KR20120120428A - Martensitic stainless steel with excellent weld characteristics, and mertensitic stainless steel material - Google Patents

Abstract

[수학식 2]

여기서, C％, N％, Ni％, Cu％, Mn％, Cr％, Si％, Al％, Mo％, Ti％는, 각각의 원소의 함유량(질량％)을 나타냄.The martensitic stainless steel is, in mass%, C: 0.003 to 0.03%, Si: 0.01 to 1.0%, Mn: 3.0 to 6.0%, P: 0.05% or less, S: 0.003% or less, Ni: 1.0 to 3.0% , Cr: 15.0 to 18.0%, Mo: 0 to 1.0%, Cu: 0 to 2.0%, Ti: 0 to 0.05%, N: 0.05% or less, Al: 0.001 to 0.1%, O: 0.005% or less The remainder is made of Fe and unavoidable impurities, the total amount of C and N is 0.060% or less, the γmax represented by the formula (1) is 80 or more, and the γpot represented by the formula (2) is 60 to 90.
[Equation 1]

&Quot; (2) "

Here, C%, N%, Ni%, Cu%, Mn%, Cr%, Si%, Al%, Mo%, and Ti% represent the content (mass%) of each element.

Description

MARTENSITIC STAINLESS STEEL WITH EXCELLENT WELD CHARACTERISTICS, AND MERTENSITIC STAINLESS STEEL MATERIAL}

The present invention is made of martensitic stainless steel and martensitic stainless steel, which are suitably used for a site requiring welding, for example, in a welded structure such as a building structure or a ship structure. The present invention relates to martensitic stainless steels having excellent impact characteristics and corrosion resistance and low cost of Ni.

This application claims priority in March 17, 2010 based on Japanese Patent Application No. 2010-60048 for which it applied to Japan, and uses the content for it here.

Since martensitic stainless steel can be easily increased in strength by quenching heat treatment, martensitic stainless steel is widely used for materials such as blades, springs and brake discs. However, since martensitic stainless steel has low toughness and poor weldability, it is not used for welding structures.

On the other hand, in steels containing 13 to 17% of Cr, by reducing the C content and adding about 3% or more of Ni, steel materials having improved toughness, weldability and corrosion resistance have been developed, and aberration runners for hydraulic power generation and oil pipes for oil wells have been developed. It is used as (for example, patent documents 1-4).

However, even in this improved martensitic stainless steel, the tempering resistance is very large. For this reason, in the tempering heat treatment for refining the properties of the final product, the heat treatment facility capability is hindered, such as the need for a long time treatment, leaving a problem that the manufacturing cost is high.

Therefore, martensitic stainless steel that does not require heat treatment for refining and manufacturing conditions that do not require dehydrogenation have been studied. The patent document 4 or martensite phase aimed at martensite single phase structure mainly consists of ferrite phase or residual. Patent Document 5 discloses a planar structure containing an austenite phase.

As disclosed in Patent Literature 4, most of the martensitic stainless steels have a Cr content in the range of 11 to 15%, lower corrosion resistance than ferritic stainless steels such as SUS430, and water repellency even in indoor environments. Generation)] may be generated. For this reason, in order to provide the outstanding corrosion resistance, it is necessary to add Mo or to increase Cr amount.

Further, in Patent Document 5, it is disclosed that it is preferable to contain at least 15% of Cr and at least 1% of Mo to improve corrosion resistance. However, the martensitic stainless steel of patent document 5 has the metal structure of the martensite phase main body containing a ferrite phase, and is not good in hot workability, and there existed a problem which often reduced the manufacturing yield of steel materials. In addition, in order to secure mechanical properties, the addition of an austenite forming element in an amount corresponding to an increase amount of Cr and Mo is required, resulting in an increase in alloy cost.

That is, steel which contains much Ni is practically used as steel which can maintain the characteristic of a base material and a weld part favorably. However, there was no inexpensive practical steel having good hot workability, corrosion resistance equivalent to that of SUS430, excellent mechanical properties, and reduced Ni content in the base metal and the welded portion.

Japanese Patent Application Laid-open No. Hei 6-306549 Japanese Patent Application Laid-open No. Hei 6-306551 Japanese Patent Application Laid-open No. Hei 2-243739 Japanese Patent Application Laid-open No. Hei 2-243740 Japanese Patent Application Laid-Open No. 2001-279392

Materials and Processes Vol. 3 (1990), 1840

In view of such a problem, the inventors of the present invention have found the component system and metal structure of inexpensive martensitic stainless steel having good hot workability and mechanical properties, and having corrosion resistance equivalent to that of SUS430, and made a practical steel material to develop a practical steel material.

As an element replacing Ni, C, N, Mn, Cu, Co and the like are assumed. However, in the above martensitic stainless steel, there are few documents regarding the steel containing a large amount of Mn, Cu, Co. As an example, Non-Patent Document 1 shows an example in which high purity stainless steel containing 17% Cr is used as a base and Ni or Mn is added. However, the example of complex addition of Ni and Mn is not disclosed, nor is the corrosion resistance considered.

By the way, since Mn is generally an element which lowers corrosion resistance, there are few examples of aggressively adding Mn in martensitic stainless steel having low corrosion resistance compared with general stainless steel. It has been questioned whether or not the desired corrosion resistance can be obtained when the amount of Cr is increased and the amount of Mn is increased. For this reason, in order to develop a practical steel material which can secure excellent hot workability and mechanical properties in addition to corrosion resistance, it may be from the technical standpoint or experience from which the method of adjusting these alloy elements was employ | adopted until then. There was no.

MEANS TO SOLVE THE PROBLEM The present inventors used the steel containing 16% Cr and 2% Ni as a base, and, about the steel containing 2% or more of Mn, about the influence which a component element and the metal structure of steel materials have on the said various characteristics It examined in detail. As a result, it has been found that by setting the contents of Cr, Ni, Mn, and other elements described later in a predetermined range, the toughness and corrosion resistance of the welded portion can be achieved while suppressing the amount of Ni having a large fluctuation in price. In addition, it was found that the mechanical properties of the base metal can be secured even if the heat treatment of the quenching and tempering required in the past is omitted by allowing the phase modulus of the steel to fall within a certain range. As mentioned above, the present invention was completed.

The requirements of the present invention are shown below.

(1) Martensitic stainless steel which is excellent in the characteristic of the weld part concerning one aspect of this invention is C: 0.003-0.03%, Si: 0.01-1.0%, Mn: 3.0-6.0%, P: 0.05% by mass% S: 0.003% or less, Ni: 1.0 to 3.0%, Cr: 15.0 to 18.0%, Mo: 0 to 1.0%, Cu: 0 to 2.0%, Ti: 0 to 0.05%, N: 0.05% or less, Al : 0.001 to 0.1%, O: 0.005% or less, and the remainder is made of Fe and unavoidable impurities. The total amount of C and N is 0.060% or less, γmax represented by the formula (1) is 80 or more, and γpot represented by the formula (2) is 60 to 90.

[Equation 1]

&Quot; (2) "

Here, C%, N%, Ni%, Cu%, Mn%, Cr%, Si%, Al%, Mo%, and Ti% represent the content (mass%) of each element.

(2) Martensitic stainless steel which is excellent in the characteristic of the weld part concerning one aspect of this invention as described in said (1) further contains Nb, and the (gamma) pot computed by Formula (3) instead of said Formula (2) is 60-. 90 may be good.

&Quot; (3) "

Here, C%, N%, Mn%, Cu%, Ni%, Si%, Cr%, Mo%, Nb%, Ti%, and Al% represent the content (mass%) of each element.

(3) Martensitic stainless steel which is excellent in the characteristic of the weld part concerning one aspect of this invention as described in said (1) or (2) may further contain either or both among V and W.

(4) Martensitic stainless steel which is excellent in the characteristic of the weld part concerning one aspect of this invention in any one of said (1)-(3) may contain Co further.

(5) Martensitic stainless steel which is excellent in the characteristic of the weld part concerning one aspect of this invention in any one of said (1)-(4) is 1 type or 2 types chosen from B, Ca, Mg, and REM. You may further contain the above.

(6) The martensitic stainless steel according to one embodiment of the present invention has the composition according to any one of the above (1) to (5), and further has a 5 to 30% ferrite phase and 0 to 20% residual austenite It contains a phase and the remainder has a structure which consists of a martensite phase.

(7) The martensitic stainless steel material according to one embodiment of the present invention described in (6) above may have a yield strength of 400 to 800 MPa.

The martensitic steel having the composition of one embodiment of the present invention exhibits an effect of being excellent in the toughness and corrosion resistance of the welded portion. Moreover, according to one form of this invention, it can be used for large welding structures, such as a building structure or a ship structure, for example, and can provide the martensitic stainless steel with low cost. In addition, since the desired characteristics can be obtained even if a long time quenching and tempering heat treatment are omitted, mass productivity can be improved. For this reason, one embodiment of the present invention can greatly contribute industrially.

Below, the reason for limitation of the chemical composition of the martensitic stainless steel of this embodiment is demonstrated first. In addition, the unit of content of each component in the following is mass%.

C is contained 0.003% or more in order to secure the strength of the steel. However, when C is contained exceeding 0.03%, intensity | strength becomes high more than necessary, and corrosion resistance and toughness in a weld part are inferior. For this reason, C content is restrict | limited to 0.003 to 0.03%. C content becomes like this. Preferably it is 0.005 to 0.025%.

Si is added 0.01% or more for deoxidation. However, when Si is added exceeding 1.0%, toughness will fall. Therefore, the upper limit of Si content is limited to 1.0%. Si content becomes like this. Preferably it is 0.2 to 0.5%.

Mn is added 3.0% or more in order to improve the toughness of a welded part. However, an increase in the amount of Mn lowers the corrosion resistance. In the steel of the present embodiment, the Mn content, γmax, γpot, and the ratio of the ferrite phase in the steel material of the present embodiment described later are closely related, and corrosion resistance deterioration accompanying an increase in the Mn content through control of the metal structure. Is suppressed. However, when Mn is contained exceeding 6.0%, desired corrosion resistance will no longer be secured. For this reason, the upper limit of Mn content is limited to 6.0%. Mn content becomes like this. Preferably it is 3.5 to 5.5%.

Since P deteriorates hot workability and toughness, P content is limited to 0.05% or less. P content becomes like this. Preferably it is 0.03% or less. In addition, P is an element unavoidably contained in steel, and its content is so preferable that it is small. However, extremely reducing results in an increase in cost, and usually P is inevitably contained at about 0.005% or more.

Since S deteriorates hot workability, toughness and corrosion resistance, S content is limited to 0.003% or less. S content becomes like this. Preferably it is 0.001% or less. In addition, S is also an element inevitably contained in steel, and its content is so preferable that it is small. However, extremely reducing results in an increase in cost, and usually S is inevitably contained in an amount of 0.0001% or more.

Ni stabilizes austenite structure and improves corrosion resistance and toughness to various acids. For this reason, Ni is contained 1.0% or more. On the other hand, Ni is an expensive alloy and limits the Ni content to 3.0% or less from the viewpoint of cost. Ni content becomes like this. Preferably it is 1.5 to 2.5%.

Cr is contained 15.0% or more in order to ensure basic corrosion resistance. On the other hand, when Cr is contained exceeding 18.0%, toughness and corrosion resistance of a weld part will be inhibited. For this reason, content of Cr is made into 15.0% or more and 18.0% or less. Cr content becomes like this. Preferably it is 16 to 17%.

Mo is a very effective element in order to further increase the corrosion resistance of stainless steel, and is an optional component (optional component) to contain as needed. Since Mo is a very expensive element, when adding in order to improve corrosion resistance, the upper limit of Mo content is made into 1.0% or less from a cost point. When adding Mo, Mo content becomes like this. Preferably it is 0.1 to 0.5%.

Cu is an element which has the effect | action which improves corrosion resistance to the acid of stainless steel further, and improves toughness, and is an arbitrary component (optional component) to contain as needed. When Cu is contained exceeding 2.0%, (epsilon) will exceed (Solubility), (epsilon) Cu will precipitate and brittleness will generate | occur | produce. For this reason, when containing Cu, the upper limit of Cu content is made into 2.0%. Cu has an effect which stabilizes an austenite phase and improves toughness. When it contains Cu, Cu content becomes like this. Preferably it is 0.2 to 1.5%.

Ti is an element which forms oxides, nitrides and sulfides in a very small amount, and refines the solidification of steel and the crystal grains of the high temperature heating structure, and is an optional component (optional component) added as necessary. When Ti is contained in excess of 0.05%, a ferrite phase is generated, and TiN is produced to impair the toughness of the steel. For this reason, when Ti is contained, the upper limit of Ti content is set to 0.05%. When it contains Ti, Ti content becomes like this. Preferably it is 0.003 to 0.020%.

N is contained 0.01% or more in order to raise the intensity | strength of a martensite phase. However, when N is contained exceeding 0.05%, strength will become high too much and toughness will fall. For this reason, N content is restrict | limited to 0.05% or less. N content becomes like this. Preferably it is 0.01 to 0.04%.

Al is an important element for deoxidation of steel and is contained together with Si to reduce oxygen in the steel. Reduction of the amount of oxygen is essential for securing toughness, and for this purpose, it is necessary to contain Al of 0.001% or more. On the other hand, Al is an element that increases the ferrite phase, and when Al is added in excess, the toughness is inhibited. When Al exceeds 0.1%, toughness fall will become remarkable. For this reason, the upper limit of Al content is set to 0.1%. Al content becomes like this. Preferably it is 0.01 to 0.05%.

O constitutes an oxide which is representative of nonmetallic inclusions, and is an element inevitably contained in steel. Therefore, the smaller the O content is, the more preferable, but extremely reducing causes an increase in cost. For this reason, O is inevitably contained about 0.001% or more normally. On the other hand, when excessive amount of O is contained, toughness is inhibited. In addition, formation of coarse cluster-shaped oxides causes surface scratches. For this reason, the upper limit of O content is made into 0.005%.

The sum of the contents of C and N (C + N) is related to the strength of the steel. When the sum (C + N) of the contents of C and N exceeds 0.060%, the strength becomes excessively high, and the toughness is inhibited. For this reason, the upper limit of the sum (C + N) of content of C and N is made into 0.060%. The sum (C + N) of the contents of C and N is preferably 0.015 to 0.050%.

(Gamma) max shown in following formula (1) is a calculation formula which predicts the maximum value of the ratio of the austenite phase produced | generated in the temperature range of 900-1000 degreeC. By increasing the value of γ max, the toughness of the steel can be increased. In the case of this embodiment, when this value of (gamma) max is less than 80%, a ferrite phase will become large too much, and it will become impossible to ensure desired toughness by residual of a ferrite band structure. For this reason, gamma max is set to 80% or more. γmax is preferably 85% or more.

Here, C%, N%, Ni%, Cu%, Mn%, Cr%, Si%, and Al% represent the content (mass%) of each element.

(Gamma) pot shown in following formula (2) is a calculation formula which shows the ratio of the martensite phase in a casting state, and respond | corresponds also to the ratio of the austenite phase at the time of hot working. In this embodiment, the range of gamma pot is determined in order to ensure hot workability. When the gamma pot becomes high, the soft ferrite phase is excessively small, so that deformation is concentrated in the ferrite phase during hot working, thereby promoting cracking. The upper limit of the gamma pot depends on the amount of Mn, S and the like affecting hot workability. However, in the present embodiment, when the gamma pot exceeds 90%, the production yield of steel materials greatly decreases. For this reason, the upper limit of gamma pot is set to 90%. On the other hand, when the gamma pot is less than 60%, C and N are concentrated on the martensite produced in the welded portion, which becomes hard, resulting in an inhomogeneous structure. Moreover, since the corrosion resistance of the martensite phase in which alloying elements, such as C, N, Mn, were concentrated, falls, the minimum of gamma pot is set to 60%. γpot is preferably 65 to 85%.

Here, C%, N%, Ni%, Cu%, Mn%, Cr%, Si%, Al%, Mo%, and Ti% represent the content (mass%) of each element.

Next, the reason for limitation of the arbitrary component (optional component) in this embodiment is demonstrated. The element demonstrated below is an optional component (optional component) added as needed.

Nb is an element effective for the refinement | miniaturization of the crystal grain of a hot rolling structure. Moreover, Nb also has the effect | action which raises corrosion resistance. The nitride and carbide which Nb forms are produced | generated in the process of hot working and heat processing, and have the effect | action which suppresses growth of crystal grains and strengthens steel and steel materials. For this purpose, you may contain Nb 0.01% or more. On the other hand, when an excess amount of Nb is added, it precipitates as an unsolubilized precipitate at the time of heating before hot rolling, and inhibits toughness. For this reason, the upper limit of Nb content is set to 0.2%. When it contains Nb, Nb content becomes like this. Preferably it is 0.03 to 0.10%.

(Gamma) pot shown in following formula (3) is a calculation formula which shows the ratio of the martensite phase in the casting state at the time of containing Nb, and respond | corresponds also to the ratio of the austenite phase at the time of hot working. In the case of containing Nb,? Pot calculated by Equation 3 to which the term of Nb is added is made into 60 to 90% instead of Equation 2. Also when it contains Nb, (gamma) pot becomes like this. Preferably it is 65 to 85%.

Here, C%, N%, Mn%, Cu%, Ni%, Si%, Cr%, Mo%, Nb%, Ti%, and Al% represent the content (mass%) of each element.

V and W are elements added in order to further improve the corrosion resistance of two-phase stainless steel.

V may be contained 0.05% or more for the purpose of improving the corrosion resistance. However, when V is contained in excess of 0.5%, coarse V-based carbonitride is produced and toughness is inferior. Therefore, the upper limit of V content is limited to 0.5%. When it contains V, V content becomes like this. Preferably it is 0.1 to 0.3%.

W, like Mo, is an element that additionally improves the corrosion resistance of stainless steel. In this embodiment, in order to improve corrosion resistance, you may contain W in 1.0% or less of range. When it contains W, content of W becomes like this. Preferably it is 0.05 to 0.5%.

That is, you may contain any one or both of V and W of content prescribed | regulated above.

Co is an effective element for improving the toughness and corrosion resistance of the steel, and may be optionally added. The content of Co is preferably 0.03% or more. When Co is contained exceeding 1.0%, since it is an expensive element, the effect corresponding to cost will not be exhibited. For this reason, the upper limit of Co content is set to 1.0%. When it contains Co, Co content becomes like this. Preferably it is 0.03 to 0.5%.

Moreover, in order to improve hot workability, you may contain B, Ca, Mg, and REM as needed.

REM is a rare earth metal, and is selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. That's it.

B, Ca, Mg, and REM are all elements which improve the hot workability of steel, and may add 1 type, or 2 or more types for the purpose. When excess amount of B, Ca, Mg, or REM is added, hot workability and toughness fall, so the upper limit of the content is determined as follows. The upper limit of each content of B and Ca is 0.0050%. The upper limit of content of Mg is 0.0030%. The upper limit of content of REM is 0.10%. Each content of B and Ca becomes like this. Preferably it is 0.0005 to 0.0030%. Content of Mg becomes like this. Preferably it is 0.0001 to 0.0015%. Content of REM becomes like this. Preferably it is 0.005 to 0.05%. In addition, content of REM is made into the sum total of content of lanthanoid rare earth elements, such as La and Ce.

Next, the reason for limitation of the martensitic stainless steel of this embodiment is demonstrated.

The martensitic stainless steel of this embodiment has the composition of the martensitic stainless steel of this embodiment mentioned above, and has a metal structure which satisfy | fills the following requirements. By adjusting the phase rate of steel materials, the mechanical characteristics and strength of a base material can be ensured.

The ferrite phase is soft and contains a certain amount to suppress excessive increase in strength and finely control grains through the biphasic mixed structure. This realizes the improvement of the toughness of the martensitic stainless steel of this embodiment. For this purpose, the ferrite phase needs at least 5%. On the other hand, since the ferrite phase itself lacks toughness, including an excess amount of ferrite phase reduces the toughness of the martensitic stainless steel of the present embodiment. In order to prevent this, the ratio of a ferrite phase is made into 30% or less. The proportion of the ferrite phase is preferably 5 to 20%.

This ferrite phase ratio is realized through the production conditions of the steel material in addition to the chemical composition,? Max and? Pot. According to a chemical composition, the ferrite phase of the said range can be implement | achieved by selecting manufacturing conditions from the range of the manufacturing conditions of normal stainless steel materials. For example, as long as it is a rolling condition, the heating temperature of hot rolling may be selected from 1150-1250 degreeC. What is necessary is just to select the finishing temperature of hot rolling from 950-700 degreeC. In addition, what is necessary is just to select the hardening heat processing temperature from 850-950 degreeC when heat-processing as needed. What is necessary is just to select tempering heat processing temperature from 550-750 degreeC. Moreover, as for the crack time of hardening heat processing temperature, about 5 to 30 minutes are preferable. In addition, the cracking time of the tempering heat treatment temperature is preferably about 10 minutes to 1 hour.

In addition, the retained austenite phase is produced when the austenite phase present at high temperature remains unchanged. This residual austenite phase is soft and increases the toughness of the steel. On the other hand, when the residual austenite phase remains excessively, the yield strength of the steel is lowered, thereby impairing the strength characteristics of the martensitic stainless steel of the present embodiment. For this reason, the upper limit of the ratio of residual austenite phase is set to 20%.

In order to control the amount of residual austenite phase, it is necessary to control Ms value (degreeC) shown by following formula (4). The chemical component is made to be equal to or higher than 200 ° C. When the value of the expression (4) becomes less than 200, the residual austenite phase rate exceeds 20% of the upper limit defined in the present embodiment. In addition, since the residual austenite phase-rate may be 0%, it is not necessary to set the upper limit of Ms value (degreeC) shown by Formula (4). Within the composition range of this embodiment, Ms value can be set high within an allowable range. In addition, the ratio of residual austenite phase can be calculated | required by X-ray measurement. The amount of retained austenite phase is preferably 3 to 15%.

Here, Cr%, Mo%, Cu%, Ni%, Mn%, Si%, C%, N% represents content (mass%) of each element.

In addition, the remainder other than the ferrite phase and the retained austenite phase is a martensite phase, and the total ratio of the three phases is 100%.

It is preferable that the yield strength of the martensitic stainless steel of this embodiment is 400-800 Mpa.

This embodiment has high strength and excellent toughness with respect to martensitic stainless steel and steel materials mainly composed of martensitic phase structure. For this reason, if yield strength is less than 400 Mpa, the application value to the high strength structural member made into the objective of this embodiment is lacking. On the other hand, when it has a yield strength exceeding 800 Mpa, even if it controls a metal structure suitably, desired weld part toughness will no longer be secured. For this reason, it is preferable that the yield strength of the martensitic stainless steel of this embodiment is 400-800 Mpa.

Example

Below, an Example is described.

Tables 1-4 show the evaluation result of the chemical composition and joint characteristics of a test steel. These steels were manufactured by the following method. 50 kg of steel ingot was manufactured by vacuum melting in the laboratory, and this steel ingot was forged, and the rolling test piece of thickness 60mm x width 110mm x length 150mm was obtained. Then, the rolling test piece was hot-rolled and thickness was 12 mm.

The chemical composition of Tables 1-3 is the result of having collected and analyzed the test piece from this hot rolled sheet steel.

In addition, other than the component described in Tables 1-3, remainder is Fe and an unavoidable impurity element. In addition, in the components shown in Tables 1-3, content of the component in which content is not described shows that it is an impurity level. In addition, REM in a table means a lanthanoid rare earth element, and content of REM has shown the sum total of those elements. And steel number A-U is an example of this invention, V-AG is a comparative example.

Welding for evaluating joint characteristics was performed as follows.

The width center part of the steel plate was cut in the rolling length direction, and the end face was cut so as to improve the V shape. Subsequently, the joint was produced by welding of 2 passes under the heat input conditions of 3.5 kJ / mm using the welding rod and flux for submerged arc welding for SUS329J3L. In this welding part, the Charpy test piece in which the V-shaped improvement of 2 mm was formed was extract | collected in the position spaced 1 mm from the boundary of a weld metal and a heat affected zone to the heat affected zone. Two tests were performed at -20 캜. The average value of the obtained impact value was shown in Table 4 as impact value 1.

Evaluation of corrosion resistance was performed as follows.

A formula potential measurement sample including a weld metal and a heat affected zone was prepared. Subsequently, using a silver-silver chloride electrode (SSE) as a reference electrode, the formula potential Vc'100 was calculated | required according to JISG0577 in 3.5% NaCl of 30 degreeC. The results are shown in Table 4.

When the impact value 1 was 35 J / cm <2> (= 27 J) or more, it judged that it was favorable. Moreover, when formula potential Vc'100 was 0.10V or more which is the average formula potential level of the base material of SUS430 steel, it was judged that corrosion resistance was favorable.

As a result, it turned out that the steel which has the composition of this embodiment is all excellent in the impact value 1 and corrosion resistance. On the other hand, in the comparative example which has a composition outside the range of this embodiment, the impact value 1 and corrosion resistance are all inferior, and the superiority of the steel of this embodiment is clear.

In addition, Table 5-Table 8 show the manufacturing conditions of a steel material of this Example, hot workability, a metal structure, and the base material characteristics of steel materials.

The rolling test piece of thickness 60mm x width 110mm x length 150mm was heated at predetermined hot-rolling heating temperature, and was then rolled so that thickness might be 12 mm by several times reduction. It showed in Table 5 and Table 6 as the temperature under final press as a hot rolling finish temperature. The magnitude | size of the edge crack which generate | occur | produced in the edge part of the steel plate obtained by this hot rolling is measured, and it evaluates as good when the maximum edge crack is 5 mm or less, and when the maximum edge crack exceeds 5 mm, It evaluated as bad and was shown to the item "hot workability" of Table 5 and Table 6.

The metal structure was investigated by the following method about the obtained steel plate as it was, or the steel plate obtained by heat-treating either or both of a quenching heat treatment and a tempering heat treatment. The plate thickness cross section was etched to reveal the metal structure (micro structure). The metal structure was observed with the optical microscope, and the area ratio of the ferrite phase was calculated | required by image analysis. Moreover, the sample of the dimension of 3 mm x 23 mm x 23 mm which makes the 1/4 part of plate | board thickness a measurement surface was produced, and the residual austenite phase rate was quantified by the X-ray diffraction method. These results were shown to the item "metal structure" of Table 7, Table 8.

Next, the tensile test and the impact test were performed by the following method.

At a right angle to the rolling direction, a round bar tensile test piece having a round shape with a diameter of 10 mm and a length of 60 mm was taken. The tensile test was done using this test piece, and 0.2% yield strength was measured.

A JIS No. 4 full-size Charpy test piece was prepared in which a V improvement of 2 mm was formed. Using this test piece, each test was performed at -60 degreeC, and the impact value was measured. The average value of the obtained impact value was shown as the impact value 2.

When the yield strength was 400 MPa or more, the yield strength was higher than that of the austenitic stainless steel, and it was judged to be good. When the impact value was 35 J / cm 2 (= 27 J) or more, it was judged to be good. As a result, in the Example corresponding to this embodiment, it turns out that hot workability, base material strength, and toughness are all favorable. In addition, it is understood from the results of Examples 34 to 37 that the strength and toughness of the base material can be secured without performing quenching or tempering heat treatment. On the other hand, in the comparative example, hot workability was insufficient, or any of the base material yield strength and the impact value 2 did not show a desired value. From the results of Comparative Examples 39 and 40, even if the steel satisfies the requirements relating to the chemical composition of the present embodiment, if the manufacturing conditions are not suitable and the metal structure does not meet the requirements of the present embodiment, desired characteristics are obtained. It turns out that it does not show.

As can be seen from the above examples and comparative examples, the present embodiment obtains martensitic stainless steel excellent in the properties of the welded part, and also satisfies the requirements for the metal structure, thereby providing excellent martensite and excellent welded properties. It became clear that a sight stainless steel was obtained.

According to one embodiment of the present invention, it is possible to provide an martensitic stainless steel material having good welded portions and low Ni content, which is economical. As a result, it is possible to provide an inexpensive high strength steel that can be applied to a large structure. In addition, since the long-term heat treatment that has conventionally been necessary can be omitted, mass productivity can be improved, which can greatly contribute industrially.

Claims (7)

In mass%,
C: 0.003 to 0.03%,
Si: 0.01% to 1.0%,
Mn: 3.0 to 6.0%,
P: not more than 0.05%
S: 0.003% or less,
Ni: 1.0 to 3.0%,
Cr: 15.0 to 18.0%,
Mo: 0% to 1.0%,
Cu: 0% to 2.0%,
Ti: 0-0.05%,
N: 0.05% or less,
Al: 0.001 to 0.1%
O: 0.005% or less,
The balance being Fe and inevitable impurities,
The total amount of C and N is 0.060% or less, γmax represented by Equation 1 is 80 or more, and γpot represented by Equation 2 is 60 to 90. .
[Equation 1]

&Quot; (2) &quot;

Here, C%, N%, Ni%, Cu%, Mn%, Cr%, Si%, Al%, Mo%, and Ti% represent the content (mass%) of each element. The martensitic stainless steel according to claim 1, further comprising Nb, wherein? -Pot of 60 to 90, which is calculated by Equation 3 instead of Equation 2, is excellent in welded properties.
&Quot; (3) &quot;

Here, C%, N%, Mn%, Cu%, Ni%, Si%, Cr%, Mo%, Nb%, Ti%, and Al% represent the content (mass%) of each element. The martensitic stainless steel according to claim 1 or 2, further comprising any one or both of V and W. The martensitic stainless steel according to any one of claims 1 to 3, further comprising Co, which is excellent in the properties of the weld. The martensitic stainless steel according to any one of claims 1 to 4, further comprising one or two or more selected from B, Ca, Mg, and REM. It has a composition as described in any one of Claims 1-5, and
A martensitic stainless steel material comprising 5 to 30% of a ferrite phase and 0 to 20% of retained austenite phase, and having a residual portion composed of a martensite phase. The martensitic stainless steel of Claim 6 whose yield strength is 400-800 Mpa.