KR960007427B1 - Making method of cold rolling austenite stainless steel sheet - Google Patents

Abstract

The austenite stainless steel sheet is prepared by (a) hot rolling steel slab that comprises C: below 0.03wt.%, Cr:15-17wt.%, Ni:2.5-3.5wt.%, Nb:0.1-0.2wt.%, Mn:2-7wt.%, N:0.05-0.15wt.%, Si:below 0.5%, and inevitable impurity; (b) annealing the hot rolled steel sheet at 1,200-1,260deg.C; (c) cold rolling the annealed steel sheet with reduction ratio of over 80%; (d) annealing at the temperature between the end point of inverse transformation of martensite-austenite and 50deg.C above the end point and; (e) cooling the annealed steel sheet by air or water.

Description

Manufacturing method of high strength austenitic precipitation hardening stainless cold rolled steel sheet

The present invention relates to a method for manufacturing a high strength austenitic stainless steel cold rolled steel sheet used as structural steel, and more particularly, to a high strength austenitic stainless steel cold rolled steel sheet used in structural materials having a tensile strength of 120Kgf / mm ² or more while requiring corrosion resistance. It relates to a manufacturing method.

Generally, austenitic precipitation hardening stainless steels are steels that are strengthened by aging by adding precipitation hardening elements C, P, N, Ti, Nb, and V to austenitic stainless steels. Precipitates are carbides, nitrides, phosphides, intermetallic compounds such as Ti and Al. These austenitic precipitation hardening steels have better corrosion resistance, workability, and weldability than martensitic and two-phase (austenitic + ferritic) precipitation hardening stainless steels. The use of which has the disadvantage of not exceeding 100 Kg / mm ² has been limited.

In addition, the conventional austenitic precipitation hardening stainless steels are heat treated for a long time at a temperature of about 700-900 ° C. for 5 hours to 24 hours or two times in order to sufficiently precipitate, and the manufacturing process is complicated. There was a drawback of being time consuming.

In order to solve these drawbacks, many attempts have been made to increase the strength of austenitic stainless steel in the past, and recently, it has been known that high strength can be achieved by refining grains using phase transformation.

As a representative example of the conventional method for increasing the strength, a metastable austenitic stainless steel causing diffusion type reverse transformation behavior (16 wt% Cr-10 wt% Ni, C, N 0.005% by weight, Mn, Si 0, 1% by weight) is cold-worked strongly at room temperature, and once the processing oil is made into martensite single phase (about 90% or more), it is annealed at a temperature near 850K and reverse transformation of martensite into austenite There is a method of obtaining a very fine austenite structure, the reverse transformation austenite obtained by the above method has a high dislocation density, fine grains can greatly improve the yield strength, which is a problem of austenitic stainless steels up to about 60kg / mm ² It was reported. (Japan Institute of Science and Technology, 74 (1988) P1052, Kagoshima University, Takamoto, Okuyama, etc.)

However, since the chemical composition of the steel used in the proposal does not contain any of the precipitation hardening elements at all, Koki et al. Adjusts the alloying composition slightly to raise the carbon content to 0.02% by weight (hereinafter referred to as "%"). In addition to the addition of 0.1% of Mo and 2% of Mo, a new high-strength steel was developed by Cr and Mo carbonitride precipitation during reverse transformation heat treatment in addition to the grain refinement effect. The chemical composition of the steel is Fe-12% Cr-9.5 % Ni-2% Mo-0.1% Mo-0.02% C. The result of using the steel to rolling of 80% cold at room temperature, as described above, and the heat treatment by changing the reverse transformation heat treatment conditions, the yield strength and high-strength austenitic tensile strength of the respective top ^{110kg / mm 2, 140kg / mm} 2 based A precipitation hardening stainless steel was developed. However, the high strength stainless steel developed by Koh, et al., Has excellent mechanical properties. However, Ni and Mo, which are expensive elements, are added in large amounts and Cr is 12.5%, which is the most effective element to improve corrosion resistance. In general, stainless steel is defined as a steel having a minimum Cr content of 12% or more, which is a very small amount. Therefore, in view of corrosion resistance, which is the biggest advantage of stainless steel, it remains very disadvantageous compared to conventional austenitic precipitation hardening stainless steel. have.

Therefore, the present invention is proposed to solve the above-mentioned problems, and partially replaces expensive Ni with relatively inexpensive Mn, N, etc. without adding Mo, and increases Cr content for the purpose of improving corrosion resistance. In order to produce austenite-based precipitation hardening stainless cold rolled steel sheet having excellent corrosion resistance and ductility and high strength by adding a small amount of Nb, an element which forms a carbide well for high strength, to be precipitated during hot rolling or reverse transformation heat treatment.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention provides a high-strength austenitic precipitation hardening stainless steel sheet manufacturing method, in weight%, C: 0.03% or less, Cr: 15-17%, Ni: 2.5-3.5%, Nb: 0.1-0.2%, Mn: Hot-rolled steel slab containing 2-7%, N: 0.05-0.15%, Si: 0.5 or less and other unavoidable impurity elements is subjected to hot-rolled sheet annealing at a temperature of 1200-1260 ° C, and reduction rate of 80% or more After cold rolling, the high strength austenitic precipitation hardening type stainless steel, which is subjected to annealing at a temperature between the martensite-austenite reverse transformation end temperature and the reverse transformation end temperature + 50 ° C., followed by air cooling or water cooling. It relates to a cold rolled steel sheet manufacturing method.

Hereinafter, the reason for limitation of component adjustment of steel according to the present invention will be described in detail.

In general, the higher the carbon content, the higher the strength of the material.However, even when a small amount of carbon is added to the steel, it stabilizes the austenite phase and lowers the martensite transformation start temperature (Ms), making it difficult to form martensite during cold working. In addition, when a large amount of carbon is added, precipitation of Cr carbides during reverse transformation heat treatment and deterioration of corrosion resistance thereof become a problem. Therefore, in the present invention, it is preferable to limit the carbon content to 0.03% or less, which is a low carbon stainless steel (eg, 304L, etc.) carbon content, which is usually produced industrially.

Meanwhile, the corrosion resistance of stainless steel is closely related to the content of solid solution Cr. It is known that the higher the solid solution amount of Cr, the better the corrosion resistance. However, when the Cr content is more than 17%, martensite transformation becomes difficult during cold rolling, which makes it difficult to obtain martensite single phase structure, and also increases the martensite / austenite reverse transformation temperature. When the austenite grains grow rapidly, on the other hand, when the Cr content is 15% or less, the corrosion resistance is greatly decreased, and the martensite transformation start temperature is increased above room temperature, thereby increasing the non-thermal elastic martensite content in the hot rolled state. In the present invention, it is preferable to limit the content of Cr to 15-17%, since the non-thermoelastic martensite is formed again in the cooling process after the reverse transformation heat treatment, and thus the austenite single phase structure cannot be obtained.

The Ni is a strong austenite forming element, when the content of Ni decreases, the Ms temperature rises and martensite is easily generated during the reverse transformation annealing, and the reverse transformation temperature also increases. Difficult to achieve. On the other hand, when too much is added, the reverse transformation temperature is lowered to suppress the growth of austenite grains, but the Ms temperature is too low, so that martensite is not produced during cold rolling. However, in the present invention, the effect of phase transformation behavior is limited to Ni, an expensive alloying element of 6% or less by adding N and Mn, which are austenitic stabilizing elements similar to Ni. In addition, if Ni is further reduced to 2% or less, more N or Mn should be added, and the maximum solubility of nitrogen in these steels is limited. When Mn is added, Ni decreases the corrosion resistance and facilitates the formation of Cr precipitates. It is desirable to limit the range of content to 2-6%.

Usually, since carbon and nitrogen are strong austenite stabilizing elements, the austenite phase is very stabilized by a small amount, and martensite single phase structure cannot be obtained by cold rolling. Therefore, in the present invention, by adding a small amount of Nb, which has a strong bonding strength with carbon and is easy to form carbides, and precipitates it as NbC during hot rolling, lowering the carbon content in steel after hot rolling, adding about 0.02% of carbon In one steel, martensite structure can be easily obtained during cold rolling, and the precipitation strengthening effect by NbC is generated during reverse transformation heat treatment, which is advantageous for high strength. At this time, it is preferable to limit the content of NbC to 0.1-0.2%. The reason is that if the content of Nb is 0.1% or less, the martensite structure during cold rolling is less because the amount of carbon and carbide formed in the steel is small. It is hard to obtain and the effect of increasing strength is small. On the other hand, adding 0.2% or more of Nb no longer improves the strength and elongation, but rather delays the formation of martensite during cold rolling due to the large amount of Nb remaining after precipitation with carbon. This is because it is not preferable to bring.

In addition, Mn is an austenite stabilizing element such as Ni, and thus has similar effects on phase transformation behavior. In the present invention, inexpensive Mn is added instead of reducing expensive Ni, wherein the amount of Mn added is 2-7. It is desirable to limit to%. The reason is that in stainless steel, Mn is added up to 7% when Ni is added instead of Ni, and when more manganese is added, the solid solution of Cr is lowered in the matrix and Cr is precipitated as the second phase. This is because a problem of deteriorating the mechanical properties occurs, and Mn is usually added to 1% or less in stainless steel. Therefore, when Mn is added in an amount of 2% or less, the effect of substituting Ni is too small, which is not preferable.

On the other hand, nitrogen is not only an austenite stabilizing element that is the same as Ni, but when added to steel, it is known to have a large solid solution strengthening or precipitation strengthening effect and increase tensile strength. When the N is added at 0.05% or less, the addition amount is small and the effect is small, and the content of N in the steel of the present invention is' the maximum nitrogen employment limit at room temperature, so the content of N is limited to 0.05-0.15%. It is preferable.

Other alloying elements, such as Si (0.5% or less) and P.S, may include a degree of entering into a typical austenitic stainless steel.

Hereinafter, a method of manufacturing a cold rolled steel sheet using the inventive steel slab having the above-described composition will be described in detail.

The stainless slab having the composition as described above is hot rolled by a conventional method, and then hot-rolled sheet annealing is performed at a temperature range of 1200-1260 ° C. At this time, the finish rolling temperature is more preferably about 950-1000 ℃ to have an austenite single-phase structure at room temperature after hot rolling during hot rolling, precipitates precipitated during hot rolling is difficult to re-use when the annealing temperature of the hot rolled sheet In the case of more than 1260 ℃, the grains become too large to damage the mechanical properties, so that the relatively large precipitates precipitated during hot rolling are re-arranged again in the matrix, so that these reclaimed elements are finely precipitated during reverse transformation annealing. It is preferable to limit plate annealing to 1200-1260 degreeC.

The cold rolling and reverse transformation heat treatment conditions of the steels subjected to the hot rolled and hot rolled sheet annealing are as follows.

The higher the amount of mardensite produced during cold rolling, the finer austenite grains can be obtained during reverse transformation, resulting in higher strength, and as much as possible cold working (at least 80%) at room temperature to ferrite scope (Ferrite-scope) It is desirable to make it produce 90% or more of processed organic martensite. However, the processing heat occurs during cold rolling, but if the processing heat is increased, the cold rolling temperature is increased, and if the rolling temperature is high, it is difficult to produce martensite, so that the rolling temperature is preferably not higher than room temperature. When the cold worked material is martensitic / austenite reverse transformation is completed, it is heated from immediately above the temperature to the reverse transformation end temperature + 50 ℃, then annealed at this temperature, and then air-cooled or water-cooled.

However, when the reverse transformation heat treatment is too high or the heat treatment time is too long, the strength of the reverse transformation austenite grains coarsened. In addition, when the grains of reverse transformation austenite become very coarse, the martensite transformation start temperature is increased, and martensite transformation may occur again during cooling after reverse transformation heat treatment, so that the final structure does not become austenite single phase. The transformation annealing temperature is preferably limited to the temperature at which the martensite / austenite reverse transformation is completed to the reverse transformation end temperature + 50 ° C.

The present invention characterized by the alloy composition and the rolling and heat treatment conditions will be described through the following examples.

EXAMPLE

Steel slabs having the composition shown in Table 1 were hot rolled at a finish rolling temperature of 1000 ° C., hot rolled annealing at 1230 ° C. for 40 minutes, and cold rolled at a thickness reduction rate of 83% at room temperature to a thickness of 1 mm. Tensile strength at room temperature with respect to the air-cooled specimens were prepared by heating the cold-rolled steels, heating the cold-rolled steels at 10 ° C./sec from reverse transformation end temperature to reverse transformation end temperature + 50 ° C. measured in Table 2 And the corrosion resistance was evaluated, the results are shown in Table 2 below.

TABLE 1

As shown in Table 1, the conventional steel (1-5) is austenitic precipitation hardening stainless steel, which is conventionally developed, and the comparative steel (6) is a steel recently developed by Koh, et al. 4) are expensive steels such as Ni and Mn, which are replaced with inexpensive elements. These are steels with increased Cr content to improve corrosion resistance.

The reverse transformation start and end temperature of the cold rolled steel sheet in Table 2 was measured while heating at a constant heating rate of 10 ℃ / second using a transformation dilatometer.

TABLE 2

As shown in Table 2, the conventional steel (1-5) can be seen that the tensile strength does not exceed 105kg / mm ² and the yield strength also does not exceed 70kg / mm ² , while the comparative steel (6) ) And invention steels have yield strength of 93 ~ 108kg / mm ² and tensile strength of 124 ~ 162kg / mm ² , but show high elongation of about 17 ~ 27%, suggesting high possibility of use as structural steel. .

In addition, these comparative steel (6) and the invention steel has a great advantage that does not need to maintain for a long time to cause precipitation, unlike the conventional steel (1-5) because precipitation occurs during reverse transformation annealing. In particular, the inventive steels do not add expensive Mo compared to the comparative steel (6), and economical chemical composition that greatly reduces the content of Ni, which is also an expensive element, by about 2-6%, and corrosion resistance due to the addition of 16% Cr It has advantages such as improvement and at the same time shows strength comparable to that of the comparative steel 6, and thus it is judged to be a new high strength austenitic precipitation hardening stainless steel.

As described above, the present invention is an economical alloy that has a carbon content that can be produced industrially while producing a large amount of martensite during cold rolling, and an expensive alloy that greatly reduces expensive Ni and improves corrosion resistance by adding Cr. Room temperature through heat treatment (air cooling after heating up to reverse transformation end temperature up to + 50 ℃) to reduce the grain size of austenite as much as possible during design and reverse transformation annealing, and to cause fine precipitation during reverse transformation annealing. In the method of making austenitic precipitation hardening stainless steel having high strength and high ductility in the future there is an effect that can be used as structural steel that requires corrosion resistance and high strength at the same time in the future.

Claims (1)

In the manufacturing method of high strength austenitic precipitation hardening stainless steel sheet, in weight%, C: 0.03% or less, Cr: 15-17%, Ni: 2.5-3.5%, Nb: 0.1-0.2%, Mn: 2-7 Hot-rolled steel slab containing%, N: 0.05-0.15%, Si: 0.5% or less and other unavoidable impurity elements, and then cold rolled hot-rolled sheet annealing at a temperature of 1200-1260 ℃ Then, after annealing at a temperature between the phase transformation end temperature +50 ℃ immediately above the martensite-austenite reverse transformation end temperature, air-cooled or water-cooled, characterized in that the high strength austenitic precipitation hardening stainless steel sheet manufacturing method.