JP2012180567A - Clad steel sheet having duplex stainless steel as mating material, and method for production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a clad steel sheet having both toughness of a base material and corrosion resistance of a mating material of alloy element-saving type duplex stainless steel, and to provide a method for producing an inexpensive clad steel sheet which is performed with less energy to be used by eliminating a solution heat treatment and is environmentally excellent.SOLUTION: In the clad steel sheet having a duplex stainless steel as a mating material, the duplex stainless steel contains, by mass, ≤0.03% C, 0.05-1.0% Si, 0.5-7.0% Mn, ≤0.05% P, ≤0.010% S, 0.1-5.0% Ni, 18.0-25.0% Cr, 0.05-0.30% N, 0.001-0.05% Al, and the balance Fe with inevitable impurities. The chromium nitride deposition temperature (TN) which is an index regarding the deposition of chromium nitride during hot rolling is 800-970°C.

Description

  The present invention relates to an inexpensive alloy element-saving duplex stainless steel clad steel sheet that does not require solution heat treatment and a method for producing the same, and a duplex stainless steel that can be used as seawater desalination equipment, tanks for transport ships, various containers, etc. The present invention relates to a clad steel plate using a laminated material.

Duplex stainless steel contains a large amount of Cr, Mo, Ni, and N, and since intermetallic compounds and nitrides are likely to precipitate, a solution heat treatment at 1000 ° C. or higher is applied to solidify the precipitates, and hot rolling It was manufactured as a steel material.
For this reason, in the production of clad steel sheets made of duplex stainless steel, the base material is carbon steel with a devised chemical composition so that mechanical properties can be secured by heat treatment at a high temperature of 1000 ° C. or higher. (Patent Document 1, etc.), or by controlling the hot rolling conditions, heat treatment is omitted to produce a duplex stainless steel clad steel sheet (Patent Document 2, etc.), or reheated during hot rolling to make a laminated material Measures such as suppressing precipitation inside (Patent Document 3 etc.) have been taken.

Recently, alloy element-saving duplex stainless steels with reduced Ni, Mo, etc. have been developed, and practical steels with greatly reduced precipitation sensitivity of intermetallic compounds have been used.
For these alloying element-saving duplex stainless steel materials, the chromium nitride is the predominant precipitate that affects the material.
Chromium nitride is a precipitate in which Cr and N are combined. In duplex stainless steel, cubic CrN or hexagonal Cr ₂ N often precipitates in ferrite grains or ferrite grain boundaries. When these chromium nitrides are produced, the impact characteristics are deteriorated, and the corrosion resistance is lowered due to the chromium-deficient layer produced as a result of precipitation.

  The present inventors have clarified the relationship between chromium nitride precipitation and component composition, and the material design based on the idea of suppressing the chromium nitride precipitation by controlling the component composition has good corrosion resistance and impact characteristics. Invented and disclosed a new alloy element-saving duplex stainless steel grade (Patent Document 4). In particular, the technique of suppressing the precipitation of chromium nitride by increasing the Mn content is reflected in the component design of a new alloy element-saving duplex stainless steel. Such alloy element-saving duplex stainless steel is already being used in various fields because of its low cost and excellent properties such as corrosion resistance.

The inventors of the present invention have paid attention to the application of the alloy element-saving duplex stainless steel to the clad steel sheet laminated material and conducted research and development. A clad steel plate is a hot-rolled steel material capable of economically obtaining composite characteristics by providing corrosion resistance to stainless steel used as a laminated material and strength / toughness and weldability to a base material.
A clad steel plate is used in a part where a stainless steel as a laminated material and a base material are structurally joined, and is generally used for applications where a plate thickness is thick and particularly strength and toughness are required. For example, seawater desalination equipment, tanks for transport ships, and the like, many of which have conventionally used austenitic stainless steel as a bonding material.
However, the trend of changing stainless steels for these applications to cheap duplex stainless steels is progressing, and there is an increasing demand for cheaper clad steel sheets in which the laminated materials are duplex stainless steels.

  By the way, in the manufacture of conventional duplex stainless steel hot-rolled steel sheets and clad steel sheets, solution treatment is indispensable. This is because, as described above, it is necessary to eliminate intermetallic compounds and chromium nitrides that lower the corrosion resistance in the duplex stainless steel. In particular, the alloying element-saving duplex stainless steel used in the clad steel sheet of the present invention has the property that nitride is likely to precipitate in the temperature range of hot working, and hot rolling is finished. In this state, the chromium nitride is dispersed in the steel material, so that the corrosion resistance is lowered.

  By performing solution heat treatment, it is possible to eliminate chromium nitride in the laminated material. However, if the solution treatment at 1000 ° C. or higher is performed, the toughness of the base material is reduced. This is an undesirable treatment from the viewpoint of use. Moreover, from the request | requirement for the further cost reduction, and the request | requirement of the energy consumption reduction in recent years, it is desired to reduce the manufacturing cost of a clad steel plate and the energy required for manufacture by omitting the solution treatment.

JP 7-292445 A Japanese Examined Patent Publication No. 4-22677 Japanese Patent Publication No. 6-36993 WO2009-119895

  The present invention is a clad steel sheet having both the toughness of the base material and the corrosion resistance of the alloy element-saving duplex stainless steel, and the production of an inexpensive clad steel sheet that eliminates solution heat treatment and uses less energy and is excellent in terms of the environment. It aims to provide a method.

In order to solve the above problems, the inventors of the present invention, in the process of joining the base material and the laminated material by hot rolling in the production process of the clad steel plate, chromium nitride is contained in the duplex stainless steel as the laminated material. If it did not precipitate, it was thought that the corrosion resistance would not be impaired even if the solution heat treatment as a subsequent step was omitted.
Therefore, we considered to find a solution by using alloy element-saving duplex stainless steel that can maintain high corrosion resistance even when the hot rolling temperature is lowered for the clad steel sheet.
And in order to obtain such alloy element-saving duplex stainless steel, the chemical composition of the hot-rolled steel material without solution heat treatment, the state of the metal structure including the hot working conditions and the precipitation amount of chromium nitride, In addition, we considered that it was necessary to obtain knowledge about the relationship between impact characteristics and corrosion resistance of steel materials, and conducted the following experiments.

  As an index for precipitation of chromium nitride during hot rolling, a new chromium nitride precipitation temperature TN is set, and steel materials having different chromium nitride precipitation temperatures TN are used, and the heating temperature of hot rolling is set to 1150 to 1250. The hot-rolled steel material having a thickness of 10 mm to 35 mm was obtained by changing the entry side temperature TF of the final finishing rolling pass of ° C. and hot rolling and the accelerated cooling start temperature TC after completion of hot rolling. And the strength, impact property, and corrosion resistance were evaluated about the obtained hot-rolled steel material and the steel material which performed solution heat treatment.

Next, the alloy element-saving duplex stainless steel obtained in the above experiment was used as a clad laminated material. The clad steel sheet having a thickness of 3 mm and a clad steel sheet thickness of 10 to 35 mm was hot-rolled. Obtained and evaluated for strength, impact properties, and corrosion resistance.
Through the above experiments, the present invention has been completed, which clearly shows a clad steel plate in which alloy element-saving duplex stainless steel is used as a bonding material and solution heat treatment is omitted.

That is, the gist of the present invention is as follows.
(1) A clad steel plate having a duplex stainless steel as a laminated material, wherein the duplex stainless steel is
% By mass
C: 0.03% or less, Si: 0.05-1.0%,
Mn: 0.5 to 7.0%, P: 0.05% or less,
S: 0.010% or less, Ni: 0.1-5.0%,
Cr: 18.0 to 25.0%, N: 0.05 to 0.30%,
Al: 0.001 to 0.05%
And the balance consists of Fe and inevitable impurities,
A clad steel plate made of a duplex stainless steel, characterized by a chromium nitride precipitation temperature TN of 800 to 970 ° C., which serves as an index for chromium nitride precipitation during hot rolling.

(2) A clad steel plate having a duplex stainless steel as a laminated material, wherein the duplex stainless steel is
% By mass
C: 0.03% or less, Si: 0.05-1.0%,
Mn: 0.5 to 7.0%, P: 0.05% or less,
S: 0.010% or less, Ni: 0.1-5.0%,
Cr: 18.0 to 25.0%, N: 0.05 to 0.30%,
Al: 0.001 to 0.05%
Further,
V: 0.05 to 0.5%, Nb: 0.01 to 0.20%,
Ti: 0.003 to 0.05%
1 type or 2 types or more chosen from, The remainder consists of Fe and an unavoidable impurity,
A clad steel plate comprising a duplex stainless steel as a combination material, wherein a chromium nitride precipitation temperature TN2 serving as a second index for precipitation of chromium nitride during hot rolling is 800 to 970 ° C.

(3) The duplex stainless steel further comprises
Mo: 1.5% or less, Cu: 2.0% or less,
W: 1.0% or less, Co: One or two or more types selected from 2.0% or less are contained, and the duplex stainless steel according to (1) or (2) above is a laminated material Clad steel sheet.
(4) The duplex stainless steel further comprises
B: 0.0050% or less, Ca: 0.0050% or less,
One phase or two or more types selected from Mg: 0.0030% or less, REM: 0.10% or less, The two-phase according to any one of (1) to (3), A clad steel plate made of stainless steel.

(5) A method for producing a clad steel plate using the duplex stainless steel according to any one of (1) to (4) as a laminated material,
When the base material and the laminated material are joined by hot rolling, the clad steel sheet that does not contain V, Nb, Ti, which are selective components in the laminated material, contains the selective component according to the following formula (1). The clad steel sheet is hot-rolled at the entry temperature TF of the final finishing rolling pass of hot rolling according to the following equation (2), and the temperature range from the entry temperature TF of the final finishing rolling pass of hot rolling to 600 ° C. Is cooled in a time of 5 minutes or less, and a method for producing a clad steel plate using a duplex stainless steel as a laminated material.
TF ≧ TN−100 (1)
TF ≧ TN2-100 (2)

(6) The clad steel sheet not containing V, Nb, Ti, which is a selective component of the laminated material, is hot according to the following formula (3), and the clad steel sheet containing the selective component is hot according to the following formula (4). By starting the accelerated cooling from the accelerated cooling start temperature TC after the end of rolling, the temperature range from the entry side temperature TF to 600 ° C. of the final finishing rolling pass of the hot rolling is cooled in a time of 5 minutes or less. The manufacturing method of the clad steel plate which uses the duplex stainless steel as described in said (5) as a laminated material.
TC ≧ TN−250 (however, TF ≧ TC) (3)
TC ≧ TN2-250 (However, TF ≧ TC) (4)

  According to the present invention, it is possible to use a clad steel plate with reduced alloying elements as a seawater desalination device, a tank for a transport ship, various containers, etc., and a steel material that is inexpensive and uses less energy for manufacturing. The area that contributes to the environment is extremely large.

Below, the reason for limitation of Claim 1 of this invention is demonstrated first. The unit% is mass%. The clad steel sheet clad steel comprising the duplex stainless steel according to claim 1 contains C, Si, Mn, P, S, Ni, Cr, N, Al, and the balance is Fe and inevitable impurities. Become.
C limits the content to 0.03% or less in order to ensure the corrosion resistance of the stainless steel. If the content exceeds 0.03%, Cr carbide is generated during hot rolling, and the corrosion resistance and toughness deteriorate.

  Si is added at 0.05% or more for deoxidation. However, if added over 1.0%, the toughness deteriorates. Therefore, the upper limit is limited to 1.0%. A preferable range is 0.2 to 0.7%.

  Mn has the effect of increasing the austenite phase and improving the toughness, and is added in an amount of 0.5% or more for the toughness of the base material and the weld. Moreover, since it has the effect of reducing the nitride precipitation temperature TN, it is preferable to add positively in this invention steel material. However, if added over 7.0%, corrosion resistance and toughness deteriorate. Therefore, the upper limit is limited to 7.0%. The preferred content is 1.0 to 6.0%, more preferably 2.0 to 5.0%.

P is an element inevitably mixed from the raw material, and is limited to 0.05% or less in order to deteriorate hot workability and toughness. Preferably it is 0.03% or less.
S is an element inevitably mixed from the raw material, and is limited to 0.010% or less in order to deteriorate the hot workability, toughness, and corrosion resistance. Preferably it is 0.0020% or less.

  Ni is contained in an amount of 0.1% or more in order to stabilize the austenite structure and improve corrosion resistance to various acids and further toughness. By increasing the Ni content, the nitride precipitation temperature can be lowered. On the other hand, the steel according to the present invention, which is an expensive alloy and uses alloy element-saving duplex stainless steel as a combination material, is limited to a content of 5.0% or less from the viewpoint of cost. The preferred content is 1.0 to 4.0%, more preferably 1.5 to 3.0%.

  Cr is contained at 18.0% or more in order to ensure basic corrosion resistance. On the other hand, if the content exceeds 25.0%, the ferrite phase fraction increases, and the toughness and the corrosion resistance of the weld zone are impaired. Therefore, the Cr content is set to 18.0% or more and 25.0% or less. A preferable content is 19.0 to 23.0%.

  N is an effective element that improves the strength and corrosion resistance by dissolving in the austenite phase. For this reason, 0.05% or more is contained. The solid solution limit increases depending on the Cr content. However, in the steel of the present invention, if it exceeds 0.30%, Cr nitride precipitates and the toughness and corrosion resistance are impaired, so the upper limit of the content Was 0.30%. A preferable content is 0.10 to 0.25%.

  Al is an important element for deoxidation of steel, and is contained together with Si in order to reduce oxygen in the steel. When the Si content exceeds 0.3%, it may not be necessary to add, but the reduction of the oxygen content is essential for securing toughness, and for this reason, the content must be 0.001% or more. . On the other hand, Al is an element having a relatively large affinity with N, and if added excessively, AlN is generated and inhibits the toughness of stainless steel. The degree depends on the N content, but when Al exceeds 0.05%, the toughness deteriorates remarkably, so the upper limit of the content is set to 0.05%. Preferably it is 0.03% or less.

  O is an unavoidable impurity, and the upper limit thereof is not particularly defined. However, O is an important element constituting an oxide that is representative of nonmetallic inclusions, and excessive inclusion inhibits toughness. In addition, the formation of coarse clustered oxides causes surface defects. Preferably it is 0.010% or less.

The reason for limitation for the remaining items of claim 1 will be described.
The chromium nitride precipitation temperature TN, which serves as an index for chromium nitride precipitation during hot rolling, is a characteristic value that is experimentally determined. The solution heat-treated steel material is subjected to water-cooling within 5 seconds after soaking at 800 to 1000 ° C. for 20 minutes, and the amount of chromium nitride deposited on the steel material after cooling is described in detail in Examples. It is determined by the electrolytic extraction residue analysis method, and is defined as the lowest temperature among the soaking temperatures at which the Cr residue amount is 0.01% or less.
The lower the TN, the lower the temperature range in which chromium nitride precipitates, so that the precipitation rate and amount of chromium nitride are suppressed, and the heat treatment of the as-rolled solution heat treatment is omitted. Corrosion resistance is maintained.

Here, the reason why the soaking temperature is defined as 800 to 1000 ° C. is that it is a general hot rolling temperature range. In the present invention, in order to prevent chromium nitride from being precipitated during hot rolling that is generally performed, the temperature range is specified.
Further, the soaking temperature is defined as 20 minutes as the time for the chromium nitride to equilibrate sufficiently. If it is less than 20 minutes, it corresponds to an area where the amount of precipitation changes drastically and it becomes difficult to obtain measurement reproducibility. Therefore, from the viewpoint of sufficiently balancing the chromium nitride and ensuring reproducibility, the soaking temperature may be set to more than 20 minutes.
After soaking, if it takes a long time for water cooling, the steel material temperature gradually decreases and chromium nitride precipitates, which results in a value different from the amount of chromium nitride at the desired temperature. It will be. Therefore, it shall be subjected to water cooling within 5 seconds after soaking.
In addition, the minimum temperature among the temperatures at which the Cr residue amount is 0.01% or less is defined as the amount of precipitation that does not adversely affect the corrosion resistance and toughness by experiments. It depends on.

In order to secure corrosion resistance and toughness, it is experimentally required that TN should be designed to be 970 ° C. or less in order to ensure corrosion resistance and toughness for alloy element-saving duplex stainless steel that does not require solution heat treatment as hot rolled. It was. Therefore, it is necessary to design a component composition such that TN is 970 ° C. or lower. Preferably it is 930 degrees C or less.
Moreover, although TN falls by reducing N content, in order to improve corrosion resistance in this invention steel, 0.05% or more of N is contained, In this case, TN shall be less than 800 degreeC. Have difficulty. Therefore, the lower limit of TN is set to 800 ° C.

  In order to reduce TN, it is effective to reduce the amount of N. However, an extreme reduction in the amount of N results in a decrease in the austenite phase ratio and a decrease in corrosion resistance of the weld. For this reason, it is necessary to appropriately design the contents of Ni, Mn, and Cu, which are the elements forming the austenite phase, and the N content.

The clad steel plate of the present invention can be obtained by limiting the chromium nitride precipitation temperature of the duplex stainless steel, which is a laminated material, to a specific temperature or lower. Therefore, as a base material of the clad steel plate, one or more kinds selected from the group consisting of ordinary steel (carbon steel) and alloy steel excluding stainless steel can be selected and used without any particular limitation. It can be appropriately selected and used according to the intended application.
Examples of alloy steel include low alloy steel, nickel steel, manganese steel, chromium molybdenum steel, high speed steel, etc., but are not limited to these, and steel with one or more elements added to ordinary steel. I just need it.

Next, the contents defined in claim 2 will be described. The laminated material of the clad steel sheet according to claim 2 contains C, Si, Mn, P, S, Ni, Cr, N, and Al, and further includes one or more selected from V, Nb, and Ti. Contained, the balance being Fe and inevitable impurities.
The present inventors have found that when alloying element-saving duplex stainless steel containing V, Nb, Ti is used as a laminated material, the behavior is different from the conventional knowledge.
That is, it has been found that the inclusion of a small amount of V, Nb, Ti in the alloy element-saving duplex stainless steel constitutes a nitride substituted with a part of chromium and increases the chromium nitride. This means that the chromium nitride deposition temperature is slightly increased. According to general conventional knowledge, it seems that the corrosion resistance deteriorates as the chromium nitride amount increases, but unexpectedly, it has been clarified that the corrosion resistance tends to improve even if the precipitation amount of chromium nitride increases. . This finding is defined in claim 2.

As described above, when a small amount of V, Nb, and Ti is contained, the allowable amount of chromium nitride increases. Therefore, for steel materials containing the optional components V, Nb, and Ti, the chromium nitride precipitation temperature is newly defined as TN2 as a second indicator for chromium nitride precipitation during hot rolling, and Cr residue It was set as the lowest temperature among the soaking temperatures at which the amount was 0.03% or less.
Needless to say, the chromium nitride precipitation temperature TN described in claim 1 is an index relating to the precipitation of chromium nitride during hot rolling in a steel material that does not contain the selective components V, Nb, and Ti.

  According to the second aspect of the present invention, if the TN2 in which the amount of Cr nitride is relaxed is 970 ° C. or less, the problem of the present invention can be solved. Preferably it is 930 degrees C or less. The means for experimentally determining TN2, the lower limit of TN2, and the method for reducing TN2 are the same as those for TN. In TN2, the minimum temperature out of the temperatures at which the Cr residue amount is 0.03% or less is defined as the amount of precipitation in which the residue amount of 0.03% or less is not adversely affected by corrosion resistance and toughness. By confirming that there is.

  The nitride and carbide formed by V are generated during the hot working and cooling of the steel material, and have the effect of increasing the corrosion resistance. Although sufficient confirmation has not been made for this reason, there is a possibility of suppressing the generation rate of chromium nitride at 700 ° C. or lower. In order to improve this corrosion resistance, 0.05% or more is contained. If the content exceeds 0.5%, coarse V-based carbonitrides are produced and the toughness deteriorates. Therefore, the upper limit is limited to 0.5%. The preferable content when added is in the range of 0.1 to 0.3%.

  Nitrides and carbides formed by Nb are produced during the hot working and cooling of the steel material, and have the effect of increasing the corrosion resistance. Although sufficient confirmation has not been made for this reason, there is a possibility of suppressing the generation rate of chromium nitride at 700 ° C. or lower. In order to improve the corrosion resistance, the content is 0.01% or more. On the other hand, excessive addition causes precipitation as an undissolved precipitate during heating before hot rolling and impairs toughness, so the upper limit of its content was set to 0.20%. The preferable content range in the case of adding is 0.03% to 0.10%.

  Ti is an element that forms oxides, nitrides and sulfides in a very small amount, and refines the solidified steel and crystal grains of the high-temperature heating structure. Further, like V and Nb, it has a property of substituting with a part of chromium nitride chromium. A Ti precipitate is formed when the content is 0.003% or more. On the other hand, if it exceeds 0.05% and is contained in the duplex stainless steel, coarse TiN is generated and the toughness of the steel is inhibited. For this reason, the upper limit of the content was set to 0.05%. A suitable content of Ti is 0.005 to 0.020%.

In Claim 3, it specified about the element which raises the corrosion resistance of a laminated material additionally. The reason for limitation of the invention according to claim 3 that further contains one or more selected from Mo, Cu, W, and Co as selective elements will be described.
Mo is a very effective element that additionally increases the corrosion resistance of stainless steel, and can be contained as necessary. In order to improve corrosion resistance, it is preferable to contain 0.2% or more. On the other hand, it is an element that promotes precipitation of intermetallic compounds. In the steel of the present invention, the upper limit is set to a content of 1.5% from the viewpoint of suppressing precipitation during hot rolling.

  Cu is an element that additionally enhances the corrosion resistance of stainless steel to acids, and it has an effect of improving toughness, so it is recommended to contain 0.3% or more. If the content exceeds 2.0%, the solid solubility will be exceeded during hot rolling and εCu will precipitate and embrittlement will occur, so the upper limit was made 2.0%. The preferable content when Cu is contained is 0.3 to 1.5%.

  W, like Mo, is an element that additionally improves the corrosion resistance of stainless steel. For the purpose of enhancing the corrosion resistance in the steel of the present invention, 1.0% is contained at the upper limit. A preferable content is 0.05 to 0.5%.

  Co is an element effective for enhancing the toughness and corrosion resistance of steel, and is selectively added. The content is preferably 0.03% or more. If the content exceeds 2.0%, it is an expensive element, so that an effect commensurate with the cost cannot be exhibited, so the upper limit was set to 2.0%. The preferable content when added is 0.03 to 1.0%.

In claim 4, B, Ca, Mg, and REM to be selectively contained as necessary to improve hot workability are limited as follows.
B, Ca, Mg, and REM are all elements that improve the hot workability of steel, and one or more of them are added for that purpose. Since excessive addition of any of B, Ca, Mg, and REM conversely decreases hot workability and toughness, the upper limit of the content is determined as follows.
B and Ca are 0.0050%, Mg is 0.0030%, and REM is 0.10%. Preferred contents are B and Ca: 0.0005 to 0.0030%, Mg: 0.0001 to 0.0015%, and REM: 0.005 to 0.05%, respectively. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

Next, the reason for limitation according to claim 5 of the present invention will be described.
A rolled clad steel sheet in which the laminated material is an alloy element-saving duplex stainless steel is manufactured by the following process. First, the joining surfaces of the base material and the laminated material having a predetermined thickness are cleaned and overlapped, and the four circumferences are joined by welding to assemble a slab. In order to increase the bonding strength, vacuum degassing, insertion of an insert material into the bonding surface, and the like are appropriately performed. This slab is subjected to normal hot rolling to produce a clad steel plate.

The entry side temperature TF of the final finishing rolling pass of hot rolling dominates the strength of the base material, and the higher the strength, the higher the strength. Moreover, when it falls too much, the precipitation amount of chromium nitride will increase in a laminated material during hot rolling, and corrosion resistance will be impaired.
In our experiments, when the TF is lower than the chromium nitride precipitation temperature by more than 100 ° C., the reduction in corrosion resistance exceeds the limit, so the lower limit of TF is set to the chromium nitride precipitation temperature−100 (° C.). It was determined.
That is, according to the following formula (1) for the clad steel sheet that does not contain the selective components V, Nb, Ti in the laminated material, the base material according to the following formula (2) for the clad steel sheet containing the selective component: The entry temperature TF of the final finishing rolling pass of hot rolling when joining the laminated material was determined.
TF ≧ TN−100 (1)
TF ≧ TN2-100 (2)

  Although the upper limit of TF is not particularly defined, it is necessary to set the upper limit of TF to about 960 ° C. in order to obtain the toughness of the base material. The temperature varies somewhat depending on the composition of the base material.

  During cooling in the temperature range from TF to 600 ° C., precipitation of chromium nitride proceeds. In order to suppress this precipitation, it is necessary to cool the steel material quickly. The precipitation rate of chromium nitride increases as the steel type has a higher chromium nitride precipitation temperature. In the present invention in which the chromium nitride deposition temperature is limited to 970 ° C. or less, when the cooling time from TF to 600 ° C. exceeds 5 minutes, the amount of chromium nitride deposited increases and the corrosion resistance is impaired. For this reason, this time was set to 5 minutes or less.

Claim 6 defines accelerated cooling after hot rolling of the clad steel plate.
Accelerated cooling after the end of hot rolling is performed in order to suppress chromium nitride precipitation in the laminated material after the end of rolling. Precipitation into the duplex stainless steel after hot rolling proceeds in a supercooled state, but the precipitation rate shows a maximum value at 600 to 800 ° C. Since the maximum value increases in accordance with the degree of supercooling from the chromium nitride precipitation temperature, it is desirable to cool immediately after finish rolling. Further, when the plate thickness exceeds 20 mm, it is preferable to perform accelerated cooling, and based on the results of experiments by the present inventors, the accelerated cooling start temperature TC is set to a chromium nitride precipitation temperature of −250 (° C.) or higher. Is specified.
That is, hot-rolling according to the following formula (3) for clad steel plates not containing V, Nb, Ti as selective components in the laminated material, and following formula (4) for clad steel plates containing the selective components Accelerated cooling start temperature TC after completion was determined.
TC ≧ TN−250 (however, TF ≧ TC) (3)
TC ≧ TN2-250 (However, TF ≧ TC) (4)

Note that it is desirable to set TC in the range from the chromium nitride precipitation temperature of −150 ° C. to the chromium nitride precipitation temperature. This accelerated cooling also has the effect of increasing the strength of the base material.
It is reasonable from the viewpoint of equipment cost that the accelerated cooling medium be mixed with water or air / water.

Examples are described below. Table 1 shows the chemical composition of the laminated material.
In addition, the part in which content is not described about the component shown in Table 1 shows that it is an impurity level, REM means a lanthanoid type rare earth element, and content shows the sum total of these elements. O is an inevitable impurity.

The chromium nitride precipitation temperature in the table was determined by the following procedure.
(1) Solution heat treatment is performed on a test steel having a thickness of 10 mm under the conditions described later.
(2) A soaking process is performed for 20 minutes at an arbitrary temperature of 800 to 1000 ° C., and then water cooling is performed within 5 seconds.
(3) Polish the surface layer of the test steel after cooling # 500.
(4) A 3 g sample is taken and electrolyzed (100 mV constant voltage) in a non-aqueous solution (3% maleic acid + 1% tetramethylammonium chloride + remaining methanol) to dissolve the matrix.
(5) The residue (= precipitate) is filtered with a 0.2 μm hole diameter filter, and the precipitate is extracted.
(6) Analyze the chemical composition of the residue and determine its chromium content. The chromium content in the residue is used as an index of the amount of chromium nitride deposited.
(7) The soaking temperature in (2) is changed variously, and the lowest temperature among soaking temperatures at which the chromium content in the residue is 0.01% or less is defined as TN. Moreover, when it contains any 1 or more types of V, Ti, and Nb, let TN2 be the minimum temperature among the soaking | heat-treating temperatures from which chromium content will be 0.03% or less.

The clad steel plate is made of a duplex stainless steel having the chemical composition shown in Table 1, and the base material is C: 0.16%, Si: 0.21%, Mn: 0.63%, P: 0.018. %, S: 0.006%, Ni: 0.01%, Cr: 0.04%, Cu: 0.02%, SS400 steel having the composition of the balance Fe and inevitable impurities is used as a material of a predetermined thickness. It was assembled by welding and used as a material for hot rolling as a slab having a thickness of 130 mm.
In the hot rolling, the laminated steel sheet was heated to a predetermined temperature of 1150 to 1220 ° C. with the laminated material side as the lower surface, and then a clad steel plate was prepared by a two-stage rolling mill in a laboratory. As hot rolling conditions, the rolling was repeated 10 to 15 times, and finish rolling was performed at 760 to 1000 ° C. so that the final sheet thickness was 10 to 35 mm, and the product was transferred to a cooling bed and allowed to cool or water cooled. . In this way, a rolled clad steel sheet having a thickness of 3 mm was obtained. And the solution heat treatment was implemented at 1000 degreeC using a part of this steel plate, and it was set as the sample for pitting corrosion potential measurement.

The pitting corrosion potential of the laminated material was measured by measuring a potential (VC′100) corresponding to a current density of 100 μA / cm ² with respect to a surface of 1 mm of the epidermis of the steel material by a method defined in JIS G0577. The steel materials before and after the solution heat treatment were measured at n = 4, and the average value was obtained. The difference between the average values is shown in Tables 2 and 3.

  For the tensile test of the clad steel plate, a JIS Z2201 No. 1A test piece (full thickness plate-like tensile test piece) was taken in the direction perpendicular to the rolling direction and measured according to JIS Z2241. Ten tensile tests were performed at room temperature. Tables 2 and 3 show the average values (MPa) of the three tensile strengths.

For the impact toughness of the clad steel plate, a JIS No. 4 Charpy test piece in which a 2 mmV machined notch was machined in the rolling direction was taken from the base material. The direction of specimen collection was such that the fracture surface propagated parallel to the rolling direction. In the impact test, a sub-size impact test piece having a parallel part width of 5 mm was obtained for a material having a thickness of 10 mm, and a full-size impact test piece having a width of 10 mm was obtained from a base material for a material having a thickness of 15 mm. For the material, a full-size impact test piece having a width of 10 mm from the center of the plate thickness was collected, and for a material having a plate thickness of 35 mm, a full-size impact test piece having a width of 10 mm centered on the base material side plate thickness of 1/4 part was collected.
The impact test was performed according to the method of JIS G2242. The test temperature was set to −20 ° C., and three impact tests were carried out with a tester with a maximum energy of 500 J specifications. Tables 2 and 3 show the results of average values (J / cm ² ) of the three impact values.

The example shown in Table 2 shows the characteristics of a clad steel plate which is made of the steel shown in Table 1 as a laminated material, finished to a plate thickness of 10 mm with a TF (hot rolling final finishing temperature) of 900 ° C., and then air-cooled.
In the steel material using the laminated material whose chromium nitride precipitation temperature is lowered to 970 ° C. or less, the solution heat treatment is omitted, and the pitting potential difference remains at a reduced amount of less than 0.10V. Further, the tensile strengths of the base materials were all 470 to 490 MPa, and the impact value at −20 ° C. was 40 to 60 J / cm ² . Thus, it is clear that the clad steel material of the present invention disclosed in claims 1 and 2 is excellent in the corrosion resistance of the laminated material and the strength and impact characteristics of the base material.

In the examples shown in Table 3, corrosion resistance, strength and impact properties were evaluated using a part of the laminated materials shown in Table 1 as clad steel plates having a thickness of 10 to 35 mm under various hot rolling conditions. Results are shown.
In the example of the present invention, the pitting corrosion potential difference between the laminated material and the solution heat-treated material is only a decrease of less than 0.10V. Moreover, the tensile strength of a base material is 400 Mpa or more, and the impact value in -20 degreeC shows 40 J / cm < ² > or more. Thus, it is clear that the clad steel sheet of the present invention manufactured under the conditions disclosed in claims 3 and 4 is excellent in corrosion resistance, strength and impact characteristics.
In Comparative Example 42, TF is a very high temperature of 1000 ° C., and therefore the impact value of the base material is low and the target impact characteristic is not achieved.

  As can be seen from the above examples, it became clear that according to the present invention, an inexpensive clad steel sheet in which alloy element-saving duplex stainless steel is used as a combined material and solution heat treatment is omitted can be obtained.

The present invention makes it possible to provide an alloyed element-saving economical clad steel plate with good corrosion resistance and toughness, which contributes to the industry such as seawater desalination equipment, tanks for transport ships, various containers, etc. However, it is extremely large.

Claims (6)

A clad steel plate made of a duplex stainless steel, the duplex stainless steel,
% By mass
C: 0.03% or less,
Si: 0.05 to 1.0%,
Mn: 0.5 to 7.0%,
P: 0.05% or less,
S: 0.010% or less,
Ni: 0.1 to 5.0%,
Cr: 18.0 to 25.0%,
N: 0.05-0.30%
Al: 0.001 to 0.05%
And the balance consists of Fe and inevitable impurities,
A clad steel plate made of a duplex stainless steel, characterized by a chromium nitride precipitation temperature TN of 800 to 970 ° C., which serves as an index for chromium nitride precipitation during hot rolling.
A clad steel plate made of a duplex stainless steel, the duplex stainless steel,
% By mass
C: 0.03% or less,
Si: 0.05 to 1.0%,
Mn: 0.5 to 7.0%,
P: 0.05% or less,
S: 0.010% or less,
Ni: 0.1 to 5.0%,
Cr: 18.0 to 25.0%,
N: 0.05-0.30%
Al: 0.001 to 0.05%
Further,
V: 0.05-0.5%
Nb: 0.01-0.20%,
Ti: 0.003 to 0.05%
1 type or 2 types or more chosen from, The remainder consists of Fe and an unavoidable impurity,
A clad steel plate comprising a duplex stainless steel as a combination material, wherein a chromium nitride precipitation temperature TN2 serving as a second index for precipitation of chromium nitride during hot rolling is 800 to 970 ° C.
The duplex stainless steel is further
Mo: 1.5% or less,
Cu: 2.0% or less,
W: 1.0% or less,
Co: A clad steel sheet comprising the duplex stainless steel according to claim 1 or 2, wherein the clad steel sheet contains at least one selected from 2.0% or less.
The duplex stainless steel is further
B: 0.0050% or less,
Ca: 0.0050% or less,
Mg: 0.0030% or less,
REM: 1 type or 2 types or more chosen from 0.10% or less are contained, The clad steel plate which uses the duplex stainless steel of any one of Claims 1-3 as a laminated material.
A method for producing a clad steel plate using the duplex stainless steel according to any one of claims 1 to 4 as a laminated material,
When the base material and the laminated material are joined by hot rolling, the clad steel sheet that does not contain V, Nb, Ti, which are selective components in the laminated material, contains the selective component according to the following formula (1). The clad steel sheet is hot-rolled at the entry temperature TF of the final finishing rolling pass of hot rolling according to the following equation (2), and the temperature range from the entry temperature TF of the final finishing rolling pass of hot rolling to 600 ° C. Is cooled in a time of 5 minutes or less, and a method for producing a clad steel plate using a duplex stainless steel as a laminated material.
TF ≧ TN−100 (1)
TF ≧ TN2-100 (2)
For clad steel sheets that do not contain V, Nb, and Ti, which are selective components in the laminated material, according to the following formula (3), and for clad steel sheets that contain the selective components, according to the following formula (4), after hot rolling is completed By starting the accelerated cooling from the accelerated cooling start temperature TC, the temperature range from the entry side temperature TF to 600 ° C. of the final finishing rolling pass of the hot rolling is cooled in a time of 5 minutes or less. A method for producing a clad steel plate using the duplex stainless steel according to Item 5 as a laminated material.
TC ≧ TN−250 (however, TF ≧ TC) (3)
TC ≧ TN2-250 (However, TF ≧ TC) (4)