JPH06279855A - Production of high strength and high toughness steel sheet - Google Patents

Abstract

PURPOSE:To produce a steel sheet whose structure is refined and excellent in low temp. toughness, at the time of subjecting a bloom contg. specified amounts of Ti, N, Nb or the like to reheating and rolling by optimizing heating condition, hot rolling conditions and cooling conditions for the bloom. CONSTITUTION:At the time of subjecting a bloom contg., by weight, 0.003 to 0.03% Ti and 0.001 to 0.006% N, furthermore contg. one or two kinds of 0.003 to 0.1% Nb or 0.01 to 0.l0% V and contg. fine TiN to heating from the Ac1 transformation pint to 950 to 1250 deg.C, it is heated at >=20 deg.C/min temp. rising rate and is held within 60min. Next, it is subjected to hot rolling under the conditions in which the cumulative rolling reduction at <=950 deg.C is regulated into >=60% and the rolling finishing temp. into 680 to 900 deg.C into a sheet material, which is water-cooled to the temp. range of 350 to 600 deg.C at 3 to 40 deg.C/sec cooling rate and is thereafter allowed to cool or is temperted to the Ac1 transformation point or below after the water cooling. Initial austenite grain size is refined, and the final refining of the structure is attained, by which the steel plate excellent in low temp. toughness can be produced.

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 high toughness steel having fine crystal grains.
It can be widely applied to the production of forged products (thick plates, hot coils, shaped steel, etc.). Further, the steel produced by the method of the present invention is excellent in ductility and can be used for various purposes such as shipbuilding, construction, bridges, pressure vessels, and automobile parts.

[0002]

2. Description of the Related Art At present, as a method of refining crystal grains of a hot-worked product, a controlled rolling technique which is a combination of addition of refining elements (Nb, V, etc.) of the crystal grain and processing (rolling / forging) is generally used. Has been adopted. For example, as disclosed in JP-A-52-128821 and JP-A-56-87622, Nb and V-added controlled rolled steels are known to have good low temperature toughness. This is due to the refinement of ferrite grains through the refinement of austenite (γ) grains. That is, Nb dissolved during reheating of the slab becomes Nb (CN) in lattice defects introduced by rolling.
As a result, strain-induced precipitation is caused, and recrystallization of γ is significantly suppressed. And many deformation zones were introduced into the unrecrystallized γ grains,
This is because the stretched γ grain boundaries and the deformation zones within the grains act as ferrite nucleation sites and make the ferrite grains fine.

On the other hand, Japanese Unexamined Patent Publication No. 52-128821 and Research No. No. 297 (1979), page 49, describes a method of containing TiN in steel to suppress coarsening of the heated γ grain size and a method of reducing the reheating temperature of the slab to reduce the initial γ grain size. The ferrite grains are made finer through the fine γ grains to improve the toughness. However, even if the elements such as Nb and Ti that are effective for the refinement of the structure are utilized, the γ grain size becomes coarse if the heating rate and the holding time at the time of heating are not appropriate, and the γ is also increased by the subsequent rolling. Not enough micronization of the tissue,
Low temperature toughness deteriorates. Further, when the slab reheating temperature is lowered, the deformation resistance during rolling becomes large, and a heavy load is applied to the rolling mill, which causes a problem in productivity, and the amount of solid solution of Nb, V, etc. decreases, so Nb, There is a problem that the toughening effect of V and the like cannot be sufficiently exhibited.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the conventional method and to clarify the optimum manufacturing conditions to provide a method for manufacturing a high strength steel sheet excellent in low temperature toughness. It is provided.

[0005]

SUMMARY OF THE INVENTION The gist of the present invention is the weight%
And Ti: 0.003 to 0.03, N: 0.001 to 0.
006, and Nb: 0.003 to 0.1 or V:
Steel pieces containing one or two of 0.01-0.10 are heated from the A _c1 transformation point to a heating temperature of 950 to 1250 ° C at 20 ° C.
After heating at a heating rate of at least / min and holding for 60 minutes or less,
Rolling is performed at a rolling reduction temperature of 680 to 900 ° C. with a cumulative reduction of 950 ° C. or less of 60% or more, and Ti: 0.003 to 0.03, N: 0.001 to 0.0% by weight.
06, and Nb: 0.003 to 0.1 or V:
Steel pieces containing one or two of 0.01-0.10 are heated from the A _c1 transformation point to a heating temperature of 950 to 1250 ° C at 20 ° C.
After heating at a heating rate of at least / min and holding for 60 minutes or less,
After rolling at a rolling reduction temperature of 680 to 900 ° C. with a cumulative reduction of 950 ° C. or less of 60% or more, a cooling rate of 3 to 4
After water-cooling at 0 ° C / sec to any temperature below 600 ° C, A _c1
It is to temper at a temperature below the transformation point.

[0006]

FUNCTION The material of the controlled rolled steel is greatly affected by the γ grain size during slab reheating, and the heated γ grain size is influenced by the slab heating temperature and the chemical composition of the steel. It is a well-known fact that the lower the heating temperature, the smaller the heated γ particle size, and the inclusion of precipitates such as TiN to suppress coarsening of the heated γ particle size. The inventors found that in the Ti and N-containing steels, even if the reheating temperature of the slab was the same, the heating γ grain size could be remarkably refined by optimizing the heating rate during heating, and the low temperature toughness The inventors of the present invention have earnestly studied proper heating and rolling conditions of Ti and Nb-containing steels for producing an extremely excellent steel, and arrived at the present invention.

The present invention will be described in detail below. A feature of the present invention is that when reheating to rolling a slab containing a certain amount of Ti, Nb, and N, by optimizing the heating conditions during slab reheating, the initial γ grain size is refined, and By optimizing the rolling and cooling conditions thereafter, the final microstructure is sought to obtain a steel sheet having excellent low temperature toughness.

First, Ti: 0.003 to 0.03 is applied to the steel piece.
%, N: 0.001 to 0.006%, and Nb:
0.003-0.1% or V: 0.01-0.10%
Must be included. Addition of Ti forms fine TiN, suppresses coarsening of the heated γ grain size, and improves low temperature toughness. Further, Nb and V contribute to refinement of the structure and precipitation hardening based on the uncrystallized γ grains during rolling, and strengthen the steel. T
The lower limits of i and N are the minimum amounts necessary for forming TiN. Excessive addition of Ti causes deterioration of HAZ toughness due to precipitation hardening of TiC.
It is limited to 3 to 0.03%. On the other hand, addition of an excessive amount of N causes deterioration of HAZ toughness due to solid solution N, so the upper limit is set to 0.
006%. The lower limits of Nb and V are the minimum amounts required to exert the toughening effect. Excessive addition of Nb and V is H
Since the AZ toughness is deteriorated, the amount of Nb added is 0.003.
.About.0.1%, and the addition amount of V is limited to 0.01 to 0.10%.

Next, when heating the steel piece containing TiN from the A _c1 transformation temperature to 950 to 1250 ° C., 20 ° C. /
It is necessary to heat at a temperature rising rate of not less than a minute and hold it within 60 minutes. In the steel containing TiN, by increasing the temperature rising rate during heating, coarsening of the γ grain size is significantly suppressed. This is because if the A _c1 transformation temperature is exceeded, the ferrite grain boundaries and carbides are transformed into spherical γ grains one after another, and if the A _c3 transformation temperature is exceeded (after complete γ conversion), uniform and fine γ grains are obtained. is there. If the rate of temperature increase is less than 20 ° C./minute, the bainite structure is transformed into acicular γ grains containing undissolved carbides, so that the acicular γ grains remain as coarse γ grains even after complete γ conversion. However, a uniform and fine γ particle size cannot be obtained.

The heating temperature of 1250 ° C. is the upper limit temperature at which the austenite grains do not become coarse during heating. On the other hand, if the heating temperature is too low, the additional alloying elements are not sufficiently solutionized,
The internal quality of steel deteriorates and sufficient material improvement effects cannot be expected. Therefore, it is necessary to set the lower limit to 950 ° C. If the holding time exceeds 60 minutes, the γ particle size becomes coarse even if the temperature rising rate is increased, so the upper limit is set to 60 minutes. In addition, in heating, heating by an electric method (so-called energization heating) is desirable, but furnace heating by normal gas combustion may also be used.

After heating, the cumulative reduction amount of 950 ° C. or less is set to 60.
% Or more, and the rolling end temperature must be 680 to 900 ° C. This is because uniform and fine γ-grains are stretched by low-temperature rolling to thoroughly refine the ferrite grain size and improve low-temperature toughness. Cumulative reduction is 60
If it is less than g, the elongation of the γ structure is insufficient and fine ferrite grains cannot be obtained. If the rolling end temperature is 900 ° C. or higher, fine ferrite grains cannot be achieved even if the cumulative reduction amount is 60% or higher. However, if the rolling end temperature is excessively lowered, excessive (γ + α) two-phase region rolling is caused, which causes deterioration of low temperature toughness. Therefore, the lower limit of the rolling end temperature is set to 680 ° C.

As the cooling condition, after rolling, it is cooled at 3 to 40 ° C./sec to a temperature of 350 ° C. to 600 ° C. and then air cooled, or any temperature of 3 to 40 ° C./sec and 600 ° C. or less after rolling. After cooling down to 10 ° C., it must be tempered at a temperature below the A _C1 transformation point. If the cooling rate is too slow or the cooling stop temperature is too high, the effect of accelerated cooling cannot be sufficiently obtained, and an appropriate microstructure cannot be obtained. On the other hand, if the cooling rate is too high or the stopping temperature is too low, a hardened structure is formed and the low temperature toughness and HIC resistance are significantly deteriorated. Also, when reheating for the purpose of tempering or dehydrogenation,
It must be below A _c1 transformation point. When reheated to a temperature above the A _c1 transformation point, strength and low temperature toughness deteriorate.

[0013]

EXAMPLES Next, examples of the present invention will be described. Table 1
Table 2 shows the chemical composition of the sample steel and Table 2 shows the manufacturing conditions and mechanical properties. Steel sheets of various thicknesses were manufactured and their mechanical properties were investigated. The tensile properties were investigated using JIS-5 No. 5 tensile test pieces, and the Charpy properties were investigated using JIS-4 No. 4 test pieces taken from the 1/4 t portion. For weldability, the peak temperature is 135
HA at -20 ° C with a simulated thermal cycle of 0 ° C
The Z toughness was evaluated. In Tables 1 and 2, steels 1 to 4 are steels of the present invention, and 5 to 16 are comparative steels. The steels 1 to 6 of the present invention exhibit extremely good low temperature toughness.

On the other hand, in Comparative Steel 5, the HAZ toughness deteriorates because the Ti content is too large. Since the comparative steel 6 has a small amount of Ti, it has a small amount of TiN effective for suppressing the coarsening of the γ grain size, and the low temperature toughness of the base material deteriorates. Since the comparative steel 7 has a small amount of Nb or V, the strength and low temperature toughness of the base material are deteriorated. The comparative steel 8 has too much Nb, so the HAZ toughness deteriorates. Since the comparative steel 9 has an excessively large amount of N, the HAZ toughness is deteriorated.

Since the comparative steel 10 has a small amount of N, it has a small amount of TiN which is effective for suppressing the coarsening of the γ grain size, and the low temperature toughness of the base material deteriorates. Comparative Steel 11 does not have sufficient strength and low temperature toughness because the slab reheating temperature is too low, and internal defects are also recognized. Comparative Steel 12 has too large a slab reheating temperature, so that the initial γ grains are large and good strength and low temperature toughness cannot be obtained. Comparative Steel 13 has a rate of temperature rise from the A _c1 transformation temperature of 2
Since it is less than 0 ° C./minute, the initial γ grains are large and good low temperature toughness cannot be obtained. Comparative Steel 14 has a large initial γ grain size because the holding time during heating is too long, and the low temperature toughness deteriorates. Comparative Steel 15 has a cumulative reduction of 60% below 950 ° C.
Therefore, the low temperature toughness deteriorates. Comparative steel 16 has a rolling end temperature of more than 900 ° C., and thus its low temperature toughness deteriorates. Comparative steel 17 has a rolling end temperature of less than 680 ° C., and thus its low temperature toughness deteriorates. Steel 18 deteriorates in low temperature toughness because of its low cooling rate. Steel 19 has low strength because the cooling stop temperature is too high. Since the tempering temperature of steel 20 is too high, strength and low temperature toughness deteriorate.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

INDUSTRIAL APPLICABILITY The present invention provides a means for inexpensively producing a steel sheet having good low temperature toughness, and can secure the safety of a steel structure produced using this steel sheet.

Claims (2)

[Claims] 1. Ti: 0.003 to 0.03 by weight%,
N: 0.001 to 0.006, and Nb: 0.00
3 to 0.1 or V: 0.01 to 0.10. Steel pieces containing one or two kinds are heated from the A _c1 transformation point to a heating temperature of 950 to 950.
Heat up to 1250 ° C at a heating rate of 20 ° C / min or more, and
After holding for less than 0 minutes, the cumulative rolling reduction below 950 ° C is 60
%, At a rolling end temperature of 680 to 900 ° C., water cooling to 350 to 600 ° C. at a cooling rate of 3 to 40 ° C./second, and then leaving to cool, followed by cooling. . 2. Ti: 0.003 to 0.03 by weight%,
N: 0.001 to 0.006, and Nb: 0.00
3 to 0.1 or V: 0.01 to 0.10. Steel pieces containing one or two kinds are heated from the A _c1 transformation point to a heating temperature of 950 to 950.
Heat up to 1250 ° C at a heating rate of 20 ° C / min or more, and
After holding for less than 0 minutes, the cumulative rolling reduction below 950 ° C is 60
%, At a rolling end temperature of 680 to 900 ° C., then water-cooled to an arbitrary temperature of 600 ° C. or less at a cooling rate of 3 to 40 ° C./second, and then tempered at a temperature of A _c1 transformation point or less. Of high strength and high toughness steel sheet.