JP3477054B2 - Steel sheet with excellent toughness of weld heat affected zone - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding heat affected zone (He).
at Affected zone (HAZ) is a steel plate having excellent toughness, and is used for various welded structures such as architecture, bridges, shipbuilding, line pipes, construction machinery, marine structures, and tanks. 2. Description of the Related Art In a heat affected zone (HAZ), the heating temperature at the time of welding becomes higher as the temperature becomes closer to the melting line, and particularly in a region near the melting line which is heated to 1400 ° C. or more, heated austenite (γ). Is remarkably coarsened, so that the HAZ structure after cooling is coarsened and the toughness is deteriorated. As a method of refining the heated γ grains of steel, “Iron and Steel” No. 62
Year (1976), No. 9, p. 1209-p. 1218, "Effect of TiN dispersion state on austenite grain size of low carbon and low alloy steel", high temperature stable precipitates such as TiN are finely dispersed in steel to increase the growth of γ grains. Pinning is generally widely known. However, TiN, which is said to have a pinning effect up to the highest temperature in steel among various carbides and nitrides,
At a high temperature of 1400 ° C. or higher, most of the effect is lost due to coarsening and dissolution of TiN as judged from the solubility product. Therefore, as in the vicinity of the melting line of HAZ, 14
There has been no conventional means for suppressing the growth of γ grains in a region heated above 00 ° C., and HAZ embrittlement in this region has been a serious problem. As means for solving such problems, JP-A-60-245768 and JP-A-60-152626 are known.
JP, JP-A-63-210235, JP-A-2-210
JP-A-250917 discloses an intragranular transformed ferrite (IntraGranuler Ferrit) in coarse gamma grains.
e: IGF) has been positively generated to improve HAZ toughness. In such a case, a grain boundary ferrite (Grain Boundary Ferr) is formed from the γ grain boundary.
item: GBF) and a coarse ferrite side plate (FSP) are likely to be coarsely generated, and the generation of these embrittlement structures and IGFs compete with each other. [0004] In order to suppress the formation of coarse GBF and FSP, it is necessary to enhance the hardenability of the γ grain boundary. However, if the hardenability is excessively increased, the coarse upper part containing island-like martensite is required. Bainite (Upper Bainite: B
U) is formed and HAZ toughness is deteriorated. On the other hand, hardenability must be considered from the viewpoint of the mechanical properties of the base material. Therefore, it is difficult to select a chemical component that satisfies both requirements sufficiently, and there is a limit in increasing the HAZ toughness.
Therefore, a wide range of welding conditions and base material (strength level)
In addition, a steel sheet having good HAZ toughness has been demanded. SUMMARY OF THE INVENTION [0005] The present invention relates to a method for producing a steel sheet having a tensile strength of 40 with good HAZ toughness under a wide range of welding conditions.
The purpose is to provide a steel sheet of 0 MPa or more. SUMMARY OF THE INVENTION The gist of the present invention is to provide a method for measuring weight%
, C: 0.02 to 0.20%, Si: 0.4% or less, Mn: 0.5 to 2.0%, P: 0.015% or less, S: 0.006% or less, Al: 0. More than 03% to 0.08%, Ti: 0.005 to 0.03%, Mg: 0.0005 to 0.005%, O: 0.002 to 0.006%, and if necessary Cu: l. 5% or less, Ni: l. 5% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, V: 0.05% or less, Ca: 0.005% or less, REM: 0.005% B: 0.0015% or less, N: 0.001 to 0.005%, and at least one of the following, the balance being iron and unavoidable impurities: sum of the contents of Ti and Mg in size is 15 wt% or more Ti
30 or more Mg-based oxides / mm ² , and at the same time, 5 μm or more and less than 0.5 μm
It is a steel sheet having excellent toughness of a weld heat-affected zone characterized by being present in an amount of 000 pieces / mm ² or more. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to achieve good HAZ toughness under a wide range of welding conditions and base material, the basic idea is to sufficiently refine the HAZ structure. H
In order to refine the AZ structure, (1) reducing the size of GBF or FSP by heating γ refinement, (2) I
It is necessary to increase the GF fraction. To achieve (1), pinning the heated γ grains by finely dispersing an oxide that is stable even at 1400 ° C. or higher, and to achieve (2), increase the oxides serving as IGF generation nuclei and increase the It is desired to increase the nucleation ability. The inventors quantitatively examined the relationship between the dispersion state and composition of the oxide and the heated γ particle size and the IGF fraction in the HAZ, and
The following new findings were obtained regarding the refinement of the structure. FIG. 1 is a graph showing the effect of the number of oxides on the 1450 ° C. heated γ particle size. As shown in FIG.
When the number of oxides having a particle size of from 0.5 to 0.5 μm is 5,000 / mm ² or more, the γ-graining becomes remarkable and the average γ particle size becomes 150 μm or less. An oxide having such a size is mainly composed of a Ti-based oxide, and a Ti-Mg-based oxide is partially present. When the number of oxides having such a size is less than 5000 / mm ² , the heated γ grains of the HAZ are insufficiently refined. FIG. 2 is a diagram showing the effect of the γ particle size at 1450 ° C. on the size of GBF or FSP. When the γ particle size is 150 μm or less, the refining of GBF or FSP becomes remarkable. FIG. 3 shows a 1450 ° C. heated γ particle size of 150 μm.
It is a figure which shows the influence of the number of oxides and the oxide composition on IGF fraction when it is about. Here, most of the oxides serving as IGF-producing nuclei have a size of 0.5 to 5 μm, and in order to express a high lGF fraction of 40% or more,
It is necessary that the composition of the oxide having a size of 0.5 to 5 μm is not less than the sum of the contents of Ti and Mg or not less than 15% by weight and the number thereof is not less than 30 / mm ² . In a Ti-Mg based oxide of this size, the composition is Ti and M
If the total content of g is less than 15% by weight or the number is less than 30 / mm ² , a high IGF fraction cannot be obtained and the HAZ structure does not become fine. FIG. 4 shows the effect of the amount of Mg on the number of oxides. The number of both the oxide of 0.05 to 0.5 μm and the oxide of 0.5 to 5 μm increases as the amount of Mg increases. Low oxide fine dispersion effect due to Mg
l, the number of oxides of 0.05 to 0.5 μm is 5000 / mm when the amount of Mg is 5 ppm or more.
^{When the} number of oxides of ² or more and 0.5 to 5 μm is 30 or more / mm ² or more, sufficient miniaturization of the HAZ structure is achieved. M
These effects are saturated when the amount of g becomes about 50 ppm. Conventional steel (for example, as described in JP-A-60-245768 described above)
JP, JP-A-60-152626, JP-A-63
In JP-A-210235 and JP-A-2-250917, a Ti-based oxide serving as an IGF-producing nucleus is 10-2.
Since the number of particles was as small as about 0 / mm ² , almost no IGF was generated with fine γ of about 150 μm. However, as shown in FIG. 4, the number of Ti—Mg-based oxides acting as IGF nuclei increases in the steel in which Ti and Mg are added in a complex manner, and as shown in FIG.
F is remarkably generated and the HAZ structure can be refined. As described above, by adding Ti and Mg in a complex manner, a large number of oxides of 0.05 to 0.5 μm are dispersed in the steel to make the heating γ finer, thereby making the GBF and FSP finer. Scale, at the same time, 0.5-5 μm Ti-Mg
High IG even with fine γ by dispersing a large number of oxides
It is a feature of the present invention to achieve an F fraction and have good HAZ toughness in a wide range of welding conditions and base metal materials. The reasons for limiting the chemical components will be described below. [0014] The lower limit of C is a minimum amount for securing the strength and the paddy properties of the base material and the welded portion. However, if the amount of C is too large, the toughness of the base material and HAZ is reduced and the weldability is deteriorated. Therefore, the upper limit is set to 0.20%. [0015] Si is contained in steel for deoxidation, but if it is too much, weldability and HAZ toughness deteriorate, so the upper limit was made 0.4%. Deoxidation of steel is sufficiently possible with Ti alone, and in order to obtain good HAZ toughness, it is desirable to use 0.3% or less of Si. Since Mn is indispensable for securing the strength and toughness of the base material and the welded portion, the lower limit is set to 0.5%. However, if the Mn content is too large, the HAZ toughness is degraded, the center segregation of the slab is promoted, and the weldability is degraded. Therefore, the upper limit is set to 2.0%. In the steel of the present invention, P and S which are impurity elements
Are set to 0.15% or less and 0.006% or less, respectively.
This is for improving the mechanical properties of Z. Reduction of P is HA
Suppressing the intergranular fracture of Z, and reducing S improves the material in the thickness direction of the base material and HAZ through reduction of MnS. P and S are preferably 0.01% or less and 0.003% or less, respectively. A1 has a deoxidizing effect, but as a result of repeated investigations on more than 0.03%, if it exceeds 0.08%, cluster-like oxide-based inclusions increase, which adversely affects the base material. I found something. Therefore, Al is 0.03
% To 0.08% or less. Ti is an essential element of the present invention, and is a Ti-based or Ti-Mg-based oxide effective for refining the HAZ structure.
And 0.005% or more to form TiN. In the present invention, in order to further refine the heating γ in the low-temperature heating region, TiN in addition to the oxide is utilized to the utmost, and a strong pinning effect is exhibited at 1350 ° C. or less. The upper limit of Ti is set to 0.03% to prevent HAZ embrittlement due to precipitation of excessive TiC. Mg is the most important element of the present invention, for example, Ni-Mg alloy, Fe-Si-Mg alloy, Si-M
The oxide fine dispersion effect is exhibited by adding the Ti and the low alloy steel as a g alloy in combination with Ti. The lower limit of the amount of Mg is H
In order to secure the minimum number of oxides necessary for the refinement of the AZ structure, and to reduce the Mg content in the oxides that are IGF-forming
It is specified as 5 ppm to increase the content. On the other hand, M
The upper limit of the amount of g is an amount at which these effects are saturated. An Mg content exceeding the upper limit is not preferred because it involves an increase in alloy cost. O is essential for forming fine oxides by combining with Mg and Ti. The lower limit is the minimum amount for securing a sufficient amount of oxide, and the upper limit is the maximum amount for securing the cleanliness of the steel and avoiding deterioration of mechanical properties. Next, Cu, Ni, Mo, Cr, Nb,
The reason for adding one or more of V, Ca, REM, B, and N will be described. Cu and Ni improve the strength and toughness of the base material without adversely affecting weldability and HAZ toughness.
The upper limit of each element is for preventing deterioration of weldability and HAZ toughness. Mo improves the strength and toughness of the base material. However, if the addition amount exceeds 0.5%, the base material toughness, weldability and HAZ toughness are impaired. Cr improves the strength of the base material. However, if the addition amount exceeds 0.5%, the base material toughness, weldability and HAZ toughness are impaired. Nb is an element effective for refining the base metal structure, and improves the strength and toughness of steel. However, if the addition amount exceeds 0.05%, the HAZ toughness deteriorates. V improves the strength of the base material but is 0.05%
If it exceeds, weldability and HAZ toughness are impaired. The reason why Ca and REM are added is to control the form of the elongated inclusion (MnS) to improve the toughness. However, the amount of each of these is 0.0
If it exceeds 05%, a large amount of coarse oxides are generated, and the toughness of the base material and HAZ is deteriorated. B improves the hardenability and improves the strength and toughness of the base material and HAZ. However, if added in excess of 0.0015%, HAZ toughness and weldability deteriorate. N forms TiN to improve HAZ toughness. The lower limit is the minimum amount for securing a sufficient amount of TiN, and the upper limit is the amount for preventing HAZ embrittlement due to solid solution N. According to the present invention, for example, a Mg alloy is added to molten steel in a molten steel ladle in a steelmaking process, a tundish or a mold of continuous casting, and a slab having a specified chemical composition such as the amount of Mg is produced by continuous casting. This is 12
It is achieved by reheating to 50 ° C. or lower and performing a working heat treatment such as controlled rolling, accelerated cooling, quenching, and tempering. EXAMPLES Table 1 shows the chemical composition of continuously cast steel, Table 2 shows the dispersion state and composition of oxides in the steel sheet, and Table 3 shows the base material and HAZ toughness. The steel sheets were welded under various welding conditions, and the Charpy impact characteristics of the molten wire (FL), which was the most embrittled portion of the HAZ, were investigated. The steel of the present invention has a TS of 450 to 8
It has a good base material having a vTrs of −80 ° C. or less at 20 MPa, and has a good HAZ toughness in the vicinity of FL where the welding heat input is 30 to 1000 kJ / cm. On the other hand, the comparative steel is inferior in HAZ toughness because the dispersion state and composition of the chemical components and oxides are not appropriate. Since the steel 6 has a small amount of Ti, the number of Ti-based oxides of 0.005 to 0.5 μm is small and the HBF-heated γ grains are coarsened, so that the GBF and FSP are also coarsened. Because of the small sum of the contents of Ti and Mg in the oxide,
The GF fraction is low, and the HAZ toughness is deteriorated. Steel 7 is T
Due to the large number of i, Ti dissolved in HAZ heated above 1400 ° C. precipitates as TiC in the cooling process and embrittles HAZ. Steel 8 has a small number of pinning particles or oxides serving as IGF-forming nuclei due to a low content of Mg.
The HAZ toughness is degraded without sufficiently refining the Z structure. At this time, the small sum of the contents of Ti and Mg in the oxide of 0.5 to 5 μm also causes a decrease in the IGF fraction. In Steel 9, the number of oxides necessary for pinning and IGF generation was not obtained because of a small amount of O, and the HAZ structure was coarsened to degrade HAZ toughness. Since the steel 10 has a large amount of O, the cleanliness of the steel is reduced, and coarse oxides that may be the starting points of fracture increase, and the HAZ toughness is deteriorated. [Table 1] [Table 2] [Table 3] According to the present invention, good HAZ toughness is achieved in a wide range of welding conditions and base metal materials, and the safety of various welded structures has been significantly improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the effect of 0.05-0.
It is a figure which shows the influence of the number of 5 micrometers oxide. FIG. 2. Effect of 1450 on particle size of GBF or FSP
It is a figure which shows the influence of ° C heating γ particle size. FIG. 3 is a graph showing the influence of the number and composition of 0.5 to 5 μm oxide on the IGF fraction. FIG. 4 is a graph showing the effect of the amount of Mg in steel on the number of oxides.

Claims (1)

(57) [Claims 1] C: 0.02 to 0.20%, Si: 0.4% or less, Mn: 0.5 to 2.0%, P: 0 by weight% 0.015% or less, S: 0.006% or less, Al: more than 0.03% to 0.08%, Ti: 0.005 to 0.03%, Mg: 0.0005 to 0.005%, O: In a steel sheet comprising 0.002 to 0.006%, the balance being iron and unavoidable impurities, when the sum of the contents of Ti and Mg in a size of 0.5 μm or more and 5 μm or less is 15% by weight or more, there is Ti-Mg-based oxide 30 / mm ² or more, at the same time oxides of magnitude of less than 0.05μm or 0.5μm is 5000 / mm ²
A steel sheet having excellent toughness in the heat-affected zone of a weld, characterized in that it exists as described above. 2. C: 0.02 to 0.20% by weight, Si: 0.4% or less, Mn: 0.5 to 2.0%, P: 0.015% or less, S: 0. 006% or less, Al: more than 0.03% to 0.08%, Ti: 0.005 to 0.03%, Mg: 0.0005 to 0.005%, O: 0.002 to 0.006%, , Cu: 1.5% or less, Ni: l. 5% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, V: 0.05% or less, Ca: 0.005% or less, REM: 0.005% B: 0.0015% or less, N: 0.001 to 0.005%, and at least one of the following, the balance being iron and unavoidable impurities: sum of the contents of Ti and Mg in size is 15 wt% or more Ti
30 or more Mg-based oxides / mm ² , and at the same time, 5 μm or more and less than 0.5 μm
A steel sheet having excellent toughness in the heat affected zone of the weld, wherein the steel sheet is present in an amount of 000 pieces / mm ² or more.