JPH0137470B2 - - Google Patents

Description

[Detailed description of the invention]

Technical field Welded parts, especially weld heat affected zones (HAZ) in high-strength steel, for welded structures that are processed and assembled as welded structures and then immersed in a hot-dip galvanizing bath to galvanize them. Cracking susceptibility due to hot-dip galvanizing (also known as GCS)
The technical content described in this specification regarding the reduction of
In particular, it is concerned with new proposals based on development results aimed at significantly improving GCS resistance without hindering productivity, especially for high-strength steels with a tensile strength of 60 Kgf/mm ² or higher. BACKGROUND ART Hot-dip galvanizing is useful from the viewpoint of rust prevention or aesthetics, particularly paintability, of steel structures, and has been widely used in the past. In recent years, due to the strong demand for larger and lighter steel structures, there has been a growing trend to increase the strength of steel materials for welded structures that should be subjected to the above-mentioned plating treatment. In particular, when steel materials with a strength exceeding 60Kgf/ ^mm2 are subjected to galvanizing treatment by immersion in a molten zinc bath after rough welding,
The tendency of frequent intergranular cracking, called plating cracking or liquid metal embrittlement, in HAZs is especially remarkable. This is because as welded structures become larger and more complex, the residual stress that accompanies welding becomes more significant for materials with higher tensile strength, which is one of the factors that promotes the above-mentioned cracking. It is. Therefore, in order to reduce GCS resistance, consideration must be given to welding conditions and immersion conditions to reduce residual stress during welding and thermal stress during immersion in a hot-dip galvanizing bath. Although removal annealing treatment has been carried out, it cannot be said to be a fundamental solution because it is not economical or impairs the properties of the base material, which is why the steel material itself needs to be improved. Prior art and its problems Regarding considerations regarding the composition of welded structural steel that requires GCS resistance, 0.005 to 0.05wt is discussed in both Japanese Patent Application Laid-Open No. 57-104656 and No. 58-84959.
% (hereinafter simply expressed as %) of Ti alone or 0.005~
It has been disclosed that the GCS resistance is improved due to the composite content of 0.03% Ti and 0.02 to 0.09% V, but as is well known, such Ti-containing steel is difficult to use when using the continuous casting method. Surface defects called so-called star cracks frequently occur on the surface of the cast surface, which not only poses a disadvantage in the process of requiring surface care prior to hot rolling, but also causes problems in that the yield inevitably decreases due to such care. I'm leaving it behind. Purpose of the Invention In a composition that does not have the above-mentioned disadvantages associated with Ti addition, it is possible to improve the GCS resistance of high-tensile welded structural steel, especially with a tensile strength of 60 Kgf/mm ² or more.
It is an object of the invention to advantageously improve this. Structure of the Invention The above object is advantageously satisfied under the composition adjustment based on the following matters. That is, C: 0.08~0.15wt%, Si: 0.10~
0.50%, Mn: 1.00~2.00%, Zr: 0.012~0.030%,
sol Al: 0.005 to 0.100%, Nb: 0.01 to 0.10% and V: 0.01 to 0.10% in the relationship of the following formula 265−293C−21Si−96Mn−370V −410Nb−35Al+534Zr≧40…(1), The remainder is the composition of Fe and impurity elements. In practical development of this invention, 0.05~
Ni, Cu, Cr and Mo as steel toughening components within the range of 1.00%, and 0.0005 to 0.0040%
B is a reinforcing component that enhances hardenability, and
It is more desirable to select 0.0005 to 0.0050% of Ce and Ca from among the above groups and add them to the steel ingredients as toughness improving ingredients by suppressing grain growth or controlling inclusion morphology. In this case, the influence of each component on GCS resistance is expressed by the following equation. 265−293C−21Si−96Mn−370V−410Nb −35Al−18Ni−128Cu−94Cr−90Mo−2454B +534Zr+1429Ce≧40…(2) When Cu is contained in a relatively large amount within the above range, it is combined with Ni. It is more desirable to do so. The key point of this invention is that it can withstand Zr in the range of 0.012 to 0.030%, especially in the composition of ingredients that can ensure a tensile strength level exceeding 60Kgf/ ^mm2 .
This comes from the knowledge that its inclusion as a component to improve GCS properties is compatible with the intended purpose. By the way, regarding Zr, in the former of the above-mentioned prior art, it is used as an element to control the form of inclusions up to 0.15%.
Although it is mentioned that Zr can be used in combination with Ti to the extent that it does not impair cleanliness, there is no mention of the direct influence or contribution of Zr on the above-mentioned GCS resistance. In this invention, the technical basis for limiting the composition of the GCS-resistant high-strength steel is as follows. C: 0.08-0.15% C is a component that most easily helps increase the strength of steel, and if it is less than 0.08%, the effect is not expected, while if it exceeds 0.15%, weldability decreases and it is not suitable for the purpose. The range is set at 0.08 to 0.15%. Si: 0.10-0.50% Si is required to be 0.10% or more to utilize deoxidizing effect and contribute to strength, but if it exceeds 0.5%, it will have a sub-effect on GCS resistance and will also cause low-temperature dust. 0.10 to 0.50% as it tends to deteriorate
shall be. Mn: 1.00 to 2.00% Mn also requires a minimum content of 1.00% to ensure strength. On the other hand, if it exceeds 2.00%, it will not only impair GCS resistance but also impair basic performance such as weldability and workability, so it should be 1.00 to 2.00%.
Limit to 2.00% range. Zr: 0.012-0.030% Zr helps to ensure GCS resistance under tensile strength of 60 Kgf/mm ² or more according to the present invention without the disadvantages mentioned in the description of the prior art It is an important component, and when the content is 0.012% or more, it has a new remarkable effect on improving the GCS resistance under the refinement of the HAZ structure, but when it exceeds 0.030%, it affects the cleanliness of the steel and deteriorates the mechanical properties. In particular, since it has the disadvantage of deteriorating toughness, it is necessary to limit it to a range of 0.012 to 0.030%. sol Al: 0.005 to 0.100% Al is 0.005 for deoxidizing effect and improving hardenability.
% or more is required, but if it exceeds 0.100%, GCS resistance
Since it causes a decrease in weldability, it is set at 0.005 to 0.100%. Nb and V: 0.01 to 0.10% Nb and V require 0.01% to increase strength, but exceeding 0.10% is accompanied by a decrease in GCS resistance and weldability, so 0.01 to 0.10%, respectively. do. Useful as a toughening component to more advantageously achieve high tensile strength as a premise for improving GCS resistance of this invention without deteriorating GCS resistance.
Ni, Cu, Cr and Mo all require at least 0.05% or more of one kind, but Ni, Cu and Cr
Mo
In addition, the enhancement of the effect of sizing the austenite grains during rolling and strengthening them reaches saturation when added in excess of 1.00%, which is economically disadvantageous, so both should be limited to 1.00%. Next, B contributes to increasing hardenability in small amounts of about 0.0005% or more, but if it exceeds 0.0040%, the effect is saturated and the hardening of the HAZ becomes severe.
A range of ~0.004% is useful. Ce is 0.0005%, which has the effect of suppressing oxysulfide and grain growth or controlling the morphology of sulfides.
It is necessary to contain more than 0.0005%, but if it exceeds 0.0005%, the cleanliness may deteriorate and the mechanical properties may be impaired.
Limited to the range of 0.0005-0.0050%. Next, S and P as impurities in steel are allowed to be within 0.020% and 0.030%, respectively, but N in particular is preferably as low as 0.012% or less, as toughness deteriorates significantly when it exceeds 0.012%. Molten steel whose composition has been adjusted according to formula (1) or (2) within the above composition range is obtained by ordinary steelmaking means, and then rolled into slabs, blooms, or billets by the ingot-forming method or continuous casting method. The high tensile strength steel of the present invention can be manufactured by processing according to conventional methods. This structural steel can be hot-dip galvanized after welding assembly, and the following describes how to avoid the cracking that tends to occur in HAZ. Now, the cracking properties of hot-dip galvanizing were evaluated as follows. The test pieces were made by cutting 10mmφ round bars from steel plates with various chemical compositions shown in Table 1 and heating them with high frequency.
After applying a heat cycle equivalent to a welding heat input of 15 KJ/cm, a circumferential notch was processed.

【table】

[Table] * ○: No cracks ×: Cracks Only the notch part of each test piece was galvanized.
Various static load stresses were applied at 470°C, when the zinc is in a molten state, and the time required for the zinc to break under the stress was measured. Ratio of the applied stress at this time to the tensile strength at 470℃ of the unplated test piece (expressed as a percentage)
The relationship between R〓 and the rupture time is shown in Figure 1. Here, R〓 is a parameter that indicates what percentage of the tensile strength of an unplated test piece was maintained at 470°C until it broke.
is at least 40%, and the higher the value, the more difficult it is to crack even when residual stress or thermal stress is large. Considering the immersion time in the plating bath in actual operation, find the value of R〓 at 400 seconds and perform multiple regression with the chemical components, (1),
The coefficients of each term on the left side of equation (2) were determined. On the other hand, in order to deal with cracking during the plating process after actual welding, a restraint joint as shown in Figure 2 was manufactured.
After immersion in a hot-dip galvanizing bath, cracks in the heat-affected zone were examined. 1-1 and 1-2 in the figure are test beads, and 2-
1 and 2-2 are restraining beads, and 3 is a test plate. The test plate 3 was assembled by assembling plates with one side ground in a cross pattern so that the fillet bead 1-1 was between the ground surfaces and the fillet bead 1-2 was between the black skins. The dimensions of the test plate are 15 mm thick, the length of the test bead L = 50 mm, and the length of each cross overhang l ₁ = l ₂
= l ₃ = l ₄ = 50 mm, and total length W of test plate 3 = 150 mm
It is. The number of each of the restraining beads 2-1 and 2-2 is 20. This constrained joint was immersed in a hot-dip galvanizing bath to check for cracks in the test beads 1-1 and 1-2, and the results are shown in Table 1. From now on, with the component composition within the scope of the claims,
Moreover, if R〓≧40 is satisfied for the calculated values on the left side of equations (1) and (2), it can be seen that no cracks have occurred in the restrained joint. Example The comparison material and the invention material whose chemical compositions are shown in Table 2 were each melted into a 100 kg steel ingot by vacuum melting.
In the table, S13, S14 and S16 are air cooled after rolling.
S11 and S12 were subjected to accelerated cooling after rolling, and S15 and S17 were subjected to quenching and tempering treatment.

【table】

[Table] * ○: No cracking, ×: Cracking ** Accelerated cooling material *** Quenched and tempered material Others are air-cooled materials Table 2 shows the tensile strength and toughness of steel materials and the regression equation according to the present invention. The R value and the crack inspection results from the above-mentioned restrained joint test are also listed. The restraint joint was manufactured as already described in Figure 2, and after degreasing, pickling, and flux treatment, it was immersed in a 455°C molten zinc bath for 6 minutes to remove the resulting buildup, and then cracks in the heat-affected zone were removed. I conducted an inspection. As is clear from this table, all the invented steels with components of this invention whose R value exceeds 40 based on the regression equation have a tensile strength of 60 Kgf/mm2 or ^more , and furthermore, no cracks have occurred in the heat affected zone of the restrained joint. do not have. Effects of the invention For welded structural steel with a tensile strength of 60Kgf/mm2 or ^more , the susceptibility to cracking of the HAZ part due to the hot-dip galvanizing treatment applied after welding has been improved in the application of continuous casting. Concomitant surface defects are advantageously reduced.

[Brief explanation of drawings]

Figure 1: Relationship graph between R value and rupture time, 2nd
Figure: Front view and side view of a restrained joint crack test specimen.

Claims (1)

[Claims] 1 C: 0.08-0.15wt%, Si: 0.10-0.50wt%, Mn: 1.00-2.00wt%, Zr: 0.012-0.030wt%, sol Al: 0.005-0.100wt%, Nb: It contains 0.01 to 0.10 wt% and V: 0.01 to 0.10 wt% in the relationship of the following formula 265-293C-21Si-96Mn+534Zr-35Al-410Nb-370V≧40, and the remainder is substantially Fe and inevitable impurities. Tensile strength 60Kgf/mm ²
A high-strength rolled steel material for welded structures having excellent hot-dip galvanizing cracking resistance, which is characterized by the above properties. 2 C: 0.08-0.15wt%, Si: 0.10-0.50wt%, Mn: 1.00-2.00wt%, Zn: 0.012-0.030wt%, sol Al: 0.005-0.100wt%, Nb: 0.01-0.10wt% and , V: 0.01 to 0.10wt% and further 0.05 to 1.00wt% of one or more of Ni, Cu, Cr, Mo, and the following formula 265−293C−21Si−96Mn+534Zr−35Al− 410Nb−370V−18Ni −128Cu−94Cr −90Mo≧40, the remainder is substantially Fe and unavoidable impurities, and has a tensile strength of 60Kgf/mm ²
A high-strength rolled steel material for welded structures having excellent hot-dip galvanizing cracking resistance, which is characterized by the above properties. 3 C: 0.08-0.15wt%, Si: 0.10-0.50wt%, Mn: 1.00-2.00wt%, Zr: 0.012-0.030wt%, sol Al: 0.005-0.100wt%, Nb: 0.01-0.10wt% and , V: 0.01 to 0.10wt% and B: 0.0005 to 0.0040wt% in the relationship of the following formula 265-293C-21Si-96Mn+534Zr-35Al-410Nb-370V-2454B≧40, and the remainder is substantially Fe and Unavoidable impurities, tensile strength 60Kgf/mm ²
A high-strength rolled steel material for welded structures having excellent hot-dip galvanizing cracking resistance, which is characterized by the above properties. 4 C: 0.08-0.15wt%, Si: 0.10-0.50wt%, Mn: 1.00-2.00wt%, Zr: 0.012-0.030wt%, sol Al: 0.005-0.100wt%, Nb: 0.01-0.10wt% and , V: 0.01~0.10wt% and further 0.05~1.00wt% of Ni, Cu, Cr, Mo.
Contains one or more species and 0.0005 to 0.0050 wt% of Ce in the relationship 265−293C−21Si−96Mn+534Zr −35Al−410Nb−370V−18Ni−128Cu −94Cr−90Mo+1429Ce≧40, and the remainder is substantially Fe. and unavoidable impurities, with a tensile strength of 60Kgf/mm ²
A high-strength rolled steel material for welded structures having excellent hot-dip galvanizing cracking resistance, which is characterized by the above properties.