JPS58120765A - Heat resistant steel - Google Patents

Abstract

PURPOSE:To enhance the weldability of a 12Cr steel while maintaining the heat resistance and creep rupture strength by adding a specified small amount of Ca to the steel. CONSTITUTION:This heat resistant 12Cr steel with superior weldability suitable for use as a material for the casing and various valves of a steam turbine has a composition consisting of 0.05-0.25% C, 0.05-0.5% Si, 0.1-0.8% Mn, 8.0- 16.0% Cr, 0.3-1.2% Mo, 0.05-0.5% V, 0.1-1.0% Ni, 0.01-0.3% Nb, 0.01-0.1% N, 0.0001-0.01% Ca and the balance Fe. Ca improves the weldability of the steel, and it is added in the form of a Ca mother alloy such as a Ca-Si alloy or an Ni-Ca alloy.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention is a heat-resistant steel with excellent weldability for applications such as steam turbine casings and Class 6 valve materials.

Conventionally, lCr-IMO-0, which has relatively excellent high-temperature strength, has been used as pulp material for steam turbine casings. 2
5V heat-resistant steel is widely used. However, recently, as steam turbines have become larger and steam temperatures have become higher, 12C, which has higher creep rupture strength and superior high temperature strength, is used for casings and valves. r-thread steel is being used. Most casings and valves are Oya's cast products, so defects inevitably occur during manufacturing.

In general, defects that occur are repaired and welded to prevent them from becoming a starting point for brittle fracture. However, 120R thread steel is highly susceptible to cracking and requires preheating during welding, but it is used in turbine casings and pulp. When welding large structures that are subject to significant constraints, such as the welding process, it is often not possible to prevent cracking by preheating alone.

Furthermore, although the weldability of conventional 120R thread steel is improved by reducing the amount of C, it has the disadvantage that its creep rupture strength decreases. In other words, weldability and creep rupture strength are contradictory properties. For this reason, it is necessary to deal with larger parts and higher temperatures (
= requires a heat-resistant steel that has both high creep rupture strength and excellent weldability.

The present invention has been made to satisfy the above-mentioned needs, and has 120
The object of the present invention is to provide a heat-resistant steel suitable for steam turbine casings and Class 6 valves, which has improved weldability without reducing the heat resistance and creep rupture strength, which are characteristics of R-thread steel. The low alloy steel according to this invention is C! 0
．． 05~0.25%, 810.05~0-5chi, Mn0
．． 1-0.8%, Or&0-16.0%, Mo0.3-
1.2q6. '1'0.05~0,5911i0.1
~1.0-1N℃0.01-Q, 3 East Summer 0.01-0.
1ch.

Ca0.0001=0.01%, and the remainder is 7s.The reason for limiting each component element is as follows.

C improves hardenability and improves creep rupture strength at high temperatures by combining with carbide-forming elements such as V, Mo, and Or. Therefore, at least 0
．． 0556 or more is required. If it exceeds 0.25'1G, the carbides become coarse and toughness deteriorates, and weldability is significantly deteriorated. 81 and Kn are deoxidizers,
It is added as a desulfurizing agent, and in order to perform sufficient deoxidation and desulfurization, it must be added at least s 10-05cm, M
nα1s or more is required.

Furthermore, since addition of large amounts of Sl and Mn deteriorates toughness, the upper limits were set at 810.5% and Mn at 0.8%.
Cr improves hardenability, ensures tensile strength and creep rupture strength, and deteriorates creep rupture strength. MO is required to be 0.3 or more in order to increase resistance to temper softening and improve hardenability and toughness. However, if it exceeds 1.2-, a large amount of carbides will be produced and the toughness will deteriorate. V is C
It is an element necessary for combining with metal to precipitate fine carbide and improving creep rupture strength, and is necessary in an amount of 0.05% or more. However, if it exceeds 0.5%, the carbides become coarse, the toughness deteriorates, and welding becomes difficult, so this range was chosen. Ml is an element that improves hardenability and toughness and is required to be at least 0.1%, but if it exceeds 1.0%, the effect is saturated, so it was set in this range. It) is
It is an element that refines crystal grains and improves toughness and is required in an amount of 0.01% or more. However, if it exceeds 0.316, segregation will occur during manufacturing and the toughness will deteriorate, so the upper limit was set at 0.3%.

N is an element that improves creep rupture strength and is required to be 0.011 or more, and if it exceeds O-1%, toughness and weldability are required, so this range was set.

(a is an element effective for improving weldability, and 0.
The effect is fully demonstrated at about 0001chi. But 0.
If it exceeds 0.01%, not only the effect will be saturated but also the toughness will deteriorate. c.
The effect is the same even if it is performed in the form of a gold alloy or other master alloy.

Next, the heat-resistant steel according to the present invention will be explained in detail using examples.

Steel samples having the compositions shown in Table 1 were melted using a high frequency induction melting furnace.

Table 1 These samples were heated to a temperature of 1050°C to austenitize and then air cooled. Next, it was heated to a temperature of 720'C, held for 12 hours, and then cooled in a furnace.A creep rupture test was conducted at 595C to determine the creep rupture strength. In addition, in order to examine weldability, Surin 1
Using ffi restrained cracking test pieces, welding was performed while changing the preheating temperature, and the average cracking in the cross section of the weld heat affected zone was determined. Welding conditions were: welding current 160m, arc voltage 25v, welding speed 19.
0 to 200 ml m, a mild steel low-hydrogen thread-coated arc welding rod was used to investigate cracks in the acutely affected zone, and was resistant to cracking.

Weld cracks were observed by polishing and corroding the cross section after 48 seconds or more after welding.

No. 1114: Comparative example-1 (fl141a), 2 (fa
ll) ml), 3 (curve C) and Example-2 (song!
Table 182 shows the results of the slit overflow restraint weld cracking test.

Table 2, Figure 2 of Kamikugeto et al. shows Example-1 when the average cracking rate is 50-.
As is clear from Figure 1, which shows the relationship between preheating temperature and C content in (@ line θ) and Comparative Example-1 (curve), the creep rupture strength of Example-2 is almost the same as that of Comparative Example. It can be seen that it has a sufficient leap rupture strength. Second
As is clear from the table, the weldability of the examples is sufficiently excellent, for example, the preheating temperature of comparative example-1 with less at:
The average cracking rate in Example 3 is 1396, which is 1396, whereas in Example 3, which has a large amount of at, it is a small value of 796, indicating that good welding can be performed. Furthermore, due to the second factor, the steel of the present invention has a low cracking rate even if the zero content is increased. That is, it has excellent weldability and high creep rupture strength.

As described above, the heat-resistant steel according to the present invention has much better weldability than conventional steels and has sufficiently high creep rupture strength, so it is extremely useful for application to casings and pulps used at high temperatures.

[Brief explanation of the drawing]

FIG. 1 is a curve diagram showing the creep rupture test results, and gJ2 diagram is a kneading diagram showing the relationship between the preheating temperature and the amount of C when the average cracking is 5096. (7317) Agent: Patent attorney Noriyuki Chika (and 1 other person) Figure 1: Hanazaki Kenuchi (h-hi) Figure 2: ex<%)

Claims (1)

[Claims] Cα05-0.25% by weight, slo, 05-0.
5%, MnO, 1"Q-8%, Or&0-16.0%,
Moα3-1.2%, Vα05-0.5 rumor, Ni
Q, 1-1.051°NbO,0l-(L3%, MO,
01-0,1%, C! a O,0001-o-o-t
*, A heat-resistant steel characterized by the remainder being F's.