JPS5941487B2 - Manufacturing method of ferrite stainless steel for welded structures - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing ferritic stainless steel, which is used as a welded structural material for oil refining equipment and various other chemical T industry equipment.

Stainless steel has many products such as pipes, bars, and steel plates, but demand for steel plates in particular is rapidly increasing.
Among these, ferritic stainless steel has low quenching hardness and sufficient corrosion resistance as a durable consumer material, so it is used more as a structural material than martensitic steel of the same type, and is being used for new purposes. However, ferritic stainless steel has the advantages of being economically cheaper and less susceptible to stress corrosion cracking than 18-8 austenitic stainless steel, but on the other hand, it has inferior toughness and workability, making it less safe. Due to this, its uses are limited, and it is often not used as a material for welded structures.
It is widely known that the reason why ferritic stainless steel has low toughness and poor workability is due to the large amounts of C and N contained in the steel.
acumOxygenDecarbur-i2ati
onProcess) We are working to reduce C and N through melting technology.

However, simply reducing C and N does not exhibit satisfactory properties; for example, when welding, especially multi-layer welding, chromium carbide (Cr23
C6) was precipitated at the grain boundaries, and the corrosion resistance was inferior to other parts, and the toughness and workability tended to deteriorate. The present inventors have attempted many experiments with the aim of providing a method for manufacturing ferrite stainless steel that eliminates the deterioration of toughness and workability caused by welding.

As a result, by adding and containing Nb and V, which fix C and N contained in steel and form fine carbon and nitrides, precipitation of chromium carbides at grain boundaries is prevented, and M
It has been found that toughness and workability can be stabilized by reducing the area ratio of Al2O3 inclusions of 5μ or more among the 2O3 inclusions to 0.01% or less (approximately 0.030 coefficient for conventional steel). The present invention was constructed based on this knowledge. That is, in the present invention, A7, Nb is added to the molten steel as necessary.
C: 0.015 or less, Si
: 0.50% or less, Mn: 0.50% or less, Cr:
12.0-30.0 section, MO: 0.2-5.0%, A
7: 0.04-0.10, Nb: 0.02-0.2
5%, N: 0.020% or less, V: 0.02 to 080% added as necessary, the remainder is substantially iron, and the Al2O3 inclusions have a diameter of 5μ or more. This is a method for producing ferrite stainless steel for welded structures, in which a steel piece in which Al2O3 inclusions occupy an area ratio of 0.01% or less is hot rolled.

The method of the present invention will be explained in detail below.

Al and Nb are added to molten steel that is melted in a melting furnace such as a converter or electric furnace, and then subjected to refining treatments such as V0 and D treatments, without losing its corrosion resistance as stainless steel. , to produce molten steel that yields steel with excellent toughness and workability.

C deteriorates toughness, workability, and intergranular corrosion resistance as the content increases. This deterioration can be prevented by adding carbonitride forming elements such as Nb and V. However, excessive content of C increases the necessary amount of added Nb and the like, thereby deteriorating the toughness of the weld heat affected zone in particular. Therefore, C is
As a welded structural material, the content does not deteriorate the toughness of the weld heat affected zone, that is, the content is 0.015 (preferably 0.0
10%) or less. Si and Mn are components that deteriorate toughness, and their permissible content is 0.50%. Cr is the main component of stainless steel, and needs to have a coefficient of 120 or more in order to obtain corrosion resistance. However, excessive Cr content exceeding 3αo% will cause sigma (σ) phase to precipitate from ferrite, especially when heated for a long time between 540 and 820°C or slowly cooled, which will deteriorate various steel properties such as toughness and workability. There is a tendency for deterioration to appear. Therefore, the Cr content is
In consideration of corrosion resistance, the ratio was set at 12.0 to 30.0. M.O.
is added as an effective component for improving corrosion resistance, particularly pitting corrosion resistance, and its effect cannot be obtained if the content is less than 0.2. Further, since excessive MO content precipitates the σ phase or insulator phase and deteriorates the toughness of the steel, the upper limit was set at 5.0%. Al is a necessary element to remove oxygen remaining in molten steel after oxygen blowing for decarburization with V, O, D, etc., but if it is less than 0.04%, the deoxidation reaction is slow and it will not be formed. The floating of Al2O3 becomes slow, and coarse Al2O3 inclusions remain in the product, degrading the toughness.

On the other hand, as the amount of A7 increases, the deoxidation reaction becomes faster and the floating of the deoxidized products is promoted, the size and amount of Al2O3 inclusions decrease, and the toughness improves.
If the content exceeds 0.1%, the amount of solid solution A7 will increase and the toughness will deteriorate as shown in Figure 1, so the upper limit should be set at 0.10%.
And so. Since Al2O3 inclusions are harder than the base metal and hardly deform, those over a certain size act as starting points for cracks.

For welded structural steel plates at room temperature, which is the object of the present invention,
The limit value of the diameter of these Al2O3 inclusions is 5μ, and if the diameter is larger than this, they begin to act as starting points for cracks. However, if the amount of inclusions of 5μ or more is small, cracks that once occur will stop, and the toughness of the steel plate will be ensured. On the other hand, if more than a certain amount of inclusions of 5μ or more are present, cracks will occur more frequently, and the cracks will become easier to connect, resulting in a deterioration in the toughness of the steel sheet.
From FIG. 2, this limit amount is 0.010% of the area ratio of Al2O3 inclusions of 5μ or more in the weld heat affected zone. N
When b is included in steel, it fixes C and N, forms fine carbon and nitrides, and prevents the precipitation of chromium carbides at grain boundaries.
It is included as an effective component for improving the toughness and intergranular corrosion resistance of the weld heat affected zone.

The effect cannot be obtained below 0.02%, and 0.25%
If the content exceeds %, the toughness will be lost. V is also added in an amount of 0.02 to 0.80% for the same reason as Nb.
In particular, when V coexists with Nb, it has the effect of further improving the above effects. The reason for this effect is not clear. In addition, in terms of fixing C and N in steel, Nb
Similarly to v, Ti is also considered. However, Ti carbon/nitride cannot be used because it is larger in size than Nb, VIII carbon/nitride and embrittles the steel. N
has the same effect as C, and excessive content increases the amount of Nb and V added and significantly deteriorates the toughness of the weld heat affected zone.
A content of 0.020% or more should be avoided. Molten steel whose composition has been adjusted by adding Al, Nb, and V as described above is
In order to reduce and reduce the amount of Al2O3 inclusions remaining in the steel, AI! A standing time is required to allow the 203 particles to float.

Figure 3 shows Al2O3 remaining in steel with different Al contents.
The area ratio occupied by Al2O3 inclusions of 5 μ or more among the inclusions is shown with respect to the standing time of the molten steel. In other words, the size and amount of Al2O3 inclusions decrease with the time the molten steel is allowed to stand, and with a standing time of 20 minutes or more, the steel can be cleaned to the extent that it does not adversely affect the properties of the steel. . The molten steel that has been allowed to stand for 20 minutes or more is then used to produce steel billets by continuous casting or ingot making and blooming methods, and the billets are then hot rolled to be used as welded structural materials such as steel plates and long steel. Used for products such as Further, according to the present invention, a hot rolled steel sheet can be further heat-treated and then used as a product. Ferrite stainless steel for welded structures with excellent toughness and workability can be manufactured by the manufacturing method of the present invention as described in 2.

Next, examples of the present invention will be described.

Table 1 shows the results of molten steel melted in an electric furnace and subjected to V, O, and D treatments, the composition of which was adjusted by adding Al, Nb, or V, and after the molten steel was allowed to stand for a period of time, it was cast and hot rolled. Indicates the ingredients, manufacturing conditions, etc.

Furthermore, Table 2 shows the mechanical properties of the hot rolled steel sheet and the weld heat affected zone at that time. That is, the toughness and workability of the ferritic stainless steel produced by the method of the present invention are superior to those of comparative steels that deviate from the method of the present invention, and the degree of deterioration due to welding heat is also small.

[Brief explanation of the drawing]

Figure 1 shows the influence of the Al content in steel on the fracture transition temperature, and Figure 2 shows the area ratio occupied by Al2O3 inclusions of 5 μ or more in the Al2O3 inclusions at the fracture surface transition temperature. FIG. 3 is a diagram showing the influence of temperature on base metal and weld heat affected zone.

Claims (1)

[Claims] 1. C: 0.0 obtained by adding Al and Nb to molten steel and allowing it to stand for 20 minutes or more, followed by continuous casting or ingot making/blooming method.
15% or less, Si: 0.50% or less, Mn: 0.50%
Below, Cr: 12.0-30.0%, Mo: 0.2-5
．． 0%, Al: 0.04-0.10%, Nb: 0.02
~0.25%, N: 0.020% or less, the remainder is substantially iron, and the area ratio of Al_2O_3 inclusions with a diameter of 5μ or more is 0.01 among the Al_2O_3 inclusions.
1. A method for producing ferritic stainless steel for welded structures, characterized by hot rolling a steel billet having an occupancy of % or less. 2 C:0 obtained by adding Al, Nb, and V to molten steel and allowing it to stand for 20 minutes or more, followed by continuous casting or ingot making/blowing method.
．． 015% or less, Si: 0.50% or less, Mn: 0.5
0% or less, Cr: 12.0-30.0%, Mo: 0.2
~5.0%, Al: 0.04~0.10%, Nb: 0.
02-0.25%, V: 0.02-0.80%, N: 0
．． Al_2 containing 020% or less with the remainder substantially consisting of iron and having a diameter of 5μ or more in Al_2O_3 inclusions
A method for producing ferritic stainless steel for welded structures, comprising hot rolling a steel piece in which O_3 inclusions occupy an area ratio of 0.01% or less.