JPH1096016A - Manufacturing method of non-heat treated high strength steel with excellent hot-dip galvanizing crack resistance - Google Patents

Abstract

(57) [Summary] [PROBLEMS] When used as a structural member such as a steel tower or a bridge, a crack that does not crack even if subjected to hot-dip galvanizing after bolt hole processing or the like is excellent in hot-dip galvanizing crack resistance. Provided is a method for producing high-quality high-strength steel. 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, Ti: 0.10 to 0.2%, the balance being Fe and inevitable impurities Steel consisting of
It is heated to 100 to 1350 ° C., hot rolling is completed at 800 ° C. or lower, and the tensile strength is set to 780 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 made into steel towers without welding work on site, plating cracking susceptibility of the base material is regarded as important, but high strength of 780 MPa or more is required. 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 780 MPa or more, and is resistant to hot-dip galvanizing cracks in bolted portions 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]

Means for Solving the Problems 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, including Ti: 0.10 to 0.2%,
1100 steel with the balance consisting of Fe and unavoidable impurities
A method for producing a non-finished high-strength steel excellent in hot-dip galvanizing crack resistance, wherein the steel is heated to about 1350 ° C, hot rolling is completed at 800 ° C or less, and the tensile strength is set to a 780 MPa class.

(2) Nb: 0.15% or less by weight ratio
V: 0.2% or less, and Nb + 0.5V + Ti ≧
(1) The method for producing a non-refined high-strength steel excellent in hot-dip galvanizing crack resistance as described in (1), wherein 0.175% is satisfied.

(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.
A steel having 25Si as a basic component and varying amounts of Ti, Nb, and V, and a steel to which Ca is added, is made into an equilateral equiangular iron by hot rolling, and has room temperature tensile strength and tensile elongation in a plating bath. The effect of added elements on the relationship with was investigated. The room temperature tensile was performed on a JIS No. 1A test piece, and the in-bath tensile test was performed on the test piece shown in FIG.

[0010] As a result, the relationship between the two is organized by the amount of Ti, and elongation by tensile in the plating bath of 20% or more, which is a measure for preventing cracking in the bolt hole processing portion due to hot-dip galvanizing due to the increase in Ti amount, It has been found that high strength steel can be secured.

That is, as shown in FIG. 1, when the addition amount of Ti is as low as 0.06%, the elongation in tension in the plating bath is low, and the elongation in high-strength steel is 20% or less.
When it is 0.10%, the elongation in tension in the plating bath becomes 20% or more even in high-strength steel.

Further, when Ca is added, the tensile elongation in the bath is improved. When one or two of Nb and V are added, an improvement effect is still observed, though not as much as Ca addition.

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 780 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 tends to occur. Therefore, 0.
It was limited to 6% 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 780 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 is not limited, since it is sufficiently reduced by the current refining technology.
Lower is more desirable.

S: unavoidable impurity level S is present in steel mainly in the form of inclusions and causes deterioration of the material due to embrittlement. However, since the current refining technology is sufficiently reduced, the upper limit 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: 0.1-0.2% Ti is an effective element which increases the strength of steel by precipitation strengthening with a small amount of addition and exhibits excellent properties in hot-dip galvanizing cracking resistance. Add more than 1%. 0.2
%, The precipitate in the steel becomes coarse and the toughness is remarkably deteriorated. Therefore, the content is limited to the range of 0.1 to 0.2%.

Nb: 0.15% or less, V: 0.2% or less Nb and V are effective elements for increasing the strength of steel by precipitation strengthening with a small amount of addition, but the Nb content is 0.15% or less. % Or V content exceeding 0.2% excessively deteriorates the toughness of the steel. Therefore, it is limited to the above value, and one or two types can be added. .

When Ti, Nb, and V are added, the relational expression of Nb + 0.5V + Ti ≧ 0.175% is satisfied as a condition for exhibiting high strength and excellent resistance to hot-dip galvanizing cracks.

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: Al is an indispensable element for deoxidation, but does not cause any problem at the current refining level addition amount, so the current inevitable impurity level was used. In the present invention, Al may be added for deoxidation, and in that case, the usual addition amount (0.005 to 0.060%) is used. 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 set 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 conditions will be described.

Heating temperature: 1100 to 1350 ° C. When 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.

[0033]

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 finish temperature of each steel by symbols A to D, respectively. The symbol A is a condition of a heating temperature of 1225 ° C. and a rolling finish temperature of 765 ° C., the symbol B is a condition of a heating temperature of 995 ° C. and a rolling finish temperature of 685 ° C., and the symbol C is a heating temperature of 1244 ° C.
C shows a condition of a rolling finish temperature of 877 ° C, and symbol D shows a condition of a heating temperature of 1365 ° C and a rolling finish temperature of 775 ° 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 the production conditions of the present invention, they have a strength of 780 MPa or more, excellent toughness, and no plating crack.

1-3 have a low C and a low heating temperature,
The strength of the 80 MPa class cannot be secured.

1-4 have a large C content and a rolling finish temperature of 87
Since it is as high as 7 ° C., the strength becomes extremely high at 959 MPa,
The toughness has deteriorated and plating cracks have occurred.

In the case of 1-5, a large amount of Si causes burning of the plating, and a high heating temperature has a high strength, a toughness is deteriorated, and a plating crack occurs.

1-6 has low strength due to low Mn content.

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 Cu is high, the toughness is deteriorated, and plating cracks occur.

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

In No. 1-10, since the amount of added Cr is large and the rolling finishing temperature is high, the strength becomes high, the toughness is low, and plating cracks occur.

In the case of 1-11, although the amount of Mo added was large, the heating temperature was low, but the strength was high and plating cracks occurred.

In the case of 1-12, since the amount of Ti added is within the claimed range, the production conditions are within the claimed range, but plating cracks occur.

1-13 indicate that the amount of Ti added is too large,
Due to the remarkably high heating temperature, the precipitate becomes coarse and the toughness is deteriorated, resulting in plating cracks.

Sample No. 1-14 has a high Nb addition amount and a high rolling finish temperature, and therefore has a high strength and has plating cracks.

In the case of 1-15, since the amount of V added was large, the heating temperature was low but the strength became high, and plating cracks occurred.

In No. 1-16, since the amount of Ca added was high and the cleanliness of the steel was inferior, the strength was at a level of 780 MPa, but cracks occurred.

The component 1-17 satisfies the claim range, but the structure is coarse, the toughness is deteriorated, and the plating crack occurs due to the remarkably high heating temperature.

The components 1-18 satisfy the claims, but have high toughness due to a high rolling finish temperature and have plating cracks.

The components 1-19 satisfy the claims, but have insufficient strength due to low heating temperature.

Since 1-20 satisfies the claims with respect to both components and production conditions, it has high strength and high toughness.
No generation of plating crack is observed.

Each of the components 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.

[0057]

[Table 1]

[0058]

The steel material produced by the method of the present invention has high strength and does not crack even when hot-dip galvanizing after bolt hole processing. 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. C: 0.08 to 0.20% by weight, 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, Ti: 0.10 to 0.2%, the balance being Fe and inevitable impurities Steel consisting of
A method for producing a non-refined high-strength steel excellent in hot-dip galvanizing cracking resistance, wherein the steel is heated to 100 to 1350 ° C, hot rolling is completed at 800 ° C or lower, and the tensile strength is set to a 780 MPa class. 2. Nb: 0.15% or less by weight, V:
0.2% or less and Nb + 0.5V + Ti ≧ 0.1
The method for producing a non-refined high-strength steel excellent in hot-dip galvanizing crack resistance according to claim 1, characterized by satisfying 75%. 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. .