JPS5873717A - Production of low temperature steel - Google Patents

Abstract

PURPOSE:To produce low temp. steel of excellent low temp. toughness and strength by controlling the micro structure and heat treatment of steel consisting basically of limited compsns. of C, Si, Mn, Ni, Al and contg. Cr, etc. according to need. CONSTITUTION:The steel contg. <=0.2% C, 0.01-0.8% Si, 0.5-4.5% Mn, 0.5- 10.0% Ni, <=0.07% Alsol as basic components, contg. >=1 kind among <=1% Cr, <=0.8% Mo, <=0.5% Cu, <=0.2% V, <=0.1% Ti, <=0.2% Nb, <=0.03% La, <=0.05% Ce, and consisting of the balance substantially Fe is hardened at 15- 60 deg.C/s cooling rate from the temp. region higher than the Ac3 point to martensite and lower bainite structure. After this steel is heated to the temp. region lower than the Ac3 point and higher than the Ac1 point, the steel is hardened at 15-60 deg.C/S cooling rate and is then tempered at temp. lower than the Ac1 point, whereby the inexpensive low temp. steel of good weldability is obtained.

Description

[Detailed description of the invention] The present invention relates to a method for manufacturing steel for low temperature use.

In recent years, the demand for liquefied gases such as LPG and LNG has increased due to the tense energy situation, and the use of steel for low-temperature tanks and large-diameter steel pipes in extremely cold regions has increased. In line with this, there is a growing demand for further improvements in low-temperature toughness and weldability for the above-mentioned steels.

As low-temperature steel materials, those containing N1 of about 1 to 10% have been developed and are widely put into practical use.

These materials are usually manufactured by undergoing treatments such as quenching, tempering, and standard tempering, but in response to the strict performance requirements of recent years, they have become armored, and the quenching (or standard) process and It has been found that a so-called QQ"l' special heat treatment furnace is effective, which performs quenching after heating in the two-phase region of 3 points or less to 1 point or more of Mu c in the middle of the tempering process.

By the way, due to the high price of alloying elements in recent years, for example, expensive N
Adding a large amount of i is becoming a problem in terms of cost efficiency, and the low-temperature steel produced by the conventional manufacturing method has the same composition as the QQ'T special heat treatment method low-temperature steel. There is a need for the development of a method for producing low-temperature steel with even higher performance.

The purpose of the present invention is to provide a method for manufacturing steel for low temperature use that can meet the above demands, and the above object can be achieved by providing the manufacturing method described in the claims. can.

That is, according to the present invention, 00.1% or less St O,,
01~o, t%t In O,! −*, t % e N
10.3~10* 0% *Muj sod O,Of
-Contains the following as basic ingredients, and if necessary, Or/-
Hereinafter e Ko O-t % or less I01LO-1% or less *
vo-J% or less-TeLO,/% or less*Nb0-
A steel containing one or more selected from the following: 0.03% or less + L&0.03% or less, with the remainder substantially consisting of y. From/j ``C7' is quenched at a cooling rate of 8 to 60℃/S to form a mixed structure of martensite and lower bainite, and then heated to a temperature range of 3 points or less and 5.1 points or more for 13''. The above object is achieved by providing a method for manufacturing steel for low temperature use, which is characterized by quenching at a cooling rate of 'G/S ~ 40 @c/8 and then tempering in a temperature range of 01 point or below. be able to.

Next, the present invention will be explained in detail.

As a result of extensive research on the QQ'T special heat treatment method, the present inventors found that low-temperature toughness and strength can be improved by appropriately controlling the microstructure and heat treatment. This led to the idea of this invention.

The reason why the basic components are limited as described above in the present invention will be explained.

0 is an effective element for improving the hardenability of steel materials and easily increasing the strength. However, if it is included in the sessha, there is a risk that it will increase the susceptibility to weld cracking. Therefore, C is 0. -% or less.

Sl is an element as effective as C in promoting deoxidation and increasing strength, and in this sense it is at least 0.0/4
It is necessary to add more than that. However, if the amount is too large, the low-moisture toughness and contact between the columns will be significantly impaired, so it is limited to the maximum θ and t%.

N1 is an extremely useful element for improving the low temperature toughness and strength of steel. In other words, since ill helps stabilize the virgin austenite that remains during heating, it is possible to advantageously control the structure obtained by subsequent heat treatment, which in turn improves the low-temperature toughness of the product. It can be said that it is an essential ingredient for improving health.

Therefore, in the present invention, N1 is added to the product Ii leopard by more than oJ-. However, N1 is expensive and costs 10%. t)-
If it is added in excess of 100%, it becomes extremely difficult to form lower bainite during cooling during hardening, so the structure targeted by the present invention as described above cannot be obtained, and the "low iI@ property" deteriorates.For this reason, The upper limit of the amount of N1 added was 10.0%.

Since Mu has the effect of improving hardenability and increasing both the strength and low-temperature toughness of steel, it is an extremely useful element in place of the expensive N1. Therefore, in the present invention, Kn is actively added.
．． The content should be 2% or more, but if it is added in excess of Lx-, the welding susceptibility will increase sharply and temper embrittlement will also increase.

Furthermore, as the hardenability improves, the formation of lower bainite decreases.
ll1, the target structure of the present invention cannot be obtained, and low-temperature toughness deteriorates. Therefore, the upper limit of Mn was set at 5%.

As is well known, MuI acts as a deoxidizing agent in the steelmaking process, and also
It is useful because it forms nitrides during hot rolling or heat treatment to refine the structure, but if there is too much, it will cause problems.
GoS-based inclusions increase and impair weldability. Therefore, the upper limit was set at 0.01%.

Further, according to the present invention, the above-mentioned O*BLeMi*
In addition to the basic components of Mu and Muj, if necessary, within a range that supplements the characteristics of the steel material (3r/% or less).

Mo O, t% or less - Ou O, Jt% or less, VD,
% or less tTi O, / % or less * lJb O-λ
% or less* La 0-03% or less and O@0.03%
Any one or two or more selected from the following may be added.

Next, the reason for limiting the composition of each of the above components will be explained.

Or is known as a solid solution hardening element, and has the effect of improving hardenability and increasing strength. However, if it is added in excess, it becomes difficult to form lower bainite during quenching, making it impossible to obtain the target structure of the present invention, resulting in a decrease in low-temperature toughness and hardening of the welded part during welding. Therefore, even if it is added, it should be less than 1-□.

No is an effective element for preventing temper embrittlement induced by the addition of a large amount of MrL. At the same time, No has a strong solid solution strengthening function and also improves hardenability. However, MO is expensive, and if added in large amounts, it will significantly harden the welded area during welding, so 0. The upper limit is t-.

O is a solid solution strengthening element, and is effective in increasing strength and improving corrosion resistance, but if added in a large amount, hot workability will be inhibited, so the upper limit was set at 0.1%.

Both VeTi and a13 are precipitation hardening elements,
This is effective for further improving strength. However, when added in large amounts, the amount of carbonitrides produced by these elements increases, and the toughness of the base metal and weld zone during welding deteriorates.
is 0. Below J%, the lower t'xi is 0. / % or less t Wb
is limited to 0.2% or less.

La*O. has the effect of spheroidizing the shape of sulfide-based nonmetallic inclusions and improving toughness. However, if a large amount is added, the low-temperature toughness will deteriorate, so each 0.03
% or less. In the present invention, in place of Lm and O., other rare earth elements or them in the form of metal can be added, but in that case, the amount added alone or in total needs to be 0.03 or less.

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

A steel containing the above-mentioned constituent elements within the permissible content range, with the remainder substantially consisting of F. is melted, formed into a steel plate or bar by a conventional method, and then heated to a temperature of Aa 3 or above. The microstructure becomes a mixed structure of martensite and lower lRi'e inite.It is quenched at a cooling rate of 7'C/S. By applying the hardening and tempering treatments, it is possible to obtain excellent low-temperature toughness that is not available in conventional methods.By concentrating the Aoki alloy elements in Koaijo quenching, rapid cooling at 13°C/8~u''c7s It becomes complete martensite.

The above treatment according to the present invention is based on the following new knowledge. That is, the mixed structure of martensite and lower bainite, which is the autera structure during quenching, is
When reheated to a temperature of 3 points or lower to 501 points or higher, each lath constituting martensite and bainite recovers and recrystallizes, turning into ferrite. Since this heptate formation occurs depending on the crystal orientation of each lath, the greater the randomness of the crocodile crystal orientation of the lath, the less the degree of coalescence between adjacent laths, which tends to become fine and random.

If the structure before heating in the "co-phase" region is martensitic (martensitic), the martensite lath is formed in fairly large units with approximate crystal orientation. There is little randomness in orientation.In contrast, in the mixed structure of lower bainite and martensite, the lower part.

Bainite lath and martensite lath mutually inhibit aggregation and refine the structure, and the presence of heterogeneous structures significantly promotes rung incubation of mutual crystal orientations. As a result, the ferrite obtained during heating in the co-phase region is not only fine with growth and coarsening suppressed, but also highly random in crystal orientation.

Furthermore, the precipitation form of the austenite phase during heating in the co-phase region also contributes to improving low-temperature toughness. In other words, the carbides precipitated in the lower bainite lath are dissolved in the early stage of heating,
Provides precipitation nuclei of austenite phase. This means that the austenite phase is finer than the case where the austenite phase precipitates only at the martensite/lath boundary.
It promotes uniform distribution and greatly contributes to the uniformity of the tissue. Diffusion and concentration of harmful elements in the matrix occur in this austenite phase, improving the toughness of the matrix. The presence of lower bainite is important because it allows the austenite phase containing harmful elements to be dispersed finely and uniformly, and it also reduces the negative effects of this phase.

A tissue in the above state /k"c/B -40"C/
By quenching at a cooling rate of 8, the austenite phase becomes martensite. This means that the concentration of alloying elements is tr+
This is because it occurs in the area a>. It has been found that this martensite recovers its low-temperature toughness through the subsequent tempering process, resulting in a product with excellent low-temperature toughness.

According to the above newly obtained findings of the present inventors, low temperature toughness is normal QQ'! This results in a level that is even better than that obtained through processing. Therefore, according to the present invention, it is not only possible to reduce the cost by reducing the amount of added elements to meet the required performance, but also to improve weldability.

Next, the present invention will be explained with reference to examples.

EXAMPLE Steel having the composition shown in Table 1 was melted to produce a thick steel plate. These were subjected to quenching, co-phase region heating quenching and tempering treatments according to the heat treatment conditions shown in No. J Mugi. These heat treatment I! ! After yield strength - tensile strength.

Regarding Charpy fracture transition temperature (vTrg) and Charpy impact absorption energy at -194℃
-"1°°0": In Table -, the test AJ
// e A/4' e Al4 t All t A
〃*屓- and MU comments are comparative examples manufactured by the conventional method of rapidly cooling from the quenching temperature to make the microstructure completely martensite. In either case, the properties as a normal chain plate for low temperature use are at a general level.

si and 7j66 are comparative examples in which a mixed structure of upper bainite F and lower bainite is formed by forced air drying after heating to the quenching temperature, and due to the mixing of upper bainite! lf
ml and low temperature toughness are significantly reduced.

A/J is a comparative example in which the structure after quenching is completely lower bainite. It has higher low-temperature toughness than A// single martensite, but lower strength.

On the other hand, the cooling rate during quenching was controlled to create a mixed structure of martensite and lower bainite.
j e A f e A/(7t A
/Kori Azt+ Δni/rim/? Both e-Todoroko/el&n and 48 have higher strength and significantly improved low-temperature toughness than the comparative examples.

In the examples, steel with the composition shown in Table 1 is shown as a representative example, but the composition is not limited to -, and the same effect can be obtained as long as the composition is within the scope of the claims of the present invention. You can get it.

As described above, according to the present invention, conventional QQ'! Low temperature steels can be produced with properties superior to those obtained by processing.

Patent applicant: Kawasaki Steel Corporation Representative patent attorney Mura 1) Osamu Masa:; :111 11

Claims (1)

[Claims] 1, ao, co-whisper - Below, Si O, 0/ ~σ, t
% e Inθe, t ”w 亭, s%* ML O,
Contains 1410, σ-, MulsoIθ, 01% or less as a basic component, and if necessary, Or / % or less
O-t % or lesse Ou OJ attack or less-V O, 2% or less* Tt o, i % or lesse ybθ knee- or lesse
Contains one or more selected from L & 0*QJ % or less* O@ 0*OJ % or less,
The steel, the remainder of which is essentially F, is quenched from a temperature range of 3 points or higher at a cooling rate of /j''c7s -u''''CAB to form a mixed structure of martensite and lower bainite, and then heated to a mixed structure of martensite and lower bainite. The following *After heating to a temperature range of 61 points or higher/! "C/B~60℃/S, a low temperature steel characterized by being quenched at a cooling rate of 60°C/S and then tempered in a temperature range of 01 points or lower. Production method.