JPH0351780B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) This invention relates to tempered high tensile strength steel used for welded structures, especially tempered (quenched and tempered) steel for high heat input welding with a strength level of 50 Kgf/mm class ² or higher. It relates to improvements in tempered high-strength steels for welding, particularly useful in applications where stress relief annealing is performed via high heat input welding. In recent years, when manufacturing large welded structures, single-sided single-sided submerged arc welding, electrogas welding,
Alternatively, there is a growing trend to adopt automatic welding that uses high heat input, such as electroslag welding. (Conventional technology) 40Kgf/ conventionally used for welded structures
mm ² , 50Kgf/mm ² class unheated steel, and 50Kgf/mm ² class or higher quenched and tempered steel, when high heat input welding is performed on any of these, the structure of the weld heat affected zone, especially the weld bond, will deteriorate. is mainly composed of coarse upper bainite, and its toughness is significantly inferior.
It was difficult to perform high heat input welding. Since then, various steel compositions of this type suitable for high heat input welding have been developed and are now being put into practical use.These high heat input welding steels generally consist of Al, B,
REM and B or Ti and B are added in combination, and both improve the toughness of the weld heat affected zone by making the weld heat affected zone structure a ferrite-pearlite structure. The formation of ferrite/pearlite in the weld heat-affected zone structure occurs because B contained in the steel precipitates as BN (boron nitride) during cooling during the welding heat cycle, and the BN assists in the nucleation of ferrite. On the other hand, Al, REM and Ti are added in combination with B.
form precipitates and inclusions in steel, and these act to promote the precipitation of BN.Therefore, Al, REM, and Ti are added in combination to BN as components that have the same effect. In some cases, Ti forms TiN (titanium nitride) in steel,
As it has the same effect as BN, addition of Ti alone as well as in the presence of B is carried out in 40Kgf/mm ^2nd grade and 50Kgf/mm ^2nd grade non-thermal treated steels, but in quenching and quenching steels of 5Kgf/mm ^2nd grade and above. In tempered tempered steel, even when Ti is added from the viewpoint of utilizing the hardenability increasing effect of B, B
are often added in combination. In accordance with common usage, the rare earth metals collectively expressed in REM here mainly refer to Mitsushi metals. For these high heat input welding steels, it has recently been discovered that the stress relief annealing (hereinafter abbreviated as SR) treatment that is generally performed after welding poses a serious problem in practical use. In other words, for 40Kgf/mm ² and 50Kgf/mm ² class non-tempered steel, the deterioration in base material toughness due to SR treatment is 2mmV.
The amount of change in fracture surface transition temperature (ΔvTrs) in the Notchi Shape impact test is at most 20°C, which is not a problem, but on the other hand, in quenched and tempered tempered steel, the base material toughness deteriorates significantly due to SR treatment. , ΔvTrs exceeds 20℃, sometimes 80℃
It has been observed that deterioration of temperatures as high as 10°C has been observed. This phenomenon of significant base metal toughness deterioration is particularly noticeable in steels that have been subjected to quenching and tempering treatment, especially steels that have added B to enable high heat input welding. Is recognized. In order to reduce the deterioration of base material toughness due to this SR treatment to at least the conventional level and to a level that does not pose a practical problem, that is, within 20°C in terms of ΔvTrs, the P content is normally included in this type of steel. We have previously proposed the invention of Japanese Patent Application No. 56-156092 for a method of reducing the amount even further. However, in order to extremely reduce the P content to a level that can meet this problem, an increase in manufacturing costs is unavoidable, and other methods are more advantageous. (Problem to be Solved by the Invention) Therefore, it is proposed to advantageously avoid deterioration of base metal toughness by SR treatment of B-containing heat-treated steel for high heat input welding without drastically reducing the P content. For this purpose, the inventor conducted the following development experiment. In other words, we investigated the deterioration behavior of base metal toughness due to SR treatment for quenched and tempered high-strength steel for high heat input welding containing Al or Ti in combination with B.
By reducing B to 0.0010% by weight (hereinafter simply expressed as a percentage for each component in the steel) and adding 0.002% or more of REM, the base material toughness can be improved by stress relief annealing without drastically reducing P. It was discovered that it is possible to reduce the deterioration to a level that does not pose a practical problem (∆vTrs of ~20°C or less), and that it also has excellent high heat input welding characteristics. (Means for Solving the Problems) In other words, the main part of achieving the above object lies in limiting the structure of each invention as follows. (1) C: 0.03-0.22%, Si: 0.02-0.80%, Mn:
0.20~2.50%, B: 0.0003~0.0010%, P:
Contains 0.010 to 0.025% and N: 0.012% or less, along with at least one of 0.005 to 0.1% Al, 0.003 to 0.07% Ti, and 0.002 to 0.008% rare earth elements (REM), with the remainder being substantially (2) A composition that further contains 0.5% or less of Mo in addition to the basic component (3) A composition that further contains 0.1% or less of V, respectively, in addition to the basic component.
Composition containing Nb (4) In addition to the basic components, 1% or less Ni and 0.1%
A composition containing either the following V and 0.8% or less of Cr or 0.5% or less of Cu (reason for limiting the steel composition) The reason for limiting the steel composition in the present invention is as follows. C: 0.03-0.22% C is required at least 0.03% in order to obtain the strength required for this type of welded structural steel, and on the other hand, it reduces the susceptibility to weld cracking during high heat input welding and the heat affected zone during high heat input welding. The upper limit is set at 0.22% from the viewpoint of toughness. Si: 0.02 to 0.80% Si is required as a deoxidizing element and as an element that increases strength, and must be at least 0.02%, but if it exceeds 0.80%, the toughness of the base material will be impaired, so it should be kept at 0.80% or less. Mn: 0.20 to 2.50% Mn is required to be 0.20% or more in order to impart ductility and strength to the base metal, but if it exceeds 2.50%, weld hardenability will significantly increase, so it should be kept at 2.50% or less. B: 0.0003 to 0.0010% B is an element that improves the toughness of the heat affected zone of high heat input welding when it coexists with Al or Ti and REM, but on the other hand, when it coexists with P, it weakens the base material by SR treatment. It is also an element that deteriorates toughness. As already mentioned, the addition of REM significantly improves the toughness of the heat-affected zone during high-heat-input welding without the need to limit P and without deteriorating the base metal toughness due to SR treatment. One of the important points is the discovery of interactions of B that can be improved. In other words, Figure 1 shows the change in shear py absorbed energy after applying a thermal cycle equivalent to a high heat input welding (110 KJ/cm) bond part.
C: 0.13%, Si: 0.25%, Mn: 1.36%, P:
0.015%, S: 0.004%, Al: 0.025%, Ti: 0.015
%, Cr: 0.10%, Ni: 0.31%, with various amounts of B up to 0.002%, as well as a comparison steel with no REM added, compared to a test steel in which 0.005% REM was added. Under REM0.005% addition
While a remarkable improvement in bond toughness is seen with B of 0.0003% or more, B increases when REM is not added.
It can be seen that even when added up to 0.0008%, the bond toughness did not improve at all. Further, FIG. 2 shows the effect of the amount of B in the above-mentioned test steel on the change in base metal toughness after the SR treatment as the difference ΔvTs in vTs (fracture surface transition temperature) before and after the treatment. According to this figure, if B is 0.0010% or less, the deterioration of base material toughness due to SR treatment will not reach 120°C, and there will be no practical problem. However, if more than 0.001% of B is added, ΔvTs deteriorates and reaches a maximum of 80°C, so as already mentioned, countermeasures such as extremely lowering P are required. According to these facts, B is limited to 0.0003-0.0010%. P: 0.010-0.025% P is an impurity element that is inevitably mixed into steel, and is generally 0.035% for this type of welded structural steel.
It is often specified at ~0.040% or less. However, in practical steel materials, it is common for the content to be around 0.010 to 0.025%, so we have taken measures that do not require reducing the amount of P.
It is sufficient to simply keep it below 0.025%, and there is no need for special measures to reduce it below 0.010% beyond the level that can be reduced by the process normally adopted for this type of steel. N0.012% N is contained in the normal steelmaking process, but 0.012%
If it exceeds 0.012%, it will impair the toughness of the base metal and the heat-affected zone of low heat input welding, so it should be limited to 0.012% or less. Al: 0.005-0.010%, Ti: 0.003-0.07% Both Al and Ti are elements that improve toughness by converting the heat-affected zone structure of high heat input welding into ferrite/pearlite when combined with B. . The lower limit of their content is determined by the minimum content at which their effects are exhibited.If they are included in large amounts, they will not only damage the heat-affected zone of high heat input welding but also the toughness of the base metal. An upper limit is specified. For Al, their content range is
0.005 to 0.1%, and 0.003 to 0.07% for Ti. Even if only one of these Al and Ti is added in combination with B, the toughness of the heat-affected zone of high heat input welding can be sufficiently improved, but even if Al, Ti, and B are added in combination, the The affected area toughness is further improved. In addition
In addition to the above-mentioned main effects, Al and Ti have a secondary effect of improving the toughness of the base material by refining the crystal grains, and both of these effects are fully exhibited within the above-mentioned content ranges. After adjusting the ingredients to the above range, REM is added.As shown in Figure 1, this REM coexists with B and improves the toughness of the heat-affected zone during high heat input welding. When P and P are present, the deterioration of base material toughness caused by SR treatment is significantly suppressed. This was mentioned earlier in Figure 2, but it should be noted that if B exceeds 0.001%, the effect will not be exhibited. Regarding this effect, especially for B, from 0.0003 to
As shown in Figure 3, the influence of the amount of REM in the above sample steel set at 0.0006% is 0.002%, while the lower limit is 0.008% so as not to impair the base metal toughness.
limited within %. Cr: 0.8% or less, Ni: 1% or less, Mo: 0.5% or less,
Cu: 0.5% or less and V, Nb: 0.1% or less Cr, Ni, Mo, and Cu are all based on hardenability enhancement and solid solution strengthening effects, and V and Nb are based on precipitation strengthening effects. However, the addition of these various elements has the common effect of increasing the strength of the base material, and therefore, at least one of them is used for each group, but for the reasons shown below, each addition is An upper limit on the amount is specified. When Cr exceeds 0.8%, it increases welding sensitivity. Ni is an expensive element and is limited to 1% or less in this type of steel for economical reasons. When Mo exceeds 0.5%, it impairs the toughness of the base metal and weld heat affected zone. If Cu exceeds 0.5%, welding susceptibility increases. When V and Nb exceed 0.1%, they impair the toughness of the base metal. Note that when B and P coexist, the phenomenon in which the base metal toughness is significantly degraded by SR treatment occurs only in quenched and tempered type tempered steel, so the above countermeasures are for quenched and tempered type tempered steel. Limited to steel.
However, it goes without saying that the quenching mentioned here includes not only normal quenching but also direct quenching immediately after rolling. Furthermore, even though the above-mentioned steel is used for high heat input welding, low heat input welding such as tack welding may be performed.
The requirement for excellent low heat input weldability can also be satisfactorily met. (Example) Steel with the composition shown in Table 1 was melted by high-frequency vacuum melting, made into a 100Kg steel ingot, and steel plates other than Steel 5, 12, and 13 were hot-rolled to a thickness of 20mm, and then heated at 930℃. Hardening treatment and tempering treatment at 640℃ were performed. Steel 5, 12, and 13 are hot rolled to 20mm thick.
Finished at 950℃, immediately quenched (direct hardening), and then tempered at 640℃. For these steel plates, () test specimens were subjected to a thermal cycle equivalent to the weld bond part of submerged arc welding with a heat input of 110 KJ/cm to examine high heat input weldability, and () stress relief annealing was applied using a welding thermal cycle reproduction device. In order to investigate the degree of deterioration of base metal toughness due to annealing, JIS No. 4 Sharpie impact test specimens were taken from test specimens before and after annealing at 580°C for 3 hours. The absorbed energy vE−20 was investigated, and the fracture surface transition temperature (vTs) was determined for the specimen in (), and the difference ΔvTs between vTs before and after annealing (vTs− after SR treatment) was determined.
vTs) before SR treatment. The results are shown in Table 2.

【table】

【table】

【table】

【table】

[Table] Steels 1 to 10 and 17 to 19 shown in Table 1 are all
In particular, Steels 1 to 5 are invention steels that do not contain Cr, Ni, Mo, Cu, V, and Nb, and satisfy the composition conditions of the specific invention. Steels 6 and 8 do not contain REM, and Steel 7 has a B content that meets the requirements. Steel 9 is a conventional steel with a composition that does not take into account high heat input weldability, and Steel 10 is a comparison steel that is outside the range, and Steel 10 has P extremely reduced as a countermeasure for deterioration of base metal toughness due to SR treatment. Conventional steel and steel 17,
18, the REM amount is outside the compatible range, and the steel
Steel No. 19 is a comparison steel in which both the REM and B amounts are outside the compatible range. As is clear from the results in Table 2, Invention Steels 1 to 5 have excellent high heat input weldability, and the deterioration of base metal toughness due to stress relief annealing treatment was evaluated by ΔvTs and was only 20°C or less in all cases. It turns out that it is something. On the other hand, Comparative Steels 6 to 8 all have high heat input weldability as excellent as the invention steel, but the deterioration of base metal toughness due to SR treatment is large with ΔvTs of 70°C or more. Furthermore, conventional steel 9 shows little deterioration in base metal toughness due to SR treatment, but vE-20 is low because no countermeasures are taken for high heat input welding. In addition, conventional steel with extremely low P10
Although it is excellent in both vE-20 and ΔvTs, it is less economical because it requires a special melting method to achieve extremely low P. Steels 11 to 15 in Table 1 all contain Cr, which is a reinforcing component,
Steel 16 is an invention steel containing at least one of Ni, Mo, and Cu as well as V and Nb, and Steel 16 is a comparison steel whose B content is outside the conforming range. As is clear from Table 2, invention steels 11 to 15 have excellent high heat input weldability, and the deterioration of base metal toughness due to SR treatment is small, whereas comparison steel 16 has high heat input properties due to excessive B content. is excellent, but the toughness of the base material deteriorates significantly due to SR treatment. Steels 17 and 18 have a large amount of REM, so the toughness of the high heat input bond is poor. Steel 19 has a large amount of both REM and B, so not only the toughness of the high heat input bond part but also the base material toughness deteriorates significantly after SR. As shown above, this steel is extremely useful as a heat-refining type high heat input welding steel with less deterioration of base metal toughness during SR treatment, which is much more advantageous than low P steel that requires special melting methods. .

[Brief explanation of drawings]

Figure 1 is a correlation graph between B content and high heat input weld bond properties, Figure 2 is a graph showing the effect of B content on the deterioration of base metal toughness due to SR treatment, and Figure 3 is REM content. FIG. 2 is a graph showing the influence of

Claims (1)

[Claims] 1 C: 0.03 to 0.22% by weight Si: 0.02 to 0.80% by weight Mn: 0.20 to 2.50% by weight B: 0.0003 to 0.0010% by weight P: 0.010 to 0.025% by weight and N: 0.012% by weight or less , 0.005~0.1 wt% Al, 0.003~0.07 wt%
Contains at least one type of Ti and 0.002 to 0.008% by weight of rare earth elements (REM)
A tempered high-strength steel for high heat input welding that exhibits little deterioration in toughness due to stress relief annealing, and is characterized by a composition that contains iron and the remainder consists essentially of iron and unavoidable impurities. 2 C: 0.03-0.22 wt% Si: 0.02-0.80 wt% Mn: 0.20-2.50 wt% B: 0.0003-0.0010 wt% P: 0.010-0.025 wt% and N: 0.012 wt% or less, 0.005-0.1 wt% Al, 0.003-0.07% by weight
Contains at least one type of Ti and 0.002 to 0.008% by weight of rare earth elements (REM)
and 0.5% by weight or less of Mo, with the remainder consisting essentially of iron and unavoidable impurities, and is a tempered high-strength steel for high heat input welding that exhibits little toughness deterioration due to stress relief annealing. . 3 C: 0.03-0.22% by weight Si: 0.02-0.80% by weight Mn: 0.20-2.50% by weight B: 0.0003-0.0010% by weight P: 0.010-0.025% by weight and N: 0.012% by weight or less, 0.005-0.1% by weight Al, 0.003-0.07% by weight
Contains at least one type of Ti and 0.002 to 0.008% by weight of rare earth elements (REM);
High heat input welding with less deterioration in toughness due to stress relief annealing, characterized by a composition that contains less than 0.1% by weight of V and less than 0.1% by weight of Nb, with the remainder essentially consisting of iron and unavoidable impurities. Tempered high tensile strength steel. 4 C: 0.03-0.22% by weight Si: 0.02-0.80% by weight Mn: 0.20-2.50% by weight B: 0.0003-0.0010% by weight P: 0.010-0.025% by weight and N: 0.012% by weight or less, 0.005-0.1% by weight Al, 0.003-0.07% by weight
Contains at least one type of Ti and 0.002 to 0.008% by weight of rare earth elements (REM);
Contains 1% by weight or less of Ni and 0.1% by weight or less of V,
Furthermore, 0.8% by weight or less of Cr or 0.5% by weight or less of Cu
A heat-treated high-strength steel for high heat input welding that exhibits little deterioration in toughness due to stress relief annealing, and is characterized by a composition containing one of the following, with the remainder consisting essentially of iron and unavoidable impurities.