JPH02163342A - High strength steel for large heat imput welding - Google Patents

Abstract

PURPOSE:To obtain the large heat imput weld joint toughness in the steel without incorporating large amounts of expensive alloy elements by specifying the contents of C, Si, Mn, Ni, Cu, Cr, Mo, etc., and the value of their parameter. CONSTITUTION:The compsn. of a high strength steel is constituted of, by weight, 0.02 to 0.06% C, 0.05 to 0.20% Si, 0.50 to 3.0% Mn, <=0.010% P, <=0.010% S, 0.03 to 0.10% Al, 1.0 to 10.0% Ni, 0.0005 to 0.0020% B and <=0.0040% N, one or more kinds among 0.01 to 1.50% Cu, 0.01 to 1.50% Cr and 0.01 to 1.50% Mo and the balance Fe with inevitable impurities. The value of the parameter (x) expressed by the equation is furthermore regulated to 18 to 24. If required, 0.005 to 0.02% Ti is incorporated into the steel.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is capable of achieving a tensile strength of the base material of 70 kgf/ms with good weld joint toughness even in adult heat welding where the welding heat input is about 8 to 1 okJ/ms. This relates to tensile strength steel of +m4 or more.

(Conventional technology) In recent years, adult heat welding has been used to improve the efficiency of welding work, but in general, the joint toughness of high-strength steel deteriorates significantly due to high heat input welding. For,
In structures requiring low-temperature toughness, the use of high-heat-input welding is often restricted for high-strength steel. Particularly in the case of high-strength steel with a tensile strength of 70 kg f/wJ or more, it is extremely difficult to ensure the low-temperature toughness of welded joints, which is a major problem.

Recently, in steels with a tensile strength of 50 kg f / mJ, as seen in Japanese Patent Application Laid-open No. 80-245788, TI oxides and nitrides have been used to strengthen the grains of the weld heat-affected zone. The low-temperature toughness of high-heat-input welds can be improved by creating a fine ferrite-pearlite structure in the weld heat-affected zone by promoting ferrite transformation and refining the austenite grain size. has become possible.

However, the tensile strength is 70 kg f / +++
In high-strength steels with a tensile strength of i or higher, improvement in low-temperature toughness cannot be expected by such measures, and in fact, the toughness may even deteriorate.

This is because high-strength steel with a tensile strength of 70) cgf/mJ or higher necessarily contains a large amount of alloying elements in order to maintain the properties of the base material, and if it contains TI oxides or nitrides, On the contrary, the hardenability decreases, resulting in an upper bainite-based structure containing a large amount of island martensite, which is harmful to toughness.

Therefore, a method for improving toughness that is completely different from that for copper treated with 50 kg f/- is required, and it is completely impossible to find a solution by extending the conventional technology.

As a technique for improving the toughness of a high heat input weld in this class of high tensile strength steel, there is a technique as disclosed in Japanese Patent Laid-Open No. 61-441G1. The technology in this publication is for BN during high heat input welding.
In order to prevent the deterioration of toughness caused by this, one of the major characteristics is that no B is added, and the aim is to achieve a Charpy fracture surface transition temperature of 0° C. or lower in the bond portion (Fusion Llnc).

However, recently, the environments in which structures are used have become more severe and diverse, and as a result, demands for low-temperature toughness have become increasingly strict for steel materials, and at the same time, various other properties have also come to be required. Therefore, it is necessary to increase the toughness of the layer at a lower cost and to be able to select from a wide range of ingredients.

(Problem to be solved by the invention) In order to improve the toughness of the adult heat welded part, it is necessary to
It is preferable to use a component that does not generate upper bainite containing a large amount of island-like martensite, which is harmful to toughness, in the coarse grain region near the usion line.

Methods for this purpose include reducing the amount of alloying elements, making the austenite grain size finer using precipitates and inclusions, or promoting intragranular transformation to reduce hardenability and create a ferrite/pearlite structure. conversely, increasing the hardenability and creating a lower bainite or mixed structure of lower bainite and martensite to refine the structure.
There are two possible ways.

The former method is difficult to adopt because high-tensile steel with a tensile strength of 70 kgf/mJ or more necessarily contains a large amount of alloying elements in order to maintain the matrix properties.

Therefore, the latter method is considered to be the method that can basically be adopted.

However, under conditions such as high heat input welding where the cooling rate from the austenite region during welding is slow, it is necessary to suppress the formation of upper bainite and create a structure consisting mainly of lower bainite or martensite. Even so, it is necessary to contain a larger amount of alloying elements than usual, which causes problems such as deterioration of other properties and making the steel material too expensive, reducing its industrial advantages.

(Means for Solving the Problems) In order to solve these problems, the present invention aims to solve the problems by reducing the toughness of high heat input welded parts even if a certain percentage of upper bainite is added to the steel without drastically increasing the amount of alloying elements in the steel material. As a result of conducting various studies on the composition of ingredients to prevent the occurrence of
2-0.06%, S 〇, 05-0.20%, Mn: 0
．． 50-3.0%, P: 0.010% or more, S: 0
．． 010% or less, AI: 0.03-0.10%, NI:
1.0-10.0%, B: 0.0005-0.00
20%, N: 0.0040% or less, and Cu, Cr,
Mo to Cu: 0.01-1.50%, Cr: 0.0
1-1. ．． 5096, Mo: 0.01-1.50%
1 tTl or two or more types, with the balance consisting of iron and unavoidable impurities, and the value of parameter X expressed by the following formula is from 18 to 24. It is tension steel.

x=[C(%)]'/2X [1+0. [34xS1(
X)]X[1+4. lOxMn(%)]x [1+0.
27×Cu(%)]X[1+ 0.52X Nl(%)
] X [L+ 2.33X Cr(%)ko×[1+
3.14X No (%)] Furthermore, the present invention is a high-strength steel for adult heat welding that can contain TI in an amount of 0.005 to 0.02%.

(Function) The present invention improves the fusion line of the welded part during large heat input welding.
As a result of various studies, we invented a method for suppressing the proportion of upper bainite in the structure that forms near e to a certain level or less by mixing appropriate alloying elements, and then minimizing the deterioration of toughness due to the upper bainite. This is what led to this.

Hereinafter, the gist of the present invention will be explained in detail based on experimental results.

The present inventors melted various steels with chemical components a within the range of components shown in Table 1 in a vacuum melting furnace, hot-rolled them, and then quenched and tempered them. A reproducible thermal cycle was added to simulate the thermal history experienced by the coarse-grained region of the heat-affected zone.

The reproducible thermal cycle conditions were a heating temperature of 1400°C, a holding time of 1 second, and two types of cooling time (Δt815) from 800 to 500°C: 80 seconds and 120 seconds. This cooling time is equivalent to a heat input of 8 to 10 for submerged arc (SAW) welding.''/I
The conditions roughly correspond to II■.

Process the material into a test piece, perform a Charpy test,
The relationship between 50% fracture surface transition temperature (vTrs) and chemical components was investigated.

Since the HAZ toughness of steel with a tensile strength of 70 kgf/- or more is most strongly affected by the microstructure, especially the amount of upper bainite, the present inventors found that it corresponds well to the microstructure of the weld heat affected zone or simulated thermal cycle material. The experimental results were organized according to the parameter X shown in the following formula.

x-[C(%)]1/2x[l+0,84xSl(%)
]×[1+ 4.10 X Mn(%)] X [1+
0.27X Cu (1%)] X[1+ 0.52X
N1 (1%)] X [1+ 2J3
)ko×[1+3.14×No(%)] FIG. 1 and FIG. 2 show the relationship between this parameter X and vTrs.

Figure 1 shows the result when Δt815 is 80 seconds, and Figure 2 shows the result when Δt815 is 80 seconds.
t815 is the result of 120 seconds. These results revealed that vTrs is organized by X and Cm for each cooling time.

Since the parameter X corresponds to the amount of upper bainite, in other words, the toughness is controlled by the amount of upper bainite and Cm, and it can be said that the toughness improves as the amount of upper bainite, Cff1, decreases.

The amount of upper bainite has the greatest effect on toughness, but under high heat input welding conditions represented by the cooling conditions of this experiment (80 to 120 seconds at Δt815),
Even if the value of parameter

However, as is clear from FIGS. 1 and 2, even if upper bainite exists, if the parameter X is 18 or more and Cm is as low as 0.02 to 0.06%,
Even if t815 is quite slow, 120 seconds, the toughness is vT.
It was found that a good value of about −60° C. or lower was exhibited at rs.

When the CQ is 0.06% or more, even if the parameter

In addition, some of the steels used in this experiment include not only B-added steel, but also
Steels that do not substantially contain B are also included, but as shown in Figures 1 and 2, the B-added steel did not show any deterioration in toughness. It was concluded that B addition is not necessarily harmful to the toughness of the hot welded joint if the composition is appropriate, and that it can be used to ensure the hardenability of the base metal.

In addition, in the present invention, the study of Cm with respect to thermal cycle toughness is carried out at 0.02% or more, and Cm is 0.02% or more.
We have not considered anything less than that. This is because the present invention relates to steel with a tensile strength of 70 kgf/mA or more, and it is difficult to ensure the base material strength with a CEI of less than 0.02%. That is, the lower limit of Cm in the present invention is determined from the viewpoint of ensuring the strength of the base material.

From the above study results, in order to improve the toughness of the heat affected zone in high heat input welding, the parameter
．． It was concluded that 0.02 to 0.06% is essential. In addition, the higher the parameter Since there is a tendency for the effect to gradually decrease, the upper limit of parameter

The reasons for limiting the parameters X and Cm have been described above. Next, the reasons for limiting the components in the present invention will be described.

Si is an element that facilitates the formation of island-shaped martensite, and since it has a particularly large negative effect on high heat input welding, its upper limit was set at 0.20%. On the other hand, if it is less than 0.05%, deoxidation becomes insufficient and internal defects in the steel material increase, so the lower limit was set at 0.05%.

If Mn is less than 0.5096, there is a problem in ensuring the strength and toughness of the base material, and if it exceeds 3.0%, tempering embrittlement becomes noticeable, so it is set in the range of 0.50 to 3.0%.

In the high-strength steel targeted by the present invention, P is set to 0.0% or less because of the need to suppress temper brittleness.

S is an element that forms MnS and reduces ductility, and its influence is particularly large in high-strength steel, so the value of 0.010
% or less.

Ap has the effect of increasing the hardenability improvement effect of B by fixing N, which inhibits the hardenability improvement effect of B, as AIN during heat treatment of the base metal, but if it is less than 0.03%, this effect is insufficient. , while 0. If it exceeds 10%, ↑■ will form unnatural precipitates, which will have a negative effect on toughness, so 0.03 to 0.
The range was set at 10%.

Ni has the effect of improving the toughness of the matrix itself in addition to the effect of improving the toughness via the parameter X.

If Nt is less than 1.0%, the effect is insufficient, and in order to stably obtain good adult heat weld joint toughness, especially in high-strength steel, N1 of 1.0% or more is required. However, N
In the present invention, the upper limit is set at 10.0% because 1 is an expensive element and if it exceeds 1090%, the susceptibility to toughness deterioration due to impurity elements increases.

B is an element that can improve the hardenability of the base metal in trace amounts, and the steel of the present invention has a small amount of C, and the amounts of other alloying elements are controlled by parameter X.
Since it cannot be added unlimitedly due to the limitation of , it is an essential element to ensure strength.

0.0 in order to fully exhibit the hardenability improvement effect after 1 day.
005% or more is required. On the other hand, the amount of B is 0.0020%
If B exceeds 0.0005 to 0.0, even if Nff1 is low, the toughness of the weld will deteriorate in high heat input welding.
The range was 0.020%.

In adult heat welding, N is an element that causes a significant deterioration of toughness, especially in B-added steel, and if its content exceeds 0.0040%, the adverse effect on toughness becomes significant, so the upper limit is set to 0.0.
040%.

Cu, Cr, and Mo are all included in the formula of parameter X in the invention, and are included in order to improve the structure. Since both have similar effects on tissue control, it is possible to contain any one or two of these, or even BFJ. However, if the content is less than 0.01%, the effect of improving the structure will not be clear, and if it exceeds 1,50%, precipitation embrittlement will become noticeable, so the range was set from 0.01 to 1,50%. .

The above are the basic components of the present invention.
．． It is possible to contain it in the range of 0.02%. That is, like Ag, TI is effective in fixing N, and is also effective in preventing coarsening of austenite grains during reheating before hot rolling or heat treatment during base material production. TI is added depending on the thickness and heating temperature.

However, if it is less than 0.005%, the austenite coarsening prevention effect is insufficient, and if it exceeds 0.020%, coarse precipitates are formed and toughness is likely to deteriorate.
Limited to a range of 0.020%.

The present invention is a high tensile strength steel for high heat input welding having the above-mentioned composition. In manufacturing, after melting in a converter or electric furnace, a slab is formed by ingot-blowing or continuous casting, and then manufactured by ordinary heating-rolling-quenching-tempering. It is also possible to manufacture by controlled cooling after heating and rolling.

When melting, it is also possible to perform a 5:1 adjustment of the components by conventionally known vacuum degassing treatment, etc., if necessary.

(Actual, I+f example) Examples are shown in Table 2.

No. 1 to IO are the steels of the present invention, and Na 11 to I5
is the comparative steel. Both plates are 30mm thick by hot rolling.
After binding, the tensile strength of the base material is approximately 75 to 90 kg f
Appropriate quenching and tempering treatment was performed to obtain /-.

Depending on the X groove, the heat input is 8.0 or 1060kJ.
A Charpy test piece was taken from a position where the center of the test piece was about 71 cm from the surface of the steel plate and subjected to a Charpy test at -BO°C. The knots were all Fuslon Line.

As can be seen from Table 2, the value of parameter X is 18 to 2.
4. The amount of C is within the range of 0.02 to [+, [)8%,
In addition, m1kl to lO, whose other components are also within the range of the present invention, exhibits extremely excellent Charpy properties even at -60°C, although they are adult heat welded.

On the other hand, comparative steel N11Li1.12 has a small value of parameter X, so No. 13 has a large amount of C, so N11L1
4 has a large amount of C and a small value of parameter X,
In addition, although the C content of Na15 and the range of parameter

Therefore, from the above examples, it is clear that if the scope of the present invention is not satisfied, a steel with excellent low-temperature toughness of the adult heat welded part at -60°C cannot be obtained.

(Effect of the invention) High-strength steel with a tensile strength of 70 kgf/d or more has a high content of alloying elements due to the requirement to ensure base metal strength, and for this reason, conventionally it has been extremely difficult to obtain excellent high heat input welded joint toughness. It was difficult.

The present invention also applies to high tensile strength steel with high alloy components.
The appropriate composition range for obtaining excellent high heat input welded joint toughness is clarified, and according to the present invention, excellent high heat input welded joint toughness can be obtained without containing large amounts of expensive alloying elements. The industrial effects are extremely significant.

[Brief explanation of the drawing]

Figure 1 shows the Charpy characteristics of steel (50% fracture i-plane i
Figure 2 shows the relationship between parameter
rs). Agent Patent Attorney Tatsuo Chanoki Parameters × 2nd District Procedural City Authorization (Spontaneous) 1. Indication of Weakness Patent Application No. 317841 of 1988 2. Name of Invention High Tension for Large Heat Input Welding Steel 3, relationship with inertia of the person making the amendment Patent applicant address 2-683 Otemachi, Chiyoda-ku, Tokyo Name (6
65) Nippon Steel Corporation Representative Yutaka Saito 4, Agent Address 3-4-5 Kuramae, Taito-ku, Tokyo T5 2
25 Parameters (2) In the specification, page 11, line 11, "In the case of 120 seconds, which is quite slow" is corrected to "In the case, which is quite slow, 120 seconds." (3) In Table 2, page 19, r (kj/■)J is r (k
J/+i+n) Corrected to J. Claim 1, Weight: C: 0.02-0.06% Si: 0.05-0.20% Mn: 0.50-3.0% P: O, OLo 96 or less S: 0 .010% or less Ap: 0.03-0.10% Ni: 1.0-10.0% B: 0.0005-0.0020% N: 0
．． 0040% or less and Cu: 0. Old ~ 1.5096 Cr: 0.01 ~ 1.50% Mo: 0. Contains L species or two or more of old ~ 1.509TS, the balance consists of iron and unavoidable impurities, and 1 of parameter X shown by the following formula
A high tensile strength steel for high heat input welding, characterized in that the straightness is from 18 to 24. x - rc (%)] 1/2x [1+ 0.84X S
i (X)] X[1+4. IO×Mn(%)]X[l
+0.27×Cu(%)]×[1+ 0.52 X N
i (%) koX [1+2.33×Cr(%)]×(1
+3. 2. The high-strength steel for high heat input welding according to claim 1, which contains Ti in a range of 0.005 to 0.02%.

Claims (1)

[Claims] 1. C: 0.02-0.06% Si: 0.05-0.20% Mn: 0.50-3.0% P: 0.010% or less S: 0.010% or less Al: 0.03-0.10% Ni: 1.0-10.0% B: 0.0005-0.0020% N: 0.0040% or less and Cu, Cr, Mo : 0.01 to 1.50% Cr; 0.01 to 1.50% Mo: 0.01 to 1.50% Contains one or more of the following, the balance being iron and inevitable impurities, and the following: A high-strength steel for high heat input welding, characterized in that the value of the parameter x expressed by the formula is from 18 to 24. x-[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(%)] 2. The high-strength steel for high heat input welding according to claim 1, which contains 0.005 to 0.02% of Ti.