JP2004131793A - Part made of niobium-containing low alloy heat resistant steel, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat resistant steel part in which cracks on forging frequently occurring in the production of a part made of niobium-containing low alloy heat resistant steel are eliminated, and the occurrence of cracks in a working process including forging is prevented, and to provide a production method therefor. <P>SOLUTION: When, with niobium-containing low alloy heat resistant steel containing 0.01 to 0.10% Nb as an object, at least a part of crystallized NbC is made to form a solid solution in a matrix by soaking, and next and forging and required mechanical working are performed, ≥60% of the crystallized NbC is allowed to form the solid solution. The progress rate into the solid solution is estimated by the formula, S=1-exp[ä-2.72×10<SP>16</SP>exp(-6.41×10<SP>5</SP>/RT)}×t] (wherein S is the progress rate into the solid solution; T is the temperature (K) of a steel ingot; t is the time (s), and R is a gas constant of 8.3114), and the heating temperature and time are decided, so that the soaking is efficiently performed. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a component for a machine structure manufactured by subjecting a steel ingot of a niobium-containing low alloy heat-resistant steel to a process including forging. The invention also relates to an industrially advantageous method for producing such components. In the present invention, “parts” include not only final mechanical parts but also materials that have room for further processing.
[0002]
[Prior art]
For example, taking up the process of manufacturing a disk (blade plate) for a gas turbine for power generation, a relatively large ingot of low-alloy heat-resistant steel is prepared and subjected to processing, including forging, to produce a material of almost product size. It is made into a part by machining it. As the low-alloy heat-resistant steel, an alloy containing a predetermined amount of niobium is preferably used. This is because, as is well known, Nb has the effect of making the structure of steel finer and increasing the toughness.
[0003]
However, the addition of Nb causes the generation of the carbide NbC. As described above, since the steel ingot is relatively large and the typical one is about 15 tons, cooling after casting does not proceed quickly, and a considerable portion of the generated NbC crystallizes. If the steel ingot crystallized with NbC is forged as it is, cracks are likely to occur. Processed products with cracks inside may be damaged during use, leading to a serious accident. Therefore, after processing, they must be found by ultrasonic inspection and then re-dissolved as defective. If it turns out that the product is defective after processing to a considerable extent, it goes without saying that the cost of manufacturing a good product that suffers from the wrinkling increases, and such a situation should be avoided as much as possible.
[0004]
Therefore, soaking is performed prior to processing a steel ingot, and the crystallized NbC is dissolved in a steel matrix to prevent cracking. Normally, heat treatment is performed at a temperature of 1200 ° C. or higher for at least 20 hours. However, it is natural that conducting soaking at a high temperature for a long time to achieve a sufficient solid solution is disadvantageous from the viewpoint of cost, contrary to the demand for energy saving, and effective improvement measures are required. Was.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a heat-resistant steel part which solves the above-mentioned problem which has been a concern when manufacturing a niobium-containing low-alloy heat-resistant steel part, and prevents the occurrence of cracks in a processing step including forging, and a method of manufacturing the same. It is to provide a method.
[0006]
[Means for Solving the Problems]
The niobium-containing low-alloy heat-resistant steel part of the present invention that achieves the above objects achieves, by weight, C: 0.3 to 1.0%, Nb: 0.01 to 0.10%, and Ni: 1.0% or less. , Cr: 3.0% or less, Mo: 1.5% or less, and V: 0.5% or less, with the balance being Fe and impurities. And a component having an area ratio of NbC of 0.004% or less.
[0007]
The method of the present invention for producing a niobium-containing low-alloy heat-resistant steel component as described above comprises, by weight, C: 0.3 to 1.0%, Nb: 0.01 to 0.10%, Ni: 1. A niobium-containing low-alloy heat-resistant steel containing 0% or less, Cr: 3.0% or less, Mo: 1.5% or less, and V: 0.5% or less, with the balance being Fe and impurities. In a part manufacturing method, at least a part of the crystallized NbC is solid-dissolved in a matrix by soaking, and then forged and subjected to necessary machining, at least 60% of the crystallized NbC is solidified. By melting, cracks that may occur during forging are prevented to obtain a component.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The reason why the alloy composition of the niobium-containing low-alloy heat-resistant steel used as the material of the component of the present invention is determined as described above is as follows.
[0009]
C: 0.3-1.0%
In order to secure the strength required for the component, 0.3% or more of C is required. Since the presence of excessive C increases the amount of NbC crystallized and makes it difficult to disappear by solid solution, a large amount of C exceeding 1.0% should not be added.
[0010]
Nb: 0.01 to 0.10%
As described above, Nb has an effect of reducing the crystal grain size, and is an important component for increasing the toughness of parts. In the Cr-Mo-V steel, the one in which 0.07% of Nb is added has a crystal grain size higher by about 0.5 to 1 in a quenching temperature range of 910 to 970 ° C than the non-added one. It is known that. This crystal grain refining effect is observed when Nb is added in an amount of 0.01% or more, but is saturated even when the added amount is increased. The preferred range of the amount added is 0.04 to 0.08%.
[0011]
Ni: 1.0% or less Ni enhances hardenability and increases the strength of parts. From the viewpoint of saturation of the effect and cost, 1.0% is made the upper limit of the addition amount.
[0012]
Cr: 3.0% or less Cr enhances hardenability and increases tempering resistance. The upper limit of 3.0% was set from the viewpoint of cost.
[0013]
Mo: 1.5% or less Mo is an element that increases the hardness and strength at high temperatures, and is an important component for heat-resistant steel. From the viewpoint of cost effectiveness, it is advisable to limit the amount of addition to 1.5%.
[0014]
V: 0.5% or less Helps to refine crystal grains together with Nb. This effect saturates at more than 0.5% addition.
[0015]
The significance of dissolving 60% or more of the crystallized NbC is, of course, to obtain a steel material having an area ratio of NbC of 0.004% or less, but the limit of 60% is due to the following experimental results described below. It is the knowledge obtained from. In other words, as shown in the graph of FIG. 1 obtained by plotting the relationship between the solid solution progress rate and the defect rate, the data shows that the defect rate becomes substantially zero when the solid solution progress rate becomes 60% or more. is there.
[0016]
If it is certain that the solid solution progress rate has reached 60% or more, there is no point in continuing soaking, but the problem is how to determine it. The inventors have found that the progress of solid solution of NbC during soaking can be estimated based on the following formula of solid solution progress rate,
S = 1-exp [{− 2.72 × 10 ¹⁶ exp (−6.41 × 10 ⁵ / RT)} × t]
Here, S: solid solution progress rate T: temperature of steel ingot (K)
t: time (seconds) R: gas constant = 8.3114
By determining the temperature and time of soaking based on this, a technique for obtaining a forging material free of defects or having no risk of generating defects was established with efficient energy consumption.
[0017]
In the above equation, the temperature of the steel ingot (T) is the ambient temperature of the heating furnace in a steady state, but a cold steel ingot is charged at the start of soaking, and the temperature substantially reaches the ambient temperature. It takes time to do so. As will be easily understood, since the molten steel poured into the mold has just solidified into a steel ingot, the temperature is still high, so it is advantageous in terms of heat consumption to shift to soaking as soon as possible. However, it has been found that early heating furnace loading results in a reduced defect rate beyond the effect of simply raising the temperature of the cold one again. This indicates that it is more advantageous to suppress the crystallization of NbC as much as possible, rather than dissolving the NbC crystallized once cooled.
[0018]
【Example】
Melting Nb-containing low-alloy heat-resistant steel having the following alloy composition (% by weight, balance Fe and impurities)
C: 0.31%, Si: 0.1%, Mn: 0.7%, Ni: 0.7%, Cr: 2.35%, Mo: 1.02%, V: 0.20% and Nb : 0.05%
Soaking was performed on the cast ingot of 15 tons at various temperatures in the range of 1240 to 1350 ° C for 5 to 10 hours. The steel ingot was cut into quarters and divided into quarters, and the solid solution progression rate was calculated based on the results of measuring the area ratio of NbC for these.
[0019]
The “area ratio A of NbC” is calculated as follows by observing a visual field of a microscope of 10 mm × 10 mm.
A = B ₀ / B _t × 100 (%)
Here, B ₀ : the total area of NbC present in the visual field B _t : the area of the visual field = 100 mm ²
The solid solution progress rate S is defined as follows.
S = (A ₀ −A ₁ ) / A ₀ × 100 (%)
Here, A ₀ : NbC area ratio before soaking A ₁ : NbC area ratio after soaking
The presence or absence of defects in the alloy at various solid solution progress rates was examined by an ultrasonic flaw detector, and the defect rates were calculated. FIG. 1 is a graph obtained by plotting the relationship between the solid solution progression rate of NbC and the defect rate of the steel slab. The relationship between the solid solution progress rate S calculated from the NbC area ratio and the soaking conditions of the steel material temperature (heating furnace atmosphere temperature) T and the heating time t was derived by a double regression analysis. (T, t).
[0021]
The inventors' experience has shown that it is more advantageous to employ a slightly higher temperature than conventional soaking in order to realize rapid solid solution of NbC. Specifically, while the conventional soaking employs the condition that heating is performed empirically, typically at 1200 ° C. inside and outside for 20 to 24 hours, in the practice of the present invention, it is 1240 to 120 ° C. The heating is preferably performed at a temperature of 1350 ° C. for 5 to 10 hours. The use of higher heating temperatures increases the amount of energy consumed per unit time, but the savings can be made much more by shortening the heating time.
[0022]
The above-mentioned S = f (T, t) equation is generally applied to a niobium-containing low-alloy heat-resistant steel having an Nb content of 0.01 to 0.10%, so if the soaking furnace atmosphere temperature T is determined. , The solid solution progress rate S with the progress of the time t is calculated. An example of the soaking time required for S to exceed 60% is as follows.
Soaking temperature (° C) 1200 1240 1260 1280 1300 1320 1340
Soaking time (hr) 625 155 85 42 23 13 7
[0023]
【The invention's effect】
The niobium-containing low-alloy heat-resistant steel part according to the present invention is forged in the processing step by controlling the amount of crystallized NbC to a critical value of 0.004% or less in area ratio. The risk of cracking has also been significantly reduced. By processing a hard-to-break material, the rate of occurrence of defective products due to cracking during processing can be significantly reduced. It goes without saying that an improvement in the yield of non-defective products contributes to efficiency of production and reduction of cost.
[0024]
The method of the present invention for producing a niobium-containing low-alloy heat-resistant steel component that is difficult to crack is used in the process of soaking the crystallized NbC into the matrix by soaking. In other words, based on the discovery of the critical fact that NbC, which causes cracking during processing, decreases to a negligible degree, a double regression analysis was used to find an equation for estimating the rate of solid solution progression, and to reduce waste soaking. This is a manufacturing method that can be implemented. As a result, since the conditions of the soaking temperature and the time which are simply determined empirically have been adopted, the disadvantage of the conventional technology that wastes a lot of energy has been solved, and the demand for energy saving has been met. This also helps reduce the cost of manufacturing heat resistant steel parts.
[Brief description of the drawings]
FIG. 1 is a graph showing experimental data obtained in the process of establishing the present invention and plotting a relationship between a solid solution progression rate of NbC crystallized in a niobium-containing low alloy heat-resistant steel and a defect rate of a steel slab.

Claims (4)

By weight, C: 0.3 to 1.0%, Nb: 0.01 to 0.10%, Ni: 1.0% or less, Cr: 3.0% or less, Mo: 1.5% or less, and V : A component formed by forming a niobium-containing low-alloy heat-resistant steel having an alloy composition of 0.5% or less, with the balance being Fe and impurities, by processing including forging, wherein the area ratio of NbC is Parts that are 0.004% or less. The component according to claim 1, wherein Nb is 0.04 to 0.08%. By weight, C: 0.3 to 1.0%, Nb: 0.01 to 0.10%, Ni: 1.0% or less, Cr: 3.0% or less, Mo: 1.5% or less, and V : Niobium-containing low-alloy heat-resistant steel containing 0.5% or less, with the balance being Fe and impurities, is cast into a steel ingot, and at least a portion of the crystallized NbC is dissolved in a matrix by soaking. Then, in a method for producing a part, which comprises forging and performing necessary machining, a component is obtained by dissolving 60% or more of the crystallized NbC to thereby prevent cracking that may occur at the time of forging. For producing parts of niobium-containing low alloy heat resistant steel. Estimating the progress of solid solution of NbC during soaking based on the following formula of solid solution progress rate,
S = 1-exp [{− 2.72 × 10 ¹⁶ exp (−6.41 × 10 ⁵ / RT)} × t]
Here, S: solid solution progress rate T: temperature of steel ingot (K)
t: time (seconds) R: gas constant = 8.3114
4. The method according to claim 3, wherein the soaking temperature and time are determined.

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