JPH1096020A - Manufacture of ts780mpa class high tensile-strength steel excellent in resistance to hot dip galvanizing crack - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel having >=TS781MPa strength and free from the occurrence of galvanizing crack in a weld zone. SOLUTION: A continuously cast slab, having a composition where one or >=2 kinds among 0.04-0.08%, by weight, C, 0.1-0.6% So, 0.8-0.6% Mn, <=0.02% P, <=0.002% S, 0.8-0.8% Cu, 0.4-1.0%Ni, 0.01-0.04% Nb, 0.01-0.05% To, 0.001-0.005% Ca, 0.002-0.006% N, 0.005-0.1% Al, <=0.0002% B, <=0.005% O, <=0.5% Cr, <=0.4% Mo, and <=0.08% V are added and which has the valance consisting of iron with impurities, is heated to <=1100 deg.C. After rolling is finished at 950-720 deg.C, water cooling is performed without delay and stopped at <=250 deg.C. Then, aging treatment is carried out at 450-650 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-alloy, high-strength steel that is hot-dip galvanized after welding for the purpose of preventing rust on steel towers, bridges, buildings, and the like.

[0002]

2. Description of the Related Art In order to prevent rust on steel towers, bridges, and buildings, a method has been widely used in which steel used for them is welded to a structural member and then hot-dip galvanized. that time,
Cracks may occur in the heat affected zone. So-called,
This is due to liquid metal embrittlement.

[0003] In order to prevent this cracking, intensive research has been made. Those achievements are iron and steel vol. 79
(1993) p. 1108-p. 1114. This document was co-authored by a fabricator and four steel companies and is currently considered the most reliable and cutting-edge technology published.
This paper describes in detail the effect of boron contamination in steel, B is less than 2 ppm, and CEZmod =
C + Si / 17 + Mn / 7.5 + Cu / 13 + Ni / 1
7 + Cr / 4.5 + Mo / 3 + V / 1.5 + Nb / 2 +
It has been clarified that, if Ti / 4.5 + 420B ≦ 0.44% is satisfied, hot-dip galvanizing cracking does not occur after welding in a steel having a tensile strength (TS) of 590 MPa.

[0004]

In the composition design of high-strength steels, elements that enhance hardenability and elements that strengthen precipitation are generally added. However, as can be seen from the CEZmod equation, almost all of the added elements deteriorate the hot-dip galvanizing cracking resistance, so a steel with a strength of at least TS780 MPa and no galvanizing cracking in the weld zone has been developed. It has been considered impossible to do so.

[0005] An object of the present invention is to provide a method for producing steel which has a strength of TS780 MPa or more and does not cause galvanizing cracking resistance in a welded portion.

[0006]

In view of the above-mentioned situation, the present inventor has determined that there is no additional element for increasing the hot-dip galvanizing cracking resistance, and to achieve both the strength of TS780 MPa or more and the galvanizing cracking resistance. I did intensive research on what kind of ingredient design was. As a result, 0.8% or more of Cu was added and ε
-The use of precipitation strengthening of Cu minimizes elements such as C, which enhance hardenability, and further improves the hot-dip galvanizing cracking resistance by adding Ti-Ca.
It has been discovered that strength of 780 MPa or more and galvanizing crack resistance can be achieved at the same time.

In the present invention, C: 0.04% to 0.08%, Si: 0.1% to 0.6%, M
n: 0.8% or more and 1.6% or less, P: 0.02% or less,
S: 0.002% or less, Cu: 0.8% or more and 1.8% or less, Ni: 0.4% or more and 1.0% or less, Nb: 0.01
% To 0.04%, Ti: 0.01% to 0.05
% Or less, Ca: 0.001% or more and 0.005% or less,
N: 0.002% to 0.006%, Al: 0.0
05% or more and 0.1% or less, B: 0.0002% or less,
O: 0.005% or less, Cr: 0.5% or less,
Mo: 0.4% or less, V: 0.08% or less, one or two or more kinds are added, and a continuous cast slab having a composition composed of iron and impurities is heated to 1100 ° C or more and 950 ° C or less. Finish rolling at 720 ° C or higher, immediately cool with water and stop water cooling at 250 ° C or lower.
Tensile strength of 780M with excellent hot-dip galvanizing crack resistance in the heat affected zone of welding characterized by aging at 50 ° C or less
This is a method for producing high-tensile steel of Pa or higher.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below. First, the reasons for limiting the component ranges will be described.

0.8% ≦ Cu ≦ 1.8% A hardened structure having a prior austenite grain boundary in which the structure of the heat-affected zone in the weld zone is clearer is more likely to cause galvanization cracking. Conventionally, in order to secure the tensile strength of the base material of 780 MPa or more,
Since a large amount of elements such as C, Mn, Cr, and Mo, which enhance hardenability, must be added, the structure of the heat-affected zone of the weld becomes a hardened structure with a prior austenite grain boundary, and galvanization cracks occur. Resulting in.

In the present invention, elements that enhance hardenability are reduced by utilizing precipitation strengthening of ε-Cu. 0.8%
If less Cu is added, the precipitation strengthening of ε-Cu that can secure the tensile strength of the base material of 780 MPa or more cannot be obtained, so that 1.8
%, The risk of deterioration of the toughness of the base material and Cu cracking increases. Therefore, the amount of Cu is set to 0.8% or more to 1.8%.
% Or less.

0.01% ≦ Ti ≦ 0.05% 0.001% ≦ Ca ≦ 0.005% The second feature of the present invention is a composite addition of Ti—Ca. In order to prevent galvanized cracks in the weld, it is important to reduce the austenite grain size in the heat-affected zone during welding heating and to precipitate ferrite in the old austenite grain size during cooling after welding. When Ca and Ti are added in combination, TiN
Significantly reduces the growth of austenite grains in the weld heat affected zone during welding heating, and acts as a nucleation site for ferrite during cooling after welding.The structure of the weld heat affected zone is thin with grain boundary ferrite precipitated. It turned out that the tissue was obtained. If Ti is less than 0.01%, a sufficient number of TiNs to obtain the above-described structure of the heat affected zone cannot be obtained. It generates TiC and leads to embrittlement of the heat affected zone. Therefore, the amount of Ti is limited to 0.01 or more and 0.05 or less. Also, if less than 0.001% of Ca is added, the above Ti
The effect of N refinement is not sufficient, and a heat-affected zone having a fine structure in which grain boundary ferrite is precipitated cannot be obtained. Also, 0.0
Addition of Ca exceeding 0.05% lowers the cleanliness of the steel and causes deterioration of toughness. Therefore, Ca is 0.001% or more and 0.001% or more.
Limited to 5% or less.

FIG. 1 summarizes the effects of adding Cu and Ti—Ca. SLM400 (described later) on the vertical axis is a galvanization resistance cracking index. The higher the value, the more excellent the galvanization resistance. Compared to conventional steels containing a large amount of C, Mn, Cr, and Mo, which are elements that improve hardenability, steels with the addition of elements such as C minimized by adding Cu improve the balance between SLM400 and TS. ing. Further, it was confirmed that the steel further added with Ti-Ga further improved the SLM.

0.04% ≦ C ≦ 0.08% C is an essential element for increasing the strength, but remarkably hardens the heat affected zone by welding. In the present invention, since precipitation strengthening of ε-Cu is used, addition of C exceeding 0.08% is not necessary to secure TS780 MPa or more. Also,
If the amount of C is less than 0.04%, it is difficult to secure TS780 MPa or more. With an addition of 0.04% or more and 0.08% or less, a strength of TS780MPa or more can be obtained, and the hardness of the heat affected zone does not harden until zinc plating cracks are caused.

0.1% ≦ Si ≦ 0.6% Si is related to the appearance after plating, and 0.1% ≦ Si ≦ 0.6%.
If it is less than 0.6% and the plating is burnt easily. Therefore, it is limited to 0.1% or more and 0.6% or less.

0.8% ≦ Mn ≦ 1.6% Mn is an essential element in view of strength and toughness.
%, It is difficult to obtain a strength of 780 MPa or more,
When the content exceeds 1.6%, the heat affected zone of the weld is significantly hardened. Therefore, the Mn content is limited to 0.8% or more and 1.6% or less.

P ≦ 0.02% P is an element that promotes the occurrence of hot cracking in the weld.
When the content exceeds 2%, the danger is significantly increased, so that the content is limited to 0.02% or less.

S ≦ 0.002% S combines with Ca to form CaS. If the content exceeds 0.002%, a CaS cluster is formed, and the toughness and weldability of the steel are significantly deteriorated. Therefore, 0.
002% or less.

0.4% ≦ Ni ≦ 1.0% Ni is effective for preventing Cu cracking and is an essential element in the present invention. For that purpose, it is necessary to add about half of the added amount of Cu, so the lower limit is limited to 0.4%. The upper limit was limited to 1.0% from the viewpoint of economy.

0.01% ≦ Nb ≦ 0.04% Nb is an essential element in the present invention on the premise of direct quenching. The strength can be significantly increased by adding a small amount.
However, if it is less than 0.01%, it is difficult to obtain a strength of 780 MPa or more, and if it exceeds 0.04%, it causes embrittlement of the steel, so it is limited to 0.01% or more and 0.04% or less. did.

0.002% ≦ N ≦ 0.006% N is an element necessary for generating TiN in the heat affected zone. If the content is less than 0.002%, TiN cannot be obtained in a sufficient number to obtain a heat-affected zone having a fine structure with grain boundary ferrite precipitated. Further, if the content of N exceeds 0.006%, the toughness of the welded portion is deteriorated. Therefore, N
Content was limited to 0.002% or more and 0.006% or less.

0.005% ≦ Al ≦ 0.1% Al is an essential element for deoxidation. If it is less than 0.005%, deoxidation is insufficient, and if it exceeds 0.1%, a large amount of alumina is generated, and the cleanliness of the steel is significantly deteriorated. Therefore, it is limited to 0.005% or more and 0.1% or less.

B ≦ 0.0002% B significantly improves the hardenability of steel. If it exceeds 0.0002%, the hot-dip galvanizing cracking resistance is significantly deteriorated, so B was limited to 0.0002% or less.

O ≦ 0.005% O deteriorates the cleanliness of steel. In the case of Ca addition, 0.0
When O is contained in more than 05%, Ca-OS-based inclusion clusters are easily formed, and the toughness of steel is deteriorated.
It was limited to 0.005% or less.

Cr ≦ 0.5% Cr is an element effective for increasing the strength of steel.
If added in excess of 5%, the weld heat affected zone will be significantly hardened, so the content is limited to 0.5% or less.

Mo ≦ 0.4% Mo is an element effective for increasing the strength of steel.
When added in excess of 4%, the heat affected zone of the weld is significantly hardened, so the content is limited to 0.4% or less.

V ≦ 0.08% V is an element effective for increasing the strength of steel by precipitation strengthening when added in a small amount, but when added in excess of 0.08%, the toughness and weldability of the steel are significantly deteriorated. Therefore, the content is limited to 0.08% or less.

The above-described SLM 400 is obtained from a test for evaluating hot-dip galvanizing cracking resistance called an NBT test. The iron and steel vol. 79 (1993) p.
1108-p. 1114 describes the method and the like. 10mm in diameter and 170m in length collected from each steel plate
After heating rapidly to 1400 ° C on a round bar sample of
A welding HAZ simulation thermal cycle of cooling between 8C and 500C in 8 seconds is provided. A round bar having a depth of 2 mm and an angle of 60 ° (a radius of curvature of the bottom of the notch of 0.25 mm and a radius of curvature of the shoulder of the notch of 2 mm) is provided on the round bar subjected to the above-described heat cycle, and then the notch is formed. Is electrodeposited with zinc to give the thermal processing cycle shown in FIG. Then, data on the rupture stress and the rupture time is collected. On the vertical axis, not the fracture stress itself in the above test but the value obtained by dividing by the fracture stress when zinc is not electrodeposited, and the horizontal axis is the fracture time, and the data is plotted as in the example of FIG. Curve regression of these plots, and the value at the intersection with the rupture time of 400 seconds is an index of hot-dip galvanizing cracking resistance called SLM400. By the way, in the above-mentioned document, SLM400 is 4
If it is 2% or more, in the case of TS590 MPa grade steel, hot-dip galvanizing cracks do not occur even when used for welded joints. However, in the TS780 MPa class, since the residual stress of the welded portion increases, even if SLM400 ≧ 42% or more,
Cracking is expected in the actual weld.

Next, the manufacturing conditions will be described.

Rolling heating temperature ≧ 1100 ° C. The reason why the rolling heating temperature is limited to 1100 ° C. or more is to secure solid solution Nb which dissolves NbCN during rolling and contributes to strength improvement. 0.04 to 0.08 in the range of the present invention
% C, 0.01-0.04% Nb, sufficient solid solution N
In order to secure b, heating at 1100 ° C. or more is necessary,
It is difficult to obtain a tensile strength of 780 MPa or more at a temperature lower than that.

720 ° C. ≦ rolling finishing temperature ≦ 950 ° C. The reason for limiting the rolling finishing temperature to 950 ° C. or lower and 720 ° C. or higher is as follows. When rolling is completed at a temperature exceeding 950 ° C., the structure becomes coarse and excellent toughness cannot be obtained.
After rolling at temperatures below 720 ° C, DQ-
This is because, even if T is performed, it is difficult to obtain sufficient tensile strength of 780 MPa or more because of insufficient sintering.

Immediately after cooling with water, the DQ treatment is performed immediately after baking sufficiently to obtain a tensile strength of 780 MPa or more. Of course, the higher the rolling finish temperature, immediately
Needless to say, there is some margin from metallurgical principles. D
The reason why the coolant for the Q treatment is limited to water is that it is the cheapest and has a large cooling capacity. In addition, the reason why the heat treatment was limited to direct quenching instead of reheating quenching is that in reheating quenching, 900
NbCN does not form a solid solution because the heating temperature is set to around ℃ 7
This is because it is difficult to obtain a tensile strength of 80 MPa or more.

Water cooling stop temperature ≦ 250 ° C. The reason why the water cooling stop temperature of the DQ treatment is limited to 250 ° C. or less is that martensitic transformation is caused to the center of the sheet thickness, and 780
This is for obtaining a tensile strength of not less than MPa.

450 ° C. ≦ aging temperature ≦ 650 ° C. The reason for limiting the aging condition to 450 ° C. or more and 650 ° C. or less is as follows. ε-Cu precipitates when heat aging is performed in the ferrite region. However, the aging temperature is 450
At temperatures below ℃, it is necessary to maintain for a very long time,
The lower limit temperature of aging was set to 450 ° C. Further, when aging is performed at a temperature exceeding 650 ° C., ε-Cu coarsens immediately, and it is difficult to obtain a desired strength. It is so-called overaging. Therefore, the aging temperature is 450 ° C. or more and 6
The temperature was limited to 50 ° C or lower.

[0034]

EXAMPLE Steel having the chemical composition shown in Table 1 was melted and continuously cast into a slab of 220 to 300 mm. Table 2 shows hot rolling conditions and DQ-T conditions. Steel plate N in Table 2
o. Are the steel No. in Table 1. It corresponds to. For example, the steel sheet No. For both FP and FP *, steel No. It has the same chemical composition as FP.

These steel sheets were subjected to a tensile test and a galvanization cracking test of restraint joints.

The galvanized crack test of the restraint joint is a test for examining the toe of the test bead 1 for cracks after preparing the cross joint shown in FIG. The number of passes of the constraining bead 2 was 18 and it was confirmed that a very high residual stress equivalent to the yield stress of the base material was acting on the toe portion of the test bead 1 by the constraining bead. Therefore, when no crack occurs in this test piece, it can be determined that no crack occurs even in hot-dip galvanizing of the welded member having the actual structure.

Table 2 also shows the test results of the test steels.

Addition of 0.8% or more of Cu to reduce the elements such as C for improving the hardenability as much as possible, further adding Ti-Ca, setting a rolling heating temperature of 1100 ° C. or more,
Rolled at 0 ° C or less and 720 ° C or more, immediately quenched, water-cooled to 250 ° C or less, and then 450 ° C or more and 65 ° C or less.
Invention steel plates FP-F aged at 0 ° C or lower
The invention steels of P ^*** , JP, KP, LP to LP ^* , MP, and NP showed TS of 780 MPa or more, and no cracks occurred in the galvanization cracking test of the restraint joint. Also,
The toughness is as good as vTs ≦ −60 ° C.

On the other hand, in the conventional steels A to C containing a large amount of elements C, Mn, Cr and Mo, which increase the hardenability,
Steel A has insufficient strength and steels B to C have cracks. Cu
Although addition of elements such as C is suppressed as much as possible by addition, Ti
In the conventional steels D to F to which Ca was not added, only the steel F to which Nb was added had a tensile strength of 780 MPa or more, but the zinc plating crack occurred in the restraint test.

Further, by adding 0.8% or more of Cu, elements such as C for improving the hardenability are reduced as much as possible.
KP1, LP2, L manufactured in a range that deviates from the rolling, DQ, and aging conditions of the present invention, although a is added.
P3 and MP1 do not have a tensile strength of 780 MPa or more.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

As is clear from the above description, when the components are designed and DQ-T is applied in accordance with the present invention, a steel having a tensile strength of 780 MPa or more can be obtained, which can be used for welding structures such as steel towers, bridges, and buildings. Even if used and hot-dip galvanized,
Cracks can be prevented. It has an extremely great effect on industry.

[Brief description of the drawings]

FIG. 1 DQ-T treated steel sheets (Steels A to FP in Table 1)
The figure which showed the relationship between the tensile strength of SLM400.

FIG. 2 is a diagram showing the size and configuration of a restrained crack test specimen.

FIG. 3 is a view showing a thermal processing cycle in an NBT test.

FIG. 4 is a diagram showing an example of data arrangement of an NBT test.

[Explanation of symbols]

1 ... test bead, 2 ... restraint bead (18 passes / 1 side), 3 ... test plate.

Claims (1)

[Claims] 1. C: 0.04% or more and 0.08% by weight
%, Si: 0.1% or more and 0.6% or less, Mn: 0.1% or less.
8% or more and 1.6% or less, P: 0.02% or less, S: 0.
002% or less, Cu: 0.8% or more and 1.8% or less, N
i: 0.4% to 1.0%, Nb: 0.01% to 0.04%, Ti: 0.01% to 0.05%, Ca: 0.001% to 0.005% Hereinafter, N:
0.002% to 0.006%, Al: 0.005
% To 0.1%, B: 0.0002% or less, O:
0.005% or less, Cr: 0.5% or less, M
o: 0.4% or less, V: 0.08% or less, one or two
A continuous cast slab having a composition containing at least one seed and the balance consisting of iron and impurities is heated to 1100 ° C. or more and heated to 9100 ° C. or more.
Rolling was completed at 50 ° C or lower and 720 ° C or higher, immediately after water cooling, and stopped at 250 ° C or lower, and then 450 ° C or higher and 65 ° C or lower.
Tensile strength of 780MP with excellent hot-dip galvanizing crack resistance in the heat affected zone of welding characterized by aging at 0 ° C or less
a. A method for producing a high-tensile steel of a or higher.