KR960005223B1 - Making method of high strength stainless cold steel sheet - Google Patents

Abstract

The stainless steel sheet for structural steel provides a combination of good corrosion resistance and high strength by adding molybdenum. The steel slab comprises, in weight percent, up to 0.03% carbon, 15 to 17% chromium, 2.5 to 3.5% nickel, 0.1 to 0.2% niobium, 3 to 4 manganese, 0.05 to 0.15% nitrogen, 0.3 to 1.0% molybdeum, silicon not exceeding 0.5%, and the balance of iron and inevitable impurities. The sheet is produced by the processes of general hot rolling the steel slab having the same chemical composition as mentioned above as a starting material, annealing of the hot rolled sheet at 1200 to 1260deg.C, cold rolling of the annealed sheet at a reduction ratio of not less than 80%, and continuous annealing of the cold rolled sheet at the temperature of the martensite/austenite inverse transformation finish temperature to 50deg.C higher than that, and continuous cooling in air or quenching in water.

Description

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

The present invention relates to a method of manufacturing Mo-added high-strength austenitic stainless cold rolled steel sheet used as structural steel, and more particularly, high-strength austenitic precipitation hardening stainless steel used in structural materials having a tensile strength of 120kgf / mm2 or more while requiring corrosion resistance It relates to a method for producing a cold rolled steel sheet.

Generally, austenitic precipitation hardening stainless steels are austenitic stainless steels that are hardened by aging by adding precipitation hardening elements C, P, N, Ti, Al, Nb, and V, etc. It occurs at the base and precipitates include intermetallic compounds such as carbides, nitrides, phosphides, Ti and Al.

Compared to martensitic and two-phase (austenitic + ferritic) precipitation hardening stainless steels, these austenitic precipitation hardening steels are superior in corrosion resistance, workability, and weldability, but the amount of alloying elements is increased, and the precipitation hardening degree is small. The strength is not more than 100kg / ㎜ has the disadvantage that its use has been limited. In addition, conventional austenitic precipitation hardening stainless steels are heat treated for a long time at a temperature of about 700 to 900 ° C. for more than 5 hours to 24 hours, or twice a heat treatment in order to sufficiently cause precipitation. This had a lot of drawbacks.

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 wt%) is cold-worked strongly at room temperature, and the processing oil is made into martensite single phase (about 90% or more), and then annealed at a temperature of around 850K to reverse transform the martensite into austenite, and the average particle size is 1 μm or less. There is a method of obtaining a very fine austenite structure, and the reverse transformation austenite obtained by the above method has a high dislocation density and fine grains, which greatly improves the yield strength, which is a problem of austenitic stainless steels, to about 60 kg / mm 2. It became. (Japan Institute of Science and Technology, 74 (1988) P1052, Kagoshima University, Takamoto, Okuyama, etc.)

However, only the refinement of the grain refinement according to the conventional method is limited in the strength improvement, and these steels also have some problems such as the addition of a large amount of expensive Ni.

Hereinafter, the present inventors applied an economical alloy design by replacing expensive Ni with relatively inexpensive Mn, N, and the like, and in addition to the above-mentioned grain refining effect, carbon or nitrogen was well formed to form carbides or nitrides in order to increase the strength. By adding a small amount of Nb, which is an element, to precipitate during hot rolling or reverse transformation heat treatment, an austenitic precipitation hardening stainless steel sheet having high strength has been developed and filed. Compared with the conventional austenitic precipitation hardening stainless steel, these steels are characterized by high strength, short heat treatment time, and low-cost alloying components. However, in terms of corrosion resistance, it is considered to be slightly inferior to conventional austenitic precipitation hardening stainless steels containing Mo and the like, and therefore, development of austenitic precipitation hardening stainless steels having better corrosion resistance is required.

Accordingly, the present invention is proposed to improve the corrosion resistance of the high strength austenitic precipitation hardening stainless steel in the prior art, and has the advantages of high tensile strength, short heat treatment time, and cheap alloy component design, but also to increase corrosion resistance and strength. The purpose of the present invention is to provide a method for manufacturing a high strength austenitic precipitation hardening stainless cold rolled steel sheet having excellent corrosion resistance by adding Mo, which is effective in all, instead of expensive Ni.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention provides a high-strength austenitic precipitation hardening stainless steel sheet production 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 3-4%, N: 0.05-0.15%, Mo: 0.3-1.0%, Sl: 0.5% or less and other unavoidable impurity elements, followed by hot-rolled annealing at 1200-1260 ℃ Cold rolling at a reduction ratio of 80% or more, followed by 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 treatment. It relates to a high strength austenitic precipitation hardening stainless steel sheet production method.

Hereinafter, the reason for limiting the compositional composition of the 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. If a large amount of carbon is added, the precipitation of Cr carbides during reverse transformation heat treatment and the deterioration of corrosion resistance due to this become a problem. Therefore, in the present invention, it is preferable to limit the carbon content to 0.03% or less, which is a carbon content range of low-carbon stainless steel (for example, 304L), which is usually produced industrially.

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

As Ni is a strong austenite forming element, when the content of Ni decreases, Ms temperature is increased, and martensite is easily generated during the reverse transformation annealing, and the reverse transformation temperature is also increased. 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 the martensite transformation start temperature on Ni and Mn similar to Ni, and Mo to replace Ni for improving corrosion resistance, thereby greatly reducing Ni, an expensive alloying element. To further reduce Ni to 2.5% or more, more N or Mn must be added, and the maximum solubility of nitrogen in these steels is limited. When Mn is added, the corrosion resistance and the occurrence of Cr precipitates are easily increased. It is desirable to limit the range to 2.5-3.5%.

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 force with carbon and is easy to form carbides, and precipitates it as NbC during hot rolling, carbon content dissolved in steel after hot rolling is lowered, thereby reducing carbon by about 0.02%. In the added steel, the 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, the content of Nb is preferably limited to 0.1-0.2%. The reason is that if the content of Nb is less than 0.1%, the amount of carbon and carbide formed in the steel is less, so the martensite structure is cold rolled. It is difficult to obtain and the strength increase effect is small. On the other hand, addition of more than 0.2% of Nb no longer improves the strength and elongation, but rather the amount of Nb remaining after the precipitation with carbon has delayed the production of martensite during cold rolling, resulting in a material cost increase. This is because it is not preferable.

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. Of course, more Mn may be added instead of Ni, but if Mn is added at 4% or more, the solid solubility of Cr decreases in the matrix, and Cr precipitates in the second phase, thereby degrading corrosion resistance and mechanical properties, and S in steel. If the amount of the inclusions is large, many inclusions such as MnS can be formed. However, when such inclusions are present in large amounts, the corrosion resistance can be reduced. In addition, since Mn is usually added to 1% or less in stainless steel, Mn is 3% or less in the present invention. When added as, the effect of substituting Ni is too small, so the content of Mn is preferably limited to 3-4%.

On the other hand, nitrogen is not only the same austenite stabilizing element as Ni, but when added to steel, it is known to have a high solid solution strengthening or precipitation strengthening effect, and in particular, an element that increases tensile strength. When the N is added at 0.05% or less, the addition amount is small, the effect of addition is small, and the content of N is limited to 0.05-0.15% because the N content of 0.15% is the maximum nitrogen employment limit at room temperature in the steel of the present invention. desirable. Mo, which dominates the characteristics of the present invention, is generally known as an element that improves corrosion resistance, and therefore, Mo-added steels are used in austenitic stainless steels.

In the present invention, it is preferable to limit the amount of Mo added to 0.3-1.0%. The reason for this is that when the amount of Mo added is 0.3% or less, the improvement of corrosion resistance is small, and when added at 1.0% or more, it becomes a factor of cost increase. However, Mo is a ferrite stabilizing element, which is undesirable because it increases the inverse transformation temperature together with Cr to obtain fine grains.

Other alloying elements such as Si (less than 0.5%), P.S, and the like may be included in the production of 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. In this case, the finish rolling temperature is more preferably about 950-1000 ℃ to have 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 is less than 1200 ℃ In the case of 1260 ° C. or more, the grains become too large to deteriorate the mechanical properties, so that the relatively large precipitates precipitated during hot rolling are re-arranged again in the matrix and these reclaimed elements are finely precipitated during reverse transformation annealing. It is preferable to limit hot rolled sheet 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 martensite produced during cold rolling, the more austenite grains can be obtained at the time of reverse transformation, so that higher strength can be achieved. It is desirable to produce 90% or more of processed organic martensite. However, the processing heat occurs during cold rolling, 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 it is preferable that the rolling temperature is not higher than room temperature. The cold worked material is heated to the reverse transformation end temperature + 50 ° C from immediately above the temperature at which the martensite / austenite reverse transformation is completed, and then annealed at this temperature, followed by air cooling or water cooling.

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 a temperature at which the martensite / austenite reverse transformation is completed to a reverse transformation end temperature of + 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

The steel slab having the composition as shown in Table 1 was hot rolled at a finish rolling temperature of 1000 ° C., hot-rolled annealing at 1230 ° C. for 40 minutes, and cold rolled at 83% thickness reduction at room temperature to a thickness of 1 mm. A cold rolled steel sheet was prepared, and the cold rolled steels were heated at 10 ° C./sec from the reverse transformation end temperature to the reverse transformation end temperature of + 50 ° C. measured in Table 2, followed by air cooling, and then the tensile strength at room temperature with respect to the air cooled specimen. 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 a conventional austenitic precipitation hardening stainless steel, and the comparative steel (6) is a low cost such as Ni, Mn, etc. Invented and replaced with elements, Invented steels (1-2) and Comparative steels (7) added 0.3%, 1%, and 2% Mo, respectively, in the same composition as that of Comparative steel (6) to improve corrosion resistance. to be.

The corrosion resistance evaluation was performed after the reverse transformation heat treatment of the comparative steel (6-7) and the inventive steel (1-2) of Table 1 at 830 ° C. for 90 seconds with sand paper (Sand Paper) # 800, 3.5% Nacl It was added to an aqueous solution of + 5% H ₂ SO ₄ , and a voltage was applied at 200 mV / min to the loaded specimen using a potentio-stat to measure the current change when a current change pitting occurred.

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

TABLE 2

As shown in Table 2, the conventional steel (1-5) and the comparative steel (7) do not exceed the yield strength of 70kg / ㎜, while the comparative steel (6) and the inventive steels yield strength of 93 ~ 108kg / ㎜, Although the tensile strength reaches 124 to 131 kg / mm2, it shows a high elongation of about 17-27%, which is highly applicable as a structural steel, and these steels are precipitated during reverse transformation annealing. Unlike), there is a big advantage that does not need to be maintained for a long time to cause precipitation. In particular, compared to the comparative steel (6), the invention steel is almost the same in tensile properties, and by replacing some of the Ni content with Mo, it can be seen that the raw material is almost similar. However, in terms of corrosion resistance, it can be seen that the invention steel has a low critical current density and a high pitting voltage, which is a measure of corrosion resistance determination. It was found to be greatly improved.

On the other hand, the comparative steel (7) to which Mo is added more than the inventive steel did not improve the corrosion resistance any more than the Mo addition amount, as well as the comparative steel (7) was found that the tensile properties rather deteriorate.

Therefore, it can be seen that the present invention is a new austenitic precipitation hardening stainless steel having excellent corrosion resistance in addition to advantages such as high tensile strength, inexpensive alloy design, and short heat treatment time.

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 greatly reduces expensive Ni and increases Mo to improve corrosion resistance. Room temperature through heat treatment (air cooling immediately after reverse transformation end temperature to reverse transformation end temperature + 50 ℃) to make the grain size of austenite as small as possible during design and reverse transformation annealing and to make 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 cold rolled steel sheet, in weight%, C: 0.03% or less, Cr: 15-17%, Ni: 2.5-7.5%, Nb: 0.1-0.2%, Mn: 3-4 Hot-rolled steel slab containing%, N: 0.05-0.15%, Mo: 0.3-1.0%, Si: 0.5% or less and other unavoidable impurity elements, and then hot-rolled annealing at a temperature of 1200-1260 ° C Cold rolling at a reduction ratio of not less than 80%, followed by annealing at a temperature between the martensite-austenite reverse transformation end temperature and the reverse transformation end temperature of + 50 ° C., followed by air cooling or water cooling. Austenitic precipitation hardening stainless steel sheet production method.