JP5849940B2 - Low yield ratio high strength steel plate with excellent weld heat affected zone toughness - Google Patents

Description

  The present invention is used for welded structures in the fields of civil engineering, architecture, and bridges, and is excellent in toughness and weld crack resistance (or hardening resistance) of weld heat-affected zone under a wide range of heat input of 15 to 900 kJ / cm, yielding. The present invention relates to a low-yield ratio high-tensile steel sheet having a strength of 440 MPa or more, a tensile strength of 590 MPa or more, and a yield ratio of 80% or less.

  In recent years, with the increase in size of welded structures, the strength and thickness of steel sheets have been increased. At the same time, improvement in welding efficiency is required from the viewpoint of improving the construction efficiency of structures and reducing construction costs, and the application range of high heat input welding is expanding. For example, in a box column used for a high-rise building, super large heat input welding such as submerged arc welding or electroslag welding in which welding heat input exceeds 400 kJ / cm is applied.

  In addition, building structures are required to have improved earthquake resistance, and in order to secure the plastic deformability of the steel plate base material, low YR characteristics with a yield ratio (YR) of 80% or less have been required. High toughness in the welded joint is also required. For example, the value of Charpy absorbed energy at 0 ° C. is 47 J or more at an important part and 27 J or more at other parts in a welded part of a column and a column-beam joint.

  In general, when high heat input welding is applied to a steel sheet, the portion where the toughness is most reduced is a region called a bond portion in the vicinity of a fusion line in a welding heat affected zone (hereinafter referred to as HAZ). The bond part is exposed to a high temperature close to the melting point during high heat input welding, and the austenite grains tend to coarsen. During subsequent cooling, low toughness structures such as upper bainite structure and island martensite are formed in the austenite grains. It's easy to do. Such HAZ is called coarse grain HAZ (hereinafter referred to as CGHAZ) because old austenite grains are coarsened.

  On the other hand, there is a region (Inter-critically reheated CGHAZ: hereinafter referred to as ICCGHAZ) that is reheated to a two-phase region by reheating by a subsequent pass in the bond portion at the time of small heat input multipass welding. In such a region, island-like martensite is easily generated and toughness is reduced.

  In high-strength steel sheets with a tensile strength exceeding 590 MPa, a large amount of alloy is often added to ensure strength, the yield ratio increases, and the bond area of large heat input welding and the two-phase region in small heat input welding In reheated HAZ (ICCGHAZ), the toughness tends to decrease. Also, in welded parts with small heat input and short bead length, such as tack welding of steel sheets and welding of hanging tools, HAZ tends to harden, and if the hardness of the hardened part exceeds HV350, cold cracking or delay Since there is a risk of destruction, the HAZ hardness is required to be less than HV350 from the viewpoint of weld crack resistance (or curing resistance). For this reason, there is a demand for the development of a high-strength steel sheet having all of a low yield ratio, excellent HAZ toughness, weld crack resistance, and the like.

  For improving HAZ toughness, fine inclusions and precipitates are dispersed in the steel to prevent coarsening of austenite grains and to function as nucleation sites of intragranular ferrite, thereby allowing the prior austenite grain structure Technology to reduce the size of the device is widespread.

  For example, Patent Document 1 discloses a technique in which TiN and a rare earth element (REM) oxysulfide (oxysulfide) are combined and finely dispersed in steel. Patent Document 2 discloses a technique for improving HAZ toughness by dispersing Ti oxide and using it as a ferrite grain nucleation site in austenite grains. In Patent Document 3, in order to improve the super-high heat input HAZ toughness, the Ca oxysulfide whose content is adjusted to an appropriate range and the shape is optimized is dispersed in the steel to improve the grain toughness. A technique for promoting nucleation of inner ferrite is disclosed.

  However, in these techniques, it is necessary to precisely control the amount of trace elements such as Ti, N, Ca, S, and O, leading to an increase in steelmaking costs and difficulty in mass production. In addition, if N content is increased in order to disperse TiN in steel in a large amount, the surface property of the continuous cast slab deteriorates, the maintenance load on the surface of the slab increases, and the yield decreases to increase costs. There is a problem of being connected.

  Patent Document 4 discloses a method of increasing the high heat input HAZ toughness by controlling the hardenability to an appropriate range by adjusting the amount of alloy element added. There are cases where the hardenability is controlled to a low range and the structure of the super large heat input weld HAZ part is ferrite + pearlite, and the structure is mainly composed of lower bainite as high hardenability. For example, in the case of a thick material exceeding 60 mm, it is difficult to maintain the strength of the base material and the welded joint at 590 MPa or more. On the other hand, in order to achieve high hardenability, there is a problem that a large amount of alloy element needs to be added. is there.

  Patent Document 5 discloses a method for reducing C to 0.02 to 0.04% and improving hardenability by adding B and other alloy elements to obtain a bainite-based structure. . In order to secure sufficient strength with thick materials, it is necessary to add a large amount of alloying elements in order to obtain high hardenability, but increase the amount of austenite stabilizing elements such as Mn, Cu, Ni etc. This may cause problems such as deterioration of the surface properties of the continuous cast slab and an increase in slab internal defects due to an increase in the hardness of the central segregation part.

  In Patent Document 6, 0.20 to 0.60% of Mo is added to steel of C: 0.07 to 0.09%, where the strength of the base material and the HAZ is easily secured, and a high heat input welded HAZ structure Is disclosed in which a high-heat-input welding HAZ toughness is improved by making bainite into a single phase and further reducing Si and P. According to this technology, elements that impair the surface properties of slabs without using advanced steelmaking technologies such as fine dispersion control of inclusions and precipitates in high-strength thick steel plates with excellent weld heat-affected zone toughness and weld crack resistance Can be obtained inexpensively in a form suitable for mass production. Even in a high heat input weld heat affected zone such as an electroslag weld zone, a toughness having Charpy absorbed energy at 0 ° C. of about 27 J or more can be stably obtained. However, in this method, Charpy absorbed energy of 47 J or more is stabilized at 0 ° C. in a reconstructed HAZ structure simulating ICCGHAZ scattered in the high heat input welding heat-affected zone or in the bond portion during small heat input multi-pass welding. It is difficult to obtain. Further, in this technique, it is necessary to contain a certain amount of S. Therefore, in the segregated portion where Mn and S are concentrated near the center of the steel plate thickness, the thickness of the plate is increased due to the presence of elongated inclusions (MnS). The Charpy absorbed energy in the direction (Z direction) may decrease. In particular, in electroslag welded joints that cover a wide range of heat-affected zone, the center segregation zone and weld heat impact may overlap, and in the Charpy impact test of the weld zone, the specimen is placed in the plate thickness direction (the crack surface is the plate surface). Since the samples are collected (in parallel), there is a problem that the absorbed energy tends to decrease.

JP 60-152626 A JP 57-51243 A JP 2005-68519 A JP 09-202936 A Japanese Patent Laid-Open No. 2000-219934 Japanese Patent Application No. 2011-208213

  The present invention solves the above-mentioned problems of the prior art, and has a yield strength of 440 MPa or higher, a tensile strength of 590 MPa or higher, and a low yield ratio of 80% or lower, which are suitable for building structures, in a plate thickness range up to 100 mm. In addition, high-strength thick steel plates with excellent toughness and weld crack resistance in super-high heat input weld heat affected zone and small heat input multi-pass weld zone are used for advanced dispersion control of inclusions and precipitates. The object is to obtain it at a low cost in a form suitable for mass production without using steelmaking techniques, without adding elements that impair the surface properties of the slab.

  The term “excellent in weld heat affected zone toughness” as used in the present invention means the center segregation contained in the bond portion (CGHAZ) and HAZ of a high heat input welded joint such as electroslag whose welding heat input exceeds 400 kJ / cm. In the case of ICCGHAZ in the small heat input multi-pass welding, the absorbed energy (vE0) at 0 ° C. in the Charpy impact test has an average toughness of 47 J or more and the welding heat input is 20 to 50 kJ / cm. The case where the absorbed energy (vE0) at 0 ° C. has an average toughness of 47 J or more. Further, “excellent in weld crack resistance” refers to a case where the maximum hardness of a 40 mm long short beat weld is less than HV350 and the weld crack sensitivity index Pcm is 0.21% or less.

  In order not to impair the manufacturability due to the occurrence of cracks in the continuous cast slab, elements such as N, B, Cu and Ni that promote slab cracking are not added as much as possible. In order to secure stable tensile strength in the base metal and the welded joint while reducing the alloying element addition amount, it is desirable that the C amount is large, but in order to obtain excellent weld crack resistance and HAZ toughness. C reduction is desirable. In order to optimize the balance, the amount of C was limited to a narrow range of 0.06 to 0.09%, and the strength was ensured by adding Mo as the main structure of bainite.

  At the same time, by reducing Si and P, the toughness of CGHAZ for super high heat input welding and ICCGHAZ for small heat input multi-pass welding is improved. The reduction of Si and P suppresses the generation of island martensite that deteriorates toughness, and further contributes to the improvement of toughness of island martensite itself. Further, reducing elements such as N, B, Cu, and Ni as much as possible contributes to the suppression of the formation of island martensite in CGHAZ for super-high heat input welding and ICCGHAZ in small heat multi-pass welding. With these component designs, it is possible to stably ensure toughness having an average Charpy absorbed energy at 0 ° C. of about 47 J or more in a heat-affected zone of a weld joint having a small heat input to a large heat input.

  Furthermore, the component design for ensuring higher toughness was examined. Conventionally, the inclusion of an appropriate amount of S has been considered preferable for improving HAZ toughness. This is because MnS dispersed in the steel promotes bainite nucleation in the prior austenite grains and refines the size of the bainite substructure called packets and blocks. However, MnS contributes to the refinement of the structure and, at the same time, causes a decrease in toughness. Therefore, by reducing S as much as possible to reduce MnS in the steel to the limit, instead of Ti, which is finer than MnS and has little adverse effect on toughness, and promotes bainite nucleation in the prior austenite grains as with MnS. It was conceived that high heat input welding HAZ toughness is improved by dispersing the nitride or carbonitride (including composite carbonitride of Ti and Nb). By controlling the balance of Ti, Nb, and N within an appropriate range, it is possible to refine the bainite substructure while reducing harmful MnS.

  The present invention is obtained by further examining the above-described knowledge, and the gist thereof is as follows.

  [1] By mass%, C: 0.06 to 0.09%, Si: 0.01 to 0.07%, Mn: 1.0 to 1.6%, P: 0.009% or less, S: 0.0007% or less, Mo: 0.20-0.60%, Al: 0.005-0.060%, Nb: 0.005-0.030%, Ti: 0.005-0 0.020%, N: 0.0025% to 0.0055% is contained so as to satisfy the following formulas (1) and (2), B: 0.0003% or less, and the following formula ( The carbon equivalent Ceq shown in 3) is 0.40 to 0.47%, the weld cracking sensitivity index Pcm shown in the following formula (4) is 0.22% or less, and the balance is Fe and inevitable impurities. A low-yield-ratio high-tensile steel plate with excellent weld heat-affected zone toughness.

3.0 × 10 ⁻⁵ ≦ (Ti + Nb / 5) × N ≦ 9.0 × 10 ⁻⁵ (1)
However, the element symbol represents mass% of each element.

2.5 ≦ (Ti + Nb / 3) /N≦5.5 (2)
However, the element symbol represents mass% of each element.

Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5
+ Mo / 4 + V / 14 (%) (3)
However, the element symbol represents mass% of each element.

Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60
+ Cr / 20 + Mo / 15 + V / 10 + 5B (%) (4)
However, the element symbol represents mass% of each element.

  [2] The composition according to [1], further comprising one or two of Cr: 0.05 to 0.60% and V: 0.005 to 0.080% in mass%. A low-yield ratio high-tensile steel sheet with excellent weld heat-affected zone toughness.

  [3] Further, by mass%, Cu: 0.05 to 0.30, Ni: 0.05 to 0.30, or 1 or 2 may be contained so as to satisfy the following formula (5): The low-yield ratio high-tensile steel sheet having excellent weld heat-affected zone toughness as described in [1] or [2].

Mn + Cu + Ni ≦ 1.6 (%) (5)
However, the element symbol represents mass% of each element.

  [4] Further, by mass%, it contains at least one selected from Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, and Mg: 0.0010 to 0.0020 The low-yield-ratio high-tensile steel sheet excellent in weld heat-affected zone toughness according to any one of [1] to [3].

  [5] Further, the cleanliness of sulfide inclusions is 0.002% or less, and the average size of nitride or carbonitride containing Ti (including composite carbonitride of Ti and Nb) is the equivalent circle diameter. The low yield ratio high tensile strength steel sheet having excellent weld heat affected zone toughness according to any one of [1] to [4], wherein the thickness is 5 to 50 nm.

  According to the present invention, it has a yield strength of 440 MPa or more, a tensile strength of 590 MPa or more, and a low yield ratio of 80% or less, which are suitable for building structures in a plate thickness range up to 100 mm, and a super high heat input welding heat. High-strength steel sheets with excellent weld heat-affected zone toughness and weld crack resistance can be mass-produced in the affected zone and small heat input multi-pass weld zone, which increases the size of the steel structure and the earthquake resistance of the steel structure. It greatly contributes to improvement and improvement of construction efficiency, and has a remarkable industrial effect.

It is a figure which shows the groove shape of a welded joint test piece. It is a figure which shows the collection position of the V notch Charpy impact test piece in (a) board thickness 100mm and (b) board thickness 40mm.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. About component composition First, the reason which prescribed | regulated the component composition of the steel of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.06 to 0.09%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material. In order to minimize the amount of other alloy elements added, the C amount is 0.06% or more. On the other hand, if it exceeds 0.09%, the weld cracking resistance and the HAZ toughness are significantly reduced, so the C content is in the range of 0.06 to 0.09%. Preferably it is the range of 0.07-0.09. More preferably, it is the range of 0.07-0.08.

Si: 0.01 to 0.07%
Si reduction is one of the most important elements of the present invention. By setting Si to 0.07% or less, generation of island martensite in HAZ is suppressed and HAZ toughness is improved. On the other hand, since Si also has an effect as a deoxidizer and an effect of increasing the strength of the base material, addition of 0.01% or more is desirable. For this reason, Si amount is taken as 0.01 to 0.07% of range. Preferably, it is 0.01 to 0.05% of range.

Mn: 1.0 to 1.6%
Mn has an effect of increasing the strength of steel, and in order to ensure a tensile strength of 590 MPa or more, addition of 1.0% or more is required. On the other hand, if it exceeds 1.6%, the concentration in the central segregation part during solidification becomes remarkable, which causes an increase in slab defects. Further, the reduction in the base material and the HAZ toughness is remarkably deteriorated. For this reason, the amount of Mn is made into the range of 1.0 to 1.6%. Preferably, it is 1.2 to 1.6% of range. More preferably, it is the range of 1.4-1.6.

P: 0.009% or less Reduction of P is one of the most important elements of the present invention. P is an element that concentrates in island martensite, promotes the formation of island martensite, and lowers the toughness of the island martensite itself. In order to improve the HAZ toughness, it is desirable to reduce it as much as possible. In particular, when combined with low Si, the amount of island martensite produced is significantly reduced and the HAZ toughness is significantly improved. When the P content is 0.009% or less, the effect of improving the HAZ toughness becomes remarkable, so the P content is 0.009% or less. Preferably, it is 0.005 or less.

S: 0.0007% or less S combines with Mn to form MnS. MnS is elongated by rolling, and when the Charpy specimen is taken in the plate thickness direction (Z direction), the absorbed energy is significantly reduced. MnS is unevenly distributed in the center segregation part at the center of the plate thickness. Since the heat-affected zone of electroslag welding covers a wide range, the center segregation zone may be included in the heat-affected zone, and the Charpy impact absorption energy at such a position is significantly reduced. In order to improve this, it is necessary to reduce S to the limit and reduce MnS as much as possible. If the S content is reduced to 0.0007% or less, the effect of MnS is hardly recognized, so the S content is set to 0.0007% or less. Preferably, it is 0.0006% or less.

Mo: 0.20 to 0.60%
Mo is an essential element essential for securing strength in the present invention. Addition of alloys such as Mn, Cu, and Ni causes problems such as deterioration of the surface properties of continuous cast slabs and problems such as an increase in slab internal defects due to an increase in the hardness of the central segregation part. In addition, in order to promote generation of island martensite of CGHAZ of super high heat input welding and ICCGHAZ in small heat input multi-pass welding and deteriorate toughness, the strength of the base material and HAZ is secured using Mo. By adding 0.20% or more, the formation of grain boundary ferrite can be suppressed, the base material and the structure of HAZ can be mainly bainite, and a tensile strength of 590 MPa or more can be obtained. If it exceeds 0.60%, the weld crack resistance deteriorates, so the Mo content is in the range of 0.20 to 0.60%. Preferably, it is the range of 0.25-0.55. More preferably, it is in the range of 0.30 to 0.50.

Al: 0.005-0.060%
Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process for high strength steels. Moreover, N in steel is fixed as AlN, and it has the effect of toughness reduction and crack generation by N. Such an effect is recognized with addition of 0.005% or more. However, when added over 0.060%, the toughness of the base material is lowered and mixed with the weld metal during welding to deteriorate the toughness. For this reason, Al amount is taken as 0.005 to 0.060% of range. Preferably, it is 0.010 to 0.045% of range. More preferably, it is the range of 0.020-0.035.

Nb: 0.005 to 0.030%
Nb is an element that has the effect of increasing the strength by precipitation strengthening and the effect of suppressing the recrystallization of austenite during controlled rolling and refining the subsequent transformation structure to strengthen the base material. Moreover, a composite carbonitride (Ti, Nb) (C, N) is formed by adding simultaneously with Ti. (Ti, Nb) (C, N) has a higher function as a nucleation site of ferrite and bainite than TiN, and contributes significantly to refinement of the HAZ structure. To obtain these effects, 0.005% or more must be added. Moreover, since addition exceeding 0.030% reduces a base material and HAZ toughness remarkably, Nb shall be 0.005 to 0.030% of range. Preferably, it is 0.008 to 0.020% of range.

Ti: 0.005-0.020%
Ti has a strong affinity for N and precipitates as TiN during solidification, and contributes to the suppression of coarsening of austenite grains in HAZ or to the enhancement of HAZ as a ferrite transformation nucleus. In order to obtain such an effect, addition of 0.005% or more is necessary. On the other hand, if added over 0.020%, the TiN particles become coarse and the above-described effects cannot be expected. For this reason, Ti amount is taken as 0.005 to 0.020% of range. Preferably, it is 0.008 to 0.020% of range.

N: 0.0025 to 0.0055%
Solid solution N promotes the formation of island-like martensite in CGHAZ for base metal, super-high heat input welding, and ICCGHAZ in small heat input multi-pass welding, and deteriorates toughness. On the other hand, when nitrides are formed in combination with Ti or Nb, the austenite grains are prevented from coarsening by the pinning effect, and function as nucleation sites for ferrite and bainite, thereby contributing to refinement of the HAZ structure. In order to obtain such an effect, it is necessary to add at least 0.0025%. On the other hand, if it exceeds 0.0055%, the toughness may be deteriorated due to excessive nitride or coarsening, so the N content is in the range of 0.0025 to 0.0055%. Preferably, it is 0.0030 to 0.0050% of range.

(Ti + Nb / 5) × N: 3.0 × 10 ^{−5 to} 9.0 × 10 ⁻⁵
When the value of (Ti + Nb / 5) × N is less than 3.0 × 10 ⁻⁵ , a sufficient number of composite nitrides (Ti, Nb) N are not generated. On the other hand, when it exceeds 9.0 × 10 ⁻⁵ , (Ti, Nb) N increases and coarsens, so that HAZ toughness decreases. Therefore, the value of (Ti + Nb / 5) × N is in the range of 3.0 × 10 ^{−5 to} 9.0 × 10 ⁻⁵ . Incidentally, preferably, 4.0 range × ^{10 -5} ~8.0 × ¹⁰ -5.

(Ti + Nb / 3) / N: 2.5-5.5
If the value of (Ti + Nb / 3) / N is less than 2.5, solid solution N remains in the steel and the HAZ toughness decreases. On the other hand, if it exceeds 5.5, the composite nitride (Ti, Nb) N is coarsened, and the HAZ toughness is lowered. Therefore, the value of (Ti + Nb / 3) / N is set to a range of 2.5 to 5.5. Preferably, it is the range of 2.5-4.0.

B: 0.0003% or less B hardens HAZ of small heat input welding to impair weld crack resistance, or promotes formation of island martensite of CGHAZ of super large heat input welding and ICCGHAZ in small heat input multi-pass welding In order to deteriorate the toughness, the B content is made 0.0003% or less.

  In the present invention, the ranges of the carbon equivalent Ceq and the weld cracking sensitivity index Pcm defined in the expressions (3) and (4) are further determined.

Carbon equivalent Ceq: 0.40 to 0.47%
In order to ensure the strength of the base metal and the HAZ with a thick material up to a maximum plate thickness of 100 mm, Ceq needs to be 0.40% or more, but if it exceeds 0.47%, the weldability decreases, Moreover, since HAZ toughness falls, carbon equivalent Ceq shall be the range of 0.40-0.47.

Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5
+ Mo / 4 + V / 14 (%) (3)
However, the element symbol represents mass% of each element.

Weld crack susceptibility index Pcm: 0.21% or less In order to maintain good low temperature crack resistance and to secure weldability that does not require preheating even for a thick material having a thickness of 60 mm or more, the weld crack sensitivity index Pcm is set to 0. 21 or less.

Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60
+ Cr / 20 + Mo / 15 + V / 10 + 5B (%) (4)
However, the element symbol represents mass% of each element.

  The above is the basic chemical component of the present invention, and the balance consists of Fe and unavoidable impurities. However, in order to obtain desired strength and toughness, one or two of Cr and V may be added as selective elements. Good.

Cr: 0.05-0.60%
Cr is an element that has little adverse effect on the slab surface properties and the central segregation part, and has little deterioration in the toughness of CGHAZ in super high heat input welding and ICCGHAZ in small heat input multi-pass welding. It can be added as necessary. In order to obtain the effect of increasing the strength, addition of 0.05% or more is necessary, and when it exceeds 0.60%, the weldability deteriorates. Therefore, when Cr is added, the Cr amount is 0.05 to 0.00. A range of 60% is preferable. More preferably, it is 0.15 to 0.35% of range.

V: 0.005-0.080%
V has an effect of increasing strength by precipitation strengthening. Although it can add as needed, in order to acquire such an effect, 0.005% or more of addition is required. Since V of 0.080% or more remarkably lowers the HAZ toughness, when V is added, the amount of V is preferably in the range of 0.005 to 0.080%. More preferably, it is 0.010 to 0.050% of range.

  Furthermore, in order to obtain desired strength and toughness, one or two of Cu and Ni may be added as selective elements.

Cu: 0.05-0.30, Ni: 0.05-0.30, Mn + Cu + Ni: 1.6% or less Cu, Ni is an element that increases the strength of steel in the same manner as Mn. In order to ensure the strength of the HAZ, it can be added as necessary.

  Cu has an action of increasing the strength of the steel, and 0.05% or more of addition is necessary to obtain this effect. However, if Cu is added over 0.30%, slab cracking is likely to occur. Therefore, when Cu is added, the Cu content is preferably in the range of 0.05 to 0.30%. More preferably, it is 0.05 to 0.15% of range.

  Ni also has an effect of increasing the strength of steel, and 0.05% or more of addition is necessary to obtain this effect. However, if Ni is added in excess of 0.30%, slab cracking is likely to occur. Therefore, when Ni is added, the Ni content is preferably in the range of 0.05 to 0.30%. More preferably, it is 0.05 to 0.20% of range.

  In addition, when adding Cu and Ni, it is further preferable to satisfy | fill following formula (5).

Mn + Cu + Ni ≦ 1.6 (%) (5)
However, the element symbol represents mass% of each element.

  The reason why Mn + Cu + Ni is 1.6% or less is that when Mn + Cu + Ni exceeds 1.6%, the concentration in the central segregation part during solidification is remarkable, the hardness of the central segregation part increases, and the slab internal defects increase. This is because it becomes easy to cause problems.

  Furthermore, for the purpose of improving the material, one or more selected from Ca, REM, and Mg may be added as a selective element.

Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%,
Mg: 0.0010 to 0.0020%
Ca, REM, and Mg are elements that contribute to improving the ductility of steel through the control of sulfide morphology. The sulfides or oxide particles of these elements are combined with MnS and act as ferrite transformation nuclei during welding, contributing to the improvement of HAZ toughness. In order to exert these effects, it is necessary to add at least 0.0005% or more of Ca, 0.0010% or more of REM or Mg. On the other hand, when Ca is added in excess of 0.0020%, an excessive amount of Ca-based inclusions may be generated, and the toughness may be reduced. For this reason, when adding Ca, it is preferable to make Ca amount into 0.0005 to 0.0020% of range. For the same reason, when REM or Mg is added, the REM content is preferably in the range of 0.0010 to 0.0030%, and the Mg content is preferably in the range of 0.0010 to 0.0020%.

  Furthermore, in order to improve the HAZ toughness, the purity of sulfide inclusions and the size of nitride or carbonitride containing Ti may be defined.

Cleanliness of sulfide inclusions: 0.002% or less The Z-direction Charpy absorbed energy is improved as the purity of sulfide inclusions elongated in the rolling direction is improved. If the cleanliness is 0.002% or less, sufficient toughness can be ensured even in the central segregation portion, and therefore the cleanliness of sulfide inclusions is preferably 0.002% or less. More preferably, it is 0.001% or less. The cleanliness of sulfide inclusions is calculated by “Microscopic test method for non-metallic inclusions by point calculation” in Appendix 1 of JIS G 0555.

Average size of nitride or carbonitride containing Ti (including composite carbonitride of Ti and Nb) is equivalent to circle diameter: 10 to 100 nm
MnS functions as a nucleation site for ferrite and bainite and contributes to the refinement of the HAZ structure, but has the same function when the S content is reduced to the limit in order to improve the Z-direction Charpy absorption energy. It is necessary to disperse the inclusions and precipitates in the steel. Ti-containing nitride (TiN and / or Ti and Nb composite nitride (Ti, Nb) N) and / or carbonitride (Ti (C, N) and / or Ti and Nb composite carbonitride ( Ti, Nb) (C, N)) has a function as a nucleation site for ferrite and bainite, similar to MnS. However, if the particle size is too small, it hardly affects the phase transformation and serves as a nucleation site. Small function. On the other hand, if it is too large, the toughness is lowered by becoming the starting point of fracture, so the average size is preferably in the range of 5 nm to 50 nm in terms of the equivalent circle diameter. More preferably, it is the range of 10 nm or more and 30 nm or less.

  In the present invention, the steel having the above-described composition is melted by a melting means such as a converter or an electric furnace, and is made into a steel material such as a slab by a conventional method such as a continuous casting method or an ingot-bundling method. However, it does not specify a method for melting or casting steel. In addition, heat treatment after rolling includes reheating quenching (RQ), direct quenching (DQ), two-phase quenching (Q '), tempering (T), controlled rolling (CR), controlled rolling + accelerated cooling (TMCP). Can be done in combination. The quenching temperature is preferably 900 ° C. or lower.

  A steel material (slab: plate thickness 250 mm) prepared in the composition shown in Table 1 by a converter-ladder refining-continuous casting method was hot rolled into steel plates of 100 mm thickness and 40 mm thickness. Table 3 shows the manufacturing conditions of each steel plate.

  A JIS No. 4 tensile test piece was sampled from the position of the obtained thickness of each thick steel plate, and a tensile test was conducted in accordance with the provisions of JIS Z 2241 to investigate the tensile properties. In addition, V-notch Charpy impact test specimens were collected from the position of 1/4 of each obtained steel plate in accordance with JIS Z 2202, and Charpy impact test was performed in accordance with JIS Z 2242. Was carried out, the absorption energy at -40 ° C (vE-40 ° C) was determined, and the base material toughness was evaluated.

  The cleanliness of the sulfide inclusions was calculated by cutting an optical microscope sample of the cross section in the rolling direction from the position of 1/2 the plate thickness, and calculating “microscopic test method of nonmetallic inclusions by point calculation” in Appendix 1 of JIS G 0555. The average size of nitride or carbonitride containing Ti (including composite carbonitride of Ti and Nb) was taken from a TEM replica sample from 1/4 position and observed at 10,000 times. Then, nitrides or carbonitrides containing 100 or more Ti were observed and calculated as the average of their equivalent circle diameters.

  Further, from each of the obtained thick steel plates, a joint test plate (size: 400 × 600 mm) was collected, and electroslag welding (welding heat input: 1000 kJ / cm) having a groove shape as shown in FIG. A welded joint was prepared. The supply wire used was JIS Z 3353 YES62 equivalent, and the flux was JIS Z 3353 FS-FG3 equivalent.

  From the obtained welded joint, as shown in FIG. 2, a V-notch Charpy impact test piece having a notch position as a bond part and a center segregation part (only a joint having a skin plate thickness of 40 mm) is collected and specified in JIS Z 2242. In accordance with the above, a Charpy impact test at a test temperature of 0 ° C. was performed to determine the absorbed energy (vE0 ° C.) of the joint bond portion at 0 ° C., and the joint toughness was evaluated. In addition, by induction heating, a double heat cycle corresponding to ICCGHAZ for small heat input multi-pass welding with an average cooling rate of 800 ° C. → 500 ° C. being 50 ° C./s with peak temperatures of 1400 ° C. and 800 ° C. was given. A V-notch Charpy impact test piece was collected from the sample, and the absorbed energy at 0 ° C. (vE 0 ° C.) was determined. The obtained results are shown in Table 2.

  Further, the short bead maximum hardness test was performed in accordance with JIS Z 3101. Using a coated arc welding rod (diameter: 4.0 mm) corresponding to JIS Z3211 E6216-N1M1 U, a welding bead is formed on a steel plate at a welding current of 170 A, a voltage of 24 V, and a welding speed of 150 mm / min. The hardness was measured to determine the maximum hardness. The obtained results are shown in Table 2.

  In addition, the evaluation of the base material characteristics is that the yield strength (YS) is 440 MPa or more, the tensile strength (TS) is 590 MPa or more, the yield ratio (YR) is 80% or less, and the Charpy absorbed energy at −40 ° C. is 47 J or more. The case where all were satisfied was regarded as passing.

  In addition, the evaluation of the weld heat affected zone toughness is based on the absorption at 0 ° C. of the Charpy impact test at the bond portion (CGHAZ) of the electroslag weld joint where the heat input of welding exceeds 400 kJ / cm and the central segregation portion included in the HAZ. The energy (vE0) has an average toughness of 47J or more, and also in the ICCGHAZ in small heat input multi-pass welding with a welding heat input of 20-50 kJ / cm, the absorbed energy (vE0) at 0 ° C. in the Charpy impact test is an average of 47J. The case where it had the above toughness was set as the pass.

  In addition, “Evaluation of resistance to weld cracking was determined to be acceptable when the maximum hardness of the short beat welded part having a length of 40 mm was less than HV350 and the weld cracking sensitivity index Pcm was 0.21% or less.

  As shown in Table 1, steel no. A to Steel No. I is an invention example in which the component composition is within the scope of the present invention. J to Steel No. Z is a comparative example whose component composition is outside the scope of the present invention. Table 2 shows the test results of the base metal and the welded portion of the steel plate manufactured using these steels. 1 to steel plate No. 1 12 and steel plate no. 31 to steel plate no. No. 42 is a steel No. 42 whose component composition is within the scope of the present invention. A to Steel No. Since I was used, excellent values were obtained for the base metal part strength, toughness, and HAZ part toughness.

  Steel plate No. 13 to steel plate No. 29 and steel plate No. 43 to steel plate No. No. 59 is a steel No. having a component composition outside the scope of the present invention. J to Steel No. This is a comparative example manufactured using Z, and any of the base material part strength, toughness, and HAZ part toughness did not satisfy the target characteristics, resulting in inferior results compared to the inventive examples.

Claims (5)

In mass%, C: 0.06-0.09%, Si: 0.01-0.07%, Mn: 1.0-1.6%, P: 0.009% or less, S: 0.0007 %: Mo: 0.20-0.60%, Al: 0.005-0.060%, Nb: 0.005-0.030%, Ti: 0.005-0.020% N: 0.0025% to 0.0055% is contained so as to satisfy the following formulas (1) and (2), and B: 0.0003% or less, and in the following formula (3) The carbon equivalent Ceq represented is 0.40 to 0.47%, the weld cracking sensitivity index Pcm represented by the following formula (4) is 0.21% or less, and the balance is composed of Fe and inevitable impurities. Low-yield ratio high-tensile steel plate with excellent weld heat-affected zone toughness.
3.0 × 10 ⁻⁵ ≦ (Ti + Nb / 5) × N ≦ 9.0 × 10 ⁻⁵ (1)
However, the element symbol represents mass% of each element.
2.5 ≦ (Ti + Nb / 3) /N≦5.5 (2)
However, the element symbol represents mass% of each element.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5
+ Mo / 4 + V / 14 (%) (3)
However, the element symbol represents mass% of each element.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60
+ Cr / 20 + Mo / 15 + V / 10 + 5B (%) (4)
However, the element symbol represents mass% of each element.   Furthermore, it contains 1 type or 2 types of Cr: 0.05-0.60%, V: 0.005-0.080% by the mass%, The welding heat influence of Claim 1 characterized by the above-mentioned. Low-yield ratio high-tensile steel sheet with excellent toughness. Furthermore, it is characterized by containing one or two of Cu: 0.05 to 0.30 and Ni: 0.05 to 0.30 so as to satisfy the following formula (5) by mass%. The low yield ratio high tensile strength steel sheet excellent in weld heat affected zone toughness according to claim 1 or 2.
Mn + Cu + Ni ≦ 1.6 (%) (5)
However, the element symbol represents mass% of each element.   Furthermore, it contains at least one selected from Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, and Mg: 0.0010 to 0.0020 by mass%. The low yield ratio high-tensile steel sheet excellent in weld heat affected zone toughness according to any one of claims 1 to 3.   Furthermore, the cleanliness of sulfide inclusions is 0.002% or less, and the average size of nitride or carbonitride containing Ti (including composite carbonitride of Ti and Nb) is 5 to 5 in terms of equivalent circle diameter. The low-yield-ratio high-tensile steel sheet having excellent weld heat-affected zone toughness according to any one of claims 1 to 4, characterized by being 50 nm.

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