BE1020801A3 - EXCELLENT STEEL IN TERMS OF TENACITY OF THE BASE METAL AND THE AREA AFFECTED BY THE HEAT OF WELDING AND METHOD OF MANUFACTURING THE SAME. - Google Patents

Description

"Excellent steel in terms of the toughness of the base metal and the area affected by the heat of welding and method of manufacturing the same"

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to steel used for welded structures such as bridges, high-rise buildings, ships and the like, and relates to a technology improving the toughness of the base metal and the toughness of a position receiving a thermal effect. during high temperature welding (which may be referred to hereinafter as "zone affected by the welding heat" or "ZAC".

2. Description of the Related Art

Associated with an increase in the size of the welded structures, the welding of thick steel plates with a plate thickness of 50 mm or more is inevitable. Therefore, from the point of view of improving the efficiency of the welding work, high heat-input welding of 50 kJ / mm or more is oriented. However, in high heat welding, there is a problem due to the fact that the ZAC structure (especially near the welded section outside the ZAC) is made coarser and the toughness of the portion is likely to be deteriorated because the ZAC is gradually cooled after being heated to a high temperature austenite region. This is a problem that must be solved to obtain such toughness in the ZAC (which may hereafter be called "ZAC toughness".

A number of technologies to prevent the deterioration of ZAC toughness during high heat welding have heretofore been proposed. As representative examples of these technologies, steel is proposed in patent literature 1-4, for example, in which the austenite grain magnification occurring in the ZAC during high heat-input welding is suppressed and the deterioration of ZAC toughness is inhibited by dispersively precipitating fine TiN in steel. However, according to these technologies, there is a problem that, when the temperature of the molten metal during welding rises to 1400 ° C. or more, in the position especially in the vicinity of the molten metal (welded section) outside the ZAC, said TiN is dissolved in solid solution and disappears due to the heat received during welding, and the deterioration of the ZAC toughness can not be sufficiently prevented.

In the patent literature 5, there is also proposed a technology in which by optimizing the numerical density of fine TiN having a grain size of 0.01 -0.1 μm, the generation of coarse TiN whose grain size exceeds 0, 1 pm is removed and the ZAC toughness is improved. However, it has been found that even when the numerical density of the fine TiN is optimized, it is not possible to obtain sufficient ZAC toughness.

On the other hand, the present applicant has proposed (patent literature 6 for example) a technology in which the ZAC toughness in a wide range of heat input is obtained by the fact that the Nb is positively contained in inclusions based on TiN present in the steel for welding and a Ti / Nb ratio is controlled so that the number of inclusions having a grain size of 0.01 -0.25 μm is 1.0 × 10 4 or more per 1 mm 2. However, even with this technology, it is inevitable that the TiN is dissolved in solid solution and disappears due to the heat received during welding, and there are cases where the ZAC toughness has deteriorated.

Meanwhile, the steel used for a welded structure is also required to be excellent in terms of the toughness of the same steel (toughness of the base metal), which is the fundamental property in addition to the ZAC toughness. Therefore, a thick steel plate in which the base metal toughness and ZAC toughness are improved has been proposed by the present applicant in the patent literature 7. According to this technology, a thick steel plate excellent in terms of Base metal toughness and ZAC toughness is provided by controlling the numerical density according to the Ti size contained in the nitride included in the steel plate and appropriately controlling the sectional ratio of the island-shaped martensite. However, the method is not necessarily highly accurate because the numerical density of each size is measured by microscopic observation and a variation in property can be caused.

[Prior art literature] [Patent literature] [Patent literature 1] Examined Japanese Patent Application Publication No. S55-26164 [Patent Literature 2] Unexamined Japanese Patent Application Publication No. S2003-166017 [Patent Literature 3] Unexamined Japanese Patent Application Publication No. S2003-213366 [Patent Literature 4] Unexamined Japanese Patent Application Publication No. S2001 -20031 [Patent Literature 5] Unexamined Japanese Patent Application Publication No. S2001 - 98340 [Patent Literature 6] Unexamined Japanese Patent Application Publication No. S2004-218010 [Patent Literature 7] Unexamined Japanese Patent Application Publication No. S2010-95781

SUMMARY OF THE INVENTION

The present invention has been developed in view of these circumstances, and its purpose is to provide excellent steel in terms of both the toughness of the base metal and ZAC toughness and a method for making said steel.

Excellent steel in terms of base metal toughness and a heat-affected area of welding in connection with the present invention that could achieve the goal is steel whose composition comprises C: 0.03. 0.16% (means% by weight, hereinafter the same for the compositions), Si: 0.25% or less (including 0%), Mn: 1-2.0%, P: 0.03% or less (not including 0%), S: 0.015% or less (not including 0%), Al: 0.05% or less (not including 0%), Ti: 0.010-0.08%, Ca: 0.0005-0.010% and N: 0.0020-0.020%, the balance comprising iron and inevitable impurities.

It is also characterized in that an amount of Ti included in the steel as Ti containing inclusions exceeding 2.0 μm relative to a total amount of Ti Q included in the steel is 0.010% or less (not including 0%), and a ratio R / Q of a value R obtained by deducting a quantity of Ti included in the steel as inclusions containing Ti exceeding 0.1 μm of the total amount of Ti Q included in the steel is 0.30-0.70.

In the present invention, the Ti-containing inclusions mean precipitates comprising at least Ti, and signifies inclusions comprising Ti such as Ti-containing nitride such as TiN, composite nitride in which a portion of Ti (approximately 50% or less in terms of atomic ratio) is substituted by other nitride elements (Nb, Zr, V and the like, for example, Ti-containing oxide and the like.) In addition, the Ti-containing oxide also comprises composite oxide in which part of the Ti (approximately 50% or less in terms of atomic ratio) is substituted by other oxide-forming elements (Si, Mn, Al, Ca, Zr, REM and the like for example) not to mention the oxide Ti (T1O2 for example).

The steel may further comprise, as other elements, (a) one or more elements selected from a group consisting of Ni: 1.5% or less (not including 0%), Cu: 1.5% or less (no 0%), Cr: 1.5% or less (not including 0%) and Mo: 1.5% or less (not including 0%), (b) Number: 0.10% or less (not included 0 %) and / or V: 0.1% or less (not including 0%), (c) B: 0.005% or less (not including 0%), (d) Zr: 0.02% or less (not included 0%) and / or REM: 0.02% or less (not including 0%), and the like.

The steel can be made by melting the steel so that Ti, N and Si satisfy an expression (1) below and then casting the molten steel after the number of inclusions containing Al2O3 (more specifically, the inclusions comprising Al203 at 80 wt.% or greater) included in the steel was tested at 10 units or less (including zero units) per 1 mm2 by flotation separation of inclusions included in the molten steel. In expression (1) below, [] expresses a content (% by weight) of each element in steel.

[Ti] x [N] <(1 x10'5) / [Si] ... (1)

However, when Si = 0% by weight, the steel is melted so that Ti and Ni satisfy an expression (2) below.

[Ti] x [N] <1x10'3 ... (2)

According to the present invention, with respect to inclusions containing Ti in steel, instead of controlling the numerical density of each size by microscopic observation as was done in the prior art, the amount of Ti included in the steel as coarse inclusions containing Ti whose size exceeds 2 μm in relation to the total amount of Ti Q included in the steel is reduced as much as possible, the amount of Ti included in the steel as inclusions containing Ti whose size exceeds 0.1 μm is quantified by a

electrolytic extraction process, an R / Q ratio of R value obtained by deducing the amount of Ti from the total amount of Ti Q to the total amount of Ti Q included in the steel is appropriately controlled, and by therefore the improvement of the toughness of the base metal and the ZAC can be achieved more precisely. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic drawing explaining the concept of the total amount of Ti Q (the total amount of Ti included in the steel as Ti containing inclusions greater than 2.0 μm in size, the amount of Ti included in the steel as inclusions containing Ti whose size exceeds 0.1 μm and 2.0 μm or less, and the quantity R of solid solution Ti (including the amount of Ti in the steel as inclusions containing Ti whose size is 0.1 μm or less) stipulated in the present invention.

Fig. 2 is a graph showing the relationship between the value of [Ti] x [N] x [Si] (Z value) and the ZAC toughness.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To improve both the toughness of the base metal and the ZAC toughness of steel, considering that, according to the method of controlling the numerical density according to the size of inclusions containing Ti determined by microscopic observation as was done in the prior art, the field of vision area being only approximately 300 mm 2 maximum, the accuracy was therefore low and there was variation in property, the present inventors have carried out investigations to provide a more accurate alternative method . At the time, the idea emerged of making good use of a method to screen the total amount of Ti in steel by size and quantifying the amount of Ti of each size (density) by combining electrolytic extraction and filtration separation by multiple membrane filters (which may simply be referred to hereinafter as filters) having a different opening (mesh), and investigations have been made on this point.

As a result, the following has been found and the present invention has been completed.

(1) With the proviso that Ti being passed through a 0.1 μm mesh filter after electrolytic extraction with a predetermined electrolytic solution is referred to as solid solution Ti, the amount of Ti in solid solution has a large effect on the improved base metal toughness and ZAC toughness of steel.

(2) A desired property can not be exercised unless the amount of Ti in solid solution is controlled while preserving equilibrium with the total amount of Ti included in the steel (more specifically a ratio of the amount of Ti in solid solution to the total amount of Ti) instead of being controlled as an absolute value.

(3) In addition, to effectively exert the desired property, simply do not control that the ratio of the amount of Ti in solid solution is not sufficient, and it is also important to appropriately control the amount of Ti included in inclusions containing Ti whose size exceeds 2 μm which does not pass through the filter (remains on the filter) having meshes of 0.2 μm.

(4) Therefore, in order to improve the base metal toughness and ZAC toughness of the steel, it is extremely important to appropriately control the ratio of the amount of Ti in solid solution to the total amount of Ti calculated by the method described above and the amount of Ti included in the steel as inclusions containing Ti whose size exceeds 2.0 μm.

In the present description, the total amount of Ti included in the sample is designated S and the amount of Ti that passes through the 0.1 μm mesh filter (which is referred to as solid solution Ti in the present invention). after electrolytic extraction (details of which will be described below) is designated R. The total amount of Ti Q is a value quantified by ICP atomic emission spectrometry after the electrolytic extraction has been performed. The amount of Ti in solid solution means the amount of Ti that passes through the filter during filtration using a 0.1 μm mesh filter after electrolytic extraction. The smallest mesh filter sold on the market is a filter having a mesh size of 0.1 μm and the Ti passing through the filter having the minimum diameter was considered to be "solid solution Ti" in the present invention even though it was present in inclusions containing Ti.

In addition, with respect to the solid solution amount R of Ti determined as described above, the amount of solid solution Ti is not measured directly but is calculated indirectly by quantifying the amount of Ti included in inclusions containing Ti whose size exceeds 0.1 μm which do not pass through the filter (remain on the filter) with a mesh size of 0.1 μm after electrolytic extraction by atomic emission spectrometry ICP, and deducting from the total quantity Ti Q included in the steel. This is because a direct quantitative analysis of the amount of Ti in solid solution is difficult.

In the present description, "Ti containing inclusions exceeding 2.0 μm" means that they do not pass through the filter (remain on the filter) when they are filtered using the 2.0 μm mesh filter after that the electrolytic extraction was carried out. In the past, it was known that coarse Ti-containing inclusions exerted a deleterious effect on ZAC toughness and the like, but, from the results of investigations by the present inventors, it was found that the amount of coarse inclusions containing Ti whose size exceeds 2.0 μm separated by the method described above exerted a particularly deleterious effect on the base metal toughness and ZAC toughness especially outside the Ti-containing coarse inclusions, and therefore, has come to appropriately control the amount of these coarse inclusions containing Ti.

First, the total amount of Ti in the steel of the present invention will be described. According to the invention, the steel is melted by the electrolytic extraction process, the extracted residue obtained after electrolytic extraction is filtered by a 0.1 μm mesh membrane filter which is the smallest mesh sold on the market, the extracted residue obtained by filtration is melted with the filter and the amount of Ti is measured by the ICP atomic emission spectrometry method. According to this method, as shown in FIG. 1, the total amount of Ti Q included in the steel is expressed as the total amount of Ti included in the steel as inclusions containing Ti whose size exceeds 0.1 pm not passing through the filter and the amount of Ti in solid solution comprising the amount of Ti included in the inclusions containing Ti passing through the filter.

According to investigations by the present inventors, it has also been found that the base metal toughness and steel ZAC toughness could be improved by adjusting the R / Q ratio of the obtained R value by deducting the amount of Ti included in the steel as inclusions containing Ti whose size exceeded 0.1 pm of the total amount of Ti Q to the total amount of Ti Q included in the steel in the range of 0.30 - 0 70. This fact has been verified in the example of the present description.

That is, the N-type steel shown in Table 1 and the D-type steel shown in Table 2 of the example are steels which are generally the same in terms of componential composition and are generally the same in terms of the amount of Ti included in the steel as coarse inclusions containing Ti exceeding 2.0 μm. However, the base metal toughness and ZAC toughness of d-type steel have been deteriorated, while the base metal toughness and ZAC toughness of N-type steel have been improved. Similarly, although the type E steel shown in Table 1 below and the type b steel shown in Table 2 below are also generally the same in terms of componential composition and amount of As described above, the base metal toughness and ZAC toughness of the type b steel have been deteriorated, while the base metal toughness and ZAC toughness of the E type steel have been improved.

The investigation shows that the ratio of the amount of Ti in solid solution to the amount of Ti added (the total amount of Ti included in the steel) was high in the type d steel, the ratio from the amount of Ti in solid solution to the amount of Ti added was low in type b steel, and it was assumed that this amount of solid solution Ti had greatly contributed to the improvement of the base metal toughness. and ZAC toughness.

After further investigations reflecting these results, it was found that the ratio R / Q of the amount of Ti in solid solution to the total amount of Ti Q included in the steel had an effect on the toughness of the base metal and the ZAC toughness.

However, it was also found that the effect of improving base metal toughness and ZAC toughness was not sufficient by simply controlling the ratio R / Q of the amount of Ti in solid solution to the amount total Ti included in the steel in a suitable range.

As a result of further investigations by the present inventors, it has also been found that the amount of Ti included in the steel as Ti containing inclusions greater than 2.0 microns in size compared to Ti-containing inclusions. whose size exceeded 0.1 pm also had an effect on base metal toughness and ZAC toughness. This fact has also been verified in the example described below. For example, the type L steel shown in Table 1 and the type a steel shown in Table 2 of the example are steels having a componential composition which is generally the same and the R / Q ratios. of all are controlled in the range of 0.30 -0.70. However, in type a, e steel, as the amount of Ti included in coarse inclusions containing Ti greater than 2.0 μm in size, both the base metal toughness and the ZAC toughness have deteriorated. . On the other hand, in type L steel, as the amount of Ti included in coarse inclusions containing Ti whose size exceeds 2.0 μm is reduced to the range stipulated in the present invention, both the toughness of the metal Basic that the ZAC toughness are excellent.

It is understood that both the base metal toughness and the ZAC toughness can be improved when the amount of Ti included in the steel as coarse inclusions containing Ti greater than 2.0 μm in size is therefore reduced. The reason for this is thought to be the following, although its details are unclear. In order to improve the ZAC toughness, miniaturization of the former y grain size by inclusions containing fine Ti (TiN precipitate for example) is performed, and a large amount of Ti is required for this. However, when the Ti included in the steel is present as coarse inclusions containing Ti whose size exceeds 2.0 pm, it is considered that the number of Ti-containing fine inclusions (TiN precipitate for example) is insufficient and not only the miniaturization of the grain size y anterior can be achieved but both the tenacity of the base metal and the ZAC toughness are also deteriorated, the coarse inclusions containing Ti themselves becoming the starting points of fracture .

The present invention will be concretely described below.

<The R / Q ratio of the R-value obtained by deducting the amount of Ti included in the steel as inclusions containing Ti whose size exceeds 0.1 pm of the total amount of Ti Q to the total amount of Ti Ti Q included in the steel should be 0.30 - 0.70>

In the present invention, the ratio of the amount of Ti in solid solution to the total amount of Ti included in the steel (hereinafter referred to as Ti ratio in solid solution) should be 0.30-0.70. When the ratio Ti in solid solution is less than 0.30, as the Ostwald growth of the TiN particles becomes manifest during the heat treatment and the welding, the Ti-containing nitride is likely to grow and the amount of nitride formation containing

Fine Ti having the effect of improving the toughness of the base metal and the ZAC toughness can not be obtained. Therefore, since the metal structure can not be miniaturized during welding, the base metal toughness and the ZAC toughness are deteriorated. Accordingly, the solid solution ratio Ti should be 0.30 or more, preferably 0.35 or more, and more preferably 0.40 or more. However, when the Ti ratio in solid solution exceeds 0.70 and becomes in excess, the amount of Ti in solid solution becomes excessively large and therefore the transformation structure formed by the former grain seal is magnified, and the toughness Base metal and ZAC toughness are deteriorated. Accordingly, the ratio of solid solution Ti should be 0.70 or less, preferably 0.65 or less and more preferably 0.60 or less.

The ratio of Ti in solid solution can be expressed as the R / Q ratio of the R value obtained by deducing the amount of Ti included in the steel as inclusions containing Ti whose size exceeds 0.1 pm of the total amount of Ti Q to the total amount of Ti Q included in the steel (see Fig. 1). That is, the R value means the total amount of the amount of Ti actually in solid solution in the steel and the amount of Ti included in the superfine inclusions that passed through the filter having 0 meshs. , 1 μm, and in the present invention, the amount of Ti included in the superfine inclusions is considered to be solid solution Ti.

<The amount of Ti included in the steel as coarse inclusions containing Ti> 2.0 μm>

According to the present invention, the amount of Ti included in the steel as coarse inclusions containing Ti greater than 2.0 μm in size must be 0.010% or less (not including 0%). When this amount of Ti exceeds 0.010%, Ti-containing coarse inclusions that become fracture starting points increase, which becomes a cause of the deterioration of base metal toughness and ZAC toughness. The amount of Ti is preferably as small as possible, preferably 0.0080% or less and more preferably 0.0050% or less.

The amount of Ti included in the steel as Ti containing inclusions exceeding 2.0 μm means the amount of Ti included in the Ti-containing inclusions extracted from the steel by the electrolytic extraction process and not passing through. through the 2.0 μm mesh filter. Inclusions containing Ti include all inclusions containing Ti and are intended to include nitride including Ti, oxide including Ti, carbide including Ti or a composite compound thereof and the like. According to the present invention, since the residue extracted by the electrolytic extraction process is melted and the amount of Ti is measured by the ICP atomic emission spectrometry method as described below, the total amount of the amount of Ti can be measured for inclusions of any compositions included in the steel as inclusions containing Ti greater than 2.0 μm in size. In addition, when the total amount of Ti included in coarse inclusions greater than 2.0 μm in size is 0.010% or less on a steel basis, the base metal toughness and ZAC toughness can be improved.

In addition, the coarse inclusions containing Ti extracted from the steel are those whose size exceeds 2.0 μm. The reason is that in inclusions containing Ti whose size was 2.0 μm or less, the effect of tenacity due to the difference in Ti amount was rarely observed.

As described above, the steel of the present invention is characterized in that the amount of Ti included in the steel as Ti containing inclusions greater than 2.0 μm in size must be 0.010% or less. and the ratio of Ti in solid solution (R / Q ratio) should be 0.30-0.70.

The componential composition of the steel of the present invention will then be described.

[C: 0.03-0.16%] C is an indispensable element for obtaining resistance and resistance can not be obtained when the amount of C is less than 0.03%. Accordingly, the amount of C should be 0.03% or more, preferably 0.04% or more and more preferably 0.05% or more. However, when the amount of C becomes excessively high, a lot of island-shaped hard martensite (MA) is formed and the base metal toughness and ZAC toughness are deteriorated. Accordingly, the amount of C should be reduced to 0.16% or less, preferably to 0.12% or less and more preferably to 0.10% or less.

[If: 0.25% or less (including 0%)]

Although Si is a useful element for achieving solid solution hardening resistance, when the amount of Si becomes excessively high, a lot of island-shaped hard martensite (MA) is formed, coarse inclusions containing Ti are formed, and the toughness of the base metal and the ZAC toughness are deteriorated. Accordingly, the amount of Si should be 0.25% or less, preferably 0.2% or less, more preferably 0.1% or less, and most preferably 0.08% or less. The amount of Si should preferably be 0.01% or more, more preferably 0.02% or more, and most preferably 0.03% or more.

[Mn: 1-2.0%]

Mn is a useful element for obtaining resistance and should be contained at 1% or more. The amount of Mn should preferably be 1.2% or more, and more preferably 1.4% or more. However, when Mn is contained in excessively high amount exceeding 2.0%, the strength increases excessively, and the base metal toughness and ZAC toughness are deteriorated. Accordingly, the amount of Mn should be 2.0% or less, preferably 1.8% or less and more preferably 1.7% or less.

[P: 0.03% or less (not including 0%)] P is an unavoidable impurity element, it is likely to cause intragranular fractures, it has a deleterious effect on both base metal toughness and ZAC toughness, and it is therefore preferable that the P content is as small as possible. Accordingly, the amount of P should be reduced to 0.03% or less, preferably to 0.02% or less and more preferably to 0.01% or less. However, it is industrially difficult to bring the amount of P into the 0% steel and P is normally contained at approximately 0.003%.

[S: 0.015% or less (not included 0%)] S is an unavoidable impurity element, it deteriorates the base metal tenacity by intergranular fracture due to intergranular segregation and coarse sulphide, and therefore it is It is preferable that the S content be as small as possible. Accordingly, the amount of S should be reduced to 0.015% or less, preferably 0.010% or less, more preferably 0.008% or less, and most preferably 0.005% or less. However, it is industrially difficult to bring the amount of S into the 0% steel and S is normally contained at approximately 0.0001%.

[Al: 0.05% or less (not including 0%)]

Although Al acts as a deoxidizing agent, Al is an element that forms inclusions containing Al 2 O 3 in the steel, it becomes a cause of the formation of Ti-containing coarse inclusions and it deteriorates the base metal toughness and toughness. ZAC when it is contained in excessively high amount. Accordingly, the amount of Al should be reduced to 0.05% or less, preferably to 0.040% or less and more preferably to 0.030% or less. The lower limit of the amount of Al is 0.0003% for example.

[Ti: 0.010-0.08%]

Ti is an N-reactive element to form nitride, miniaturize the structure of the metal and improve the toughness of the base metal. Accordingly, Ti should be contained to 0.01% or more, preferably 0.012% or more and more preferably 0.015% or more. However, when the amount of Ti becomes excessively high, many Ti-containing coarse inclusions are formed and the base metal toughness and ZAC toughness are deteriorated. Accordingly, the amount of Ti should be 0.08% or less, preferably 0.07% or less, more preferably 0.06% or less, and most preferably 0.05% or less.

[Ca: 0.0005-0.010%]

It is an element preventing the crystallization of Ti-containing coarse inclusions and improving base metal toughness and ZAC toughness. Accordingly, it should be contained at 0.0005% or more, preferably 0.0008% or more and more preferably 0.001% or more. However, when the amount of Ca becomes excessively high, coarse Ca-containing oxide is formed and the toughness of the base metal is deteriorated. Accordingly, the amount of Ca should be 0.010% or less, preferably 0.008% or less and more preferably 0.006% or less.

[N: 0.0020-0.020%] N is a Ti-containing nitride-forming element, preventing austenite grain magnification by anchoring effect to miniaturize the structure and improving base metal toughness and toughness ZAC. In addition, the Ti-containing nitride also acts to promote intragranular ferritic transformation and contributes to miniaturize the structure to improve base metal toughness and ZAC toughness. In order to perform these actions, the amount of N must be 0.0020% or more, preferably 0.0030% or more and more preferably 0.0040% or more. However, when the amount of N becomes excessively high, the amount of N in solid solution increases, aging after deformation is caused and the toughness of the base metal and the ZAC toughness are deteriorated. Accordingly, the amount of N should be 0.020% or less, preferably 0.018% or less and more preferably 0.016% or less.

The basic componential composition of the steel of the present invention is as described above, and the balance consists of iron and unavoidable impurities. The mixture of elements incorporated because of the situations of the raw materials, materials, production facilities and the like (Sn, As, Pb and the like for example) is admitted as unavoidable impurities. In addition, it is also effective to positively contain the elements described below, and the property of the steel is further enhanced depending on the type of composition.

[One or more elements selected from a group consisting of Ni: 1.5% or less (not including 0%), Cu: 1.5% or less (not including 0%), Cr: 1.5% or less ( not including 0%) and Mo: 1.5% or less (not including 0%)]

Ni, Cu, Cr and Mo are all elements that act effectively to increase the strength of the steel and this effect is increased when the content of these elements is increased. However, to effectively exert this effect, it is preferable that the content of any of these elements is 0.05% or more. The content of any of Ni, Cu, Cr and Mo should preferably be 0.10% or more. However, when the content of these elements becomes excessively high, the resistance increases excessively, and the base metal toughness and ZAC toughness are negatively deteriorated. Accordingly, the content of any of these elements should preferably be reduced to 1.5% or less. Ni, Cu, Cr and Mo should preferably be 1.2% or less, and more preferably 1% or less.

[Number: 0,10% or less (not including 0%) and / or V: 0,1% or less (not including 0%)]

Nb and V are carbonitride precipitating elements, reducing the magnification of the austenite grain and thus improving the toughness of the base metal. In order to exert these effects effectively, Nb should preferably be contained at 0.002% or more, more preferably 0.005% or more, and most preferably 0.010% or more. However, when the amount of Nb becomes excessively high, the carbonitride is made coarser and the toughness of the base metal is negatively deteriorated. Accordingly, the amount of Nb should be 0.10% or less, preferably 0.08% or less, more preferably 0.06% or less, and most preferably 0.04% or less. In addition, V should preferably be present at an occurrence of 0.002% or more, and more preferably 0.005% or more. However, when the amount of V becomes excessively high, the coarse carbonitride is precipitated and the toughness of the base metal is negatively deteriorated. Accordingly, the amount of V should preferably be 0.1% or less and more preferably 0.08% or less.

[B: [B: 0.005% or less (not including 0%)] B is an effective element for reducing coarse ferrite formation at the grain boundary and improving base metal toughness and ZAC toughness. Although these effects are increased when the B content is increased, in order to exert these effects effectively, it is preferable that it be contained at 0.0005% or more. The amount of B should preferably be 0.0010% or more, and more preferably 0.0013% or more. However, when the amount of B becomes excessively high, BN is precipitated on the austenitic grain seal and the base metal toughness and ZAC toughness are deteriorated. Accordingly, the amount of B should preferably be 0.005% or less, more preferably 0.004% or less, and most preferably 0.003% or less.

[Zr: 0.02% or less (not including 0%) and / or REM: 0.02% or less (not including 0%)]

Zr and REM (rare earth element) contribute to the miniaturization of oxides and the improvement of the ZAC toughness. Although these effects are increased when the content of these elements is increased, in order to exert these effects effectively, it is preferable that they be contained at 0.0001% or more. Zr and REM must both preferably be 0.0005% or more. However, when their amount is excessively high, the oxide becomes coarse and deteriorates the base metal toughness and the ZAC toughness, and it is therefore preferable to reduce both to 0.02% or less. The amount of Zr and REM should preferably be 0.018% or less and more preferably 0.015% or less.

Further, in the present invention, REM is meant to include lanthanides (15 elements from La to Lu) plus Sc (scandium) and Y (yttrium). Of these elements, it is preferable that it contain at least one element selected from the group consisting of La, Ce and Y, and it is more preferable that it contains La and / or Ce.

A method of producing the steel of the present invention will next be described. As described above, to reduce the amount of Ti included in the steel as coarse inclusions containing Ti greater than 2.0 μm in size at a predetermined amount or less and to control the ratio of Ti in solution solid in the steel within a predetermined range, the steel can be melted so that Ti, N and Si satisfy the expression (1) below and is then cast after the number of inclusions containing Al 2 O 3 included in the The steel was tested to 10 units or less (including 0 units) per 1 mm2 by flotation separation of inclusions included in the molten steel. In expression (1) below, [] expresses a content (% by weight) of each element in steel.

[Ti] x [N] <(1x10'5) / [Si] ... (I)

However, when Si = 0% by weight, the steel is melted so that Ti and Ni satisfy the expression (2) below.

[Ti] x [N] <1x10'3 ... (2)

The reasons for stipulating each requirement are described below.

<Ti, N and Si balance>

When melting the steel, the composition should be adjusted so that Ti, N and Si satisfy the expression (1) below. In expression (1) below, [Ti] x [N] on the left expresses a Ti and N product with permissible solubility, and it was known that coarse inclusions containing Ti greater than 2.0 pm were formed in casting when this value exceeded a constant value. In addition, as a result of the investigations of the present inventors, it was known that the product with permissible solubility was affected by the amount of Si in the steel. That is, it was found that the value of [Ti] x [N] varied as a function of Si concentration in steel, and that the value of [Ti] x [N] decreased and the formation of Ti-containing coarse inclusions was reduced as the amount of Si increased. Accordingly, in order to appropriately control the amount of Ti included as Ti-containing coarse inclusions and the Ti ratio in solid solution, the composition should be adjusted so that the amounts of Ti, N and Si in the steel satisfy the ratio of expression (1) below. Expression (1) below is an expression made by the present inventors after carrying out various experiments. Expression (1) below may be modified to Expression (1a) below, and the composition may be adjusted to meet Expression (1a). When the value on the left side of the expression (1a) below is designated Z, the value Z should preferably be 5x10'6 or less, and more preferably 1x10'6 or less.

[Ti] x [N] <(1x10'5) / [Si] ... (1) [Ti] x [N] x [Si] <(1x10'6) ... (1a)

In addition, when Si is not added and the Si content of the steel is 0% by weight in the molten steel, the steel can be melted so that Ti and N satisfy the expression ( 2) below. Expression (2) below is calculated by substituting Si = 0.01% by weight (the lowest limit value of Si in the example described below) in Expression (1) above.

[Ti] x [N] <1x10'3 ... (2) <Flotation separation of inclusions>

After melting, the steel must be cast after the number of inclusions containing Al2O3 included in the steel has been controlled to 10 units or less (including zero units) per 1 mm2 by flotation separation of inclusions included in the molten steel. In the present invention, inclusions containing Al2O3 mean inclusions containing

Al203 up to 80% by mass or more. Although Ti-containing inclusions are known to be crystallized with oxide such as Al 2 O 3 and the like and are generally nuclei, in the steel of the componential composition as set forth in the present invention, inclusions are considered containing Ti are formed with inclusions containing Al 2 O 3 which are the nuclei of crystallization. In general, it is called heterogeneous nucleation. Accordingly, when the numerical density of inclusions containing Al 2 O 3 in the molten steel is reduced, the amount of Ti included in the steel as Ti containing inclusions greater than 2.0 microns in size can be reduced and the Ti ratio in solid solution can be controlled in a suitable range.

The numerical density of inclusions containing Al 2 O 3 must be 10 units or less (including zero units) per 1 mm 2 in the field of vision area. When the numerical density exceeds 10 units / mm 2, inclusions containing Ti are made coarser, the amount of Ti in solid solution can not be obtained, the ratio of Ti in solid solution becomes less than 0.3 and the tenacity of the metal Basic and ZAC toughness are deteriorated. The numerical density should preferably be 8.0 units per mm 2 or less and more preferably 6.0 units / mm 2 or less.

The numerical density of the inclusions containing Al 2 O 3 can be adjusted by flotation separation of the inclusions (mainly oxide inclusions) included in the molten steel of the molten steel. As a method of flotation separation of inclusions, it is preferable to flocculate and amalgamate the oxide using a gas-brewing refining apparatus such as an LF (pocket furnace) and the like, for example, and a flotation apparatus. ebb type vacuum degassing refining such as RH (Ruhrstahl Hausen) and the like and promoting flotation separation of Al 2 O 3 containing oxide. When the RH-type degassing refining apparatus is used and the reflux gas flow rate is 100-200 Nm3 / h, the time after adding Al to the molten steel until it stops Reflux (reflux time) should preferably be 5 min or more, and more preferably 10 min or more for example. It is preferable to extend the reflux time because the numerical density of inclusions containing Al 2 O 3 can thus be reduced, but since the productivity is deteriorated, the upper limit is approximately 90 min.

After flotation separation of inclusions included in the molten steel, the steel may be cast and hot rolled (as well as cold rolled if necessary) by a normal process. More specifically, the rolling can be carried out for 600 s or less of the cooling time at 1400-1500 ° C in casting, at 1050-1200 ° C x 2-5 h pre-rolling heating conditions and at 750 ° C or above the final rolling temperature, and the cooling after completion of rolling can be carried out at 2-15 ° C / s average cooling rate and 300-500 ° C cooling off temperature.

The shape of the steel of the present invention is not particularly limited and the steel can be used as a thick steel plate for example. As defined in JIS, the thick steel plate means a plate having a thickness of 3.0 mm or more in general. The thick steel plate can be used as a material for structures such as bridges, high buildings, ships and the like, for example, and is excellent in terms of base metal toughness and toughness ZAC not only in low to medium heat welding but also in high heat welding. The steel of the present invention exhibits excellent ZAC toughness even when high heat welding of 50 kJ / mm or more of caloric intake is performed for a steel plate having a thickness of 50 mm or more by example, therefore, a preferred aspect applies to a steel plate of this thickness but the application is not limited to the steel plate of a thickness of 50 mm or more and the application to a Steel plate of lesser thickness is not excluded.

Although the invention is explained below with specific reference to examples, the present invention is not limited to the examples below, and it is obvious that the present invention can also be implemented with added modifications. suitably within the scope adaptable to the purposes described above and below, and are to be included in the technical scope of the present invention.

[Examples] The steel having the componential composition shown in Table 1 and Table 2 below (the balance being iron and the inevitable impurities) was melted, cast into a slab (the cross section was 150 mx 250 mm) after flotation separation of inclusions included in the molten steel of the molten steel, was then hot rolled and a hot rolled steel plate with a thickness of 80 mm was obtained.

With regard to hot rolling, the rolling was carried out for 600 s or less of the cooling time at 1400-1500 ° C in casting, at 1100 ° C x 3 h pre-rolling heating conditions and at 780 ° C or above the final rolling temperature, and after rolling, the cooling at 450 ° C was carried out at a rate of 6 ° C / s of the average cooling rate and at 450 ° C of the cooling off temperature .

In Table 1, REM was added as a 50% La-containing mischmetal and approximately 25% Ce. In addition, in Table 1 and Table 2 below, the element is not contained.

Based on the amount of Ti, the amount of N and the amount of Si shown in Table 1 and Table 2 below, the value of [Ti] x [N] x [Si] (Z value) was calculated and the result is shown in Table 3. In addition, for Type A steel in Table 1 below, the value of [Ti] x [N] was calculated and the result The calculation is shown in the Z value column. In addition, in Table 3 below, "αΕ-Β" means Μαχ10'6 ".

The inclusions included in the molten steel were separated by flotation of the molten steel, the flow of reflux gas in RH being 100-200 Nm3 / h and the time after addition of Al until the gas was stopped. reflux (reflux time) being varied. The reflux time is shown in Table 3 below. Further, Nos. 29 and 31 in Table 3 below are examples in which the steel was cast without flotation separation of the inclusions included in the molten steel of the molten steel.

After separating the inclusions from the molten steel by flotation and before casting, the numerical density of the inclusions containing Al 2 O 3 in the molten steel was examined by the procedure described below.

[Numeric Density of Inclusions Containing Al2O3] The molten steel was withdrawn from a continuous tundish using a bucket sampler (approximately 35 mm ID x approximately 50 mm in height) and was solidified by cooling. to air. The steel obtained by solidification was removed from the bucket sampler, cut at a horizontal plane at a location approximately 10 mm from the bottom of the sample, the cut surface was polished and made up the sample to observe the inclusions. The sample for observing the inclusions was observed using a ΕΡΜΑ (electron microprobe, "JXA-8500F" manufactured by JEOL Ltd.), the number of particles whose circle equivalent diameter was 0.2 μm or more was measured. , and the componential composition of the particle was analyzed quantitatively. The observation conditions were: 20 kV of the accelerating voltage, 0.01 μΑ of the sampling current, 1-5 cm2 around the central part of the polished surface of the field of view and 100 units or more of the number of particles to be analyzed, and the componential composition of the particle was analyzed semi quantitatively using a characteristic X-ray detector of energy dispersive type (EDS). The elements of the analysis were Al, Mn, Si, Ti, Zr, Ca, La, Ce and O, the detected concentration of all the elements was converted to oxide and was normalized and the Al 2 O 3 concentration was then been obtained. Of all the inclusions detected, those containing Al2O3 at 80% by mass or more were considered inclusions containing Al2O3. The number of inclusions containing Al 2 O 3 was converted to units per 1 mm 2 and the numerical density was obtained. The numerical density of the inclusions containing Al 2 O 3 is given in Table 3 below.

Then, the following items were measured by the procedures described below for the hot rolled steel plate manufactured as described above.

(a) The amount of Ti included in the steel as coarse inclusions containing Ti whose size exceeds 2.0 pm in relation to the total amount of Ti included in the steel.

(b) The ratio R / Q of the value obtained by deducting the amount of Ti included in the steel as inclusions containing Ti whose size exceeds 0.1 pm of the total amount of Ti Q to the total quantity of Ti Q included in the steel (ratio of Ti to t solid solution) (c) Toughness of the base metal (d) ZAC tenacity when the base metal is welded These results are shown in Table 3 below.

[(a) Quantity of Ti included in the steel as coarse inclusions containing Ti whose size exceeds 2.0 pm in relation to the total amount of Ti included in the steel]

A specimen (15 mm deep x 15 mm wide x 5 mm long) was cut from each hot-rolled steel plate so that the axis passes through the position at depth t / 4 (t being thickness of the plate) relative to the surface of the hot-rolled steel plate and electrolytically stripped with an electrical current of 500 A / m2 or less at room temperature with the electrolyte which was a solution 2% triethanolamine-1% tetramethylammonium chloride-methanol. After the electrolytic extraction, the extracted residue was filtered using a 2.0 μm membrane filter.

Then, the extracted residue remained on the filter during filtration (inclusions larger than 2.0 μm) was placed in a platinum crucible with the filter, heated by a gas burner and incinerated. Then, an alkali stream (mixture of sodium carbonate and sodium tetraborate) was added, the extracted residue was again heated by the gas burner and melted. Then, hydrochloric acid 18% vol. The melt was added to a solution state, then placed in a volumetric flask into which pure water was added to obtain 50 ml of analytical liquid. The concentration of Ti in the analysis liquid was measured by the ICP atomic emission spectrometry method and the amount of Ti included in the steel as inclusions containing Ti whose size exceeded 2.0 pm a. been measured. The result of the measurement is shown in Table 3 below.

In addition, generally, as the inclusions are coarser, the number of inclusions remaining in the steel is smaller, therefore when observing a polished sample with a microscope, which is an ordinary method for To examine inclusions, it was difficult to be aware of the amount of Ti present as coarse inclusions. However, according to the present method of analysis in which electrolytic extraction and membrane filter filtration are combined, the total amount of Ti included in the steel and included in inclusions larger than 2.0 μm can be measured, therefore the measurement error is small and an accurate measurement can be obtained.

[(b) R / Q ratio (Ti ratio in solid solution)]

Instead of filtering using a membrane filter with a mesh of 2.0 μιη as above in (a), the filtration was carried out using a membrane filter with a mesh of 0.1 μιτι, and the amount of Ti included in the steel as inclusions containing Ti whose size exceeded 0.1 μιτι was measured.

In addition, at a location near the specimens cut from hot-rolled steel plates in (a) above, specimens of the same size were cut separately, all specimens were melted according to JIS G 1258- 1 "dissolving process in acids and molten potassium disulfate", the concentration of Ti in the solution was measured by the ICP atomic emission spectrometry method, and the total amount of Ti Q included in the steel has been measured.

Then, the R-value obtained by subtracting the amount of Ti included in the steel as Ti-containing inclusions greater than 1.0 μm in size of the total amount of Ti Q was obtained. This is the equivalent of the amount of Ti in solid solution. The ratio R / Q of the value R to the total amount of Ti Q included in the steel was obtained. The obtained R / Q ratio is shown in Table 3 below.

[(c) Base metal toughness] At depth t / 4 (where t is the thickness of the plate) relative to the surface of each hot-rolled steel plate, a flexural test specimen by Charpy notched specimen impact (JIS Z 2201 specimen No. 4) was taken in the rolling direction, the Charpy notched impact test was performed at 60 ° C based on JIS Z 2242 , and the energy absorbed (vE.6o) was measured. At this stage, the energy absorbed (νΕ_6ο) was measured for three specimens, and the minimum value for these specimens was obtained. Those with 100 J or more of the minimum value of νΕ-βο were considered excellent in terms of toughness of the base metal.

[(d ZAC toughness when the base metal is welded] At the point of depth t / 4 (where t is the thickness of the plate) relative to the surface of each hot-rolled steel plate, a specimen for Charpy notched impact test (Specimen No. 4 of JIS Z 2201) was taken in the rolling direction, a thermal cycling test simulating high heat-input welding was performed, and ZAC toughness when the hot-rolled steel plate (base metal) was evaluated and at this stage, during the thermal cycling test, the specimen was heated to 1400 ° C, held for 60 seconds, then It was cooled in the 800-500 ° C temperature range for 500 s, and a heat cycle equivalent to 55 kJ in terms of caloric intake during welding was given.The Charpy Notch Impact Test made at -40 ° C based on JIS Z 2242, and energy ab At this stage, the absorbed energy (vE.40) was measured for three specimens, and the minimum value for these specimens was obtained. Those with 100 J or more of the minimum value of vE.4o were considered excellent in terms of ZAC toughness.

In addition, in fig. 2, the ratio between the value of [Ti] x [N] x [Si] (Z value) and the ZAC toughness is shown in a graph. In fig. 2, the results of Examples (No. 1-25) of the invention shown in Table 3 below are represented by 0 and the results of those whose value Z deviated from the range recommended by the present invention ( No. 26, 28, 30, 31) of the comparative examples are represented by. From fig. 2, it is known that there is correlation between the Z value and the ZAC toughness, and the ZAC toughness can be improved by reducing the Z value to 1.0E-05 (1.0x10'5) or less or, when the steel does not contain Si (steel type A in Table 1), reducing the Z value to 1.0E-03 (1.0x10'3) or less.

From Table 1 in Table 3 below, the following study is possible. Nos. 1-25 are known to be examples which satisfy the requirements of the present invention, the componential composition has been suitably adjusted, the amount of Ti included in the steel as coarse inclusions containing Ti a It was reduced to 0.010% or less, Ti in the appropriate amount was solid solution in the steel and, therefore, an excellent steel plate in terms of base metal toughness and ZAC toughness was obtained.

On the other hand, Nos. 26-46 are the examples deviating from any of the requirements set forth in the present invention and are inferior in terms of either base metal toughness or ZAC toughness. The details concerning them are described below.

Nos. 26 and 30 are examples in which [Ti] x [N] x [Si] exceeds 1.0x10'5, the amount of Ti included in the steel as inclusions containing Ti of which the size exceeds 2.0 μm is excessively high and, as a result, the base metal toughness and the ZAC toughness have been deteriorated. Nos. 27 and 29 are examples in which the ratio R / Q deviates from the predetermined range, and the amount of Ti in solid solution in the steel is excessively high. As a result, the toughness of the base metal and the ZAC toughness have been deteriorated. Nos. 28 and 31 are examples in which [Ti] x [N] x [Si] exceeds 1.0x10'5, the amount of Ti included in the steel as inclusions containing Ti of which the size exceeds 2.0 μm is excessively high, the amount of solid solution Ti in the steel is excessively low and, therefore, the ratio R / Q is less than the predetermined range. As a result, the toughness of the base metal and the ZAC toughness have been deteriorated.

Nos. 32-49,44-44 are all examples not satisfying the componential composition stipulated in the present invention. In No. 32, the C content of the steel plate exceeds the range stipulated in the present invention, and the base metal toughness and ZAC toughness have been deteriorated. The deterioration of the base metal toughness and the ZAC toughness is believed to be due to the increase in the formation of hard martensite (MA) in the form of islands. In No. 33, the Si content of the steel plate exceeds the range stipulated in the present invention, and the base metal toughness and ZAC toughness have been deteriorated. The deterioration of the base metal toughness and the ZAC toughness is believed to be due to the increase in the formation of hard martensite (MA) in the form of islands. In No. 34, the Mn content of the steel plate exceeds the range stipulated in the present invention, the strength of the steel plate has been too much increased and, therefore, the toughness of the base metal and the ZAC toughness have been deteriorated.

In No. 35, the P content of the steel plate exceeds the range stipulated in the present invention, and the base metal toughness and ZAC toughness have been deteriorated. In No. 36, the S content of the steel plate exceeds the range stipulated in the present invention, the ZAC toughness is excellent but the toughness of the base metal has been deteriorated. In No. 37, the Al content of the steel plate exceeds the range stipulated in the present invention, and the base metal toughness and ZAC toughness have been deteriorated. In No. 38, the Ti content of the steel plate did not reach the range stipulated in the present invention, and although the ZAC toughness was excellent but the toughness of the base metal was deteriorated. In No. 39, the Ti content of the steel plate exceeds the range stipulated in the present invention, and the base metal toughness and ZAC toughness have been deteriorated.

No. 40 is a reference example, the Nb content added as a selective element exceeds the range stipulated in the present invention, and the toughness of the base metal has been deteriorated.

In No. 41, the Ca content of the steel plate did not reach the range stipulated in the present invention, and the base metal toughness and ZAC toughness were deteriorated. In No. 42, the Ca content of the steel plate exceeds the range stipulated in the present invention, and the base metal toughness and ZAC toughness have been deteriorated. In No. 43, the N content of the steel plate did not reach the range stipulated in the present invention, and the base metal toughness and ZAC toughness were deteriorated. No. 44 is the example in which the N content of the steel plate exceeds the range stipulated in the present invention, the R / Q ratio has not reached the predetermined range and the amount of Ti in solid solution in steel is too small. As a result, the toughness of the base metal and the ZAC toughness have been deteriorated.

No. 45 and No. 46 are reference examples, the Ni or Cu content added as a selective element exceeds the range stipulated in the present invention and the base metal toughness and ZAC toughness have been deteriorated.

Furthermore, in Nos. 28, 29, 31 and 41, since the time after the addition of Al until the reflux gas (reflux time) stops in RH is too short, the inclusions included in molten steel are not sufficiently separated by flotation of the molten steel. As a result, the toughness of the base metal and the ZAC toughness have been deteriorated.

[Table 1]

Claims (3)

1. Excellent steel in terms of the toughness of the base metal and an area affected by the welding heat of which the steel composition comprises: C: 0.03-0.16% (means% by weight, hereinafter the same thing with regard to the compositions); If: 0.25% or less (including 0%); Mn: 1 - 2.0%; P: 0.03% or less (not included 0%); S: 0.015% or less (not including 0%); Al: 0.05% or less (not including 0%); Ti: 0.010 - 0.08%; Ca: 0.0005-0.010%; and N: 0.0020 - 0.020%; the balance comprising iron and unavoidable impurities, wherein an amount of Ti included in the steel as Ti containing inclusions exceeding 2.0 μm is 0.010% or less (not including 0%), and R / Q is 0.30 - 0.70, that is, a ratio of R to Q in which Q is a total amount of Ti in the steel and R is a value obtained by deducting a quantity of Ti included in the steel as inclusions containing Ti exceeding 0.1 pm of the Q value. The steel of claim 1, wherein the steel composition further comprises at least one of groups (a) to (d) below as other elements, (a) one or more members selected from a group consisting of Ni: 1.5% or less (not included 0%), Cu: 1.5% or less (not including 0%), Cr: 1.5% or less (not including 0%) and Mo: 1.5 % or less (not including 0%), (b) one or more elements selected from a group consisting of Nb: 0.10% or less (not including 0%) and / V: 0.1% or less (not including 0 %), (c) B: 0.005% or less (not including 0%), (d) one or more elements selected from a group consisting of Zr: 0.02% or less (not including 0%) and / or REM: 0.02% or less (not including 0%). A method of manufacturing steel excellent in toughness of the base metal and a heat-weldable zone as claimed in claim 1 or claim 2, comprising the steps of: melting the steel so that Ti, N and Si satisfy an expression (1) below, provided, when Si = 0% by weight, to melt the steel so that Ti and Ni satisfy an expression (2) below: and then cast the molten steel after the number of inclusions containing Al 2 O 3 included in the steel has been controlled to 10 units or less (including zero units) per 1 mm 2 per flotation separation of inclusions included in the molten steel.

in which [] expresses a content (% by weight) in each element of the steel.