JPS6054374B2 - Method for manufacturing austenitic steel plates and steel strips - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing austenitic steel plates and steel strips.

Ferritic steel is used as a low-temperature steel from room temperature to about -100° C. and has been put into practical use.

However, because it is ferromagnetic, it is not suitable as a structural member that is particularly sensitive to magnetism. On the other hand, SUS304 (18% Cr-8
Austenitic stainless steel, represented by Ni steel (%Ni steel), has excellent low-temperature toughness and is non-magnetic, but is expensive. Against this background, research has been carried out on inexpensive and non-magnetic austenitic steel, and a steel in which expensive Ni has been replaced with Mn has been developed. (For example, Japanese Patent Publication No. 49-10892,
(See Japanese Patent Publication No. 55-51423) However, all of these relate to hot rolled steel materials. Recently, the fusion M
In basic and applied research in future energy development fields such as HD power generation, giant superconducting magnets are needed for proton and electron acceleration and plasma confinement.

The need for non-magnetic steel for these structural members is increasing. In particular, from the viewpoint of magnetic loss and low-temperature toughness, a laminated structure in which thin plates of 3 wm or less are stacked is also attracting attention. When such plates are produced by hot rolling, not only does the load on the rolling mill become extremely high, but the flatness of the plates is poor, which tends to cause problems in consistency when the plates are stacked one on top of the other. The inventors of the present invention conducted research aimed at compensating for the drawbacks of the conventional method as well as obtaining a material that is inexpensive and has even higher strength. As a result, the inventors have developed an inexpensive and stable non-magnetic steel by replacing Ni with Mn. It has also been found that by adding 20% or more cold rolling after hot rolling, a material with excellent shape and high strength can be obtained by refining the crystal grains after final annealing.

That is, the gist of the present invention is as follows.

(1) C: 0.70% or less, Si: 2.5% or less, Mn
: 9 to 35%, Cr: 0.5 to 19%, Ni: 8% or less, N: 0.50% or less, A]: 2.0% or less, Ca: 0
．． A cold-rolled austenite characterized by hot-rolling and cold-rolling a steel billet containing 20% or less and the remainder consisting of iron and unavoidable impurities at 800 to 1150°C. Method of manufacturing steel plates and steel strips.

(2) C: 0.70% or less, Si: 2.5% or less, Mn
: 9 to 35%, Cr: 0.5 to 19%, Ni: 8% or less, N: 0.50% or less, A1: 2.0% or less, Ca: 0
．． In addition to 02% or less, MO, W, CO, Cu, Nb,
One or more types of Tj, ■, Zr are MO, W, CO
, 0.01 to 4.0% in total for Cu, Nb, T
A steel billet containing a total of 0.01 to 1.5% of i, V, and Zr, with the balance consisting of iron and unavoidable impurities is hot rolled and then cold rolled to 800% or more. ~
A method for producing cold-rolled austenitic steel sheets and steel strips, characterized by annealing at 1150°C.

The components and manufacturing method of the steel constituting the present invention will be explained in detail below.

Although C stabilizes the austenite phase and improves strength, it tends to deteriorate weldability, particularly the toughness of the weld zone.

In order to maintain good weldability, the upper limit was set to 0.70%. S1 is necessary for steel manufacturing as a deoxidizer and improves strength.

However, excessive content tends to generate a ferrite phase.
For this reason, the upper limit was set at 2.5%. Mn is an effective element for obtaining an austenite phase, and 9% or more of Mn is required to obtain nonmagnetism in austenite.

Moreover, if it exceeds 35%, the effect is saturated, so Mn
The range was 9 to 35%. Like Mn, Cr is an element necessary to obtain a stable austenite phase, and its lower limit is 0.5%.

However, if it exceeds 19%, a ferrite phase is likely to form, so the upper limit was set at 19%. Further, Cr is an element that improves corrosion resistance, and in that respect as well, the above range is preferable. Ni stabilizes the austenite phase and improves the toughness of hot rolled steel.

However, the upper limit of Ni to obtain austenite by combining Mn and Cr in appropriate amounts in the present invention is 8%. More than this amount of Ni is not only excessive for austenitization, but also causes a significant increase in cost and is not effective. Therefore, the upper limit of Ni was set at 8%. Like C, N stabilizes the austenite phase and improves strength. However, when it exceeds 0.50%, the toughness of the weld heat affected zone deteriorates. Therefore, the upper limit of N was set to 0.50%. A
Like Si, 1 is a strong deoxidizing element, and also has the effect of precipitating as a nitride to prevent coarsening of crystal grains, thereby improving strength.

However, if it exceeds 2.0%, the effect is saturated. Therefore, the upper limit of Al7 was set to 2.0%. Ca is an effective element for improving hot workability.

However, excessive content reduces cleanliness, so the upper limit should be set at 0.
02%. The present invention provides a steel billet having the above-mentioned composition, and further contains MO, MO,
One or two of W, CO, Cu, Nb, Ti, V, Zr
Use a steel billet containing more than 100% of

MO, w, CO, Cu, Nb, Ti, V, and Zr not only improve the strength of the austenite base, but also precipitate as carbon and nitrides, and have the effect of suppressing coarsening of crystal grains.

Therefore, within the range that does not cause deterioration of hot workability and toughness,
The total amount of MO, W, CO, and Cu is 0.01 to 4.
0%, and the total amount of Nb, Ti, V, and Zr is 0.0
1 to 1.5%. In addition to the above, unavoidable impurities such as P and S are preferably each contained at 0.08% or less. Cold rolled steel sheets and steel strips according to the present invention are manufactured as follows. That is, molten steel is produced using a melting furnace such as a converter or an electric furnace and, if necessary, a vacuum degassing device, and then subjected to ingot making, blooming, or continuous casting to form steel billets. Subsequently, this steel piece is hot rolled to form a hot rolled steel strip. The finishing temperature at this time is preferably 500'C or higher. Further, cooling after hot rolling may be performed by any cooling method such as air cooling or water cooling. Thereafter, this hot rolled steel strip is softened and annealed as required, and then cold rolled once or twice. In addition, in the case of one-time cold rolling, the rolling reduction ratio is set to 20% or more, and in the case of two-time cold rolling, the rolling reduction ratio of the second cold rolling is set to 20% or more. Furthermore, this cold rolled steel strip is 800 to 115
The steel is annealed in step (3) to produce a cold rolled steel strip or steel sheet product. Water cooling is preferable for cooling after annealing, but air cooling may also be used. The manufacturing method using cold rolling, which is a feature of the present invention, is different from the conventional hot rolling method in that the cold rolling makes it possible to control the austenite crystal grains after subsequent annealing.

The strength of austenite depends on the crystal grain size, and the finer the crystal grains, the higher the strength. That is, cold rolling increases strain accumulation and generates many recrystallization nuclei during annealing. Further, due to dislocations and stacking faults introduced by cold rolling, fine carbon and nitrides precipitate within the grains during temperature rise or holding during annealing. As mentioned above,
By performing cold rolling, many recrystallization nuclei are generated during annealing of the cold rolled steel strip, and the growth of crystal grains is suppressed by fine precipitates, resulting in extremely fine crystal grains and high strength. It is possible to

In this way, the present invention uses completely austenite, adds atomizing elements such as Nb and Ti as necessary, and combines this with a cold rolling process to increase the strength of the material. It is something. Figure 1 shows 25%M
The mechanical properties of n-5%Cr-1%Ni-0.04%N steel were hot-rolled, then cold-rolled at various rolling reductions, further annealed at 1000°C for 1 minute, and then water-cooled. It is something.

Cold-rolled products have finer grains and higher strength than non-cold-rolled products. Further, even if cold rolling is performed, if the rolling reduction is less than 20%, sufficient strain will be accumulated, less recrystallized nuclei will be generated during annealing, and carbon and nitrides will not be finely precipitated. Therefore, the crystal grains tend to become coarse and the strength is low. Therefore, it is necessary that the reduction ratio in cold rolling immediately before annealing is 20% or more. In particular, fine crystal grains can be stably obtained at a rolling reduction ratio of 40% or more. Figure 2 shows 25%Mn-5%C
This figure shows the mechanical properties of r-1%Ni-0.04%N steel when it was cold rolled, annealed at various temperatures for 1 minute and 5 minutes, and then cooled with water. When annealed at less than 800° C., the material is not recrystallized, has low elongation, and is inferior in workability required as a thin plate.

Moreover, if it exceeds 115(1)C, the crystal grains will become coarse and the strength required as a structural member will decrease. This tendency does not differ clearly depending on the annealing time when annealing is performed for a short time of 5 minutes or less.
Further, long-time annealing exceeding 5 minutes is unsuitable in terms of productivity and economy. As mentioned above, the appropriate annealing temperature is 800 to 1150°C. Next, examples of the present invention will be described.

Table 1 shows the relationship between the composition, cold working rate, annealing temperature after cold rolling, and material properties of steel melted in an electric furnace.

However, the annealing temperature is 1 minute. According to the method of the present invention, the 0.2% proof stress is 25k9f/i or more and the elongation is 50%.
In addition to exhibiting the above properties, it also exhibits good hot workability and a good shape.

The magnetic permeability also shows an excellent value for use in non-magnetic structures. On the other hand, Comparative Methods 1 and 2 do not contain Ca and exhibit almost the same mechanical properties as the method of the present invention, but are inferior in hot workability.

In Comparative Methods 3 and 4, the annealing temperature after cold rolling is below the lower limit of the method of the present invention, and the elongation is lower than that of the method of the present invention.
Poor secondary workability.

Comparative method 5 has an annealing temperature exceeding the upper limit of the range of the method of the present invention, and both 0.2% proof stress and tensile strength are lower than those obtained by the method of the present invention, making it unsuitable for use as a structural member.

Comparative method 6 omits cold rolling, and comparative method 7 has a low cold working rate, so both have poor shapes and low strength, making them unsuitable as structural members.

[Brief explanation of the drawing]

Figure 1 shows 0.02%C-0.86%Si-25.6%M
A slab of n-0.97%Ni-4.67%Cr-0.04%N steel was heated to 1200°C, soaked, hot rolled, and
After cold rolling with a reduction of ~80%, 1 at 1000'C
A diagram showing the relationship between cold rolling reduction and mechanical properties of steel material that has been water-cooled after being annealed for minutes. Figure 2 shows the relationship between hot rolled steel material manufactured in the same manner as in Figure 1 and cold rolled at a reduction ratio of 70%. After that, 7
It is a figure showing the relationship between the annealing temperature and the tensile properties of steel materials annealed for 1 minute (O mark) and 5 minutes (● mark) at each temperature of 00 to 1250 °C.

Claims (1)

[Claims] 1 C: 0.70% or less, Si: 2.5% or less, Mn:
9 to 35%, Cr: 0.5 to 19%, Ni: 8% or less,
N: 0.50% or less, Al: 2.0% or less, Ca: 0.
02% or less, with the balance consisting of iron and unavoidable impurities, which is hot rolled and cold rolled to 20% or more, and then annealed at 800 to 1150°C. Method for manufacturing rolled austenitic steel plates and steel strips. 2 C: 0.70% or less, Si: 2.5% or less, Mn:
9 to 35%, Cr: 0.5 to 19%, Ni: 8% or less,
N: 0.50% or less, Al: 2.0% or less, Ca: 0.
In addition to 0.2% or less, Mo, W, Co, Cu, Nb, T
One or more of i, V, and Zr are combined with Mo, W, Co,
The total amount of Cu is 0.01-4.0%, Nb, Ti
, V, and Zr in a total amount of 0.01 to 1.5%, and the balance is iron and unavoidable impurities. After hot rolling and cold rolling of 20% or more,
A method for producing cold rolled austenitic steel sheets and steel strips, characterized by annealing at ~1150°C.