JPH08104920A - Production of high-strength austenitic stainless steel sheet - Google Patents

Abstract

PURPOSE: To provide a producing method in which rolling efficiency is high and the production cost is inexpensive in the method for producing a high- strength austenitic stainless steel sheet for eliminating final annealing. CONSTITUTION: An austenitic stainless steel contg., by weight, <=0.040% C, <=1% Si, <=2% Mn, 16 to 20% Cr, 7 to 15% Ni, <=0.25% N and 0.003 to 0.020% Ti is heated to the temp. range of 1150 to 1300 deg.C and is thereafter subjected to primary hot rolling at the temp. range above 950 deg.C, and the rolling is once interrupted. The steel is water-cooled so as to regulate the temp. to the range of 900 to 700 deg.C and is subjected to secondary hot rolling in the temp. range of 900 to 700 deg.C at 5 to 30% cumulative draft, and the rolling is finished. After that, it is air cooled to produce the high strength austenitic stainless steel sheet excellent in rolling efficiency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high strength austenitic stainless steel sheet which is inexpensive to produce.

[0002]

2. Description of the Related Art SUS304 is the main type of stainless steel thick plate, which exceeds 80% of the actual usage. Austenitic stainless steel is subjected to final annealing in a temperature range of 1000 ° C. to 1150 ° C., which is referred to as solution treatment, for ordinary products for the purpose of homogenizing the structure and improving corrosion resistance due to solid solution of carbide. In recent years, various methods have been proposed for manufacturing thick plate products while cooling after rolling with the aim of omitting the final annealing.

In these methods, only the final annealing disclosed in Japanese Examined Patent Publication No. 6-15692 is omitted, and the strength enhancement and the final annealing disclosed in Japanese Examined Patent Publication No. 5-75809 are omitted. It is divided into

The present invention relates to the method of Strengthening means the yield strength required for structural materials (0.2%
Proof strength). Austenitic stainless steel has a low yield strength, which limits its application as a structural material.

It is known that the yield strength of austenitic stainless steel can be improved by refining the crystal grains and introducing dislocations. For that purpose, in the low recrystallization temperature range and the non-recrystallization temperature range. Must be rolled.

The method described in the above Japanese Patent Publication No. 5-75809 is a method in which primary rolling is performed in a high temperature range and then secondary rolling is performed in a low temperature range to increase the strength. There is a problem that cracking occurs when the reduction rate of the primary reduction in the high temperature range is increased. Even if the cracks are fine, the surface defects become larger due to the deterioration of hot workability associated with the secondary low temperature rolling. In addition, the rolling efficiency is not good because the cooling time is required between the interruption of the primary rolling and the secondary rolling.

The cooling after the secondary rolling is generally forced cooling in order to prevent sensitization due to precipitation of Cr carbide. However, water cooling deteriorates the flatness of the steel sheet, and the steel sheet is run in a cooling facility. In some cases, it may be impossible, and a straightening step may be required, which causes a reduction in rolling efficiency.

Conventionally, austenitic stainless steel C
It is known that the rate of carbide precipitation decreases by decreasing the amount, and there are conditions under which sensitization does not occur even in the as-rolled state. However, there is little knowledge about the corrosion resistance of the welded part of the air-cooled steel plate, and since forced cooling can more completely prevent sensitization, corrosion resistance can be secured, so the method of cooling is actually adopted. Didn't.

As described above, in the method of producing an as-rolled stainless steel sheet by the double rolling method, the rolling cost and the yield loss are high, and the cost improvement commensurate with the omission of the final annealing cannot be realized. Had. However, there are no documents that disclose important cost factors such as rolling efficiency and hot work flaws.

[0010]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above problems, and in a method for producing a high-strength austenitic stainless steel sheet for omitting final annealing, the production method has a high rolling efficiency and a low production cost. It provides a method.

[0011]

As a method for producing an austenitic stainless steel sheet having high strength as rolled, the present inventors have focused on a double rolling method in which high strength is obtained by low temperature rolling, and in order to improve the cost. The following findings were obtained as a result of diligent research on the influence of the composition of the steel material and the rolling conditions on the omission of the maintenance process and the enhancement of strength by preventing the occurrence of hot work flaws.

A) In order to prevent hot work flaws, a small amount of Ti is added and primary hot rolling is performed at a temperature as high as possible so that surface flaws can be effectively prevented even when heavy working is performed.

B) If the end temperature of the primary hot rolling is 950 ° C. or higher, recrystallization is completed within about 5 seconds and a fine-grained microstructure is obtained. Being able to achieve high strength without forming large expanded grains.

C) A predetermined strength can be obtained by carrying out the secondary rolling at a sufficiently low temperature and a minimum required rolling ratio of 5 to 30% for increasing the strength.

E) Immediately lowering the temperature of the steel material by performing water cooling after high-temperature primary rolling also greatly improves rolling efficiency, and this water cooling does not adversely affect corrosion resistance and mechanical properties.

F) C content is controlled to 0.04% or less, preferably 0.025% or less to prevent sensitization even if it is allowed to cool instead of water cooling after the secondary hot rolling. Therefore, when cooling with water, it is possible to omit the flatness correction step that was essential before water cooling.

The present invention has been completed based on the above findings, and the gist thereof is "% by weight, C: 0.040%.
Hereinafter, Si: 1% or less, Mn: 2% or less, Cr: 16 to
20%, Ni: 7 to 15%, N: 0.25% or less, T
i: Austenitic stainless steel containing 0.003 to 0.020% is heated to a temperature range of 1150 to 1300 ° C., then primary hot rolling is performed in a temperature range of up to 950 ° C. to temporarily stop the rolling, and Water-cool it so that the temperature is within the range of 900-700 ℃, 900-700 ℃
Method for producing high strength austenitic stainless steel sheet with excellent rolling efficiency, which is characterized by performing secondary hot rolling with a cumulative reduction of 5 to 30% in the temperature range of 1 to finish rolling, and then allowing to cool. is there.

[0018]

Next, the reasons and functions for limiting the content of each component element and the manufacturing conditions will be described.

C: C must be reduced to prevent it from becoming a Cr carbide and deteriorating the corrosion resistance when the steel sheet is allowed to cool after secondary hot rolling, and is made 0.04% or less.

Cr: Cr is the main element responsible for the corrosion resistance of stainless steel, and 16% or more is necessary to obtain sufficient corrosion resistance. If it exceeds 20%, the hot workability deteriorates, so the upper limit is 20%. And

Si, Mn: Si and Mn are basic elements involved in deoxidation of steel and need to be added. However, excessive addition may impair workability. Therefore, they are limited to 1% or less and 2% or less, respectively. .

Ni: Ni is an essential element for forming an austenite structure and for ensuring acid resistance. For that purpose, 7% or more is necessary, and if it exceeds 15%, the cost becomes high. And

N: N is added to improve the strength and corrosion resistance of austenitic stainless steel. The addition of 0.25% or more deteriorates the hot workability and raises the manufacturing cost, so the upper limit was made 0.25% or less.

Ti: Continuously cast slabs have oscillation marks formed on the surface during casting, but usually the surface is polished and removed to remove it. However, a coarse solidified structure is formed below the oscillation mark, which often causes surface defects on the thick plate. Subcutaneous 1 mm
Therefore, coarse particles having a crystal grain size of 2 to 3 mm may be present.

From the viewpoint of hot workability, it is not preferable to strongly reduce such a slab having a low hot deformability at the initial stage of rolling. In the present invention, since the rolling reduction per pass is increased in the primary rolling in the high temperature region in order to increase the rolling efficiency, it is necessary to sufficiently enhance the hot workability of the slab in the cast state.

Therefore, Ti is precipitated as an oxide or nitride of Ti in the solidification process to refine the solidification structure, enhance the hot workability of the slab and prevent the generation of surface defects during rolling, and It is added to improve strength and toughness.
If it is less than 0.003%, the effect of preventing surface defects is insufficient, and if it exceeds 0.02%, the oxides and nitrides of Ti become coarse and the grain refining effect is reduced.
Was made 0.02%.

In addition, the contents of the elements generally contained in the austenitic stainless steel to which the present invention is applied will be described below.

P: P is an element harmful to corrosion resistance, and
It is desirable to limit it to 04% or less.

S: S is an element that reduces hot workability and toughness, and is preferably reduced to 0.020% or less.

Cu: Cu may be added in order to improve general corrosion resistance. Excessive addition lowers the hot workability, so the content is preferably 1.0% or less.

Mo: Mo is an element to be added in order to improve the corrosion resistance of stainless steel. However, it is preferable to add Mo in an amount of 4% or less because adding a large amount will increase the cost.

Al: Al is an element having a very large deoxidizing ability of steel, and may be added together with Si and Mn for deoxidizing. For this purpose, it is necessary to add 0.003% or more. Deoxidation reduces oxide inclusions and provides high toughness. On the other hand, excessive addition causes hardening of the steel and may deteriorate workability, so it is desirable to add 0.1% or less.

V, Nb: These elements are added as necessary to increase the strength of stainless steel. For this purpose, it is necessary to add 0.01% or more. On the other hand, excessive addition of 0.5% or more reduces toughness. Therefore, it is desirable that the content of each is 0.5% or less.

In addition, 0.0 to improve hot workability.
1% or less of Ca, Mg, B and 0.1% or less of Zr,
Y, La, and Ce can be contained.

Next, the reason for defining the manufacturing conditions will be described.

Heating temperature: Since austenitic stainless steel has a high resistance to hot deformation, it is difficult to roll at a heating temperature of less than 1150 ° C. On the other hand, if heated above 1300 ° C, the oxide scale grows abnormally and the surface of the steel sheet becomes rough, so it was defined as 1150 to 1300 ° C.

Primary hot rolling / rolling interruption: In the present invention, hot workability is achieved by separately performing the main primary rolling for rolling to a target plate thickness and the secondary rolling for imparting strength. It is possible to improve and improve rolling efficiency.

The temperature at which the primary rolling ends is set to 950 ° C. or higher because the temperature of 950 ° C. or higher corresponds to the recrystallization temperature range of the austenitic stainless steel, and the strain caused by the primary rolling is released immediately after the rolling to recrystallize. is there.

The following tests were conducted to determine the end temperature of the primary reduction.

SUS304L with a carbon content of 0.015%
60mm thickness x 100 cut from a continuous cast slab of steel
60 mm width x 130 mm long steel slab is heated to 1230 ° C and then 60
→ 50 → 40 → 32 → 25mm thickness, set the rolling interruption temperature to T1, start cooling with spray water within 5 seconds, cool to 850 ° C, then add 12% hot rolling at 800 ° C to add 22mm After thickening, it was air-cooled. The rolling interruption temperature was adjusted from 1020 ° C to 880 ° C by adjusting the pitch to which the rolling pass was applied. The microstructure of the cross section of the obtained steel sheet in the direction perpendicular to the rolling direction was subjected to oxalic acid electrolytic corrosion to determine the grain size and the degree of sensitization. Although no sensitization was observed in any of them, a mixed grain structure was formed at a rolling interruption temperature of 920 to 960 ° C.

FIG. 1 shows the results of the above test and shows the primary rolling end temperature and the degree of grain refinement.

It can be seen from the figure that 90% or more of the particles are finely divided when the primary rolling end temperature is 950 ° C. or more.

It was also confirmed that there was no precipitation of carbides even in the welded part using the steel sheet thus produced as a base material, and that it had the same level of corrosion resistance as in the solution state.

Performing primary rolling at 950 ° C. or higher,
In addition to the improvement of hot workability due to addition of Ti and Ti, it is possible to prevent the occurrence of surface flaws on the steel sheet even in the rolling of strong working. Further, since it is a high temperature region rolling, the hot deformation resistance is small, so that the rolling speed can be made high and the rolling efficiency is improved.

Water cooling: Recrystallization is completed instantly in order to finish the main primary hot rolling at a high temperature. For this reason, it is possible to obtain a uniform recrystallized structure even if cooling is performed immediately after the interruption of rolling. Further, by cooling with water, the waiting time during the rolling interruption can be significantly shortened, and the rolling efficiency can be improved.

Secondary rolling: Secondary rolling is performed in a temperature range of 900 to 700 ° C. to obtain a strength of 350 MPa or more. 90
The temperature of 0 ° C. or lower is in the non-recrystallization temperature range of austenitic stainless steel, and the strain due to rolling can be effectively utilized for increasing the strength. On the other hand, the reason why the temperature is 700 ° C. or higher is that if the temperature is lower than 700 ° C., hot deformation resistance becomes high and rolling becomes difficult.

If the rolling reduction of the secondary rolling is less than 5%, the strength cannot be expected to increase. On the other hand, if it exceeds 30%, the strength is excessively increased, leading to a decrease in toughness and the occurrence of surface defects. It was set to 30%. In normal thick plate rolling, rolling of 2 to 3 passes may be sufficient.

Cooling after secondary rolling: In the conventional water cooling process,
Flatness correction before water cooling was necessary, which significantly reduced the rolling efficiency. That is, when a steel plate having poor flatness is water-cooled, the flatness may be deteriorated rapidly during water cooling, which makes it difficult to pass the plate through the water-cooling device. Therefore, the straightening step before water-cooling is essential. On the other hand, by cooling after rolling, deterioration of flatness can be prevented, a straightening step is not required, and rolling efficiency is significantly improved.

[0049]

Example A 200 mm thick continuously cast slab having the composition shown in Table 1 was melted and hot rolled under the conditions shown in Table 2 to manufacture a thick steel plate. Table 2 shows rolling results and evaluation results.

[0050]

[Table 1]

[0051]

[Table 2]

In the table, the "water cooling" column shows the presence or absence of water cooling after the rolling interruption, and the "cooling" column shows the cooling method of the steel sheet after the rolling. "Water-cooled" with "-" is a method of finishing at a high temperature without interruption of rolling, and those with "-" in the primary reduction end temperature and reduction ratio column are at low temperature while waiting for each pass without interruption of rolling. The finished method is shown.

The unit weight of the slab is 6 to 10 tons, and the rolling efficiency (ton / ton /
hr) was calculated.

The surface defects were judged by reworking the surface defects generated after rolling with a grinder, and ◯,
It was evaluated as Δ or ×. ○ indicates that the area ratio of rework is 1%
Less than, Δ is 1 to 3%, × is all-surface care, XX is all-surface care, and ear cracking has occurred. 1 after rolling
Tensile test pieces were sampled in the direction perpendicular to the rolling direction from the / 4 width part, and
The 2% proof stress was measured. In addition, the corrosion resistance of the micro-sample of the plate thickness section of this portion was evaluated by the oxalic acid electrolytic corrosion test specified in JIS G0571. The case of the step structure was judged as ◯, the dual structure and the ditch structure were judged as ×.

Further, V-grooves were applied to the obtained steel sheets 1 to 7 to perform a covered arc multilayer welding (the welding rod is Y308L).
Or Y316L, interlayer temperature is 150 ° C or less), after removing the bead on the surface, a test piece with a width of 30 mm, a length of 50 mm and a thickness of 5 mm is taken from the surface layer so as to include the molten metal and the HAZ part, and is specified in JIS 0573. Boiled 65%
As a result of performing a nitric acid test (Huiy test), the average values of corrosion weight loss for 5 cycles were all good values of 0.3 g / m ² h or less.

In Table 2, the test number 8 of the comparative example is primary rolling 10
After finishing at 50 ° C., the material was allowed to cool to the secondary rolling, so about 2 minutes was required as a waiting time, so the rolling efficiency was lowered. Further, in Test No. 9, the flatness was corrected by a hot leveler and spray cooling was carried out before the final cooling, so that the rolling efficiency was lowered because the transfer time to the leveler and the correction time were required.

Test Nos. 11, 12, and 13 are results obtained by using F steel. F steel is SUS304 steel, and the amounts of C and Ti are out of the range specified by the present invention. Test Nos. 11 and 13, which were water-cooled after the final rolling, were excellent in corrosion resistance due to the suppression of carbide precipitation, but had a low Ti content, so under high temperature strong rolling +
Due to the low-temperature finish rolling, surface defects frequently occurred, and it took time to maintain the defects and the rolling efficiency decreased. On the other hand, test number 12 which was left to cool had poor corrosion resistance.

Test No. 16 has been subjected to solution annealing, so that it has good corrosion resistance but low strength.

14 and 15 show the results using G steel,
Since the Ti content is low and N and Mo are contained in the steel, which is a type of steel that is likely to have flaws, surface flaws frequently occur. On the other hand, No. 5 is an example of the present invention using C steel, which contained N and Mo but had good surface properties due to the effect of adding Ti.

[0060]

As described above, according to the manufacturing method of the present invention, it is possible to manufacture a high-strength austenitic stainless steel sheet with less surface defects during hot rolling, excellent rolling efficiency, good flatness, and excellent corrosion resistance. , Brings a useful effect on the industry.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a finish temperature of primary hot rolling and a degree of grain refining.

Claims (1)

[Claims] 1. C: 0.040% or less, Si:
1% or less, Mn: 2% or less, Cr: 16 to 20%, N
i: 7 to 15%, N: 0.25% or less, Ti: 0.00
After heating austenitic stainless steel containing 3 to 0.020% to a temperature range of 1150 to 1300 ° C., 950
Primary hot rolling is performed in a temperature range up to ℃, the rolling is temporarily stopped, water cooling is performed so that the temperature of the steel material falls within a range of 900 to 700 ° C.
A method for producing a high-strength austenitic stainless steel sheet with excellent rolling efficiency, characterized by performing secondary hot rolling with a cumulative reduction of -30%, finishing rolling, and then allowing to cool.