JPS63128118A - Manufacture of low temperature steel having superior toughness at low temperature - Google Patents

Abstract

PURPOSE:To obtain a low temp. steel having superior toughness at low temp. at low cost by restricting the amts. of sol. Al and N in a low temp. Ni steel and treating the steel by direct quenching so as to precipitate fine AlN and to form fine austenite grains. CONSTITUTION:A low temp. steel having a compsn. consisting of, by weight, <=0.1% C, <=0.5% Si, <=1% Mn, 8-10% Ni, 0.02-0.04% sol. Al, 0.004-0.006% N and the balance Fe with inevitable impurities is manufactured by refining and continuously cast into a slab. This slab is heated to <=1,100 deg.C, hot rolled and immediately quenched. It is then tempered at the Ac1 transformation point or below to obtained a low temp. steel having superior toughness at low temp. The resulting low temp. steel is suitable for use as a material for LNG transportation and storage containers.

Description

[Detailed description of the invention] [Industrial application field] In recent years, 9% Ni has been used as a material for transportation and storage containers for LNG, etc.
f14 is becoming more and more widely used.

Since these materials are used under harsh conditions of -160°C or lower, they are desired to have more stable low-temperature toughness than ordinary materials.

On the other hand, recently, in order to reduce energy consumption, the so-called direct quenching method, in which quenching is performed immediately after hot rolling, has been widely adopted. This method is being adopted not only for thick eIl pipes but also for seamless steel pipes, etc., but compared to normal reheating and quenching, the grain size tends to become coarser (for low m
ffl properties tend to deteriorate.

The present invention relates to a method for producing low-temperature steel having excellent low-temperature toughness, particularly 9% Nigti, which is produced by a direct quenching method.

[Prior art and its problems]

The reason for the variation in low-temperature toughness of 9% N
is considered to be precipitated. For this reason, conventionally sol, Al
Au coarsened by reducing 1ff1 or Nff1
Attempts have been made to suppress the precipitation of N, and stable low-temperature toughness has been obtained in the usual F and Atfl injection methods.

However, when 9%N1g4 is produced by a direct quenching method, a phenomenon is observed in which low-temperature toughness deteriorates when the sol, Al1 amount, or Nfft becomes too low.

The reason for this is that in the direct quenching method, compared to the reheating quenching method, the crystal grain size is determined by the heating of the slag and the subsequent hot rolling process, and since the influence of Takagata remains than the reheating quenching temperature, the grain size is generally more likely to become coarse. However, sol, Al2ff
If the amounts of i and N are large, IN will precipitate, blocking movement of grain boundaries and reducing the growth of crystal grains. Although the quenching method can obtain almost the same crystal grain size as the reheating quenching method, sol.

, l ffi or Nm becomes too low (too much, or the precipitation of l!N becomes too small), there is nothing to inhibit the growth of crystal grains, and the crystal grain size in the direct quenching method becomes coarse, resulting in poor low-temperature toughness.

In order to prevent this grain size coarsening in the direct quenching method, it is possible to lower the heating temperature of the steel billet, but A
If the amount of 12m precipitated is too small, the effect is not sufficient, and if the heating temperature of the steel billet is too low, rolling may become impossible in terms of mill power.

[Means for solving problems]

The present invention aims to prevent the growth of crystal grains on paper and to precipitate IN of an appropriate size without deteriorating low-temperature toughness.
We propose optimal manufacturing conditions that limit the amount of ol, Al1m, and N to ensure low-temperature toughness.

The present invention has a weight ratio of C≦0.1%, St≦0.5%, M
n≦0.0%, including Ni 8.0-10.0%, balance F
e and unavoidable impurities, and so 1. A4
0.02-0.04%, N 0.004-0.006%
A low-temperature steel with excellent low-temperature toughness, which is characterized by hot-rolling a steel for low-temperature use after heating it to a hot water temperature of 1100°C or less, immediately quenching it, and subsequently tempering it at a temperature below A c + transformation point. Concerning the manufacturing method of steel.

Furthermore, a method for manufacturing a low temperature steel characterized by containing MO≦0.5% is inspired.

Next, the reasons for limiting each component will be shown.

This is because C is inevitably included in the refining process and is the cheapest element for ensuring strength, but if it exceeds 0.1%, it adversely affects low temperature toughness.

Si is included in the refining process for deoxidation, but 0.
If it exceeds 5%, low temperature toughness will be adversely affected.

Mn is added to improve hardenability and ensure strength, but even if it is added in an amount exceeding 1.0%, the effect is small (economically unfavorable).

Ni is an essential element to ensure low m f and II properties, but if it is less than 8.0%, its effect is small;
Even if it is added in excess of 1%, the effect will be saturated and this is not economically preferable.

Mo may be added as necessary to ensure strength, but if it exceeds 0.5%, it is unfavorable in terms of low-temperature toughness.

The above component is a basic component of 9%N1g1, but the present invention is to limit sol, the amount of Al2, and the amount of N in addition to the above component.

Al is an element that is inevitably included in the refining process for deoxidation, but its influence is due to the precipitation of Al2N.
Great for low temperature toughness of steel. When A4N precipitates as fine precipitates, it prevents movement of grain boundaries, especially austenite grain boundaries during the hot rolling process, that is, prevents austenite grain boundaries from becoming larger due to recrystallization, and promotes grain refinement. ,
Although it has a positive effect on low-temperature toughness, if it precipitates as coarse precipitates, it becomes a starting point for cracking, which is not preferable in terms of low-temperature toughness. Therefore, sol and Al1ff1 are 0.02%
less than 2%, sufficient amount of A1 to precipitate fine A4N
If it exceeds 0.04%, coarse AuN will precipitate.

Nff1 is an element necessary to precipitate /lN, but if Ilt is 0.004%, A4 N is difficult to precipitate, and if the N content exceeds 0.006%, coarse IN is precipitated. As shown in Figure 1, in directly quenched materials, when the Al content is in the range of 0.02 to 0.04%, Nfil is 0.
．． Low temperature toughness (vE'-1
90''C: Good Charpy impact value at -11°C.

Unavoidable impurities include p, s, etc., but from the viewpoint of low-temperature toughness, it is preferable that these be low. Although P is not particularly limited here, it is preferably 0.020% or less, and optimally 0.01% or less if possible.

Although S is not particularly limited here, it is preferably o, oto% or less, and optimally 0.001% or less if possible.

The reason why hot rolling is carried out after heating at a heating temperature of 1100° C. or lower is that heating at a heating temperature exceeding 1100° C. causes grains to coarsen and the low temperature toughness after hot rolling to deteriorate.

The reason for quenching immediately after hot rolling is that the purpose of the present invention is to provide a method for producing 9% Ni steel that can be directly quenched. Tempering is not particularly different from normal tempering;
It is sufficient to temper it below the Ac+ transformation point.

〔Example〕

Ig! 9% Ni steel with the chemical composition shown in Table 1! After being melted in a top-blown converter, it was continuously cast into a slab. These continuously cast slabs were heated to 1050°C and hot a thick plate rolling was carried out.
(1) Direct quenching, tempering, (2) Reheating quenching, and tempering. Shock value vE -19 at 196℃ at that time
The temperature at 6°C was investigated and shown in Table 2.

As shown in Table 2, 9%N+f of codes A and B in the example of the present invention
In 14 Il! High vE -196 even in contact hardening method
9%N1g of codes C and E, which have a lower amount of N than that shown in °C
4 shows a low vE of -198℃ in the direct quenching method, and 9% Ni steel with high N content shows a low vE of -106℃ in the direct quenching method and the reheat quenching method. show.

(The following is a blank space) [Effect of the invention] By limiting the sol, Afl amount, and Nn of 9%N1fJ4 as in the present invention, fine lN is precipitated and fine austenite grains are obtained, and as a result, conventionally stable The problem of 9% Ni steel using the direct quenching method, where low-temperature toughness could not be obtained, was resolved.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between NQ and vE-196°C in the direct quenching method and the reheating quenching method. 0 20 40 60 80 100
12ON amount (ppm)

Claims (2)

[Claims] (1) Weight ratio: C≦0.1%, Si≦0.5%, Mn
≦1.0%, Ni8.0 to 10.0%, the balance consisting of Fe and inevitable impurities, and sol. Al0.0
A low-temperature steel containing 2 to 0.04% and 0.004 to 0.006% of N is heated to a heating temperature of 1100°C or lower, then hot rolled, immediately quenched, and then sintered at a temperature of Ac_1 transformation point or lower. A method for producing low-temperature steel with excellent low-temperature toughness. (2) The method for producing a low-temperature steel with excellent low-temperature toughness according to claim 1, characterized in that the low-temperature steel further contains Mo≦0.5% by weight.