JPH0470386B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention is applicable to COD (Crack Opening) of welded parts.
Excellent tensile strength (displacement) properties of 80Kg
This relates to high tensile strength steel of f/mm ² or higher. [Conventional technology] In recent years, development of offshore oil resources has been actively promoted.
Development sea areas are also developing into colder sea areas. As a result, the size of marine structures being installed is also increasing, and the steel materials used are tending to have higher strength. In addition to the fact that structures are used at low temperatures, the safety of structures normally constructed by welding has become increasingly important. Traditionally, the toughness of welds, which determines the safety of structures, has been evaluated not only by measuring the fracture toughness value, but also by the Shapey test, but in recent years,
It is now being evaluated by COD tests to meet the BS5762 standard. The COD test has the advantage of being able to detect microscopic embrittlement that cannot be detected with the Shapey test, which can be used directly in the design of structures.
It is becoming established as a safety evaluation test. Since the COD test sensitively reflects the microscopic embrittlement of the material, it has been quite difficult to develop steel materials that stably exhibit high COD values, but recently we have finally achieved a high tensile strength of around 50Kgf/ ^mm2 . In steel,
It is becoming possible to manufacture steel materials that exhibit excellent COD properties. Furthermore, if the tensile strength is up to about 60 kgf/mm ² , it is considered possible to ensure COD properties in principle by the same means as for 50 kgf/mm ² class steel. Currently, high-strength heat-treated tensile steel is being developed.
Although some technologies are being developed to improve COD properties (e.g., Japanese Patent Application Laid-open No. 1988-9854), very little research has been done on ultra-high tensile strength steels with tensile strengths of 80 to 100 Kgf/mm2 ^or higher. The current situation is that there is no such thing. In particular, in high-strength steel of this level, the controlling factors of COD characteristics are completely different from those of 50 to 60 Kgf/mm ² steel, so it is impossible to find a solution by extending the conventional technology. [Problems to be Solved by the Invention] The present invention provides a steel material with excellent COD properties and a tensile strength of 80 Kgf/mm ² or more for multi-layer welded joints that are welded at a welding heat input of about 1 to 6 Kj/mm. is intended to provide. [Means to solve the problem] If the toughness of the material is poor, brittle fracture will occur early in the COD test, so the limit COD value (hereinafter referred to as δc) is naturally high enough to guarantee safety.
Therefore, in order to improve the COD characteristics of welded joints, weld heat affected zone (hereinafter referred to as
It is called the HAZ department. ) The entire structure must exhibit excellent toughness. High strength material with tensile strength of 80Kgf/mm2 or ^more
The area where the toughness deteriorates the most in the HAZ is the area where the coarse grains heated to a high temperature near the weld bond are reheated by the heat of the subsequent bead to approximately 900-1000℃, just above the Ac ₃ transformation point (hereinafter referred to as , coarse grain + Ac ₃ region). In order to improve the toughness of this region and thus make the entire HAZ exhibit excellent toughness, we aimed to change the structure of this region from an upper bainite-based structure to a lower bainite- and martensite-based structure. It became clear that it was necessary to design the ingredients. If such measures improve toughness,
In a COD test, the material undergoes ductile failure until the end, or after the load decreases past the maximum load point, as seen in the load-clip gauge displacement curve.
Becomes brittle fracture. (Hereinafter, these types of fractures will be referred to as m-value fractures.) The value of δc in these fracture modes is generally defined by the apparent COD value at the point of maximum load (hereinafter, the value of δc in such m-value fracture
is called δm. ). For steels with a tensile strength of 50 to 60 Kgf/ ^mm2 , if the m-value fracture occurs, the limit COD value, δm, will be sufficiently high, so in order to guarantee the COD for this class of steel, only the toughness should be improved. That's enough. However, the tensile strength targeted by the present invention is 80Kgf/
In the case of high-strength steel of mm ² or more, improvement of toughness is a prerequisite for COD guarantee, but even if the toughness is sufficiently increased to cause m-value fracture, the value of δm is not necessarily sufficiently high (for example, usually required limits
It was found that the COD value level was 0.2 to 0.25 mm). Therefore, the present inventors investigated ways to improve toughness as well as measures to improve δm, and based on the findings, invented a high tensile steel with excellent welded joint COD characteristics and a tensile strength of 80 Kgf/mm ² or more. Hereinafter, the gist of the present invention will be divided into a method for improving toughness and a method for improving Δm, and will be explained in detail based on experimental results. First, in order to improve toughness, it is necessary to minimize the proportion of upper bainite in the structure of the coarse grain + Ac ₃ region. The main reason for this is that when there is a large amount of upper bainite structure, the amount of island martensite, which is harmful to toughness, increases and its size also becomes coarser. Steel with a tensile strength of 80Kgf/ ^mm2 or higher must contain a certain amount of alloy to ensure the strength of the base metal, so it basically has a composition system with extremely high hardenability. . Therefore, in order to suppress the formation of upper bainite in the structure of the coarse grain + Ac ₃ region, it is better to further increase the alloy content and increase the hardenability to create a structure consisting mainly of lower bainite or martensite. It is more practical than reducing the hardenability to create a ferrite + pearlite structure. Therefore, the present inventors investigated conditions for suppressing upper bainite formation under thermal cycle conditions corresponding to the coarse grain + Ac ₃ region through a simulated thermal cycle test. Steel having the chemical composition shown in Table 1 was melted in a 30 kg vacuum melting furnace, hot-rolled, quenched and tempered, and a simulated thermal cycle test shown in FIG. 2 was conducted on the material. 1 to simulate the coarse grain + Ac ₃ region.
A heat cycle test was conducted twice, with the maximum heating temperature of the first heating being 1400°C and the maximum heating temperature of the second heating being 900°C. The cooling pattern simulated actual multilayer submerged (SAW) welding, and the cooling time from 800°C to 500°C was set at 40 seconds, assuming the larger welding heat input. In this way, for multi-layer welding, the cooling time on the side with a larger heat input was chosen because it is more difficult to suppress upper bainite formation with a longer cooling time, and under these conditions upper bainite can be suppressed. If so, we thought that there would be no problem with a smaller heat input than this condition. From materials that have undergone such thermal sun test,
Test piece (thickness = 10mm, width = 20mm, span = 80mm,
A total notch length (including the fatigue notch part = approximately 10 mm) was fabricated, and a COD test was conducted at -30°C. The reason why we set the COD test temperature to -30℃ is because currently COD tests generally require a temperature of about -10℃, but in this study, we took into account the small-scale test, so the actual plate thickness -10℃ for a plate thickness of about 50mm
This is because the intention was to compensate for the small thickness of the plate with temperature in order to make it equivalent to the test in . As a result of comparing the test results with the optical microscopic structure, as shown in Figure 3, there is a certain relationship between the chemical composition of the steel and the amount of upper bainite in the optical microscopic structure of the simulated thermal cycle material. It has been found that by using the parameter x expressed by the following formula, the amount of upper bainite in the coarse grain + Ac ₃ region can be determined almost uniquely. x=0.32×b×[C(%)] ^1/2 ×[1+0.64×Si(%)]×[1+4.10×Mn(%)]×[1+0.27×Cu(%)]×[ 1 + 0.52 × Ni (%)] × [1 + 2.33 × Cr (%)] [1 + 3.14 × Mo (%)] However, the value of b is when the N content is less than 30 ppm
Set it as 1.3, and set it as 1 if it is 30ppm or more. Furthermore, Figure 4 shows the relationship between the amount of upper bainite and δc at -30°C. Considering that the toughness has been improved by m-value fracture, this figure shows that the amount of upper bainite should be approximately 10% or less. It turns out that there is. Therefore, in order to obtain excellent toughness, it can be seen from FIG. 3 that the parameter x must satisfy x≧12. However, the content range of each element related to the parameter x is limited for the reasons listed below. C is required to be at least 0.02% to ensure the strength of the base metal, but as described later, in order to improve δm,
Since it needs to be 0.08% or less, it is set in the range of 0.2 to 0.08%. Si is an element that facilitates the formation of island-like martensite, and if it exceeds 0.50%, there is a problem with the toughness of the base material.
If it is less than 0.05%, deoxidation will be insufficient and internal defects in the steel will increase, so the content is set in the range of 0.05% to 0.50%. If Mn is less than 0.50%, there is a problem in ensuring the strength and toughness of the base metal, and if it exceeds 3.0%, tempering embrittlement becomes noticeable, so it was set in the range of 0.50% to 3.0%. Cu ensures the strength of the base material through precipitation strengthening.
0.60% or more is required, but if it exceeds 2.0%, the susceptibility to temper embrittlement and SR embrittlement increases rapidly.
The range was 0.60% to 2.0%. Ni has the effect of improving the toughness of the matrix itself in addition to the effect of improving the toughness through the effect of the parameter x, but for this purpose it is necessary to contain more than 2.0%. However, Ni is expensive and 10.0
If the content exceeds 2.0% to 10.0%, the susceptibility to toughness deterioration due to trace elements increases. B contributes to tissue improvement as the value of b in the formula of parameter x, but in order to obtain this effect, 0.0003
% or more. Also, 0.0030%
If the content is too high, it will have an adverse effect on toughness, so it is set in the range of 0.0003% to 0.0030%. When the amount of B is within this range and the amount of N is less than 30 ppm, B contributes to the improvement of the structure, and the degree of this is 1.3, which is the value of b in the formula for the parameter The value of b becomes 1 because the proportion of the particles that are fixed as such increases and does not contribute to tissue improvement. Cr has the effect of increasing hardenability and strength, but
If it is less than 0.01%, the effect of improving the structure will not be clear even if it is contained, and if it exceeds 1.50%, precipitation embrittlement will become noticeable, so it was set in the range of 0.01 to 1.50%. Like Cr, Mo is also effective in increasing hardenability and strength, but if it is less than 0.40%, the effect of improving the structure is not clear, and if it exceeds 1.50%, precipitation embrittlement becomes noticeable. The range was from more than 1.50% to 1.50% or less. Next, a method for improving Δm will be described. A comparative study of steels with various strength levels that underwent m-value fracture revealed that the higher the hardness of the HAZ, the lower the δm. The first reason for this is that the higher the hardness, the easier the occurrence and propagation of ductile cracks, but in addition to the hardness, if there are many elongated inclusions such as MnS, δm may decrease. If the hardness of the HAZ part can only be reduced, for example by lowering the carbon equivalent, then the value of the parameter It turns out. Therefore, instead of simply lowering the hardness, taking measures to lower the HAZ hardness while maintaining the parameter x≧12 improves toughness, that is, m-value fracture and The absolute value (Δm) of the COD value is also necessary to ensure a sufficiently high value. The condition of the parameter x≧12 is a region where upper bainite is hardly produced, that is, a region where the structure is mainly from lower bainite to martensite. Reducing the amount of C is considered to be the most effective way to lower the hardness while maintaining such a structure. In other words, if the amount of C is lowered, the parameter x will be constant at 12 or more, and if the contents of other elements are appropriately adjusted, the structure will be constant. As the amount of C decreases, a decrease in strength can be expected. Conversely, if the amount of other elements is lowered and the amount of C is increased instead, the hardness will increase under conditions of a constant structure, which is not preferable. Moreover, even if only the contents of other elements are adjusted while keeping the amount of C constant, no substantial decrease in strength can be expected. Therefore, the following experiment was conducted to examine what level of C content is appropriate for ensuring δm. The experiment was conducted using a simulated thermal cycle test. The number of cycles and maximum heating temperature are the same as in Figure 2, 1400℃.
and two heat cycles at 900℃. However, from 800
Considering that the cooling time to 500°C tends to decrease as the δm becomes harder, 20 seconds was selected as a condition simulating the low heat input side. Although the specific chemical composition of the test steel is omitted, the first
C content is 0.015-0.16% based on steel type K in the table.
By changing the amount of C within the range of
The amounts of Ni, Cu, CR, and Mo were adjusted so that the value of parameter x fell within the range of 13 to 15. The COD test is exactly the same as the method described above. The results are shown in FIG. Since all steel types satisfy the condition of x≧12, they all undergo m-value fracture. Obviously, as the amount of C increases, δm decreases rapidly. Strictly speaking, a fracture mechanics study is required to determine the absolute value of δm, but in order to satisfy the current standard COD requirement of δc = 0.2 to 0.25 mm from the design side,
It can be seen from Figure 1 that the C content needs to be 0.08% or less. This study result was based on a small test with a plate thickness of 10 mm and a ligament size (specimen width - initial notch length) of approximately 10 mm; however, the plate thickness of most actual high-strength steel products is larger than this, and COD The ligament size of the test piece is also larger than in this experiment, and it is thought that the obtained Δm is correspondingly larger due to the size effect. Therefore, the results of this study are on the safe side from the perspective of the test piece size, so if the C amount is within the range of the present invention, the actual thickness of the actual welded joint will be
In the COD test, a value of δm that is sufficiently higher than 0.25 mm should be obtained. From the above considerations, the upper limit of the amount of C is limited from the point of view of securing δm. On the other hand, the reason why the lower limit was set to 0.02% was not a requirement from HAZ toughness or Δm, but was for the purpose of ensuring base material strength. Limiting the amount of C is most important for improving δm, but in addition, limiting the amount of S, Ca, REM
It is also necessary to consider the addition of That is, since changes in δm are directly linked to the occurrence and propagation characteristics of ductile cracks, it is effective to reduce δm and to control the morphology of inclusions, particularly elongated MnS, which have a large effect on this. The amount of S needs to be 0.005% or less in order to reduce the amount of MnS and prevent a decrease in δm. Ca, REM
has almost the same effect on the morphology control of MnS, so it is possible to contain either one or both of them. However, if Ca+REM is less than 0.003%, the morphology control is insufficient; on the other hand, Ca+REM
When REM exceeds 0.02%, coarse inclusions tend to form;
Since Ca itself becomes a starting point for brittle fracture and deteriorates toughness, Ca+REM was set in the range of 0.003 to 0.02%. Since welded bond parts are exposed to extremely high temperatures,
MnS elongated during hot rolling also re-dissolves and finely re-precipitates during cooling, so the effect of elongated inclusions on δm is relatively small, and only in this region is it necessary to control the morphology of inclusions by Ca and REM. Although MnS may not have a large effect, in the base metal or in the HAZ area close to the base metal, MnS retains its morphology during rolling, so controlling the morphology of inclusions with Ca and REM is effective in improving δm in this region. be. In an actual joint, unlike a simulated thermal cycle test, there is a high possibility that various structures coexist at the bottom of the notch, even if it is a notch in a welded bond.
There is great significance in controlling the morphology of inclusions. Finally, we will explain the reasons for limiting ingredients other than those mentioned above. First, in high-strength steel, P is set to 0.010% or less in order to easily cause intergranular fracture in the coarse grain region of the weld. A needs to be contained in an amount of 0.010% or more in order to perform sufficient deoxidation to prevent internal defects in steel materials, and since exceeding 0.10% is harmful to toughness, the upper limit was set at 0.10%. N is an element that promotes the formation of island-like martensite and inhibits its decomposition.If N exceeds 0.0050%, the toughness will decrease significantly, so the upper limit should be set at 0.0050%.
And so. In addition, one of the features of the present invention is to keep the amount of C low in order to improve δm, but there is a concern that the strength of the base material cannot be ensured accordingly, so the strength of the base material can be increased by adding Nb and V. It is planned. Both of them increase the strength of the base material through precipitation strengthening and have almost the same effect, so it is possible to contain one or both of them, but Nb
If +V is less than 0.010%, the effect of increasing strength is not clear, and if it exceeds 0.10%, precipitation embrittlement becomes noticeable, so Nb+V was set in the range of 0.010 to 0.10%. The above is the basic component system of the present invention, but in the present invention, Ti can also be contained in a range of 0.005 to 0.015%. That is, since Ti can improve the strength and toughness of the base material through grain refinement of austenite and fixation of N, Ti is included in accordance with the characteristics required for the base material. however,
If it is less than 0.005%, there is no effect, and if it exceeds 0.015%, coarse precipitates tend to be formed and the toughness deteriorates, so the content is set in the range of 0.005 to 0.015%. [Example] Table 2 shows the results of comparing No. 1 to No. 13 inventive wires and No. 14 to No. 23 comparative wires. Both plates were hot-rolled to a thickness of 50 mm, and then hardened and tempered to create the material. A welded joint was prepared under the welding conditions shown in Table 3 and the groove shown in FIG. 5, and a COD test was conducted at -10°C in accordance with the BS5762 standard. S is the base material. The direction in which the specimen was collected was the direction in which the notch was parallel to the rolling direction (direction C), and the cross-sectional dimensions of the COD specimen were 50 x 100 mm. There were two types of notch positions: at the weld bond and at the HAZ boundary. As can be seen from Table 2, in the steel of the present invention, by limiting each component and at the same time making sure that the parameter x satisfies x≧12, the weld bond part,
It shows very good COD characteristics with HAZ boundary,
Most of them have m-value breaks. Some steel types with parameters close to 12 do not undergo m-value fracture, but even in that case, δc maintains a high value. Since the amount of C is limited, the COD value (δm) at the m-value break is a sufficiently high value. On the other hand, comparative steels No. 14 to No. 16 satisfy the limited range of each component, but their toughness is poor because the parameter Only COD values can be obtained. Also,
Comparative steels No. 17 to No. 19 satisfy the condition of parameter I can't. On the other hand, although the value of the parameter x in comparative lines No. 20 to No. 23 sufficiently satisfies the condition of x≧12, C, S, Ca, and REM, which closely affect δm, are outside the scope of the present invention. Therefore, the m value is broken, but
The value of Δm is considerably lower than that of the steel of the present invention. Therefore, from the above examples, it is clear that if the scope of the present invention is not satisfied, it is impossible to obtain a steel with excellent COD properties in both brittleness and ductility.

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[Table] [Effects of the Invention] As is clear from the above examples, according to the present invention, the welded part has excellent tensile strength and COD characteristics.
It is possible to provide high tensile strength steel of 80Kgf/mm ² or more, and the industrial effect is extremely significant.

[Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between δm and the amount of C;
The figure shows the simulated thermal cycle test conditions, and Figure 3 shows the proportion of upper bainite in the structure and the parameters x
FIG. 4 is a chart showing the relationship between δc and the proportion of upper bainite in the structure, and FIG. 5 is a diagram showing the dimensions and shape of the groove used in the examples.

Claims (1)

[Claims] 1% by weight: C 0.02% to 0.08% Si 0.05% to 0.50% Mn 0.50% to 3.0% P 0.010% or less S 0.005% or less A 0.010% to 0.10% Ni more than 2.0% to 10.0% Contains Cu 0.60% to 2.0% B 0.0003% to 0.0030% N 0.0050% or less Cr 0.01% to 1.50% Mo More than 0.40% to 1.50%, and one or two of Nb and V in the range of Nb + V = 0.010 to 0.10% Contains one or two of Ca and REM in the range of Ca + REM = 0.003 to 0.02%, with the balance consisting of iron and unavoidable impurities, and the value of the parameter x shown by the following formula is x≧12 High tensile strength steel with excellent COD properties at welded parts. x=0.32×b×[C(%)] ^1/2 ×[1+0.64×Si
(%)] × [1 + 4.10 × Mn (%)] × [1 + 0.27 × Cu
(%)] × [1+0.52×Ni (%)]×[1+2.33×Cr
(%)] × [1 + 3.14 × Mo (%)] However, the value of b is when the N content is less than 30 ppm
Set it as 1.3, and set it as 1 if it is 30ppm or more. 2 In weight%, C 0.02% to 0.08% Si 0.05% to 0.50% Mn 0.50% to 3.0% P 0.010% or less S 0.005% or less A 0.010% to 0.10% Ni More than 2.0% to 10.0% Cu 0.60% to 2.0% Contains B 0.0003% to 0.0030% N 0.0050% or less Ti 0.005% to 0.015% Cr 0.01% to 1.50% Mo More than 0.40% to 1.50%, and one or both of Nb and V in the range of Nb + V = 0.010 to 0.10% Contains one or two of Ca and REM in the range of Ca + REM = 0.003 to 0.02%, with the balance consisting of iron and unavoidable impurities, and the value of the parameter x shown by the following formula is x≧12 High tensile strength steel with excellent COD properties at welded parts. x=0.32×b×[C(%)] ^1/2 ×[1+0.64×Si
(%)] × [1 + 4.10 × Mn (%)] × [1 + 0.27 × Cu
(%)] × [1+0.52×Ni (%)]×[1+2.33×Cr
(%)] × [1 + 3.14 × Mo (%)] However, the value of b is when the N content is less than 30 ppm
Set it as 1.3, and set it as 1 if it is 30ppm or more.