JPH062037A - Method for producing non-heat treated high strength steel with a tensile strength of 70 kgf / mm (2) or more - Google Patents

Abstract

(57) [Summary] [Purpose] Proposal of a method for manufacturing high-strength, high-strength steel with a tensile strength of 70 kgf / mm ² or more at low cost without tempering. [Composition] C: 0.04-0.08%, Si: 0.15-
0.35%, Mn: 1.30 to 2.00%, Cu: 0.30% or less, Ni: 0.70
% Or less, Nb: 0.015 to 0.060%, V: 0.015 to 0.100
%, The balance Fe and other unavoidable impurities
After heating at 1150 ° C or higher, when rolling, 40% or more of processing in the unrecrystallized γ region is performed, and immediately after controlled cooling at a cooling rate of 5 ° C / sec or more and a cooling stop temperature of 550 ° C or less,
A method for producing a steel sheet which is subjected to a precipitation treatment of heating at an appropriate temperature between 550 and 650 ° C and air cooling.

Description

Detailed Description of the Invention

[0001]

[Field of Industrial Application] The present invention relates to a pipe material, an industrial machine,
The present invention mainly relates to a method for manufacturing a high-tensile steel plate having a plate thickness of 15 mm or less, which is used for boom materials for construction machines, arm materials, and the like.

[0002]

2. Description of the Related Art Cu precipitation high-strength steel (ASTM A710) has been standardized as a steel subjected to tempering as a precipitation treatment type. On the other hand, the Nb-V component system forced cooling (A
There is a method of setting the cooling stop temperature of CC) to a low temperature to obtain strength, but in this method, there are cases in which strength varies widely and strength cannot be sufficiently guaranteed.

Therefore, in order to stably produce steel having a tensile strength of 70 kgf / mm ² , generally, a tempering treatment is adopted, but in this case, the DWTT characteristic which is the weight drop characteristic is inferior,
In addition, mass production has a problem of capacity, and the cost is significantly increased.

[0004]

As described above, in the high-strength steel sheet, the above-mentioned problems are encountered in the ACC method in order to ensure high strength and high toughness and to uniformly stabilize the material in the sheet thickness direction and the sheet width direction. Therefore, there is a problem that the manufacturing cost is high and the lead time is long because it is manufactured from heat-treated steel in the past.

The present invention satisfies the need for Nb-
It is an object of the present invention to provide a method for producing a high-strength steel sheet having a tensile strength of 70 kgf / mm ² or more while adopting ACC as a V-based component system while maintaining toughness.

[0006]

The present invention, in% by weight,
C: 0.04 to 0.08%, Si: 0.15 to 0.35%, Mn: 1.30 to 2.00
%, Cu: 0.30% or less, Ni: 0.70% or less, Nb: 0.015 ~
0.060%, V: 0.015 to 0.100%, Mo: 0.10% or less, Ti:
After heating a slab containing 0.020% or less and Al: 0.06% or less and the balance Fe and other unavoidable impurities at 1150 ° C. or higher, when rolling, 40% or more of processing in the unrecrystallized γ region is performed, Immediately, cooling rate 5 ℃ / sec or more, cooling stop temperature 5
After performing the following control cooling 50 ° C., heated at an appropriate temperature between 550 to 650 ° C., the tensile and characterized by applying deposition process of cooling intensity 70 kgf / mm ² or more non-tempered high tensile It is a method of manufacturing a steel sheet.

[0007]

First, the reasons for limiting the chemical components will be described. C: 0.04% by weight (hereinafter referred to as%) or more is necessary to obtain the required strength, and if the amount is added too much, problems occur in weldability, so the upper limit is made 0.08%. Si: 0.15% or more is required for deoxidation. On the other hand, if the amount of Si added is increased, the strength increases, but if it exceeds 0.35%, the toughness deteriorates, so the upper limit is made 0.35%.

Mn: 1.30% to give ductility and strength to the base material
The above additions are necessary. On the other hand, if it exceeds 2.00%, the weld hardenability is significantly increased, so the upper limit was made 2.00%. Cu: Strength of the base material is improved, but addition of a large amount deteriorates the toughness of the base material, so the upper limit was made 0.30%. Ni: An upper limit of 0.70% in order to improve toughness and strength, but an expensive element and to keep it within the range necessary to guarantee the characteristics.

Nb, V: Elements important for the present invention, both of which are precipitation hardening type elements, and their strength can be increased without increasing C, Mn and the like. Further, it has an effect of expanding the unrecrystallized region in hot rolling to reduce the ferrite grains after transformation and improving the toughness. However, if the content is less than 0.015%, the above effect is not sufficient, and if it exceeds 0.060% for Nb and 0.100% for V, the effect is not improved, and rather the toughness of the welded portion is deteriorated The upper limit is 0.060% and the upper limit of V is 0.100%.

Mo: Improves the strength of the base material, but addition of a large amount causes a problem in the heat-affected zone weldability of the weld metal, so the upper limit is 0.10.
%. Ti: Addition of a small amount has the effect of improving the toughness of the weld heat affected zone, but addition of a large amount deteriorates the toughness of the base metal, so the upper limit was made 0.020%. Al: The toughness of the base material is improved by refining the crystal grains,
The effect is not increased even if added in a large amount exceeding 0.06%, so the upper limit was made 0.06%.

Next, the reasons for limiting the rolling conditions will be described.
The heating temperature is set to 1150 ° C. or higher because Nb and V carbonitrides must be solid-dissolved in the base material. In the unrecrystallized region
The reason why 40% or more is applied is to make the ferrite grains after recrystallization fine and to maintain toughness.

In the controlled cooling, the cooling rate is set to a relatively high rate of 5 ° C./sec or more and the cooling stop temperature is set to 550 ° C. or less in order to obtain sufficient strength. Although the strength and toughness are increased by the above-mentioned chemical components, heating temperature, processing conditions and controlled cooling, there is unevenness in the material properties in the plate, and the properties required for the entire plate are not sufficiently satisfied. Uniform characteristics can be obtained in the entire plate by performing a precipitation treatment of heating to 650 ° C and cooling with air.

[0013]

EXAMPLES Table 1 shows the chemical composition. AP15LX x 80
A steel plate having a corresponding thickness of 10.1 mm was manufactured under the conditions shown in Table 2. The mechanical properties are also shown in Table 2. The tensile strength of the steel of the comparative example not subjected to the tempering treatment does not reach 70 kgf / mm ² , but the steel of the example of the present invention achieves 70 kgf / mm ² or more.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

According to the present invention, it becomes possible to manufacture a high-strength, high-strength steel having a tensile strength of 70 kgf / mm ² or more at low cost without performing heat treatment.

Claims (1)

[Claims] 1. C: 0.04 to 0.08% and Si: 0.
15-0.35%, Mn: 1.30-2.00%, Cu: 0.30% or less, Ni:
0.70% or less, Nb: 0.015 to 0.060%, V: 0.015 to 0.1
00%, Mo: 0.1% or less, Ti: 0.020% or less, Al: 0.06%
After heating the slab consisting of the balance Fe and other unavoidable impurities at 1150 ° C or higher, including the following, 40% or more of the processing in the unrecrystallized γ region is performed when rolling, and the cooling rate is 5 ° C immediately.
/ Sec or more, controlled cooling at a cooling stop temperature of 550 ° C or less, then heated at an appropriate temperature between 550 and 650 ° C, and subjected to precipitation treatment by air cooling Tensile strength 70 kgf /
A method for producing a non-heat treated high-strength steel sheet of mm ² or more.