JPH07242939A - Production of high strength steel plate having excellent low temperature toughness - Google Patents

Abstract

PURPOSE:To produce a thick steel plate exceedingly excellent in TS, in rolling in the austenitic unrecrystallized region and the two phase region of austenite and ferrite, by controlling the rolling temp. by utilizing the draft per pass. CONSTITUTION:At the time of subjecting a slab to hot rolling, the cumulative draft at >=950 deg.C is regulated to >=30%, successively, rolling in which the cumulative draft at the Ar3 -20 deg.C to the Ar3+180 deg.C is regulated to >=60% and, as for >=60% of the total time of passes, the draft per pass is regulated to >=15% is executed, and after that, air cooling or accelerated cooling is executed. In this way, intensive rolling is executed in the nallow temp. range in the vicinity of the Ar3, and the remarkable increase of the ferritic nuclear forming sites and the recovery and recrystallization of ferrite are enabled to refine the ferritic grains to a limit, by which excellent low temp. toughness and strength can be attained. As a result, the steel plate with >=15mm plate thickness having exceedingly excellent low temp. toughness of >=500MPa TS can be obtd.

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-strength steel sheet having a thickness of 15 mm or more having excellent low temperature toughness, and is preferably applied to a thick plate mill in the steel industry.

[0002]

2. Description of the Related Art In order to improve the low temperature toughness of high strength steel with the progress of thermo-mechanical treatment technology, the heating temperature of the cast slab is lowered to suppress the coarsening of the initial austenite grains, and Nb and Ti.
The microstructure is refined by adding alloy elements such as, controlled rolling and accelerated cooling. In particular, rolling in the austenite unrecrystallized region and austenite-ferrite two-phase region in controlled rolling increases the ferrite nucleation sites by stretching the austenite grains and introducing a deformation zone into the grains, and recovery / regeneration of the processed ferrite. It is extremely effective in improving the low temperature toughness and strength by refining the ferrite grains by crystallization. As a method for increasing the ferrite nucleation site in rolling in the austenite unrecrystallized region, for example, Proceedings of Microalloying
75 (1975), p120, it is effective to increase the cumulative reduction amount, for example, to lower the rolling temperature as is known in iron and steel 60 (1974) 11, S557.

Further, as a method of refining ferrite grains in rolling in the austenite-ferrite two-phase region to obtain excellent low temperature toughness, for example, iron and steel 65 (1979) 9, p1425
As is well known, it is effective to increase the cumulative reduction amount in a high temperature range where the processed ferrite is recovered and recrystallized. Therefore, for refining ferrite grains, Ar
It is desirable to increase the cumulative reduction by isothermal rolling near ₃ (transformation start temperature). The reduction ratio per pass in a general thick plate mill is at most about 15% at most, and in such conventional rolling, the temperature drop of the steel sheet becomes large during rolling, so it is necessary to heat the steel sheet from the outside. Without it, it was difficult to carry out strength rolling within a narrow temperature range near Ar ₃ . For example, JP-A-63-050426
In the publication, the rolling temperature range is Ar _{3 to} Ar ₃ + 150 ° C.
As described above, the temperature range of 150 ° C. is specified. Therefore,
In conventional rolling, the cumulative rolling reduction in the vicinity of Ar ₃ cannot be made sufficiently large, so there is a limit to the refinement of ferrite grains, and it has been difficult to obtain excellent low temperature toughness in high strength steel sheets.

[0004]

The present invention has a TS of 500.
It is an object of the present invention to provide a method for manufacturing a high-strength steel sheet having a markedly excellent low temperature toughness of at least MPa.

[0005]

The gist of the present invention is that the cumulative reduction amount at 950 ° C. or higher is 30 at the time of hot rolling of a steel slab.
% Or more, then the cumulative reduction amount at Ar ₃ -20 ° C to Ar ₃ + 80 ° C is 60% or more, and 60% of all passes.
The above is to perform rolling with a rolling reduction of 15% or more per one pass, and then perform air cooling or accelerated cooling. Hereinafter, the present invention will be described in detail.

Controlled rolling is performed by rolling in the austenite recrystallization region to make the austenite grains uniformly fine, and subsequently rolling in the austenite unrecrystallized region to stretch the austenite grains and introduce a deformation zone into the grains. This is a technique for increasing the ferrite nucleation site, and further for recovering and recrystallizing the ferrite processed by rolling in the austenite-ferrite two-phase region to make the ferrite grains fine. The technical idea of the present invention is to control the rolling temperature by utilizing the rolling reduction per pass in rolling in the austenite unrecrystallized region and the austenite-ferrite two-phase region, so that the temperature within a narrow temperature range near Ar ₃ can be achieved. In order to achieve excellent low temperature toughness and strength, the ferrite grains are refined to the utmost limit by carrying out a strength rolling at a high temperature to significantly increase the ferrite nucleation site and recover and recrystallize the ferrite.

Rolling temperature and 1 in rolling below 950 ° C.
As a result of the inventors' earnest study on the relationship with the rolling reduction per pass, as shown in FIG. 1, the rolling reduction per pass is 15
%, The decrease in the temperature of the steel sheet during rolling was significantly reduced, and it became clear that rolling within a narrow temperature range of 100 ° C. or less is possible. The reason why rolling with such a small temperature drop is possible is that the heat removal amount of the roll is reduced and the heat generation amount of the processing is increased due to the reduction of the number of passes. The inventors of the present invention utilize such a rolling having a small temperature drop to carry out rolling having a strength such that the cumulative reduction amount becomes 60% or more within a narrow temperature range of Ar ₃ −20 ° C. to Ar ₃ + 80 ° C. Has invented a method for obtaining extremely fine ferrite grains.

First, in rolling at 950 ° C. or higher, the cumulative reduction amount must be 30% or more. This is to obtain uniform and fine austenite grains by rolling in the austenite recrystallization region. Austenite grains recrystallize almost completely during rolling at 950 ° C or higher. If the cumulative reduction amount at 950 ° C. or higher is less than 30%, grain refinement due to recrystallization becomes insufficient, and rolling is performed in the austenite unrecrystallized region with some coarse recrystallized grains. A mixed grain structure containing ferrite grains is formed and the low temperature toughness deteriorates.

Subsequently, Ar ₃ -20 ° C. to Ar ₃ + 80 ° C.
The cumulative rolling reduction at 60% or more and the total number of passes is 60
% Or more, rolling with a rolling reduction of 15% or more per pass must be performed. This is a feature of the present invention.
This is a new method for significantly reducing the ferrite nucleation sites of austenite grains by performing strength rolling in the vicinity of r _{3 and also} for recovering and recrystallizing the ferrite to miniaturize the ferrite grains to the limit.

FIG. 2 shows the effect of Ar on the average ferrite grain size.
₃ shows the effect of cumulative reduction ratio at -20 ℃ ~Ar ₃ + 80 ℃. If the cumulative reduction amount at Ar ₃ −20 ° C. to Ar ₃ + 80 ° C. is less than 60%, the formation of ferrite nucleation sites and the recovery / recrystallization of ferrite will be insufficient, and the ferrite grains will not be sufficiently miniaturized. When the rolling temperature is less than Ar ₃ -20 ° C, the generation of work ferrite becomes remarkable and the low temperature toughness deteriorates. When the ratio of the number of passes that the rolling reduction per one pass of 15% or more is less than 60%, drop in temperature of the steel strip during rolling is increased, the cumulative amount of reduction at _{Ar 3 -20 ℃ ~Ar 3 + 80} ℃ Of 60% or more cannot be rolled.

The cooling conditions after rolling are not particularly specified. Further, tempering the cooled steel sheet to a temperature of Ac ₁ or lower does not impair the characteristics of the steel of the present invention. The chemical composition of steel desirable for applying the present invention is C ≦ 0.30%, Si ≦ 0.6%, and Mn ≦% by weight.
2.0%, Al ≦ 0.10%, Ni + Cr + Mo + Cu
≦ 2.0%, Nb + Ti + V + B + Ca ≦ 0.2% with the balance being Fe and inevitable impurities.

[0012]

EXAMPLES Table 1 shows the chemical composition of the steel slab. Table 2 shows the steel plate manufacturing conditions. Table 3 shows the mechanical properties of the steel sheet.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

[Table 3]

Steels 1 to 6 in Tables 2 and 3 are steels of the present invention, and steels 7 to 11 are comparative steels. The steel of the present invention has high strength (T
S ≧ 500 MPa and high toughness (vTrs ≦ −140 ° C., BDW
TT85% Shear FATT ≦ −50 ° C.), while the comparative steel has high strength but poor low temperature toughness. Steel 7 has a rolling reduction of less than 15% for all passes in rolling below 950 ° C.
r ₃ -20 ° C. to Ar cumulative reduction ratio at ₃ + 80 ° C. 60%
And the low temperature toughness is deteriorated because the ferrite grains are not sufficiently refined. Steel 8 has a reduction ratio of 15% or more per pass in rolling at less than 950 ° C, and the number of passes is less than 60%. Therefore, Ar ₃ -20 ° C to Ar ₃ +
Since the cumulative reduction amount at 80 ° C is less than 60% and the rolling end temperature is less than Ar ₃ -20 ° C, the ferrite grains are not sufficiently refined, and the generation of worked ferrite becomes remarkable, resulting in low temperature toughness. Deteriorated.

Steel 9 has an excessively high rolling start temperature below 950 ° C., so the cumulative rolling reduction at Ar ₃ -20 ° C. to Ar ₃ + 80 ° C. is less than 60%, and the ferrite grains are sufficiently refined. However, the low temperature toughness is deteriorated. Steel 10 is 95
Since the rolling start temperature below 0 ° C. is too low, the rolling finish temperature becomes less than Ar ₃ −20 ° C., the generation of work ferrite becomes remarkable, and the low temperature toughness deteriorates. Steel 11 is 95
Since the rolling start temperature below 0 ° C is too low, Ar ₃ -2
The cumulative rolling reduction at 0 ° C to Ar ₃ + 80 ° C is less than 60%, and the rolling end temperature is less than Ar ₃ -20 ° C. It becomes remarkable and the low temperature toughness is deteriorated.

[0018]

The high-strength thick steel sheet produced by the method of the present invention has much lower temperature toughness than conventional steels, and the benefits of the present invention are great.

[Brief description of drawings]

FIG. 1 is a chart showing the effect of the rolling reduction per pass on the rolling temperature in rolling (plate thickness 15 mm) in which the cumulative rolling reduction is less than 950 ° C. and 60%.

[2] on the ferrite grain size in average Ar ₃ -20 ° C.
Chart showing the effect of cumulative reduction ratio at ~Ar ₃ + 80 ℃.

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[Procedure amendment]

[Submission date] September 2, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 1

[Correction method] Change

[Correction content]

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location B21B 37/76

Claims (1)

[Claims] 1. When hot rolling a steel slab, the cumulative reduction amount at 950 ° C. or higher is set to 30% or higher, and then Ar ₃ -20.
℃ ~ Ar ₃ + 800 ℃ cumulative reduction amount of 60% or more,
Moreover, the rolling reduction rate per pass is 1 for 60% or more of all passes.
A method for producing a high-strength steel sheet having excellent low-temperature toughness, which comprises rolling at 5% or more and then performing air cooling or accelerated cooling.