TWI623622B - Warm rolling of steels containing metastable austenite - Google Patents

Abstract

Warming the metastable steel before or during cold rolling inhibits the transformation of austenite to the granules, resulting in lower mill load and higher shrinkage under similar loads. The warm rolled steel has enhanced mechanical properties when compared to the same amount of steel reduced by cold rolling. Warm rolling and subsequent annealing also produce better mechanical properties achieved in the same amount of cold rolling and then annealed. The metastable steel which has been warm rolled exhibits enhanced strength and ductility upon subsequent room temperature rolling (cold rolling).

Description

Warm rolling of steel containing metastable austenite

Cold rolling of metastable austenitic steels can be challenging due to the deformation-induced transition of metastable austenite to higher strength masculine bulk phases. Cold rolling of this steel results in a significant increase in mill load. The steel also needs to undergo annealing to partially or completely austenitize before further cold reduction can be performed.

The present invention relates to raising the temperature of a material prior to or during cold rolling to inhibit the transformation of austenite to granules. This can result in lower mill loads and higher reductions under similar loads. The ability to reduce the amount of material can also cause less intermediate annealing before the material can reach the final specification. Surprisingly, the warm rolled steel has shown enhanced mechanical properties when compared to the same amount of steel reduced by cold rolling. Warm rolling and subsequent annealing also produce better mechanical properties achieved in the same amount of cold rolling and then annealed. The steel which has been warm rolled shows increased strength and ductility at subsequent room temperature rolling (cold rolling).

Previously, warm rolling has been avoided in production environments due to the fact that warm rolling can cause damage to rolling equipment and existing risks associated with oil heating as a lubricant. This application shows that the benefits of warm rolling can be achieved at moderate temperatures and there are no extensive line modifications.

Figure 1 shows that the % of the lost field in the metastable steel varies with the % reduction from the warm rolling and cold rolling.

Figure 2 shows that the % elongation of metastable steel varies with the % reduction from cold rolling and warm rolling.

Figure 3 (a) shows the true stress-true strain curve of the metastable steel after warm rolling and then cold rolling.

Figure 3(b) shows the true stress-true strain curve of the cold rolled metastable steel in two passes.

priority

The application is filed on January 14, 2016. The title of the application is WARM ROLLING OF STEELS CONTAINING METASTABLE AUSTENITE, US Provisional Application No. 62/278,567, and filed on October 12, 2016, titled WARM ROLLING OF STEELS The priority of U.S. Provisional Application Serial No. 62/407,001, the disclosure of which is incorporated herein by reference.

The present invention relates to a steel containing a large amount of metastable austenite (10% - 100% austenite), which is called "meta-stable steel". If the austenite transforms into a granule loose body during mechanical deformation, it is considered to be a metastable state. This Ma Tian loose body is called a deformation-induced Ma Tian bulk. The steel containing this metastable austenite may be carbon steel or stainless steel.

There are several ways to characterize the stability of austenite. One way is to calculate the instability factor (IF) based on the chemical composition of austenite. This factor is set forth in U.S. Patent No. 3,599,320, the disclosure of which is hereby incorporated by reference in its entirety, in which the IF is defined as: IF = 37.193 - 51.248 (C%) - 0.4677 (Cr%) - 1.67 (Cu%) - 1.0174 (Mn%) - 34.396 (N%) - 2.5884 (Ni%) Equation 1

Steels with calculated IF values of 0-2.9 are classified as "slight metastability" and steels with an IF greater than 2.9 Classified as "moderate metastability". The method of the present invention is most meaningful for steels containing metastable austenite having an IF greater than 2.9.

Another technique for characterizing the stability of austenite is to calculate or measure the temperature referred to as the temperature of M _d 30 . For a given metastable steel composition, 50% of the austenite is converted to a granule dispersion when deformed to a true strain at a temperature of M _d 30 . For a given metastable steel composition, the temperature of the M _d is higher than the temperature at which no mass of the matrix is formed during deformation. Temperatures of M _d and M _d 30 are well known in the art. In addition to empirical determination, the M _d 30 temperature of a particular steel composition can also be calculated by one of several equations found in the literature, including the following equations: such as Nohara, K., Ono, Y. And Ohashi, N.1977. Composition and Grain-Size Dependencies of Strain-Induced Martensitic Transformation in Metastable Austenitic Stainless Steels. Journal of Iron and Steel Institute of Japan, 63(5), pp. 212-222 (the disclosure of which is The teachings are 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

As Angel, T.1954.Formation of Martensite in Austenitic Stainless Steels.Journal of the Iron and Steel Institute, 177 (5), on pages 165-174 (the disclosure of which is incorporated herein by reference) taught: 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 temperature of the M _d 30 of the austenite of the given metastable steel composition, the more unstable the austenite. The temperature of M _d 30 in the metastable austenite is higher than the temperature of M _s (the temperature at which the mass of the hemp field is released from the field).

Steel with a large amount of metastable austenite becomes rapidly hardened with the transformation of austenite into a higher strength of the field. Cold rolling of these steels remains a challenge as higher conversions can exceed mill capacity. It is then necessary to anneal these steels to form some or all of the austenite before it can be further rolled. If the transformation of austenite to the granules is inhibited during rolling, the steel can be rolled to a thinner gauge with a lower mill load. One way to suppress this transition is to warm the material before or during cold rolling. Warm rolling has been shown to have the added benefit of producing better mechanical properties.

The method of the present application involves rolling the metastable steels while the steel is warm. It is considered to be warm when the metastable steel temperature is above room temperature (typically about 80 °F). For certain embodiments, the steel is heated to a temperature near or above the metastable state M specific steel composition of the temperature _d. In other embodiments, the steel is allowed to warm to a temperature above the temperature of M _d 30 of the particular metastable steel composition. Typically, the metastable steel is not heated to a temperature greater than 250 °F.

The wire rod of this material can be heated by one of the following methods or a combination thereof:

I. The wire rod is heated in a furnace/oven and thereafter placed on a rolling line.

II. The wire rod on the production line is heated by using a heater and then cold rolled.

III. Warming the coolant on the mill and then rolling the steel material. This can be implemented in several ways. One way is to turn off the cooling tower on the mill and run some other material to warm the coolant before rolling the metastable steel. Other methods of warming the coolant prior to rolling will be apparent to those skilled in the art.

The metastable steel is melted, cast, hot rolled and annealed prior to cold rolling (if applicable), depending on the typical metal fabrication process for the particular composition. During cold rolling of metastable steel, at least one "cold rolling" pass is when the steel is warm (ie, when the steel is above 80 °F but not greater than 250 °F and is near or above a specific metastable steel when M _d at or above the temperature of the composition of the metastable state M specific steel composition of the temperature of the temperature _d 30) "warm rolling" of the embodiment passes. This warm rolling pass may be one or more of the first, second or any subsequent "cold rolling" steps.

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

Example 1

The metastable steel is prepared by melting a molten steel having a chemical property of an unstable factor of 6.8. The molten steel is continuously cast into a cast slab. The slab is reheated to 2300 °F and hot rolled to a thickness of 0.175", wherein the crimp temperature is 1000 ° F. The tropics are then pickled to remove the scale. The pickled tropical section is cold rolled and warm rolled. For the purpose of warm rolling, the tropical section is heated in a furnace to a desired temperature and rolled to a desired specification.

Figure 1 shows the amount of shift in the field of the field from the cold rolling and warm rolling of the metastable steel. At the same reduction, the amount of loose ground in each warm rolled steel is significantly smaller than that in cold rolled steel, which is rolled at room temperature. The benefits of warm rolling are visible at low temperatures (150 °F in this example), but the higher the temperature during warm rolling (250 °F in this example), the lower the amount of mash formed.

Figure 2 shows the % elongation of metastable steel after warm rolling and cold rolling to different reduced amounts. Surprisingly, the warm rolling causes the % elongation to increase to a specific amount of reduction and then begins to decrease. The benefits of warm rolling can be adjusted by varying the amount of shrinkage performed at a certain temperature or by changing the temperature. On the other hand, cold rolling at room temperature always results in a decrease in the % elongation of metastable steel.

Example 2

Another metastable steel is prepared by selecting a chemical with an instability factor of 13. The molten steel is cast into an ingot. After trimming the ingot, four test bars of 5.75" (W) x 2.75" (T) x 2.75" (L) were obtained. The trimmed ingots were soaked at 2200 °F and hot rolled to 0.2. ", where the finishing temperature is 1100 °F. Then pickle the tropics to remove the scale. Will be pickled The tropical section is cold rolled and warm rolled at different temperatures. For warm rolling purposes, the tropical section is heated in the furnace to the desired temperature and rolled to the desired specifications.

In this metastable steel, both warm rolling and subsequent cold rolling show an increase in strength and elongation %. In the absence of prior warm rolling, as expected, the same steel showed an increase in strength but a decrease in % elongation. Figure 3 (a) shows true stress-strain data from metastable steel that has been warm rolled by 30% and then cold rolled to a different reduced amount at room temperature. In Figs. 3(a) and 3(b), "WR" means warm rolling and "RT" means cold rolling at room temperature. 30% warm rolling followed by another 10% cold rolling showed an increase in both elongation and strength. As shown in Figure 3(b), the same material shows an increase in final tensile strength ("UTS") but a decrease in elongation when cold rolled 30% and then cold rolled 0-30% at room temperature, as will expected. In addition, the benefits of warm rolling can be adjusted by varying the amount of shrinkage performed at a certain temperature or by changing the temperature.

Example 3

The metastable steel of Example 1 above shows the effect of warm rolling on the steel containing metastable austenite, as further shown by the test data illustrated in Tables 1 and 2 below, which compares the already fully annealed metastable The austenitic steel (wire rod 1) and the properties of the medium-temperature austenitic steel (wire rod 2) which are 25% warm rolled in the apparatus.

Example 4

The effect of warm rolling on anisotropy was also investigated for the metastable steel of Example 1. Anisotropy can have a significant effect on subsequent forming. Warm rolling helps to control the anisotropy of the mechanical properties of the metastable steel.

The effect of warm rolling is further demonstrated by the data illustrated in Table 3 below compared to cold rolling. For the two sets of rolling, the initial tropical is the same. A set of warm rolling (at about 250 °F) to different reductions (10%, 15%, and 20%) and another set of cold rolling to a similar reduction. In the case of cold rolled samples The elongation in the longitudinal (L) and transverse (T) orientations varies greatly. The higher the reduction, the greater the difference. However, in the case of warm rolling, the difference is much smaller.

Claims (6)

A method of rolling metastable steel comprising the steps of: a. selecting a metastable steel having an unstable factor (IF) greater than or equal to 2.9, wherein the IF is calculated by the following equation: IF = 37.193-51.248 (C%) - 0.4677 (Cr%) - 1.67 (Cu%) - 1.0174 (Mn%) - 34.396 (N%) - 2.5884 (Ni%); b. Warm the metastable steel until rolling Greater than 80 °F and less than or equal to 250 °F, and a temperature near or above the M _d temperature of the particular metastable steel composition or above the temperature of the M _d 30 of the particular metastable steel composition; c. rolling the medium Steady state steel. The method of claim 1, wherein the temperature of the M _d 30 of the metastable steel is calculated according to the following equation: M _d 30=551-462 (C%+N%)-68*Cb%-13.7*Cr %-29 (Cu%+Ni%)-8.1*Mn%-18.5*Mo%-9.2*Si%. The method of claim 1, wherein the temperature of the M _d 30 of the metastable steel is calculated according to the following equation: M _d 30=413-462*(C%+N%)-13.7*Cr%-8.1* Mn%-18.5*Mo%-9.5*Ni%-9.2*Si%. The method of claim 1, 2 or 3, further comprising wherein after rolling, at room temperature The step of rolling the metastable steel. The method of claim 4, further comprising the step of annealing the metastable steel and then rolling at room temperature. The method of claim 1, 2 or 3, further comprising the step of: rolling the metastable steel in a second rolling step after rolling and prior to the second rolling step, The steady state steel is raised to greater than 80 °F and less than or equal to 250 °F, and is at or above the M _d temperature of the particular metastable steel composition or higher than the M _d 30 temperature of the particular metastable steel composition The temperature.