JP2005220379A - Method for producing non-heat treated high strength thick steel plate excellent in toughness at extra-large heat input weld heat-affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To produce a high strength thick steel plate excellent in toughness at an extra-large heat input weld heat-affected zone, which has ≥590 MPa tensile strength, ≤80% low yield ratio and excellent HAZ toughness even in an extra-large heat input welding of >400 KJ/cm, in a non-heat treatment. <P>SOLUTION: A steel block having the composition containing 0.05-0.13% C, and the suitable contents of Si, Mn, P, S and Al and further, 0.005-0.030% Ti, 0.0030-0.0070% N, 0.0005-0.0050% Ca, 0.0010-0.0030% O and 0.35-0.44% carbon equivalent Ceq and satisfying 0.2-0.8 ACR, is heated into the range of 1000-1300°C, and after applying hot-rolling at not lower than Ar<SB>3</SB>transformation point to the rolling-finish temperature, accelerating cooling treatment is applied till ≤600°C at 1-20°C/s average cooling speed. In this way, the thick steel plate having ≥590 MPa TS, ≤80% YR and showing the excellent HAZ toughness even in the case of such welding heat affected part of the extra-large heat input welding as to exceed 400 KJ/cm welding heat input, can be obtained with the non-heat treatment. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a thick steel plate suitable for use in a building structure, and in particular, it has a tensile strength (TS) of 590 MPa or more, a yield ratio of 80% or less, and an excellent adjustment of super-high heat input heat affected zone toughness. It relates to a high-quality, high-strength steel plate. In the present invention, the thick steel plate refers to a steel plate having a thickness of 20 mm or more. Further, “super large heat input welding” in the present invention refers to welding in which the welding heat input exceeds 400 kJ / cm.

  In recent years, with the increase in size and span of building structures, there has been a demand for thicker and higher strength steel used. On the other hand, from the viewpoint of the safety of steel structures, reduction of the yield ratio of the steel material used is required. By reducing the yield ratio, even if a stress higher than the yield point is applied, the stress allowed until failure increases, and the uniform elongation increases, so that the steel material is excellent in plastic deformability.

  Such a building structure is finished into a desired shape structure by welding. However, as pointed out during the Hyogoken-Nanbu Earthquake, if a welded structure is subjected to an abrupt and large load such as during an earthquake, brittle fracture occurs mainly in the weld area before sufficient plastic deformation occurs. May occur. For this reason, in recent years, steel materials for welded structures are required to have good toughness including welds. For example, a column-beam large heat input weld is required to have high toughness such that Charpy absorbed energy at 0 ° C. exceeds 70 J. There are similar requirements for the welded portion of the box column.

  On the other hand, from the viewpoint of improving the efficiency of construction and reducing the cost of implementation, improvement in welding efficiency is required, and the application range of high-efficiency large heat input welding is expanding. For example, for box columns for building structures, supermerged heat input submerged arc welding, electroslag welding, or the like with a welding heat input exceeding 400 kJ / cm is applied.

  Generally, when high heat input welding is applied to a steel material, the most serious problem is toughness deterioration in the bond portion of the weld heat affected zone (hereinafter also referred to as HAZ). This bond portion is exposed to a high temperature just below the melting point during high heat input welding, and the austenite crystal grains are most likely to be coarsened, and the subsequent cooling is likely to transform into a brittle upper bainite structure. Further, in this bond portion, a brittle structure such as a woodman-stetten structure or island martensite is easily generated, which also causes a decrease in toughness. In particular, in a high-strength thick steel plate having a tensile strength exceeding 590 MPa, it is common to add a large amount of alloy to ensure the strength, so the yield ratio tends to increase and the HAZ toughness also decreases. For this reason, development of the high strength thick steel plate which combines the low yield ratio and the outstanding HAZ toughness is desired.

  In response to such a demand, for example, Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4 propose a method for producing a low-yield ratio high-strength steel. The techniques described in Patent Document 1 and Patent Document 2 are both direct quenching methods in which quenching is performed immediately after rolling, the cooling start after rolling is delayed, and about 5 to 60% of ferrite is precipitated, followed by rapid cooling. Thus, it has a two-phase structure of ferrite phase + hard phase. As a result, high strength and low yield ratio are realized. On the other hand, the technique described in Patent Document 3 achieves high strength and low yield ratio by quenching after being kept in the ferrite precipitation temperature range and by forming a two-phase structure of ferrite + hard phase. . Moreover, in the technique described in Patent Document 4, after hot-rolling the steel sheet, it is heated again to the two-phase region of ferrite + austenite, and after quenching, tempering is performed to increase the strength and lower the yield ratio. Has achieved. However, with the techniques described in Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4, it is possible to achieve sufficient HAZ toughness even if high strength and low yield ratio can be achieved. There is a problem that has not been reached.

  For such problems, for example, Patent Literature 5, Patent Literature 6, Patent Literature 7, and Patent Literature 8 propose techniques for improving the HAZ toughness of high heat input welding. In Patent Document 5, aiming to improve the weld bond toughness of 100 kJ / cm, a rare earth element and Ti are added in combination, and fine particles are dispersed in the steel to suppress austenite grain growth. Techniques for improving the toughness of steel have been proposed. Patent Document 6 proposes a technique for finely dispersing Ti oxide to increase the toughness of the high heat input HAZ. Patent Document 7 proposes a technique for improving the toughness of high heat input HAZ by finely dispersing Ti oxide and using it as a nucleation site for ferrite transformation. In Patent Document 8, in order to thoroughly reduce solid solution N, Ti and a sufficient Al amount are contained, and further, Ca oxide is used as a fine oxide, and HAZ toughness in super-high heat input welding. High-strength steel sheets that improve the strength have been proposed.

However, even with the techniques described in Patent Document 5, Patent Document 6, Patent Document 7, and Patent Document 8, the tensile strength is a high strength of 590 MPa or more and the base material yield ratio is a low yield ratio of 80% or less. Furthermore, it has been difficult to stably maintain excellent HAZ toughness even in super-high heat input welding in which the heat input of welding exceeds 400 kJ / cm.
Japanese Patent Publication No.58-10442 JP-A 62-77419 JP-A-2-34721 Japanese Patent Laid-Open No. 4-99817 JP-A-60-184663 JP-A-57-51243 JP-A-60-245768 Japanese Patent Laid-Open No. 2001-107177

The present invention solves the above-mentioned problems of the prior art, and has a tensile strength of 590 MPa or more and a low yield ratio of 80% or less, and also in super-high heat input welding in which the welding heat input exceeds 400 kJ / cm. It aims at proposing the manufacturing method of the non-tempered high-strength thick steel plate which was excellent in the super-high heat-input welding heat affected zone toughness which has the outstanding HAZ toughness. In the present invention, “excellent toughness of the super-high heat input welding heat-affected zone” means that the Charpy absorbed energy vE _{0 at} 0 ° C. of the super-high heat input welding heat-affected zone having a welding heat input exceeding 400 kJ / cm is 70 J or more. The case where it has.

  In order to achieve the above-mentioned problems, the present inventors have intensively studied various factors affecting strength, yield ratio, and HAZ toughness. As a result, in order to ensure high toughness in the super-high heat input HAZ with a welding heat input exceeding 400 kJ / cm, the austenite grain coarsening is suppressed in the region heated to a high temperature during welding, and ferrite transformation is promoted during cooling We found that the dispersion of transformation nuclei is important. To achieve this, fine dispersion of TiN by strict component adjustment and adjustment of the amount of dissolved oxygen at the time of melting Ca addition to 0.0010 to 0.0050%, and the addition amount of Ca, S, O is ACR It has been found that adjusting to satisfy 0.2 to 0.8% is essential to suppress coarsening of austenite grains and promote ferrite transformation. In addition, in order to make the tensile strength of the base metal to be 590 MPa or more and to achieve both the base material strength and toughness, it is important to adjust the carbon equivalent Ceq in the range of 0.35 to 0.44% in addition to strict component adjustment. Also found out.

  Furthermore, after performing hot rolling on the steel material whose components are adjusted as described above, it is subjected to accelerated cooling treatment in which the cooling rate and the cooling stop temperature are optimized, so that the excellent super heat input HAZ toughness and the tensile strength described above are obtained. It has been found that it has a strength of 590 MPa or more, a low yield ratio of 80% or less, and a high toughness base material characteristic.

The present invention has been completed based on the above findings and further studies. That is, the present invention is mass%, C: 0.05 to 0.13%, Si: 0.05 to 0.50%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.0050% or less, Al: 0.1% or less, Ti : 0.005 to 0.030%, N: 0.0030 to 0.0070%, Ca: 0.0005 to 0.0050%, O: 0.0010 to 0.0030%, the following formula (1)
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Ni, Cu, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by is 0.35-0.44%, and the following formula (2)
ACR = {Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) ………………… （2）
(Where Ca, O, S: content of each element (mass%))
A steel material having a composition defined by ACR of 0.2 to 0.8 and the balance consisting of Fe and inevitable impurities is heated to a range of 1000 ° C to 1300 ° C, and the rolling end temperature is Ar ₃ transformation point or higher. After being subjected to hot rolling, it is subjected to accelerated cooling treatment to cool to a temperature of 600 ° C. or lower at an average cooling rate of 1 to 20 ° C./s, tensile strength: 590 MPa or more, yield ratio: 80% In addition to the above composition, in addition to the above composition, Cu is 1.0% or less, and Cu: 1.0% or less. Ni: It is preferable to set it as the composition containing 1 type or 2 types chosen from 2.0% or less. Moreover, in this invention, in addition to each said composition, it is mass%, Cr: 0.7% or less, Contains one or more selected from Mo: 1.0% or less, Nb: 0.05% or less, V: 0.2% or less Further, in the present invention, in addition to each of the above-mentioned compositions, the composition further contains one or two selected in mass% from REM: 0.02% or less and Mg: 0.005% or less. A composition is preferred.

According to the present invention, the tensile strength is 590 MPa or more, and the yield ratio is 80% or less.
Super high heat input welding exceeding 400kJ / cm High strength thick steel plate with excellent HAZ toughness can be manufactured at low cost, greatly increasing the size of steel structures, improving the earthquake resistance of steel structures, and improving construction efficiency Contributes and has a remarkable industrial effect.

  First, the reasons for limiting the components of the steel material used in the present invention will be specifically described. In addition, unless otherwise indicated, the% display regarding a component shall mean mass%.

C: 0.05-0.13%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material. In the present invention, the content of 0.05% or more is required to obtain the above-described effects. On the other hand, the content exceeding 0.13% degrades the HAZ toughness and weld crack resistance. For this reason, C was limited to the range of 0.05 to 0.13%. In addition, Preferably, it is 0.05 to 0.12%.

Si: 0.05-0.50%
Si acts as a deoxidizer and needs to be at least 0.05% for steelmaking. However, if it exceeds 0.50%, the toughness of the base material deteriorates and island martensite is generated in the super-high heat input HAZ. , HAZ toughness is significantly deteriorated. For this reason, Si was limited to the range of 0.05 to 0.50%. In addition, Preferably, it is 0.05 to 0.35%.

Mn: 0.5-2.0%
Mn has the effect of increasing the strength of steel, and in the present invention, it is necessary to contain 0.5% or more in order to ensure a tensile strength of 590 MPa or more. On the other hand, if the content exceeds 2.0%, the toughness and the HAZ toughness of the base material deteriorate significantly. For this reason, Mn was limited to the range of 0.5 to 2.0%. In addition, Preferably, it is 0.6 to 1.5%.

P: 0.02% or less P is an element that increases the strength of steel and deteriorates toughness. In particular, P is desirably reduced as much as possible in order to deteriorate the toughness of welds. This tendency becomes remarkable when P exceeds 0.02%, so 0.02% was made the upper limit. In addition, since excessive P reduction raises refining cost and becomes economically disadvantageous, it is desirable to set it as 0.005% or more.

S: 0.0050% or less S is finely crystallized at the solidification stage as CaS particles by combining with Ca. These fine particles act as ferrite transformation nuclei during cooling after hot rolling, and the structure after accelerated cooling is made into a two-phase structure of soft ferrite phase and hard bainite phase, contributing to lower yield ratio of the base metal To do. Furthermore, at the time of welding, it precipitates as MnS on CaS particles, acts as a ferrite transformation nucleus, and has the effect of improving the weld zone toughness. Such an effect is noticeable when the content is 0.0005% or more. On the other hand, if the content exceeds 0.0050%, the toughness of the base metal and the welded portion is deteriorated. For this reason, S was limited to 0.0050% or less.

Al: 0.1% or less
Al acts as a deoxidizer and is most commonly used in the high-strength steel deoxidation process. In addition, N in the steel is fixed as AlN, which prevents coarsening of crystal grains and contributes to improvement of the toughness of the base material. Such an effect is noticeable when the content is 0.005% or more. On the other hand, when the content exceeds 0.1%, the toughness of the base material is lowered, and the toughness is deteriorated by being mixed into the weld metal part during welding. For this reason, Al was limited to 0.1% or less. In addition, Preferably, it is 0.01 to 0.07%.

Ti: 0.005-0.030%
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 strengthening HAZ as a ferrite transformation nucleus. Also, during accelerated cooling after hot rolling, TiN formed in the melting stage acts as a transformation nucleus that generates a ferrite phase, and the structure after accelerated cooling is divided into a soft ferrite phase and a hard bainite phase. As phase structure, it contributes to lower yield ratio of the base metal. In order to acquire such an effect, 0.005% or more needs to be contained. On the other hand, if the content exceeds 0.030%, the TiN particles become coarse and the above-described effects cannot be expected. For this reason, Ti was limited to 0.005 to 0.030% of range. In addition, Preferably, it is 0.010 to 0.030%.

N: 0.0030-0.0070%
N combines with Ti and precipitates as TiN to suppress the coarsening of the austenite grains in the HAZ or contribute to the toughening of the HAZ as a ferrite transformation nucleus. Also, during accelerated cooling after hot rolling, TiN formed in the melting stage acts as a transformation nucleus that produces a ferrite phase, and the structure after accelerated cooling is divided into a soft ferrite phase and a hard bainite phase. As phase structure, it contributes to lower yield ratio of the base metal. In order to secure the necessary amount of TiN having such an effect, N needs to be contained by 0.0030% or more. On the other hand, when the content exceeds 0.0070%, in HAZ heated to a temperature at which TiN is dissolved, the amount of solid solution N increases and the HAZ toughness significantly decreases. For this reason, N is limited to 0.0030-0.0070%.

Ca: 0.0005 to 0.0050%
Ca is an element that contributes to improving the ductility of steel through the form control of sulfide. In order to exert such an effect, it is necessary to contain at least 0.0005%, but even if it exceeds 0.0050%, the effect is saturated. For this reason, in this invention, Ca was limited to 0.0005 to 0.0050% of range. In the present invention, as will be described later, after adjusting the dissolved oxygen content immediately before the addition of Ca to 0.0050% or less, Ca is added to suppress the formation of Ca oxide and to crystallize CaS. CaS crystallizes in molten steel at a temperature lower than that of oxides, so that fine and uniform dispersion is possible in the steel. Such fine CaS particles are combined with MnS and act as ferrite transformation nuclei during welding, contributing to the improvement of HAZ toughness. Furthermore, fine CaS particles act as ferrite transformation nuclei during accelerated cooling after hot rolling, and the structure after accelerated cooling is made into a two-phase structure of a soft ferrite phase and a hard bainite phase. Contributes to yielding.

O: 0.0010 to 0.0030%
O is contained as an unavoidable impurity and is present as an oxide in the steel, reducing the cleanliness. For this reason, in the present invention, it is preferable to reduce as much as possible. However, since excessive reduction leads to an increase in refining cost, it is limited to 0.0010% or more. On the other hand, when the oxygen content exceeds 0.0030%, CaO inclusions are coarsened and adversely affect toughness. For this reason, in this invention, O was limited to 0.0010 to 0.0030% of range. In the present invention, in order to crystallize Ca as CaS, O having a strong binding force with Ca strengthens degassing or introduces a deoxidizing agent before adding Ca, so that O in molten steel is It is preferable to reduce it to 0.0050% or less.

Carbon equivalent Ceq: 0.35-0.44%
In the present invention, the content of each component is adjusted so that the carbon equivalent Ceq is 0.35 to 0.44% within the above-described component range. The carbon equivalent Ceq used in the present invention is represented by the following formula (1):
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Ni, Cu, Cr, Mo, V: content of each element (mass%))
Defined by

  If Ceq is less than 0.35%, the hardenability at the time of accelerated cooling after rolling becomes insufficient, and the structure after accelerated cooling becomes a ferrite main structure, so that a desired tensile strength of 590 MPa or more cannot be secured. On the other hand, if Ceq exceeds 0.44%, the base metal toughness after accelerated cooling after rolling deteriorates significantly. For this reason, in the present invention, Ceq is limited to a range of 0.35 to 0.44%.

ACR: 0.2 to 0.8
In the present invention, the amount of dissolved oxygen in molten steel at the time of Ca addition is adjusted to 0.0010 to 0.0050%, and then Ca, S and O are expressed by the following formula (2):
ACR = {Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) (2)
(Ca, O, S: content of each element (mass%))
The ACR defined by is adjusted to satisfy 0.2 to 0.8.

  When ACR is less than 0.2, Ca does not crystallize, so S precipitates in the form of MnS alone. Since this MnS is elongated by rolling during the production of the steel sheet and does not disperse uniformly and finely, it causes a decrease in the toughness of the base metal and does not contribute to an improvement in the weld HAZ toughness. On the other hand, when the ACR exceeds 0.8, S is fixed by Ca and S which becomes MnS is insufficient, and MnS does not precipitate on CaS that acts as a ferrite nuclei. Therefore, improvement in HAZ toughness cannot be expected. Only when the ACR satisfies 0.2 to 0.8 is a composite sulfide in which MnS is deposited on CaS. The presence of this composite sulfide functions as a ferrite transformation nucleus, the HAZ structure is refined, and the HAZ toughness is improved.

  In the present invention, in addition to the basic components described above, one or two selected from Cu: 1.0% or less, Ni: 2.0% or less, and / or Cr: 0.7% or less, Mo: 1.0% Hereinafter, Nb: 0.05% or less, V: One or more selected from 0.2% or less, and / or REM: 0.02% or less, Mg: 0.005% or less Two types can be contained.

One or two selected from Cu: 1.0% or less, Ni: 2.0% or less
Both Cu and Ni can increase strength while maintaining high toughness, and also have a small effect on HAZ toughness. Therefore, Cu and Ni are useful elements for increasing strength. it can. Such an effect becomes remarkable when Cu: 0.1% or more and Ni: 0.1% or more are contained. On the other hand, if the Cu content exceeds 1.0%, hot brittleness occurs and the surface properties of the steel sheet deteriorate. Moreover, even if it contains Ni exceeding 2.0%, an effect will be saturated and the effect corresponding to content will not be expectable, but it becomes economically disadvantageous. For this reason, it is preferable to limit Cu to 1.0% or less and Ni to 2.0% or less. More preferably, Cu is 0.2 to 0.7% and Ni is 0.2 to 1.7%.

One or more selected from Cr: 0.7% or less, Mo: 1.0% or less, Nb: 0.05% or less, V: 0.2% or less
Cr, Mo, Nb, and V are all elements that contribute to improving the strength of steel, and can be selected and contained as necessary. Such an effect becomes remarkable when Cr: 0.05% or more, Mo: 0.05% or more, Nb: 0.005% or more, and V: 0.01% or more. On the other hand, when Cr exceeds 0.7%, HAZ toughness deteriorates. When Mo exceeds 1.0%, the base metal toughness and HAZ toughness deteriorate. When Nb exceeds 0.05%, the base metal toughness and HAZ toughness deteriorate. When the content exceeds V: 0.2%, the HAZ toughness deteriorates. Therefore, it is desirable to limit Cr to 0.7% or less, Mo to 1.0% or less, Nb to 0.05% or less, and V to 0.2% or less.

One or two selected from REM: 0.02% or less, Mg: 0.005% or less
REM and Mg are both elements that contribute to toughness improvement, and can be selected and contained as necessary. Such an effect becomes remarkable when the content of REM is 0.002% or more. On the other hand, even if the content exceeds 0.02%, the effect is saturated, so REM has an upper limit of 0.02%. Mg is a useful element that improves toughness through refinement of crystal grains, and this effect becomes significant when Mg is contained in an amount of 0.001% or more, but even if contained in excess of 0.005%, the effect is significant. Because of saturation, 0.005% was made the upper limit.

  The balance other than the components described above is Fe and inevitable impurities.

  The method for producing the steel material used in the present invention is not particularly limited, and any ordinary melting method and casting method can be applied.

  The molten steel with the above composition is melted in a normal melting method such as a converter, electric furnace, vacuum melting furnace, etc., and after controlling the gas components using a deoxidation treatment or degassing process, the CaSi It is preferable to use a steel material (slab) by a normal casting method such as a continuous casting method after controlling inclusions by adding wires.

  In addition, in order to crystallize Ca as CaS at the time of melting, O having a strong binding force with Ca strengthens degassing or introduces a deoxidizing agent before adding Ca, thereby adding O in molten steel. It is preferable to reduce it to 0.0050% or less. Moreover, in this invention, after adjusting the amount of molten steel oxygen at the time of Ca addition to 0.0050% or less, it is preferable to add and adjust Ca and S so that ACR may satisfy 0.2-0.8.

  Next, these steel materials are reheated to 1000 ° C to 1300 ° C. If the reheating temperature is less than 1000 ° C., the deformation resistance in hot rolling becomes high and the rolling reduction per pass cannot be made large. Therefore, the number of rolling passes increases, and the rolling efficiency decreases, and the steel material The casting defect in (slab) may not be crimped. On the other hand, when the reheating temperature exceeds 1300 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. For this reason, the reheating temperature of the steel material was limited to a range of 1000 to 1300 ° C.

The reheated steel material is hot-rolled so that the rolling end temperature is equal to or higher than the Ar ₃ transformation point, thereby obtaining a thick steel plate having a predetermined thickness. The hot rolling conditions are not particularly limited as long as a predetermined plate thickness and shape can be satisfied except that the rolling end temperature is the Ar ₃ transformation point. In the case of a very thick steel plate having a plate thickness exceeding 80 mm, it is desirable to secure at least one rolling pass at which the rolling reduction per pass is 15% or more for zaku pressure bonding.

When the rolling end temperature is less than the Ar ₃ transformation point, the deformation resistance increases, the rolling load increases, and the load on the rolling mill increases. Further, in order to lower the rolling end temperature of the thick material to less than the Ar ₃ transformation point, it is necessary to wait in the middle of rolling, which greatly hinders productivity. For this reason, in the present invention, the rolling end temperature is limited to the Ar ₃ transformation point or higher.

The Ar ₃ transformation point is approximately the following formula: Ar ₃ = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb
(Here, C, Si, Mn, Ni, Cr, Mo, Cu, Nb: content of each element (mass%))
Can be organized.

  After the hot rolling is finished, the thick steel plate is immediately subjected to accelerated cooling to 600 ° C. or less at an average cooling rate of 1 to 20 ° C./s. When the cooling rate of the accelerated cooling process is less than 1 ° C./s, the microstructure after the accelerated cooling process becomes a ferrite main structure, and thus the target tensile strength of 590 MPa or more cannot be satisfied. On the other hand, if the cooling rate exceeds 20 ° C./s, the microstructure after the accelerated cooling treatment becomes a bainite single-phase structure, and the target yield ratio of 80% or less cannot be satisfied. Only after the cooling rate of the accelerated cooling treatment after hot rolling satisfies the range of 1 to 20 ° C./s, the microstructure after the accelerated cooling treatment is mainly composed of bainite and other fine inclusions such as TiN. It becomes a bainite + ferrite two-phase structure in which a ferrite phase is generated. This bainite + ferrite two-phase structure effectively acts on high strength and low yield ratio. In addition, the cooling rate as used in the field of this invention shall mean the average cooling rate from the cooling start to 600 degreeC in the plate | board thickness 1/4 position of a thick steel plate.

  In the present invention, the cooling stop temperature of the accelerated cooling process is set to 600 ° C. or lower. If the cooling stop temperature exceeds 600 ° C, the target tensile strength of 590 MPa or more cannot be secured. The cooling stop temperature is preferably room temperature or higher. The thick steel plate is allowed to cool to the room temperature from the cooling stop temperature of the accelerated cooling process.

  By using the steel material having the above composition, hot rolling under the above conditions and accelerated cooling treatment after hot rolling, high strength with a tensile strength of 590 MPa or more and low yield with a yield ratio of 80% or less. It is possible to easily manufacture a thick steel plate that combines the ratio and has excellent HAZ toughness during super-high heat input welding.

  A steel material (slab: plate thickness 310 mm) prepared in the converter shown in Table 1 by the converter-ladder refining-continuous casting method is shown in Table 2 by hot rolling and accelerated cooling treatment under the conditions shown in Table 2. A thick steel plate was used.

A JIS No. 4 tensile test piece was collected from the position of the thickness ¼ of each thick steel plate obtained, and a tensile test was conducted in accordance with the provisions of JIS Z 2241 to investigate the tensile characteristics. In addition, V-notch test specimens were collected from ¼ position of each thick steel plate obtained according to JIS Z 2202, and Charpy impact test was conducted according to JIS Z 2242. Absorbed energy (vE ₀ ) at 0 ° C. was determined to evaluate the base material toughness.

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

From the obtained welded joint, a V-notch test piece having a notch position as a bond portion as shown in FIG. 2 is collected and subjected to a Charpy impact test at a test temperature of 0 ° C. in accordance with the provisions of JIS Z 2242. Then, the absorbed energy (vE ₀ ) at 0 ° C. of the joint bond part was determined, and the joint toughness was evaluated.

  The obtained results are shown in Table 3.

Each of the present invention has a base material characteristic of a tensile strength of 590 MPa or more, a yield strength of 80% or less, a low yield ratio, and a high toughness of absorbed energy (vE ₀ ) of 100 J or more at 0 ° C. Weld heat input: Even when super-high heat input welding is performed at 1000 kJ / cm, vE _{0 of} the bond portion is 100 J or more, indicating excellent HAZ toughness. On the other hand, in a comparative example that is out of the scope of the present invention, one or more of the base material strength, yield ratio, base material toughness, and HAZ toughness do not satisfy the target value.

It is explanatory drawing which shows the groove shape of the electroslag welded joint used in the Example. It is explanatory drawing which shows the extraction | collection point of the Charpy impact test piece from the electroslag welded joint part in an Example.

Claims (4)

mass%
C: 0.05 to 0.13%, Si: 0.05 to 0.50%,
Mn: 0.5 to 2.0%, P: 0.02% or less,
S: 0.0050% or less, Al: 0.1% or less,
Ti: 0.005-0.030%, N: 0.0030-0.0070%,
Ca: 0.0005 to 0.0050%, O: 0.0010 to 0.0030%
The carbon equivalent Ceq defined by the following formula (1) is 0.35 to 0.44%, the ACR defined by the following formula (2) is 0.2 to 0.8, and the balance is Fe and inevitable impurities. A steel material having a composition as follows is heated to a range of 1000 ° C. to 1300 ° C. and subjected to hot rolling at a rolling end temperature equal to or higher than the Ar ₃ transformation point, and then an average cooling rate of 1 to 20 ° C./s is 600. Non-tempered high-strength thick steel plate with excellent tensile strength: 590MPa and yield ratio: 80% or less Manufacturing method.
Record
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Here, C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (mass%)
ACR = {Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) ………………… （2）
Here, Ca, O, S: Content of each element (mass%)   2. The composition according to claim 1, further comprising, in addition to the composition, at least 1% or 2 types selected from Cu: 1.0% or less and Ni: 2.0% or less in mass%. Manufacturing method of non-tempered high strength thick steel plate.   In addition to the above composition, the composition further contains at least one selected from mass: Cr: 0.7% or less, Mo: 1.0% or less, Nb: 0.05% or less, and V: 0.2% or less. The method for producing a non-tempered high strength thick steel plate according to claim 1 or 2.   The composition according to any one of claims 1 to 3, wherein, in addition to the composition, the composition further includes one or two kinds selected from mass%, REM: 0.02% or less, and Mg: 0.005% or less. The manufacturing method of the non-tempered high strength thick steel plate in any one.

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