KR20180095662A - Hot rolling of steel containing metastable austenite - Google Patents

Abstract

Heating the metastable steel before or during cold rolling suppresses transformation of the austenite into martensite, thus lowering the mill load and decreasing the amount at a similar load. Warm-rolled steel improves mechanical properties compared to steel by cold rolling and reduces the same amount. Warm rolling after the subsequent annealing also obtains mechanical properties superior to those achieved in the same amount of cold rolling and subsequent annealed materials. The metastable steel warm-rolled at the subsequent room temperature rolling (cold rolling) also improves both strength and ductility.

Description

Hot rolling of steel containing metastable austenite

This application claims the benefit of US Provisional Application Serial No. 62 / 278,567 filed on January 14, 2016 entitled "Warm rolling of steels containing metastable austenite" and US Provisional Patent Application filed on October 12, No. 62 / 407,001, entitled Hot Rolling of Steel Containing Metastable Austenite, the disclosures of which are incorporated herein by reference.

Cold rolling of steel containing metastable austenite may be challenging due to strain induced transformation into metastable austenite to high strength martensite phase.

The cold rolling of such steel significantly increases the mill load. In addition, the steel needs to undergo annealing (s) for partial or total austenitization before further cooling reduction is performed.

The present invention includes heating the material before or during cold rolling to suppress the transformation from austenite to martensite. This may result in lower mill loading and greater reduction at similar loads. The ability to further reduce the material leads to less intermediate annealing before the material reaches the final gauge. Surprisingly, preheated rolled steel shows improved mechanical properties compared to steel by the same amount reduced by cold rolling. Warm rolling after subsequent annealing also provides better mechanical properties than achieved with the same amount of cold rolled material. Hot rolled steel at subsequent room temperature rolling (cold rolling) shows improved strength and ductility.

Hot rolling has been avoided in the manufacturing environment due to the risk associated with heating of the oil conventionally used as lubricating oil as well as damage to the rolling equipment. This application shows that the advantage of warm rolling can be achieved without modifying the extension line at a mild temperature.

Figure 1 shows the ratio of martensite in metastable steels as a function of the rate of reduction due to warm rolling and cold rolling.
Figure 2 shows the elongation (%) of metastable steel as a function of the rate of reduction due to cold rolling and hot rolling.
Figure 3 (a) shows the actual stress-strain curve for cold-rolled metastable steel after warm rolling.
Fig. 3 (b) shows the actual stress-strain curve for the metastable steel cold-rolled in two passes.

The present invention is a steel containing a significant amount of metastable austenite (10% to 100% austenite), which is referred to as "metastable steel. &Quot; Austenite is regarded as metastable when converted to martensite during mechanical deformation. Such martensite is referred to as strain-induced martensite. The steel containing such metastable austenite may be carbon steel or stainless steel.

There are several ways to characterize the stability of austenite. One method is to calculate the austenite instability factor (IF) based on the chemical composition of the austenite. This factor is described in U.S. Pat. No. 3,599,320, the disclosure of which is incorporated herein by reference, which defines the IF as follows:

IF = 37.193-51.248 (% C) -0.4677 (% Cr) -1.0174 (% Mn) -34.396 (% N) -2.5884 (% Ni) Equation 1

Steel with a calculated IF value of 0 to 2.9 is classified as "slightly metastable " and steels with an IF of 2.9 or greater are classified as " generally metastable ". The method of the present invention has the most significance for steel containing metastable austenite with an IF greater than 2.9.

Another technique for characterizing the stability of austenite is to calculate or measure what is known as the M _d 30 temperature. For a given metastable steel composition, at a strain of 0.3 actual strain at an M _d 30 temperature, 50% of the austenite is transformed into martensite. For a given metastable steel composition, the M _d temperature is the temperature at which martensite is not formed upon deformation. The M _d and M _d 30 temperatures are well known in the art. In addition to being empirically determined, the M _d 30 temperature for a particular steel composition can also be calculated by one of several equations found in the literature, including:

Composition and particle size dependence of strain-induced martensitic transformation in metastable austenitic stainless steels, as taught by Nohara, K., Ono, Y. and Ohashi, N. 1977. Journal of the Japan Iron and Steel Association, 63 (5), pages 212-222 (the disclosure of which is incorporated herein by reference):

M _d 30 = 551-462 (% C +% N) -68 *% Cb-13.7 * Cr-29 (% Cu +% Ni) -8.1 *% Mn-18.5 *% Mo -9.2 *% Si Equation 2

Martenite formation of austenitic stainless steels, as taught by Angel T. 1954. Journal of the Iron and Steel Association, 177 (5), 165-174 (the disclosure of which is incorporated herein by reference):

M _d 30 = 413-462 * (% C +% N) -13.7 *% Cr-8.1 *% Mn-18.5 *% Mo-9.5 *

Equation 3

The higher the M _d 30 temperature of austenite of a given metastable steel composition, the more unstable austenite is. The M _d 30 temperature of this metastable austenite is higher than the M _s temperature (martensite starting temperature of the thermal martensite).

Steels with significant amounts of metastable austenite work quickly cure as austenite transforms into high strength martensite. Cold rolling of such steels may present a challenging problem because such work hardening and final martensite may require loads that may exceed the performance of the mill. These metastable steels need to be annealed to form some or all of the austenite before being further cold rolled. If the transformation from austenite to martensite can be suppressed during rolling, the steel can be rolled to a thinner gauge at a lower rolling load. One way to suppress this transformation is to warm the material before cold rolling or during cold rolling. Warm rolling has been shown to have the additional advantage of providing better mechanical properties.

The method of the present application comprises rolling metastable steels while the steel is warmed. If the metastable steel temperature is above room temperature (typically about 80 ° F), it is considered warm. In certain embodiments, the steel is heated to a temperature near or above the M _d temperature for a particular metastable steel composition. In another embodiment, the steel is heated to a temperature above the M _d 30 temperature for a particular metastable steel composition. In another embodiment, the metastable steel is warmed to a temperature of less than 250 < 0 > F.

Coils of this material may be warmed in a manner which is obvious to one of ordinary skill in the art, including one or a combination of the following methods:

I. Boil the coil in a furnace / oven before placing the coil on the rolling line.

II. The coil of the line is heated using a heater before cold rolling.

III. The coolant is warmed from the mill before rolling the steel material. This can be done in several ways. One method is to turn the cooling tower off the rolling mill and warm the coolant before rolling the metastable steel using other materials. Other methods of warming the coolant prior to rolling will be apparent to those skilled in the art.

Metastable steels are melted, cast, hot rolled and annealed prior to cold rolling (if applicable) according to conventional metal-making processes for a particular composition. During the cold rolling process of metastable steel, at least one "cold rolled" pass is a "hot rolling" pass during which the steel is warmed, i. E. In some embodiments, the steel is warmed to a temperature of 250 ℉ or less. In another embodiment, metastable steel is warmed to a temperature near or above the M _d temperature for a particular metastable steel composition. In another embodiment, metastable steel is heated to a temperature near or above the M _d 30 temperature for a particular metastable steel composition. This warm rolling pass may be at least one of the first, second or any subsequent "cold rolling"

In some embodiments of the present invention, metastable steel may be annealed after at least one warm rolling step. For example, during "cold rolling" processing, metastable steel may be warm rolled in a first pass, annealed, and then cold rolled in a second pass (at room temperature).

Example 1

Unstable steels were prepared by heat melting a chemical with an instability factor of 8.5 and M _d 30 (Nohara) = 447.6 ° F. Heat was continuously applied into the slab. The slab was reheated to 2300 DEG F and hot rolled to a thickness of 0.175 "at a coiling temperature of 1000 DEG F. The hot band was pickled to remove scale. For warm rolling purposes, the high temperature band section was warmed to the desired temperature in the furnace and rolled to the desired gauge.

Figure 1 shows the amount of martensitic transformation from cold and warm rolling of this metastable steel. At the same reduction, the amount of martensite in each hot rolled steel is significantly less than cold rolled steel rolled at room temperature. The advantage of warm rolling can be seen at low temperatures (150 ° F in this example), but the higher the temperature in warm rolling (250 ° F in this example) the less the amount of martensite formed.

Fig. 2 shows the elongation (%) of metastable steel at different decreasing amounts after warm rolling and cold rolling. Surprisingly, warm rolling increased elongation (%) up to a certain amount of reduction before descending rebound. The advantage of warm rolling can be tailored by varying the amount of reduction performed at temperature or by varying the temperature. On the other hand, cold rolling at room temperature always leads to a decrease in elongation (%) with respect to metastable steel.

Example 2

Other quasi-stable steels were prepared by choosing chemicals with an unstable factor of 13.11 and M _d 30 (Nohara) = 227.6 ° F. Heat was injected into the ingot. After trimming the ingot, four bars of 5.75 "(W) x 2.75" (T) x 2.75 "(L) were obtained. The trimmed ingot was housed at 2200 ° F. and at a finishing temperature of 1100 ° F, a 0.2"Lt; / RTI > The high temperature band was then pickled to remove the scale. Sections of the hot-rolled bands were cold-rolled and warm-rolled at different temperatures. For warm rolling purposes, the high temperature band sections were warmed to the desired temperature in the furnace and rolled to the desired gauge.

In these metastable steels, hot rolling followed by cold rolling showed both increased strength and elongation. Without previous warm rolling, the same steels exhibited an increase in strength, but as expected, a reduction in percent yield (%). Figure 3 (a) shows the actual stress strain data from 30% warm rolling at room temperature with various reductions and subsequently cold-rolled metastable steels. 3 (a) and 3 (b), "WR" represents warm rolling and "RT" represents cold rolling at room temperature. When 30% warm rolling followed by an additional 10% cold rolling, both elongation and strength were found to increase. As shown in Fig. 3 (b), the same material showed an increase in ultimate tensile strength (UTS) but a decrease in elongation as expected when cold rolling followed by 30% cold rolling at 0-30% room temperature . Again, the benefits of warm rolling can be tailored by varying the amount of reduction performed at temperature or by varying the temperature.

Example 3

The metastable steels of Example 1 above exhibit the effect of warm rolling on steel containing metastable austenite as further shown by the test data set forth in Tables 1 and 2 below, The properties of the steel containing the annealed metastable austenite (Coil 1) are compared to those of steel (Coil 2) containing 25% warm-rolled metastable austenite in the plant.

Example 4

The effect of warm rolling on anisotropy was also studied in the metastable steel of Example 1. Anisotropy can have a significant impact on subsequent molding. Warm rolling manages anisotropy in the mechanical properties of metastable steel.

The effect of warm rolling compared to cold rolling is further demonstrated by the data presented in Table 3 below. The initial high temperature band was the same in both rolling sets. One set was warm rolled (~ 250 ° F) with various reductions (10, 15 and 20%) and the other set was cold rolled with similar reduction. In the case of cold-rolled samples, the elongation in the longitudinal direction (L) and in the transverse direction (T) are considerably different. The larger the reduction, the bigger the difference. However, the difference in warm rolling is much smaller.

Claims (9)

In a method for rolling metastable steels,
a. Selecting a metastable steel having an instability factor (IF) of at least 2.9, wherein IF is expressed by the following equation:
Selecting the metastable steel calculated by IF = 37.193-51.248 (% C) -0.4677 (% Cr) -1.0174 (% Mn) -34.396 (% N) -2.5884 (% Ni);
b. Prior to rolling, warming the metastable steel to a warming temperature of greater than 80 < 0 >F; And
c. And rolling said metastable steel. ≪ RTI ID = 0.0 > 21. < / RTI > The method according to claim 1,
Wherein said warming temperature is near or above the M _d temperature for a particular metastable steel composition. The method according to claim 1,
Wherein said warming temperature is near or above the M _d 30 temperature for a particular metastable steel composition. The method according to claim 1,
Wherein the heating temperature is no more than 250 < 0 > F. The method of claim 3,
The temperature of M _d 30 of the metastable steel is given by the following equation:
M _d 30 = 551-462 (% C +% N) -68 *% Cb-13.7 * Cr-29 (% Cu +% Ni) -8.1 *% Mn- A method of rolling metastable steel. The method of claim 3,
The temperature of M _d 30 of the metastable steel is given by the following equation:
M _d 30 = 413-462 * (% C +% N) -13.7 *% Cr-8.1 *% Mn-18.5 *% Mo-9.5 *% Ni-9.2 * How to. 7. The method according to any one of claims 1 to 6,
Further comprising, after rolling, the metastable steel is rolled at room temperature. 8. The method of claim 7,
Wherein the metastable steel further comprises annealing prior to rolling at room temperature. 7. The method according to any one of claims 1 to 6,
Further comprising, after rolling, the metastable steel is rolled in a second rolling step, and before the second rolling step, the metastable steel is warmed to a heating temperature of greater than 80 DEG F. The metastable steel is rolled How to.

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