JPH1096015A - Manufacturing method of non-heat treated high strength steel with excellent hot-dip galvanizing crack resistance - Google Patents

Abstract

(57) [Summary] [Problem] Hot-dip galvanizing cracking characteristics that do not cause cracking even if hot-dip galvanizing processing is performed after bolt holes and the like after drilling holes in structural members such as steel towers and bridges Provided is a method for producing a non-heat treated high-strength steel excellent in heat resistance. SOLUTION: C: 0.08 to 0.20% in weight ratio, S
i: 0.60% or less, Mn: 1.0 to 2.0%, Cu:
2.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V:
Steel containing 0.1% or less, Nb + 0.5V + Ti ≧ 0.08%, and the balance being Fe and unavoidable impurities
It heats to 00-1350 degreeC, completes hot rolling below 800 degreeC, and sets a tensile strength to 690 Mpa class.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a non-refined high-strength steel excellent in hot-dip galvanizing crack resistance mainly used as a steel material for a steel tower.

[0002]

2. Description of the Related Art In recent years, high-strength steel materials for the purpose of reducing the weight of steel materials used have been actively used in various fields. Such a tendency has also appeared in steel materials for power transmission towers, and steel materials having a tensile strength of 590 MPa class are currently used. In addition, large power transmission towers are often constructed in the mountains, and further higher tension is required to reduce costs in transporting materials.

[0003] After being constructed, steel materials for steel towers are subjected to hot-dip galvanizing to make them maintenance-free. Since steel sections for steel towers (e.g., equilateral equiangular section steels) can be used as steel towers without performing welding work on site, plating cracking susceptibility of the base material is regarded as important. In the case of high-strength section steel, plating cracking may occur from a drilled portion for bolt connection of the section steel at the time of plating treatment, which hinders high strength.

A high strength steel to be hot-dip galvanized has been disclosed in Japanese Patent Application Laid-Open Nos.
Techniques such as 9-1316 have been proposed,
All of them are aimed at preventing cracks generated in the welded portion, and at present, there is little knowledge from the viewpoint of preventing cracks from being formed in bolted portions in high-strength steel.

[0005]

SUMMARY OF THE INVENTION The present invention is to fundamentally solve the above-mentioned problems, and has a high strength of 690 MPa or more, and is resistant to hot-dip galvanizing cracks in a bolted portion of a base material. An object of the present invention is to provide a method for producing a non-heat treated high-tensile steel having excellent characteristics.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to achieve this object. (1) C: 0.08 to 0.20% by weight, Si: 0.
60% or less, Mn: 1.0 to 2.0%, Cu: 2.0%
Ni: 2.0% or less, Cr: 1.0% or less, M
o: 1.0% or less, Nb: 0.1% or less, V: 0.1%
Hereinafter, steel containing Nb + 0.5V + Ti ≧ 0.08%, the balance being Fe and unavoidable impurities, is 1100-1
A method for producing a non-refined high-strength steel excellent in hot-dip galvanizing cracking resistance, wherein the steel is heated to 350 ° C., hot rolling is completed at 800 ° C. or less, and the tensile strength is set to 690 MPa class.

(2) The method for producing a non-refined high-strength steel excellent in hot-dip galvanizing crack resistance as described in (1), characterized by containing not more than 0.2% by weight of Ti.

(3) A non-heat treated high-strength steel excellent in hot-dip galvanizing crack resistance according to (1) or (2), wherein Ca: 0.004% or less is added in a weight ratio. It is a manufacturing method.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have prepared 0.11C-0.
25Si is used as a basic component, Mn, Nb, V, and the amount of Ti added are changed, and even steel including Ca is added to form an equilateral thick angle iron by hot rolling. The effect of added elements on the relationship was discussed. Room temperature tension is JIS No. 1A test piece.
The test pieces shown in the table below were used.

[0010] As a result, the relationship between the two is organized by the value of Nb + 0.5V + Ti, and in the case of high-strength steel, when drilling a bolt hole or the like, Nb + 0.5V + Ti ≧ 0.08.
It was found that it was necessary to satisfy (In general, it is said that when the elongation is 20% or more in a hot dip galvanizing bath, cracking in the bolted portion can be prevented.) That is, as shown in FIG. 5
When the added amount of V + Ti is 0.06%, the elongation in the tension in the plating bath is low, and the elongation in the high-strength steel is 20% or less. It was found that the elongation exceeded 20%, and that elongation was further increased by the addition of Ti, Ca or Ti-Ca. It should be noted that elements not contained in the formula are calculated as 0.

Hereinafter, the reasons for limiting the added components will be described.

C: 0.08 to 0.20% C is an essential element for increasing the strength. If it is less than 0.08%, it is difficult to obtain a strength of 690 MPa or more.
If it exceeds 20%, the toughness of the steel is significantly deteriorated.
It was limited to not less than 08% and not more than 0.20%.

Si: 0.60% or less Si is related to the appearance after plating, and if it exceeds 0.6%, plating burn is likely to occur. Therefore, 0.
It was limited to 60% or less.

Mn: 1.0 to 2.0% Mn is an essential element in view of strength and toughness.
%, It is difficult to obtain a strength of 690 MPa or more,
If it exceeds 2.0%, the hardenability increases, coarse bainite is formed, and the toughness is significantly deteriorated.
% To 2.0%.

P: unavoidable impurity level P segregates at grain boundaries and degrades toughness, but the upper limit value is not limited because it is sufficiently reduced by the current refining technology.
Lower is more desirable.

S: Inevitable impurity level S is mainly present in the form of inclusions in steel and causes deterioration of the material due to embrittlement. However, since the current refining technology is sufficiently reduced, the upper limit value is limited. No, but lower is better.

Cu: 2.0% or less Cu is an effective element for increasing the strength of steel.
When added in excess of 0%, Cu cracks are likely to occur. Therefore, it was limited to 2.0% or less.

Ni: 2.0% or less Ni is an element effective for increasing the strength and toughness of steel, but is limited to 2.0% or less in consideration of economy.

Cr: 1.0% or less Cr is an element effective for increasing the strength of steel.
If added in excess of 0%, the toughness of the steel deteriorates.
It was limited to 1.0% or less.

Mo: 1.0% or less Mo is an element effective for increasing the strength of steel.
If added in excess of 0%, the toughness of the steel will be significantly degraded, so it was limited to 1.0% or less.

Ti: 0.2% or less Ti is an effective element because it enhances the strength of steel by precipitation strengthening with a small amount of addition, and exhibits excellent properties in hot-dip galvanizing cracking resistance. If added in excess of, the precipitates in the steel become coarse and the toughness of the steel deteriorates significantly.
It was limited to 2% or less, and could be added as needed.

Nb: 0.1% or less, V: 0.1% or less Nb and V are effective elements for increasing the strength of steel by precipitation strengthening with a small amount of addition, but exceeding 0.1%. If added excessively, the toughness of the steel is remarkably deteriorated.

Nb + 0.5V + Ti ≧ 0.08% Ti, Nb, and V are all effective elements for increasing the strength of steel by precipitation strengthening, but have a strength of 690 MPa or more and are resistant to hot-dip galvanizing cracks. As a condition showing excellent characteristics, it is assumed that a relational expression of Nb + 0.5V + Ti ≧ 0.08% is satisfied. Note that these elements can be added as needed.

Ca: 0.004% or less Ca is the only element that can significantly improve the hot-dip galvanizing crack resistance by adding Ca. But,
If it is added in excess of 0.004%, Ca-OS clusters are generated and the cleanliness of the steel decreases. Therefore, Ca was limited to 0.004% or less.

Al: In the present invention, Al may be added for deoxidation. In this case, the usual addition amount (0.
005 to 0.060%). Treated as inevitable impurities in Si deoxidation in section steel and the like.

B: Inevitable impurity level B improves the hardenability of steel, but significantly deteriorates the hot-dip galvanizing cracking resistance of the welded portion. Therefore, when welding, B is controlled to 2 ppm or less.

Since the steel of the present invention is not intended for welding in principle, the upper limit of B is controlled to about 5 ppm. However, B is preferably as low as possible, and if welding is inevitable, the above is followed.

Next, the reasons for limiting the manufacturing method will be described.

Heating temperature: 1100 to 1350 ° C. If the heating temperature is 1100 ° C. or less, sufficient strength cannot be obtained because the added element expected to strengthen precipitation does not completely form a solid solution. Further, when heated to 1350 ° C. or higher, the heated particle size becomes extremely coarse, and the toughness is deteriorated. Therefore, the heating temperature was limited to the range of 1100 to 1350 ° C.

Rolling finish temperature: 800 ° C. or less If rolling is completed at a temperature exceeding 800 ° C., the structure becomes coarse and sufficient toughness cannot be obtained. Therefore, the rolling finishing temperature was limited to 800 ° C. or less.

[0031]

Examples are shown in Table 1. Each steel was rolled into an equilateral equal thickness angle steel under the respective manufacturing conditions shown in the table. The term of presence or absence of plating cracks in the table, as in the actual construction for rolled angle iron,
This is a result of confirming whether or not a crack is generated from a drilled portion after immersing the jointing bolt in a hot-dip galvanizing bath after drilling.

The term "manufacturing" in Table 1 shows the conditions of the heating temperature and the rolling finishing temperature of each steel by symbols A to D, respectively. Symbol A is a condition at a heating temperature of 1230 ° C. and a rolling finish temperature of 775 ° C., symbol B is a condition at a heating temperature of 995 ° C. and a rolling finish temperature of 715 ° C., and symbol C is a heating temperature of 1255.
C shows a condition of a rolling finishing temperature of 867 ° C, and symbol D shows a condition of a heating temperature of 1384 ° C and a rolling finishing temperature of 795 ° C.

The term “TS” in Table 1 is based on JIS1
Tensile strength at normal temperature (MPa) investigated using A test piece
It is shown.

The term “vTrs” in Table 1 indicates the fracture surface transition temperature (° C.) of each steel.

Since 1-1 and 1-2 satisfy the chemical components and production conditions of the present invention, they have a strength of 690 MPa or more, excellent toughness, and no plating crack.

1-3 has a low C and a low heating temperature.
90 MPa class strength cannot be secured.

1-4 have a large C content and a rolling finish temperature of 85
Since the temperature is as high as 0 ° C., the strength becomes higher than 810 MPa, the toughness is deteriorated, and plating cracks occur.

In the case of 1-5, a large amount of Si causes plating burn, and a high heating temperature results in high strength, poor toughness, and plating cracks.

1-6 has low strength due to low Mn content, and the tensile strength does not reach 690 MPa.

In the case of 1-7, since the amount of Mn is large, the heating temperature is low but the strength is high, and plating cracks occur.

In 1-8, since the content of Cu is large and the rolling finish temperature is high, the toughness is deteriorated and plating cracks occur.

Since 1-9 contains a large amount of Ni and a remarkably high heating temperature, the strength is high, the toughness is deteriorated, and plating cracks occur.

In the case of 1-10, since the amount of added Cr is large, the production conditions satisfy the claims of the present invention, but plating cracks occur.

In the case of 1-11, since the amount of Mo added is large, the heating temperature is low, but the strength is high and plating cracks occur.

In No. 1-12, the added amount of Nb + V + Ti was small and the rolling finish temperature was high, so that the toughness was low and plating cracks occurred.

In the case of 1-13, since the amount of Ti added is too large, the precipitate becomes coarse, and as a result, plating cracks occur.

1-14 has a high strength due to a large amount of Nb added, and has plating cracks.

In No. 1-15, since the amount of V added is large and the rolling finishing temperature is high, the toughness is deteriorated and plating cracks occur.

In No. 1-16, since the Ca content was high and the cleanliness of the steel was inferior, the strength was at a level of 690 MPa, but cracks occurred.

Although the components 1-17 satisfy the claims, the heating temperature is extremely high, so that the structure becomes coarse, the toughness is deteriorated, and plating cracks occur.

1-18 has high strength and high toughness because both components and manufacturing conditions satisfy the claims.
No generation of plating crack is observed.

The component 1-19 satisfies the claims, but the rolling finish temperature is high, so that the toughness is low and plating cracks occur.

The component 1-20 satisfies the claims, but the heating temperature is extremely high, so that the structure becomes coarse, the toughness is deteriorated, and plating cracks occur.

Each of 1-21, 1-22, and 1-23 satisfies the claims with respect to the components and the manufacturing conditions, so that all of them have high strength and do not generate plating cracks.

[0055]

[Table 1]

[0056]

The steel material produced by the method of the present invention has high strength and does not crack even after hot-dip galvanizing after boring (or after welding and boring). Therefore, when used for steel towers, bridges, and buildings, they greatly contribute to weight reduction of these structures, and are extremely useful in industry.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the room temperature strength TS (MPa) of a steel material and the elongation (%) of a tensile test in a plating bath.

FIG. 2 is a diagram showing a tensile test piece for examining embrittlement of a base material in molten zinc.

Claims (3)

[Claims] 1. A weight ratio of C: 0.08 to 0.20%, S
i: 0.60% or less, Mn: 1.0 to 2.0%, Cu:
2.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V:
Steel containing 0.1% or less, Nb + 0.5V + Ti ≧ 0.08%, and the balance being Fe and unavoidable impurities
A method for producing a non-refined high-strength steel excellent in hot-dip galvanizing crack resistance, wherein the steel is heated to 00 to 1350 ° C, hot rolling is completed at 800 ° C or lower, and the tensile strength is set to a 690 MPa class. 2. The method for producing a non-heat treated high-strength steel excellent in hot-dip galvanizing crack resistance according to claim 1, wherein the steel contains Ti: 0.2% or less by weight. 3. The method for producing a non-refined high-strength steel excellent in hot-dip galvanizing crack resistance according to claim 1, wherein Ca: 0.004% or less is added in a weight ratio. .