JPH0517820A - Production of high tensile strength steel for structural use having galvanizing crack resistance - Google Patents

Abstract

PURPOSE:To produce a high tensile strength steel material for structural use having galvanizing crack resistance by subjecting a slab of steel with specific composition to hot rolling, accelerated cooling, and tempering treatment under respectively specified conditions. CONSTITUTION:A slab of a steel which has a composition containing, by weight, 0.02-0.20% C, 0.10-0.30% Si, 0.50-1.80% Mn, 0.005-0.100% Al, <0.0002% B, and <50ppm N or further containing one or >=2 kinds among <1.0% Cu, <1.0% Ni, <0.5% Cr, <0.5% Mo, <0.10% V, <0.08% Nb, and <0.02% Ti and satisfying an expression I is heated up to 900-1250 deg.C and then hot-rolled at a temp. between the Ar3 transformation point and the recrystallization finishing temp. at >=20% draft. The hot rolled stock is rapidly cooled by air cooling from a temp. between (Ar3-5 deg.C) and (Ar3-50 deg.C) down to <=200 deg.C at 15-100 deg.C/sec cooling rate and tempered at >=450 deg.C. By this method, the occurrence of cracks in a weld heat-affected zone at the time when this steel material is subjected to hot-dip galvanizing after welding can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field in which a structural member is welded and then hot-dip Zn plated for rust prevention such as a bridge, a building and a steel tower.

[0002]

2. Description of the Related Art Conventionally, the above structural members have been used for the purpose of rust prevention.
Hot-dip Zn plating is widely used after welding. During the hot-dip Zn plating, cracks often occur in the heat-affected zone of the structural member. Several proposals have been made so far as measures to prevent this cracking. For example, JP-A-62-50448 and JP-A-2-57669 propose measures for increasing the plating crack resistance of the weld heat affected zone by satisfying a certain relationship with the amount of alloying elements in the steel material. Japanese Patent Laid-Open No. 2-145721 proposes a measure to obtain a steel material having a low yield ratio by forming a structure containing acicular ferrite.

[0003]

Due to the increase in the tensile strength of the steel material accompanying the increase in size of the structure, the residual welding stress and the thermal stress at the time of plating increase, and the Zn plating cracks that occur when plastic strain occurs are as described above. The current situation is that new technology is required without seeing the solution even with the proposals made. INDUSTRIAL APPLICABILITY The present invention improves the cracking susceptibility of solid metal grain boundaries by the chemical composition and microstructure of steel materials, and achieves a low yield strength during Zn plating (450 ° C.) even when plastic strain occurs. It is an object of the present invention to provide a method for producing a high-strength steel for a Zn-resistant plating cracking structure, which improves plastic strain followability and solves the above problems.

[0004]

In order to solve the above problems, the gist of the present invention is as follows. (1) C: 0.02 to 0.20% Si: 0.10 to 0.30% Mn: 0.50 to 1.80% Al: 0.005 to 0.100% B: 0. 0002% or less N: Containing 50 ppm or less, consisting of balance Fe and unavoidable impurities, and simultaneously satisfying S _LM ⁴⁰⁰ = 227-320C-10Si-76Mn-50Cu-30Ni-92Cr-88Mo-220V-200Nb + 200Ti ≧ 53. the steel slab was heated to 900 ° C. or higher 1250 ° C. or less and then subjected to rolling reduction ≧ 20% of the heat rolling recrystallization finish temperature or less and Ar ₃ point or more, then cooling the resultant hot rolled sheet Ar ₃ 200 from the temperature of -5 ° C to Ar ₃ -50 ° C
A method for producing a high-strength steel for Zn-plated crack resistant structure, which comprises accelerating cooling to 15 ° C. or less at a cooling rate of 15 to 100 ° C. and tempering at 450 ° C. or more.

(2)% by weight C: 0.20 to 0.20% Si: 0.10 to 0.30% Mn: 0.50 to 1.80% Al: 0.005 to 0.100% B : 0.0002% or less N: Contains 50 ppm or less, and further Cu: 1.0% or less Ni: 1.0% or less Cr: 0.5% or less Mo: 0.5% or less V: 0.10% or less nb: 0.08% or less Ti: contain one or two or more of 0.02% or less, and the balance Fe and unavoidable impurities, and ^{_{S LM 400 = 227-320C-10Si-}} 76Mn-50Cu-30Ni Steel pieces satisfying -92 Cr-88Mo-220V-200Nb + 200Ti ≧ 53 at the same time are heated to 900 ° C. or higher and 1250 ° C. or lower, and then hot-rolled at a rolling reduction of 20% or less at a recrystallization end temperature or less and at Ar ₃ points or more. Was given and obtained 200 from the temperature of the Ar ₃ -5 ℃ ~Ar ₃ -50 ℃ by air cooling the rolled sheet
A method for producing a high-strength steel for Zn-plated crack resistant structure, which comprises accelerating cooling to 15 ° C. or less at a cooling rate of 15 to 100 ° C. and tempering at 450 ° C. or more.

As will be described below, the structural steel to which the present invention is applied has been confirmed to be a conventional welded structural steel in order to obtain the required material. It is composed of the type and amount of the additional element that is determined based on the relationship of the effect. Each element and the reason for adding it are shown below. C is added as an effective component for improving the strength of steel. If it is less than 0.02%, the strength required for structural steel cannot be obtained, and if it exceeds 0.20%, the welded portion is not welded. Precipitates island-shaped martensite, which significantly deteriorates the toughness of the steel.

Si is effective as a deoxidizing element for molten steel, but if added in excess, it causes the iron-zinc alloy layer to develop abnormally during Zn plating and causes a deterioration of the appearance called plating burn. % Or less. Further, if the addition amount is small, deoxidation is insufficient, so the content was made 0.10% or more. Mn is a component that improves the strength of steel, and is 0.5
%, It is necessary to add it in an amount of not less than 1.%, but if added in excess, the toughness of the welded portion will decrease, so 1.80% is made the upper limit.

Al, like Si, is an element effective in deoxidation and needs to be added in an amount of 0.005% or more. However, since excessive addition deteriorates the toughness of the welded portion, the upper limit is 0.100.
%. B is an effective element that improves the hardenability of steel. However, an excessive content increases the Zn plating cracking susceptibility, so the content is regulated to 0.0002% or less in consideration of the effect of B and the Zn plating cracking property.

The above are the basic components of the steel targeted by the present invention. For the purpose of increasing the strength of the base metal or improving the joint toughness, Cu: 1.0% or less depending on the required properties. , Ni: 1.0% or less, Cr: 0.5% or less, Mo:
0.5% or less, V: 0.10% or less, Nb: 0.08%
Hereinafter, one or more of Ti: 0.02% or less can be contained.

The addition amount of each of the above components is S_LM ⁴⁰⁰= 227-320C-10Si-76Mn-50Cu-30Ni-92 Cr-88Mo-220V-200Nb + 200Ti ≧ 53 It is necessary to satisfy the Zn plating cracking susceptibility index
It's a ground. The reason is S _LM ⁴⁰⁰Is less than 53, Zn resistance
This is because the plating cracking property deteriorates.

The heating temperature of the steel slab thus produced is set to 900 ° C. as a lower limit in consideration of the usual heating conditions for this kind of steel slab, that is, the workability of rolling due to the temperature decrease during rolling.
The upper limit is 1250 ° C to prevent coarsening of austenite. If the total rolling reduction below the recrystallization end temperature and above the Ar ₃ point is less than 20%, good toughness and effective ferrite precipitation cannot be obtained due to poor grain refinement, so below the recrystallization end temperature and above the Ar ₃ point. The rolling reduction ratio is set to 20% or more.

If the cooling start temperature is higher than Ar ₃ -5 ° C, precipitation of ferrite will be insufficient and yield strength at 450 ° C will not decrease, and if it is lower than Ar ₃ -50 ° C, ferrite will be excessively precipitated and the room temperature strength will be low. Since it does not satisfy 60 kgf / mm ² grade steel, it was set to Ar ₃ −5 ° C. or lower and Ar ₃ −50 ° C. or higher. The cooling rate is 15 to satisfy the required strength.
It was set to 100 ° C./sec.

In the production of 50 kgf / mm ² grade steel, the cooling start temperature is below Ar ₃ -5 ° C. and Ar ₃ -8.
There is no reservation even if the temperature is 0 ° C or higher.

[0014]

In order to solve the problems of the prior art, the present inventors repeated various experimental studies using general structural steel having the following chemical composition. The part of the actual structure where cracking occurs frequently is the fillet weld. The causes are the presence of high welding residual stress, the addition of secondary stress due to the deformation due to thermal stress during immersion in the plating bath, and the liquid metal embrittlement due to the penetration of molten Zn into the grain boundaries. Therefore, in order to improve the Zn plating cracking resistance, it is effective to suppress liquid metal embrittlement due to molten Zn, reduce the yield strength at 450 ° C., which is the temperature for performing Zn plating, and increase the plastic deformability.

Therefore, the present inventors repeated various experiments in order to establish a manufacturing method for effectively lowering the yield strength at 450 ° C. while maintaining the strength at room temperature. As a result, by precipitating ferrite, the yield strength at 450 ° C can be markedly reduced compared to conventional high-strength steel, plastic strain followability is improved, stress concentration is prevented, and cracking in the Zn bath is prevented. It was found that it is possible to carry out hot rolling at a rolling reduction of 20% or less at the recrystallization end temperature or lower and at the Ar ₃ point or higher, and then air-cooling the obtained hot-rolled sheet to Ar ₃ −5 ° C. It has been found that by accelerating cooling from the temperature of Ar _{3 to} 50 ° C. to 200 ° C. or lower, stable strength at normal temperature can be obtained, and the yield strength at 450 ° C. can be effectively reduced.

It is considered that this is because the combination of the above-mentioned limited rolling and cooling conditions makes it possible to obtain the metal structure suitable for satisfying the ordinary temperature strength and lowering the yield strength at 450 ° C. for the first time. The present invention is based on these findings.

[0017]

EXAMPLES The effects of the present invention will be specifically described below with reference to examples. Table 1 shows the compositions of the tested steels. Tables 2 and 3 show rolling, cooling, tempering conditions, normal temperature, high temperature strength, ferrite area ratio and Zn plating crack resistance. The Zn plating cracking resistance was evaluated by making a full-scale plate girder model shown in FIG. 1 with a steel plate having a plate thickness of 9 mm, immersing it in a molten Zn plating bath at 450 ° C., and then examining the presence or absence of plating cracks in each HAZ part. .

As is clear from Tables 2 and 3, all steel Nos. 1 to 7 as examples of the present invention are 60 kgf / mm ² grade steel and _SLM.
⁴⁰⁰ is 53% or more, and the yield strength at 450 ° C is 35 kg.
With a material of f / mm ² or less, the target strength was satisfied, Zn plating crack resistance was also good, and the required material could be obtained. Steel No. 8-13 Any S _LM is Meanwhile comparative steel
^{Although 400} is 53% or more, steel Nos. 8 and 11 do not have the required strength, and steel Nos. 9, 10, 12, and 13 have a small amount of ferrite precipitation and have high strength at 450 ° C, causing cracking. did. Steel Nos. 14 and 15 were able to obtain the target strength and microstructure, but both had S _LM ⁴⁰⁰ of less than 53% and cracking occurred.

However, with regard to Steel Nos. 8 and 11, the cooling start temperature is Ar ₃ -60 ° C., which satisfies 50 kgf / mm ² .

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

As is apparent from the above description, the alloy element
In addition to limiting the content of each element, S _LM ⁴⁰⁰By the formula
Steel strips with limited combinations of
Decomposes ferrite by combining the conditions in a limited way
The yield strength at the zinc plating temperature of 450 ℃.
Lowering and galvanizing the welded structure
For members that generate contact residual stress and plating thermal stress
However, it is clear that galvanization cracks can be prevented.
Therefore, the present invention has a great industrial effect.
Can be said.

[Brief description of drawings]

1 shows a plate girder model for evaluation of Zn plating crack resistance, (a) is a side view of the plate girder model for evaluation, (b) is a sectional view taken along line AA of (a), and (c) is. It is a BB sectional view of (a).

Claims (2)

[Claims] 1. By weight%, C: 0.02 to 0.20% Si: 0.10 to 0.30% Mn: 0.50 to 1.80% Al: 0.005 to 0.100% B: 0.0002% or less N: contains 50ppm or less, and the balance Fe and unavoidable impurities, and the ^{_{S LM 400 = 227-320C-10Si-}} 76Mn-50Cu-30Ni-92 Cr-88Mo-220V-200Nb + 200Ti ≧ 53 simultaneously Satisfying steel slabs are heated to 900 ° C. or higher and 1250 ° C. or lower, and then hot-rolled at a recrystallization end temperature or lower and an Ar ₃ point or higher with a rolling reduction of ≧ 20%, and air-cooling the obtained hot-rolled sheet. From the temperature of Ar ₃ −5 ° C. to Ar ₃ −50 ° C. to 200
A method for producing a high-strength steel for Zn-plated crack resistant structure, which comprises accelerating cooling to 15 ° C. or less at a cooling rate of 15 to 100 ° C. and tempering at 450 ° C. or more. 2. By weight%, C: 0.02 to 0.20% Si: 0.10 to 0.30% Mn: 0.50 to 1.80% Al: 0.005 to 0.100% B: 0.0002% or less N: 50 ppm or less, Cu: 1.0% or less Ni: 1.0% or less Cr: 0.5% or less Mo: 0.5% or less V: 0.10% or less Nb : 0.08% or less Ti: contain one or two or more of 0.02% or less, and the balance Fe and unavoidable impurities, and ^{_{S LM 400 = 227-320C-10Si-}} 76Mn-50Cu-30Ni- 92 Cr-88Mo-220V-200Nb + 200Ti ≧ 53 A steel piece that simultaneously satisfies 53 is heated to 900 ° C. or higher and 1250 ° C. or lower, and then hot-rolled at a rolling reduction of 20% or less at a recrystallization end temperature or less and at an Ar ₃ point or more. Hot rolled sheet obtained by applying 200 from the temperature of the Ar ₃ -5 ℃ ~Ar ₃ -50 ℃ and air cooling
A method for producing a high-strength steel for Zn-plated crack resistant structure, which comprises accelerating cooling to 15 ° C. or less at a cooling rate of 15 to 100 ° C. and tempering at 450 ° C. or more.