EP1736562A1

EP1736562A1 - Thick high strength steel plate having excellent low temperature toughness in welding heat affected zone caused by high heat input welding

Info

Publication number: EP1736562A1
Application number: EP05730695A
Authority: EP
Inventors: Minoru; c/o NIPPON STEEL CORPORATION ITO; Akihiko; c/o NIPPON STEEL CORPORATION KOJIMA; Masanori; c/o NIPPON STEEL CORPORATION MINAGAWA; Yoichi;c/o NIPPON STEEL CORPORATION TANAKA; Toshiei; c/o NIPPON STEEL CORPORATION HASEGAWA; Jun; c/o NIPPON STEEL CORPORATION OTANI
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2004-04-07
Filing date: 2005-04-06
Publication date: 2006-12-27
Also published as: SG151274A1; KR100839262B1; JP4660250B2; TWI295693B; JP2005320624A; WO2005098068A1; US20070181223A1; TW200538561A; EP1736562A4; NO20065095L; KR20060130700A

Abstract

The present invention provides a high-strength thick steel plate having a plate thickness of 50 to 80 mm and a tensile strength of 490 to 570 MPa which is able to realize an excellent HAZ toughness even when welding with a heat input of 20 to 100 kJ/mm is conducted and is characterized by containing, by wt%, 0.03-0.14% of C, 0.30% or less of Si, 0.8-2.0% of Mn, 0.02% or less of P, 0.005% or less of S, 0.8-4.0% of Ni, 0.003-0.040% of Nb, 0.001-0.040% of Al, 0.0010-0.0100% of N, and 0.005-0.030% of Ti, where Ni and Mn satisfy equation [1], and the balance of iron and unavoidable impurities:

Ni / Mn \geq 10 \times Ceq - 3 (0.36 < Ceq < 0.42)

where, Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to a high strength thick steel plate excellent in low temperature toughness at heat affected zone (hereinafter referred to as an "HAZ") used for ships, offshore structures, medium/high rise buildings, bridges, and so forth, more particularly relates to a steel plate having a thickness of 50 mm or more and a tensile strength of 490 to 570 MPa and having an excellent welded joint even in a case where welding with a welding heat input of 20 to 100 kJ/mm is conducted.

2. Description of the Related

In recent years, the demands on the material properties of steel materials for welding used in large sized structures such as ships, offshore structures, medium/high rise buildings, and bridges have become increasingly tough. Especially, among these structures, the use of steel plates having large thicknesses exceeding 50 mm and having tensile strength of around 570 MPa has been increasing. Further, in order to promote higher efficiency of the welding, for the welding of such high-strength thick steel plates, one-pass welding by a large heat input welding process such as electro-gas welding, electo-slag welding, etc. has been investigated. The demands on the HAZ toughness have become increasingly tough in the same way as on the toughness of the base material per se.
Many proposals have been made hitherto paying attention to the HAZ toughness of steel plate to which a large heat input welding process is applied. For example, Japanese Unexamined Patent Publication (Kokai) No. 55-026164 discloses an invention of securing fine Ti nitrides in the steel so as to reduce the austenite grain size in the HAZ and thereby improve the toughness. Further, Japanese Unexamined Patent Publication (Kokai) No. 03-264614 proposes an invention making use of complex precipitates of Ti nitrides and MnS as transformation nuclei of ferrite so as to improve the HAZ toughness. Further, Japanese Unexamined Patent Publication (Kokai) No. 04-143246 proposes an invention making use of complex precipitates of Ti nitrides and BN as precipitation nuclei of grain boundary ferrite so as to improve the HAZ toughness.
However, Ti nitrides end up becoming almost completely dissolved in the vicinity of the border with a welded metal in HAZ where the highest temperature reached exceeds 1400°C (hereinafter also referred to as a "weld bond portion"). As a result, there is a problem that the effect of improvement of the toughness is lowered. For this reason, in steel utilizing the Ti nitrides as described above, it is difficult to meet the recent tough demands for the HAZ toughness or the required characteristics of the HAZ toughness in ultra-large heat input welding.
Steels containing Ti oxides as a method of improving the toughness in the vicinity of this weld bond portion are being used in various fields such as thick plates and steel shapes. For example, in the field of thick steel plates, as described in the inventions disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-079745 and Japanese Unexamined Patent Publication (Kokai) No. 61-117245 , steel containing Ti oxides is very effective for improving toughness at the large heat input weld portion, and is promising in application to high tensile steels. This principle is that the Ti nitrides, MnS, etc. precipitate in the middle of temperature drop after welding using the Ti oxides stable even at the melting point of the steel as precipitation sites, fine ferrite is generated using these as sites, and as a result the production of coarse ferrite harmful to the toughness is suppressed and deterioration of the toughness can be prevented.
However, such Ti oxides involve a problem that the number of particles dispersed into the steel cannot be increased that much. The reason is the coarsening or aggregation of the Ti oxides. It is believed that if the Ti oxides particles are increased, coarse Ti oxide particles of 5 µm or more, i.e., so-called inclusions, end up increasing. These inclusions of 5 µm or more size become initiation sites for fracture of a structure or cause a drop of the toughness and therefore are harmful. Therefore, this should be avoided. For this reason, in order to achieve a further improvement of the HAZ toughness, it was necessary to make use of oxides which are hard to coarsen and agglomerate and are more finely dispersed than Ti oxides.
Further, many such methods of dispersion of such Ti oxides into steel are based on adding Ti into molten steel substantially not containing any Al or other strong deoxidizing elements. However, it is difficult to control the number and degree of dispersion of Ti oxide particles in steel by just adding Ti into the melt. Further, it is also difficult to control the number and degree of dispersion of precipitates such as TiN and MnS. For this reason, in steel in which Ti oxide particles are dispersed by only Ti deoxidation, there was the problem for example that neither a sufficient number of Ti oxide particles nor a stable toughness of thick plate in the thickness direction could be obtained.
With respect to such problems, Japanese Unexamined Patent Publication (Kokai) No. 06-293937 and Japanese Unexamined Patent Publication (Kokai) No. 10-183295 disclose inventions making use of Ti-Al complex oxides and Ti, Al, and Ca complex oxides produced by the addition of A1 immediately after the addition of Ti or the complex addition of Al and Ca. By such inventions, it became possible to greatly improve HAZ toughness in the large heat input welding.

SUMMARY OF THE INVENTION

However, with the conventional means of reducing the austenite grain size of the HAZ or generating ferrite by using precipitates as transformation nuclei of the ferrite, it is necessary to increase the alloy elements in order to secure a tensile strength of 490 MPa or more when the plate thickness is 50 mm or more. In this case, the hardness of the HAZ rises and, at the same time, the production of MA (martensite-austenite constituent) degrading the toughness becomes remarkable. Therefore, a sufficient HAZ toughness such as the E grade (-20°C guarantee) in for example the shipbuilding field cannot be stably secured. In addition, when the tensile strength becomes more than 570 MPa, the required HAZ toughness cannot be obtained.
Therefore, an object of the present invention is to provide a high-strength thick steel plate excellent in the low temperature toughness of the heat affected zone resulting from large heat input welding, which can realize excellent HAZ toughness even in a case of welding with a heat input of 20 to 100 kJ/mm for steel plate having a thickness of 50 to 80 mm, and a tensile strength of 490 to 570 MPa.
The inventors discovered that by defining the amount of addition of Ni and Ni/Mn ratio, the above problems could be advantageously solved. They engaged in extensive study and thereby completed the present invention for the first time. The gist thereof is as follows:

(1) A high-strength thick steel plate excellent in low temperature toughness at a heat affected zone resulting from large heat input welding characterized by containing, by wt%, 0.03-0.14% of C, 0.30% or less of Si, 0.8-2.0% of Mn, 0.02% or less of P, 0.005% or less of S, 0.001-0.040% of Al, 0.0010-0.0100% of N, 0.8-4.0% of Ni, 0.005-0.030% of Ti, and 0.003-0.040% of Nb, where Ni and Mn satisfy Equation [1], and a balance of iron and unavoidable impurities: $Ni / Mn \geq 10 \times Ceq - 3 (0.36 < Ceq < 0.42)$

where, Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
(2) A high-strength thick steel plate excellent in low temperature toughness at a heat affected zone resulting from large heat input welding as set forth in (1), characterized by further containing, by wt%, one or more of 0.0003-0.0050% of Ca, 0.0003-0.0050% of Mg, 0.001-0.030% of an REM and containing at least 100/mm² of grains of an oxide containing 0.0010-0.0050% of O and having an equivalent circle diameter of 0.005 to 0.5 µm.
(3) A high-strength thick steel plate excellent in low temperature toughness at a heat affected zone resulting from large heat input welding as set forth in (1) or (2), characterized by further containing, by wt%, 0.0005-0.0050% of B.
(4) A high-strength thick steel plate excellent in low temperature toughness at a heat affected zone resulting from large heat input welding as set forth in any one of (1) to (3), characterized by further containing, by wt%, one or more of 0.1-0.5% of Cr, 0.01-0.5% of Mo, 0.005-0.10% of V, and 0.1-1.0% of Cu.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of a welding heat cycle corresponding to 45 kJ/mm.
FIG. 2 is a graph of the relationships among Ni/Mn, Ceq, and the simulated HAZ toughness.
FIG. 3 is a graph of an effect of improvement of the simulated HAZ toughness due to dispersion of fine oxides or B addition.

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed explanation of the present invention will be given below.
Hitherto, as the means for improving the HAZ toughness, as previously mentioned, it has been considered to suppress an austenite grain growth at a high temperature. The most effective method as that means is pinning an austenite grain boundaries by dispersed particles so as to stop the movement of the grain boundaries. This pinning is extremely effective for reduction of the reheated austenite grain size at the HAZ even in the case where a large heat input such as 20 to 100 kJ/mm is applied. However, in steel material wherein the amount of alloy elements added is increased in order to raise the strength and a carbon equivalent (Ceq) indicating both the weldability of the steel and a quench bardenability in terms of chemical composition becomes 0.36 or more, the hardness of the HAZ becomes higher. Therefore, there arises the new problem that a sufficient HAZ toughness cannot be obtained even when the re-heated austenite grains become finer due to the pinning. In the case where the hardness of the HAZ becomes high, it is necessary to improve the toughness of the base material per se.
Therefore, the inventors intensively studied the optimal chemical composition for improving the toughness of the base material itself for improvement of the HAZ toughness in the case where the Ceq was as high as 0.36 to 0.42, which becomes necessary for the high-strength thick steel plate. It has been conventionally known that Ni is an effective element improving the toughness of the matrix. However, it has not been known whether it is effective for improving the HAZ toughness in case of a high Ceq of 0.36 to 0.42. Therefore, the inventors first studied the influence of the amount of the Ni addition. For the study, they predetermined that an 0.003% or more of Nb addition is effective for securing the base material strength. For evaluation of the HAZ toughness, they employed the ductility/brittleness transition temperature in a Charpy impact test (vTrs) when imparting a heat cycle corresponding to electro-gas welding (heat input of 45 kJ/mm) shown in FIG. 1.
As a result of their studying the influence of the amount of addition of Ni, first they learned that the required toughness could not be obtained when the Ni was less than 0.8%. Further, even when the Ni was 0.8% or more, they observed cases where the HAZ toughness was not improved and cases where conversely the HAZ toughness was lowered. Therefore, they engaged in further intensive studies including the relationships with the other added elements and Ceq. As a result, they discovered that the HAZ toughness was related to the Ceq and Ni/Mn ratio as shown in FIG. 2 when the Ceq is 0.36 to 0.42 in this way. FIG. 2 plots the synthetic HAZ toughness (vTrs) of the steel material used for the study classified for each Ceq with the Ni/Mn ratio plotted on the abscissa. From FIG. 2, in steel wherein the following relationship stands: $Ni / Mn \geq 10 \times Ceq - 3$

a good toughness in terms of vTrs of -15°C or less was obtained. As the reason why steel not satisfying equation [1] cannot obtain a sufficient HAZ toughness, it can be considered that the amount of addition of Ni is not sufficient and the effect of increasing the toughness of the matrix is small, or that even if a large amount of Ni is contained, MA is produced in the HAZ due to an excessive addition of Mn and the effect of Ni of increasing the toughness disappears. Note that the inventors engaged in a similar study on the heat cycle of the heat input corresponding to a heat input of 100 kJ/mm for the steel used in the above described study, and as a result confirmed that a good synthetic HAZ toughness is obtained in steel satisfying the relationship of equation [1] even in the case of a heat input of 100 kJ/mm.
By the above mentioned studies, the inventors found that the HAZ toughness was improved by the addition of Ni in an amount of 0.8% or more satisfying equation [1]. The inventors further studied regarding the improvement of the HAZ toughness. They studied the following three methods as methods of improving the HAZ toughness. First is the method of suppressing the coarsening of the austenite grains at a high temperature, since in large heat input welding, the holding time at a high temperature becomes a long period. Therefore, the austenite grains coarsen, which lowers the HAZ toughness. Second is the method of suppressing the coarsening of the grain boundary ferrite, since the cooling time after the welding is long in large heat input welding, so the ferrite generated from the austenite grain boundaries coarsen. The coarse grain boundary ferrite becomes the cause of a drop in the HAZ toughness. Third is the method of refining the HAZ microstructure itself.
Concerning the first method of suppressing the coarsening of the austenite grains, for example, as described in Japanese Unexamined Patent Publication (Kokai) No. 10-183295 , the method of dispersing fine oxides is effective. In this publication, in the dispersion of the fine oxides, the amount of dissolved oxygen in the steel melt is adjusted by an equilibrium reaction with Si in the deoxidation process. Further, the following deoxidation is conducted in a sequence of Ti, Al, and Ca. Then, by this method, oxides having a particle size of 0.01-1.0 µm are dispersed to 5 x 10³ to 1 x 10⁵/mm².
Therefore, the inventors intensively studied a method of further improving the HAZ toughness by dispersing fine oxides in a process containing 0.03% of Nb and adding 0.8% or more of Ni in the case of a Ceq as high as 0.36 to 0.42. First, for the method of dispersing fine oxides, they found the fact that, in such a process, by adjusting the amount of dissolved oxygen of the steel melt to 0.0010-0.0050% in the deoxidation process, then first deoxidizing the steel melt with Ti, and then deoxidizing the steel melt with Al, and further adding one or more of Ca, Mg, and REM, it is possible to disperse fine oxide particles having an equivalent circle diameter of 0.005 to 0.5 µm to 100/mm² or more. Further, by this dispersion of the fine oxides, the coarsening of the austenite grains at the holding time at a high temperature in the welding was suppressed, and the HAZ toughness could be further improved. As an example, the result of comparison with the HAZ toughness obtained by only adding a proper amount of Ni is shown in FIG. 3. Note that, the larger the amount of Ni, the finer the produced oxides, and the larger the number of particles. When the amount of Ni is 1.5% or more, it even becomes 1000/mm² or more. This is discovered this time. Further, for the amount of Si in the steel melt, the larger the amount of Si, the harder the oxide to form. Therefore, it was clarified from this study that the amount of Si was preferably 0.30% or less and further preferably 0.20% or less. On the other hand, in the case where the amount of dissolved oxygen before the Ti deoxidation exceeds 0.050% and the case where the sequence of the deoxidation elements is different, the oxide coarsens and the sufficient amount of fine oxide cannot be obtained. Therefore, almost no effect of suppression of coarsening of the austenite grains can be obtained. Note that the number of grains of the oxides having the equivalent circle diameter of 0.005 to 0.5 µm was obtained by preparing an extraction replica from the steel plate as the base material. Observing this under an electron microscope with X10000 magnification in at least 100 fields (10000 µm² or larger observation area), and observing particles less than 0.1 µm by properly raising the magnification. The inventors conducted elemental analysis at each particle having a diameter of 0.005 to 0.5 µm observed and counted the oxide particles.
Next, the inventors intensively studied the suppression of the coarsening of the grain boundary ferrite and the refining of the HAZ microstructure as above described second and third methods of improvement of the HAZ toughness. As a result, the inventors clarified that the addition of B was effective particularly in the case where large heat input welding corresponding to 20 to 100 kJ/mm was conducted in a process where the Ceq was as high as 0.36 to 0.42 and Ni was added in an amount of 0.8% or more. The reason for that, in terms of the suppression of the coarsening of the grain boundary ferrite, is the suppression of the production of grain boundary ferrite by a segregation of the solute B at the re-heated austenite grain boundaries. Further, in terms of the increased refined HAZ microstructure, in the case where the cooling rate is slow in the large heat input welding, the B nitrides precipitated at the austenite grain boundaries and in the inclusions in the austenite grains due to the addition of B, and a large number of fine ferrite grains of several micrometers using those as nuclei are present at the austenite grain boundaries and in the grains, whereby the HAZ structure is made finer. The inventors compared the improvement of the HAZ toughness by the addition of B with the HAZ toughness obtained by only properly adding Ni. The results are shown in FIG. 3. It is seen that the HAZ toughness is further improved by the addition of B. Further, FIG. 3 shows the HAZ toughness in the case where the B is added in addition to the method of dispersing the above mentioned fine oxides. The HAZ toughness is further improved by the dispersion of the fine oxides and the addition of B. It is considered that the improvement is due to the increase of the oxides acting as precipitation sites of the BN and thereby the greater refined HAZ microstructure due to the increase of the ferrite using the BN as the nuclei.
Further, from the viewpoints of securing the strength and improvement of the corrosion resistance, in addition to the above described conditions, the inventors also studied the HAZ toughness when Cu, Cr, Mo, and V were added. As a result, they found that the HAZ toughness was not greatly lowered when they were added within ranges of 0.1-0.4%, 0.1-0.5%, 0.03-0.2%, and 0.005-0.050%.
Note that, the method of production of the steel plate according to the present invention is not particularly limited. The steel plate may be produced by any known method. For example, a slab is formed from steel melt adjusted to the preferred composition described above by a continuous casting method, then is heated to 1000 to 1250°C, then is hot rolled.
Next, an explanation will be given on the reasons for limitation of the ingredients of the steel materials used in the present invention. Below, the wt% in the compositions will be simply described as %.
C is an ingredient effective for improving the strength of the steel, so the lower limit is made 0.03%. An excess addition produces large amounts of carbide and MA and remarkably lowers the HAZ toughness, therefore the upper limit was made 0.14%.
Si is an ingredient necessary for securing the strength of the base material and deoxidation, but in order to prevent the drop in the toughness due to the hardening of the HAZ, the upper limit was made 0.30%. When utilizing an oxide, the upper limit of the content is made 0.20% or less in order to prevent the reduction of the oxygen concentration in the molten steel.
Mn is an ingredient effective for securing the strength and toughness of the base material and must be added in an amount of 0.8% or more, but the upper limit was made 2.0% within the range where the toughness, cracking property etc. of the welding zone were permissible. Further, concerning the upper limit of Mn, it is necessary to satisfy equation [1] indicating the relationship among the Ceq, Mn amount, and the Ni amount. This is based on the newly found fact by this study that the increase of Mn becomes the cause of production of a large amount of MA in the HAZ microstructure in the case where the Ceq is high and the effect of improvement of the HAZ toughness by Ni disappears. $Ni / Mn \geq 10 \times Ceq - 3$
P is desirably as little as possible, but in order to reduce this industrially, enormous costs are entailed, so the range of content was made 0.02 or less.
S is desirably as little as possible, but in order to reduce this industrially, enormous costs are entailed, so the range of content was made 0.005 or less.
Ni is an important element in the present invention and must be added in an amount of at least 0.8%. Further, concerning the lower limit of Ni, it is necessary to satisfy equation [1] showing the relationship of Ceq, the amount of Mn, and the amount of Ni. The upper limit was made 4.0% from the viewpoint of the production cost. $Ni / Mn \geq 10 \times Ceq - 3$
Nb is an element effective for improving the strength of the base material by improving the quench bardenability, so is added in an amount of 0.003% or more. However, if a lot of Nb is added, the MA becomes easy to be produced in the HAZ regardless of the Ni/Mn ratio, while when it is added in an amount larger than 0.040%, a large amount of coarse MA having a long axis of 5 µm is produced in the HAZ and greatly reduces the HAZ toughness. Therefore, the upper limit of Nb is made 0.040%. Note that, in order to obtain a higher toughness, preferably the amount of Nb is suppressed to 0.020% or less where almost no coarse MA having a long axis of 5 µm is produced in the case of the Ni/Mn ratio satisfying the above mentioned equation [1]. In order to stably obtain further higher toughness, it is preferred to suppress the amount of Nb to 0.010% or less where almost no MA having a long axis of 3 µm or more is generated in the case of an Ni/Mn ratio satisfying the above mentioned equation [1].
Al is an important deoxidation element, so the lower limit was made 0.001%. Further, when a large amount of A1 is present, the surface quality of the slab is deteriorated, so the upper limit was made 0.040%.
Ti is added in an amount of 0.005% or more according to need in order to produce the Ti nitride and the Ti-containing oxide particles which become pinning sites necessary for suppressing the coarsening of the re-heated austenite grains. However, its excess addition increases the amount of dissolved Ti and induces a drop in the HAZ toughness, therefore 0.030% was made the upper limit.
N is adjusted in the amount of addition, if necessary, in order to produce the Ti nitride and the B nitride particles at the austenite grain boundaries and in the grains during the cooling after the welding. In order to form the B nitride by binding with B, N must be added in an amount of 0.0010% or more, but its excess addition increases the amount of dissolved N and induces a drop in the HAZ toughness, so 0.0100% was made the upper limit.
Ca is added in an amount of 0.0003% or more, if necessary, in order to produce the Ca-based oxide particles acting as pinning grains necessary for suppressing the coarsening of the re-heated austenite grains. However, excess addition produces coarse inclusions, so 0.0050% was made the upper limit.
Mg is added in an amount of 0.0003% or more, if necessary, in order to generate the Mg-based oxide particles acting as pinning grains necessary for suppressing the coarsening of the re-heated austenite grains. However, excess addition produces coarse inclusions, so 0.0050% was made the upper limit.
A REM is added in the amount of 0.,0001% or more, if necessary, in order to produce the REM-based oxide particles acting as pinning sites necessary for suppressing the coarsening of the re-heated austenite grains. However, excess addition produces coarse inclusions, so 0.030% was made the upper limit. Further, the "REM" mentioned here represent Ce and La, and the amount of addition is the total amount of the two.
B is added in an amount of 0.0005% or more, if necessary, in order to cause the dissolved B to segregate at the austenite grain boundaries during the cooling after the welding and suppress the production of the grain boundary ferrite and further to produce BN at the austenite grain boundaries and in the grains. However, its excess addition increases the amount of dissolved B, greatly raises the HAZ hardness, and induces a drop in the HAZ toughness, so 0.0050% was made the upper limit.
Cu is added in an amount of 0.1% or more, if necessary, in order to improve the strength and corrosion resistance of the steel. The effect thereof is saturated at 1.0%, so the upper limit was made 1.0, but when it exceeds 0.4%, MA becomes easy to be generated and the HAZ toughness is lowered, therefore 0.4% or less is preferred.
Cr is added in an amount of 0.1% or more, if necessary, in order to improve the corrosion resistance of the steel, but its excess addition induces a drop in the HAZ toughness due to the generation of MA, so 0.5% was made the upper limit.
Mo is an element effective for improving the strength and the corrosion resistance of the base material and is added in an amount of 0.01% or more, if necessary. The effect thereof is saturated at 0.5%, so the upper limit was made 1.0, but its excess addition induces a drop in the HAZ toughness due to the generation of MA, so 0.2% or less is preferred.
V is an element effective for improving the strength of the base material and is added in an amount of 0.005% or more, if necessary. The effect thereof is saturated at 0.5%, so the upper limit was made 0.10%, but its excess addition induces a drop in the HAZ toughness due to the generation of MA, so 0.050% or less is preferred.

Example 1

Slabs were prepared by continuously casting the steel melt having the chemical compositions shown in Table 1. For D23-D31 and D46-D49, the amounts of dissolved oxygen of the steel melt were adjusted to 0.0010%-0.0050% by Si before charging the Ti, then Ti was used for deoxidization, then Al was used for deoxidation, then any of Ca, Mg, or REM was added for deoxidation. The slabs were re-heated at 1100 to 1250°C, then were hot rolled by the following two methods to produce steel plates having plate thicknesses of 50 to 80 mm. One method was to roll the plate at a surface temperature within a range of 750-900°C, then cool it by water at a plate surface temperature within the temperature range of 200-400°C after recalescence (described as TMCP in Table 2). The other method of production is cooling with water down to room temperature after hot rolling, then tempering within a range of 500-600°C (described as DQ-T in Table 2).
Table 2 shows the production conditions, plate thicknesses, and mechanical properties of the steel plates. Further, for D23-D31 and D46-D49, the numbers of fine oxide particles having equivalent circle diameters of 0.005 to 0.5 µm measured at any location of the steel plates were additionally described. The number of the oxide particles are found by preparing an extraction replica from any portion of the steel plate, observing this under an electron microscope with X10000 magnification in 100 fields or more (10000 µm² or more in observation area), and observing particles less than 0.1 µm by properly raising the magnification. Elemental analysis was conducted for each observed particle having a diameter of 0.005 to 0.5 µm and the oxide particles were counted. All of the steel plates among D23-D31 and D46-D49 had fine oxide particles having equivalent circle diameters of 0.01 to 0.5 µm dispersed to 100/mm² within the range of the present invention. Note that it is seen from the comparison of D46 and D47 and D48 and D49 wherein elements other than Si are almost equal, that the smaller the amount of Si, i.e. 0.20% or less, the larger the amount of the oxides.
Each of these steel plates was made to abut against another steel plate and subjected to vertical one-pass butt welding in using electro-gas welding (EGW) or electro-slag welding (ESW) having welding heat inputs of 20 to 100 kJ/mm. Then, in the HAZ located at the center portion of the plate thickness (t/2), notches were formed at two locations, that is, the HAZ separated from the FL (Fusion Line) by 1 mm (HAZ 1 mm) and the FL. A Charpy impact test was conducted at -40°C. Table 2 shows the welding conditions and HAZ toughnesses. In the Charpy impact test here, use was made of JIS No. 4 2 mm V-notch full size test pieces. Further, Table 2 also describes the former austenite grain size in FL-HAZ 1 mm. The "former austenite grain size in FL-HAZ 1 mm" described here is the average grain size obtained by measuring the grain size of the former austenite grains contained within a 2 mm range in the thickness direction centered by t/2 and the FL-HAZ 1 mm range by the cross-sectional method. Note that, the measurement was conducted by using particulate ferrite connected in a form of a net as the grain boundaries of the former austenite grains.
D1 to D49 are steels of the present invention. The chemical compositions of the steels are properly controlled, therefore the large heat input HAZ toughness at -40°C is good while satisfying the predetermined base material performances. Further, in D23-D31 and D46-D49 obtained by dispersing the fine oxide particles, the former austenite grain size in FL-HAZ 1 mm becomes finer than those of the others, i.e. 200 µm or less, and the large heat input HAZ toughness at -40°C becomes further higher. Further, D20 aiming at increasing the refined HAZ structure by adding B has a better HAZ toughness in comparison with D19 without addition of B and containing addition elements other than B in the equal amounts and exhibits a higher performance for also the large heat input HAZ toughness at -40°C.
On the other hand, the Comparative Steels C1 to C17 do not contain sufficient Ni for satisfying equation [1] or properly control the chemical compositions of the steels, so the large heat input HAZ toughness is insufficient.
As described above, the present invention provides thick steel plate satisfying the excellent toughness demands regarding destruction for ships, offshore structures, medium/high rise buildings, etc.

Claims

A high-strength thick steel plate excellent in low temperature toughness at heat affected zone resulting from large heat input welding, characterized by containing, by wt%,
C: 0.03-0.14%,

Si: 0.30% or less,

Mn: 0.8-2.0%,

P: 0.02% or less,

S: 0.005% or less,

Al: 0.001-0.040%,

N: 0.0010-0.0100%,

Ni: 0.8-4.0%,

Ti: 0.005-0.030%, and

Nb: 0.003-0.040%,
where Ni and Mn satisfy equation [1], and the balance of iron and unavoidable impurities: $Ni / Mn \geq 10 \times Ceq - 3 (0.36 < Ceq < 0.42)$

where, Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
A high-strength thick steel plate excellent in low temperature toughness at heat affected zone resulting from large heat input welding according to claim 1,
characterized by further containing, by wt%,
one or more of:

Ca: 0.0003-0.0050%,

Mg: 0.0003-0.0050%, and

REM: 0.001-0.030% and
contains at least 100/mm² of oxide particles containing
O: 0.0010-0.0050%
and having a equivalent circle diameter of 0.005 to 0.5 µm.
A high-strength thick steel plate excellent in low temperature toughness at heat affected zone resulting from large heat input welding according to claim 1 or 2,
characterized by further containing, by wt%,
B: 0.0005-0.0050%.
A high-strength thick steel plate excellent in low temperature toughness at heat affected zone resulting from large heat input welding according to any one of claims 1 to 3, characterized by further containing, by wt%,
one or more of:

Cr: 0.1-0.5%,

Mo: 0.01-0.5%,

V: 0.005-0.10%, and

Cu: 0.1-1.0%.