JP3371715B2 - Method for producing TS780 MPa class steel excellent in hot-dip galvanizing crack resistance - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-alloy, high-strength steel which is hot-dip galvanized after welding to prevent rust on steel towers, bridges, buildings and the like. About. 2. Description of the Related Art In order to prevent rust of 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 situation, the present inventor has determined that there is no additional element that enhances the hot-dip galvanizing cracking resistance, and that it has a strength of at least TS780 MPa and a galvanizing cracking resistance. We have intensively researched what component design and manufacturing conditions are compatible. As a result, the hot-dip galvanizing crack resistance was significantly improved by the addition of Ti—Ca, and both were added in combination, and Ceqm (= C + Mn / 20 + Si /
30 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 1
5 + V / 10 + 5B + 1.0Nb) was found to be able to achieve both strength of more than TS780MPa and resistance to galvanizing cracking by designing the composition of 0.23% or more and 0.27% or less and directly quenching and tempering under appropriate conditions. did. According to the present invention, C: 0.06% to 0.12%, Si: 0.1% to 0.6%, M
n: 1.0% or more and 2.0% or less, P: 0.02% or less,
S: 0.002% or less, Nb: 0.01% or more and 0.06
% Or less, Ti: 0.01% or more and 0.05% or less, Ca:
0.001% or more and 0.005% or less, N: 0.002%
Not less than 0.006% and Al: not less than 0.005% and 0.1
%, B: 0.0002% or less, O: 0.005% or less, Cu: 0.6% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.6 % Or less, V:
0.1% or less, one or more kinds are added, the balance is composed of iron and impurities, and the combined value of these elements Ceqm = C + Mn / 20 + Si / 30 + Cu /
20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10
+ 5B + 1.0Nb is 0.23% ≦ Ceqm ≦ 0.2
Continuous cast slabs having a composition of 7%
The effect of welding heat characterized in that the rolling was completed by heating at a temperature of at least 00 ° C and rolling at a temperature of at least 950 ° C and at a temperature of at least 720 ° C. This is a method for producing a high-strength steel having a tensile strength of 780 MPa or more, which is excellent in hot-dip galvanizing cracking resistance of a portion. The details of the present invention will be described below. First, the reasons for limiting the component ranges will be described. 0.01% ≦ Nb ≦ 0.06% 0.23% ≦ Ceqm First, in the present invention, it is a first object to obtain a TS780 MPa grade steel. Nb is an element effective for significantly increasing the strength when added in a small amount, and is an essential element in the present invention. If the addition is less than 0.01%, it is difficult to obtain a strength of 780 MPa or more, and if the addition exceeds 0.06%, the steel becomes brittle. Therefore, the addition is limited to 0.01% or more and 0.06% or less. Also, Nb is an element that is difficult to indicate as C equivalent, despite being an element that increases strength. The reason for this is that the contribution to strength differs depending on the rolling and heat treatment conditions. However, on the premise that the so-called DQ-T treatment is performed after the rolling heating temperature at which solid-solution Nb is sufficiently obtained and the steel is directly quenched after rolling and then tempered, as shown in FIG. 1, Ceqm (= C + Mn / 20 + Si / 30 + Cu)
/ 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 1
It was found that the tensile strength can be arranged by the C equivalent equation of 0 + 5B + 1.0Nb). If Ceqm is controlled to 0.23% or more, TS78 can be obtained in a range of a plate thickness of 40 mm or less.
It turned out that 0 MPa or more can be obtained. [0010] A second object of the present invention is to prevent galvanized cracks in the heat affected zone of the weld by 0.01% ≦ Ti ≦ 0.05% 0.001% ≦ Ca ≦ 0.005% Ceqm ≦ 0.27%. It is to prevent. This is achieved by controlling the combined addition of Ti—Ca and Ceqm to 0.27% or less. 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. When Ca and Ti are added in combination, T
iN became remarkably thin and suppressed the growth of austenite grains in the weld heat affected zone during welding heating, and acted as a nucleation site 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. As a result, as shown in FIG.
When a is added in combination, Ceqm is 0.23% or more and 0.2% or more.
It was found that the galvanized crack in the welded portion could be prevented in the range of 7% or less. 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 Ti content is limited to 0.01% or more and 0.05% or less. Further, if Ca is added in an amount of less than 0.001%, the effect of miniaturizing TiN is not sufficient, and a heat-affected zone having a fine structure in which grain boundary ferrite is precipitated cannot be obtained. Also,
Addition of Ca exceeding 0.005% lowers the cleanliness of the steel and causes deterioration of toughness. Therefore, Ca is limited to 0.001% or more and 0.005% or less. 0.06% ≦ C ≦ 0.12% C is an essential element for increasing the strength. 0.06%
If less than it is difficult to obtain a strength of 780 MPa or more,
If it exceeds 0.12%, the toughness and weldability of the steel are remarkably deteriorated, so the content is limited to 0.06% or more and 0.12% or less. 0.1% ≦ Si ≦ 0.6% Si is related to the appearance after plating, and is 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. 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 the content exceeds 2.0%, the weldability is significantly deteriorated.
n: Limited to 1.0% or more and 2.0% 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. 0.002% ≦ 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
The 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 remarkably 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. 0.00 when Ca is added
If the content of O exceeds 5%, Ca-OS-based inclusion clusters are easily formed, and the toughness of steel is deteriorated.
It was limited to 0.005% or less. Cu ≦ 0.6% Cu is an effective element for increasing the strength of steel.
When added in excess of 6%, Cu cracks are likely to occur.
Therefore, it was limited to 0.6% or less. Ni ≦ 1.0% Ni is an element effective for improving the strength up and toughness of steel, but is limited to 1.0% or less in consideration of economic efficiency. Cr ≦ 1.0% Cr is an element effective for increasing the strength of steel.
If added in excess of 0%, the toughness and weldability of the steel deteriorate, so the content was limited to 1.0% or less. Mo ≦ 0.6% Mo is an effective element for increasing the strength of steel.
If added in excess of 6%, the toughness and weldability of the steel will be significantly deteriorated, so the content is limited to 0.6% or less. V ≦ 0.1% 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 0.1%, the toughness and weldability of the steel are significantly deteriorated. For this purpose, the content is limited to 0.1% or less. 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. 0.06 to 0.12 in the range of the present invention
% C, 0.01-0.06% 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, the steel is not sufficiently quenched and it is difficult to obtain a tensile strength of 780 MPa or more. [Immediate water cooling] The reason why the DQ treatment is performed immediately thereafter is to sufficiently sinter and obtain a tensile strength of 780 MPa or more. Of course, it goes without saying that the higher the finishing temperature of the rolling, the more room there is, even immediately, from the metallurgical principle. The reason why the refrigerant for the DQ treatment is limited to water is that it is the cheapest and has a large cooling capacity. The reason for limiting the heat treatment to direct quenching instead of reheating quenching is that in reheating quenching, a heating temperature of about 900 ° C. is usually set so that NbCN does not form a solid solution and a tensile strength of 780 MPa or more is obtained. Because it is difficult. 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. 550 ° C. ≦ tempering temperature ≦ 650 ° C. The reason for limiting the tempering temperature to 550 ° C. or more and 650 ° C. or less is as follows. If the temperature is lower than 550 ° C., excellent toughness cannot be obtained. If the temperature exceeds 650 ° C. , tempering softening is remarkable, and it is difficult to obtain a tensile strength of 780 MPa or more. EXAMPLE Steel having the chemical composition shown in Table 1 was melted and continuously cast to form 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 EP and EP1, steel N in Table 1
o. It has the same chemical composition as EP. These steel sheets were subjected to a tensile test and a galvanizing crack test of a restraint joint. The galvanized crack test of the restraint joint is a test for preparing the cross joint shown in FIG. 3, immersing it in a zinc bath at 470 ° C., plating, and then examining the toe portion of the test bead 1 for cracks. 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 shows the test results of the test steels.
Conventional steel sheets B to I to which Ti-Ca has not been added have cracks in the galvanization cracking test of restraint joints. Conventional steel sheet A did not crack even in the restraint joint galvanization cracking test, but since Ceqm was less than 0.23%, it was 780.
TS higher than MPa was not obtained. Conventional steel plate HH-I
For I, although Ti-Ca was added, since Ceqm exceeded 0.27, cracks occurred in the galvanization cracking test of the restricted joint. Ga—Ti is added, and Ceqm is 0.2
It has a component composition of 3% or more and 0.27% or less, and has a temperature of 1100 ° C.
The above-mentioned rolling heating temperature is set, and the roll is finished at 950 ° C or lower and 720 ° C or higher, immediately quenched, water-cooled to 250 ° C or lower, and then tempered at 550 ° C or higher and 650 ° C or lower, CP, DP, EP , FP, GP, JP, K
The developed steels of P and LP show TS of 780MPa or more,
No cracking occurred in the galvanized cracking test of the restraint joint. Further, the invention steel also has excellent toughness. However, Ca-Ti is added and Ceqm
Despite having a component composition of 0.23% or more and 0.27% or less, steel that does not satisfy the production conditions of the present invention cannot have a strength of 780 MPa or more or has extremely low toughness. I understand. [Table 1] [Table 2] As is apparent from the above description, when the components are designed and DQ-T is applied according to the present invention, a steel having a tensile strength of 780 MPa or more can be obtained, and welding of steel towers, bridges, buildings and the like can be obtained. Even if it is used for the structure and is subjected to hot-dip galvanization,
Cracks can be prevented. It has an extremely great effect on industry.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the tensile strength and Ceqm of DQ-T treated steel sheet.
FIG. The test steels are steels A to I in Table 1. FIG. 2 is a view of a zinc plating restraint cracking test result showing the relationship between Ceqm and the effect of adding Ti—Ca. No. 1 for test steel
Steels A to G, steels CP to FP and steels HH to II in the table. FIG. 3 is a diagram showing the size and configuration of a restrained crack test specimen. [Description of Signs] 1 ... test bead, 2 ... restraint bead (18 passes / 1 side), 3 ... test plate.

Continuation of front page (56) References JP-A-8-269545 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38 / 60

Claims (1)

(57) [Claims 1] C: 0.06% or more and 0.12% 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, Nb: 0.01% or more and 0.06% or less,
Ti: 0.01% or more and 0.05% or less, Ca: 0.00
1% or more and 0.005% or less, N: 0.002% or more.
006% or less, Al: 0.005% or more and 0.1% or less,
B: 0.0002% or less, O: 0.005% or less, Cu: 0.6% or less, Ni: 1.0% or less, Cr:
One or more of 1.0% or less, Mo: 0.6% or less, and V: 0.1% or less are added, and the balance consists of iron and impurities, and the value of the combination of these elements Ceqm = C + Mn / 20 + Si / 30 + Cu / 20 +
Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
+ 1.0Nb is a continuous cast slab having a composition of 0.23% ≦ Ceqm ≦ 0.27%, is heated to 1100 ° C. or more, finishes rolling at 950 ° C. or less and 720 ° C. or more, and immediately cooled with water. After stopping the water cooling below ℃,
Tensile strength 78 excellent in hot-dip galvanizing cracking resistance of the heat-affected zone of the weld characterized by tempering at 50 ° C. or less.
Method for producing high-strength steel of 0 MPa or more