JP4901262B2 - Thick steel plate with excellent toughness of heat affected zone - Google Patents

Description

  The present invention relates to a thick steel plate excellent in the toughness of a heat affected zone (HAZ) in a high heat input welded joint. The steel plate to which the present invention is applied can be manufactured at a low cost in the steel industry, and is used for welding structures such as shipbuilding, architecture, bridges, tanks, offshore structures, line pipes, etc. It is suitable when high high heat input welding is performed and the required level of weld zone toughness is high.

  In recent years, major requirements for welded structures such as shipbuilding and construction are to increase the size of structures, increase the efficiency of construction, and improve safety against destruction. In response to such trends, thicker steel plates for welded structures are required to have higher HAZ toughness when high-efficiency large heat input welding is applied. On the other hand, both the producers and users of such thick steel plates are increasingly focusing on economic efficiency, and there is a demand for thick steel plates that are cheaper than before.

  The conventional technology for increasing the high heat input welding HAZ toughness of thick steel plates is generally aimed at refining the structure of the HAZ near the fusion line. As a method of refining the HAZ structure, there is a method of refining the HAZ structure by utilizing precipitates existing in the grain boundaries and grains of γ as α transformation nuclei. In order to make this function effectively, it is desirable to disperse as many precipitates as possible in the α-transformation with high nucleation ability.

In this HAZ structure refinement method, the effect of VN is known as a precipitate having a high nucleation ability of α transformation. Since VN has good lattice matching with α, a large amount of α is generated by using VN precipitated in γ grain boundaries and γ grains as transformation nuclei, the structure is refined, and HAZ toughness is improved. As a conventional large heat input welding HAZ toughness technique using such a VN effect, for example, there is Patent Document 1. This contains 1000 / mm ² or more of VN composite precipitates containing Ca, Mg of 10 to 500 nm. Specifically, by allowing VN to coexist with pin oxides or sulfides containing Ca and Mg for pinning in the γ grains, the γ grains are made 70 to 170 μm in HAZ, and Charpy absorbed energy at −40 ° C. ( This is a technique for stably securing toughness of vE-40) of 100 J or more.

JP 2005-298900 A

  However, in the fine grain γ of 70 to 170 μm, after the α transformation on the grain boundary, the hardenability increases due to the concentration of carbon in the grain and the bainite transformation precedes, so the α transformation occurs in the grain. It will be difficult. Furthermore, since fine particles of 10 to 500 nm effective for pinning γ are difficult to function as intragranular transformation nuclei, there are cases where the intragranular refinement is insufficient. For this reason, the case where the HAZ toughness does not show a necessary value may occur.

  An object of the present invention is, for example, a thick steel plate having a thickness of 40 to 100 mm and a yield strength of 390 to 500 MPa class, and a Charpy impact absorption energy at −40 ° C. in HAZ welded with a welding heat input of 20 to 100 kJ / mm. The average value is to stably show 100 J or more.

The present invention has been made as a result of intensive studies in order to solve the above-described problems, and the gist of the present invention is the following contents as described in the claims.
(1) In a thick steel plate having a weld heat affected zone,
% By mass
C: 0.03-0.15%
Si: 0.1-0.5%
Mn: 0.5-1.8%
P: ≤0.01%
S: 0.001 to 0.01%
Al: 0.01 to 0.1%
V: 0.01 to 0.15%
N: 0.002 to 0.008%
O: 0.001 to 0.004%
Ca: 0.0003 to 0.005%
Ni: 0.05-2%
Satisfies the formula (1) calculated using mass%, the balance is a chemical component composed of iron and unavoidable impurities, and an oxide containing V nitride and Ca having an equivalent circle diameter of 0.1 to 1 μm and / or composite precipitates of sulfides 100 / mm ² or more, and contains less than 1000 / mm ^2, wherein the prior austenite grain size of the weld heat affected zone is 200～800Myuemu, rafters Thick steel plate with excellent toughness of heat-welded heat affected zone.
3 ≦ V / N ≦ 15 (1)

(2) In mass%,
Cu: 0.05 to 0.8 %
C r: 0.05-0.8%
Mo: 0.05-0.8%
Nb: 0.005-0.03%
Ti: 0.005-0.02%
B: 0.0003 to 0.003%
The thick steel plate excellent in toughness of the high heat input welding heat-affected zone according to (1), further comprising one or more of the following.

  According to the present invention, for example, in a thick steel plate having a thickness of 40 to 100 mm and a yield strength of 390 to 500 MPa class, an average value of Charpy impact absorption energy at −40 ° C. in HAZ welded with a welding heat input of 20 to 100 kJ / mm. However, it comes to show 100J or more stably.

  The aim of the present invention is to draw out the VN effect under a low N content in order to avoid surface cracking of a continuous cast slab when the α transformation nucleus effect by VN is used in high heat input welding HAZ. . In response to this technical problem, the inventors conducted basic research and obtained the following new findings.

That is, even in a low N content, V is 0.01 to 0.15%, N is 0.002 to 0.008%, V / N is 3 to 15, and Ca-containing oxide or sulfide and V nitride are used. It has been found that precipitates with an equivalent circle diameter of 0.1 to 1 μm, which are α transformation nuclei, can be dispersed in a large amount of 100 to 1000 pieces / mm ² by composite precipitation. Further, by setting the old γ grain size in the HAZ to 200 to 800 μm, α transformation from the grain boundary and inside the grain is promoted with the above precipitate as the nucleus, and the coarse bainite structure is suppressed and the HAZ structure is refined. It was found that it was possible.

The appropriate balance of V and N for extracting the VN effect is shown in the following equation (1).
3 ≦ V / N ≦ 15 (1)
If V / N is less than 3, the formation of VN precipitates is insufficient, and it is difficult to develop the VN effect. If V / N is greater than 15, that is, if N is excessively contained, the remaining N after VN formation is dissolved in the matrix and causes embrittlement. Therefore, it is necessary to add V and N in the balance of formula (1).

  When Ti, Nb, B, etc., which are nitride forming elements stronger than V are added, TiN, NbN, BN, etc. are on the high temperature side of the γ phase prior to the precipitation of VN in the HAZ during cooling. Although it is necessary to consider the precipitation, in the component range according to the present invention, if the VN precipitation is controlled using the formula (1) as a guide, the HAZ structure can be refined industrially and the HAZ toughness can be improved.

The important point of the present invention is that by using Ca-containing oxide and / or sulfide as a VN precipitation site, a composite precipitate of VN and Ca-containing oxide and / or sulfide is generated in addition to VN. However, the use of this composite precipitate as an α transformation nucleus is to greatly increase the α transformation nucleus. Dispersion of Ca-containing oxides and / or sulfides in steel is performed by adding Si, Mn, and Al to perform preliminary deoxidation in molten steel refining, and then adding Ca to perform final deoxidation and desulfurization. Then, continuous casting is possible.

The reason why the particle diameter is 0.1 to 1 μm in terms of the equivalent circle diameter is that if it is less than 0.1 μm, the effect as an α transformation nucleus is small. Further, if it is larger than 1 μm, it becomes a starting point of brittle fracture, and conversely, toughness is lowered.
The number density of particles 100 / mm ² or more, the reason for the less than 1000 / mm ^2, since 100 / mm ² less than that sufficient α transformation does not occur, bainite develops, HAZ structure fine It is because it does not become. On the other hand, if it is 1000 / mm ² or more, the old γ particle size becomes fine due to the pinning effect, and it becomes difficult to make the old γ particle size 200 to 800 μm. In order to make the old γ particle size 200-800 μm, it is preferably 900 / mm ² or more.

  The reason why the old γ grain size of the HAZ is 200 to 800 μm after welding and heating is smaller than 200 μm, as described above, is hardenability due to carbon concentration in the grains after α transformation on the grain boundaries. This is because bainite transformation precedes, and intragranular transformation hardly occurs. However, if it is larger than 800 μm, the grain boundary α becomes coarse and the toughness decreases, so this is the upper limit.

The reason for limiting the chemical components will be described in detail below.
C is required to be 0.03% or more in order to ensure the strength of the base material. However, if it exceeds 0.15%, the second phase of embrittlement increases and the HAZ toughness decreases. 0.15%.

  Si is an essential element as a deoxidizing element in addition to ensuring strength, and in order to obtain the effect, addition of 0.1% or more is necessary, and when it exceeds 0.5%, MA (which is harmful to toughness) Since there is a tendency to deteriorate the high heat input weld HAZ toughness by promoting the formation of martensite-authentite constituent), the amount is set to 0.1 to 0.5%.

  Mn is required to be 0.5% or more in order to economically secure the strength and toughness of the base material. However, if Mn is added in excess of 1.8%, the hazard of central segregation becomes remarkable and the toughness of the base material and HAZ is impaired, so this is the upper limit.

P is an impurity element and needs to be reduced to 0.01% or less in order to stably secure the HAZ toughness.
S is required to be 0.001% or more in order to form a Ca-containing sulfide to be a VN precipitation site. However, if S exceeds 0.01%, coarse sulfides are formed and the toughness of the base material and HAZ is impaired, so this is the upper limit.

  Al is necessary for deoxidation and to reduce O, which is an impurity element, to 0.004% or less. In addition to Al, Si also contributes to deoxidation, but even if Si is added, it is difficult to stably suppress O to 0.004% or less without 0.01% or more of Al. However, when Al exceeds 0.1%, an alumina-based coarse oxide or a cluster thereof is generated, and the toughness of the base material and the HAZ is impaired, so this is the upper limit.

  V is the most important element in the present invention. In the cooling process of high heat input welding, VN precipitates in the γ grain boundaries and γ grains, and this functions as α transformation nuclei, leading to refinement of the HAZ structure. For this purpose, the lower limit V required is 0.01%. If V exceeds 0.15%, precipitation hardening by V carbides becomes prominent in HAZ after α transformation, and HAZ toughness deteriorates. Therefore, this is the upper limit.

  N is an important element in the present invention. In order to stably avoid the surface cracking of the continuous cast slab, it is necessary to suppress it to 0.008% or less, which is the upper limit. However, in order to precipitate VN in HAZ, 0.002% or more is necessary, and this is the lower limit.

  O is required to be 0.001% or more in order to secure oxide particles. However, if it exceeds 0.004%, coarse oxides are produced, and there is concern about the harmful effect that these act as the starting point of brittle fracture.

  Ca is an important element in the present invention. Ca addition oxide and sulfide can be formed by adding 0.0003% or more in consideration of the addition order to the molten steel described above. If it is less than 0.0003%, the amount is insufficient. However, excessive addition exceeding 0.005% becomes the starting point of brittle fracture due to the coarsening of oxides and sulfides, and the toughness is lowered, so this is the upper limit.

  Cu, Ni, Cr, Mo, and B are added to ensure the required strength of the base material. Each element has the effect of increasing the hardenability during the γ → α transformation in the cooling process after thick plate rolling and increasing the strength of the base material. The lower limit of the effect of these elements is 0.05% for Cu, Ni, Cr, and Mo, and 0.0003% for B. However, if there are too many of these elements, the HAZ toughness and weldability deteriorate, so an upper limit must be provided. The upper limit of Cu, Cr, and Mo is 0.8%, the upper limit of Ni is 2%, and the upper limit of B is 0.003%.

  Nb is added to ensure both the strength and toughness of the base material. Nb refines the rolled γ structure, increases hardenability during the γ → α transformation, and precipitates after the α transformation, thereby contributing to the toughening of the base material. The lower limit for exerting this effect is 0.005%. However, if Nb is added over 0.03%, the HAZ hardens and becomes brittle, so this is the upper limit.

  Ti is added to increase the toughness of the base material and the HAZ. TiN is formed to suppress the growth of γ grains during slab heating or HAZ (the region where the maximum heating temperature ≦ 1350 ° C.), and has the effect of increasing the toughness by refining the α structure after transformation. The lower limit for exerting this effect is 0.005%. However, if Ti is added in excess of 0.02%, the HAZ toughness is hardened and becomes brittle, so this is the upper limit.

  Next, the example of the manufacturing method of the steel plate to which this invention is applied is demonstrated. In the steelmaking process of the iron and steel industry, Si, Mn, and Al are added to molten steel for preliminary deoxidation, then Ca is added for final deoxidation and desulfurization, and a slab is made by continuous casting. As described above, by making the addition order of Si, Mn, Al, and Ca in this way, Ca-containing oxides and sulfides can be dispersed in the steel. Then, the steel slab is reheated during or after cooling during casting, a steel plate having a predetermined thickness is formed by thick plate rolling, and air-cooled or water-cooled after rolling. In the middle of water cooling, water cooling may be stopped and air cooling may be performed. By performing an appropriate heat treatment after cooling, the strength and toughness of the base material may be adjusted.

  Steel strip was produced by continuous casting by controlling the deoxidation and desulfurization procedures and chemical composition of the molten steel in the steel making process. . Heat treatment was performed as necessary to produce a thick steel plate having a yield strength of 390 to 500 MPa. Table 1 shows the chemical composition of steel and the number of pinned particles, and Table 2 shows the steel sheet manufacturing method, mechanical properties, and HAZ toughness of one-pass welded joints. In Table 1, the unit of the component is% by mass.

The number of pinning particles was measured by the following method. Create an extracted replica material from any location on the steel sheet, and observe the area of 1000 μm ² or more at a magnification of 10,000 to 50,000 times using a transmission electron microscope (TEM), and target V of 0.1 to 1 μm in size The number of composite precipitates of nitride or / and V nitride and Ca-containing oxide or / and sulfide was measured and converted to the number per unit area.

  In addition, as shown in FIG. 1, a butt joint was produced using an electrogas welding method. Then, as shown in FIG. 1, a notch was made in HAZ or FL 1 mm away from melt melting line (FL), and a Charpy impact test was conducted at -40 ° C. The Charpy impact test here was based on JIS Z 2242, and a V-notch test piece of JIS Z 2202 was used. Moreover, the result of the Charpy impact test has shown the average value of three test pieces, and the yield stress (YS) and the tensile stress (TS) have shown the average value of two test pieces.

  Steels 1 to 5 are steels of the present invention. Since the chemical composition of steel, the density of composite precipitates, and the prior γ grain size are appropriately controlled, vE-40 has achieved a good HAZ toughness exceeding 100 J. Yes.

In Steel 6, since V is excessively added and V / N greatly deviates from the requirement of the present invention, vE-40 is greatly lowered due to precipitation embrittlement of V carbide. Further, since Ca is not added, the fact that the particle number density deviates from the requirement of the present invention is also a cause of toughness reduction.
In Steel 7, since N is excessively added and V / N is out of the requirement of the present invention, vE-40 is greatly lowered due to solute N embrittlement.
In Steel 8, since V is not added and V / N greatly deviates from the requirement of the present invention, no intragranular transformation is seen at all, and vE-40 is greatly reduced because the entire intragranular structure has a bainite structure. is doing.
Steel 9 satisfies the requirements of the present invention in terms of the V / N ratio, but since the particle number density does not satisfy the requirements of the present invention because Ca is not added, the amount of intragranular transformation is insufficient, and vE-40 is It is falling.

  As described above, by applying the present invention, in HAZ having a thickness of 40 to 100 mm and a yield strength of 390 to 500 MPa class and welded with a welding heat input of 20 to 100 kJ / mm, −40 ° C. It was confirmed that a thick steel plate having an average value of Charpy impact absorption energy at 100 J or more can be provided.

It is a figure which shows the extraction | collection point of the Charpy test piece in an electrogas welding butt joint.

Claims (2)

In thick steel plate with weld heat affected zone,
% By mass
C: 0.03-0.15%
Si: 0.1-0.5%
Mn: 0.5-1.8%
P: ≤0.01%
S: 0.001 to 0.01%
Al: 0.01 to 0.1%
V: 0.01 to 0.15%
N: 0.002 to 0.008%
O: 0.001 to 0.004%
Ca: 0.0003 to 0.005%
Ni: 0.05-2%
Satisfies the formula (1) calculated using mass%, the balance is a chemical component composed of iron and unavoidable impurities, and an oxide containing V nitride and Ca having an equivalent circle diameter of 0.1 to 1 μm and / or composite precipitates of sulfides 100 / mm ² or more, and contains less than 1000 / mm ^2, wherein the prior austenite grain size of the weld heat affected zone is 200～800Myuemu, rafters Thick steel plate with excellent toughness of heat-welded heat affected zone.
3 ≦ V / N ≦ 15 (1) % By mass
Cu: 0.05 to 0.8 %
C r: 0.05-0.8%
Mo: 0.05-0.8%
Nb: 0.005-0.03%
Ti: 0.005-0.02%
B: 0.0003 to 0.003%
The thick steel plate excellent in toughness of the high heat input welding heat-affected zone according to claim 1, further comprising one or more of the following.

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