JP5406230B2 - Alloy element-saving duplex stainless steel hot rolled steel material and method for producing the same - Google Patents

Description

  The present invention relates to an inexpensive alloy element-saving duplex stainless steel hot-rolled steel material in which solution heat treatment is omitted, and a method for producing the same, and in particular, a high-performance seawater desalination apparatus, tanks for transport vessels, various containers and the like. The present invention relates to a strength duplex stainless steel hot-rolled steel material and a method for producing the same.

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.
Recently, however, 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 come to be 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 have good corrosion resistance and impact characteristics by material design based on the idea of controlling the component composition to suppress chromium nitride precipitation. Invented and disclosed a new alloy element-saving duplex stainless steel type (Patent Document 1). 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 alloy element-saving duplex stainless steel is also expected to be used as a hot-rolled steel sheet. A hot-rolled steel sheet is generally not thick and therefore has a large thickness, and is particularly used for applications requiring strength and toughness. For example, seawater desalination equipment, tanks for transport ships, and the like, austenitic stainless steel has been used for many of them.
However, duplex stainless steel is generally stronger than austenitic stainless steel, and has the advantage that it can be thinned while maintaining the required strength, and the cost is low because the amount of expensive elements used is small. For these reasons, duplex stainless steel hot rolled steel is already being used in some of the above applications.

On the other hand, the problem of toughness is a problem when using a duplex stainless steel hot rolled steel material. That is, the duplex stainless steel has a ferrite phase in addition to an austenite phase that is generally considered not to cause brittle fracture, and thus exhibits a ductile-brittle transition in impact toughness like a ferritic stainless steel. It has poor toughness.
Many studies have been made on this problem, and the inventors of the present invention have also clarified the relationship between the steel structure and toughness of the duplex stainless steel hot rolled steel sheet in Patent Document 2 and controlled the chemical composition and heat treatment method. It is disclosed that toughness can be improved.

  By the way, in the manufacture of conventional duplex stainless steel hot-rolled steel sheets including Patent Document 2, solution heat treatment is indispensable. This is because it is necessary to solve the problem of chromium nitride and toughness, which are problems in the duplex stainless steel as described above. In particular, the alloy element-saving duplex stainless steel targeted by the present invention has the property that nitride is likely to precipitate in the temperature range of hot working, and chromium nitride is a steel material after hot rolling is completed. Dispersion in the interior will reduce impact properties and corrosion resistance.

  Although it is possible to eliminate chromium nitride by performing solution heat treatment, the strength decreases when solution heat treatment is performed. Therefore, it can be said that the treatment is not preferable in terms of the use of the hot-rolled steel sheet. Moreover, from the request | requirement for the further cost reduction, and the request | requirement of energy consumption reduction in recent years, it is desired to reduce the manufacturing cost and the energy required for manufacture by omitting the solution heat treatment.

WO2009-119895 JP 2010-84220 A

  In addition to the above-described component design of alloy element-saving duplex stainless steel, the present inventors have made a solution by repeatedly researching hot-rolled steel that maintains strength, impact properties, and corrosion resistance even when hot-rolled. It was an object of the invention to obtain an alloy element-saving duplex stainless steel hot-rolled steel material that is inexpensive, uses less energy, and is environmentally friendly, omitting heat treatment.

  In order to solve the above-mentioned problems, the inventors of the present invention have included a metal composition including a chemical composition of hot-rolled steel material in which solution heat treatment is omitted for alloy element-saving duplex stainless steel, hot working conditions, precipitation amount of chromium nitride, and the like. The following experiments were conducted on the assumption that it was necessary to obtain knowledge about the state of the structure, the impact characteristics of steel, and the relationship between corrosion resistance and the like.

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 6 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 the 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.
Through the above experiments, the present invention has been completed, which clearly shows an inexpensive alloy element-saving duplex stainless steel hot-rolled steel material in which solution heat treatment is omitted.

That is, the gist of the present invention is as follows.
(1) In 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,
The chromium nitride precipitation temperature TN, which is an index related to precipitation of chromium nitride during hot rolling, is 960 ° C. or less,
Alloy element-saving duplex stainless steel with a yield strength of 50 MPa or more higher than that of hot-rolled steel that has undergone solution heat treatment that is water-cooled to room temperature after soaking at 1000 ° C. for 5 minutes, and that eliminates solution heat treatment as hot-rolled Hot rolled steel.
(Here, the chromium nitride precipitation temperature TN is obtained by subjecting the steel material that has undergone solution heat treatment to 800-1000 ° C. for 20 minutes and then subjecting the steel material to water cooling within 5 seconds. Is determined by an electrolytic extraction residue analysis method for non-metallic inclusions, and is defined as the lowest temperature among soaking temperatures at which the Cr residue amount is 0.01% or less.)

(2) In 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 chromium nitride precipitation temperature TN2 serving as a second index for precipitation of chromium nitride during hot rolling is 960 ° C. or less,
Alloy element-saving duplex stainless steel with a yield strength of 50 MPa or more higher than that of hot-rolled steel that has undergone solution heat treatment that is water-cooled to room temperature after soaking at 1000 ° C. for 5 minutes, and that eliminates solution heat treatment as hot-rolled Hot rolled steel.
(Here, the chromium nitride precipitation temperature TN2 is obtained by subjecting the solution heat-treated steel to water cooling within 5 seconds after soaking at 800-1000 ° C. for 20 minutes, and the amount of chromium nitride deposited on the steel after cooling. Is determined by an electrolytic extraction residue analysis method for non-metallic inclusions, and is defined as the lowest temperature among soaking temperatures at which the Cr residue amount is 0.03% or less.)

(3) Furthermore,
Mo: 1.5% or less, Cu: 2.0% or less,
It contains 1 type or 2 types or more chosen from W: 1.0% or less, Co: 2.0% or less, Solution forming as hot-rolling as described in said (1) or (2) characterized by the above-mentioned Alloy element saving duplex stainless steel hot rolled steel without heat treatment.
(4) Furthermore,
B: 0.0050% or less, Ca: 0.0050% or less,
The hot according to any one of (1) to (3) above, which contains one or more selected from Mg: 0.0030% or less, REM: 0.10% or less Alloy element-saving duplex stainless steel hot rolled steel that eliminates the solution heat treatment as it is rolled.

(5) A method for producing an alloying element-saving duplex stainless steel hot rolled steel material in which the solution heat treatment as hot-rolling according to any one of (1) to (4) is omitted,
The steel material that does not contain the selective components V, Nb, and Ti is in accordance with the following formula (1), and the steel material that contains the selective component is in accordance with the following formula (2). A method for producing an alloying element-saving duplex stainless steel hot rolled steel, characterized in that the temperature range from the side temperature TF to 600 ° C is cooled in a time of 5 minutes or less.
TF ≧ TN−30 (1)
TF ≧ TN2-30 (2)

(6) According to the following formula (3) for a steel material with a thickness of more than 20 mm that does not contain V, Nb, Ti, which are selective components, the following (4) for a steel material with a thickness of more than 20 mm that contains the selective component In accordance with the equation, by starting the accelerated cooling from the accelerated cooling start temperature TC after the end of the hot rolling, the temperature range from the entry side temperature TF to 600 ° C. of the final finishing rolling pass of the hot rolling is within 5 minutes or less The method for producing an alloying element-saving duplex stainless steel hot-rolled steel as described in (5) above, wherein the cooling is performed.
TN −200 ≦ TC ≦ TN + 50 (however, TF ≧ TC) (3)
TN2-200 ≦ TC ≦ TN2 + 50 (however, TF ≧ TC) (4)

  The alloy element-saving duplex stainless steel hot-rolled steel according to the present invention can be used by making it thinner than conventional steel as seawater desalination equipment, tanks for transport ships, various containers, etc. There is a tremendous contribution to the industrial and environmental aspects, such as steel with less energy.

Below, the reason for limitation of Claim 1 of this invention is demonstrated first. The unit% is mass%. The duplex stainless steel hot-rolled steel material according to claim 1 contains C, Si, Mn, P, S, Ni, Cr, N, and Al, and the balance consists of Fe and inevitable impurities.
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 it exceeds 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 toughness. Moreover, since it has the effect of lowering the chromium nitride precipitation temperature TN, it is preferable to add it positively in the steel material of the present invention. Add 0.5% or more for toughness of base metal and weld. 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 is intended for alloy element-saving duplex stainless steel, 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 in claim 1 will be described.
The chromium nitride precipitation temperature TN, which serves as an index for precipitation of chromium nitride 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 impact properties are reduced with the solution heat treatment as hot rolled 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.

It is experimental that it is necessary to design TN at 960 ° C. or lower in order to ensure corrosion resistance and toughness for alloy element-saving duplex stainless steel hot-rolled steel material in which solution heat treatment as hot rolling is omitted. Asked for. Therefore, it is necessary to design a component composition such that TN is 960 ° C. or lower. When TN exceeds 960 ° C., chromium nitride precipitates during hot rolling, and the pitting corrosion potential difference and impact characteristics deteriorate. 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 content and N content of Ni, Mn, and Cu, which are elements that form the austenite phase.

  The alloy element-saving duplex stainless steel hot-rolled steel material in which the solution heat treatment as hot rolled according to the present invention is omitted has a yield strength higher by 50 MPa or more than the hot-rolled steel material subjected to the solution heat treatment. In general, when solution heat treatment is performed, the strength decreases. In the present invention, strain remains in the hot-rolled state to obtain high strength. In other words, the solution heat treatment is omitted so that the strength is not lowered. Generally, the strength is lowered when the rolling finishing temperature is raised to approach the solution heat treatment temperature. In the steel of the present invention, as described later, the rolling finishing temperature is lowered, and the chromium nitride precipitation temperature is lowered so that the strength can be increased, so that high strength can be easily obtained.

Next, the contents defined in claim 2 will be described. The duplex stainless steel hot-rolled steel material according to claim 2 contains C, Si, Mn, P, S, Ni, Cr, N, Al, and one or more selected from V, Nb, Ti. The balance consists of Fe and inevitable impurities.
The present inventors have found that in the case of steel containing one or more selected from V, Nb, and Ti, the behavior is different from the conventional knowledge.
That is, it has been a general knowledge that the corrosion resistance deteriorates as the amount of chromium nitride increases. However, when a small amount of V, Nb, and Ti is contained, the amount of precipitation of chromium nitride is unexpectedly large. It has been clarified that the corrosion resistance has a tendency to improve even if it 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 selective components V, Nb, and Ti, the chromium nitride precipitation temperature TN2 is newly defined as a second index regarding the precipitation of chromium nitride during hot rolling, and the amount of Cr residue Was the lowest temperature of the soaking temperature at which 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 960 ° 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 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 chromium nitride precipitation temperature TN is first decreased by decreasing the chromium concentration and the nitrogen concentration, but can be decreased by adding an austenite stabilizing element. In Claim 2, precipitation of chromium nitride is controlled by containing a very small amount of V, Nb and Ti, which are nitride-forming elements stronger than chromium, and the corrosion resistance of hot-rolled steel is improved.

  That is, in the research by the present inventors, a nitride in which a part of chromium is substituted by a small amount of V, Nb, and Ti contained in the alloy element-saving duplex stainless steel is formed, and the precipitation of the nitride Although it has the effect of slightly increasing the temperature, it has been surprisingly found that the corrosion resistance tends to improve even if the amount of chromium nitride deposited increases. Claim 2 is disclosed as an invention including such an additional effect of adding trace elements.

  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 the corrosion resistance, V is contained in an amount of 0.05% or more. 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, Nb is contained in an amount of 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 also has a property of substituting for 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 corrosion resistance 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 set 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.
The entry temperature TF of the final finishing rolling pass of hot rolling is an important factor governing the strength of the hot-rolled steel material, and the higher the strength, the higher the strength. On the other hand, if it is too low, the amount of chromium nitride deposited during hot rolling increases, and the toughness and corrosion resistance are impaired. On the other hand, if the TF is too high, the strength is lowered as if the solution heat treatment was performed, and the object of the present invention omitting the solution heat treatment cannot be achieved.

In our experiments, when TF is below 30 ° C. below the chromium nitride precipitation temperature, the decrease in toughness and corrosion resistance exceeds the limit. Therefore, the lower limit of TF is reduced to the chromium nitride precipitation temperature −30 ( ° C).
That is, the final finishing rolling pass of hot rolling according to the following formula (1) for steel materials not containing the selective components V, Nb, Ti, and the following formula (2) for steel materials containing the selective components: Entry side temperature TF was determined.
TF ≧ TN−30 (1)
TF ≧ TN2-30 (2)

  Further, although the upper limit of TF is not particularly defined, the alloy element-saving duplex stainless steel hot-rolled steel material in which the yield strength is 50 MPa or more higher than that of the hot-rolled steel material subjected to the solution heat treatment and the solution heat treatment as hot rolled is omitted. In order to obtain, it is necessary to make it less than 1000 degreeC. The solution heat treatment temperature varies somewhat depending on the steel composition. In the case of the steel of the present invention, the solution heat treatment is 950 to 1050 ° C. × 5 minutes, but the confirmation of the yield strength difference in the examples is 1000 ° C. × 5 minutes. went.

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 960 ° C. or less, when the cooling time from TF to 600 ° C. exceeds 5 minutes, the amount of chromium nitride deposited increases, and the toughness and corrosion resistance are impaired.
For this reason, in the manufacturing method of alloy element-saving duplex stainless steel hot rolled steel according to the present invention, the temperature range from the entry side temperature TF to 600 ° C. of the final finishing rolling pass of hot rolling is 5 minutes or less. It was decided to cool. A steel material having a thickness of 20 mm or less can substantially satisfy this restriction by air cooling.

Claim 6 defines accelerated cooling after the hot rolling of the alloy element-saving duplex stainless steel hot rolled steel according to the present invention.
Accelerated cooling after the end of hot rolling is performed to suppress chromium nitride precipitation in the steel after the end of rolling. Precipitation into the steel material after hot rolling proceeds in a supercooled state, but the precipitation rate shows a maximum value in 600 to 800 ° C. Since the maximum value increases according to the degree of supercooling from the chromium nitride precipitation temperature, it is necessary to cool immediately after finish rolling.

Therefore, it is preferable to perform accelerated cooling for a steel material having a thickness of more than 20 mm, 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 −200 (° C.) or higher. Is specified.
That is, the steel material having a thickness of more than 20 mm that does not contain the selective components V, Nb, and Ti is in accordance with the following formula (3). The steel material having a thickness of more than 20 mm that contains the selective component is represented by the following formula (4) Accelerating cooling start temperature TC after the end of hot rolling is determined.
TN −200 ≦ TC ≦ TN + 50 (however, TF ≧ TC) (3)
TN2-200 ≦ TC ≦ TN2 + 50 (however, TF ≧ TC) (4)

And in the manufacturing method of the alloy element saving type duplex stainless steel hot rolled steel material having a plate thickness exceeding 20 mm according to the present invention, the hot rolling is started by starting the accelerated cooling from the accelerated cooling start temperature TC after the end of the hot rolling. In the final finishing rolling pass, the temperature range from the entry temperature TF to 600 ° C. was cooled in 5 minutes or less.
The purpose of the present invention is to cool the temperature region from TF to 600 ° C. in 5 minutes or less to suppress the precipitation of chromium nitride, thereby reducing the alloy element-saving duplex stainless steel heat excellent in toughness and corrosion resistance. Since it is to obtain a rolled steel material, not only steel materials with a thickness of 20 mm or less, but also steel materials with a thickness of more than 20 mm can be cooled in a temperature range from TF to 600 ° C. in 5 minutes or less. It is not necessarily necessary. For example, in the case where cooling from TF to 600 ° C. can be performed by air cooling or standing cooling in 5 minutes or less, it is not always necessary to perform accelerated cooling. The purpose of performing accelerated cooling in the middle of cooling from TF or with the start of cooling from TF is that steel materials with a thickness of more than 20 mm may not be cooled from TF to 600 ° C. in less than 5 minutes. In order to avoid this, accelerated cooling is started from TC which is the optimum temperature.

Further, if the accelerated cooling start temperature TC is extremely increased, the corrosion resistance may be lowered. Therefore, the upper limit of TC is set to the chromium nitride precipitation temperature +50 (° C.) or less. It is desirable to set the TC in the range from the chromium nitride precipitation temperature of −150 ° C. to the chromium nitride precipitation temperature.
In addition, it is reasonable from the viewpoint of equipment cost to perform the accelerated cooling medium by water or air-water mixing.

Examples are described below. Table 1 shows the chemical composition of the test steel.
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.

For all the steels, a steel piece having a thickness of 140 mm was used as a hot rolled material.
Hot rolling is performed at a predetermined temperature of 1150 to 1250 ° C., and then carried out by a two-stage rolling mill in a laboratory, repeatedly subjected to reduction, and subjected to final finish rolling at 780 to 1080 ° C., with a final thickness of 6 Rolled to ˜35 mm.
This hot-rolled steel material was halved and solution heat treatment was performed on one steel material. In the solution heat treatment, the steel material was inserted into a heat treatment furnace set at 1000 ° C., extracted after a soaking time of 5 minutes, and then cooled to room temperature.

  The tensile test of the hot-rolled steel material is performed using ASTM 13B-shaped plate-shaped test pieces for materials having a plate thickness of 6 mm, and round bar tensile test pieces having a parallel portion having a diameter of 8 mm for materials having a plate thickness of 10 mm. For the 35 mm material, 10 mm diameter round bar tensile specimens were taken in the direction perpendicular to the rolling direction. In addition, about the material of plate | board thickness 30 and 35 mm, it extract | collected centering on ¼ part of plate | board thickness. Tables 2 and 3 show the difference in yield strength before and after the solution heat treatment.

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

The impact toughness of the hot-rolled steel material was sampled in a direction in which the fracture surface propagated parallel to the rolling direction using a JIS No. 4 Charpy test piece in which a 2 mmV machined notch was machined in the rolling direction.
It should be noted that the 6 mm material is 1/2 size, the 10 mm material is 3/4 size Charpy test piece, and the 20 mm thickness material is the full size Charpy test piece at the center of the plate thickness, 30 mm thick. The 35 mm material was evaluated with a full-size Charpy test piece collected centering on 1/4 part of the plate thickness.
The test temperature was −20 ° C., and an impact test was performed with a tester with a maximum energy of 500 J. Tables 2 and 3 show the results of average values (J / cm ² ) of the three impact values.

The Example shown in Table 2 has shown the characteristic of the hot-rolled steel materials which finished the steel shown in Table 1 to plate thickness 10mm by making TF (hot rolling final finishing temperature) 930 degreeC, and air-cooled after that.
In a steel material in which the chromium nitride precipitation temperature is lowered to 960 ° C. or less, the solution heat treatment is omitted, the strength difference from the solution heat treatment material is 50 MPa or more, and the pitting potential is only a decrease of 0.05 V or less. . Moreover, the impact value in -20 degreeC shows 50 J / cm < ² > or more. Thus, it is clear that the hot-rolled steel material of the present invention disclosed in claims 1 and 2 is excellent in strength, corrosion resistance, and impact characteristics.

In the examples shown in Table 3, a part of the steel shown in Table 1 was used to obtain a hot rolled steel material having a thickness of 6 to 35 mm under various hot rolling conditions, and the strength, corrosion resistance, and impact characteristics were evaluated. Results are shown.
In the example of the present invention, the strength difference from the solution heat treatment material is 50 MPa or more, and the pitting corrosion potential is only a decrease of 0.05 V or less. Moreover, the impact value in -20 degreeC shows 50 J / cm < ² > or more. Thus, it is clear that the hot-rolled steel material of the present invention manufactured under the conditions disclosed in claims 3 and 4 is excellent in strength, corrosion resistance, and impact characteristics.
In Comparative Example 42, TF is a very high temperature exceeding the solution treatment temperature. Therefore, the strength is low and the target strength is not reached.

  As can be seen from the above examples, it has been clarified that the present invention provides a high-strength duplex stainless steel hot-rolled steel material in which solution heat treatment is omitted.

The present invention makes it possible to provide a high-strength, high-strength, high-strength duplex stainless steel material that saves alloying elements and contributes to the industry, such as seawater desalination equipment, tanks for transport vessels, and various containers. The place to do is extremely large.

Claims (6)

% 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,
The chromium nitride precipitation temperature TN, which is an index related to precipitation of chromium nitride during hot rolling, is 960 ° C. or less,
Alloy element-saving duplex stainless steel with a yield strength of 50 MPa or more higher than that of hot-rolled steel that has undergone solution heat treatment that is water-cooled to room temperature after soaking at 1000 ° C. for 5 minutes, and that eliminates solution heat treatment as hot-rolled Hot rolled steel.
(Here, the chromium nitride precipitation temperature TN is obtained by subjecting the steel material that has undergone solution heat treatment to 800-1000 ° C. for 20 minutes and then subjecting the steel material to water cooling within 5 seconds. Is determined by an electrolytic extraction residue analysis method for non-metallic inclusions, and is defined as the lowest temperature among soaking temperatures at which the Cr residue amount is 0.01% or less.) % 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 chromium nitride precipitation temperature TN2 serving as a second index for precipitation of chromium nitride during hot rolling is 960 ° C. or less,
Alloy element-saving duplex stainless steel with a yield strength of 50 MPa or more higher than that of hot-rolled steel that has undergone solution heat treatment that is water-cooled to room temperature after soaking at 1000 ° C. for 5 minutes, and that eliminates solution heat treatment as hot-rolled Hot rolled steel.
(Here, the chromium nitride precipitation temperature TN2 is obtained by subjecting the solution heat-treated steel to water cooling within 5 seconds after soaking at 800-1000 ° C. for 20 minutes, and the amount of chromium nitride deposited on the steel after cooling. Is determined by an electrolytic extraction residue analysis method for non-metallic inclusions, and is defined as the lowest temperature among soaking temperatures at which the Cr residue amount is 0.03% or less.) Furthermore,
Mo: 1.5% or less,
Cu: 2.0% or less,
W: 1.0% or less,
The alloy element-saving duplex stainless steel omitting the solution heat treatment as hot-rolled according to claim 1 or 2, characterized in that it contains one or more selected from Co: 2.0% or less Hot rolled steel. Furthermore,
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, The alloy element which omitted the solution heat treatment as hot-rolling of any one of Claims 1-3 characterized by the above-mentioned Saving type duplex stainless steel hot rolled steel. It is a manufacturing method of the alloy element saving type duplex stainless steel hot rolled steel material which omitted the solution heat treatment as hot-rolling according to any one of claims 1 to 4,
The steel material that does not contain the selective components V, Nb, and Ti is in accordance with the following formula (1), and the steel material that contains the selective component is in accordance with the following formula (2). A method for producing an alloying element-saving duplex stainless steel hot rolled steel, characterized in that the temperature range from the side temperature TF to 600 ° C is cooled in a time of 5 minutes or less.
TF ≧ TN−30 (1)
TF ≧ TN2-30 (2) According to the following formula (3) for steel materials with a thickness of more than 20 mm that do not contain V, Nb, Ti, which are selective components, By starting accelerated cooling from the accelerated cooling start temperature TC after the end of hot rolling, the temperature range from the entry side temperature TF to 600 ° C. of the final finishing rolling pass of hot rolling is cooled in 5 minutes or less. The method for producing an alloying element-saving duplex stainless steel hot-rolled steel according to claim 5.
TN −200 ≦ TC ≦ TN + 50 (however, TF ≧ TC) (3)
TN2-200 ≦ TC ≦ TN2 + 50 (however, TF ≧ TC) (4)