JPS61253344A - Steel plate for high heat input welding and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a steel plate for high heat input welding having superior tensile strength and forming a weld heat-affected zone which is not embrittled by added B by regulating the carbon equiv. of a steel plate contg. a rare earth element or Ti and the B content in the steel plate to a specified value each. CONSTITUTION:A steel slab having a composition contg. 0.03-0.15% C, 0.01-0.50% Si, 0.50-1.80% Mn, 0.005-0.08% Al, 0.0003-0.0050% B, <0.01% N and <=0.03% in total of at least one among <0.03% each of Ti, Ca and REM and having 0.28-0.42% carbon equiv. (Ceq) represented by formula 1 is heated to 900-1,050 deg.C and rolled at <=30% draft per one pass to manufacture a steel plate for high heat input welding having superior tensile strength and forming a weld heat-affected zone which is not embrittled. In the composition, B contains <=0.0005% soluble B and the quantitative relation among Ti, B and N satisfies formulae 2, 3.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a steel plate for high heat input welding and a method for manufacturing the same.
2,50 711m2 or 60 K9/■2te,
The present invention relates to a steel plate for high heat input welding that can prevent embrittlement of a heat-affected zone (hereinafter referred to as HAZ zone) due to B, and a method for manufacturing the same.

(Prior art) As there is a growing demand for high efficiency welding, we have developed a new technology to meet this demand.
The rare earth element (hereinafter referred to as REM)-B steel material described above is
Furthermore, a Ti-B steel material described in JP-A No. 50-80911 was developed, and the coarse grain H near the bond part welded on one side
It has become possible to obtain excellent toughness even in the AZ region.

(Problem to be solved by the invention) However, when the B-added steel sheet is subjected to SAW welding on both sides or welding with a lower heat input, fine HA
A phenomenon of embrittlement is observed in the Z section and the α-r two-phase heating region, and this phenomenon is particularly noticeable when water-cooled copper welding is used, such as in electrogas welding, and the lower the nitrogen content, the more the embrittlement becomes. The present inventors have discovered through detailed examination and research that there is a tendency for this to occur.

The present inventors performed electrogas welding on two types of 11'60 steel with a plate thickness of 25 mm and a heat input of 75 KJ/cva to examine the toughness of the joints. The results are shown in Table 1 along with the steel composition. Furthermore, in order to investigate the embrittlement temperature range, a HAZ reproduction test was conducted using a high-frequency heating type thermal cycle simulator. The results are shown in FIG. It can be seen from the same figure that steel IK becomes brittle in the heating range of 800 to 1000°C, even in the heating range of 1350°C, which corresponds to the bond part.

Further, through optical and electron microscopy observations, the present inventors have found that the above-mentioned embrittlement is due to the formation of island-like martensite by solid solution BK. This means that B-free steel 2 has a microstructure of ferrite + pearlite in the 800P-1000°C heated zone, resulting in high toughness, but B-added steel I has island-like marten. It is thought that the solid solution B that generates sites has a quenching effect, resulting in a decrease in toughness.

By the way, in steels with low H content among B-added HT80 steels, the phenomenon of formation of island-like martensite from solid solution BK is not observed; Specific embrittlement is observed in B-containing steel.

Figure 2 shows the cooling time Δt from 800°C to 500°C, which corresponds to the change in welding heat input, based on experiments conducted by the present inventors.
8 when changing within the range of 10 to 500 seconds
FIG. 2 is a diagram showing the results of an investigation of WE-40 in the 00° C. heating section using @IK listed in Table 1. From the figure, it can be seen that the shorter the cooling time Δt and the faster the cooling rate, the more likely embrittlement occurs.

According to the above-mentioned research by the present inventors, REM-Ti-containing steel 2 heated to a temperature of 1350°C in the bond area of a high heat input weld joint.
It was found that the toughness of steel can be improved by adding B, i.e., by changing the composition to steel 1.
It has been found that steel IK heated to 1000° C. becomes embrittled. Such embrittlement is thought to be due to the formation of island-shaped martensite due to the improvement in hardenability due to solid solution BK as described above.

The object of the present invention is to provide a means for eliminating and solving the problems of the law, and the above object can be achieved by providing the steel sheet and the method for manufacturing the same as described in the claims. can.

That is, in the present invention, 00.03 to 0.15%, Si 0.01 = 0.01%.
50%, In0.50-1.80%, kl O,
005~0.08%, BO,0003~0.005
0%, NO, 0.01% or less, and further Ti0.0
3% or less, Oa 0.03% or less, REM 0.0
Contains at least one selected from 3% or less in a total amount of 0.03% or less, and has the formula (1) shown below.
) Calculated carbon equivalent Ceq is 0.28 to 0.42
% of high heat input welding steel: The Ti content and the total B content satisfy the following formulas (2) and (3), and the solid solution B of the total B is 0.0005% or less. Steel plate for high heat input welding.

aeq: (Q+ In/6 + Cu+ Ni/15
+Cr +Mo+V15) % ・= (1) Ti/
N≦3.5・・・・・・・・・・・・
・−・・・・・・・・・・(2) −0.003%≦
(N-(ilB"4r,s'fl))60
．． 003%...-(3) and its manufacturing method 0 As mentioned above, conventional iron and steel Vo1.63 Na
2p312 and Japanese Patent Application Laid-open No. 50-80911, and also conducted a follow-up study of the change in the shape of B from the time of steel production to the time of rolling of B-added steel. As a result, we arrived at the present invention. Next, the reason why the composition of the steel sheet is limited in the present invention will be explained.

C is 0.03%! ! If D is less than 0.15%, it will not only be difficult to ensure the strength of the steel plate, but also the joint will soften significantly during high heat input welding, while if D is more than 0.15%, the pound toughness during high heat input welding, which is the objective of the present invention, will be reduced. C deteriorates significantly, so C is 0.
O81, which needs to be in the range of 0.03 to 0.15%, is an element that is inevitably included together with l for deoxidation, but if it is less than 0.01%, deoxidation will be insufficient. In addition, the strength of the steel plate decreases, and on the other hand, if Si exceeds 0.50%, the toughness of the HAZ part of the high heat input welded joint deteriorates, so Si is 0.0%.
O In, which needs to be in the range of 1 to 0.50%, is less than 0.50%, the high heat input welded joint becomes soft, and the strength and toughness of the weld metal decreases;
%, it causes deterioration of the toughness of the high heat input welded joint flAZ, so Mn needs to be in the range of 0.50 to 1.80%.

If ht is less than o or oos%, AjN will not have the effect of refining the base metal and the toughness will be low, while if it is more than 0.08%, the toughness of the high heat input weld metal will deteriorate, so Hao,
Must be within the range of oos~0.08%, 0.0
Best results are obtained when the content is between 1 and 0.05%.

B is an essential component for improving the toughness of the HAZ part of high heat input welded joints, and B is 0.0003%! Little girl and Hmu 2
On the other hand, if B is 0.0050≦more than 0.0050, a grain boundary KB-containing structure will be formed in the coarse-grained part of a high-heat-input welded joint, resulting in brittleness. 00
Must be within 50%, especially 0.000
The most preferable range is 4 to 0.0020%.Ti
, (a) When at least one type of REM is used in combination, the toughness of the coarse grain part of a high heat input welded joint is further improved0
That is, B is 0.0003 to o, oos.

% added, BN is formed in the cooling process starting from unmelted REM-, Oa-oxysulfide or TiN in the heat-affected zone of high heat input welding, and free N
The improvement of the HAZ structure can be effectively achieved by reducing the amount of ferrite and by causing BN to generate ferrite.

At that time, if the Ti content is more than 0.03%, the excess Ti will not only generate coarse TiN, but also a considerable amount will be dissolved in solid solution in the HAZ heated to 1300°C or higher. Since embrittlement occurs, Ti must be kept at 0.030% or less, especially when it is 0.020% or less, HAZ
The toughness of the part is effectively improved.

REM generates REM-oxysulfide and B
It is an element that acts as a precipitation nucleus for N and is effective in improving the toughness of coarse grain areas during high heat input welding, but if it exceeds 0.030%, excessive R
In steel for high heat input welding to which EM-0Xi (16 is generated, which deteriorates the cleanliness of the steel and impairs the properties in the thickness direction), Ti and/or REM are added to the steel for high heat input welding. HA by the presence of 030% or less
Toughness of Z part improves 0 Therefore, TiK is 0.00
04-0.030%, 0.0030 for REM
Good results can be obtained when ~0.030% 0 REM
A similar effect can be obtained by adding Ca instead of Ca, and it is necessary to keep the amount below 0.030%. Since toughness deteriorates, the total amount of N needs to be 0.01% or less.

When the ratio of Ti and N is greater than 3.5, excess Ti becomes solid solution T.
Ti/N needs to be 3.5 or less because it causes embrittlement.

When Ceq (carbon equivalent) is less than 0.28%, softening occurs in the 9 HAZ part due to high heat input welding, while when Ceq (carbon equivalent) is less than 0.28%, softening occurs in the HAZ part.
Ceq O, 28-06 does not cause the toughness of IAZ coarse grains to deteriorate, but also causes significant embrittlement that is characteristic of B-added steel in the heating range of 800-1000°C.
It needs to be within the range of 42%. Furthermore, in the present invention, the inventors have newly discovered that the above-mentioned embrittlement can be prevented by reducing the amount of solid solution B in the steel sheet to 0,00% or less.

Next, the present invention will be explained using research data.OCO, 10
%, Si 0.25%, Mn 1.5%, P 0
．． 015%.

Using steel plates obtained from various small vacuum steel ingots with the basic composition of SO, 0.005% and A10.03%, and varying the Ti, B, and N contents, the maximum heating temperature was 800℃.
A thermal cycle reproduction test was conducted at 405elC with a cooling time of 800°C to 500°C, and the Charpy absorbed energy (vE-4n) at -40°C was determined. Figure 3 shows these results in terms of the relationship between the amount of solid solution B and the 800°C reproduced HAZ toughness.If the solid solution Bii is 0.0005% or less, embrittlement does not occur in the 800°C heating range. I understand that.

Figure 4 is a columnar graph showing the results of a follow-up investigation of the amount of solid solution B in continuous cast slabs of Steel 1 until it reaches the steel plate after blooming and thick plate rolling.X is the percentage of B in the form of BN. .

Y is Fe2. (Be) % B in the form of 6, 2 is solid solution B
%, and when the slab heating temperature for rolling is high, solid solution B
It can be seen that the amount Y is smaller than that of BjlY in the slab state. Next, the slabs having the composition shown in Table 1 were rolled to a thickness of 25 tllK while changing the heating temperature within the range of 800 to 1400°C. 0 Figure 5 is a diagram showing the relationship between the above-mentioned slab heating temperature and the amount of solid solution B. In order to reduce the amount of solid solution B in the steel plate to 0,00% or less, the heating temperature of the slab must be 10%.
It turns out that it is necessary to keep the temperature below 50°C. However, 80
At 0°C, normal rolling is difficult and the distortion is large, so it is preferable that the heating temperature be within the range of 900 to 1050°C.

Next, when large amounts of Ti and B are added compared to the amount of N, 1
Even at slab heating temperatures below 050°C, the amount of solid solution B is large and embrittlement occurs, as shown in Figure 3. Therefore, using the following equation (4), the toughness of the HAZ part reproduced at 1350°C and 800°C was calculated. The relationship with the (n) value is summarized in Figure 6.

(n) value = N - (-B + -'I'i)
---(4) 10.8 47.9 When the above (n) value is -0,0030% to 0.0030%, coarse particles (AZ part (1350℃ heating part) Oyohi fine grain H
It is clear that the toughness of the AZ part (800°C heated part) is improved at the same time. That is, the (n) value is -0,003
%) If small, K 800 due to excess B or excess T1
℃ heating part becomes brittle and reverse KO.
becomes excessive and the 1350°C heated part becomes brittle ◇ Therefore,
According to the present invention, it has the above-mentioned component composition, and Ti/N≦3.5 -0.003%≦N-(-Ri-B+''-Ti)≦0
．． If the amounts of KTi, B, and N are controlled so that 0.003% 10.8 47.9 and the heating temperature of the steel billet is within the range of 900 to 1050°C, K and
The amount of solid solution B in the steel plate must be 0.005% or less, coarse grain HAZ
It has been newly discovered that a steel for high heat input welding can be obtained which has excellent toughness in both the fine-grained HAZ part and the fine-grained HAZ part.

Note that the slabs that can be used in the present invention are not limited to slabs obtained by the continuous casting method, but may also be slabs obtained by the ingot forming method. Next, the present invention will be described with reference to embodiments.

Example 1 Continuously cast slabs with a thickness of 240°C of jI3 to 18 shown in Table 2 (
cooling rate of 5℃/min or more) at 1000TI each.
A thermal cycle reproduction specimen was collected from a steel plate obtained by rolling to 25 degrees after CK heating, and a specimen heated to 1350°C and a specimen heated to 800°C
800℃ to 500℃ for specimens heated to
A surface welding heat simulation was performed so that the cooling time Δt until the welding time was 1iosec. The Charpy absorbed energy at 40°C for one of the specimens thus obtained is shown in Table 3. For steel plates having the compositions of steels 6, 12 and 16 with an On value of -0,003% or less, solid solution The amount of B is 0.000
When it was 9% or more, embrittlement occurred when heated to 800°C. However, inventive steels 3 to 5, where the n value is -0,003%, υ is large.
BK for 8-11, 14, 15.17 and 18
It was confirmed that no embrittlement occurred due to Comparative steels 7 and 13 do not experience embrittlement at 800°C, but their n value is higher than 0.003%, and due to excess H, embrittlement occurs at 1350°C.
The toughness has decreased.

Table 3 Thermal cycle reproduction test results Table 4 Thermal cycle reproduction test results Example 2 Shown in Table 3! Using $14 and IO, the slab was rolled to a thickness of 251111 m by changing the heating temperature of 900°C, 1100°C, and 1300°C, and a thermal cycle reproduction test of 800°C heating (cooling time Δ from 800°C to 500°C
The results of the above tests are shown in Table 4. Even with the slab having the composition of the present invention, the heating temperature of the slab during blooming or thick plate rolling was as low as 900°C. For example, type K heated to 800℃ does not cause embrittlement, but 11
It was confirmed that embrittlement occurs at temperatures as high as 00°C or 1300°C.

(Effects of the Invention) The high heat input welding steel of the present invention provides good toughness in the entire two parts of the process, from shielded arc welding with a small heat input to normal submerged arc welding and single-sided welding. It will be done.

[Brief explanation of drawings]

Figure 1 shows the relationship between the toughness and the maximum heating temperature in °C when the reproduction 1iAZ part test was carried out for steels 1 and 2 having the compositions shown in Table 1 with a cooling time of Δt of 40 sec from 800°C to 500°C. Diagram showing relationships. FIG. 2 is a diagram showing the relationship between HAZ toughness and cooling time Δt sea from 800°C to soo t: in a 800°C heating reproduction HAZ test of steel I. FIG. 3 is a diagram showing the relationship between the amount of solid solution B in a steel plate and the toughness of the HAZ part after heating at 800°C. FIG. 4 is a diagram showing changes in the structuring acid ratio of the B precipitation form in steel slabs, one that was bloomed at 1,200°C, and one that was further plate-rolled at 1,150°C after blooming. FIG. 5 is a diagram showing the relationship between slab heating temperature and solid solution B amount. Figure 6 shows 1350℃ heating and 800℃ heating reproduction ■Mu2
Relationship between part toughness and [G(14B+-Lee T1)] attack 10.
8 47.9 It is a diagram showing the section.

Claims (1)

[Claims] 1. C0.03-0.15%, Si0.01-0.50
%, Mn 0.50-1.80%, Al 0.005-0.
08%, B0.0003-0.0050%, N0.01
% or less, and further includes Ti0.03% or less, Ca0.0
Contains at least one selected from 3% or less, REM 0.03% or less in a total amount of 0.03% or less,
And the carbon equivalent Ce calculated by the formula (1) shown below
In steel for high heat input welding where q is 0.28 to 0.42%: The Ti amount and the total B amount satisfy the following formulas (2) and (3), and the solid solution B of the total B is A steel plate for high heat input welding, characterized in that the heat input is 0.0005% or less. Ceq=(C+Mn/6+Cu+Ni/15+Cr+M
o+V/5)%...(1) Ti/N≦3.5(2) -0.003%≦{N-[(14/10.8)B+(1
4/47.9) Ti]}≦0.003%...(3)2
, C0.03-0.15%, Si0.01-0.50%
, Mn0.50-1.80%, Al0.005-0.0
8%, B0.0003-0.0050%, N0.01%
Contains the following, furthermore Ti0.03% or less, Ca0.03
% or less, REM 0.03% or less in a total amount of 0.03% or less, and the carbon equivalent Ceq calculated by the formula (1) shown below is 0.
．． 28 to 0.42%, and the Ti content and total B content satisfy the following formulas (2) and (3).
A method for manufacturing a steel plate for high heat input welding, characterized by rolling the steel plate at 0 to 1050°C at a rolling reduction rate of 30% or less per pass, and reducing the solid solution B in the thus obtained steel plate to 0.0005% or less. Ceq=(C+Mn/6+Cu+Ni/15+Cr+M
o+V/5)%...(1) Ti/N≦3.5...(
2) -0.003%≦{N-[14/10.8)B+(14
/47.9)Ti)]≦0.003%...(3)