JPS6254847B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention aims to provide an economical method for manufacturing a high-Mn non-magnetic cold-rolled steel sheet as an alternative to conventional austenitic stainless steel. Many high-Mn-based austenitic steels have been developed and offered as highly economical non-magnetic steels, but all of these austenitic steels have a basic structure consisting mainly of thick plates and sections. Since it is provided as a steel for general use, it is manufactured using a compositional design that places the most emphasis on strength, with secondary properties such as corrosion resistance or wear resistance. By the way, recently, economically advantageous cold-rolled size (plate thickness
There is a demand for the use of high-Mn steel (with a diameter of 1.4 mm or less and a dimensional accuracy and surface condition comparable to those of cold-rolled steel sheets), but in order to use the conventional high-Mn steel mentioned above as a cold-rolled material, it is difficult to manufacture it. There are several important issues that remain unresolved. That is, the first problem is that all conventional component steels are designed to increase the low yield strength, which is an inherent characteristic of austenitic steel, from the viewpoint of ensuring strength as a structural material. Due to the large amount of work hardening, gauge down by cold rolling becomes extremely difficult, and rolling becomes impossible without a powerful rolling mill such as a Zenzi mill. In other words, the resistance during cold rolling is small.
A steel type that can be rolled on a regular rolling mill is required.
Second, since the cold-rolled materials mentioned above are often used after being subjected to surface chemical conversion treatment or coating, they are required to have good surface treatment properties. The steel composition must be such that a surface oxide film during annealing is difficult to form even in a normal annealing atmosphere for mild steel sheets, or the oxide film must be easily removed on the line. In addition, since such cold-rolled materials usually undergo high-level forming mainly through press forming,
It has good formability, and in many cases, the tensile strength is below the strength limit of about 70 kg/mm ² for forming, and of course the basic property of non-magnetic properties does not deteriorate even when subjected to severe forming. This is also required. The present invention solves the problems of the basic elements in producing cold-rolled materials as described above, and provides an economical method for producing non-magnetic cold-rolled steel sheets. That is, the product according to the present invention contains 0.15% or less C, 0.2% or less Si, 28 wt% (hereinafter simply referred to as %).
% to 36% Mn, 0.015% or less S, the balance
Steel that is composed of Fe and inevitable elements, has a composition that satisfies 37・C (%) + Mn (%) ≧ 32.5 (%), and has the above composition so as to fully satisfy the press formability as described above. After hot rolling and cooling, the annealing temperature (T°C) is set at T≧0.19 (CR) ^3/2 +770 in relation to the cold rolling ratio (CR%), and the magnetic permeability is characterized by continuous annealing.
This is a method for manufacturing a high manganese nonmagnetic cold rolled steel sheet with a molecular weight of 1.02 or less. Describing the reasons for defining the present invention as described above, first of all, the reasons for the limitations regarding the components of steel are as follows. C is the most useful element in stabilizing austenite in balance with Mn, but
On the other hand, this C is also an element that significantly increases the strength of expanded austenite, so in order to lower the deformation resistance during cold rolling and ensure cold rollability, it is also necessary to use advanced pressing or other forms by the user. Considering this, the upper limit of the amount of C was set at 0.15% in order to achieve a practical strength (for example, tensile strength of 70Kg/mm ² ).
And so. Si is an undesirable element in the steel sheet of the present invention due to the surface properties of the steel sheet during the hot rolling or cold rolling annealing process (scale generation, reduction in pickling properties), and the upper limit is
Limited to 0.2%. If it is less than this, no adverse effects of Si will occur. Mn is the basic element for stable austenite formation. For this reason, although it varies depending on the C content, it is necessary to be 28% or more and satisfy the following: 37×C (%) + Mn (%)≧32.5 (%). This is necessary to maintain stable non-magnetic properties with magnetic permeability of 1.02 or less even in areas where the cold-rolled annealed material is subjected to high strain at the edges of forming or punching, and also to maintain the work hardenability of the steel material. In order to reduce the rolling load during cold rolling by reducing the rolling load as much as possible, we conducted extensive tests to find a composition system that satisfies both the objectives of improving so-called cold rolling properties and facilitating the forming of steel sheets. It is something. Regarding the latter, conventional nonmagnetic steels with a C-Mn balance are generally prone to phase transformation during processing, but this transformation product (ε phase) is nonmagnetic like austenite, so it has low permeability. Magnetic property is maintained. However, work hardening was found to be greater than that of austenite, which is more stable.
The lower limit value of the amount of Mn mentioned above is limited for the above reasons. This situation is shown in Figure 2. That is, steels with various amounts of C and Mn are finished at 850℃, coiled and hot rolled at 700℃,
Its tensile strength (TS) and magnetic permeability after 60% cold rolling are shown. The tensile strength is a parameter indicating difficulty in terms of strength in terms of cold rolling load and press formability of the steel material, and for the purpose of the present invention, it needs to be 70 Kg/mm ² or less. Magnetic permeability is a basic property for non-magnetic properties and must be 1.02 or less. In Fig. 2, steels that satisfy both of these parameter values are shown as ○, and steels that do not satisfy either one or both of them are shown as ●.Firstly, in terms of strength, C must be 0.15 or less. Furthermore, the lower limit of the C-Mn balance could be approximately indicated by the line Mn+37C=32.5. Looking in detail, in a low C region such as 0.05%, the C-Mn balance is out of the appropriate range due to magnetic permeability, while in a region with a slightly higher C content, it is due to strength rather than permeability. This corresponds to the above explanation that steels with a C-Mn balance in this range are not necessarily stable austenite, but ε-martensite is formed through processing, which does not increase permeability but increases work hardening. There is. On the other hand, even if Mn exceeds 36%, the above-mentioned contribution to austenite stabilization does not change, which tends to cause problems in steel manufacturing, and is also disadvantageous in terms of cost, so 36% was adopted as the upper limit. Since S is an element that impairs ductility, the upper limit was set at 0.015% to ensure that it does not have an adverse effect. Although not specified in the present invention, Cr
Since it is effective for corrosion resistance, it can be added in an amount up to 2% if necessary. This is because if it exceeds 2%, an oxide film is likely to form during the annealing process, causing problems such as deterioration of magnetic permeability and, conversely, the need for a special pickling line to remove the film. Slabs or continuously cast slabs made from Al-killed ingots satisfying the above basic components are hot-rolled and cold-rolled and then continuously softened and annealed in a conventional manner. The recrystallization softening temperature of the steel produced by the method of the present invention is as high as 750° C. or higher, and grain growth is difficult, so a considerably high temperature annealing is required. In the present invention, in consideration of such circumstances, continuous annealing was performed from the viewpoint of thermoeconomic efficiency and uniformity of the steel sheet material. The annealing temperature is a characteristic of short-time heating in continuous processing, which causes grain growth as well as recrystallization, and is determined according to the cold rolling rate in order to obtain an appropriate structure and grain size, that is, strength and ductility. It is necessary to set T0.19 (CR) ^3/2 +770 (℃). The annealing effect has extremely high temperature dependence,
If the temperature is appropriate, a sufficient effect can be obtained with annealing time of several minutes. In the steel produced by the method of the present invention, there is a good correlation between the structure (grain size) after cold rolling annealing, strength, ductility, and yield ratio (yield strength/tensile strength), and If this is insufficient, the yield ratio will be high, the strength will be high, and the ductility will be low. In other words, the yield ratio is useful as a parameter that indicates the appropriateness of the structure during annealing, and the attached Figure 1 shows the yield ratio obtained for the 0.08C-30%Mn material with respect to the cold rolling rate and annealing temperature. It shows the ratio. It is known that the yield ratio when sufficient ductility is achieved by softening annealing corresponds to a yield ratio of 0.45 or less, and it can be seen that the conditions for a yield ratio of 0.45 or less are obtained in the region above the solid line in the figure. In other words, the solid line is
It gives the appropriate lower limit of the annealing temperature, T〓0.19
(CR) ^3/2 +770. It should be noted that annealing at a higher temperature than necessary causes coarsening of crystal grains, resulting in a decrease in yield ratio or deterioration of ductility. In addition, thermal economy, equipment deterioration, and loss also increase, so the upper limit annealing temperature is not particularly specified in the present invention, but is preferably lower limit +70°C or lower. Specific embodiments of the present invention will be described below. Example 1 The composition of the test steel used by the present inventors is shown in Table 1 below.

[Table] These melted steels A to E were hot rolled to 2.3 mm, cold rolled to 1.0 mmt (cold rolling ratio 56.5%), and then briefly annealed at 900°C for 3 minutes. That is, the tensile test values and magnetic permeability of the steel plate obtained in this way are as shown in Table 2 below, and A1,
Both B1 and C1 materials satisfy the chemical composition of steel materials that are subject to the manufacturing method of the present invention, whereas D1 material has a C-Mn balance that is higher than the specified one.
It has a low Mn content, and is an E1 material or has a Si content above the specified value.

[Table] In other words, A1, B1, and C1 materials all have a yield ratio of 0.45 or less, a tensile strength of 70Kg/ ^mm2 or less, and some
Some of them are less than 60Kg/mm ² and can be used as thin steel sheets, and their magnetic permeability is 1.01.
Excellent as below. On the other hand, D1 material has low ductility and magnetic permeability of 1.1 or more, so it cannot be used as a non-magnetic steel. Even though the E1 material has the same tensile properties as the B1 and C1 materials, a significant oxide film is formed on the surface after annealing, and the oxide film cannot be removed sufficiently even after pickling, resulting in a high magnetic permeability. The desired low magnetic permeability has not been achieved. Example 2 Using Steel A and Steel B in Table 1, the final plate thickness was 1.5 after hot rolling under various cold rolling rates and annealing conditions.
A thin steel plate of mmt was obtained. The manufacturing conditions at that time, the tensile test values and magnetic permeability of the obtained thin steel sheets are summarized in Table 3 below.

[Table] In other words, A2, A4, B2, and B3 materials all satisfy the conditions of the present invention, so they have good tensile properties, as well as excellent magnetic permeability and surface treatment properties, and can be used in various presses as non-magnetic thin steel sheets. It has sufficient properties to be used for molding. On the other hand, it is clear that other materials are good in terms of magnetic permeability and surface treatment properties, but are not necessarily preferable in terms of tensile properties. In other words, the A3 and B4 materials are annealed at 800°C, which would not satisfy the second invention unless the annealing temperature is 822°C or higher under the above cold rolling rate. Even if a longer annealing time is adopted, the ductility remains low and is not satisfactory for severe press forming purposes. Similarly, A5 material according to the present invention
Where it should be annealed at 843℃ or above, it is annealed at 820℃ below.
However, it still does not have sufficient tensile properties for press molding. It has been confirmed that in the manufacturing method of the present invention as described above, the magnetic permeability hardly changes even if severe processing is performed. According to the present invention as explained above, a high manganese non-magnetic steel having excellent non-magnetic performance with a magnetic permeability of 1.02 or less and good surface treatment properties can be precisely obtained, and it has industrial effects. In addition to these properties, it also has a tensile strength of about 55 to 70 kg/ ^mm2 , and high ductility that enables high-level forming processing, making it possible to appropriately manufacture motor cases with complex configurations. The industrial effect is great.

[Brief explanation of the drawing]

The drawings show the technical content of the present invention, and
The figure is a chart showing the relationship between the yield ratio of the steel plate obtained and the cold rolling reduction and annealing temperature. However, in this drawing, the numbers shown in circles are the yield ratio (%)
It is. FIG. 2 is a drawing for determining the lower limit value of the Mn content.

Claims (1)

[Claims] 1 Contains C: 0.15wt% or less, Si: 0.2wt% or less, Mn: 28 to 36wt%, S: 0.015wt% or less, with the balance consisting of Fe and inevitable impurities, and 37C ( %)+Mn(%)≧32.5 After hot rolling and cold rolling, the annealing temperature (T°C) was determined according to the following formula in relation to the cold rolling rate (CR%), and T ≧0.19 (CR) ^3/2 +770℃ A method for manufacturing high manganese nonmagnetic steel sheets characterized by continuous annealing to achieve a magnetic permeability of 1.02 or less.