0.0100% or less, (4) Mg: 0.0050% or less, (5) Hf: 0.050% or less and Zr: 0.100% or less, one or two, (6) An element selected from any one or two groups of Co: 2.50% or less and W: 2.50 or less may be contained alone or in combination.

In the preferred method for producing a high tensile strength steel sheet, the steel having the above components in the production method of the present invention is heated to an Ar 3 point to 1300 ° C., followed by hot rolling at least 50% of the total reduction in the partial recrystallization temperature range, followed by cooling. Furthermore, it reheats in the austenite ferrite two-phase area | region, and adjusts the quantity of MA and hardness ratio with respect to bainitic ferrite (hardness of MA / hardness of bainitic ferrite). In addition, after reheating in the two-phase region, it can be tempered so that the MA is decomposed to disappear at a temperature below the Ar1 point. By performing such tempering, the base metal toughness can be further improved. The partial recrystallization temperature range is 20 to 80% by volume when a steel sheet test piece having an austenite particle diameter of 100 ± 10 μm is pressed at a strain rate of 10 / sec and a strain rate of 0.2, and 10 seconds later, for example, by freezing the tissue by water cooling. Refers to the temperature range where recrystallization occurs.

The point of the steel sheet component of the present invention is to limit the amount of Nb, V and Mo that adversely affects the HAZ toughness, the base metal toughness after limiting the amount of C (DL ≤ 2.80), Mn, an quenchability improving element Ni and Cu were actively added (AS ≧ 4.00). First, the structure and characteristic which arise by hot rolling by the steel component of the steel plate of this invention are demonstrated with reference to a CCT drawing.

BRIEF DESCRIPTION OF THE DRAWINGS The CCT figure of the ultra-low C type steel (A) which actively added Mn, Ni, and Cu concerning this invention, the conventional high C type steel (B1), and the low C type steel (B2). In the figure, BF represents bainitic ferrite, GBF represents granule bainitic ferrite, M represents martensite, B represents bainite, and F represents ferrite.

In the drawing, in the steel sheet (A) of the present invention, the cooling after hot rolling is 85% or more, more preferably 90% or more in terms of the area ratio of any of the high cooling rate CR1 and the low cooling rate CR2. Will be created. Even when a thick plate having a thickness of 50 mm or more without special quenching and tempering heat treatment is performed by such a structure mainly composed of BF (sometimes a small amount of GBF is formed in the remainder), the mechanical properties of the base material are 780 MPa or more. It is obtained and also has excellent toughness. In addition, in any of the high cooling rate CR1 and the low cooling rate CR2, as described above, since the matrix structure mainly becomes BF having low cooling rate sensitivity, the hardness of the HAZ is reduced under low heat input welding conditions. Improved low temperature crack resistance) and HAZ toughness can be secured even under high heat input welding conditions.

On the other hand, in the conventional high C-based steel B1, a considerable amount of M is generated at the high cooling rate CR1, and therefore, the cooling rate sensitivity of the hardness is large. It was difficult to balance toughness. In addition, in the conventional high C steel (B1) and low C-based steel (B2), F or GBF is generated at the medium cooling rate or the low cooling rate (CR2), thereby producing coarse but massive MA. The base material strength and toughness fell, and the toughness of HAZ at the time of large heat welding could not be ensured.

Next, the relationship between the acoustic anisotropy of the steel sheet and the flatness ratio of the old austenite particles (γ particles) will be described. Regarding acoustic anisotropy, the transverse wave speed ratio C _SL / C _SC specified in JIS Z 3060 (Short wave sound speed value C _SL obtained as the vibration direction in the L direction (rolling direction) and the C direction (rolling perpendicular direction) (m / sec ) And C _SC (m / sec).

The present inventors investigated the relationship between the shear wave speed ratio (C _SL / C _SC ) and the flatness ratio of the spherical γ particles so that the shear wave speed ratio C _SL / C _{SC is set} to a low value of 1.020 or less, that is, low acoustic anisotropy. . The result is shown in FIG. It can be seen from FIG. 3 that low acoustic anisotropy with a transverse sound speed ratio of 1.020 or less is achieved when the flatness ratio of the spherical γ particles is 3.0 or less (minimum value 1.0). From the viewpoint of acoustic anisotropy, the flatness ratio of the spherical? Particles is preferably 1.8 or less, and more preferably 1.6 or less. 3 is obtained from the examples described later.

Furthermore, the investigation by the present inventors shows that there is a close relationship between the average circle equivalent diameter of the spherical γ particles and the base material toughness (vE- ₅₀ ). Although FIG. 4 shows the relationship between the average equivalent circle diameter of the sphere γ particles and the base material toughness (vE- ₅₀ ), the smaller the average equivalent circle diameter of the sphere γ particles from FIG. 4, the better the base material toughness (vE- ₅₀ ) is. It can be seen that. From this, the average circle equivalent diameter of the spherical? Particles is preferably 70 µm, preferably 60 µm or less, and more preferably 40 µm or less. 4 is obtained from the examples described later.

In general, since MA is harder than the mother phase, it is known that the yield ratio decreases when MA remains, but the toughness of the base metal also decreases. For example, when the yield ratio is to be 85% or less in ordinary steel, it is necessary to leave the MA amount about 10% or more. On the other hand, when such an amount of MA remains, the toughness of the base metal is remarkably lowered, so that desired characteristics cannot be exhibited. For this reason, it is common to use steel of the component system which is hard to produce MA so that MA does not remain, and forcibly cools after rolling. Nevertheless, if MA remains, a method of further decomposing MA by tempering is employed.

However, as a result of the inventor's concrete investigation, when the steel of the component of the present invention of AS≥4.00 and DL≤2.80 is used, by adjusting the balance between the hardness of MA and the amount of MA to the hardness of BF (parent phase), In other words, it is understood that the yield ratio can be 85% or less while maintaining the base material toughness by making the MA amount 10% or less in area ratio and making the hardness ratio to MA BF 1.10 times or more.

FIG. 5 illustrates hardness ratios for BF of MA for steel materials having a substantially same level of MA content and base material toughness (about 2 to 4% of MA, about 120 to 140J of base material toughness [vE- ₅₀ ]). The relationship between yield ratio is summarized, and it turns out that a yield ratio will be 85% or less when the hardness ratio is 1.10 times or more from the said figure. In particular, when the amount of MA exceeds 10 area%, even when the hardness ratio is 1.10 times or more, the base metal toughness deteriorates (see sample numbers 66 and 75 in Table 9 and Example No. 80 in Table 10 in Examples described later). 5 is obtained from the examples described later.

In the case of the steel of the present invention, the reason why the yield ratio decreases even if the amount of MA is reduced by increasing the hardness of MA is not clear, but is inferred to be based on the following reasons. That is, in this invention, although the quantity and hardness of MA are adjusted by performing a 2-phase area | region heat processing after hot rolling, it is thought that the hardness of MA raises because the concentration of C to MA arises at this time of heat processing. And at that time, it is estimated that the movable potential was introduce | transduced around martensite in MA according to C concentration. It is thought that the situation which is easy to surrender by the introduction of this movable potential is obtained.

Since it is a feature of the present invention to effectively utilize the presence of such MA, the MA must necessarily exist. In order to exert the effect of MA effectively, it is preferable to exist 0.5 area% or more, More preferably, it is 1.0 area% or more, More preferably, it is desirable to exist 2.0 area% or more. In particular, if the amount of MA exceeds 10 area%, the base material may be affected. Therefore, the upper limit of the amount of MA is 10 area%, preferably 9.0 area% or less, more preferably 8.0 area% or less, Even more preferably, it is preferably set to 7.0 area% or less.

Next, the reason for component limitation of the high tensile strength steel plate of this invention is demonstrated concretely. All units are mass%.

C: 0.010 to 0.080%

C is an element necessary to secure the base material strength. If it is less than 0.010%, even if it actively adds Mn, Ni, and Cu which are hardenability improvement elements, it will not be able to ensure the base material strength of 780 Mpa or more. On the other hand, if it exceeds 0.080%, martensite rather than bainitic ferrite is generated on the high cooling rate side, and the low temperature crack resistance is deteriorated. By adding 0.010% or more of C and limiting it to 0.080% or less, and simultaneously adding an appropriate amount of Mn, Ni, Cu and Cr, both the low temperature crack resistance of the HAZ during small heat welding and the strength of the base metal can be made compatible. The toughness of the HAZ of the city can be improved. For this reason, the lower limit of the amount of C is made 0.010%, preferably 0.030%, while the upper limit thereof is made 0.080%, preferably 0.060%.

Si: 0.02 to 0.50%

Si is an element having a deoxidation action, and when the amount of Si is less than 0.02%, the effect is too small, while when it exceeds 0.50%, weldability and base material toughness deteriorate. For this reason, the minimum of Si amount is made into 0.02%, and the upper limit is made into 0.50%, Preferably it is 0.20%.

Mn: 1.10-3.00%, Ni: 0.40-2.50%, Cu: 1.60% or less

These elements have the effect of improving the hardenability, and are easy to produce bainitic ferrite from high cooling rate to low cooling rate, and by their active addition and extremely low C, the HAZ toughness and resistance at the time of small heat input welding It is compatible with low temperature cracking and can also improve the substrate strength, toughness and HAZ toughness in large heat input welding.

That is, Mn improves hardenability and is effective for securing strength and toughness. If it is less than 1.10%, this action is too small, whereas if it exceeds 3.00%, low temperature toughness deteriorates. For this reason, the minimum of Mn amount is 1.10%, Preferably it is 1.30%, More preferably, it is 1.40%, and the upper limit is 3.00%, Preferably it is 2.20%, More preferably, it is 2.10%.

Ni also improves the low temperature toughness and hardenability of the steel and improves the strength while also being effective in preventing hot cracking and hot welding cracking. If the amount of Ni is less than 0.40%, these effects are too small. On the other hand, if the amount of Ni exceeds 2.50%, scale marks tend to occur. For this reason, the lower limit of the amount of Ni is 0.40%, preferably 0.50%, and the upper limit thereof is 2.50%, preferably 2.00%.

Cu improves hardenability, although not as much as Mo, Mn, Ni, and Cr, and also improves the strength of the base metal by solid solution strengthening and precipitation strengthening. In order to effectively express this action, it is preferably added at least 0.10%, more preferably at least 0.50%, even more preferably at least 0.80%. In particular, if the content exceeds 1.60%, the base metal toughness and the HAZ toughness at the time of heat input welding are lowered, so the upper limit of the amount of Cu is 1.60%, preferably 1.20%.

AS value: 4.00 or more

The amount of Mn, Ni, and Cu added is closely related to the strength of the base metal, and Cu is about twice as high in strength as the Mn and Ni. In order to make the base material strength 780 MPa or more in the range of high cooling rate to low cooling rate, Mn, Ni, and so on, the AS value is 4.00 or more, preferably 4.20 or more, more preferably 4.40 or more, as is apparent from the examples described later. Cu needs to be added.

P: 0.030% or less

P, which is an impurity element, is adversely affected by the toughness of the base metal and the weld portion, and is therefore 0.030% or less. Preferably, you may be 0.010% or less.

S: 0.010% or less

S is an element that forms MnS and lowers ductility, and particularly, the effect is large in high strength steel, and therefore it is preferably set to 0.010% or less, preferably 0.005% or less.

Al: 0.200% or less

Al has the effect of improving the base material toughness by deoxidation and miniaturization of the microstructure. In order to express such an effect effectively, the addition is preferably at least 0.010%, more preferably at least 0.020%. In particular, when excessively added, the toughness of the base metal is lowered, so the upper limit is made 0.200%. Preferably it is 0.060% or less.

N: 0.0100% or less

N combines with Ti mentioned later, forms TiN, refines austenite particles during large heat input welding, and has the effect of improving HAZ toughness. In order to express such an effect effectively, the addition is preferably at least 0.0020%, more preferably at least 0.0040%. However, excessive addition of N adversely affects the base metal toughness and the HAZ toughness, so the upper limit thereof is made 0.0100%, preferably 0.0080%, and more preferably 0.0060% or less.

Cr: 0.30 to 2.00%

Cr increases the strength of the base metal and the welded part, but when the amount of Cr is less than 0.30%, such an effect is too small. On the other hand, when Cr exceeds 2.00%, the weldability or the HAZ toughness is deteriorated. For this reason, the minimum of Cr amount is 0.30%, Preferably it is 0.50%, More preferably, it is 0.70%, The upper limit is 2.00%, Preferably it is 1.50%, More preferably, it is 1.00%.

Mo: 0.10 to 1.10%

Mo is effective for securing hardenability and securing strength, and has an effect of preventing temper brittleness. If the amount of Mo is less than 0.10%, this action is too small, so the lower limit of the amount of Mo is made 0.10%, preferably 0.15%. On the other hand, Mo has a recrystallization inhibiting action, and when added excessively, it becomes coarse austenite particles after rolling, coarsening the bainite block (block of bainitic ferrite) after transformation and deteriorating the toughness of the base material. In addition, Mo is easy to segregate at the austenite grain boundary, and when added excessively, Mo decreases the frequency of nucleation during transformation, coarsens the bainite block after transformation, and deteriorates base material toughness and HAZ toughness. For this reason, the upper limit of Mo amount is made 1.10%, Preferably it is 0.60%.

DL value: Below 2.80

Mo and the below-mentioned Nb and V have the effect | action which improves hardenability, On the other hand, coarse a bainite block, and deteriorate base material toughness and HAZ toughness. Such deterioration of the toughness of the base metal is not the same for each element. When Mo is 1 by an experiment by the inventor or the like, Nb is about 12 times and V is about 4 times. As apparent from the examples below, the addition of Mo, Nb, and V is suppressed so that the DL value is 2.80 or less, preferably 2.50 or less, more preferably 2.00 or less, thereby suppressing coarsening of the bainite block. By setting the reduction amount in the region of AS≥4.00 and the partial recrystallization temperature to 50% or more of the total rolling reduction, the average circle equivalent diameter of the spherical γ particles is refined to about 70 µm or less, and the base material toughness of vE _-50 ≥ 100J or more is reduced. It can ensure and can also have favorable HAZ toughness.

Ti: 0.002 to 0.030%

Ti is an element that is effective in miniaturizing austenite particles of HAZ and improving HAZ toughness at the time of welding by forming a nitride by bonding with N. If the amount of Ti is less than 0.002%, the refining effect is too small, while if the amount of Ti exceeds 0.030%, the HAZ toughness is deteriorated. For this reason, the minimum of Ti amount is made into 0.002%, Preferably it is 0.005%, and the upper limit is made into 0.030%, Preferably it is 0.020%.

The steel sheet of the present invention is formed by the balance Fe and impurities in addition to the above components, but does not prevent the addition of an element that further improves the characteristics in a range that does not impair the action and effect of the components. For example, (1) B in the following range, (2) any one or two of Nb and V in the following range, (3) any one or more of Ca and REM in the following range, (4) Mg in the following range, (5) any one or two of Zr and Hf in the following ranges, and (6) an element selected from any one or two of Co and W in the following ranges may be added alone or in combination. Can be.

B: 0.0050% or less

B has the effect of improving hardenability and improving HAZ toughness. In particular, the effect is great when welding heat input is large. In order to express this action effectively, an addition of 0.0005% or more is preferred. In particular, in large amounts, the base metal toughness and HAZ toughness are deteriorated. For this reason, the upper limit of the amount of B is made into 0.0050%, Preferably it is 0.045%. More preferably, it is 0.0010 to 0.0040%.

Nb: 0.10% or less

Solid solution Nb has the effect of improving the hardenability of the base material and improving the base metal strength and the weld joint strength, and can be added as necessary. On the other hand, since the solid solution Nb suppresses the recovery of the processed austenite and suppresses recrystallization, it becomes coarse austenite particles after rolling, coarsening the bainite block after transformation and significantly lowering the base material toughness. In addition, Nb tends to segregate in the austenite grain boundary. When added excessively, Nb lowers the frequency of nucleation during transformation, coarsens the bainite block after transformation, and deteriorates base material toughness and HAZ toughness. For this reason, the upper limit of Nb amount is made into 0.10%, Preferably it is 0.020%, More preferably, it is 0.015%.

V: 0.30% or less

V has the effect of raising hardenability and temper softening resistance by addition of a small amount, and can be added as needed. On the other hand, since V suppresses the recovery of processed austenite and suppresses recrystallization, it becomes coarse austenite particles after rolling and coarsens the bainite block after transformation, thereby significantly reducing the base metal toughness. In addition, V is easy to segregate at the austenite grain boundary, and when excessively added, the frequency of nucleation during transformation is reduced, coarsening of the bainite block after transformation is deteriorated, and the base metal toughness and HAZ toughness are degraded. For this reason, the upper limit of the amount of V is made 0.30%. Preferably it is 0.20% or less, More preferably, it is 0.10% or less.

Ca: 0.0050% or less, REM: 0.0100% or less

Ca and REM have the effect of reducing anisotropy by controlling the shape of inclusions which spheroidize MnS. At Ca: greater than 0.0050% and REM: greater than 0.0100%, the toughness of the base material is deteriorated because the added amount is excessive. For this reason, the upper limit of Ca amount is 0.0050%, Preferably it is 0.0030%, and the upper limit of REM is 0.0100%, Preferably it is 0.0070%. In order to utilize each said element effectively, it is preferable to contain Ca: 0.0005% or more, REM: 0.0010% or more.

Mg: 0.0050% or less

Mg forms MgO and has the effect | action which improves HAZ toughness by suppressing the coarsening of the austenite particle of HAZ. In order to utilize this effect effectively, it is preferable to contain Mg: 0.0001% or more. At Mg: more than 0.0050%, the toughness of the base material is rather deteriorated because the added amount is excessive. For this reason, the upper limit of Mg amount is made into 0.0050%, Preferably it is 0.0035%.

Zr: 0.100% or less, Hf: 0.050% or less

Zr and Hf form an nitride with N like Ti to refine the austenite particles of HAZ during welding, and are effective elements for improving HAZ toughness. However, excessive addition decreases the base metal toughness and the HAZ toughness. For this reason, the upper limit of the amount of Zr is made 0.100% and the upper limit of the amount of Hf is made 0.050%.

Co: 2.50% or less, W: 2.50% or less

Co and W are effective elements in order to improve hardenability and to secure strength easily, and W has the effect of further improving temper softening resistance. On the other hand, when excessively added, base material toughness and HAZ toughness deteriorate rather. For this reason, the upper limit of Co amount and W amount is made 2.50%, Preferably it is 1.00%.

Next, the manufacturing method of the resistive ratio high tensile strength steel plate of this invention is demonstrated.

In the production method of the present invention, it is assumed that steel having the chemical composition is used, and hot rolling conditions must be strictly managed in order to control the form of the spherical? Particles. In addition, after obtaining a hot-rolled steel sheet having a structure predominantly BF, reheating is performed in a two-phase region for adjusting the yield ratio, that is, for adjusting the amount and hardness of MA, and tempering to adjust the base material toughness as necessary. Is carried out. The other process and conditions at the time of manufacturing the steel plate of this invention are not specifically limited, The manufacturing process and conditions (temperature, time, etc.) of the high tension steel plate normally used can be employ | adopted suitably.

In the rolling conditions in the production method of the present invention, the steel pieces are heated to an Ar 3 point to 1300 ° C. to completely austenite, and then hot rolling is performed. In hot rolling, it is particularly important to roll at least 50% of the total reduction, preferably at least 70% of the total reduction in the region of partial recrystallization temperature. By rolling in such a temperature range, the average flatness and the average circle equivalent diameter are controlled to predetermined values so as to equalize the shape of the spherical? Particles in the steel sheet by using a phenomenon called partial recrystallization, as is apparent from the examples described later. can do.

Since the said partial recrystallization temperature area | region changes with the chemical composition of a steel plate, it is preferable to irradiate the temperature area | region by appropriate experiment before performing hot rolling. That is, a steel sheet test piece having the same chemical composition as that of the steel sheet to be manufactured is prepared, and the test piece is heated to a temperature at which the austenite particle diameter becomes 100 ± 10 μm, and then the test piece is subjected to a strain rate of 10 / sec and a strain rate of 0.2. 10 seconds after the reduction, the temperature range in which the recrystallized grain becomes 20 to 80% by volume when the tissue is frozen by, for example, water cooling, is determined in advance.

The cooling conditions after the said hot rolling are air-cooled or ice-cooled to the temperature below Bs point (about 200 degreeC). Since the amount of MA is adjusted by the reheating treatment in a two-phase region to be performed later, it may be quenching (in this case, it is difficult to produce MA) or air cooling, but in order to increase the amount of BF in the steel structure more water-cooling It is desirable to. The water cooling here means that a cooling rate is about 3 degree-C / sec or more, and it is preferable to set it as 5 degree-C / sec or more.

After cooling after hot rolling, a predetermined low-temperature transformation structure is obtained, and after cooling, reheating is typically performed in a two-phase region of about 700 to 900 ° C. in order to reduce the yield ratio. Thereby, MA is produced from BF (parent phase), carbon is concentrated in MA, and the amount of MA is adjusted to 10 area% or less, and the hardness ratio of MA to BF is 1.10 or more.

In addition, after the reheating, it may be tempered at a temperature below the Ar1 point as necessary. In this case, the holding temperature and the holding time are adjusted so that the MA is not completely decomposed and lost. Such tempering can improve the base metal toughness while realizing a resistance ratio. Cooling from the tempering temperature is not particularly limited, and air cooling is preferred.

Next, although an Example is given and this invention is demonstrated more concretely, this invention is not limited by these Examples.

Example

The steel shown in Tables 1-5 was melted by the usual solvent method, and it was made into slab, and as shown in Tables 6-10, after heating on the conditions shown in the table | surface, hot rolling is performed and the temperature of 200 degrees C or less is carried out. After air-cooling (average cooling rate: about 5 to 10 ° C / sec), reheating was performed for about 15 to 30 minutes in the two-phase region, and further tempered as needed, followed by cooling by air cooling. In addition, each sample of Tables 6-10 was manufactured using the steel of the same number of Tables 1-5. In addition, the reduction amount in the partial recrystallization temperature range shows the ratio (%) of the reduction rate rolled to the partial recrystallization temperature range with respect to the total reduction ratio in hot rolling.

About the obtained hot rolled sheet, the structure observation test piece was extract | collected from the 1/4 site | part of the plate | board thickness of a hot rolled sheet, and the optical microscope observation (magnification 400x) was performed, and BF is a main body, and remainder is MA or MA and trace amount It was composed of GBF. In order to measure these area fractions and the like, the tissue observation test piece was corroded with 2% nitric acid-ethanol solution (collectively, 2% nital solution), and then the tissue was photographed at 1000 times magnification using a scanning electron microscope (SEM). The photographed image was analyzed using image analysis software (named Image-Pro, manufactured by Pranetron, Inc.) to obtain area ratios of BF, GBF and MA.

In addition, the hardness of BF (parent shape) and MA was measured 5 points at a load of 1 g using a micro picker tester (manufactured by Akaishi Co., Ltd.), and the average value of three points excluding the highest value and the lowest value was obtained. The hardness ratio with respect to BF of MA was calculated | required.

In addition, using the above-described tissue observation test piece, the flatness ratio and the equivalent circle diameter of the spherical γ particles were determined by the following methods. The mirror-polished test piece is subjected to corrosion treatment using AGS liquid, 2% nital liquid, or the like manufactured by Yamamoto Scientific Tool Research. Corrosion conditions are 5 to 10 minutes at room temperature in the case of the AGS liquid, and 5 to 30 seconds at room temperature in the case of 2% nital liquid. The specimen after corrosion is observed at 400 times magnification using an optical microscope, and photographing is performed. The obtained micrographs were subjected to image analysis using image analysis software (named Image-Pro P1us, manufactured by Media Cybernetics) to obtain a circle equivalent diameter, and to obtain the spherical γ particle long axis and short axis length. The value of (long axis / short axis) is calculated.

Moreover, the acoustic anisotropy was investigated using the sample steel plate. Acoustic anisotropy, according to JIS Z 3060, transverse sound speed C _SL (m / sec) in the L direction (rolling direction) and transverse sound speed value C _SC (m / sec) in the C direction (rolling perpendicular direction). Was measured, and the shear wave speed ratio C _SL / C _SC was calculated and evaluated by this.

In addition, the tensile test and the impact test were implemented with the following method, and the mechanical property of the base material was investigated.

The tensile test was carried out using a JIS No. 4 test piece taken from the plate thickness 1/4 site of each steel plate, and the yield ratio (YS / TS × 100%) was measured by measuring 0.2% yield strength (YS) and tensile strength (TS). Saved. In addition, the impact test performed the Charpy impact test at -50 degreeC using the JIS No. 4 test piece extract | collected from the plate | board thickness 1/4 site | part of each steel plate, and calculated | required the absorption energy (vE- ₅₀ ). In this invention, TS≥780MPa, YR <85%, and base material toughness vE- _{50 <} = 100 (J) were made into the pass level.

In addition, although both tensile strength was 780 Mpa or more and the base material toughness vE- _{50 <} = 100 (J) and the pass criterion was not reached, some HAZ toughness and low temperature crack resistance were investigated as the following method.

HAZ toughness was welded (submerged arc welding) at a heat input of 5 kJ / mm, 10 kJ / mm, and 15 kJ / mm, and a JIS No. 4 test piece was collected from the test piece sampling site 3 shown in FIG. , Charpy impact test was conducted to determine the absorbed energy (vE-40) of the bond portion, and vE _-40 ? 80J was set as the pass level. In the figure, 1 is a steel plate, 2 is a welded metal part, and 3 is a test piece collecting part, and is located on the improvement opening side at the center of the plate thickness. Heat input welding with a heat input greater than 15 kJ / mm was carried out to see the effect of alloying elements when the cooling rate was very slow.

Low temperature crack resistance According to the y type welding crack test method prescribed | regulated to JISZ3158, coating arc welding was performed at the heat input of 1.7 kJ / mm, and the root crack prevention preheating temperature was measured. Preheating temperature of 0 degreeC shows that the steel plate provided for the test was welded in the state cooled to 0 degreeC, and a crack did not generate | occur | produce after welding.

The investigation results are shown together in Tables 6 to 10. Moreover, FIG. 3 (sample numbers 1-12, 89-93 which plotted) and the relationship of the average circular equivalent diameter of a spherical-gamma particle and a base material toughness are shown in FIG. (Sample Nos. 1 to 4), the relationship between the hardness ratio of MA to BF and YR is shown in FIG. 5 (plot No. 43, 49 to 52, 96 to 99), and the relationship between AS value and tensile strength is shown in FIG. Plotted sample numbers 11, 18 to 20, 62, 73 and 84).

It can be seen from FIG. 3 that low acoustic anisotropy with a flatness ratio of sphere γ particles of 3.0 or less and a transverse sound velocity ratio of 1.020 or less is obtained. In addition, from FIG. 4, as the average circle equivalent diameter of the spherical γ particles is refined, the base material toughness (vE _-50 ) is improved, and the average energy equivalent diameter of the spherical γ particles is 70 μm or less, so that the absorption energy is about 100 J or more. It can be seen that. 5 shows that when the hardness ratio is 1.1 or more, a high strength steel sheet having a resistivity ratio of 85% or less is obtained. 6, it turns out that a high strength steel plate whose tensile strength is 780 Mpa or more is obtained by making AS value 4.00 or more.

Further, from Tables 6 to 8, the invention examples show that the vE- ₅₀ is all 100J or more in terms of the base material toughness, and no root cracking occurs even when the steel plate temperature is 0 ° C in terms of low temperature crack resistance, and the base material toughness and low temperature cracking are prevented. Excellent in sex Moreover, also regarding HAZ toughness, it was confirmed that the bond part was excellent in any of small heat input welding and large heat input welding. In addition, in the invention example, it was confirmed that addition of 0.0005% or more of B always yielded excellent HAZ toughness of 150 J or more even when a large heat input welding of more than 15 kJ / mm was performed.

On the other hand, as shown in Tables 9 and 10, the comparative example in which the alloy composition (including the AS value and the DL value) is out of the invention range has a tensile strength of less than 780 MPa or a base material toughness of vE ₋ , even if the manufacturing conditions are appropriate. ₅₀ became less than ₁₀₀ J and did not reach the acceptance level. In addition, even if the alloy composition was in the invention range as in Sample Nos. 84, 87, 89 to 93, the manufacturing conditions were inadequate, and when the rolling reduction in the partial recrystallization temperature range was less than 50%, the acoustic anisotropy was greater than 1.020 and the acoustic anisotropy deteriorated. . In addition, as shown in Sample Nos. 96 to 99, even if the components, the heating hot rolling conditions, and the tempering temperature were appropriate, the YR was higher than 85% and the target level was not reached because the reheating was not performed in the two-phase region. .

According to the high-strength steel sheet of the present invention, C is extremely low and Mn, Ni, and Cu are actively added so that the AS value is 4.00 or more, while Mo, Nb, and V are added so that the DL value is 2.80 or less. Because of the adjustment, the microstructure mainly composed of bainitic ferrite hardly constrained from crack propagation even when the plate thickness is thicker than 50 mm without being restrained by the height of the cooling rate after hot rolling can be used. It has excellent weldability (welding crack resistance, HAZ toughness). In addition, since the hardness of MA was increased by reheating in the two-phase region to make the hardness ratio of the bainitic ferrite of MA to 1.10 or more, a high strength of 780 MPa or more and a resistivity ratio of 85% or less can be realized, and the former austenite particles By setting the average flatness of to 1.0 to 3.0, the acoustic anisotropy can be reduced, and the defect detection work during welding construction can be simplified.

Claims (10)

As mass%, C: 0.010 to 0.080%, Si: 0.02 to 0.50%, Mn: 1.10-3.00%, Cu: 1.60% or less, Ni: 0.40 to 2.50%, P: 0.030% or less, S: 0.010% or less, Al: 0.200% or less, N: 0.0100% or less, Cr: 0.30 to 2.00%, Mo: 0.10 to 1.10% and Ti: 0.002 to 0.030% Wherein the balance is composed of Fe and unavoidable impurities, and the AS value and DL value defined by the following formula are AS≥4.00, DL≤2.80, and the tissue at the plate thickness 1/4 site is MA (Martensite). -Austenite Constituent: mainly composed of bainitic ferrite containing up to 10 area% (except 0%) of martensite and austenite, and the average long / short axis of the old austenite particles is 1.0 to 3.0 In addition, the hardness ratio to the bainitic ferrite of the MA is 1.10 or more, characterized in that the low anisotropy, weldability high tensile strength steel sheet excellent in weldability. AS = [Mn] + [Ni] + 2 × [Cu] DL = 2.5 × [Mo] + 30 × [Nb] + 10 × [V] ([X] shows content (mass%) of the element X) The method of claim 1, In the above structure, further, the average value of the equivalent circle diameter of the old austenite particles is 70 µm or less. The method of claim 1, B: A resistive ratio high tensile strength steel sheet further containing 0.0050% or less. The method of claim 1, Nb: 0.10% or less and V: 0.30% or less of any one or two of the resistance to high ratio high tensile strength steel sheet. The method of claim 1, A resistive high tensile strength steel sheet, which further contains any one or two of Ca: 0.0050% or less and rare earth element: 0.0100% or less. The method of claim 1, Mg: A resistive high tensile strength steel sheet, which further contains 0.0050% or less. The method of claim 1, A resistance-ratio high tensile strength steel sheet further comprising any one or two of Hf: 0.050% or less and Zr: 0.100% or less. The method of claim 1, Co: 2.50% or less and W: 2.50% or less, any one or two or more of the resistive ratio high tensile strength steel sheet. Heating the steel having the component according to any one of claims 1 to 8 to an Ar 3 point to 1300 ° C .; When the steel sheet test piece with the austenite grain size of 100 ± 10 μm was pressed at a strain rate of 10 / sec and a strain rate of 0.2, and the tissue was frozen 10 seconds later, the entire recrystallization temperature range was 20 to 80% by volume. After hot rolling at least 50% of the reduction amount; Cool; In addition, reheating in the two-phase region of austenite ferrite to adjust the amount of MA and the hardness ratio for bainitic ferrite  A method for producing a resistive high tensile strength high strength steel sheet, characterized by having low acoustic anisotropy and excellent weldability according to any one of claims 1 to 8. The method of claim 9, And after reheating in the two-phase region, tempering treatment is performed so that the MA is decomposed and lost at a temperature below the Ar1 point.

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