KR102249721B1 - Warm rolling of steel containing metastable austenite - Google Patents

Abstract

Warming the metastable steel before or during cold rolling suppresses the transformation of austenite to martensite, so the mill load is low and a larger amount is reduced at similar loads. Warm rolled steel has improved mechanical properties compared to steel by cold rolling, reducing the same amount. Warm rolling after subsequent annealing also obtains mechanical properties superior to those achieved with the same amount of cold rolling and subsequently annealed material. In the subsequent room temperature rolling (cold rolling), the warm-rolled metastable steel also improves both strength and ductility.

Description

Warm rolling of steel containing metastable austenite

This application is a U.S. Provisional Application Serial No. 62/278,567 filed January 14, 2016 (Title of the Invention: Warm Rolling Steel Containing Metastable Austenite) and U.S. Provisional Application Serial Filed October 12, 2016 It claims priority to No. 62/407,001 (name of the invention: warm rolling of steel containing metastable austenite), the disclosures of which are incorporated herein by reference.

Cold rolling of steels containing metastable austenite can be challenging due to strain-induced transformation of metastable austenite into a high strength martensite phase.

(Patent Document 1) Japanese Patent Publication No. Hei 07-68584 (July 26, 1995)

Cold rolling of these steels significantly increases the mill load. In addition, the steel needs to undergo an anneal(s) for partial or full austenitization before further performing the cooling reduction.

The present invention includes heating the material before or during cold rolling to suppress the transformation from austenite to martensite. This can lead to lower mill loads and larger reductions at similar loads. The ability to reduce the material further leads to less intermediate annealing before the material reaches the final gauge. Surprisingly, preheated rolled steel showed improved mechanical properties compared to steel by the same amount reduced by cold rolling. Warm rolling after subsequent annealing also results in mechanical properties superior to those achieved with the same amount of cold rolled material. On subsequent room temperature rolling (cold rolling), the warm rolled steel shows an improvement in both strength and ductility.

Conventionally, warm rolling has been avoided in a manufacturing environment because there is a risk of damage to rolling equipment as well as a risk related to heating of oil used as a lubricant. The present application shows that the benefits of warm rolling can be achieved at mild temperatures without extension line deformation.

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

The present invention is a steel containing a significant amount of metastable austenite (10% to 100% austenite), referred to as "metastable steel". Austenite is considered metastable if it converts to martensite upon mechanical deformation. Such martensite is called deformation-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 austenite chemical composition. This factor is described in U.S. Patent 3,599,320, the disclosure of which is incorporated herein by reference, which defines IF as:

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

Stills with a calculated IF value of 0 to 2.9 are classified as "slightly metastable", and stills with an IF greater than 2.9 are classified as "moderately metastable". The method of the present invention has the most important implications for steels 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, _{upon deformation to 0.3 actual strain at 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. M _d and M _d 30 temperatures are well known in the art. In addition to being determined empirically, 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 in 1977. Journal of the Japan Steel Association, pages 63(5), 212-222 (the disclosure content 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

Martensitic formation of austenitic stainless steels, as taught by Angel T. in 1954. Journal of the Iron Works Association, 177(5), pages 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*%Ni-9.2*%Si

Equation 3

_{The higher the M d} 30 temperature of austenite of a given metastable steel composition, the more unstable the austenite becomes. _{The M d} 30 temperature of this metastable austenite _{is higher than the M s} temperature (the martensitic starting temperature of thermal martensite).

Steel with a significant amount of metastable austenite processing hardens rapidly as the austenite transforms into high strength martensite. Cold rolling of these steels can be presented as a challenging problem as these work hardening and final martensite can require loads that can exceed the performance of the mill. These metastable steels need to be annealed to form some or all of the austenite before further cold rolling. If the transformation from austenite to martensite during rolling can be suppressed, the steel can be rolled into a thinner gauge with a lower rolling load. One way to suppress this transformation is to warm the material before or during cold rolling. It has been shown that warm rolling has the additional advantage of leading to better mechanical properties.

The method of the present application involves rolling metastable steel while the steel is warming. A metastable steel temperature is considered warm if it exceeds room temperature (typically about 26°C). In certain embodiments, the steel is warmed to a temperature near or above the _{M d temperature for a particular metastable steel composition.} In another example, the steel is warmed 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 121° C. or less.

Coils of such material may be warmed in a manner apparent to a person skilled in the art, including one or a combination of the following methods:

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

II. Before cold rolling, a heater is used to warm the coils of the line.

III. The coolant is warmed in the mill before rolling the steel material. This can be done in several ways. One way is to turn off the cooling tower in the rolling mill and warm up the coolant before rolling the metastable steel with another material. 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 the conventional metal-making process for a particular composition. During the cold rolling process of metastable steel, at least one “cold rolling” pass is a “warm rolling” pass performed while the steel is warming, ie while the steel is at a temperature above 26°C. In some examples, the steel is warmed to a temperature of 121° C. or less. In another embodiment, the metastable steel is warmed to a temperature near or above the _{M d temperature for a particular metastable steel composition.} In another embodiment, the metastable steel is warmed to a temperature near or above the _{M d 30 temperature for a particular metastable steel composition.} This warm rolling pass may be one or more of the first, second or any subsequent “cold rolling” steps.

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

Example 1

The unstable steel was prepared by heating and melting a chemical agent having an instability factor of 8.5 and M _{d 30 (Nohara) = 230°C.} Heat was continuously applied into the slab. The slab was reheated to 1260[deg.] C. and hot rolled to a thickness of 0.175" at a coiling temperature of 537[deg.] C. The hot band was pickled to remove scale. Sections of the pickled hot band were cold rolled and warm rolled. For warm rolling purposes, the hot band sections were warmed to the desired temperature in a furnace and rolled to the desired gauge.

1 shows the amount of martensitic transformation from cold and warm rolling of this metastable steel. At the same amount of reduction, the amount of martensite in each hot-rolled steel is significantly less than that of cold-rolled steel rolled at room temperature. The advantage of warm rolling can be seen at low temperatures (65° C. in this example), but the higher the temperature during warm rolling (121° C. in this example), the less the amount of martensite formed.

2 shows the elongation (%) of metastable steel with different reduction amounts after hot rolling and cold rolling. Surprisingly, warm rolling increased the elongation (%) until a certain amount of decrease before the start of the descent. The benefits of warm rolling can be tailored by varying the temperature or by varying the amount of reduction carried out in temperature. On the other hand, cold rolling at room temperature always leads to a decrease in the elongation (%) for metastable steel.

Example 2

Other metastable steels were prepared by choosing a chemical agent with an instability factor of 13.11 and M _d 30 (Nohara) = 108°C. Heat was put into the ingot. After trimming the ingot, four bars of 5.75" (W) x 2.75" (T) x 2.75" (L) were obtained. This trimmed ingot was held at 1204° C. and 0.2" at a finishing temperature of 593° C. It was hot rolled into a furnace. The hot band was then pickled to remove scale. The sections of the pickling hot band were cold rolled and warm rolled at different temperatures. For the purpose of warm rolling, the hot band section was warmed to the desired temperature in a furnace and rolled to the desired gauge.

In this metastable steel, it was found that both the strength and the elongation increased in the cold rolling followed by the warm rolling. In the absence of previous warm rolling, the same steel exhibited an increase in strength but, as expected, a decrease in elongation (%). Figure 3(a) shows actual stress strain data from 30% warm rolled and subsequently cold rolled metastable steel at room temperature with various reductions. In Figs. 3(a) and 3(b), "WR" denotes warm rolling and "RT" denotes cold rolling at room temperature. When performing 30% warm rolling followed by an additional 10% cold rolling, both elongation and strength were found to increase. As shown in Fig. 3(b), when cold rolling of 30% followed by additional cold rolling at room temperature of 0 to 30%, the same material was found to increase the final tensile strength (UTS) but decrease the elongation as expected. . Again, the benefits of warm rolling can be tailored by varying the temperature or by varying the amount of reduction carried out in temperature.

Example 3

The metastable steel of Example 1 shows the effect of warm rolling on the steel containing metastable austenite as further presented by the test data shown in Tables 1 and 2 below, and Tables 1 and 2 are completely The properties of the annealed metastable austenite-containing steel (coil 1) are compared with the properties of the steel (coil 2) containing 25% warm-rolled metastable austenite at the factory.

Example 4

The effect of warm rolling on anisotropy was also studied in the metastable steel of Example 1. Anisotropy can have a significant effect on subsequent shaping. 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 shown in Table 3 below. The initial hot bands were the same for both roll sets. One set was warm rolled (@~121° C.) with various reductions (10, 15 and 20%), and the other set was cold rolled with similar reductions. In the case of a cold-rolled sample, the elongation in the longitudinal direction (L) and the transverse direction (T) is quite different. The larger the reduction, the larger the difference. However, in the case of warm rolling, the difference is much smaller.

Claims (9)

In the method of rolling metastable steel,
a. Selecting a metastable steel having an instability factor (IF) of 2.9 or greater, IF is 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. Hot rolling the metastable steel;
c. Heating the metastable steel to a heating temperature of more than 26° C. and not more than 121° C. after the hot rolling step and before the cold rolling step or during the cold rolling step; And
d. A method of rolling the metastable steel comprising the step of rolling the metastable steel. The method of claim 1,
The warming temperature is near or above _{the M d} 30 temperature for a particular metastable steel composition,
_{The M d} 30 temperature for the metastable steel is the following equations:
M _d 30 =551-462(%C+%N)-68*%Cb-13.7*Cr-29(%Cu+%Ni)-8.1*%Mn-18.5*%Mo-9.2*%Si; And
M _d 30 = 413-462*(%C + %N)-13.7*%Cr-8.1*%Mn-18.5*%Mo-9.5*%Ni-9.2*%Si
Method of rolling metastable steel, calculated according to any one of. The method according to claim 1 or 2,
After the step of rolling the metastable steel (d), the method of rolling the metastable steel further comprising the step of rolling the metastable steel at room temperature. The method of claim 3,
Prior to rolling at room temperature, the method of rolling the metastable steel further comprising the step of annealing the metastable steel. The method according to claim 1 or 2,
The step (d) of rolling the metastable steel is a first rolling step,
The method comprises the steps of: after the first rolling step, the metastable steel is rolled in a second rolling step, and before the second rolling step, the metastable steel is heated to a heating temperature of more than 26°C and not more than 121°C. A method of rolling a metastable steel further comprising a. The method of claim 5,
After the first rolling step and before the second rolling step, the method of rolling the metastable steel further comprising the step of annealing the metastable steel. delete delete delete

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