JPS607005B2 - Manufacturing method for low temperature steel bars - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing birch steel that has high strength and toughness even at extremely low temperatures, particularly at -120°C or lower.

It is well known that, for example, reinforced concrete steel has been used for reinforced concrete structures in cold and polar regions, reinforced concrete freezers, tanks for liquefied gases such as LNG and LPG, etc. All of these reinforcing bars comply with JIS○3112.
manufactured in accordance with the provisions set forth in J.
Since IS standards were established with the assumption that they would be used at room temperature or higher, they would lack strength and toughness if exposed to extremely low temperatures as described above. .

Therefore, in recent years, efforts have been made to develop steel steel that maintains a certain level of high strength and high ballistic properties even when exposed to extremely low temperatures, as well as the above-mentioned low temperatures. At present, steel steel with low-temperature properties has not been obtained.

The present inventors have stated that ``From the above-mentioned viewpoint, in particular,
As a result of conducting research to obtain steel that has high strength and high quality even at extremely low temperatures of C: 0.02 to 0.1
7%, Si: 0.7% or less, Mn: 0.6 to 2.0%,
sol. Contains AI: 0.01 to 0.07%, and further contains "AI" as a strength improving component and toughness improving component as necessary.
Cu: 0.50% or less, Ni: 3% or less, Cr: 0.5
% or less, Mo: 0.20% or less, V: 0.15% or less,
Nb: 0.15% or less, T core 15% or less, Zr: 0.
One or two of the following: L: 0.01% or less, Ce: 0.01% or less, and Ca: 001% or less.
The steel is specified as having a composition (the above weight %, the following % means weight %), and the rest is Fe and unavoidable impurities.
After heating to a temperature within the temperature range of transformation point 125,000, rolling is performed under conditions such that the area reduction rate is 60% or more,
In this case, if only the final groove rolling from oval to round is austenite-ferrite two-phase rolling with a cross-section reduction rate of 10% or more, and rolling is performed by air cooling,
Initial austenite grains are refined by heating in the pre-rolling step, and further, by the rolling, ferrite grains are made into pongee grains by recrystallization of austenite grains, and ferrite grains are created by increasing ferrite precipitation nuclei by rolling in the final recrystallized austenite region. The grains are refined to produce a fine ferrite + barrite structure, and in this case, austenite is formed only once in the final groove rolling.

The ferrite two-phase rolling improves toughness due to the formation of separations (layered cracks parallel to the sheet surface that appear in the Charpy test), so the Yokozuna with the specified final dimensions obtained as a result is 1120mm. Even when exposed to extremely low temperatures, it maintains extremely high strength and excellent toughness, and if necessary, this steel can be forced to cool immediately after rolling without hardening, or be subjected to AC,
They found that tempering at a temperature below the transformation point also has a significant effect on improving strength and plowability. This invention was made based on the above findings, and the reason why the composition and rolling conditions of the steel were limited as described above will be explained below. [a- Composition of Steel: C component is contained in the steel in order to give it a certain strength, but if the content is less than 0.02%, the desired high strength cannot be secured; If the content exceeds 17%, the toughness deteriorates, so the content was set at 0.02 to 0.17%.

In addition, the Si component has a deoxidizing and reinforcing effect, and 0.7
% is allowed, but the LO. If the content exceeds 7%, the toughness will be significantly lowered, so the upper limit was set at 0.7%. The Mn component has the effect of forming a solid solution in the base material, strengthening it as a solid solution, and improving strength and resistance, but if its content is less than 0.6%, the desired effect cannot be obtained, On the other hand, if the content exceeds 2.0%, the toughness and weldability deteriorate, so the content was set at 0.6 to 2.0%. Furthermore, the N component has an excellent granulation effect, but its content is sol.A.
If I is less than 0.01%, desired pongee granulation cannot be achieved; If the AI content exceeds 0.07%, the amount of nonmetallic inclusions will increase rapidly and the ballability of the steel will deteriorate, so the content should be reduced to 0.07%.
It was set at 1% to 0.07%.

In addition, in the copper of this invention, the above-mentioned LCeL and Ca spheroidize the A-based inclusions and improve the impact value in ductile castles, but when added in excess of 0.01%, the effect is saturated. Not only that, but also the amount of inclusions increases and the impact value deteriorates, so the upper limit of each is set to 0.
．． It was set as 01%. Also Cu, Ni, Cr, Mo, V, N
b, Ti, and Zr are elements that improve the impact value or at least reduce the deterioration of the impact value and increase the strength.

Cu increases the strength with almost no deterioration in impact value, but if added in excess of 0.5%, the problem of surface cracking of the steel material occurs, so the content was set to 0.5% or less.

Ni is particularly effective in improving low-temperature toughness, but if it exceeds 3%, a bainite structure will result and the rutting properties will deteriorate if rolled as is, so the content is set to 3% or less. Since Cr deteriorates weldability if it exceeds 0.5%, it is set to 0.5% or less. Mo is 0.20
If the content exceeds 0.20%, a bainitic structure will result, resulting in significant deterioration of plowability and weldability, so the content was set to 0.20% or less. Nb, V, Ti, and Zr all increase the strength by precipitation of their respective carbides, but none of them
．． If added in excess of 15%, the strength-increasing effect will be saturated and a bainite structure will develop, degrading toughness, so the upper limit for each is set at 0.15%.

'b} Heating temperature Pongee If the heating temperature for granulation is lower than the Ac3 transformation point, it will not be possible to form a uniform and fine austenite structure.On the other hand, if it is heated to a temperature exceeding the Ac3 transformation point +250oo, grain growth will be inhibited. Since the pongee granulation became so severe that it was impossible to achieve the desired pongee granulation, the heating temperature for pongee granulation was determined to be in the temperature range of Ac3 transformation point to Ac3 transformation point +25000.

{c) Rolling conditions The fine-grained ferrite + pearlite structure achieves the desired high strength and
To obtain high toughness, the total reduction rate indicated by the area reduction rate is 6
0% or more is required.

It can also be obtained by low-temperature curling separation (layered separation seen on the fracture surface in the Charpy impact test). Since this separation has the effect of lowering the triaxial stress of fracture, in addition to the effect of refining the ferrite crystal grains, it also significantly lowers the Charpy fracture surface transition temperature. Conventionally, it was thought that separation did not occur in Kaba steel rolling, but
Separation occurs only in the final rolling (final groove rolling from oval to round) when a reduction of 10% or more is applied at a temperature of the ferrite + austenite mixed structure below the A transformation point of the steel. Therefore, the area reduction rate is 60
% or more, only the final rolling is austenite + ferrite 2
A restriction was applied to apply a reduction of 10% or more at phase temperature.
In addition, in other passes of 850 qo or more, the recrystallized austenite is turned into pongee grains, so the 850 qo or more is 10% or more per pass.
~Until one pass before the final rolling, one pass of rolling has little effect on the non-recrystallized steel, but the higher the total rolling reduction, the better. By the way, in the method of the present invention, it is preferable that the rolling reduction in the austenite castle is substantially 50% or more, but the upper limit of the rolling reduction is not particularly limited.

In other words, the actual upper limit of the rolling reduction rate is determined by the billet dimensions of the material, but from the point of view of product characteristics, there is virtually no upper critical value of the rolling reduction rate. Furthermore, as mentioned above, rolling in ferrite ten austenite two phase castle requires a reduction ratio of at least 10%, but in the method of the present invention, which limits rolling in this region to one time, The higher the rolling reduction ratio is, the better it is, and the upper limit of the rolling reduction ratio cannot be determined from the characteristics of the product obtained. However, when considering the capacity of the rolling mill, the rolling reduction ratio is 10 to 5.
It is practical to set it to 0%. The birch steel that has undergone the above-mentioned rolling process is air-cooled or forcedly cooled, but forced cooling that causes hardening at this time is not preferable because it deteriorates the toughness of the steel.

It is desirable to have a fine ferrite + pearlite structure. In addition, after the steel has been cooled once after rolling, it is
, even if the material is tempered to a temperature below the transformation point, the effects of the present invention will not be lost in any way. Next, the method for manufacturing Shinko steel of the present invention will be explained using examples and comparing with comparative examples.

Example: Steels having the compositions shown in Table 1 were melted by the usual melting method, and then made into billets of 15 ribs x 15 ratios, and then heated and rolled under the heating and rolling conditions shown in Table 1. By rolling, the present invention steel steels 1 to 12 and comparative steel steels 1 to 6 each having a diameter of 32 mm were manufactured.

In addition, all of the comparative Hagane steels were manufactured under conditions in which some of the manufacturing conditions were outside the scope of this invention, and the relevant conditions are marked with an asterisk and are shown in Table 1. .
Next, a tensile test and an impact test were conducted on the obtained Hansei Steel 1 to 12 of the present invention and Comparative Kaba Steels 1 to 6, and in the tensile test, the tensile strength (T.S.),
The yield point (Y.S.) and elongation (EI) were measured, and in the impact test, the fracture surface transition temperature (vTs) and -120
The V notch energy value (vE-120) at 00 was measured.

These measurement results are also shown in Table 1. 1 of Table 1 Q main) The remaining component is essentially old e.

As shown in Table 1-2, Comparative Shinko Steels 1 to 6, which were manufactured under manufacturing conditions outside the scope of the present invention, had poor impact properties. , the present invention Shinko 1 produced according to the production conditions of the present invention
It is clear that all of Nos. 1 to 12 have high strength and high toughness, and particularly exhibit extremely excellent toughness even at extremely low temperatures of -1,200°. As mentioned above, according to the method for manufacturing shimagane of the present invention, it is possible to produce ashashagane that maintains high strength and high ballability even at extremely low temperatures of -112,000 ℃.

Claims (1)

[Claims] 1% by weight, C: 0.02-0.17%, Si: 0.
7% or less, Mn: 0.6-2.0%, sol, Al: 0
．． 01 to 0.07%, with the remainder consisting of Fe and unavoidable impurities.
Heating to a temperature between C_3 transformation point + 250℃, the total cross-sectional area reduction rate is 60% or more, and only the final groove rolling from oval to round has a cross-sectional area reduction rate of 10% or more in the austenite-ferrite two-phase region. A method for manufacturing low-temperature steel bars, which involves rolling. 2% by weight, C: 0.02-0.17%, Si: 0.
7% or less, Mn: 0.6-2.0%, sol, Al: 0
．． Contains 01 to 0.07%, and La: 0.01
% or less, Ce: 0.01% or less and Ca: 0.01%
A steel having a composition that also contains one or more of the following, with the remainder consisting of Fe and unavoidable impurities, is
AC_3 Heating to a temperature between the transformation point + 250℃, the total cross-sectional area reduction rate is 60% or more, and only the final groove rolling from oval to round is austenite ferrite 2.
A method for producing a steel bar for low temperature use, characterized by rolling with a reduction in area of 10% or more in a phase region. 3% by weight, C: 0.02-0.17%, Si: 0.
7% or less, Mn: 0.6-2.0%, Sol, Al: 0
．． Contains 01 to 0.07% and Cu: 0.50
% or less, Ni: 3.0% or less, Cr: 0.5% or less, M
o: 0.20% or less, V: 0.15% or less, Nb: 0.
15% or less, Ti: 0.15% or less, Zr: 0.15%
A steel having a composition that also contains one or more of the following, with the remainder consisting of Fe and unavoidable impurities, is
AC_3 Heating to a temperature between the transformation point + 250℃, the total cross-sectional area reduction rate is 60% or more, and only the final groove rolling from oval to round is austenite ferrite 2.
A method for producing a steel bar for low temperature use, characterized by rolling with a reduction in area of 10% or more in a phase region. 4% by weight, C: 0.02-017%, Si: 0.7
% or less, Mn: 0.6-2.0%, sol, Al: 0.
Contains 01 to 0.07%, and La: 0.01%
Below, one or more of Ce: 0.01% or less and Ca: 0.01% or less, Cu: 0.50% or less, N
i: 3.0% or less, Cr: 0.5% or less, Mo: 0.2
0% or less, V: 0.15% or less, Nb: 0.15% or less, Ti: 0.15% or less, Zr: 0.15% or less, and the remainder is Fe and unavoidable Steel having a composition consisting of impurities is heated to a temperature between AC_3 transformation point and AC_3 transformation point +250°C, and the total cross-sectional area reduction rate is 60% or more, and only the final groove rolling from oval to round is changed to austenite. 10 in ferrite 2 phase area
A method for producing a steel bar for low temperature use, characterized by rolling with a reduction in area of % or more.