JPH0126797B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for welding high tensile strength steel, and particularly to a method for welding high tensile strength steel having a tensile strength of 80 Kg/mm ² class or higher without preheating. In recent years, steel structures have become increasingly larger, and the demand for high-strength steel has increased significantly year by year in order to reduce weight, manufacturing costs, and manufacturing time. 60Kg/mm ² class (hereinafter Kg/mm ² class is abbreviated as kilo class) high tensile strength steel is widely used in various fields. However, the 80kg class high-strength steel currently on the market requires the inclusion of a large amount of alloying elements to ensure high strength, resulting in a high P _CM (described later) and extreme susceptibility to weld cracking. worsens.
Of course, preheating the base material before welding is an effective way to prevent cold cracking during welding, but
Kilo-class high-tensile strength steel requires a considerably higher preheating temperature than 60-kg high-strength steel, and is currently only used in very few applications due to difficulties in welding workability. Under these circumstances, various measures to prevent cold cracking during welding have been studied, and the following methods have been proposed, for example. As for welding materials, we have reduced the amount of hydrogen and made them resistant to moisture absorption. Relaxation of restraint stress in the design and construction of structures. For steel materials, reduction of weld hardenability. However, even with these methods, preheating to a considerably high temperature is essential for welding 80 kg class high-strength steel, and no technology has been proposed that can completely prevent weld cracking without preheating. The present inventors have been conducting various researches for some time in the hope of developing a method that would be able to weld 80 kg class high-strength steel without preheating and without causing weld cracking, and in particular, the composition of both the steel material and the welding material. I thought that the above purpose could be achieved by identifying the
We have been conducting research along these lines. As a result, I learned that the above objectives could be achieved brilliantly by adjusting the chemical composition of the steel material as shown below, and by using welding materials with a limited chemical composition as shown below. The present invention was finally completed. That is, the composition of the welding method according to the present invention is as follows: C: 0.04 to 0.09% (weight %: the same applies hereinafter), Si:
0.55% or less, Mn: 0.6-1.4%, Cr: over 0.3% 1.0%
Below, Mo: more than 0.3% and 0.8% or less, V: 0.1% or less,
B: 0.0005-0.002%, Al: 0.04-0.1% and N:
0.002 to 0.004% is an essential component, the remainder is iron and unavoidable impurities, and P _CM calculated by the following formula [] is 0.25% or less, and the quenching temperature (T
Tensile strength 80 obtained by quenching and tempering steel whose relationship between Al content and N content in 〓) satisfies the following formula []
80Kg/mm ² with excellent weld cracking susceptibility over Kg/mm ²
Using grade high tensile strength steel as the base material, []...P _CM = C (%) + 1/30Si (%) + 1/20Mn
(%) +1/20Cu (%) +1/60Ni (%) +1/20Cr (%) +1/15Mo (%) +1/10V (%) +5B (%) The following composition: CaCO ₃ 40-60%, CaF ₂ 10-25%, Mn 3-5%,
Low-strength steel for high-strength steel made by coating a mild steel core with a coating material containing 14-8% Ni, 0.5-2.0% Cr, and 0.5-2.0% Mo, and having a potential hydrogen content of 40PPm or less at 350-2000℃. The gist is that the base metal is welded using a hydrogen-based coated arc welding rod. In this case, the performance of the welded part can be further improved by using 80 kg class high tensile strength steel containing Cu: 0.5% or less and/or Ni: 1.5% or less in addition to the above-mentioned essential components. The structure, operation and effect of the present invention will be explained below separately for steel materials and welding materials, but the following does not limit the present invention, and may be implemented with appropriate changes in accordance with the spirit of the above and below. All of these are within the scope of the present technology. First, steel materials will be explained. The present inventors believe that in order to achieve the above-mentioned purpose,
We believe that it is necessary to improve the weld cracking resistance and susceptibility of 80 kg class high tensile strength steel, and as a guideline we use the 60 kg class high tensile strength steel, which is currently relatively widely used, as a standard, and develop products with equivalent or superior weld cracking susceptibility. Research was conducted with the aim of developing 80 kg class high tensile strength steel. By the way, weld cracking susceptibility depends on the maximum hardness of the steel material and P _CM
It is known that they are inversely proportional to each other. Therefore 60
In order to ensure weld cracking resistance and susceptibility comparable to that of kg-class steel, it is necessary to ensure maximum hardness and P _CM of 80 kg-class steel comparable to that of 60 kg-class steel. By the way, the maximum hardness of conventional 60 kg class steel is Hv360 or less.
Since P _CM is 0.25 or less, in order to achieve the above objective, it is necessary to suppress the maximum hardness and P _CM to the same level or lower than the above. On the other hand, maximum hardness is closely related to C content,
It is also known that in order to lower the maximum hardness, the C content can be reduced. For example, in Figure 1,
This is a graph showing the results of maximum hardness tests conducted on prototype steels with various C contents.From this result, in order to keep the maximum hardness below Hv360, the C content should be kept below 0.09%. I understand that. But C
If the content is reduced, the hardness of the steel material will inevitably decrease, so in order to maintain a high tensile strength of about 80 kg class, the amounts of other alloying elements must be increased. Based on the above knowledge, the present inventors have developed an 80 kg class high tensile strength steel that has a weld cracking susceptibility equal to or higher than that of 60 kg class high tensile strength steel, that is, the highest hardness steel.
With the goal of developing steel materials with Hv360 or less, P _CM of 0.25% or less, and of course tensile strength of 80Kg/mm ² or more, we first investigated the effects of alloying elements, especially B, on the hardenability of low carbon steel. We considered this. It is generally known that the hardenability multiple F _B of B increases as the C content decreases, as shown in equation (A). F _B = 1 + 1.5 (0.9 - C%) ... (A) However, these relationships have been confirmed only in regions where the amount of C exceeds 0.10%, and the incineration of B in the low C region below this has been confirmed. The incidence multiple has only been estimated. However, based on the results of geominy edge quenching tests conducted on steel materials with various C contents, the relationship between the C content and the hardenability multiple of B was determined, and the results showed the trends shown in Figure 2. In particular, it was found that the hardenability multiple of B in the region where the C content is 0.10% or less shows a value higher than the tendency estimated from the above equation (A) (dashed line in the figure), although there is considerable variation. . The reason why such a result was obtained is expected to be that in the low C region, the precipitation of the B-C compound is significantly delayed and the amount of precipitation at the γ grain boundaries is significantly reduced. However, as is clear from FIG. 2, the variation in the hardenability multiple of B is extremely large, so in order to reliably obtain the effect of B, it is necessary to reduce the variation as much as possible. Therefore, the present invention has proceeded with the study of specific measures to reduce the above-mentioned variations. When the hardenability multiple of B for materials with a C content of 0.10% or less was organized in relation to the N content, the results shown in Figure 3 were obtained, and by regulating the N content, A new fact was confirmed that the variation in the hardenability multiple of B becomes extremely small, and that the hardenability multiple improves significantly in the region where the N content is 40 PPm or less. The reason for this has not been completely elucidated, but in regions with low N content, the amount of BN and AlN precipitated at grain boundaries is small, and ferrite transformation during cooling is significantly delayed and martensite is formed. it is conceivable that. Incidentally, Al is a useful element as a deoxidizing and crystal grain adjusting element, but as the inventors have confirmed through experiments,
In addition to the above-mentioned effects, Al has an extremely important effect of stably securing the amount of solid solution B necessary for improving hardenability in relation to the B and N contents. In order to effectively exert these effects, at least
It was necessary to contain 0.04% or more. But Al
Even if the amount exceeds 0.10%, the above effects will reach a saturated state and will not improve any further.In fact, the amount of alumina-based inclusions will increase and the toughness and properties in the thickness direction will deteriorate significantly, so 0.10% It should be kept below. In addition, as mentioned above, Al has a close relationship with the N content that affects the hardenability multiple of B, and in order to ensure a satisfactory hardenability multiple, it is necessary to adjust the quenching temperature (T The components must be adjusted so that the Al content and N content in 〓) satisfy the above formula []. That is, in the present invention, N and Al in low C steel
It is possible to obtain 80kg-class high-strength steel with good hardenability by adjusting the content of Al to within a specified range and creating a certain relationship between the amount of Al and the amount of N at the quenching temperature. can. For example, Figure 4 shows the necessary range of Al and N content when the quenching temperature is 900°C, and the shaded range is essential for obtaining steel that meets the purpose of the present invention. It is a requirement. The quenching temperature can be determined appropriately depending on the composition of the steel material, but the most preferable temperature is (Ac ₃ +50
The temperature is ℃). Next, we will provide a supplementary explanation of the reason for limiting the composition range of the steel material as described above. It is desirable that C be as small as possible from the viewpoint of weld hardenability, and in order to obtain the maximum hardness of Hv360 or less, it should be suppressed to 0.09% or less. However, in 80 kg class high-strength steel, when the C content is less than 0.04%,
Since it will not be possible to obtain the quenching hardness necessary to ensure a tensile strength of 80Kg/mm2 or ^more ,
% or more. In the case of this type of killed steel, Si is an element that is inevitably included in the deoxidation process, and also has the effect of increasing strength. However, too much content impairs weldability and toughness, so the upper limit was set at 0.55%. Mn is an essential element to ensure the strength and toughness required for 80 kg class high tensile strength steel, and in order to exhibit these effects significantly, Mn must be at least 0.6
% or more. However, if the Mn content is too high, problems will appear in terms of micro-segregation and weldability, so it should be kept below 1.4%. Cr has the effect of increasing hardenability, and has a value of 80Kg/mm ²
In order to secure tensile strength of at least
It is necessary to contain more than 0.3%. However, if it is too large, weldability deteriorates, so it must be kept below 1.0%. Mo has the effect of further increasing the hardenability of the steel material and the resistance to temper softening, and it is necessary to add more than 0.3% to ensure a tensile strength of 80 Kg/mm ² or more. However, if it is contained in a large amount, the weld heat-affected zone will harden, the weld cracking resistance will deteriorate, and the cost will also increase, so it should be kept at 0.8% or less. When added in a small amount, V has the effect of increasing temper softening resistance and strength, but if it is added too much, the toughness will deteriorate significantly, so it should be kept at 0.1% or less. B has the effect of significantly increasing the hardenability of steel even when added in a very small amount, and in order to exhibit these effects significantly, it must be contained in an amount of 0.0005% or more.
However, if the content exceeds 0.002%, a large amount of B-C compounds will be generated at the austenite grain boundaries during quenching, resulting in deterioration of hardenability and toughness. Al should be in the range of 0.04-0.1% for the reasons detailed above. N is generally considered to be a harmful element for B-containing steel, but as mentioned above, in steel materials with a low N content of 0.004% or less, the hardenability factor of B is significantly improved. and
Even if it is less than 0.002%, hardenability will further improve due to the low N effect, but since the amount of AlN that contributes to the refinement of γ grains will be insufficient, the γ grains will become coarser and the toughness will decrease, so if it is contained at least 0.002% or more. It is necessary to do so. Cu has the effect of increasing strength when added in a small amount, but if it is added in excess of 0.5%, no further improvement in strength is seen, and on the contrary, it deteriorates weldability and surface quality, so it is less than 0.5%. should be stopped. Ni has the effect of increasing strength and toughness, but
Their effect reaches a saturation level at approximately 1.5%, and addition beyond that level is undesirable because it only increases cost. Note that P _CM is used as an index for estimating the preheating temperature required to prevent weld cracking. For conventional 60 kg class high strength steel, this P _CM is typically 0.25
% or less, and in order to improve the welding conditions of 80 kg class high tensile strength steel to the same level as 60 kg class steel, this P _CM should be used.
It is necessary to keep it below 0.25%. As described above, in the present invention, the chemical components of 80 kg class high tensile strength steel, especially the content of N and B, the relationship between the content of N and Al at a given quenching temperature, and P _CM etc. are adjusted appropriately. Therefore, it is possible to obtain a weld cracking susceptibility comparable to or exceeding that of 60 kg class high-strength steel, but even when using this steel, there are certain steps required to prevent weld cracking from occurring without preheating. It is necessary to use a coated arc welding rod having the following composition, and the final objective of the present invention can only be achieved by a combination of the two. That is, in the present invention, CaCO ₃ 40
~60%, _CaF2 10~25%, Mn3~5%, Ni4~8
%, including Cr0.5~20%, Mo0.5~20% and 350
A low-hydrogen coated arc welding rod for high-strength steel must be used, in which a mild steel core is coated with a coating material that has a potential hydrogen content of 40 PPm or less in the temperature range of ~2000°C. The reason for the above numerical limitation is as follows. Ni has the effect of increasing the notch toughness of the weld metal and must be contained at least 4% or more.
Moreover, if it exceeds 8%, the toughness on the high temperature side decreases and weld cracking becomes more likely to occur, so it should be kept below this. If Mn is contained in an amount of 3% or more, it is effective as a deoxidizing agent and in improving the strength and toughness of the weld metal and in preventing hot cracking, but if it exceeds 5%, the toughness deteriorates and cold cracking becomes more likely to occur. Cr and Mo contribute to improving the strength of weld metal. Since the weld metal has a strength of 80 kg class, it is necessary to add 0.5% or more of each, but if too much, the toughness and cracking resistance tend to decrease.
% or less is preferable. Furthermore, what is extremely important in the present invention is that
Potential hydrogen content in the temperature range of 350 to 2000℃ (Generally, welding rods are dried at about 350℃,
This is the measured value of the amount of hydrogen generated when a dry welding rod is heated to a high temperature of 2000℃. )40PPm
This is something that must be done. That is, the present inventors used 80 kg class high-strength steel (steel shown in Table 1 A) that meets the requirements of the present invention as a base material, and recycled it at 350°C as a coating material for an 80 kg class low hydrogen-based coated arc welding rod. Items with different potential hydrogen amounts after drying ((2nd
JIS Z 3158 diagonal Y-shaped weld cracking test was conducted using (Table). Figure 5 shows the observation results of the root cracking rate at 25°C. As is clear from the results shown in Figure 5, the root cracking rate decreases rapidly when the potential hydrogen amount reaches a boundary of about 80 PPm, and becomes zero when the potential hydrogen amount becomes 40 PPm or less. From these results, in the present invention, it is essential to specify the type and content of elements contained in the coating material and to reduce the potential hydrogen amount to 40 PPm or less. As a means to reduce the amount of potential hydrogen in the coating material to 40 PPm or less, carefully select raw materials with low potential hydrogen (for example, raw materials that do not contain water of crystallization), raw materials with low hygroscopicity, and binders, and Examples include methods for appropriately controlling temperature and time. The present invention is roughly constructed as described above, and by using 80kg class high tensile strength steel with a specific composition and a specific 80kg class coated arc welding rod, no preheating is required as will be clear from the following examples. This makes it possible to perform welding without weld cracking, and provides an ideal technique for welding 80kg class high-strength steel. Next, examples of the present invention will be shown. Example 1 An 80 kg class steel plate having the composition shown in Table 1 was used as the base material, and a mild steel core wire was coated with a coating material having the composition shown in Table 2 (inventive bars 1 and 3, conventional bars 1 and 5). A diagonal Y-shaped welding cracking test was conducted in accordance with JIS Z 3158 using a coated arc welding rod made of the following materials. The results are summarized in Table 3.

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Claims (1)

[Claims] 1 C: 0.04 to 0.09% (weight %: the same applies hereinafter), Si:
0.55% or less, Mn: 0.6-1.4%, Cr: over 0.3% 1.0%
Below, Mo: more than 0.3% and 0.8% or less, V: 0.1% or less,
B: 0.0005-0.0020%, Al: 0.04-0.1% and N:
0.002 to 0.004% is an essential component, the remainder is iron and unavoidable impurities, and P _CM calculated by the following formula [] is 0.25% or less, and the quenching temperature (T
Tensile strength obtained by quenching and tempering steel whose relationship between Al content and N content in 〓) satisfies the following formula []
80Kg/mm ² or more with excellent weld cracking resistance and susceptibility
mm Class ² high tensile strength steel is used as the base material, []...P _CM = C (%) + 1/30Si (%) + 1/20Mn
(%) +1/20Cu (%) +1/60Ni (%) +1/20Cr (%) +1/15Mo (%) +1/10V (%) +5B (%) The following composition: CaCO ₃ 40-60%, CaF ₂ 10-25%, Mn 3-5%,
For high-strength steel made by coating a mild steel core with a coating material containing 4 to 8% Ni, 0.5 to 2.0% Cr, and 0.5 to 2.0% Mo, and having a potential hydrogen content of 40 PPm or less at 350 to 2000°C. A method for welding high-strength steel, the method comprising welding the base metal using a low-hydrogen coated arc welding rod. 2 C: 0.04 to 0.09% (weight%: the same applies below), Si:
0.55% or less, Mn: 0.6-1.4%, Cr: over 0.3% 1.0%
Below, Mo: more than 0.3% and 0.8% or less, V: 0.1% or less,
B: 0.0005-0.002%, Al: 0.04-0.1% and N:
In addition to 0.002-0.004% as an essential component, Cu: 0.5
% or less and/or Ni: 1.5% or less,
Furthermore, the remainder is iron and unavoidable impurities, P _CM calculated by the following formula [] is 0.25% or less, and the relationship between the amount of Al and the amount of N at the quenching temperature (T〓) satisfies the formula [] below. The base material is 80Kg/mm class ² high tensile strength steel, which has a tensile strength of 80Kg/mm2 or ^more and has excellent weld cracking resistance and susceptibility, obtained by quenching and tempering the steel that satisfies the requirements []... P _CM = C (%) + 1/30Si (%) + 1/20Mn
(%) +1/20Cu (%) +1/60Ni (%) +1/20Cr (%) +1/15Mo (%) +1/10V (%) +5B (%) The following composition: CaCO ₃ 40-60%, CaF ₂ 10-25%, Mn 3-5%,
For high-strength steel made by coating a mild steel core with a coating material containing 4 to 8% Ni, 0.5 to 2.0% Cr, and 0.5 to 2.0% Mo, and having a potential hydrogen content of 40 PPm or less at 350 to 2000°C. A method for welding high-strength steel, the method comprising welding the base metal using a low-hydrogen coated arc welding rod.