JPH1096018A - Method for producing TS780 MPa class high strength steel excellent in hot-dip galvanizing crack resistance - Google Patents

Abstract

(57) [Summary] [Problem] To provide a method for producing steel that has a strength of TS 780 MPa or more and does not generate galvanizing crack resistance in a welded part. %
Hereinafter, Si: 0.1% or more and 0.6% or less, Mn: 1.0 or less
% To 2.0%, P: 0.02% or less, S: 0.0
02% or less, Cu: 0.6% or less, Ni: 1.0% or less, Ti: 0.04% to 0.14%, N: 0.0
06% to 0.010%, Al: 0.005% to 0.1%, B: 0.0002%, O: 0.00
Continuously cast slab having a composition of 5% or less and the balance of iron and impurities is heated to 1100 ° C or more, and rolled at 720 ° C or less with a total reduction of 40% or more and 600 ° C or more. A method for producing a non-heat treated 800 MPa class steel sheet having excellent plating cracking properties.

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. An object of the present invention is to provide a method for producing steel that has a strength of TS780 MPa or more and does not generate galvanizing crack resistance in a welded portion.

[0005]

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, elements such as C, which enhance hardenability, are reduced as much as possible by utilizing the precipitation strengthening of TiC and the work strengthening by two-phase rolling, and the addition of N further significantly improves the hot-dip galvanizing cracking resistance. , TS780
It has been discovered that strength of not less than MPa and galvanizing crack resistance can be achieved at the same time.

The present invention relates to (1) C: 0.06% by weight.
% To 0.14%, Si: 0.1% to 0.6%, Mn: 1.0% to 2.0%, P: 0.02%
Hereinafter, S: 0.002% or less, Cu: 0.6% or less, N
i: 1.0% or less, Ti: 0.04% to 0.14%, N: 0.006% to 0.010%, Al:
A continuous cast slab having a composition of 0.005% or more and 0.1% or less, B: 0.0002% or less, O: 0.005% or less, and the balance consisting of iron and impurities is heated to 1100 ° C or more and heated to 720 ° C. Total rolling reduction of 40% or more and 600 ° C below
A method for producing a non-heat-treated 800 MPa class steel sheet having excellent galvanization resistance in a heat-affected zone of a weld, characterized by finishing rolling as described above.

(2) By weight%, Nb: 0.01% or more and 0.04% or less, Cr: 0.5% or less, Mo: 0.4%
Hereinafter, a method for producing a non-heat treated 800 MPa class steel sheet excellent in galvanization crack resistance of a weld heat affected zone according to (1), wherein one or more of V: 0.08% or less is added. .

[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. 0.04% ≦ Ti ≦ 0.14% A hardened structure having a prior austenite grain boundary in which the structure of the heat-affected zone of the weld 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 work strengthening by two-phase rolling and precipitation strengthening of TiC. If less than 0.04% of Ti is added, precipitation strengthening of TiC sufficient to secure the tensile strength of the base material of 780 MPa or more cannot be obtained, and if more than 0.14% of Ti is added, toughness is deteriorated. Therefore, the amount of Ti is set to 0.04% or more.
It was limited to 14% or less.

0.006% ≦ N ≦ 0.010% The second feature of the present invention is high N addition. 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 austenite grain size during cooling after welding.
By adding Ti and nitriding to obtain a high N component, Ti
N increased remarkably, and suppressed the growth of austenite grains in the weld heat affected zone during welding heating, and acted as nucleation sites for ferrite during cooling after welding, and grain boundary ferrite precipitated in the structure of the weld heat affected zone. It turned out that a thin tissue could be obtained. If the amount of N is less than 0.006%, the number of TiN is insufficient, and a heat-affected zone having a fine structure in which grain boundary ferrite is precipitated cannot be obtained. Further, when N exceeds 0.010%, the toughness of steel is deteriorated. Therefore, N is limited to 0.006% or more and 0.010% or less.

FIG. 1 summarizes the effects of the addition of high Ti and high N. 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 the conventional steels (DQ-T treatment) containing a large amount of elements such as C, Mn, Cr and Mo, which improve the hardenability, C is added by using the strengthening of the work by two-phase rolling and adding high Ti. SLM4 is a steel that minimizes the addition of elements
The balance between 00 and TS is improved. Further, in the steel to which high N was added, further improvement in SLM was confirmed.

0.06% ≦ C ≦ 0.14% C is an essential element for increasing the strength, but significantly hardens the heat affected zone of the weld. If the C content is less than 0.06%, T
It is difficult to secure S780 MPa or more. Also,
Addition of C in excess of 0.14% causes the heat affected zone to be significantly hardened, making it difficult to prevent galvanizing cracking. Therefore, it is limited to 0.06% or more and 0.14% or less.

0.1% ≦ Si ≦ 0.6% Si is related to the appearance after plating, and
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.

1.0% ≦ Mn ≦ 2.0% Mn is an essential element in view of strength and toughness.
%, It is difficult to obtain a strength of 780 MPa or more,
If it exceeds 2.0%, 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 forms MnS, and lamella tear is easily generated in a welded portion. If the content exceeds 0.002%, lamellar tears may be generated in places where the constraint conditions are severe such as cross welds. Therefore, S is limited to 0.002% or less.

Cu ≦ 0.6% Cu is an element that increases the strength of the base material and, as can be seen from the CEZmod equation, is less likely to deteriorate the galvanizing resistance of the welded portion. It is. However, the addition of Cu exceeding 0.6% increases the possibility of Cu cracking, so the upper limit was limited to 0.6%.

Ni ≦ 1.0% Ni enhances the strength of the base material by solid solution strengthening, and furthermore, CEZm
As can be seen from the equation of od, it is an element that is hardly deteriorated in galvanizing cracking resistance of the welded portion, and thus is an essential element in the present invention. From the viewpoint of economy, the upper limit was limited to 1.0%.

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 degrades the cleanliness of steel. O exceeding 0.005%
Contains lead to deterioration of the ductility and toughness of steel.
Limited to 5% or less.

0.01% ≦ Nb ≦ 0.04% Nb is an element that precipitates as NbC and is effective in increasing the strength of the base material. If the addition is less than 0.01%, it is difficult to obtain the effect. On the other hand, if the addition exceeds 0.04%, the solid solution Nb or the hardenability in the weld is increased, and the hot-dip galvanizing cracking resistance is significantly deteriorated. Therefore,
It is limited to 0.01% or more and 0.04% 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 effective element 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.

Note that the SLM 400 described above is an NBT
It is obtained from a test called a test for evaluating hot-dip galvanizing cracking resistance. The iron and steel vol. 79 (1993)
p. 1108-p. 1114 describes the method and the like. 10mm diameter, 170 length sampled from each steel plate
After heating rapidly to 1400 ° C.
A welding HAZ simulation thermal cycle of cooling between 0 ° C. and 500 ° C. 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 a thermal processing cycle as shown in FIG. And
Collect data on rupture stress and rupture time. The vertical axis takes the value divided by the breaking stress when zinc is not electrodeposited instead of the breaking stress itself in the above test, and the horizontal axis takes the breaking time,
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. Incidentally, according to the above-mentioned literature, if the SLM400 is 42% or more, in the case of TS590 MPa class steel, hot-dip galvanizing cracks do not occur even when used for welded joints. However, in the TS780 MPa class, since the residual stress in the welded portion increases, cracks are expected to occur in the actual welded portion even if SLM400 ≧ 42% or more.

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 Ti which dissolves TiC during rolling and contributes to strength improvement. 0.08 to 0.14% of the range of the present invention
C, 0.04-0.12% Ti, sufficient solid solution Ti
In order to secure the tensile strength, heating at 1100 ° C. or higher is necessary, and it is difficult to obtain a tensile strength of 780 MPa or higher at a temperature lower than 1100 ° C.

The reason why the rolling was limited to 720% or less and the total draft was 40% or more at 720 ° C or less is as follows. It is a feature of the present invention to utilize work strengthening in the two-phase region to increase the base metal strength. 720
If the total draft is less than 40% at a temperature of not more than 40 ° C., sufficient work strengthening cannot be obtained, and it is difficult to obtain a tensile strength of 780 MPa or more. Further, even when a large total draft is obtained at 720 ° C. or more, recovery and recrystallization occur, and sufficient work strengthening cannot be obtained, and it is difficult to obtain a tensile strength of 780 MPa or more. Therefore, one of the rolling conditions was limited to 720 ° C. or less and the total draft was 40% or more.

600 ° C. ≦ rolling finishing temperature Further, when rolling is finished at a low temperature of less than 650 ° C., work hardening is remarkable and toughness is significantly deteriorated. Therefore, the rolling finish temperature was limited to 650 ° C. or higher.

[0030]

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. In Table 2, the steel sheets No. 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 portion 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 0.04% or more of Ti to reduce the elements such as C for improving hardenability as much as possible, further addition of 0.006% or more of N, and setting of a rolling heating temperature of 1100 ° C. or more,
Developed steel sheets FP-FP *, JP, KP, LP-LP with a total draft of 40% below 0 ° C and finished at 650 ° C or more
The developed steels of *, MP, and NP showed TS of 780 MPa or more, and no crack occurred in the galvanized cracking test of the restraint joint. Further, 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 that improve the hardenability,
Steel A has insufficient strength and steels B to C have cracks. High T
In the conventional steels D to F to which addition of elements such as C is suppressed as much as possible by addition of i, but high N is not added, 0.069%
Only Ti-added steel F has a tensile strength of 780 MPa or more, but has a zinc plating crack in the restraint test.

Further, elements such as C, which enhances quenchability by adding 0.04% or more of Ti, are reduced as much as possible, and the addition of high N is attempted.
% Of steel plate FP2 or 11
The steel plate JP1 deviating from the rolling heating condition of 00 ° C. or more does not have a tensile strength of 780 MPa or more. Also,
Steel sheet FP deviating from the finish temperature condition of 650 ° C or more
No. 1 has remarkably low toughness with vTs = −25 ° C.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

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,
Cracking can be prevented, which has an extremely great effect on industry.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the tensile strength of a steel sheet and SLM400. The test steels are steels A to FP in Table 1.

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 (2)

[Claims] 1. C: 0.06% or more and 0.14% by weight
%, Si: 0.1% or more and 0.6% or less, Mn: 1.% or less.
0% or more and 2.0% or less, P: 0.02% or less, S: 0.
002% or less, Cu: 0.6% or less, Ni: 1.0% or less, Ti: 0.04% or more and 0.14% or less, N: 0.0
06% to 0.010%, Al: 0.005% to 0.1%, B: 0.0002%, O: 0.00
Welding heat characterized in that a continuous cast slab having a composition of 5% or less and the balance of iron and impurities is heated to 1100 ° C or more and rolled at 720 ° C or less and a total reduction of 40% or more and 600 ° C or more. A method for producing a non-heat treated 800 MPa grade steel sheet having excellent resistance to galvanized cracking of an affected area. 2. Nb: 0.01% or more and 0.0% by weight.
4% or less, Cr: 0.5% or less, Mo: 0.4% or less,
The method for producing a non-heat-treated 800 MPa class steel sheet excellent in galvanizing crack resistance of a weld heat affected zone according to claim 1, wherein one or more kinds of V: 0.08% or less are further added.