JPS6067622A - Preparation of low carbon equivalent steel for large heat input welding reduced in welding joint softening - Google Patents

Abstract

PURPOSE:To prepare low carbon equivalent steel for large heat input welding reduced in welding joint softening, by heating steel comprising C, Si, Mn, Nb, V and Fe in a specific composition and compositional relation to an austenite region, and hot rolling the same before cooling. CONSTITUTION:Steel which contains, on a wt. basis, 0.10-0.23% C, 0.10% or less Si, 0.90% or less Mn and one or more of 0.005- 0.050% Nb and 0.005%-0.1% V, and if necessary, further contains at least one or more of 0.50% or less Cu, 0.50% or less N, 0.50% or less Cr, 0.30% or less Mo and 0.10% or less Ti, and satisfies (Mn+Si)/C<=10, Ceq=Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14<=0.30 and comprises the remainder of Fe and ineviatable impurities, is heated to an austenite region and hot rolled and, thereafter, cooled at a cooling speed of 1-40 deg.C/ sec to obtain large heat input welding low carbon equivalent steel having tensile strength of 45kgf/mm.<2> or more which is 3kgf/mm.<2> or more higher than the tensile strength of a rolled material cooled by air after hot rolling and reduced in welding joint softening.

Description

[Detailed Description of the Invention] The present invention relates to a steel for dog heat input welding that causes less softening of welded joints.
Specifically, it has excellent weld cracking resistance during dwarf heat welding and toughness of the weld heat affected zone during high heat input welding, and has a tensile strength of 45 kg f/- or more with little softening of high heat input welded joints. This invention relates to a method for producing low carbon equivalent steel.

In recent years, with the aim of improving welding efficiency, there has been a demand for steel materials that do not require preheating during welding and can be applied to high heat input welding. In order to obtain such steel materials, the most effective method is to reduce the carbon equivalent of steel.
On the other hand, this method causes a decrease in the strength of the steel material. For this reason,
Conventionally, after hot rolling steel with a reduced carbon equivalent, the steel is strengthened by increasing the cooling rate, or by quenching after reheating.

However, when steel strengthened in this way is subjected to large heat input welding, softening occurs in the weld heat-affected zone, and the degree of this softening increases in proportion to the amount of increase in strengthening. A major problem is that the joint strength is insufficient. The reason why steel strengthened in this way softens in the weld heat affected zone during large heat input welding is because the welding heat causes the structure of the weld heat affected zone to become a low strength structure containing ferrite 1-, and when it is tempered. This is because the above-mentioned effect of increasing strength due to accelerated cooling and quenching disappears.

Conventionally, increasing the amount of C in steel or adding Nb or However, as a result of intensive research by the present inventors, we found that by suppressing the carbon equivalent to 0.30% or less, the strength of the weld heat affected zone increases as the C content in the steel increases. It is effective to reduce the softening of welded joints by increasing the amount of alloying elements such as Si and Mn, and also by reducing the amount of alloying elements such as Si and Mn.
It has been found that the above-mentioned softening can be further reduced by adding b or V.

That is, the present inventors provided steel with a predetermined chemical composition, accelerated cooling after normal hot rolling, or air cooling after normal hot rolling, heating to the austenite region again, and then quenching. However, by further tempering the steel that has been heat-treated in this way as necessary, steel that has undergone strength compensation by lowering the carbon equivalent has excellent weld cracking resistance and high heat input welding heat. It was discovered that this method not only improves the toughness of the affected zone, but also reduces the softening of the welded joint, thereby making the resulting steel possess all three properties.

Therefore, one of the methods for manufacturing low carbon equivalent steel with less softening of welded joints according to the present invention is as follows: weight% Te C0.10-0.23%, Si 0.10%
Contains 0.90% sunflower and N
bO.

005 to 0.050%, Vo, and at least one selected from 005 to 0.1%, where (Mn+Si
)/C≦10, and after heating the steel consisting of iron and unavoidable impurities to the austenite region and hot rolling, the tensile strength of the as-rolled material that is air-cooled after hot rolling is increased. is 3
Tensile strength increased by more than kg f /- 45 kg
Another method according to the present invention, characterized by obtaining a low carbon steel for high heat input welding with a f/mm or more, contains 1t% Teco, 10-0.23%, SiO, 10% or less and M
Contains N 0.90% or less, and N b 0゜0
05 to 0.050% and at least one selected from VO, OO5 to 0.1%, where (Mn + S
i) A steel that satisfies /C≦10 and consists of the remainder iron and unavoidable impurities is heated to the austenite range, hot rolled, air cooled, then heated again to the austenite range, and then quenched. It is characterized by a tensile strength that is 3 kg f/- or more higher than that of an as-rolled material that is air-cooled after inter-rolling.

First, the chemical components of the steel in the present invention will be explained.

In the method of the present invention, C is an essential element in order to obtain steel with reduced softening of the weld heat affected zone of high heat input welded joints, but when it is less than 0.10%, its effect is reduced. The upper limit is set at 0.10%, while 0.25
%, the weld heat-affected zone toughness during large heat input welding deteriorates, so the first limit is set to 0.25%.

Both St and Mn are elements useful for increasing the strength of steel, but if they are added in too large a quantity, it is necessary to reduce the amount of C to ensure weld heat-affected zone toughness and weld cracking resistance. As is clear from the above, the effect of reducing the softening of the joint is reduced. Therefore, the upper limits of the amounts of Si and Mn added should be 0.10% and 0.90%, respectively, and the relationship between (Mn+Si)/C≦10 should be satisfied, and the amount of Si and Mn should be adjusted to It is necessary to regulate the total amount.

In the present invention, Nb is used in the process described below in order to increase the strength of the steel material through precipitation strengthening and to suppress softening of the weld heat affected zone.
However, when the amount of Nb added is less than 0.0 O5%, the effect is insufficient;
If it exceeds 0%, the toughness of the weld heat affected zone deteriorates, so the addition amount range is limited to 0.005 to 0.050%.

Like Nb, ■ is also effective in precipitation strengthening, and is a useful element for strengthening steel and suppressing softening of the weld heat affected zone.
When it is less than 0.005%, the effect is insufficient, and when it is added in a large amount exceeding 0.10%,
0.0 because it deteriorates the base metal strength and the toughness of the weld heat affected zone.
0.005% to 0.10%. Note that either Nb or V may be added alone (or both may be added in combination).

In the present invention, in addition to the above-mentioned elements, the steel contains Cu,
At least one selected from Ni5Cr, MO, and Ti
It may contain a seed element.

Cu, Nis Cr, and MO are effective elements for adjusting the strength of steel and are also useful elements for reducing joint softening, but when added in too large a quantity, they reduce the toughness of the weld heat affected zone. Therefore, Cu,, Ni and C
Regarding r, the upper limit is set to 0.50%, and M
The upper limit for o is 0.30%. These elements may be added alone or in combination of two or more.

Ti is an effective element for improving the toughness of the weld heat-affected zone by dispersing fine nitrides and increasing strength by precipitating nitrides, but when added in large amounts exceeding 0.1%, it and deteriorates the weld heat affected zone toughness, so the upper limit should be set to 0.
1%.

According to one method of the present invention, by limiting the steel composition as described above, this steel is heated to an austenitic region, and after hot rolling, it is cooled at a cooling rate of 1 to b. Accordingly, even if the tensile strength is increased by 3 kg f/- or more compared to the as-rolled material which is then tempered and air-cooled after hot rolling, the welded joint can still be welded during high heat input welding of 60 KJ/cm or more. Tensile strength with little softening of 45 kg f
/- or more low carbon equivalent steel can be obtained.

According to another method of the present invention, the steel having the above-mentioned chemical composition is heated to the austenitic region, hot-rolled, air-cooled, the steel thus obtained is heated again to the austenitic region, and then Even if the tensile strength is increased by 3 kg f/- or more compared to the as-pressed material which is air-cooled after hot rolling by quenching and further tempering if necessary, the above-mentioned Similar to the method, the welded joint has a tensile strength of 45k with little softening even during large heat input welding of 60KJ/cm or more.
A low carbon equivalent steel having g f /- or more can be obtained.

As mentioned above, in the case of conventional steel, when the strength of low carbon equivalent steel is increased by heat treatment after hot rolling, the resulting steel has the above-mentioned strength in the weld heat affected zone during high heat input welding. Softening increases in proportion to the amount of rise, resulting in insufficient joint strength. Thus, short gauge tensile test specimens commonly used for measuring the tensile strength of welded joints, i.e., JIS Z 3
According to the No. 121 test piece, if the tensile strength of the heat-treated steel is 3 kgf/- or more higher than that of the as-rolled material that has been air-cooled after hot working, the joint tensile strength will be lower than that of the base material and the welding heat will be affected. It breaks at the part. However, according to the present invention,
Even if the tensile strength of the as-rolled material is increased by 3 kg f/- or more through heat treatment after hot rolling, the tensile strength of the joint is greater than that of the base metal, so the welded joint of conventional steel is It will not break in the heat affected zone of the weld.

In particular, in conventional steel, when the welding heat input exceeds 60 KJ/c+n, the degree and width of the weld heat-affected zone softens and the joint strength decreases significantly, but the steel obtained by the method of the present invention Even after welding with a large heat input of 60 KJ/cm or more, the tensile strength of the joint remains greater than that of the base metal, so it will not break in the weld heat affected zone.

However, due to the prescribed heat treatment after hot rolling, the tensile strength is 3 times higher than that of the as-rolled material that has been air-cooled after hot rolling.
If the increase is less than kg f/mm, the tensile strength of the joint is constrained by the strength of the weld metal and base metal and cannot be lower than that of the base metal. That is, the method of the present invention is particularly significant for steel whose tensile strength has been increased by 3 kg f/- by heat treatment after normal hot rolling.

Furthermore, the steel obtained according to the present invention has a tensile strength of 45k.
This is particularly significant when obtaining steel with gf/+n+f or more. In the case of steel with a tensile strength of 45 kg f/- or less, there is no need to adopt a strengthening method such as accelerated cooling or direct quenching as in the present invention, and it can be easily strengthened even if the carbon equivalent is 0.30% or less. This is because it is possible to obtain the required tensile strength for This is because.

The present invention will be explained below with reference to Examples.

Example Steel having the chemical composition shown in Table 1 is heated to a normal austenite range of 950 to 1150°C, hot rolled, and then heated to a temperature of 750 to 8
Finish rolling was carried out at a temperature of 50°C.

Immediately after this, accelerated cooling was performed to a temperature of 5 to 0°C or less to obtain a steel plate with a thickness of 251,111 mm. This method will be referred to as Method I.

Separately, steel with the chemical composition shown in Table 1 was heated to 950 to 1150℃.
After heating and hot rolling to the normal austenite region of
After cooling in air and then heating again to the austenite region, quenching and tempering were performed to obtain a steel plate with a thickness of 251 m. This method is referred to as method (■).

The mechanical properties and impact resistance properties of the heat-treated steel base material thus obtained are shown in Table 2 together with the mechanical properties of the as-rolled material. Table 2 also shows the properties of joints obtained by electroslag welding heat-treated steel. According to the inventive steel, the tensile strength of the welded joint is always greater than that of the base metal, and the impact resistance of the bonded part is also poor; Even if the strength of the welded joint is lower than that of the base metal, as seen in the steels consisting of J and K, and the strength of the welded joint is greater than that of the base metal, as seen in the steels J and K, The impact resistance of the bond part is poor.

Further, steels having steel codes A (invention steel) and M (comparative steel) were welded using various welding methods, and the tensile strength of the welded joints was determined. Table 3 also shows the difference obtained by subtracting the base metal tensile strength from the welded joint tensile strength. As the results are shown in Fig. 1, the steel obtained by the method of the present invention has a capacity of 60 KJ.
Even in the case of high heat input welding of / am or more, the joint strength is greater than the base metal strength. On the other hand, in the case of comparative steel,
When high heat input welding of 60 KJ/cm or more is performed, the joint tensile strength becomes smaller than that of the base metal due to softening of the weld heat affected zone.

Next, Table 4 shows the results of hot rolling steels with steel codes A and M, followed by accelerated cooling at various cooling rates, and electroslag arc welding of the steels thus obtained. show. Table 4 also shows the difference in tensile strength between the base material obtained by accelerated cooling and the as-rolled material. Figure 2 shows this in a graph.According to the steel of the present invention, even if the strength increase due to accelerated cooling is 3 kg f/- or more, the tensile strength of the welded joint is higher than the base metal tensile strength. It's big, but
In the case of comparative steel, on the other hand, the joint tensile strength is smaller.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the welding heat input and the difference in tensile strength between the welded joint and the base metal, and Figure 2 is the relationship between the strength increase in the base metal and the difference in tensile strength between the welded joint and the base metal. This is a graph showing.

Claims (4)

[Claims] (1) C0.10-0.23% by weight, Si 0.
Contains 10% or less and Mn 0.90% or less,
NbO, 005-0.050% and Vo, 005-0.
Contains at least one species selected from 1% of (M
A steel satisfying n+Si)/C≦10 and consisting of the balance iron and unavoidable impurities is heated to an austenite region and hot rolled. A method for producing a low carbon equivalent steel for high heat input welding having a tensile strength of 45 kf/- or more with little softening of welded joints, characterized in that the strength is increased by 3 kg f/- or more. (2) C0.10-0.23% by weight, S i O,
10% or less and M n 0.90% or less, Nb 0.005 to 0.050% and Vo, 005 to
At least one species selected from 0.1% and Cu 0
．． 50% or less, Ni 0.50% or less, CrO, 50%
Hereinafter, a steel containing at least one selected from Mo 0.30% or less and Ti 0.10% or less, satisfying (Mn+Si)/C≦10, and having the balance iron and unavoidable impurities is in the austenite range. A welded joint with less softening, characterized in that the tensile strength is increased by 3 kgf/- or more compared to the as-rolled material which is heated to 1-b and air-cooled after hot rolling. A method for producing low carbon equivalent steel for high heat input welding having a strength of 45 kg f/mm or more. (3) @fi1% CO,1O-0,23%, SiO
,]O% or less and Mn 0.90% or less, and NbO,OO5-0.050% and Vo,0
Contains at least one selected from 05 to 0.1%,
A steel that satisfies (Mn + Si)/C≦10 and consists of the remainder iron and unavoidable impurities is heated to the austenite range, hot rolled, air cooled, then heated again to the austenite range, and then quenched. By doing so, the tensile strength is increased by 3 kgf/- or more compared to the as-rolled material that is air-cooled after hot rolling.The welded joint has a tensile strength of 45 kgf/- with less softening.
- A method for producing a low carbon equivalent steel for high heat input welding as described above. (4) C0.10-0.23% by weight, Si 0.
Contains 10% or less and Mn 0.90% or less,
NbO, 005-0.050% and Vo, 005-0.
At least one species selected from 1% and Cu 0.5
24 ti 40 b 4 14, the steel consisting of iron and unavoidable impurities is heated to the austenitic region, hot rolled, air cooled, then heated again to the austenitic region, and then quenched to produce a steel after hot rolling. A welded joint with a tensile strength of 45 k with less softening, characterized by a tensile strength that is 3 kg f/mm or more higher than that of air-cooled as-rolled material.
A method for producing a low carbon equivalent steel for large heat input welding having g f /- or more.