JP2007138271A - High yield ratio high tensile strength steel plate having excellent toughness in weld heat-affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high yield ratio high tensile strength steel plate having a tensile strength in a class of 570 MPa, in which high heat input HAZ toughness is improved as much as possible. <P>SOLUTION: The high yield ratio high tensile strength steel plate has a suitably controlled chemical componential composition; wherein, HM value prescribed by the following equation (1) satisfies 0.10 to <0.25%, HG value prescribed by the following equation (2) satisfies 0.02 to <0.08%, and HB value prescribed by the following equation (3) satisfies ≤0.0%, respectively, and also has a structure in which the fraction of bainite is ≥90 area%. Formula (1)-(3) are: (1) HM=[C]+[Mn]/30+[Cr]/30+[Mo]/5+[Si]/5; (2) HG=-[C]+[Mn]/25+[Cr]/25-[Mo]/30-[Si]/10; and (3) HB=-[Cr]/10+[Mn]/10-[Nb]; wherein [C], [Mn], [Cr], [Mo], [Si] and [Nb] denote the contents (mass%) of C, Mn, Cr, Mo, Si and Nb, respectively. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-yield ratio high-tensile steel sheet having excellent toughness in a weld heat affected zone (HAZ) and having a tensile strength of 570 MPa or more.

  High yield ratio high tensile steel sheets having a tensile strength of 570 MPa or more are used as materials for various building structures and bridges. Architectural concepts and the like are constructed by welding high-tensile steel plates, but the characteristics required for high-tensile steel plates include the weld heat affected zone (HAZ) when high heat input welding is applied. It is necessary that the toughness is good.

  In addition, it is sometimes required that the yield ratio [yield strength / tensile strength × 100 (%)] is small (that is, the plastic deformability is high) for the application of ultimate strength design for earthquakes. In the case of building use, 80% or less), and from the viewpoint of reduction of steel materials (steel weight), it is preferable that the yield is high yield (the yield ratio is 80 or more) depending on the use.

  As a technique for improving the HAZ toughness in a high-tensile steel having a tensile strength of 570 MPa or more, for example, a technique shown in Patent Document 1 has been proposed. In this technology, C is extremely low and the bainite phase is made into a basic structure (low temperature transformation bainite structure), thereby suppressing the formation of island martensite phase (MA phase) during high heat input welding and quenching. Active elements Mn and Cr (Mo if necessary) are positively added so as to satisfy the predetermined relational expression, and V and Nb, which are elements that lower the high heat input HAZ toughness, are added to the predetermined relational expression. In order to satisfy the above, B is further added.

  Making C a very low bainite structure (hereinafter referred to as “ultra-low C bainite structure”) is effective in suppressing the formation of the MA phase and improving the high heat input HAZ toughness. It cannot always be said that the control of the HAZ structure is properly performed only by making the extremely low C bainite structure, and in some cases, sufficient large heat input HAZ toughness may not be obtained.

  On the other hand, Patent Document 2 discloses a very low C (C content: 0.03% or less) and less dependence on the cooling rate by optimizing the amounts of Nb and B (that is, less variation in material). ) A technique for producing ultra-low C bainitic steel has been proposed. This technology also suppresses coarsening of prior austenite grains in HAZ by uniformly dispersing oxide inclusions (Ti, Ca, Al, and REM oxides) from the viewpoint of improving high heat input HAZ toughness. It has also been shown to do.

However, if the welding heat input becomes large, there is a limit to the coarsening of the prior austenite grains in the HAZ, and the large heat input HAZ toughness may not be improved only by suppressing the coarsening of the prior austenite grains.
Japanese Patent No. 3606021 Patent Claim etc. JP, 2000-345239, A Claims etc.

  The present invention has been made to solve such problems in the prior art, and an object of the present invention is to provide a high-yield-ratio high-tensile steel sheet having a tensile strength of 570 MPa that has improved high heat input HAZ toughness as much as possible. It is in.

The high-yield ratio high-tensile steel sheet of the present invention that has achieved the above-mentioned object is: C: 0.01 to 0.05% (meaning mass%, hereinafter the same), Si: 1.0% or less (0% Not contained), Mn: 0.5 to 2.0%, P: 0.5 or less (including 0%), S: 0.01% or less (including 0%), Al: 0.01 to 0. 07%, Cr: 0.5-2.0%, Mo: 0.5% or less (including 0%), Nb: 0.0020-0.030%, Ti: 0.005-0.03%, B: 0.0005 to 0.0030%, Ca: 0.0005 to 0.005%, N: 0.0020 to 0.0080%, respectively, and the HM value defined by the following formula (1) is HG value 0.10% or more, less than 0.25%, HG value specified by the following formula (2) is 0.02% or more, less than 0.08%, and the following formula (3) 0.0% or less respectively satisfied, and bainite fraction of those with the spirit to a point which is 90 area% or more tissues.
HM = [C] + [Mn] / 30 + [Cr] / 30 + [Mo] / 5 + [Si] / 5 (1)
HG =-[C] + [Mn] / 25 + [Cr] / 25- [Mo] / 30- [Si] / 10 (2)
HB =-[Cr] / 10 + [Mn] / 10- [Nb] (3)
However, [C], [Mn], [Cr], [Mo], [Si] and [Nb] indicate the contents (mass%) of C, Mn, Cr, Mo, Si and Nb, respectively.

  In the high yield ratio high tensile strength steel sheet of the present invention, if necessary, (a) Cu: 3.0% or less (not including 0%) and / or Ni: 3.0% or less (not including 0%), (B) V: 0.05% or less (excluding 0%), (c) Mg: 0.005% or less (not including 0%), (d) Zr: 0.005% or less (excluding 0%) (E) rare earth elements: 0.0003 to 0.03%, etc. are also effective, and the properties of the high-tensile steel sheet can be further improved depending on the components contained.

  In the high-strength steel sheet of the present invention, the MA phase amount, the prior austenite grain size, and the bainite block size, which are factors affecting HAZ toughness, are stipulated by strictly defining the chemical composition, A bainite high-yield ratio high-tensile steel sheet having a tensile strength of 570 MPa that can stably ensure good HAZ toughness can be realized, and such a high-tensile steel sheet is extremely useful as a material for various building structures.

  As steel plates for obtaining good HAZ toughness, steel plates having an extremely low C bainite structure are widely used. The present inventors examined the means from various angles in order to further improve the HAZ toughness based on the steel sheet having such a structure.

  In the techniques proposed so far, as the factors affecting the HAZ toughness, the MA phase amount in HAZ, the prior austenite grain size, and the like are known. In addition to these factors, it was found that appropriately controlling the size of the structural units (bainite blocks) in the prior austenite grains is also an important factor.

  In the elements (C, Si, Mn, Cr, Mo, Nb, etc.) generally contained in the high-tensile steel, the present inventors have (1) the amount of the MA phase and (2) the prior austenite. Further detailed investigations were made on the influence of particle size and (3) bainite block size on each factor. As a result, it has been found that if the relational expression is defined by a specific element for each of the above requirements, all the requirements are good, and the HAZ toughness is remarkably improved, and the present invention has been completed. In the present invention, the HM value, the HG value, and the HB value respectively defined by the specific elements as in the above formulas (1) to (3) need to satisfy a predetermined range. The reason is as follows.

0.10 (%) ≤ HM <0.25 (%)
In order to improve the HAZ toughness, it is necessary to reduce as much as possible the amount of the MA phase that is the starting point of fracture in the HAZ. The M-A phase is a phase in which martensite and retained austenite are precipitated in the structure due to the concentration of C in the structure and the lowering of the transformation temperature in that portion. Therefore, in order to reduce the MA phase, it is effective to reduce the C content itself. In order to reduce the MA phase, it is necessary to reduce the amount of retained austenite by reducing the austenite stabilizing elements (Mn, Cr, Mo, Si, etc.). However, if the C content and the austenite stabilizing element are excessively reduced, there arises a problem that the strength cannot be secured. That is, in the steel sheet in which the HM value is less than 0.25%, the MA phase in the HAZ is sufficiently reduced to exhibit good toughness. On the other hand, when the HM value is less than 0.10%, the hardenability is lowered, the ultra-low C bainite structure is not sufficiently generated, and the structure is mainly composed of ferrite, and the strength required for the steel sheet cannot be ensured.

0.02 (%) ≦ HG <0.08 (%)
In order to suppress the coarsening of the prior austenite grains in the HAZ, it is common to use TiN. In the present invention, TiN is also used, and if it is within the above range, it is sufficiently effective. In addition, from the viewpoint of pinning austenite grain growth by nitrides, carbides, oxides, etc. and delaying grain growth itself, the results of quantitative investigation of the effects of C, Mn, Cr, Mo, Si, led to the HG value It was. In a steel sheet having an HG value of less than 0.08%, coarsening of prior austenite grains in the HAZ will be suppressed, and good toughness will be exhibited. On the other hand, when the HG value is less than 0.02%, an extremely low C bainite structure is not sufficiently generated, and the strength required for the steel sheet cannot be secured.

HB ≦ 0.0 (%)
By reducing the structure unit (bainite block) in the prior austenite grains in the HAZ, the crack propagation resistance at the time of fracture increases, and the HAZ toughness can be improved. In a steel sheet having an HB value of 0.0% or less, a large number of bainite laths having different orientations are developed from the former austenite grains in the HAZ, and good toughness is exhibited. There are other elements that promote the refinement of the bainite block size (for example, Cu, Ni, etc.), but their effects are small, and in the present invention, they are defined only by the above three elements. In particular, it is characterized by a large amount of Cr.

  The high-tensile steel sheet of the present invention is based on a bainite structure, but such a bainite structure is useful for securing a strength of 570 MPa or more despite the extremely low C. Generally, in line pipes and the like, high strength is realized by mainly using a ferrite structure, but in a ferrite structure, it is necessary to realize high strength as fine ferrite by performing low temperature rolling. . On the other hand, the bainite structure can achieve high strength even at high temperature rolling, and is useful for improving productivity. However, in order to exhibit these effects, 100 area% does not necessarily need to be a bainite structure, and what is necessary is just 90 area% or more by a bainite fraction. Examples of structures other than bainite include martensite and ferrite.

  The bainite structure in the present invention includes “steel bainite photo collection-1” [edited by Japan Iron and Steel Institute, bainite research group: (1992). 4] including bainitic ferrite or granular bainitic ferrite introduced in [4]. The bainite structure (extremely low C bainite structure) in which the amount of C is extremely reduced is excellent in strength and toughness, and can be obtained by setting the chemical composition within the range specified in the present invention.

  In the high-strength steel sheet of the present invention, it is also an important requirement to strictly adjust the chemical composition, but the reason for limiting the range is as follows.

C: 0.01 to 0.05%
C is an element effective for increasing the strength of high-strength steel, and needs to be contained in an amount of 0.01% or more in order to ensure a desired strength. However, when C is excessively contained, a large amount of MA phase or cementite is formed, and it becomes difficult to stably generate an extremely low C bainite structure. For these reasons, the upper limit needs to be 0.05%.

Si: 1.0% or less (excluding 0%)
Si is an effective element for increasing the strength of steel by solid solution strengthening regardless of the cooling conditions. However, if it is excessively contained, a large amount of island martensite phase (MA phase) is added to the steel (base material). It precipitates in and deteriorates toughness. For these reasons, the upper limit was made 1.0%. In addition, the upper limit with preferable Si content is 0.5%.

Mn: 0.5 to 2.0%
Mn is an element effective for strengthening steel by generating an extremely low C bainite structure. In order to exert such an effect, Mn needs to be contained in an amount of 0.5% or more. However, if Mn is excessively contained, the toughness of the base material is deteriorated, so the upper limit is made 2.0%. The minimum with preferable Mn content is 0.7%, and a preferable upper limit is 1.8%.

P: 0.5% or less (including 0%) and S: 0.02% or less (including 0%)
P is an impurity that segregates in crystal grains and adversely affects ductility and toughness. Therefore, it is preferable that P be as small as possible (including 0%). It is good to suppress to 5% or less. Further, S is an impurity because it reacts with alloy elements in the steel material to form various inclusions and adversely affects the ductility and toughness of the steel material. Therefore, it is preferable that S be as small as possible (including 0%). ), It is better to suppress to 0.02% or less in consideration of inevitably mixed.

Al: 0.01 to 0.07%
Al is an element effective as a deoxidizing agent, and is an element that increases the solid solution amount of B by fixing N in the steel material. Thereby, the effect of improving hardenability by B is improved. In order to exert such effects, the Al content needs to be 0.01% or more. However, if contained excessively, a large amount of island-like martensite phase (MA phase) is precipitated in the steel material (base material) to deteriorate toughness. For these reasons, the upper limit was made 0.07%. In addition, the minimum with preferable Al content is 0.02%, and a preferable upper limit is 0.05%.

Cr: 0.5 to 2.0%
Cr is an important element for obtaining an extremely low C bainite structure. It is also effective for reducing the bainite block size in the HAZ structure. Furthermore, it is an element effective in improving the hardenability and ensuring the strength of the steel material. In order to exert these effects, it is necessary to contain 0.5% or more of Cr. However, if the Cr content is excessive and exceeds 2.0%, coarse precipitates are formed, so that the toughness of both the base material and the HAZ deteriorates. In addition, the minimum with preferable Cr content is 0.7%, and a preferable upper limit is 1.8%.

Mo: 0.5% or less (including 0%)
Mo is an element effective in improving the hardenability and improving the strength. However, if it is excessively contained in excess of 0.5%, it becomes a coarse hardened phase, so both the toughness of the base material and the HAZ deteriorate. To do. In addition, in order to obtain an extremely low C bainite structure in the present invention, it is not necessarily a necessary element, and it may not be added. However, when Mo is not included, the above formulas (1) and (2) need to be calculated as not including Mo. The upper limit with preferable Mo content is 0.4%.

Nb: 0.005 to 0.030%
Nb is an important element for obtaining an extremely low C bainite structure. It is also effective for reducing the bainite block size in the HAZ structure. Furthermore, it is an effective element for ensuring the strength of the steel material. In order to exhibit these effects, it is necessary to contain Nb 0.005% or more. However, even if the Nb content becomes excessive and exceeds 0.030%, the effect is saturated. In addition, the minimum with preferable Nb content is 0.010%, and a preferable upper limit is 0.025%.

Ti: 0.005 to 0.03%
Ti is an element effective for forming nitrides, suppressing coarsening of prior austenite grains during high heat input welding, and improving HAZ toughness. In order to exert such effects, the Ti content needs to be 0.005% or more. However, if Ti is contained excessively, coarse inclusions are precipitated and the HAZ toughness is deteriorated on the contrary, so the upper limit is made 0.03%. In addition, the minimum with preferable Ti content is 0.010%, and a preferable upper limit is 0.025%.

B: 0.0005 to 0.0030%
B is an important element for obtaining an extremely low C bainite structure. It also works effectively in improving hardenability and suppressing ferrite transformation. For that purpose, B must be contained in an amount of 0.0005% or more. However, when B is contained excessively, not only the effect is saturated, but also inclusions (B nitride) in the HAZ structure increase and the HAZ toughness decreases, so the upper limit of the B content is 0.0030. % Is required. In addition, the minimum with preferable B content is 0.0007%, and a preferable upper limit is 0.002%.

Ca: 0.0005 to 0.005%
Ca has an effect of reducing the inclusion shape anisotropy, and is an effective element for improving the HAZ toughness. In order to exert such an effect, it is necessary to contain 0.0005% or more. However, even if it exceeds 0.005%, inclusions become coarse and the HAZ toughness deteriorates. In addition, the minimum with preferable Ca content is 0.001%, and a preferable upper limit is 0.004%.

N: 0.0020 to 0.0080%
In order to ensure high toughness in the high heat input welding HAZ, it is effective to prevent TiO from coarsening by precipitating TiN in the prior austenite grains. In order to exert such effects, the N content needs to be 0.0020% or more. However, if the N content becomes excessive and exceeds 0.0080%, coarse TiN precipitates and becomes the starting point of fracture. In addition, the minimum with preferable N content is 0.003%, and a preferable upper limit is 0.007%.

  In the high yield ratio high tensile strength steel sheet of the present invention, if necessary, (a) Cu: 3.0% or less (not including 0%) and / or Ni: 3.0% or less (not including 0%), (B) V: 0.05% or less (excluding 0%), (c) Mg: 0.005% or less (not including 0%), (d) Zr: 0.005% or less (excluding 0%) (E) rare earth elements: 0.0003 to 0.03%, etc. are also effective, but the reasons for limiting the range when these components are contained are as follows.

Cu: 3.0% or less and / or Ni: 3.0%
Cu and Ni are effective elements for improving the base material strength. These effects increase as the content increases. However, if the content is excessive, the formation of the MA phase is promoted during welding and the HAZ toughness deteriorates. And

V: 0.05% or less (excluding 0%)
V is an element effective for improving the strength of the base metal. However, if it is excessively contained in an amount exceeding 0.05%, precipitates are formed in the HAZ part and the HAZ toughness is lowered.

Mg: 0.005% or less (excluding 0%)
Mg is an element that finely disperses the oxide that becomes the nucleus of TiN precipitation and contributes to the improvement of the toughness of the HAZ. % Or less.

Zr: 0.005% or less (excluding 0%)
Zr, like Ti, is an element that forms nitrides and oxides and prevents coarsening of the prior austenite grains in the HAZ part and improves HAZ toughness. Since the material becomes coarse and the HAZ toughness deteriorates, it should be 0.005% or less.

Rare earth elements: 0.0003-0.03%
The rare earth element (REM) is an element effective for reducing the anisotropy of the inclusion shape and improving the HAZ toughness, like Ca. In order to exhibit such an effect, it is preferable to contain 0.0003% or more. However, if the content of REM exceeds 0.03%, the inclusions become coarse and the HAZ toughness decreases instead.

  In the high-tensile steel sheet of the present invention, in addition to the above components, it consists of Fe and inevitable impurities, but it can also contain a trace amount component (allowable component) to the extent that it does not impede its properties. Are also included in the scope of the present invention.

  In order to manufacture the steel sheet of the present invention, basically, a slab or a steel slab satisfying the chemical composition as described above is produced by a continuous casting method or an ingot forming method, and this is hot-rolled-cooled-heat treated. However, in order to obtain an extremely low C bainite structure, it is preferable to include the following steps (A) and (B).

(A) The slab or steel slab is heated to 1000 to 1300 ° C., and after hot rolling is completed at a rolling finish temperature of 700 ° C. or higher, air cooling is performed.
(B) The slab or steel slab is heated to 1000 to 1300 ° C., and after hot rolling is completed at a rolling finish temperature of 700 ° C. or higher, water cooling is performed at a cooling rate of 1 to 50 ° C./second to 500 ° C. or lower.

  The above manufacturing method basically forms a bainite structure by performing hot rolling after sufficient austenite state and then cooling. In the steps (A) and (B), if the heating temperature is less than 1000 ° C., a sufficient austenite state cannot be obtained, and if the heating temperature exceeds 1300 ° C., the initial austenite grains become coarse, and as a result The product has low toughness. The rolling finishing temperature is set to 700 ° C. or more from the viewpoint of productivity.

  After hot rolling is finished, a bainite structure is obtained because it is a component design that suppresses ferrite transformation by air cooling, but in some cases, it is accelerated to 500 ° C. or less at a cooling rate of 1 to 50 ° C./second. It may be cooled. This is because the structure is supercooled and a good ultra-low C bainite structure is obtained. In addition, when implementing accelerated cooling, since it is necessary to cool until the production | generation of a bainite structure is completed, it cools to 500 degrees C or less.

  In addition to the above manufacturing process, it is also useful to perform a tempering treatment in a temperature range of 500 to 700 ° C. as necessary, which further increases the yield ratio and toughness.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and any design modifications may be made in accordance with the gist of the present invention. It is included in the range.

Example 1
Steel sheets having chemical composition shown in Tables 1 and 2 were used, and steel sheets were produced under the production conditions shown in Tables 3 and 4 below. Tables 1 and 2 also show the HM value, HG value, and HB value defined in the present invention.

For each steel plate obtained, the bainite fraction, the tensile properties (0.2% yield strength σ _0.2 , tensile strength TS, yield ratio) of the steel (base material), impact properties (fracture surface transition temperature vTrs), resistance Welding cold cracking property, HAZ toughness and the like were measured by the following methods.

[Bainite fraction (area ratio)]
After mirror polishing, a test piece was collected from t / 4 (t is the plate thickness) of each steel plate, etched with a 2% nitric acid-ethanol solution (a nital solution), and then 400 times using an optical microscope in five fields of view. Observation was performed, and the bainite fraction (area%) in the steel structure was measured by image analysis. At this time, all lath structures other than ferrite (including polygonal ferrite and pseudopolygonal ferrite) were regarded as bainite.

[Tensile properties of steel sheet]
A JIS Z 2201 No. 4 test piece was taken from t / 4 (t is the plate thickness) of the steel sheet and subjected to a tensile test in accordance with JIS Z 2241. Yield strength (0.2% proof stress: σ _0.2 ), tensile strength ( TS) and yield ratio (yield strength / tensile strength × 100%: YR) were measured. In the present invention, the tensile strength TS: 570 MPa or more and the yield ratio YR: 80% or more were regarded as acceptable.

[Toughness of steel sheet]
A JIS Z 2202 V-notch test piece is taken in the L direction (rolling direction) from t / 4 of the steel plate, and a Charpy impact test is conducted in accordance with JIS Z 2242. The brittle fracture surface ratio of the Charpy test piece is 50%. The temperature was approximated and measured as the fracture surface transition temperature (vTrs). The target was vTrs of −50 ° C. or lower.

[Weld cold crack resistance]
According to the JIS Z 3158 y-type weld cracking test method, the coated arc welding was performed at a heat input of 1.5 KJ / mm, the cross-sectional cracking rate was measured at a preheating temperature of 25 ° C., and the cracking rate was set to 0%.

[Welding HAZ toughness]
A HAZ reproduction test was conducted. After heating 1400 ° C. × 5 seconds to a test piece taken from a steel plate [5 test pieces each having a size of 12.5 × 32 × 55 (mm)], the heat input corresponds to 10 KJ / mm [from 800 to 500 ° C. Cooled in 80 seconds] A thermal cycle test was conducted. Thereafter, two Charpy impact test pieces (JIS Z 2202 V notch test pieces) were collected from each test piece, and the average impact absorption energy vE- ₁₅ at −15 ° C. was obtained with 10 pieces for each steel plate. An average of 100 J or more was accepted.

  These results are shown in Tables 5 and 6 below, and can be considered as follows from these results. First, test no. 1 to 11 satisfy the requirements stipulated in the present invention, the toughness of the steel sheet (base material) satisfies the target, and the weldability is good with no preheating, and the heat input is 10 KJ / mm. The HAZ toughness also sufficiently satisfies the target average of 100 J or more.

  In contrast, test no. Nos. 12 to 36 lack any of the requirements defined in the present invention, and any of the characteristics is deteriorated. Of these, test no. No. 12 has a C content exceeding the specified range (steel type A1 in Table 1) and has a structure containing coarse carbides, and both the base metal toughness and the HAZ toughness are reduced. . In addition, Test No. In No. 13, the Si content exceeds the range specified in the present invention (steel type B1 in Table 1), and the HM value also exceeds the upper limit, so the amount of MA is very large. Both the base material toughness and the HAZ toughness are reduced.

  Test No. In No. 14, the Mn content is less than the range specified in the present invention (steel type C1 in Table 1), and since the hardenability is remarkably reduced, ferrite is precipitated in the base material, and the strength is reduced. ing. Test No. In No. 15, the Mn content exceeds the range specified by the present invention (steel type D1 in Table 1), and coarse precipitates are formed, so that both the base metal toughness and the HAZ toughness are reduced. .

  Test No. In No. 16, the Cr content is less than the range specified in the present invention (steel type E1 in Table 1), and since the hardenability is remarkably reduced, ferrite is precipitated in the base material, and the strength is reduced. is doing. Test No. In No. 17, the Cr content exceeds the range specified in the present invention (steel type F1 in Table 1), and coarse precipitates are formed, so that both the base metal toughness and the HAZ toughness are reduced. is doing.

  Test No. In No. 18, the Ti content exceeds the range specified in the present invention (steel type G1 in Table 1), and it is expected that coarse inclusions are generated in the HAZ part, and the HAZ toughness is It has deteriorated. Test No. In No. 19, the B content exceeds the range specified in the present invention (steel type H1 in Table 2), and it is expected that coarse inclusions are formed in the HAZ part, and the HAZ toughness is It has deteriorated. Test No. No. 20 has a Mo content exceeding the range defined in the present invention (steel type I1 in Table 2), and has a structure including a coarse hardened phase. Either of base metal toughness or HAZ toughness Has also declined.

  Test No. In No. 21, the V content exceeds the preferable range of the present invention (steel type J1 in Table 2), and it is expected that coarse inclusions are formed in the HAZ part, and the HAZ toughness deteriorates. is doing. Test No. In No. 22, the Cu content exceeds the preferable range of the present invention (steel type K1 in Table 2), and it is expected that the production amount of the MA phase in the HAZ part is increased. HAZ toughness is degraded. Test No. In No. 23, the Ni content exceeds the preferable range of the present invention (steel type L1 in Table 2), and the production amount of the MA phase in the HAZ part is expected to increase. HAZ toughness is degraded.

  Test No. In No. 24, the Nb content exceeds the range specified in the present invention (steel type M1 in Table 2), and it is expected that the amount of inclusions in the HAZ part is increased, and the HAZ toughness is deteriorated. is doing. Test No. In No. 25, the Ca content exceeds the preferable range of the present invention (steel type N1 in Table 2), and it is expected that coarse inclusions are formed in the HAZ part, and the HAZ toughness deteriorates. is doing.

  Test No. Nos. 26 and 27 are those in which the HM value does not satisfy the range specified in the present invention (steel types O1 and P1 in Table 2), the hardenability is reduced, and ferrite is generated in the base material. Is expected and the strength is low.

  Test No. Nos. 28 and 29 are those in which the HM value exceeds the range defined in the present invention (steel types Q1 and R1 in Table 2), and it is expected that a large amount of MA phase is generated in the HAZ part. HAZ toughness is degraded.

  Test No. Nos. 30 and 31 are those in which the HG value does not satisfy the range specified in the present invention (steel types S1 and T1 in Table 2), the hardenability is lowered, and ferrite is generated in the base material. Is expected and the strength is low.

  Test No. Nos. 22 and 33 have an HG value exceeding the range specified in the present invention (steel types U1 and V1 in Table 2), and it is expected that the prior austenite grains are very large in the HAZ part. The toughness has deteriorated.

  Test No. 34 to 36 have an HG value exceeding the range specified in the present invention (steel types W1, X1, Y1 in Table 2), and the austenite grains are hardly divided in the HAZ part, and the block size Is expected to increase, and the HAZ toughness is degraded.

Example 2
A HAZ reproduction test was conducted in the same manner as described above except that the steel type A shown in Table 1 was used and the amount of heat input was changed. At this time, the heat cycle was tested by changing the cooling time from 800 to 500 ° C. so as to correspond to the heat input of 1 to 20 KJ / mm. The heat input is 1 KJ / mm, the cooling time is 10 seconds, the heat input is 2 KJ / mm, the cooling time is 20 seconds, the heat input is 5 KJ / mm, the cooling time is 40 seconds, the heat input is 7 KJ / mm, the cooling time is 60 seconds, and the heat input is 15 KJ / mm. The cooling time is 120 seconds for mm, and the cooling time is 160 seconds for a heat input of 20 KJ / mm.

Thereafter, two Charpy impact test pieces (JIS Z 2202 V notch test pieces) were collected from each test piece, and the average impact absorption energy vE- ₁₅ at −15 ° C. was obtained with 10 pieces for each steel plate.

  The results are shown in Table 7 below, and it can be seen that the high-tensile steel sheet of the present invention exhibits excellent HAZ toughness up to a heat input of 20 KJ / mm.

Claims (6)

C: 0.01 to 0.05% (meaning of mass%, the same applies hereinafter), Si: 1.0% or less (not including 0%), Mn: 0.5 to 2.0%, P: 0.0. 5 or less (including 0%), S: 0.01% or less (including 0%), Al: 0.01 to 0.07%, Cr: 0.5 to 2.0%, Mo: 0.5 % Or less (including 0%), Nb: 0.0020-0.030%, Ti: 0.005-0.03%, B: 0.0005-0.0030%, Ca: 0.0005-0. 005%, N: 0.0020 to 0.0080%, respectively, and the HM value defined by the following formula (1) is 0.10% or more and less than 0.25%, defined by the following formula (2) The HG value is 0.02% or more and less than 0.08%, and the HB value defined by the following formula (3) satisfies 0.0% or less, and the bainite fraction is 90%. High yield ratio high-strength steel sheet excellent in toughness of the heat affected zone, characterized in that the product% or more tissues.
HM = [C] + [Mn] / 30 + [Cr] / 30 + [Mo] / 5 + [Si] / 5 (1)
HG =-[C] + [Mn] / 25 + [Cr] / 25- [Mo] / 30- [Si] / 10 (2)
HB =-[Cr] / 10 + [Mn] / 10- [Nb] (3)
However, [C], [Mn], [Cr], [Mo], [Si] and [Nb] indicate the contents (mass%) of C, Mn, Cr, Mo, Si and Nb, respectively.   The high yield ratio high-tensile steel sheet according to claim 1, which contains Cu: 3.0% or less (not including 0%) and / or Ni: 3.0% or less (not including 0%).   The high yield ratio high tensile strength steel sheet according to claim 1 or 2, wherein V: 0.05% or less (not including 0%).   The high yield strength high tensile strength steel sheet according to any one of claims 1 to 3, which contains Mg: 0.005% or less (excluding 0%).   Zr: 0.005% or less (0% is not included) The high yield ratio high-tensile steel sheet according to any one of claims 1 to 4.   The high-yield ratio high-tensile steel sheet according to any one of claims 1 to 5, which contains rare earth elements: 0.0003 to 0.03%.