JPS6250448A - High-tension low-alloy steel having superior resistance to cracking by plating in weld heat-affected zone - Google Patents

Abstract

PURPOSE:To improve the resistance of the weld heat-affected zone of a high- tension low-alloy steel to cracking by plating by restricting the amounts of C, Mn, Si, Ti, Ca, Al, etc., and satisfying certain relation among the amounts. CONSTITUTION:This high-tension low-alloy steel consists of, by weight, 0.03-0.15% C, 0.6-2% Mn, 0.05-0.3% Si, 0.005-0.03% Ti, 0.001-0.005% Ca, 0.005-0.1% Al, one ore more among <=0.014% N, etc., and the balance Fe and has <=0.28 T value calculated by a formula T=C+Si/30+Mn/10-S/2+Cr/10+Mo/20+V/3+Nb/3-2Ti+3B.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a low-alloy high-strength steel having excellent resistance to cracking in a weld heat-affected zone.

(Prior art) Zn has been used in steel structures from the perspective of rust prevention of steel materials.
Plating methods such as these are widely used. These platings are extremely effective in preventing rust, but depending on the composition of the steel, the shape of the structure, and the immersion method in the plating bath, cracks may occur in the weld heat affected zone, which is considered a problem.

This phenomenon has long been known as liquid metal embrittlement, but in recent years, as structures have become larger and the steel materials used have become stronger, cracks have been occurring frequently, and various studies have been conducted to improve this phenomenon. Progress has been made and some results have been published.

For example, Tetsu to Hagane, March 1982 (vot69-A5
), page 364 entitled ``Hot-dip zinc embrittlement in welds'' shows experimental results regarding molten-zinc embrittlement in the weld heat-affected zone. According to this, the susceptibility to embrittlement due to molten Zn has a good correlation with the hardness of the weld heat affected zone, and in order to reduce this, it is effective to reduce the hardness of the weld heat affected zone. is clear.

On the other hand, the present inventors have so far reported in Japanese Unexamined Patent Application Publication No. 58-849 regarding steel materials with excellent resistance to welding heat-affected zone cracking.
No. 59, JP-A No. 59-50157, JP-A-Sho 5
Publication No. 9-126754 or Japanese Patent Application No. 1987-733
We have made proposals through issues such as No. 01.

However, even with this proposed technology, the shape of the weld bead toe, which is the cause of plating, becomes more severe, and the welding joint becomes more difficult due to a decrease in welding heat input, and the structure suffers from an increase in hardness due to changes in the heat-affected zone structure. Regarding parts, there is still room for consideration in terms of prevention of cracks, such as the need for strict restrictions on plating.

(Problems to be Solved by the Invention) The present invention has been made in response to the above-mentioned needs, and its purpose is to provide a low-alloy high-strength steel that has excellent resistance to cracking in the welded heat-affected zone. It is something.

(Means and effects for solving the problem) The present inventors
As a result of extensive experiments and research, Ti against scratches,
In addition to discovering a significant improvement effect with the addition of Ca complex, we also succeeded in quantifying the influence of various alloying elements, and by satisfying a certain relationship between each alloying element, we were able to improve the weld heat affected zone of steel. The present invention has been completed by discovering a new technology not previously known that enables a further dramatic improvement in resistance to plating resistance due to molten Zn embrittlement. The areas where the weight is 0003~0.15%, the weight is 0.6~2.0%, and the Si
O,05~0.30%, TiO,005~0
．． 030%, CaO, more than 0010 to 0.0050
% or less, Al0.005-0.100%, NO,014
% or less, further Cu 2% or less, Ni 2% or less, C
Contains one or more of the following: r 0.5% or less, Mo 0.5% or less, Vo 15% or less, Nb 0.15% or less, Bo, 0020% or less, the remainder consisting of Fe and inevitable impurities, and T value = C+ Si/30+Mn/10-S
/2 + Cr/10 +Mo/20 +V/3 +N
The present invention is a low-alloy high-strength steel having excellent plating resistance in a weld heat-affected zone and characterized by simultaneously satisfying b/3-2Ti +3 B<0.28.

The present invention will be explained in detail below.

First, the reason for limiting the content of each element to the above-mentioned range will be described.

First, C is an essential element to increase strength, and 55 K
In order to ensure a strength of 17wm2 or more, 0.03% or more is required. From the point of HAZ plating, 0.15
%, cracks will appear, so the upper limit was set at 0.15%. Therefore, the amount of C is 0.03 to 0.1
It was limited to 5%.

Seed, like C, is an essential element for ensuring the strength of steel materials, and in order to ensure the strength of steel materials of 55 Kp/rtm2 or more, 0.
．． 60% or more is required. Also, if it exceeds 2.0%, H
AZ plating cracks are extremely likely to occur, so 2.
．． Must be less than 0%.

Therefore, the amount of Mn was limited to 0.60% to 2.0%.

Si is essential as a deoxidizing element and also to ensure strength, and a minimum of 0.05% is required, but from the viewpoint of HAZ plating resistance, the lower the better, and if it exceeds 0.30k, significant plating will occur. Therefore, it is necessary to limit the upper limit to 0.30%. Therefore, the amount of Si was set to 005 to 0130%.

Improve the plating resistance of HAz by adding Ti in combination with Ca. The minimum amount required for this is 0.005%
However, if the content is less than 0.005%, no effect will be observed. C
When added in combination with a, if the amount of Ti exceeds 0.030%, the composite effect will no longer exist and the plating resistance will deteriorate significantly. Therefore, Ti is 0.005 to 0.03
It was limited to 0%.

By adding Ca in combination with Ti, the plating resistance of HAZ is significantly improved. The amount of Ca is 0.0010
% or less, there is no combined effect with Ti, and 0.0050
If added in excess of %, not only will the composite effect be lost, but the toughness of the steel will also deteriorate. Therefore, Ca was limited to more than 0.0010% to 0.0050% or less.

Al is essential as a deoxidizing element, and the amount of Al is required to be at least 0.005%. Furthermore, even if the content exceeds 0.100%, the deoxidizing effect remains almost unchanged and the plating resistance deteriorates, so the upper limit was set at 0.100%. Therefore, A
l was limited to 0005-0.100%.

If N content exceeds 0.014%, the mechanical properties and weldability will be significantly deteriorated, so it is limited to 0.014% or less.

In addition, in the present invention, the strength of the base metal, the weld heat affected zone,
Cu%Ni, Cr, Mo, V to improve toughness
, Nb, and B may be contained.

First, Cu and Ni are effective elements for improving the strength and toughness of steel materials, without impairing their resistance to cracking.
Cu%N
i was limited to 2.0% or less.

Cr and Mo are effective elements for improving hardenability and improving the strength and toughness of the base metal, but if each is added in excess of 0.5%, weldability and cracking resistance will be significantly impaired. Tame, C
r and Mo were each limited to 0.5% or less.

V and Nb are effective in refining crystal grains and improving toughness when added in trace amounts, and are expected to increase strength through precipitation hardening, but when added in excess of 0.15% of each, weldability and resistance deteriorate. v and N'b are each 0.1, as this will impair the resistance to adhesion.
It was limited to 5% or less.

When B is added in a very small amount, it is effective in improving hardenability and ensuring strength, but even if it is added in excess of 0.0020%, the effect will be saturated and the cracking resistance will be significantly impaired. So, 0.0
0.020% or less.

Furthermore, in the present invention, in addition to the above-mentioned component limitations, in order to maintain the weld heat-affected zone's resistance to cracking, a certain relationship T value E= C+ St/30 +Mn/ is established between the contents of each component. 10-S/2
+Cr/10 +Mo/20+V/3+Nb/3-
It is necessary to satisfy 2Ti +3 B<0.28 at the same time, and this is based on the knowledge obtained from the following experiment.

That is, a round bar with a length of 170mm and a diameter of 10mm was processed from a large number of materials combined in various ways within the range of the ingredients limited by the present invention, and a heat cycle was applied to the bar with a heat input equivalent to 17,000 joules/mm. , after reproducing the HAZ, the angle is 60°,
A circumferential notch with a notch tip radius of 0.5ff1ml and a depth of 211111 mm was processed, Zn plating was applied, and then the heat/stress cycle shown in Figure 1 was applied to reproduce the actual plating conditions and a simulation test was conducted. The correlation between the breaking strength and the breaking time was determined.

An example of the test results is shown in Figure 2. In the figure, the horizontal axis is the rupture time, the vertical axis A is the rupture strength of the specimen without Zn plating, and Z
This is the value obtained by dividing the breaking strength of the n-plated specimen.

Now, in Figure 2, taking 400 seconds on the horizontal axis as a reference,
At this time, the corresponding value of A (denoted as K) can be found for the 6 pieces. In this case, 400 seconds was set as the standard in consideration of the actual turning time.

Furthermore, a restrained joint test was conducted as a test to reproduce molten Zn plating on an actual plating line.

This restrained joint test is a test method in which a test specimen is prepared as shown in FIG. 3, and then immersed in a Zn bath at 470° C. After plating, cracks are generated in the toe portion of the test bead 1.

In the figure, 1 is a test bead, 2 is a restraining bead, and 3 is a restraining bead.
are test plates, each dimension is between t=15 and ht=
hz = ha = h4 = 50 rta, w = 1
5Qm, t=7511D11. The number of restraint beads 2 is 25
It is. The reason why the restraint bead was set to 'l 5 pass is that
This is because this restraining bead can impart a high residual stress to the test bead toe, and if no cracking occurs under these conditions, it can be naturally determined that no cracking occurs in the actual structure.

The crack resistance of steel materials was evaluated using the above two test methods. That is, in this way, a large number of values are obtained, and the correlation between these values and the chemical composition is analyzed by multiple regression analysis, and the carbon equivalent formula related to the susceptibility to scratching, T value = C +
Si/304Mn/10- S/2+ Cr/10
+Mo/20 +V/3 +Nb/3-2 Ti +3
I got a B.

Figure 4 shows the relationship between the K value and the value (hereinafter referred to as T value) calculated by recessing the steel composition of a number of steels used in the experiment. In the restrained joint test, no cracks were observed, and the mark (*) indicates that cracks occurred.

As is clear from the figure, within the range of the limited components of the present invention, T value = C+ Si/30 +Mn/10- S/2
+Cr/10 +Mo/20 +V/3 +Nb/3-
It is clear that Zn plating cracking can be completely prevented by satisfying 2Ti + 3 B <0.28.

The effects of the present invention will be illustrated in more detail with reference to Examples below.

(Example) Steel with the composition shown in Table 1 is melted in a 50 to 9 vacuum melting furnace, then controlled rolling and normal rolling, followed by direct quenching or quenching and tempering, according to standards, etc., to 55 to 7 Q K5/++ oa2
grade steel plate. The mechanical properties of the steel plates are also listed in Table 1. These steel plates were processed to reduce the thickness from one side, and two restrained joint specimens as shown in Fig. 3 were manufactured.2. Repeated experiments were performed.

The dimensions in Figure 3 are t=15mm, h□=h2=1
13 = h4 = 5 Q rtm, W: 150
rm, t=75m. Also, restraint/1! The number of spaces was set to 25 in all cases.

After completion of welding, the restrained joint specimen was placed in a Zn bath at 470℃ for 10 minutes.
After immersion plating for a minute, the test bead was inspected by magnetic flaw detection to determine the presence or absence of cracks. The results of this cracking test and the values obtained in the simulated test are also listed in Table 1.

As is clear from the same table, the steel of the present invention can completely prevent molten Zn plating cracking even in the restrained joint test, which is a very severe test to reproduce molten Zn plating cracking.

(Effect of the invention) As is clear from the above examples, C, Mn, Si,
Ti, Ca, Al, N, and also Cu, Ni
.

The content of one or more of Cr, Mo, V, Nb, and B is specified, and the T value E-C+ Si/30 + Mn/10-
S/2 +Cr/10 +Mo/20 +V/3 +N
By limiting the components to satisfy b/3-2Ti+3B<0.28, plating cracking in the weld heat affected zone can be completely prevented, and the industrial effect is extremely significant.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the stress and thermal cycle of the notched round bar tensile test, Figure 2 is a diagram showing an example of the notched round bar tensile test results, and Figures 3 (a) and (b) are restraint joint test methods. FIG. 4 is a diagram showing the correlation between the T value and the T value and the correspondence between the results of the restrained joint test. 1...Test bead 2...Restriction bead 3...Test plate Figure 1 Figure 2

Claims (1)

[Claims] In weight percent, C: 0.03% to 0.15% Mn: 0.6% to 2.0% Si: 0.05 to 0.30% Ti: 0.005 to 0.030 % Ca: more than 0.0010 to less than 0.0050% Al: 0.
005 to 0.100% N: 0.014% or less Cu: 2.0% or less Ni: 2.0% or less Cr: 0.5% or less Mo: 0.5% or less V: 0.15% or less Contains one or more of Nb: 0.15% or less B: 0.0020% or less, the balance consists of Fe and unavoidable impurities, and T value ≡ C + Si / 30 + Mn / 10 -
S/2+Cr/10+Mo/20+V/3+Nb/3-
A low-alloy high-strength steel with excellent plating resistance in a weld heat-affected zone, characterized by satisfying 2Ti+3B≦0.28.