JPS6213523A - Production of steel bar for low temperature use - Google Patents

Abstract

PURPOSE:To produce a steel bar having superior toughness at low temp. by regulating the components of a steel and subjecting the steel to hot rolling, rapid cooling and reheating under specified conditions. CONSTITUTION:The composition of a steel is composed of, by weight, 0.05-0.3% C, <=0.4% Si, 0.5-3% Mn, 0.5-2% Ni, 0.01-0.09% acid-sol. Al, <=0.1% Nb and/or <=0.1% Ti and the balance Fe with inevitable impurities. The steel is heated to <=1,100 deg.C, hot rolled at 700-850 deg.C finishing temp., rapidly cooled at once to <=300 deg.C surface temp. and reheated to 400-550 deg.C surface temp.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a method for producing a low-temperature steel bar used as a foundation for an LNG tank.

(ri) Conventional technology Steel rods used for foundations for LNG tanks, especially for semi-underground LNG tanks
The steel rods used in tanks are exposed to low temperatures, so ordinary copper rods become brittle at temperatures below -100°C.

Semi-underground LNG tank #''l: This tank is mainly used overseas because the construction cost is lower than that of an underground type.It is also used for applications in the low temperature range as mentioned above.

The steel bars that can be used include high NI steel, high manganese steel, and low C produced by low temperature heating + low temperature EE rolling (controlled!llE rolling).
-High Mn -Nh or TI added steels.

(c) Problems to be Solved by the Invention However, these conventional steel bars have the following serious problems.

That is, although the high Ni steel has extremely excellent low toughness, it is disadvantageous in terms of cost because it contains about 9% -Nt and the cost of the teeth is extremely high. Bars are manufactured by hot rolling, then air cooling from 770 to 920 C, repeated twice (2 degrees Celsius), and then tempered at 550 to 620 C, but in order to make steel bars for reinforcing bars, heat treatment is 90Next, as for the above-mentioned high Mn steel, 13% Mn and 15% Mn are used.
Mn + 18% Mn p 25% Mn steels have been developed, but these high Mn steels are as expensive as high Ni steels, and Mn is added during the melting process. There was a problem in that the fumes were scattered and became extremely problematic for the working environment, and often became an obstacle when producing in large quantities.

Moreover, the above-mentioned low 〇-high Mn -Nh or T! Additive steel is
Although it is molded by a manufacturing method that includes the steps of low-temperature heating and low-temperature rolling (controlled rolling), it has the following problems. That is, the above manufacturing method secures weldability at a low temperature, lowers kr5 by increasing Mn'i, delays recrystallization by adding alloying elements such as Nh or 'Pi, and lowers the finishing temperature. This method improves low-temperature toughness by controlling grains (by rolling directly above Ar5), but
Furthermore, improvement in low @ toughness is desired.

As a method for this, JP-A-57-114638 discloses a method in which quenching and tempering is performed directly after the above-mentioned controlled mEE elongation.

In this method, the finishing temperature is maintained at t-750 to 850°C and the steel is rapidly cooled to produce a reinforcing steel bar.

The present inventors conducted research on such a controlled rolling + direct surface hardening method and obtained the following new findings.

That is, when a steel bar with a low-high Mn-Nb system having the composition shown in Table 1 is manufactured by controlled rolling + direct surface hardening under the conditions shown in Table 2, the steel bar with the composition shown in FIG. i area (-
Unable to satisfy the desired Charpy value at 120°C), -1
It was confirmed that it is difficult to ensure toughness in the 40 to -170°C region.

It has been determined that this phenomenon is due to the fact that the mechanism that generally improves the toughness of steel at low temperatures by making the crystal grains finer is not stable at low temperatures when using fine grain surface hardening and tempering methods. .

Therefore, the steels listed in Table 1 are suitable for supplying inexpensive steels for in-line heat treatment in hot rolling lines, but it is difficult to ensure a property in the low temperature range of -140 to -170°C. It was found that this was difficult and that further improvements were needed for low-temperature steel bars.

The present invention has been made in view of this situation.

Table 1 (wt%) Table 2) Means for solving the problems, that is, the present invention, is as follows: 0: 0.05 to 0.30 wt% e81:≦
0.40 wt%.

Mn: 0.5-3. Owt% 1 N+: (
1,5-2. Owt% H8o#A#: n, 01
~0.09 wtK and Nb≦0.1 wt and T
Contains one or two of I≦Q, l wt%, and the remainder F
Steel T-11 consisting of E and unavoidable impurities is heated to a temperature below 100°C and hot rolled to ensure a finishing temperature of 700 to 850°C, and then immediately rapidly cooled to a surface temperature of 30°C.
This is a method for manufacturing a steel bar for low temperature use, which is characterized by heating the steel bar to 0°C or lower and then reheating it so that the surface temperature becomes 400 to 550°C.

The m action 0 has a strong effect on the formation of the austenitic structure (and is an inexpensive element, so it is desirable to increase it within an allowable range, but if the amount of 0 is large, weldability will be poor). Since the strength and toughness of the welded part are adversely affected, the maximum value of 0 amount is set to 0.
．． It was set at 30%. The reason why the lower limit values t-o and os are set is that if the value is less than this, problems such as a decrease in strength will occur.

Mn is an element along with O for stabilizing the austenite structure, and moreover, by lowering the Ar6 transformation point, it is possible to control EE elongation at a relatively low temperature, that is, to refine the crystal grains in the austenite region. Is possible.

Steel rods for reinforcing bars are generally used by welding and joining, so the kr5 transformation point increases by keeping the 0 content low, but by adding 0.5X or more of Mn, the F
) It has the effect of lowering the kr5 transformation point and facilitating controlled rolling for grain refinement at a relatively low degree.

On the other hand, Mn is 3. If too much O is included, the toughness and weldability will deteriorate, so the content should be 0.5 to 3.
I set it to 0.

Furthermore, A# has an excellent grain refining effect, but if its content is less than 0.01 in 8o#A#, desired grain refining cannot be achieved. On the other hand, if the content exceeds 0.09 (for So#A#), the amount of nonmetallic inclusions will increase rapidly and the toughness of the steel will deteriorate. I decided to make friends on .09.

8i has a deoxidizing and reinforcing effect, and if it is added in a large amount, the toughness will be significantly lowered because it is a ferrite phase-strengthening element. Therefore, it was set to 0.40 or less.

In addition, Nb and Ti have the function of retarding the recrystallization behavior after hot rolling in the austenite phase, and in order to improve low-temperature toughness, the finer the grain size of ferrite and hinocoolite, the finer the A, the better the properties. One or both of the important elements 6, 9, Nb and Ti are each 0.
．． It is preferable to contain less than 10%. The cost of alloying elements such as Nb or Ti is high (if more than 10 is added, the cost becomes extremely high and the steel bar is no longer a low-cost low-temperature steel bar).

It is a steel that stabilizes the N1 austenite structure and has the effect of improving low temperature toughness.
It has the effect of significantly improving toughness.

However, it is also an expensive element and adding more than 2.0 would be costly, so the amount should be reduced from 0.5 to 2.0.
I set it to 2.0.

Next, the manufacturing method of the present invention will be explained.

(Al heating method: In order to refine the grains, the lower the heating temperature, the better; however, the alloying elements Nb or Ti can be sufficiently dissolved in solid solution and precipitate after hot rolling, contributing sufficiently to grain refinement. As temperature, 1100'c6 is sufficient, so set the upper limit to 1100'c6.
°C Toji9.

(B) EE rolling conditions The rolling conditions are fine ferrite grains.In order to obtain fine ferrite grains, it is desirable that the so-called finishing temperature after hot rolling be as low as possible. On the other hand, if the finishing temperature is too low, warm working results in straining the steel structure, resulting in a sharp decline in low-temperature toughness. This is shown in FIG. 2, and it can be seen that a finishing temperature of around 700 DEG C. to 800 DEG C. gives the best bendability.

Table 3 shows the number of steel sets used to obtain the data shown in Figure 2.
Table 4 shows the manufacturing method.

Generally, rolling while controlling the rolling temperature near the Ar s transformation point is called controlled rolling, and grain control during hot rolling is important.

(0) Cooling condition As shown in Fig. 1, the cooling condition is such that the surface of the steel bar 110 immediately after hot rolling is rapidly cooled in a cooling trough 9, and the surface is made into a martensitic structure.After passing through the cooling trough, the center of the steel bar is heated. [by] tempering.

In Figure 1, 5 is a heating furnace, 6 is a rolling mill, ? -8-10 indicates a thermometer.

In general, quenched and tempered structures can significantly improve low-temperature toughness, but if quenching is excessive and tempering is insufficient, low-temperature toughness will be poor. Therefore, it is important to optimize the recuperation temperature, and it is necessary to control the recuperation temperature by injecting an appropriate amount of water. investigated the relationship between recuperation temperature and low-temperature toughness.The results are shown in Figure 3.As is clear from this figure,
The temperature range was set to 400 to 550°C because low-temperature toughness is good at a reheating temperature around 50°C.

Tables 5 and 6 show the composition and manufacturing conditions of the steel used to obtain the data shown in FIG. 3 above.

What is more important here is that as shown in Fig. 1, the cooling trough 9 performs rapid cooling immediately after rolling, but the reading on the thermometer 10 on the outlet side of the cooling does not exceed the temperature of the steel material. It is necessary to rapidly cool the site to a temperature below 300°C.

If such rapid cooling is not performed, a martensitic structure cannot be obtained in the surface layer of the steel bar, and a tempered martensitic structure with excellent low m toughness cannot be obtained in the surface layer.

In addition, the thermometer 8 is installed in a position that is sufficiently accessible for the cooling trough 9, and is used to manage the recuperation of the steel material, and one or more thermometers are installed on the same line to cool the steel material. It is important to ensure that the surface temperature of the steel bar reaches its maximum value.

(f) Examples Regarding the components of the method of the present invention, low m toughness was further investigated with various components. The results are shown in Table 9. Tables 7 and 8 show the steel composition and manufacturing conditions.

Table 8 *Average value (3 test pieces) As shown in Table 9, the conventional 3.5 Ni steel of 1 has good low-temperature toughness, but is expensive because it contains a large amount of Ni.

Steels &2 and 3 are conventional steels that do not contain Ni, but their low temperature toughness (vli'-120) is 10 kr-1 on average.
On the other hand, the steels Nos. 4 to 8 according to the present invention are conventional steels with a small amount of Ni added, but the rolling and heat treatment according to the present invention have lower low-temperature toughness. Extremely good.

(g) Effects of the invention As described above, if the method of the present invention is used, it will have excellent low-temperature toughness as a structural steel bar used at low temperatures, and will have a higher Ni content than the conventional one.
Steel has very little Ni il, and inexpensive steel rods can be obtained. In addition, the manufacturing process does not include a step of straightening the bends in the steel rod, so the process is simple and has the advantage of not causing fume scattering. Therefore, the method of the present invention is an extremely useful method industrially.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the rolling line according to the present invention, FIG. 2 is a diagram showing the relationship between the finishing temperature and Charpy impact value of the reinforcing bar according to the present invention, and FIG. 3 is a diagram showing the relationship between the reinforcing steel bar according to the present invention. FIG. 4 is a diagram showing the relationship between the finishing temperature and impact value of conventional steel. Agent Patent attorney Masaaki Akizawa Two people outside the party 7i'1 Figure 71'2 Figure 2F Finished with Motohito') ff
pli i±slip ga 3 figure-12 pide 1; A・Keru low blended leather
Figure-tzo”cr=Oke 5hla Special I Sleeve Finishing, Tori Finishing f±klJ ff Upward ff, Tori Voluntary Procedures Amendment June 20, 1985

Claims (1)

[Claims] (1) C: 0.05-0.30wt%, Si:≦0.4
0wt%, Mn: 0.5-3.0wt%, Ni: 0.5-2.0w
t%, Sol. Al: 0.01 to 0.09 wt%, and Nb≦
Steel containing one or two of Ti≦0.1wt% and Ti≦0.1wt%, with the remainder being Fe and unavoidable impurities, is heated to a temperature of 1100°C or less to ensure a finishing temperature of 700 to 850°C. 1. A method for producing a steel bar for low temperature use, which is characterized in that after hot rolling, immediately quenching is performed to reduce the surface temperature to 300°C or less, and then reheating is carried out so that the surface temperature is 400 to 550°C.