JPS6020447B2 - Method for manufacturing low carbon aluminum killed steel with excellent nitrate stress corrosion cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing low carbon aluminum killed steel with excellent nitrate stress corrosion cracking resistance.

Cracks may occur near the welded parts of the steel shell of hot blast stoves and high-temperature heating furnaces. When we investigated the cause of this cracking, we found that NO2 generated by the decomposition of air and gas at high temperatures condenses on the inside of the steel shell as nitrates, and the steel shell is exposed to a so-called nitrate stress corrosion environment, causing stress corrosion cracking. It turns out that there is a cause that causes this. Therefore, the steel skin material for the high temperature furnace is required to have nitrate stress corrosion cracking resistance. Conventionally, semi-killed medium carbon steel has been generally used as the skin material for this type of high-temperature furnace, but this steel is highly susceptible to stress corrosion cracking in hot-rolled or normalized materials, and is prone to nitrate stress corrosion. There is a concern that it may crack relatively easily when used in an environmental environment.

The object of the present invention is to provide a method for producing steel that has extremely high resistance to nitrate stress corrosion cracking and can be used in high-temperature furnace shells to effectively prevent cracking. Regarding stress corrosion cracking, external factors such as environment and added stress,
Furthermore, it is generally known that the frequency of cracking varies depending on the metal structure and alloy composition of the steel itself.

The present inventors investigated the effects of the metallographic structure and constituent components of steel on stress corrosion cracking resistance, especially in a nitrate environment.
We conducted detailed experiments and studies.

As a result, the following facts were found. In other words, steel medium C is
Most of the amount above the Oka solubility limit precipitates at grain boundaries and within the grains as pearlite and cementite, but this grain boundary precipitation of pearlite and cementite poses no problem and even serves as a resistance to stress corrosion cracking. However, C that remains in solid solution with other substances causes polarization to the grain boundaries due to various thermal histories during the manufacturing and construction (welding, etc.) stages of steel materials, and this has a significant negative effect on nitrate stress corrosion cracking resistance. effect. Although the degree is milder than that of C, N is also harmful if it is free in a solid solution state, just like C. Microstructure-wise, nitrate stress corrosion cracking is not unrelated to the fineness or fineness of the crystal grains, and it can be said that pongee grains are preferable. Incidentally, in semi-killed medium carbon steel conventionally used for the steel shell of high-temperature furnaces, most of the C precipitates as pearlite or cementite, but about 0.03% remains in solid solution; Therefore, it is inevitable that the crystal grains become coarse, and therefore strong resistance to stress corrosion cracking in a nitrate environment cannot be obtained. The present invention has been made in view of the above findings, and aims to reduce the amount of carbon and further reduce the amount of harmful solid dissolved carbon. The idea is to fix N as carbonitride by adding Nb, and furthermore, Zr and Ti, and to make it grainy by adding AI, thereby ensuring good resistance to nitrate stress corrosion cracking.

That is, in the present invention, 0.025 to 0.07%, Sio.
5% or less, Mno. 3-3.0%, Sol. A.I.O.
01 to 0.10%, Nb (7 to 15) x C (%), and in some cases Zro. 01-0.50%,
Tio. Copper containing one or two types of 0.01 to 0.50%, the remainder consisting of Fe and unavoidable impurities, is heated to a temperature range of Ac3 point (0°C) to 1150 qo and subjected to a leveling treatment. The summary of this paper is a method for manufacturing low carbon alkylated steel with excellent nitrate stress corrosion cracking resistance.

Below, the reasons for limiting the steel composition and heat treatment conditions in the present invention will be described in detail for each requirement. C: From the point of view of reducing solid solution C, which is harmful to nitrate stress corrosion cracking resistance, it is preferable that the amount of C is as low as possible; 0 from the relationship.
0.025% or more is required.

Furthermore, if the amount of C exceeds 0.07%, it is impossible to ensure complete C fixation no matter how much Nb is added, so CO
．． It was limited to 0.025% to 0.07%. Si: An element necessary for deoxidation, but if it exceeds 0.5%, it will have a negative effect on corrosion resistance.

Mn: At least 0.3% or more is required to ensure strength, but if it exceeds 3%, it changes to an acicular ferrite type transformed shelf weave, increasing stress corrosion cracking susceptibility.

Sol. AI: It has the effect of not only deoxidizing but also refining crystal grains, and it is necessary to contain at least 0.01% or more.

However, if it exceeds 0.10%, the amount of nonmetallic inclusions in the steel increases, which is undesirable, so it is set at 0.01 to 0.10%. Nb: This is the most characteristic component in the steel subject to the present invention, and is an element added to fix C and N.

The amount added should be determined in relation to the amount of C, and 7
If less than ×C (%) is added, a large amount of C that is not fixed to Nb is generated, which diffuses and segregates at grain boundaries, resulting in insufficient nitrate stress corrosion cracking resistance. On the other hand, if this becomes 15 × C (%), conversely, there will be more Nb that does not combine with C or N, and this will cause grain boundary precipitation in the form of Fe2Nb, leading to grain boundary arms. is (7-15)×C(
%). Zr, Ti: Like Nb, they are carbonitride-forming elements, and in particular, they have a stronger N fixing effect than Nb, and are effective in increasing nitrate stress corrosion cracking resistance when added in combination with Nb. , Zr, and Ti have no effect unless they are added in an amount of 0.01% or more.

On the other hand, Zr and Ti are both 0.50 in consideration of cost.
% or less. The competitive heating temperature for heat treatment is
If the Ae3 point is not 830°C or higher, the dawn-leveling effect cannot be expected, and if the temperature exceeds 1150°C, the precipitation of Nb and C will be small and the amount of hard carbon will be large, and sufficient stress corrosion cracking resistance will be obtained. I can't do it.

Next, the implementation effects of the present invention will be explained in detail.

Steel wind (G) having the components shown in Table 1 is melted and heated at a temperature of 1
20 pi○, hot rolled at a finishing temperature of 950°C or higher,
A steel plate with a skin thickness of 26 was obtained, and for each steel plate, a: as hot rolled, b: air cooling after holding at 950qo x lh, C: 1250o
A nitrate stress corrosion cracking test was conducted after holding at 0xlh and then air cooling. In the test, a plate-like piece measuring 2 bars thick x 1 thick x 75 thick in the rolling direction was taken from a steel plate, bent into a U-shape and applied with a force of 5 skin restraints to form a test piece. A method was used in which the samples were immersed in N03)2 solution (120°C) for 50 hours. The results are shown in Table 2.

In the same table, XX: Cracks occur, ×: Cracks of less than half the wall thickness are easily recognized with the naked eye, △: Cracks occur, but they are extremely small and difficult to notice without magnification with an optical microscope.
0: No cracks, respectively. Table 1 (wt Tsutomu) In the upper table of Table 2, no cracking was recorded for all of the steels ■ to ■ that were heat treated 'b} in which both the steel composition and the heat treatment were in accordance with the method of the present invention.

Even if the steel composition is ■ ~ plate, 'a' without heat treatment or heat treatment [C
} cracks are visible. Also, the steel composition is (E'
~(G), without heat treatment or heat treatment [C', which is not in accordance with the provisions of the present invention in any respect, and even if the heat treatment is 'b' and is based on the present invention, the essential steel components However, [E--(G)] cannot prevent the occurrence of cracks. As can be seen from the above description, the method of the present invention makes it possible to produce materials that exhibit extremely high resistance to nitrate stress corrosion cracking, and therefore the present invention is suitable for use in materials exposed to nitrate stress corrosion environments, e.g. It can be said to be extremely effective as a measure to prevent cracking of furnace shells, etc.

Claims (1)

[Claims] 1 C0.025 to 0.07%, Si 0.5% or less, M
n0.3-3.0%, Sol. Al0.01~0.10
%, Nb(7-15) x C%, the balance is Fe and unavoidable impurities.
A method for producing low carbon aluminum killed steel with excellent nitrate stress corrosion cracking resistance, which is characterized by heating to a temperature range of °C and performing normalizing treatment. 2 C0.025-0.07%, Si0.5% or less, M
n0.3-3.0%, Sol. Al0.01~0.10
%, Nb(7-15)×C%, and further contains Zr0.
A steel containing one or two of 0.01 to 0.50% and 0.01 to 0.50% of Ti, with the remainder consisting of Fe and unavoidable impurities is heated to a temperature range of Ac_3 point to 1150°C,
A method for producing low carbon aluminum killed steel with excellent nitrate stress corrosion cracking resistance, which comprises performing a normalizing treatment.