JPS6119687B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a high strength stainless steel plate. In recent years, from the perspective of energy conservation, efforts have been made to reduce the weight of transportation vehicles such as automobiles and trains. Particularly, in vehicles, there is a tendency for the weight of vehicles to increase due to the increase in air conditioning equipment, equipment due to mutual access, etc., and reducing the weight of the vehicle body has become an important issue. Reducing plate thickness by increasing the strength and corrosion resistance of materials is an effective means of reducing the weight of transportation vehicles. These materials are subjected to complex processing and welding during car body manufacturing, so they are required to have high strength and high corrosion resistance, as well as workability and weldability (including strength of welded parts, corrosion characteristics, etc.). . In particular, when the material is made with high strength,
Although the deterioration of workability becomes noticeable, such a phenomenon is generally an unavoidable problem. For this reason, it is necessary to use materials that place emphasis on strength and materials that place emphasis on workability depending on the application. Unstable austenitic stainless steel produces deformation-induced martensite when processed at room temperature, increasing its strength. By changing the processing rate, the amount of martensite produced can be changed, and the strength level can be easily changed. can. Furthermore, when considering the high corrosion resistance inherent in stainless steel, it can be said that it is the most suitable material to meet the above requirements. Conventionally, when manufacturing high-strength stainless steel sheets using austenitic stainless steel,
After solution heat treatment, temper rolling was performed. That is, by changing the reduction ratio during skin pass rolling, it is possible to manufacture a material having arbitrary high strength.
However, simply selecting the optimum conditions for the rolling reduction of skin pass rolling has problems such as poor workability and corrosion resistance even though desired strength can be obtained. In particular, when high strength and workability are required, the yield ratio (σ ₀ .
₂ /σ _B ) and high elongation at break (El) are required, and conventional manufacturing methods have not been able to stably produce such materials. The present invention provides a balance between strength and ductility after temper rolling,
Furthermore, considering intergranular corrosion, etc., the composition range of austenitic stainless steel is C; 0.04 to 0.07.
%, N; 0.05-0.15%, Si; 1.0% or less, Mn; 0.3
~2.0%, Ni: 6.0~8.0%, Cr: 16.0~20.0%, with the balance consisting of Fe and other elements unavoidable in steelmaking.
~500℃ at an average cooling rate of 10℃/sec to 100℃/sec, maintain the material temperature in the roll gap at 30 to 50℃, and reduce the reduction rate during skin pass rolling as shown in the table below. By selecting a value that is
0.2% proof stress is 50Kg/mm ^2nd grade, 60Kg/mm ^2nd grade, 80Kg/mm ²
This makes it possible to manufacture high-strength stainless steel sheets with superior workability.

[Table] First, the influence of general components on mechanical properties is shown, and then the reasons for limiting each component are explained.
Figure 1 shows the influence of components on strength.
1.1~0.8mm by known cold rolling and annealing from 45Kg steel ingot
The measurements were taken on a material that was made into a thick steel plate and then temper-rolled in a temper rolling mill to a thickness of 0.8 mm.
As shown in the figure, C increases yield strength by 0.2% and decreases tensile strength on the low strength side. However, on the high strength side, the lower the C, the lower the tensile strength, and the C is 0.04%
This is particularly noticeable below. N is 0.2% like C
Increase yield strength and decrease tensile strength. The effect is greater than C. Ni shows no effect on 0.2% proof stress, but significantly reduces tensile strength. Cr and Mn show the same tendency as Ni, but their effectiveness is half that of Ni. Figure 2 comprehensively summarizes the influence of components on elongation at break. The manufacturing process is the same as in Figure 1, with C0.02~0.15%, Mn1~
2%, 6-9% Ni, 15-18% Cr, and 0.02-0.2% N. 1/ as shown in the figure
2Mn+Ni−1/12(20−Cr) ² +15(C+N)(%)
As the value increases, the elongation increases. As described above, each component increases elongation, and alloying elements other than C and N significantly decrease tensile strength. As a result, the yield ratio increases. In order to obtain the best balance between strength and ductility, it is important to sufficiently control the proportions of C, N, and other alloying elements. The reasons for limiting the various components will be explained based on the above facts. While C stabilizes the austenite phase and suppresses the formation of a deformation-induced martensite phase, it also functions to solid-solution strengthen the austenite phase and the formed martensite phase, and the larger the amount, the more advantageous it is in terms of strength. When the C content is less than 0.04%, high strength cannot be obtained unless an excessive amount of martensitic phase is generated, resulting in deterioration of ductility. On the other hand, the upper limit of the amount of C was regulated based on susceptibility to intergranular corrosion. That is, FIG. 3 shows the results of a 10% oxalic acid electrolytic etch test performed on materials with different amounts of C while changing the cooling rate between 900°C and 500°C. As shown in the same figure, it can be seen that when C exceeds 0.07%, susceptibility to intergranular corrosion becomes sensitive. Therefore, the range of C was set to 0.04% to 0.07%. N has the same function as C, and a larger amount is advantageous in terms of strength, but if it exceeds 0.15%, it causes blowholes, and if it is less than 0.05%, the austenite phase becomes too unstable. For this reason, the range of N is
It was set at 0.05-0.15%. Si is necessary for steelmaking as a deoxidizing agent, but 1.0%
If it exceeds 1.0%, the ferrite forming ability becomes strong and hot workability deteriorates, so the upper limit was set at 1.0%. Mn is an element that stabilizes the austenite phase, and when it is less than 0.3%, the austenite stability becomes extremely unstable. Moreover, if it exceeds 2.0%, it will adversely affect the surface properties of the steel sheet due to scaling.
Therefore, the Mn content was set in the range of 0.3 to 2.0%. Ni is an element that stabilizes the austenite phase, and its effect is twice that of Mn. If it exceeds 8.0%, the austenite phase becomes too stable, increasing the yield ratio and deteriorating workability. If it is less than 6.0%, the austenite phase becomes too unstable and ductility deteriorates. Therefore, the range of Ni was set to 6.0 to 8.0%. Cr has a great effect on corrosion resistance and ferrite formation. If it is less than 16.0%, corrosion resistance will deteriorate;
If it exceeds 20%, the amount of ferrite increases and hot workability deteriorates. For this reason, the range of Cr was limited to 16.0% to 20.0%. Note that Cr is said to be an element that suppresses the formation of deformation-induced martensite20.
% or less, the effect is small. Austenitic stainless steel with these components is solution heat treated and heated at 10℃/s between 900℃ and 500℃.
Cool at an average cooling rate of ec or more and 100°C/sec or less. When cooling from the solution heat treatment temperature, 900℃~
If the average cooling rate during 500℃ is slower than 10℃/sec, the product becomes sensitive to intergranular corrosion. Moreover, cooling at an average cooling rate exceeding 100° C./sec during this period is not practically useful. Next, the reason for limiting the rolling reduction during temper rolling will be described. Figure 4 shows 0.06C−0.6Si−1.3Mn−7.3Ni
-17.3Cr-0.08N steel was subjected to temper rolling using a test rolling mill, and the relationship between 0.2% proof stress and rolling reduction was investigated. As shown in Figure 4, as the rolling reduction rate increases, the 0.2% yield strength increases, and as the rolling reduction rate increases to approximately 5, 10, and 20
%, the 0.2% yield strength becomes 50, 60, 80Kg/Hz. In the composition range of the present invention, even if Si, Mn, Ni, and Cr are changed, the rolling reduction ratio to obtain the target strength hardly changes. However, when C and N increase, 0.2
The % yield strength increases and the reduction rate to obtain the target strength decreases. This tendency becomes more noticeable on the high strength side. For this reason, the range of rolling reduction during temper rolling is 50Kg/mm and 3 for grade ² .
-7%, 7-13% for 60Kg/mm ^2nd grade, and 18-26% for 80Kg/mm ^2nd grade. In addition, the temperature at the roll gap was limited to 30 to 50°C because if the temperature is lower than 30°C, the fluctuation of the 0.2% yield strength in response to changes in rolling reduction will increase, which will increase the variation in material quality after temper rolling. It is. Moreover, when the temperature exceeds 50°C, the formation of deformation-induced martensite decreases, making it difficult to obtain high strength. The present invention will be explained below with reference to Examples. Table 1
These are the results of manufacturing tests conducted using actual equipment. Steels with higher Ni and Cr content than those of the present invention have lower elongation and higher yield ratio. For this reason, workability is poor. In addition, materials with high C and low N have elongation and yield ratio that are almost the same as the steel of the present invention (steel produced according to the present invention), but when the cooling rate after solution heat treatment is 15°C/sec, the material has a mixed structure. The corrosion resistance is inferior to the components of the present invention. In addition, in thick materials where the cooling rate is slow, intergranular corrosion of the weld is unavoidable, resulting in deterioration of the corrosion resistance and strength characteristics of the weld. As can be seen in the examples above, the steel of the present invention has excellent workability and corrosion resistance, and in particular, steel of the 60 kg/mm ² class or lower class has excellent workability and can be processed into complex shapes. Also, there is no problem with simple processing even with 80Kg/mm ^2nd class steel. By using materials with such high strength, high workability, and high corrosion resistance, it is possible to easily reduce the weight of transportation systems such as vehicles, and this has many advantages when considering the arrival of an energy-saving era.

【table】

【table】 [Brief explanation of the drawing]

Figure 1 is a diagram showing the influence of components on strength, Figure 2 is a diagram showing the influence of components on elongation at break,
Figure 3 shows materials with a basic composition of 17Cr-7Ni and varying C.
After cooling by varying the cooling rate between ℃10%
A figure showing the results of an oxalic acid electrolytic etch test (○ in the figure means a step-like structure, ● means a mixed structure or a groove-like structure), Figure 4 is 0.06C−0.6Si−1.3Mn−
7.3Ni-17.3Cr-0.08N steel is subjected to temper rolling in a laboratory and is a diagram showing the relationship between 0.2% yield strength and rolling reduction during temper rolling.

Claims (1)

[Claims] 1. C: 0.04-0.07%, N: 0.05-0.05% by weight
0.15%, Si; 1.0 or less, Mn; 0.3-2.0%, Ni; 6.0
~8.0%, Cr; Contains 16.0~20.0%, the balance is Fe
and austenitic stainless steel, which consists of elements unavoidable in steel manufacturing, are solution heat treated at 900 to 500℃.
The material temperature in the roll gap is maintained at 30 to 50°C, and cold conditioning is performed under the following conditions depending on the strength required for the product. A method for manufacturing a high-strength stainless steel sheet, characterized by performing quality rolling. 【table】