JPH0613733B2 - High toughness steel plate manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a thick steel sheet having high toughness.

[Prior Art] The properties of steel materials are determined by the chemical composition and heat treatment. Recently, it has become possible to manufacture a thick steel plate having good strength and toughness by a controlled rolling method which mainly involves rolling at low temperature and an accelerated cooling method which is followed by cooling after rolling.

These technologies are disclosed in Japanese Examined Patent Publication No. 49-7291 and Japanese Examined Patent Publication No. 57-2100.
No. 7 and Japanese Patent Publication No. 59-14535.

In summary of these technologies, after rolling austenite at high temperature to recrystallize it into fine grains, by rolling unrecrystallized austenite at low temperature, flattening of austenite is achieved and nucleation during ferrite transformation is achieved. It will increase the site.

By such a method, the ferrite crystal grains are made finer,
The toughness of steel is improved.

[Problems to be Solved by the Invention] However, in the conventional method, there is a limit to grain refinement by recrystallization of austenite, and it is also possible to increase nucleation sites during ferrite transformation by rolling unrecrystallized austenite. Due to the limit, it was not possible to make ferrite crystal grains finer than a certain value, and it was difficult to obtain toughness of a certain value or more.

[Means for Solving Problems] The present invention is a method for producing a high-toughness steel plate which can advantageously eliminate the above-mentioned drawbacks of the conventional method, and the gist thereof is C: 0.03 to 0.25. %, Si: 0.01 to 0.5%, Mn: 0.5
0-1.8%, Al: 0.005-0.10%, N: 0.001-0.010%
If necessary, Nb: 0.05% or less, Ti: 0.05% or less,
Cu: 0.5% or less, Ni: 1.5% or less, Mo: 0.5% or less, Cr: 1.0% or less, V: 0.05% or less, B: 0.002% or less, and one or more kinds are contained, and the balance is Fe and After casting steel consisting of unavoidable impurities, without cooling, or by heating the cold piece to a temperature of Ac ₃ points or higher, 900
Rolling is started at a temperature of ℃ or less, rolling with a maximum rolling reduction of 10% or less per pass and the longest interpass time of 15 seconds or less is added, and 100 seconds or more and 1000 seconds or less after the rolling is completed. The method is characterized in that accelerated cooling is performed after leaving or holding for the time.

The present invention will be described in detail below.

First, the reasons for limiting the components of the steel material of the present invention will be described.

C is an indispensable element for strengthening steel materials, and 0.03
If it is less than 0.1%, it is difficult to obtain the required high strength, and if it exceeds 0.25%, the toughness of the welded part is impaired.
% Or less.

Si is an element effective in promoting deoxidation and increasing strength, so 0.01% or more is added, but if added too much, weldability deteriorates, so it is kept to 0.5% or less.

Since Mn is effective as an element for improving low temperature toughness,
Add 0.5% or more, but if added over 1.8%, weld cracking may be promoted, so keep it below 1.8%.

Since Al is effective as a deoxidizer, 0.005% or more is added, but an excessive amount of Al forms inclusions harmful to the material, so the upper limit was made 0.1%.

N forms a nitride together with Al and is effective in refining the crystal grains, but an excessive amount of N impairs the toughness of the welded portion and therefore 0.001
% To 0.010% or less.

Both Nb and Ti are effective in refining the crystal grains when added in a small amount, and therefore, an amount that does not deteriorate the toughness of the welded portion may be added. Therefore, the upper limit of the addition amount is Nb and Ti.
0.05%.

Cu, Ni, Cr, and Mo are all known as elements for improving hardenability, and when added to the steel of the present invention, the strength of the steel can be increased, but excessive addition impairs weldability. Therefore, Cu is 0.5% or less and Ni is
It was limited to 1.5% or less, Cr to 1.0% or less, and Mo to 0.5% or less.

V is effective in increasing the strength due to the precipitation effect, but excessive addition will impair the toughness, so the upper limit was made 0.05%.

B is known as an element that improves the hardenability, and when added to the steel of the present invention, the strength of the steel can be increased, but excessive addition increases the precipitates of B and impairs toughness. Therefore, the upper limit was made 0.002%.

Next, the technical idea of the present invention will be described.

Conventionally, as a processing method for improving the toughness of thick steel plates, it has been considered effective to combine rolling in the recrystallization temperature range of austenite and rolling in the non-recrystallization temperature range of austenite.

This is because by rolling the austenite that has been refined by recrystallization in the unrecrystallized temperature region, flattening the austenite and refining the ferrite crystal grains by increasing nucleation sites in the ferrite transformation,
This is a method for improving toughness.

In general, whether or not austenite recrystallizes depends on the rolling temperature, the amount of rolling strain, and the time of leaving or holding after rolling, and the higher the rolling temperature and the larger the amount of rolling strain, the recrystallization starts in a short time after rolling. .

Furthermore, the grain size of austenite after rolling recrystallization depends on the strain amount during rolling, and the larger the strain amount during rolling, the finer the grain size after rolling recrystallization.

Therefore, when rolling in a temperature range where high-temperature recrystallization easily starts as in the conventional method, recrystallization occurs in each pass of rolling, and the grain size after rolling recrystallization is equal to the strain amount in each pass. Since it will be dominated, rolling grain refinement cannot be expected so much.

Regarding rolling in the non-recrystallization temperature region that is continuously performed, no matter how large the cumulative reduction ratio in the non-recrystallization temperature region is, there is a limit to the flattening of austenite, so the nucleation site during ferrite transformation is kept constant. It was difficult to increase above the value. That is, the conventional method has a limit in making the ferrite crystal grains fine and improving the toughness.

However, the present inventors have discovered a new fact that makes it possible to overcome the above-mentioned limit, and have invented a method for producing a thick steel sheet having excellent toughness at the center portion of the thickness based on the fact.

As described above, whether or not austenite is recrystallized is determined by the rolling temperature, the amount of rolling strain, and the time left or held after rolling, so the rolling start temperature is kept low,
Moreover, by controlling the rolling reduction and the time between passes in each pass, it is possible to suppress the recrystallization of austenite during the entire rolling process.

Furthermore, if you leave or hold for an appropriate time after rolling,
It is possible for the first time to cause recrystallization of austenite during that time.

At the time of this recrystallization, strain corresponding to the total cumulative rolling reduction is accumulated in austenite, and the austenite grain size after rolling recrystallization is the austenite grain size obtained by a conventional method of repeating recrystallization for each pass. It becomes much finer than.

Further, by accelerating cooling of such a microstructure of extremely fine austenite, far finer ferrite crystal grains are obtained as compared with the ferrite crystal grains obtained by accelerating cooling of unrecrystallized austenite conventionally. This makes it possible to manufacture a steel plate with extremely high toughness.

The present inventors have derived the method for producing a high-toughness steel plate of the present invention based on the above new findings.

The reasons for limiting the manufacturing method will be described in detail below.

In the present invention, the cast piece may be directly rolled without forming the cold piece after casting, or the cold piece after casting may be reheated and rolled. The heating temperature is Ac ₃ or higher, and it is not necessary to set an upper limit.

If the rolling start temperature is too high, it is difficult to suppress recrystallization of austenite, so the upper limit is set to 900 ° C.
In order to suppress recrystallization of austenite during all rolling, the maximum rolling reduction per pass is 10% or less,
The longest inter-pass time is 15 seconds.

If the rolling reduction per pass exceeds 10% or the maximum interpass time exceeds 15 seconds, recrystallization of austenite cannot be suppressed. If both the rolling reduction per pass and the time between passes are smaller than the above values, there is no problem and it is not necessary to set a lower limit.

Furthermore, in order to cause recrystallization of austenite only after the completion of rolling, it is left for a certain period of time or held,
Recrystallization does not occur within 100 seconds, and austenite coarsens due to grain growth after completion of recrystallization within 1000 seconds. Therefore, the time of leaving or holding is 100 seconds or more and 1000 seconds or more.

Accelerated cooling is a means for obtaining fine ferrite from the fine-grained austenite obtained by such a method, so both the cooling rate and the cooling stop temperature may be selected according to the desired strength, and in particular the range is determined. No need.

[Examples] First, when the method of the present invention and the comparative method shown in Table 2 were applied to the steels of the present invention and the comparative steels having the components shown in Table 1, the strength and toughness shown in Table 3 were obtained. Inventive steels showed excellent properties.

[Effects of the Invention] The present invention is capable of accelerating and cooling an extremely fine metal structure of austenite to obtain fine ferrite crystal grains, and has a large effect of producing a high toughness thick steel plate.

Claims (8)

[Claims] 1. By weight%, C: 0.03 to 0.25%, Si: 0.01 to 0.5%, Mn: 0.50 to 1.8%, Al: 0.005 to 0.10%, N: 0.001 to 0.010%, the balance being Fe and unavoidable. After casting the steel made of impurities into cold pieces after casting, or by heating the cold pieces to a temperature of Ac ₃ points or higher and starting rolling at a temperature of 900 ° C or lower, the maximum 1
Characterized by adding rolling with a rolling reduction of 10% or less per pass and the longest interpass time of 15 seconds or less, and accelerating cooling after leaving or holding for a time of 100 seconds or more and 1000 seconds or less after completion of rolling And a method of manufacturing a high toughness steel plate. 2. By weight%, C: 0.03 to 0.25%, Si: 0.01 to 0.5%, Mn: 0.50 to 1.8%, Al: 0.005 to 0.10%, N: 0.001 to 0.010%, and further Nb: 0.05% or less. , Ti: 0.05% or less, or the like, the steel having the balance of Fe and unavoidable impurities is not cast into cold pieces after casting, or the cold pieces are heated to a temperature of Ac ₃ points or more and 900 ° C or less. Start rolling at the temperature of 1
Characterized by adding rolling with a rolling reduction of 10% or less per pass and the longest interpass time of 15 seconds or less, and accelerating cooling after leaving or holding for a time of 100 seconds or more and 1000 seconds or less after completion of rolling And a method of manufacturing a high toughness steel plate. 3. By weight%, C: 0.03 to 0.25%, Si: 0.01 to 0.5%, Mn: 0.50 to 1.8%, Al: 0.005 to 0.10%, N: 0.001 to 0.010%, and further Cu: 0.5% or less. , Ni: 1.5% or less, Mo: 0.5% or less, Cr: 1.0% or less, or one or more of the following: Steel with balance of Fe and unavoidable impurities is not cast into cold pieces after casting or with cold pieces. Ac Heated to a temperature of ₃ points or higher, and started rolling at a temperature of 900 ° C or lower, with a maximum of 1
Characterized by adding rolling with a rolling reduction of 10% or less per pass and the longest interpass time of 15 seconds or less, and accelerating cooling after leaving or holding for a time of 100 seconds or more and 1000 seconds or less after completion of rolling And a method of manufacturing a high toughness steel plate. 4. By weight%, C: 0.03 to 0.25%, Si: 0.01 to 0.5%, Mn: 0.50 to 1.8%, Al: 0.005 to 0.10%, N: 0.001 to 0.010%, V: 0.05% or less, After casting steel with the balance Fe and unavoidable impurities after casting, the cold pieces are heated to a temperature of Ac ₃ points or higher, and rolling is started at a temperature of 900 ° C. or lower to reach a maximum of 1
Characterized by adding rolling with a rolling reduction of 10% or less per pass and the longest interpass time of 15 seconds or less, and accelerating cooling after leaving or holding for a time of 100 seconds or more and 1000 seconds or less after completion of rolling And a method of manufacturing a high toughness steel plate. 5. By weight%, C: 0.03-0.25%, Si: 0.01-0.5%, Mn: 0.50-1.8%, Al: 0.005-0.10%, N: 0.001-0.010%, B: 0.002% or less, After casting steel with the balance Fe and unavoidable impurities after casting, the cold pieces are heated to a temperature of Ac ₃ points or higher, and rolling is started at a temperature of 900 ° C. or lower to reach a maximum of 1
Characterized by adding rolling with a rolling reduction of 10% or less per pass and the longest interpass time of 15 seconds or less, and accelerating cooling after leaving or holding for a time of 100 seconds or more and 1000 seconds or less after completion of rolling And a method of manufacturing a high toughness steel plate. 6. In% by weight, C: 0.03 to 0.25%, Si: 0.01 to 0.5%, Mn: 0.50 to 1.8%, Al: 0.005 to 0.10%, N: 0.001 to 0.010%, and further Nb: 0.05% or less. , Ti: 0.05% or less, 1 or 2 types and Cu: 0.5% or less, Ni: 1.5% or less, Mo: 0.5% or less, Cr: 1.0% or less, 1 type or 2 or more, and the balance is Fe and unavoidable. After casting steel consisting of mechanical impurities, it is not cooled, or the cold piece is heated to a temperature of Ac ₃ points or higher, and rolling is started at a temperature of 900 ° C or lower.
Characterized by adding rolling with a rolling reduction of 10% or less per pass and the longest interpass time of 15 seconds or less, and accelerating cooling after leaving or holding for a time of 100 seconds or more and 1000 seconds or less after completion of rolling And a method of manufacturing a high toughness steel plate. 7. By weight%, C: 0.03 to 0.25%, Si: 0.01 to 0.5%, Mn: 0.50 to 1.8%, Al: 0.005 to 0.10%, N: 0.001 to 0.010%, and further Cu: 0.5% or less. , Ni: 1.5% or less, Mo: 0.5% or less, Cr: 1.0% or less, one or two or more, and V: 0.05% or less, B: 0.002% or less, and a balance of Fe and inevitable impurities. After casting, do not make cold pieces, or heat the cold pieces to a temperature of Ac ₃ points or more, and start rolling at a temperature of 900 ° C or less,
Characterized by adding rolling with a rolling reduction of 10% or less per pass and the longest interpass time of 15 seconds or less, and accelerating cooling after leaving or holding for a time of 100 seconds or more and 1000 seconds or less after completion of rolling And a method of manufacturing a high toughness steel plate. 8. By weight%, C: 0.03 to 0.25%, Si: 0.01 to 0.5%, Mn: 0.50 to 1.8%, Al: 0.005 to 0.10%, N: 0.001 to 0.010%, and further Nb: 0.05% or less. , Ti: 0.05% or less, 1 or 2 types and Cu: 0.5% or less, Ni: 1.5% or less, Mo: 0.5% or less, Cr: 1.0% or less, 1 or 2 or more types, and V: 0.05% Hereinafter, B: 0.002% or less, steel with the balance being Fe and inevitable impurities is not cast into cold pieces after casting, or the cold pieces are heated to a temperature of Ac ₃ points or higher and rolled at a temperature of 900 ° C or lower. Start and maximum 1
Characterized by adding rolling with a rolling reduction of 10% or less per pass and the longest interpass time of 15 seconds or less, and accelerating cooling after leaving or holding for a time of 100 seconds or more and 1000 seconds or less after completion of rolling And a method of manufacturing a high toughness steel plate.