JP2007239049A - High yield ratio high tensile strength steel plate having excellent fatigue crack propagation suppression and toughness in weld heat affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high yield ratio high tensile strength steel sheet in which fatigue crack propagation resistance can be made more excellent, and further, HAZ (heat affected zone) toughness can be improved as well as possible by suitably prescribing each crystal orientation relation in a steel sheet essentially consisting of bainite. <P>SOLUTION: The high yield ratio high tensile strength steel plate has a suitably prescribed chemical componential composition, and in which PM value prescribed by formula; PM=[C]+[Mn]/30+[Cr]/23+[Mo]/5+[Si]/5+[Cu]/50+[Ni]/50 satisfies <0.27%, and also has a structure essentially consisting of a bainitic phase, where, in the case the regions surrounded by large angle grain boundaries in which the orientation difference between two crystals is ≥15° are crystal grains, the average diameter of the equivalent circle of the crystal grains is ≤15 μm, and the ratio at which the orientation difference between the adjoining crystal grains is 55 to 60° is ≥0.3; wherein, [C], [Mn], [Cr], [Mo], [Si], [Cu] and [Ni] denote the content of each element (mass%), respectively. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-yield ratio high-tensile steel sheet used in various applications such as civil engineering, architecture, bridges, offshore structures, pipes, ships, storage, construction machinery, etc. The present invention relates to a high-yield-ratio high-tensile steel sheet mainly composed of a bainite phase that has a sufficient fatigue life and is excellent in toughness in a weld heat-affected zone (HAZ).

  There are many structural materials that are applied to the above-mentioned various applications, and many stresses are repeatedly applied. Therefore, in order to ensure the safety of structural materials, the steel sheets used as raw materials must have good fatigue characteristics. Is extremely important in design.

  The fatigue process of steel materials can be broadly divided into two processes, namely, the generation of cracks in stress-concentrated portions and the progress of cracks once generated. In general machine parts, the occurrence of macroscopic cracks is considered as a use limit, and there is almost no design that allows the cracks to propagate. However, in a welded structure, even if a fatigue crack occurs, it does not immediately break, and before this crack reaches the final stage, it is discovered by periodic inspection etc., and the cracked part is repaired, Or if the crack does not grow to the length that will lead to final failure within the period of use, the structure will be able to withstand sufficient use even if there is a crack.

  By the way, in welded structures, there are many weld toes and defects as stress-concentrated parts, and it is almost impossible technically and economically to completely prevent the occurrence of fatigue cracks. Is not a good idea. That is, in order to improve the fatigue life of the welded structure, it is effective to significantly extend the crack propagation life from the state in which the crack already exists, rather than preventing the occurrence of the crack itself. For that purpose, the design which makes the progress rate of the crack of steel materials as slow as possible becomes an important matter.

  Various techniques have been proposed as techniques for suppressing the rate of fatigue crack growth. For example, Patent Document 1 discloses a two-phase structure of a hard phase and a soft phase, and bending of a crack at the soft phase / hard phase boundary. A technique for suppressing the crack growth rate by stopping and branching has been proposed.

  However, this technique requires a structure that basically includes a soft ferrite structure, and there is a problem that high-strength steel cannot be manufactured.

  As a technique relating to the relationship between fatigue crack growth resistance and crystal orientation, for example, in Patent Document 2, when the normal direction of the steel sheet surface is ND, the (100) plane of α iron is parallel to ND {(100) // The boundary between the crystal grain having ND} and the crystal grain having the orientation {(111) // ND} in which the (111) plane of α iron is parallel to ND is at least 30 μm along the crack propagation direction Or the ratio of the α (111) plane strength ratio and the α (100) plane strength ratio inside the steel plate at the measurement plane parallel to the steel plate surface is 1.25 to 2.0. It has been proposed that the steel sheet has excellent progress (propagation) characteristics.

  The steel sheet used under high stress is more interested in fatigue properties. However, since the ferrite structure is mainly used in the above technique (for example, 70 area% or more), the tensile strength is 390 to 390. It can only deal with a strength class of about 490 MPa and cannot be applied to a high yield ratio high strength steel plate with a tensile strength of 570 MPa or more. In addition, when the tension is increased mainly with a ferrite structure, it is necessary to add a large amount of alloying elements and supplement with precipitation strengthening, etc., but there is a limit, and the toughness of the weld heat affected zone deteriorates. It will end up. Furthermore, it is known that a structure mainly composed of a bainite phase (sometimes referred to simply as “bainite structure”) is generated with a certain orientation relationship with austenite. The crystal orientation cannot be controlled.

In Patent Document 3, in a bainite structure or a martensite structure, the maximum tensile / compressive strain is ± 0.012, the repetition rate is 0.5 Hz, and a wave number 12 gradually increasing / gradually decreasing load up to the maximum strain is applied 15 times. Fatigue cracks in which the repeated softening parameter represented by the ratio σ ₁ / σ ₁₅ of the stress σ ₁ at the time of _one maximum strain and the stress σ ₁₅ at the time of the maximum strain of ₁₅ is 0.65 or more and 0.95 or less Steel materials with excellent progress characteristics have been proposed. In this technique, it is said that repeated softening is caused by rearrangement of dislocations when repeated stress is applied, and the higher the dislocation density at the crack tip, the easier the softening occurs, which is effective in suppressing fatigue crack growth. It is shown that there is.

  In the above technique, in order to increase the dislocation density, it is necessary to perform substantially direct quenching or online quenching as shown in the examples. However, with online heat treatment, productivity is expected to decrease, and even if quenching is performed directly on the line, the strength increase due to the introduction of dislocations must be taken into account, so that the strength does not become too high. There is a restriction that it must be a low component. As a result, the degree of freedom in component design is low, and it is considered that other characteristics are deteriorated. Even if the softening parameter as described above is within the specified range, there are examples in which the fracture surface transition temperature vTrs exceeds 0 ° C., and the characteristics as a structure may not be sufficiently satisfied.

  On the other hand, the high-tensile steel plate used for the above-mentioned various applications is to construct various structures by welding, but as a characteristic required for the high-tensile steel plate, when applying high heat input welding It is necessary that the toughness of the weld heat affected zone (HAZ) 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.

  For example, a technique as shown in Patent Document 4 has been proposed as a technique for improving HAZ toughness in a high-tensile steel having a tensile strength of 570 MPa or more. 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 be said that the control of the HAZ structure is properly performed only by using the extremely low C bainite structure, and in some cases, sufficient large heat input HAZ toughness may not be obtained.

  Patent Document 5 discloses that the cooling rate dependency is small (ie, the variation in the material is small) by optimizing the amount of Nb and B at an extremely low C (C content: 0.03% or less). A technique for producing extremely 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.

In addition, the above-described technology for improving the HAZ toughness is not considered at all for the above-described fatigue crack properties, and it is desired to establish a technology that combines both HAZ toughness and fatigue crack properties. Is the actual situation.
Japanese Patent No. 3298544 Patent Claims etc. JP, 2000-17379, A Claims, etc. JP, 2004-27355, A Claims etc. Japanese Patent No. 3606021 Patent Claim etc. JP, 2000-345239, A Claims etc.

  The present invention has been made paying attention to the above-mentioned circumstances, and the purpose of the present invention is to further improve fatigue crack propagation resistance by appropriately defining each crystal orientation relationship in a steel sheet mainly composed of bainite. An object of the present invention is to provide a high-yield ratio high-tensile steel sheet that is excellent and can improve HAZ toughness as much as possible.

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%, the same shall apply hereinafter), Si: 1.0% or less (0% Mn: 0.5 to 2.0%, P: 0.5 or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.01 -0.07%, Cr: 0.4-2.0%, Nb: 0.001-0.050%, Ti: 0.005-0.03%, B: 0.0005-0.0030%, Ca: 0.0005-0.005%, N: 0.0020-0.010%, respectively, PM value specified by the following formula (1) satisfies less than 0.27%, and bainite When a region composed of a structure mainly composed of a phase and surrounded by a large-angle grain boundary where the orientation difference between two crystals is 15 ° or more is defined as a crystal grain, Equivalent diameter average yen is not more 15μm or less, the proportion orientation difference between the crystal grains adjacent is 55 to 60 ° is intended to include the features in that not less than 0.3. In the present invention, “mainly composed of bainite” means a state in which the bainite phase occupies 90 area% or more in the structure.
PM = [C] + [Mn] / 30 + [Cr] / 23 + [Mo] / 5 + [Si] / 5
+ [Cu] / 50 + [Ni] / 50 (1)
However, [C], [Mn], [Cr], [Mo], [Si], [Cu] and [Ni] are the contents (mass of C, Mn, Cr, Mo, Si, Cu and Ni, respectively). %).

  In the steel sheet of the present invention, except for the above basic components (remainder), the remainder is iron and inevitable impurities, but if necessary, (a) Mo: 0.5% or less (not including 0%), (b ) Cu: 2.0% or less (not including 0%) and / or Ni: 2.0% or less (not including 0%), (c) V: 0.05% or less (not including 0%) (D) Mg: 0.005% or less (not including 0%), (e) Zr: 0.005% or less (not including 0%), (f) Rare earth elements: 0.0003 to 0.003 %, Etc. are also preferable, and the characteristics of the high-tensile steel sheet are further improved depending on the components contained.

  In the present invention, in the steel sheet having a structure mainly composed of bainite, each crystal orientation relationship is appropriately defined, and the MA phase amount and the chemical composition that are factors affecting the HAZ toughness are strictly controlled. By optimizing, it is possible to realize high-tensile steel sheets that are excellent in suppressing fatigue crack growth and that can secure good HAZ toughness. It is useful as a material for structural materials for various uses such as construction machinery.

  In order to solve the above-mentioned problems, the present inventors have focused on a steel sheet having a bainite structure, and studied means for suppressing the fatigue crack growth rate in the steel sheet from various angles. As a result, the following knowledge was obtained. That is, in the bainite structure as described above, it is generated with some orientation relationship with respect to austenite, but it is selected depending on the chemical composition of the steel sheet, the formation temperature of the structure, other conditions, etc. It has been found that the fatigue crack growth is suppressed particularly at the grain boundaries having a certain crystal orientation difference. And when the crystal orientation distribution was appropriately defined, it was found that a steel sheet capable of satisfactorily suppressing the progress of fatigue cracks can be realized.

  On the other hand, as steel plates for obtaining good HAZ toughness, those having an extremely low C bainite structure are widely used, but the present inventors have further improved their HAZ toughness based on steel plates having such a structure. In order to do this, the means were examined from various angles. As a result, if the relationship is defined by the element (C, Si, Mn, Cr, Mo, Cu, Ni, etc.) generally contained in the high-tensile steel as shown in the above formula (1), MA The inventors have found that the phase amount is appropriately controlled and the HAZ toughness is remarkably improved, and the present invention has been completed. Hereinafter, along with the background of the completion of the present invention, the operational effects of the requirements defined in the present invention will be described.

  In a single-phase structure mainly composed of bainite phase, it is considered that the grain boundary acts as a resistance to crack growth, but if the frequency of the collision between the grain boundary and the crack is increased during crack growth, the crack growth is suppressed. It was considered possible. That is, it was found that the frequency of collision with cracks should be increased by making the grain boundaries finer. However, at a small-angle grain boundary (small tilt boundary) where the orientation difference between both ends forming a grain boundary is small (for example, a small tilt boundary), the grain boundary energy is small and the effect is small, so the orientation difference is 15 ° or more. It is necessary to target a large-angle grain boundary (large tilt boundary). It has also been found that the larger the ratio of the orientation difference between adjacent crystal grains in the large-angle grain boundary is 55 to 60 °, the more effective the crack growth is (see FIG. 1 described later).

  That is, a crystal grain surrounded by a large-angle grain boundary having an orientation difference of 15 ° or more, and having an average diameter (equivalent circle diameter) of 15 μm or less when converted into a circle of the same area, In the distribution of misorientation of adjacent crystal grains, the ratio of misorientation of 55 to 60 ° is 0.3 or more (30% or more), thereby realizing a steel plate having an excellent fatigue crack progress suppressing effect. It is. The “orientation difference” is also referred to as “shift angle” or “inclination angle”, and may be hereinafter referred to as “crystal orientation difference”. In order to measure such a crystal orientation difference, an EBSP method (Electoron Backscattering Pattern method) may be employed.

  On the other hand, in order to improve the HAZ toughness, it is necessary to reduce the amount of the MA phase that is the starting point of fracture in the HAZ as much as possible. 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 effective to reduce the amount of retained austenite by reducing the austenite stabilizing elements (Mn, Cr, Mo, Si, etc.). From such a viewpoint, in a steel sheet in which the PM value defined by the above formula (1) is less than 0.27%, the MA phase in the HAZ is sufficiently reduced to exhibit good toughness. However, if the C content or the austenite stabilizing element is excessively reduced, there arises a problem that the strength cannot be ensured, so it is necessary to ensure the basic minimum content (this point will be described later).

  In the above formula (1), elements that are not included in the basic chemical component such as Mo, Cu, Ni, etc. are also defined, but these elements also affect the HAZ toughness. When contained if necessary, these contents also need to be included in the calculation of the PM value. Therefore, when these elements are not contained, the amount of these elements may be calculated as 0 from the above equation (1).

The coefficient of each element defined by the above equation (1) is obtained by experiment, and this point will be described next. For each steel type having the chemical composition shown in Table 1 below, the absorbed energy vE- ₁₅ when the thermal cycle test is performed is obtained (detailed measurement method will be described later), and the steel type (table) is the base (base). In comparison with No. 1 steel type A1), the influence of Cr, Mn, Ni, Mo, Si and Cu on the toughness of the steel sheet was examined.

First, regarding C, when the content of C is X ₁ and the absorbed energy vE _-15 is Y ₁ , steel types A1 (C content: 0.038%, vE _-15 : 77J) and A2 (C content) : 0.054%, vE- ₁₅ : 30J) can be expressed by an approximate expression such as the following expression (2).
Y ₁ = −2937.5X ₁ +188.63 (2)

Similarly, regarding Cr, Mn, Ni, Mo, Si and Cu, if the relationship between the content and vE- ₁₅ is shown, it can be expressed by an approximate expression (linear approximation) such as the following expressions (3) to (8).
[About Cr: Steel types A1 and A3]
Y ₂ = −128.1 X ₂ +193.59 (3)
(However, X _2: Cr content, Y _2: vE _-15)
[About Mn: Steel types A1 and A4]
_{Y 3 = -98.0X 3 +233.92 ... (} 4)
(However, X ₃ : Mn content, Y ₃ : vE _-15 )
[About Ni: Steel types A1 and A5]
Y ₄ = −58.4X ₄ +91.6 (5)
(However, X _4: Ni content, Y _4: vE _-15)
[About Mo: Steel types A1, A6 and A7]
Y ₅ = −570.0X ₅ +268.0 (6)
_(However, X 5: Mo _content, Y 5: vE _-15)
[About Si: Steel types A1 and A8]
Y ₆ = −560.0 X ₆ +301 (7)
(However, X _6: Si content, Y _6: vE _-15)
[About Cu: Steel types A9 and A10]
_{Y 7 = -59.0X 7 +257.8 ... (} 8)
(However, X _7: Cu content, Y _7: vE _-15)

  By comparing the coefficient of the above expression (3) with the coefficient (slope) of the expressions (3) to (8), the coefficient of each element when the coefficient of C is 1 can be obtained. For example, for Cr, (−2938 / −128) ≈23 (the same applies to other elements). As a result, the relationship of the formula (1) was obtained.

  The high-tensile steel sheet of the present invention is mainly composed of a bainite structure. (The chemical component composition will be described later). In addition, by using an extremely low C system, in the HAZ structure, the amount of the MA phase can be reduced, and hardening at the time of small heat input is reduced, so preheating at welding is eliminated (free). There is also an advantage of being able to. However, in order to exert these effects, 100 area% does not necessarily need to be a bainite structure, and it may be 90 area% or more in terms of the bainite fraction. Examples of structures other than bainite include martensite and ferrite.

  The steel plate of the present invention contains C, Si, Mn, P, S, Al, Cr, Nb, Ti, B, Ca and N, which are basic components for the chemical component composition, The reasons for limiting the ranges of these components are 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 the C content is excessive (when it becomes medium carbon or high carbon), the variant selected by changing the transformation mechanism of bainite is changed, or the extremely low C bainite structure aimed at in the present invention is stably obtained. Therefore, it is necessary to make it 0.05% or less. Further, when C is excessive, the block size becomes coarse, the proportion of small-angle grain boundaries increases, and the proportion of large-angle grain boundaries having a crystal orientation difference of 55 to 60 ° tends to decrease.

[Si: 1.0% or less (excluding 0%)]
Si is an effective element for increasing the strength of the steel by solid solution strengthening regardless of the cooling conditions. However, when 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 0.6%.

[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 0.6%, and a preferable upper limit is 1.9%.

[P: 0.5% or less (not including 0%) and S: 0.02% or less (not including 0%)]
P is an impurity that segregates in the crystal grains and adversely affects the ductility and toughness, so it is preferable that it be as small as possible. Is good. S is an impurity because it reacts with alloy elements in the steel material to form various inclusions, which adversely affects the ductility and toughness of the steel material. In consideration of this, it is preferable to suppress it to 0.02% or less.

[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, the island-like martensite phase (MA phase) is precipitated in a large amount on 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 0.06%.

[Cr: 0.4 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. Moreover, the effect of suppressing transformation and lowering the bainite transformation start temperature Bs is also exhibited. Bainite transforms with a KS relationship (Kurdjiumov-Sachs relationship) during transformation, but by transforming at low temperature, a fine block consisting of a single variant (so-called sibling) is generated. become. In order to exert these effects, it is necessary to contain 0.4% 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 0.5%, and a preferable upper limit is 1.8%.

[Nb: 0.001 to 0.050%]
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. And similarly to Cr, the effect which suppresses transformation and lowers the bainite transformation start temperature Bs is also exhibited. In order to exhibit these effects, it is necessary to contain Nb 0.001% or more. However, the effect is saturated even if the Nb content becomes excessive and exceeds 0.050%. In addition, the minimum with preferable Nb content is 0.002%, and a preferable upper limit is 0.045%.

[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. Moreover, like Cr and Nb, the effect of suppressing transformation and lowering the bainite transformation start temperature Bs is also exhibited. 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. Furthermore, like Cr and Nb, the effect of suppressing transformation and lowering the bainite transformation start temperature Bs is also exhibited. 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 0.0025%.

[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 0.010%]
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 0.010%, 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 0.008%.

  The basic components in the high-strength steel sheet of the present invention are as described above, and the balance is composed of iron and inevitable impurities (for example, O), but if necessary, (a) Mo: 0.5% or less (0 (B) Cu: 2.0% or less (not including 0%) and / or Ni: 2.0% or less (not including 0%), (c) V: 0.05% (D) Mg: 0.005% or less (not including 0%), (e) Zr: 0.005% or less (not including 0%), (f) Rare earth elements : It is also effective to contain 0.0003 to 0.003%, etc., but the reason for limiting the range when these components are contained is as follows.

[Mo: 0.5% or less (excluding 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. Mo also suppresses the transformation and exhibits the effect of lowering the bainite transformation start temperature Bs. By transforming bainite at a low temperature, fine blocks composed of a single variant (so-called siblings) are generated. 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 equation (1) needs to be calculated as not including Mo. The upper limit with preferable Mo content is 0.45%.

[Cu: 2.0% or less (not including 0%) and / or Ni: 2.0% (not including 0%)]
Cu and Ni are effective elements for improving the base material strength. Moreover, these elements also exhibit the effect | action which suppresses transformation and reduces the bainite transformation start temperature Bs. 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. It is preferable that However, when Cu or Ni is not included, the above equation (1) needs to be calculated as not including Cu or Ni. The upper limit with preferable content of these elements is 1.5%.

[V: 0.05% or less (excluding 0%)]
V is an element effective for improving the strength of the base metal, and also exhibits the effect of suppressing transformation and lowering the bainite transformation start temperature Bs. However, if it is excessively contained in excess of 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 to 0.003%]
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 0.003%, the inclusions become coarse and the HAZ toughness decreases instead.

The high-tensile steel sheet of the present invention is composed of a structure mainly composed of bainite. However, by cooling in the austenite state, it becomes a supercooled state, lowers the Ar ₃ transformation point, and can have a bainite structure. . In particular, when the transformation is performed at a low temperature, the movable distance of atoms at the transformation is reduced, and the transformation behavior is also changed from the diffusion transformation to the shear transformation, and the generation of the corresponding grain boundary is promoted. As specific manufacturing conditions, it is desirable to apply the following manufacturing methods (1) to (5).
(1) After heating in a temperature range of 950 to 1250 ° C. and finishing rolling in a temperature range of Ar ₃ transformation point to 900 ° C., accelerated cooling to 450 ° C. or less with a cooling rate of 5 ° C./second or more (for example, water cooling ).
(2) After heating in the temperature range of 950 to 1250 ° C. and finishing rolling in the temperature range of Ar ₃ transformation point to 900 ° C., accelerated cooling to 450 ° C. or less with a cooling rate of 5 ° C./second or more (for example, water cooling) ) And then tempering in the temperature range of 500-700 ° C.
(3) After heating to a temperature range of 950 to 1250 ° C. and finishing rolling in a temperature range of Ar ₃ transformation point to 900 ° C., heating is again performed to a temperature equal to or higher than the Ac ₃ transformation point, and then the cooling rate is set to 5 ° C. / Cool to 450 ° C. or less for at least 2 seconds.
(4) After heating in the temperature range of 950 to 1250 ° C. and finishing the rolling in the temperature range of Ar ₃ transformation point to 900 ° C., it is again heated to a temperature not lower than the Ac ₃ transformation point and the cooling rate is 5 ° C./second. As mentioned above, it cools to 450 degrees C or less, and continues a tempering process in a 500-700 degreeC temperature range.
(5) After heating to a temperature range of 950 to 1250 ° C. and rolling in the recrystallization temperature range, cooling is performed to a temperature range of 600 to 700 ° C. at a cooling rate of 1 ° C./second or more, and then at that temperature. Rolling with a reduction rate of 30% or more is performed in the state of supercooled austenite, and then accelerated cooling is performed again.

In the methods (1) to (4), the cooling stop temperature is set to 450 ° C. or lower because it is necessary to cool to a temperature mainly composed of a bainite structure. If the heating temperature is less than 950 ° C., the austenite state is not sufficiently achieved. However, if the heating temperature exceeds 1250 ° C., the austenite grains become coarse, and the large-angle grain diameter after transformation also becomes coarse. In the case of reheating after rolling, since it is necessary to make the austenite state completely, it is necessary to set the temperature to the Ac ₃ transformation point temperature or higher. In the above method (5), many deformation zones can be introduced by rolling austenite at a low temperature, and nucleation sites increase due to the ausforming (processing heat treatment) effect. The fatigue crack growth suppressing effect can be enhanced.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Example 1]
Steel sheets (steel types A to R) having chemical composition shown in Table 2 below were used, and steel sheets were manufactured under the manufacturing conditions shown in Table 3 below. Table 2 also shows PM values defined in the present invention. Steel types A to M in Table 2 satisfy the chemical composition defined in the present invention, and steel types N to R are out of any of the requirements (chemical composition, PM value) defined in the present invention. It is a thing.

  For each steel plate obtained, the bainite fraction, the tensile properties of the steel (base material) [0.2% proof stress (YS), tensile strength (TS), yield ratio (YR: YS / TS)], impact properties ( (Fracture surface transition temperature vTrs), welding cold cracking resistance, HAZ toughness, etc. are measured by the following methods, large-angle grain boundary diameter (average equivalent circle diameter), crystal orientation difference of 55-60 °, fatigue crack growth The speed and the like were measured by the following method. These results are collectively shown in Table 4 below.

[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: YS), tensile strength (TS ), Yield ratio (yield strength / tensile strength × 100%: YR) was 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 was sampled in the L direction (rolling direction) from t / 4 of the steel plate and subjected to a Charpy impact test in accordance with JIS Z 2242. The brittle fracture surface ratio of the Charpy test piece was 50%. Was measured as a fracture surface transition temperature (vTrs). The target was vTrs of −20 ° C. or less.

[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.

[Large-angle grain boundary diameter (average equivalent circle diameter)]
The cross section parallel to the rolling direction of the steel sheet was measured by FE-SEM-EBSP (electron backscatter diffraction image method using an electron emission scanning electron microscope). Specifically, an EBSP apparatus (trade name: “OIM”) manufactured by Tex SEM Laboratories is used in combination with EF-SEM, and a grain having a tilt angle (crystal orientation difference) of 15 ° or more is used as a crystal grain boundary. The diameter was measured. The measurement conditions at this time were measurement area: 200 μm, measurement step: 0.5 μm interval, and measurement points with confidence index (Confidence Index) indicating the reliability of the measurement direction were excluded from the analysis target. . The average value of the crystal grain sizes thus obtained was calculated and used as the average crystal grain size in the present invention. Incidentally, those having a crystal grain size of 2.0 μm or less were judged as measurement noise and excluded from the target of calculating the average value of the crystal grain size.

[Proportion of crystal orientation difference of 55-60 °]
By measuring the orientation difference at each grain boundary with OIM automatic analysis software, the ratio of the crystal orientation difference of 55-60 ° was determined (calculated).

[Fatigue crack growth rate]
In accordance with ASTM E647, a fatigue crack growth rate was determined by carrying out a fatigue crack growth test using a compact test piece. At this time, the value in the stable growth region ΔK = 20 (MPa · √m) where the Paris law defined by the following equation (9) is satisfied was evaluated as a representative value. Regarding the evaluation and criteria of the fatigue crack growth rate, the normal steel material has a growth rate of about 4 to 6 × 10 ⁻⁵ mm / cycle (when ΔK = 20), so 3.5 × 10 ^−5. The standard was mm / cycle or less.
da / dn = C (ΔK) ^m (9)
Here, a: crack length, n: number of repetitions, C, m: constants determined by matters such as material and load, respectively.

From the results in Table 4, it can be considered as follows. Test No. 1 to 11 satisfy the requirements stipulated in the present invention, exhibit a sufficient fatigue crack growth inhibiting effect (growth rate of 3.5 × 10 ⁻⁵ mm / cycle or less) and toughness. It can be seen that both the HAZ toughness show good values.

  In contrast, test no. Nos. 12 to 18 lack any of the requirements defined in the present invention, and none of them exhibits the effect of suppressing fatigue progress. That is, test no. In the case of 12, 13, the crystal orientation difference is low at a rate of 55-60 °, and the fatigue crack growth rate is high. Of these, test no. No. 13 does not have a bainite structure, and fatigue characteristics are also deteriorated.

  Test No. In the case of No. 14, the C content is excessive (steel type N in Table 2), the extremely low C bainite structure targeted in the present invention cannot be obtained, and the HAZ toughness is deteriorated. In addition, Test No. In the case of No. 15, the C content is further excessive (steel type O in Table 2), the amount of MA in the HAZ structure is promoted, the toughness is deteriorated, and the fatigue strength is also deteriorated. .

  Test No. In No. 16, the Mn content is excessive (steel type P in Table 2), and the HAZ toughness is deteriorated. In addition, Test No. In No. 17, the Cr content is excessive (steel type Q in Table 2), and the HAZ toughness is deteriorated. Furthermore, test no. In No. 18, Mo content is excessive (steel type R in Table 2), and HAZ toughness is deteriorated.

Based on the results in Table 4, FIG. 1 shows the relationship between the PM value and vE- ₁₅ , and FIG. 2 shows the relationship between the rate of the crystal orientation difference of 55-60 ° and the fatigue crack growth rate. Exhibiting excellent toughness by setting the PM value to less than 0.27%, and the fatigue crack growth rate being sufficiently low by setting the ratio of the crystal orientation difference of 55-60 ° to 0.3 or more I understand.

[Example 2]
Using the steel type A shown in Table 1, a HAZ reproduction test was conducted in the same manner as in the example except that the heat input was changed. A heat cycle test was performed by changing the cooling time to 800 to 500 ° C. so as to correspond to the heat input at this time: 1 to 20 KJ. 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 5 below, and it can be seen that the high-tensile steel sheet (steel type A) of the present invention exhibits excellent HAZ toughness up to a heat input of 20 KJ / mm.

It is a graph which shows the relationship between PM value and vE- ₁₅ . It is a graph which shows the relationship between the ratio of a crystal orientation difference of 55-60 degrees, and a fatigue crack growth rate.

Claims (7)

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: 0.00. 5 or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.01 to 0.07%, Cr: 0.4 to 2.0%, Nb: 0 0.001 to 0.050%, Ti: 0.005 to 0.03%, B: 0.0005 to 0.0030%, Ca: 0.0005 to 0.005%, N: 0.0020 to 0.010 And a PM value defined by the following formula (1) satisfying less than 0.27%, consisting of a structure mainly composed of a bainite phase, and a large angle where the orientation difference between two crystals is 15 ° or more. When the region surrounded by the grain boundary is a crystal grain, the average equivalent circle diameter of the crystal grain is 15 μm or less, and the orientation difference between adjacent crystal grains is 55 μm. A high-yield ratio high-tensile steel sheet excellent in fatigue crack growth inhibition and toughness of the weld heat-affected zone, characterized in that the ratio of ˜60 ° is 0.3 or more.
PM = [C] + [Mn] / 30 + [Cr] / 23 + [Mo] / 5 + [Si] / 5
+ [Cu] / 50 + [Ni] / 50 (1)
However, [C], [Mn], [Cr], [Mo], [Si], [Cu] and [Ni] are the contents (mass of C, Mn, Cr, Mo, Si, Cu and Ni, respectively). %).   The high-yield ratio high-tensile steel sheet according to claim 1, which contains Mo: 0.5% or less (not including 0%).   3. High yield ratio high tension according to claim 1 or 2, which contains Cu: 2.0% or less (not including 0%) and / or Ni: 2.0% or less (not including 0%). steel sheet.   The high yield ratio high-tensile steel sheet according to any one of claims 1 to 3, 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 4, which contains Mg: 0.005% or less (not including 0%).   The high yield ratio high-tensile steel sheet according to any one of claims 1 to 5, which contains Zr: 0.005% or less (excluding 0%).   The high-yield ratio high-tensile steel sheet according to any one of claims 1 to 6, which contains rare earth elements: 0.0003 to 0.003%.

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