JPH1096055A - Rolled or normalize high strength steels excellent in hot dip galvanizing cracking resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of crackings from the opening part of a bolt hole for joining sections at the time of plating, at the time of producing rolled or normalized high strength sections used for an iron tower for power transmission, by specifying its compsn. and adding Ti, Nb and V thereto. SOLUTION: The steel contains, by weight, 0.08 to 0.20% C, <=0.6% Si, 1.0 to 2.0% Mn, <=2.0% Cu, <=2.0% Ni, <=1.0% Cr, <=1.0% Mo, 0.10 to 0.20% Ti, and the balance Fe with inevitable impurities. Preferably, <=0.15% Nb and <=0.2% V and furthermore incorporated therein, and also, the conditions of Nb+0.5V+Ti>=0.175% are allowed to satisfy. More preferably, <=0.004% Ca is furthermore added thereto. This steel is subjected to hot rolling to form a high strength section of a 690MPa class, and its tensile elongation percentage in a hot dip galvanizing bath shows >=20% to prevent the generation of cracking from the periphery of the bolt hole.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[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 relate to steel materials that prevent cracks generated in welded portions, and at present there is little knowledge about high-strength steels from the viewpoint of preventing cracks from being formed in bolted holes.

[0005]

SUMMARY OF THE INVENTION The present invention fundamentally solves the above-mentioned problems, and provides a non-heat treated high-strength steel excellent in hot-dip galvanizing crack resistance of a base material. Things.

[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.
6% or less, Mn: 1.0 to 2.0%, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo:
A non-refined high-strength steel excellent in hot-dip galvanizing crack resistance, containing 1.0% or less and Ti: 0.10 to 0.2%, with the balance being Fe and unavoidable impurities.

(2) Nb: 0.15% or less by weight ratio
V: 0.2% or less, and Nb + 0.5V + Ti ≧
The non-refined high-strength steel excellent in hot-dip galvanizing crack resistance described in (1), wherein the steel satisfies 0.175%. (3) The non-refined high-strength steel excellent in hot-dip galvanizing crack resistance described in (1) or (2), wherein Ca: 0.004% or less by weight is added.

[0008]

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 was added was made into an equilateral equiangular iron by hot rolling, and the tensile test piece shown in FIG. Samples were taken and the effect of added elements on the relationship between room temperature tensile strength and tensile elongation in the plating bath was examined.

As a result, the relationship between the two is organized by the amount of Ti, and when Ti ≧ 0.10%, the elongation by tension in a plating bath is a measure for preventing cracking in a bolt hole processed portion by hot-dip galvanizing. It has been found that 20% or more is secured even in high-strength steel.

That is, as shown in FIG. 1, when the Ti content 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.
If it exceeds 0.10%, the elongation in tension in the plating bath will be 20% or more even in high strength steel.

When Ca is further 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.

The steel of the present invention has a slab heating temperature of 1100 to 1
If hot rolling is performed at 350 ° C. and a rolling end temperature of 850 ° C. or less, excellent hot-dip galvanizing characteristics can be ensured regardless of the type of plate, section steel or the like.

Hereinafter, the reasons for limiting the added components will be described. C: 0.08 to 0.20% C is an element essential for increasing the strength. If it is less than 0.08%, it is difficult to obtain high strength, and if it exceeds 0.20%, the toughness of the steel is significantly deteriorated.
Limited to 20% or less.

Si: 0.6% 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.
If it is less than 2.0%, it is difficult to obtain high strength, and if it exceeds 2.0%, hardenability is increased, coarse bainite is generated, and toughness is significantly deteriorated. Limited to. P: Inevitable impurity level P segregates at the grain boundaries and degrades toughness, but the upper limit value is not limited, since it is sufficiently reduced by the current refining technology.
Lower is more desirable. S: unavoidable 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 has sufficiently reduced it, the upper limit is not limited. Lower is more desirable. Cu: 2.0% or less Cu is an element effective 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 to 0.2% 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. Add the amount of 0.
If added in excess of 2%, the precipitates of the steel become coarse and the toughness is remarkably deteriorated. Therefore, the range is limited to 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 exceeds 0.15%. Or, if the V content exceeds 0.2% and is excessively added, the toughness of the steel is remarkably deteriorated. Therefore, it is limited to the above value, and one or two types can be added.

In the case where 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. However, since there is no problem with the addition amount at the current refining level, 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 0.60%) is used. S for steel
It is treated as an inevitable impurity in i-deoxidation. B: Inevitable impurity level While B significantly improves the hardenability of steel, it significantly deteriorates the hot-dip galvanizing cracking resistance of the welded portion. Therefore, when B is welded, it is controlled to 2 ppm or less. The steel of the present invention is not targeted for welding work in principle, and has a hole drilling degree, and the upper limit of B is about 5 ppm. However, lower B is more desirable.

[0017]

Examples are shown in Table 1. The steel of the present invention has, for example, a slab heating temperature of 1100 to 1350 ° C and a rolling end temperature of 85.
It is manufactured by hot rolling at a temperature of 0 ° C. or less, but in the examples, all steels were rolled into equilateral equiangular angle steel. As for the presence or absence of plating cracks in the table, the rolled angle iron is pierced in a hot-dip galvanizing bath after drilling holes for joining bolts in the same manner as in actual It is the result of confirming whether or not it occurs.

1-3 have low strength due to low C. Also, 1-6 has low strength due to low Mn. Since 1-12 is low Ti, the intensity level is 780
Although it is of the MPa class, cracks have occurred in the perforated portions.

1-5, 1-7, 1-8, 1-9, 1-1
0, 1-11, 1-13, 1-14, and 1-15 are Si, Mn, Cu, Ni, Cr, Mo, Ti, and N, respectively.
Since the added amounts of b and V are high and the strength is too high at 900 MPa or more, cracking occurs.

In No. 1-16, the Ca content is high and the cleanliness of the steel is inferior, so that the strength is at a level of 780 MPa, but cracks occur. On the other hand, 1-1, 1-2, 1-17, 1
-18, 1-19, and 1-20 are all high-strength steels and have no cracks because the components of the test steels all satisfy the range of the present invention.

[0021]

[Table 1]

[0022]

According to the present invention, a hot-dip galvanizing crack which does not cause a crack in a bolt-hole processing part even if it is subjected to hot-dip galvanizing after a bolt hole is formed in a structural member such as a steel tower or a bridge. A non-heat treated high-strength steel having excellent characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between the room temperature strength TS (MPa) of a steel material and the elongation (y) 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: 0.6% 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 Non-refined high-strength steel with excellent hot-dip galvanizing crack resistance, characterized by being made of 2. Nb: 0.15% or less by weight, V:
0.2% or less and Nb + 0.5V + Ti ≧ 0.1
The non-heat-treated high-strength steel excellent in hot-dip galvanizing crack resistance according to claim 1, wherein the steel satisfies 75%. 3. The non-finished high-strength steel with excellent hot-dip galvanizing crack resistance according to claim 1, wherein Ca: 0.004% or less is added in a weight ratio.