JPH04272156A - Steel plate hardly causing haz cracking in high heat input welded square joint part of steel-frame box pillar - Google Patents

Abstract

PURPOSE:To prevent HAZ (heat-affected zone) cracking caused in the high heat input welded square joint part of a steel-frame box pillar for a multistory building, etc. CONSTITUTION:This steel plate hardly causing HAZ cracking in the high heat input welded square joint part of a steel-frame box pillar consists of 0.10-0.20% C, 0.05-0.50% Si, 0.50-1.80% Mn, <=0.010% P, <=0.002% S, 0.005-0.020% Nb, 0.005-0.020% Ti, 0.0008-0.0035% Ca, 0.020-0.080% Al and the balance Fe with inevitable impurities and contains <=0.008% A type inclusions in the segregated part whose hardness Hv is higher than the nonsegregated part by >=30%. The amt. of the A type inclusions is measured by a microscopic test of nonmetallic inclusions in steel stipulated by JIS G0555.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a steel plate that is less likely to cause HAZ cracking in the high heat input welded corner joint of a steel box column, and more specifically, the present invention relates to a steel plate that is less likely to cause HAZ cracking in a high heat input welded corner joint of a steel box column. The present invention relates to a steel plate that can prevent minute cracks in the weld heat affected zone (HAZ) that may occur in high heat input welded angle joints of box columns for high-rise buildings.

0002

[Prior Art and Problems to be Solved by the Invention] In recent years, buildings have become taller and have larger spans in order to make effective use of land and improve the comfort of living spaces.
The use of thick steel plates on the order of mm has been increasing. As the wall thickness increases, the amount of welding required increases, and in order to suppress this, high heat input welding, which can reduce the number of welding steps compared to conventional methods, is increasingly being applied.

[0003] This increase in heat input in welding is also progressing in square joint welding of box columns, with heat input of 10 to 100 kJ/mm.
Submerged arc welding with a high heat input has come to be applied, but low-temperature cracking may occur in the HAZ, which poses a problem in ensuring the safety of structures.

[0004] This cold cracking occurs when HA is rapidly cooled after welding.
Hardening of Z, diffusion and accumulation of hydrogen from weld metal to HAZ,
It is caused by a combination of three factors: thermal stress associated with welding heat, and until now it has mainly been a problem when low heat input welding is applied to high-strength steel plates with high carbon equivalents, resulting in large hardening of the HAZ. .

On the other hand, cold cracking in the HAZ that occurs in high-heat-input welded square joints of box columns also occurs when high-heat-input welding is applied to steel plates with relatively low carbon equivalents; It is thought that factors different from those in the conventionally well-known cold cracking are at play. Possible factors for this are A-based inclusions and a large degree of restriction. This point will be explained below.

[0006] Since the A-based inclusions extend long in the rolling direction and have sharp tips, voids created by molecularized hydrogen that accumulates at the interface with the base steel tend to become the starting point of cracks. On the other hand, in the corner joint part, the diaphragm is inserted and the four sides are welded, so it is thought that the degree of restraint is increased. In these points, in high heat input welded square joints of box columns, HA
It is thought that cracks occur even though low-temperature cracking is less likely to occur due to the hardening of Z.

By the way, cracks near welds caused by A-based inclusions include lamellar tear and butt weld H.
Inclusion aperture cracking in AZ is known, and reduction of MnS and addition of Ca for this purpose have been proposed as countermeasures against these (Japanese Patent Laid-Open No. 52-9617, Japanese Patent Laid-open No. 54
(Refer to Publication No.-47814).

H of high heat input weld angle joint of steel box column
In order to prevent AZ cracking, since the cause is thought to be the same as these cracks, it is necessary to reduce MnS and C for that purpose.
It is inferred that the addition of a would be effective. However, in high heat input welded corner joints of steel box columns, the welding heat input is extremely large and thermal strain is large, or the degree of restraint is large, compared to the above-mentioned known cracks near the weld. Even if a steel plate with the same inclusion level as proposed in the patent publication is used, cracking may not be completely prevented. The results of experiments actually conducted by the present inventors show that it is not possible to prevent HAZ cracking at high heat input weld angle joints of steel box columns by simply taking the measures proposed in the above patent publication. was confirmed.

[0009] As described above, the conventional measures to prevent cracks near welds caused by A-based inclusions are: was not sufficient.

[0010] In view of the above circumstances, the present inventors conducted research aimed at preventing HAZ cracking in the case of high heat input welded angle joints of steel box columns where cracking is likely to occur. This is what was done.

[0011]

[Means for Solving the Problems] The present inventors have intensively studied the effects of various factors on the occurrence of HAZ cracking in high heat input weld angle joints of steel box columns. As a result, in high heat input welded corner joints of steel box columns where cracks are likely to occur due to the direction of stress due to thermal strain and restraint, A
In addition to reducing system inclusions, we found that the combined addition of a small amount of Ti and Nb is effective in preventing HAZ cracking.

That is, in the present invention, C: 0.10 to 0
．． 20%, Si: 0.05-0.50%, Mn: 0.5
0-1.80%, P: 0.010% or less, S: 0
．． 002% or less, Nb: 0.005 to 0.020
%, Ti: 0.005-0.020%, Ca: 0.0
008-0.0035%, Al: 0.020-0.0
In the segregated part of the base material that contains 80% and is harder than the non-segregated part by HV30, the amount of A-based inclusions measured by the microscopic test method for nonmetallic inclusions in steel specified in JIS G0555 is 0.008%. It is a steel plate in which HAZ cracking is less likely to occur at the high heat input weld angle joint part of the steel box column where the remainder is unavoidable impurities.

[0013] In addition to the above alloy component composition, Cu:
0.05-0.50%, Ni: 0.05-0.50%,
Cr: 0.05-0.50%, Mo: 0.05-0.5
0%, V: 0.005 to 0.050%, B: 0
．． 0003 to 0.0020%, one or more of them may be contained.

[0014]

[Operation] The present invention will be explained in more detail below. The present inventors have developed four types of TMCP type 490N/mm2 class steel plates (thickness 80mm) for construction use, with different levels of segregation and inclusions, and two types of steel plates within the range proposed in the above-mentioned patent publication. A total of six types are available, and the HA of the corner joint part
We investigated the occurrence of Z cracks. The chemical composition of these steel plates,
Table 1 shows the segregation and inclusion levels. In addition, welding conditions adopted [welding conditions ■: 2-electrode multi-layer welding submerged arc welding (SAW), welding conditions ■: CO2 underlay welding + 2-electrode multi-layer welding SAW, welding conditions ■: 3-electrode 1-layer welding SAW,
Types] are shown in Table 2. In addition, in these welding operations, the flux was dried in the same way as in normal construction, and in the case of multilayer welding, the time and temperature between passes were effectively controlled to prevent hydrogen accumulation between passes.

The above investigation results are also shown in Table 1. As is clear from Table 1, S (0.002%, 0.0
01%), Nb (0.008, 0.010%)
, Ti (0.011, 0.012%), Ca (0
．． 0020, 0.0015%) and HV
In the segregation area of 180 or more, JIS G0555 (
It can be seen that in the case of steels C and D in which the amount of A-based inclusions was 0.008% or less according to the method for microscopic examination of nonmetallic inclusions in steel, no cracking occurred under any of the welding conditions. On the other hand, steel A, B, or Nb, which has a higher S content, lower Ca content, and the amount of A-based inclusions in the segregated part is 0.008% or more than steels C and D.
Cracks occur in steels E and F, which are conventional steels to which Ti and Ti are not added in combination.

[0016]

[Table 1]

[0017]

[Table 2]

Based on such research, the present invention provides S
, the contents of Nb, Ti, and Ca and the amount of A-based inclusions in the segregated area were determined.

Next, the reason for specifying the content of each chemical component will be explained.

[0020] C is an element necessary to ensure the strength of a high-strength steel plate for steel frames, and if the content is less than 0.10%, it is difficult to obtain a tensile strength of 490 N/mm2 class. Moreover, if it is added in an amount exceeding 0.20%, the weld cracking resistance will be impaired, which is not preferable. Therefore, the content of C is set in the range of 0.10 to 0.20%.

[0021]Si is an element necessary for deoxidation, but if the content is less than 0.05%, this effect is small;
If it is added in excess of more than %, weldability and toughness will deteriorate, which is not preferable. Therefore, the Si content is 0.05
The range is 0.50%.

Mn is an element necessary to improve hardenability and ensure strength within the plate thickness, but if the content is 0.
If it is less than 50%, such effects cannot be sufficiently obtained, and if it is added in excess of 1.80%, weldability and toughness are deteriorated, which is not preferable. Therefore, the Mn content is in the range of 0.50 to 1.80%.

[0023] P is an element that is likely to segregate, and when it is present in large amounts, the toughness of the segregated portion deteriorates, making HAZ cracking more likely to occur. Therefore, the smaller the amount of P, the better, but considering economic efficiency, the content is set to 0.010% or less.

[0024] S is a factor in the formation of A-based inclusions (most of which are MnS) that become the starting point of HAZ cracking, and the smaller the amount, the better. However, the upper limit of MnS (JIS
In order to achieve the amount of A-based inclusions (0.008% or less due to G0555) while taking economic efficiency into consideration, the S content is set to 0.002% or less.

[0025] Nb increases the strength and toughness of the steel plate through its grain refining action. Therefore, Nb
By adding Nb, the amount of reduction necessary to ensure the specified strength and toughness can be made smaller than when no Nb is added, reducing the amount of stretched MnS that has a negative effect on HAZ cracking. It is thought that this will be possible. To obtain this effect, it is necessary to add 0.005% or more, and addition of more than 0.020% deteriorates weldability and toughness. Therefore, the Nb content is in the range of 0.005 to 0.020%.

Ti has the effect of preventing coarsening of austenite crystal grains during heating of the slab, and combined with the crystal grain refining effect of Nb, the amount of reduction necessary to ensure the specified strength and toughness is reduced by Nb. This makes it possible to reduce the amount of stretched MnS that has an adverse effect on HAZ cracking. In order to obtain this effect, it is necessary to add Ti in an amount of 0.005% or more, and if it is added in an amount exceeding 0.020%, the toughness of the base material will be impaired. Therefore, the Ti content is in the range of 0.005 to 0.020%.

Ca has a spheroidizing effect on A-based inclusions, and H
It is effective in reducing stretched MnS, which has a negative effect on AZ cracking, but if the content is less than 0.0008%, a sufficient effect cannot be obtained, and if it is added in excess of 0.0035%, inclusions will increase. Toughness deteriorates. Therefore, C
The content of a is in the range of 0.0008 to 0.0035%.

[0028] Al is a deoxidizing element, and the content is 0.0
If it is less than 20%, there is little such effect, and if it is 0.0
Addition of more than 80% results in deterioration of toughness. Therefore, the Al content is 0.020-0.080%
The range shall be .

[0029] In addition, Cu, Ni, Cr, Mo,
One or more types of V and B may be contained depending on the target plate thickness, strength, and toughness level, and the content in that case will be explained.

[0030]Cu is an element effective in increasing strength through solid solution strengthening and precipitation strengthening, but if the content is less than 0.05%, such effects cannot be fully exhibited;
When added in excess of 50%, hot workability deteriorates and cracks are likely to occur on the surface of the steel sheet. Therefore, the Cu content is 0
．． The range is 0.05% to 0.50%.

[0031] Ni has the effect of improving toughness, but if the content is less than 0.05%, this effect cannot be sufficiently obtained, and if it is added in excess of 0.50%, scale defects are likely to occur. This also increases costs. Therefore, the Ni content is in the range of 0.05 to 0.50%.

[0032] Cr is an element effective in improving hardenability, but if the content is less than 0.05%, the effect will not be fully exhibited, and if it is added in excess of 0.50%, weldability will be impaired. Therefore, the content of Cr is 0.05-0.50%
The range shall be .

Mo is an element that improves hardenability, but if the content is less than 0.05%, a sufficient effect cannot be obtained, and if it is added in excess of more than 0.50%, weldability deteriorates and costs increase. The Mo content is 0.0.
The range is 5 to 0.50%.

[0034]V is an element that increases hardenability when added in a small amount, and in order to obtain this effect, it is necessary to add 0.005% or more, and when added in excess of 0.050%, it impairs weldability. harm Therefore, the content of V is 0
．． The range is 0.005% to 0.050%.

[0035] B is an element that improves hardenability when added in a trace amount, but this effect cannot be obtained when the content is less than 0.0003%, and when added in excess of 0.0020%, the toughness deteriorates. . Therefore, the content of B is 0.
The range is 0003% to 0.0020%.

Next, the reason for specifying segregation and inclusions will be explained. The origin of HAZ cracking is the A-based inclusions present in the segregated area, but in experiments conducted by the present inventors, cracks occurred in the segregated area where the hardness difference with respect to the non-segregating area was less than HV30. There was no. This is the reason why a segregated part that is harder than a non-segregated part by HV30 or more was targeted as a segregated part.

[0037] The results of investigating the influence of the amount of A-based inclusions on HAZ cracking in the above-mentioned segregated area are as shown in Table 1 above, and the widely used JIS G0555 (
HAZ cracking can only be prevented if the amount of A-based inclusions measured by the Microscopic Test Method for Nonmetallic Inclusions in Steel is 0.008% or less. As described above, in the present invention, JIS
The amount of A-based inclusions measured by G0555 was 0.008%.
The reason for the stipulation is as follows.

[0038]

[Example] Invention steel plates G to K and comparison steel plates L to P having the chemical composition and the amount of A-based inclusions in the segregated areas shown in Table 3 were prepared, and welding conditions were selected from the welding conditions shown in Table 2 above for each. Welding work was carried out using the same method, and the presence or absence of HAZ cracking was investigated. Table 3 shows the welding conditions at that time, the thickness of the steel plate, and the investigation results.
Shown below.

As is clear from Table 3, the steel sheets G~
No HAZ cracking was observed in any of the samples K. On the other hand, comparative steel sheet L that does not contain Nb and Ti, comparative steel sheet M that does not contain Nb, comparative steel sheet N that does not contain Ti and Ca, and comparative steel sheet O that contains a large amount of S and does not contain Ti.
Comparative steel sheets P containing a large amount of , P and no Nb, Ti, or Ca all had a large amount of A-based inclusions and HAZ cracking was observed.

[0040]

[Table 3]

[0041]

As described above, the steel plate according to the present invention can prevent HAZ cracking that occurs at high heat input weld angle joints of box columns used in high-rise buildings.

Claims (2)

[Claims] [Claim 1] C: 0.10 to 0.20%, Si:
0.05-0.50%, Mn: 0.50-1.80%,
P: 0.010% or less, S: 0.002% or less, Nb: 0.005 to 0.020%, Ti: 0.0
05-0.020%, Ca: 0.0008-0.0
035%, Al: 0.020 to 0.080%, and in the segregated part of the base material that is harder than the non-segregated part by HV30, A by the microscopic test method for nonmetallic inclusions in steel specified in JIS G0555. The measured amount of system inclusions is 0
．． 008% or less, and the remainder consists of unavoidable impurities. A steel plate that is less likely to cause HAZ cracking in a high heat input weld angle joint of a steel box column. 2. In addition to the above alloy component composition, Cu: 0.
05-0.50%, Ni: 0.05-0.50%, Cr
:0.05~0.50%, Mo:0.05~0.50%
, V: 0.005 to 0.050%, B: 0.0
2. The steel plate according to claim 1, wherein HAZ cracking is less likely to occur in a high heat input welded corner joint of a steel box column.