JP4012497B2 - High strength steel with excellent weld heat affected zone toughness and method for producing the same - Google Patents

Description

  The present invention has a high tensile strength of 550 MPa or more, specifically, the tensile strength of the steel sheet in the rolling direction is 550 MPa or more. As a structural material or a machine part, welding is mainly applied during assembly and construction. And high strength steels that require the same specifications as the base metal for their joint properties, such as shipbuilding, bridges, various steel materials for construction, and steel used in the manufacture of pressure-resistant storage containers used at temperatures below room temperature. .

  So-called low carbon steel having a carbon content of 0.3% or less is excellent in workability and weldability and is used in many structures. Buildings, vehicles, ships, industrial machines, etc. are composed of these low carbon steels to form a skeleton, an inner partition or an outer shell, and mainly bear the strength required by the structure. In “soft steel” with reduced C, technological development for increasing its strength as much as possible and reducing the weight of the structure has been actively conducted. Structures made of welded structures have gained the safety gained by increasing the specific strength (strength per mass) of steel, increasing the size, making the structure more complex, and increasing the strength.

  However, increasing the strength of low-carbon steel requires the addition of a large amount of alloying elements other than carbon, or complicates equipment for strictly controlling the crystal structure during steel production, resulting in higher strength or processing. Instead of gaining productivity, there has been a problem with an increase in productivity or production cost. In particular, in recent years, attempts have been made to shorten the welding process, which is unavoidable at the time of manufacturing a structure, as much as possible, and development of a technique for increasing welding heat input has been promoted. As a result, the heat input during welding often exceeds 50,000 J / cm, with some being welded at a heat input exceeding 100,000 J / cm for some buildings and 1 million J / cm for buildings. Yes. In the case of such high welding heat input, the material to be welded is greatly affected by heat, exposed to a temperature as high as 1400 ° C. in the immediate vicinity of the molten metal, and exposed to a temperature of about 720 ° C. or more, which is the A1 transformation point of steel. The width of the “welding heat affected zone” becomes wider. As a result, the structure of the low carbon steel whose structure has been strictly controlled is lost due to the influence of the heat of welding, and has to be an uncontrollable structure determined by the subsequent cooling rate of the joint. Even if a heat treatment at an annealing level for removing residual stress is performed after welding, re-heating above the transformation point is not performed, so that it is difficult to subsequently change the structure thus formed.

  Even in such heat-affected zone, the structure is required to maintain the same characteristics as the non-heat-affected zone as much as possible. The technology for this will be studied.

  Patent Document 1, Patent Document 2 and the like describe techniques related to steel materials using nitrides and oxides that are difficult to decompose at high temperatures in order to prevent an increase in the crystal structure, particularly the old γ grain size in the vicinity of the weld bond. However, even if these techniques are applied to the high-tensile steels targeted in the present invention, it is difficult to reproduce the form at the time of manufacturing the crystal structure performed in order to express the strength of the material. Even when toughness can be ensured, the problem that it is difficult to ensure the strength in the heat-affected zone has been unsolved.

  On the other hand, a method of adding Ni, Cr, Mo or the like to improve the hardenability of the steel material and ensure the strength is naturally appropriate as a concept of alloy design. However, Ni and Mo are expensive elements, and it is not practical to add them in large amounts to structural steel, for example, exceeding 5%. When the amount of addition is limited because the cost increase is hated, the effect is small and the increase in cost becomes a problem, so it is difficult to be a practical solution. In addition, Cr tends to cause precipitation embrittlement and loses toughness in exchange for an increase in strength. This is the same even when a large amount of elements such as Nb, V, and Ti are added, and when trying to obtain the characteristics of the heat-affected zone of high-strength steel in a well-balanced state, it was almost industrially clogged. .

  On the other hand, although the mechanism is unknown, many techniques have been developed with a focus on heat-resistant steel as a technique for improving the strength of the material by adding W. Patent Document 3, Patent Document 4, and Patent Document 5 describe a steel material containing 0.01 to 3.5% of W for the purpose of improving its creep rupture strength in a heat resistant steel containing 0.8% or more of Cr. There is a description. However, these are all intended to improve the creep characteristics, and for the compatibility of strength and toughness in the weld heat affected zone, the use temperature of the heat-resistant steel is at least 400 ° C., so there is almost no requirement for toughness. However, even if there are cracks during construction or damage during hydraulic testing, the welding conditions with high heat input are high-temperature and high-pressure power plants or petrochemical plants that are originally made of heat-resistant steel. In view of the brittleness of welded joints, this is not adopted at all. Therefore, the addition of W is not for controlling the characteristics of the heat affected zone of large heat input, and the existence form in the steel is naturally different. The characteristics of the heat-affected zone of the high heat input welding to be performed are not taken into account, and due to its chemical composition, toughness may inevitably decrease significantly when high heat input welding is applied. It was customary.

  In addition, there is an example in which the technique of adding W to a structural material used at room temperature or lower is applied to improve the characteristics of other steel materials. Patent Document 6 discloses a method for producing a thick steel plate in which the crystal grain size is finely controlled, and describes a steel material to which W is added in a range of 2.0% or less. However, in this case, since W is added for the purpose of improving the hardenability of the material, there is no description of the precipitation ratio, and therefore, no technique has been found for using the solid solution strengthening effectively. In Patent Document 7, there is a description of a technique related to a method of controlling the temperature distribution in the width direction of the steel sheet to uniformly control the in-plane crystal grains throughout the steel sheet, but also in this case, the addition of W improves the hardenability. There is no description of a technique for limiting the amount of precipitation for the purpose. That is, there is no description of a technique that actively uses the solid solution strengthening of W. Similarly, in Patent Document 8, although there is a description regarding a steel sheet manufacturing method and a steel sheet aiming at scale uniformity on the surface of the steel sheet, there is no knowledge about W precipitation control in the same way as the above technique, and solid solution strengthening is active. Usage is not considered.

  In Patent Document 9 and Patent Document 10, there is a description relating to a technique for improving the fatigue strength of the weld heat-affected zone, and there is a description in which 0.01 to 2.0% of W is added to act by precipitation strengthening or solid solution strengthening. . However, there is no mention of the precipitation ratio, and there is no knowledge of precipitation as an intermetallic compound, and the aim is to improve the strength of the steel by simply adding W, and naturally the amount of precipitation is controlled. If these techniques are not used, the strength improvement effect with little toughness reduction due to the solid solution W, which is the effect of the present invention, cannot be obtained. Therefore, even with the above prior art, a technique for ensuring the same strength and toughness characteristics as the base material in the heat-affected zone even during high heat input welding in a high strength steel having a strength of 550 MP or more cannot be realized. The problem was left.

Japanese Patent Publication No.57-19744 Japanese Patent No. 3256118 Japanese Patent Laid-Open No. 10-46290 Japanese Patent Laid-Open No. 8-225884 JP-A-9-217146 Japanese Patent No. 2633743 JP-A-4-350119 Japanese Patent Laid-Open No. 9-271806 JP 7-331382 A Japanese Patent Laid-Open No. 2003-3229

  The present invention has a problem with conventional high-strength steel, that is, when it is difficult to simultaneously increase the strength and toughness of the weld heat-affected zone of low-carbon steel, the industrial productivity is high, and the amount of alloy addition is low in cost. We provide new steel materials that enable alloy design under conditions that do not have a significant impact.

The present invention is essential to ensure the toughness of the weld bond in the high heat input welding in which the heat input exceeds 50,000 J / cm in the above-described problems of the conventional steel, that is, the high strength steel having a tensile strength of 520 MPa or more. The joints described above are made to give strength equal to or higher than that of the base material, and the gist thereof is as follows.
(1) By mass%, C: 0.001 to 0.05%, Si: 0.01 to 0.50%, Mn: 0.10 to 3.0%, W: 0.10 to 1.0% And further limited to P ≦ 0.03%, S ≦ 0.02%, O ≦ 0.01%, Nb: 0.005 to 0.017%, V: 0.005 to 0.045 %, Ti: 0.005 to 0.041%, Zr: 0.005 to 0.1%, or a steel composition comprising the balance of Fe and unavoidable impurities, and welding. The amount of precipitated W is sometimes 1% or less of the added W amount in the heat-affected zone that is sometimes exposed to the Ac1 point or higher of the steel material, and at the same time, the amount of precipitated W in the base material other than the heat-affected zone is 10% or less of the added W amount, Further, the LP value represented by the following formula is 2.5 or less in order to suppress precipitation of the intermetallic compound Laves phase containing W. A high-strength steel excellent in weld heat-affected zone toughness with a tensile strength of 550 MPa or more.
LP = 3 × (% Si) + (% W) + 2 × (% Cr) + 0.5 × (% Mo)
(2) Moreover, in mass% Mo: characterized in that it contains 0.01% to 1.0%, the (1) high with excellent tensile strength 550MPa or more weld heat-affected zone toughness according to Tensile steel.
(3) Moreover, in mass%, Ni: 0.01~5.0%, Cu : 0.01~1.0%, Cr: 0.10~1.0%, B: 0.0003~0. A high-tensile steel excellent in weld heat-affected zone toughness having a tensile strength of 550 MPa or more as described in (1) or (2) above, comprising 005% of one kind or two or more kinds.
(4) Further, by mass%, Ca: 0.0003 to 0.005%, Mg: 0.0003 to
0.005%, Ba: 0.0003 to 0.005%, Y: 0.0005 to 0.10%, C
Any one of the above (1) to (3), characterized by containing one or more of e: 0.0005 to 0.10% and La: 0.0005 to 0.10%. A high-strength steel excellent in weld heat-affected zone toughness with a tensile strength of 550 MPa or more.
(5) Moreover, in mass% Al: 0.002 to 0.20%, Ta: characterized by containing one or two of 0.002 to 0.20%, the above (1) (4) A high-tensile steel excellent in weld heat-affected zone toughness having a tensile strength of 550 MPa or more.
(6) The ratio of the intermetallic compound Fe ₂ W type Laves phase in the heat-affected zone exposed to the Ac1 point or higher of the steel material during welding is 1% or less as the precipitated W amount with respect to the added W amount, High tensile steel excellent in weld heat-affected zone toughness having a tensile strength of 550 MPa or more according to any one of (1) to (5) above.
(7) The segregation degree of W obtained by dividing the maximum value of the segregation W concentration analysis value in the steel material by characteristic X-ray measurement by the amount of addition of W is 1.5 or less, (1) to (1) above High tensile steel excellent in weld heat-affected zone toughness having a tensile strength of 550 MPa or more according to any one of (6).
(8) The steel having the steel composition described in any one of (1) to (7) above is subjected to a cooling step after hot working such as hot rolling or forging, or normalizing, tempering, In the cooling step after heat treatment such as annealing, the heat effect exposed to the Ac1 point or higher of the steel material during welding is characterized by limiting the holding time or passing time in the temperature range of 400 to 700 ° C. to within 30 hours. The amount of precipitated W in the part is 1% or less of the amount of added W, and at the same time the amount of precipitated W in the base material other than the heat-affected zone is 10% or less of the amount of added W. Furthermore, the LP value represented by the following formula is A method for producing a high-strength steel excellent in weld heat-affected zone toughness having a tensile strength of 550 MPa or more, which is 2.5 or less in order to suppress precipitation of an intermetallic compound Laves phase containing W.
LP = 3 × (% Si) + (% W) + 2 × (% Cr) + 0.5 × (% Mo)

  By applying the present invention, a high strength steel material having a small alloy addition amount and a tensile strength of 520 MPa or more is required to produce a structure having high joint toughness and strength even when welding with high heat input is used. It can be supplied at low cost.

  First, the reason why the mass proportions of various constituent elements of the steel described in the present invention are determined as described in the claims will be described in detail below.

  C: It is in steel and accumulates in the vicinity of dislocations, controls the material deformation behavior, and plays the role of facilitating the transition of the crystal lattice form from a simple BCC structure to a metastable structure such as BCT. It forms carbides with various transition elements and contributes to strengthening. In the present invention, at least 0.001% is necessary to express these factors, and when added in excess of 0.05%, W partially precipitates as carbides, and the W precipitation ratio of the steel of the present invention, the base material 10% or less and 1% or less cannot be maintained in the weld heat affected zone, so the maximum addition amount was set to 0.05%. However, as described above, the presence of C is preferably less for the purpose of keeping W in a solid solution state and mainly using it as a solid solution strengthening element. In the range of 0.001 to 0.05%, it is preferably less than 0.04%, and more preferably less than 0.015%, in order to completely eliminate the precipitation of W as a carbide.

  Si: useful as a deoxidizing element during refining of steel materials, and at the same time, when dissolved in steel, contributes to solid solution strengthening or improvement of hardenability, and is useful for increasing the strength of the material and controlling the structure. The minimum amount required for deoxidation is specified as 0.01%, and addition of 0.5% or more may significantly impair the toughness of the steel material, so its component range is set to 0.01 to 0.5%. .

  Mn: Controls deoxidation similarly to Si, and contributes to improving the hardenability of the material in a solid solution state. If it is less than 0.10%, it is insufficient for deoxidation, and if added over 3.0%, the material may be significantly embrittled at the segregation site. %.

W: An essential element that is essential in the present invention and that dominates the material characteristics. Mainly in steel, it is in a solid solution state and gives large strain to the crystal lattice. It has been clarified by detailed studies by the present inventors that the solid solution strengthening due to this is large and the toughness is not greatly reduced. The component range is that the minimum amount required for strengthening is 0.10%, and if added over 1.0%, an Fe ₂ W type intermetallic compound is formed and the material strengthening ability is reduced, and at the same time large The presence of the intermetallic compound may cause embrittlement, so the range of addition was limited to 0.10 to 1.0%. However, the effect of the present invention cannot be sufficiently obtained simply by taking W as a component of steel and adding it.

  W exhibits the above-mentioned effect only in a solid solution state. In the case of precipitation, not only strengthening corresponding to the added W is not realized, but also embrittlement may occur as described above. It can be seen that when the amount of precipitated W exceeds 1% of the amount of added W in the heat-affected zone exposed to the Ac1 point or more of the steel during welding, the absorbed energy may be less than 47 J, as shown in FIG. If the amount of precipitated W in the base material other than the heat-affected zone exceeds 10% of the amount of added W, the solid solution strengthening cannot be maintained and the strength does not reach 550 MPa. Therefore, in the present invention, the precipitation W amount is 1% or less of the added W amount in the heat-affected zone (hereinafter referred to as “welding heat-affected zone”) exposed to the Ac1 point or more of the steel material during welding. At the same time, the amount of precipitated W was 10% or less of the amount of added W in the base material other than the heat-affected zone.

  The precipitation forms of W include carbides containing W or Laves phase type intermetallic compounds, both of which reduce toughness. The precipitation temperature range of both is 700 degrees C or less, and it precipitates when it is kept at a high temperature of 400 degrees C or more for a long time. It was experimentally confirmed that precipitation started at about 700 ° C. for 30 hours and at about 400 ° C. for about 10,000 hours. These precipitates are unstable at a high temperature of 700 ° C. or higher, and when added within the scope of the present invention, as will be described later, if the degree of segregation is suppressed to 1.5 or less, all of them are dissolved and dissolved. As a result, it became clear. Accordingly, in a normal manufacturing process, if the steel material is kept in a temperature range of 400 to 700 ° C. for more than 30 hours in a heat treatment before making the steel product into a final product, for example, quenching, normalizing, tempering, annealing, etc., In both the material part and the heat-affected part, W is deposited exceeding the limit of the present invention. When the steel plate is thin or when the steel ingot is small, it is not held for 30 hours or more in the temperature range of 400 to 700 ° C. However, such a thermal history may be taken with a thick plate or a large steel ingot. . In the steel according to the present invention, it is necessary to strictly limit the passage time or the holding time in the above temperature range, including this case, including an additive case (composite heat treatment such as quenching and tempering or repeated tempering treatment).

In a welded joint, when the structure is controlled to sufficiently secure the toughness of the material, the material strength often decreases. In such a case, the addition of W is effective, and when the strength does not reach 550 MPa only by the structure control, the strength can be secured by the addition of W. However, in this case as well, if the cooling is delayed to prevent hot cracking of the material after welding, or if stress relief annealing for reducing the residual stress of the joint is performed for a long time, the temperature range is maintained and It is necessary to pay attention to the passage time and control it to 30 hours or less in total. In the case of a base metal near the weld heat affected zone that is not reheated to 700 ° C. during welding, in addition to the tempering process in the manufacturing process, the effect of welding heat and subsequent stress relief annealing are added, and therefore this time limit is exceeded. There are many. Therefore, until the material is completed as a structure, it is necessary to strictly limit the temperature range and the passage time. Claim 6 is an invention made on the basis of the findings of such research results. In the temperature range of 700 ° C. to 400 ° C., W precipitates exclusively (approximately 90% or more) as an Fe ₂ W type Laves phase in the case of the steel of the present invention. It tends to precipitate coarsely mainly at grain boundaries. The welding heat-affected zone has a part where the crystal grains are particularly coarsened, and the toughness is lowered. In order to prevent this decrease, the proportion of precipitation of W, particularly the precipitation of Fe ₂ W occupying substantially all of the precipitation of W in the heat affected zone is 1% of the amount of added W as the amount of precipitation W. It was confirmed experimentally that it should not be exceeded. Note that almost all of the Fe ₂ W type Laves phase is dissolved again and decomposes at 700 ° C. or higher. Therefore, in the welding heat-affected zone that is reheated above the Ac1 transformation point (around 720 ° C.), when exposed to 400 to 700 ° C. for a long time in the subsequent heating or cooling step, the entire amount precipitates at grain boundaries, making it more tough. It was found to have a bad influence. Therefore, the ratio of the intermetallic compound Fe ₂ W type Laves phase is limited to 1% or less with respect to the added W amount as the precipitated W amount.

Even when the above W precipitation suppression and segregation suppression techniques are used, a large amount of Si, Cr, and Mo exists in the steel, and the precipitation promotion parameter LP value of the intermetallic compound Laves phase represented by the following formula is 2.5. It exceeded the solubility product as a Laves phase of W in steel, and it was experimentally confirmed that precipitation of intermetallic compounds may occur.
LP = 3 × (% Si) + (% W) + 2 × (% Cr) + 0.5 × (% Mo)

This parameter is obtained by melting steel in which various elements described in the claims of the present invention other than W and Si are added in a steel by using a high frequency induction vacuum heating furnace in a laboratory to form an ingot, followed by heat. In the weld heat affected zone of the joint obtained by multiple passes by TIG welding or SMAW welding under the condition of the amount of Fe ₂ W precipitated in the steel material and the heat input of 50,000 J / cm or more. The amount of Fe ₂ W present in the steel is quantified by chemical analysis, such as by spectrophotometry, after the substrate is dissolved and removed by constant-potential electrolysis in an organic acid solution and the remaining steel precipitate is collected by a filter. When the LP value exceeds 2.5, Fe ₂ W precipitation in the steel material increases, and as a result, the W precipitation amount exceeds 10% in the base metal or 1% in the weld heat affected zone, and the strength becomes 550 MPa. The toughness of the joint Not obtain toughness being determined, for example, in the Charpy impact test, it can not be obtained 47J at the use temperature of the structure was found experimentally. The coefficients related to the mass ratio of each constituent element in the LP value formula are all approximate values when the experimental results are arranged with respect to the presence or absence of precipitation of the Laves phase, assuming a one-dimensional addition rule using the least square method as an empirical formula. It is. FIG. 1 is a graph showing the relationship between the parameter LP and the amount of precipitation of the Laves phase. Apparently, the relationship between the LP value on the horizontal axis and the amount of precipitation of the Laves phase greatly changes with the threshold value. The precipitation of the Laves phase cannot be restricted unless the LP value is controlled to 2.5 or less. FIG. 2 shows the Charpy absorbed energy at 0 ° C. of the heat-affected zone of the steel of the present invention having a precipitation ratio of W including the Laves phase (total precipitation of W / amount of added W) × 100% and strength of 600 to 660 MPa. The relationship was shown. When the precipitation ratio in the weld heat affected zone of W exceeds 1%, it can be seen that the absorbed energy may be less than 47J.

In the steel of the present invention, impurities P, S, and O have an effect of impairing the toughness of the base metal as well as the weld heat-affected zone. Irrespective of this, toughness deterioration may occur. In order to prevent this, P was strictly limited to less than 0.03%, S was less than 0.02%, and O was less than 0.01%.
Further, the following selective elements can be added to increase the strength of the steel material, and the structure can be controlled to enhance the effect of the present invention. That is, one or more of Nb, Ti, V, Zr precipitation strengthening and non-recrystallization temperature control elements are controlled and added.

The above is the core technology of the present invention, but the following selective elements can be further added to increase the strength of the steel material, and the structure can be controlled to further enhance the effects of the present invention. That is , Mo , which is a precipitation-strengthening / non-recrystallization temperature control element, or one or more of Ni, Cu , Cr, B hardenability improving elements, or further Ca, Mg, Ba, Y, Ce, La sulfide shape control or a fine oxide dispersion state control elements one or more of, in addition further Al, 2 primary deoxidizer or carbides of Ta, one of a nitride forming element or two or more in addition is there. The reason for limiting the addition range of these selective elements will be described below.

Nb combines with nitrogen and carbon and precipitates as NbC or NbN, thereby strengthening the material by precipitation. It is known that if the precipitation strengthening is appropriately controlled, the strength can be improved without greatly reducing the toughness. The added amount is effective from 0.005%, and if added over 0.017 %, the precipitate becomes coarse and the material becomes brittle, so the range of addition is limited to 0.005 to 0.017 %. did.

V also contributes to precipitation strengthening like Nb. Although the amount of precipitation is not as large as Nb, it is known that it precipitates together with Nb, and further precipitation strengthening can be expected by composite addition. If 0.005% or more is not added, the effect is not manifested, and if it is added over 0.045 %, it precipitates as coarse carbonitride, resulting in embrittlement of the steel material. 005% to 0.045 %.

Ti has a particularly high affinity with N and mainly becomes TiN to fix nitrogen. TiN itself is relatively coarse and hardly contributes to precipitation strengthening, but when N content is low, it contributes to precipitation strengthening as TiC. When precipitation of TiN is promoted, the effect of adding B described later is promoted, so it may be added for effective utilization of B. The effect is manifested from 0.005%, and if it is 0.041 % or more, coarse carbonitride may be produced and the material may be deteriorated, so the addition range was limited to 0.005 to 0.041 %.

  Zr has the same effect as Ti and has the highest affinity with N. However, it is coarse as a carbide and needs to be added in consideration of the amount of carbon and nitrogen when used as a carbide or nitride. is there. The effect is manifested from 0.005% and causes significant embrittlement at 0.1% or more, so the addition range was limited to 0.005% to 0.1%.

Mo is a hardenability improving element, but at the same time has a high solid solution strengthening ability, and precipitates as Mo ₂ C, showing a strong secondary hardening phenomenon. If 0.01% or more is not added, precipitation strengthening is not observed, and if it exceeds 1.0%, the strengthening becomes high and the material becomes brittle, so the range of addition is limited to 0.01 to 1.0%. did. Note that the precipitation force of Mo with C is extremely high and it is difficult to control the precipitation, so that an effect equivalent to that of W cannot be obtained. Therefore, in the present invention, it is only added as a selective element.

  Ni contributes to increasing the toughness of the base material and is often used to improve the toughness of the material. In addition, there are significant advantages such as an increase in strength due to an improvement in hardenability due to a lower transformation point. On the other hand, however, the addition of a large amount cannot be used positively because the element is expensive, and is harmful from the viewpoint of the formation of retained austenite at the time of non-diffusion transformation. The effect of addition is manifested from 0.01%, and there is a concern that addition of 5.0% or more may change the crystal lattice structure of the steel material from BCC to FCC. Limited to 5.0%.

  Cu is an austenite-forming element and contributes to the hardenability of the material by slightly dissolving in the austenite phase, and contributes to precipitation strengthening in the form of epsilon Cu in the ferrite phase. Since excessive addition induces precipitation of the grain boundary film-like Cu, the limiting content in the present invention is defined as 0.01% at which the effect is exhibited and 1.0% at which the grain boundary precipitation does not occur frequently.

  Cr is an element that is effective in improving the hardenability and is also effective in the corrosion resistance of the material. However, it is easy to generate a hard second phase such as island martensite and the strength can be improved, but the toughness is easily lost. Therefore, at least 0.10% is necessary from the viewpoint of improving hardenability, and the addition amount is limited to 1.0% from the viewpoint of maintaining toughness.

  B is an element that has the effect of suppressing nucleation of crystal grains from the grain boundary, and has a strong tendency to segregate to the grain boundary even with a very small amount, so that high hardenability can be obtained by addition of a small amount. The effect is already manifested from 0.0003%, and if added over 0.005%, grain boundary embrittlement or embrittlement due to boride precipitation occurs, so the range of addition is 0.0003% to 0.005%. Restricted to. In order to further enhance the effect of B, the aforementioned simultaneous addition of Ti is effective.

  Ca is useful for detoxifying MnS as a sulfide form control element, and as a result, contributes to improving the toughness of the material. If 0.0003% or more is not added, there is no effect, and if adding over 0.005%, the affinity with oxygen is high, and thus CaO clusters are formed, and the toughness is lowered. Therefore, the addition range is limited to 0.0003% to 0.005%.

  Similarly, Mg has a sulfide form control function. Further, it has a high deoxidizing power, and when it is finely dispersed in the steel as an oxide or oxysulfide, it functions as a grain boundary movement obstacle and has the effect of improving the toughness of the weld heat affected zone. The effect is manifested from 0.0003%. Addition of 0.005% or more causes a strong deoxidizing power to wear out the refractory in the steel making process, and raises the concentration of impurities in the steel. It was limited to 0003% to 0.005%.

  Ba has a deoxidizing power that surpasses Ca and Mg, and at the same time has a sulfide morphology control ability. It is effective for improving the toughness of the material from 0.0003%. Addition of 0.005% or more remarkably accelerates the wear of the refractory, so the addition range is limited to 0.0003% to 0.005%.

  Y, Ce, and La, which are known as rare earth elements, all have sulfide form control ability. These sulfide form control effects are manifested from 0.0005%, and addition exceeding 0.10% tends to induce clogging of the immersion nozzle at the time of casting in the steel making process. The addition range was limited to 0.0005% to 0.10%.

  It is inevitable to solve the problem of segregation of W in order to be able to strengthen the material by solid solution W, which is the greatest feature of the present invention, in an industrially stable manner. W has a large solid-liquid distribution ratio when the molten iron solidifies, and therefore microsegregation in the solidified structure becomes normal. As a result of research, it has been clarified that segregation is stronger than Mn having a high degree of segregation, and it may be 1.6 to 1.7 on average and 2.0 depending on the solidification conditions. This suggests that when a maximum of 1.0% W is added, a 2.0% W concentrated portion may be partially formed.

  It is easy to analyze such microsegregation, and it is generally possible today to create an X-ray intensity distribution map under observation with an electron microscope using a characteristic X-ray analyzer equipped with an image processing apparatus. Therefore, it is possible to reduce a partial variation in the W concentration by reducing the amount of segregation, for example, by performing homogenization annealing in the steel material manufacturing process. In the present invention, excessive addition of W induces the precipitation, so the addition amount is limited to a maximum of 1.0%. In this case, the W precipitation ratio described in claim 1 is used as an intermetallic compound or carbide. As a condition for not exceeding, it is possible to quantitatively confirm that the segregation of W is reduced by measurement using the characteristic X-ray analyzer, and to maintain the quality of the steel material. Here, as an index for quantitative determination, the degree of segregation = (maximum value of segregation W concentration analysis value in steel by characteristic X-ray measurement) / (W addition amount) is introduced, and this value is 1.5 or less. In this case, it was experimentally confirmed that W did not precipitate more than the value described in claim 1 in either the base material part or the heat-affected part, and it was set as a condition to be included in the present invention. The achievement means for setting the segregation degree of W to 1.5 or less may be any means such as heat treatment or high-temperature processing. FIG. 3 shows the relationship between the segregation degree and the base material toughness (Charpy absorbed energy) at 0 ° C. When the segregation degree exceeded 1.5, it was confirmed by wet chemical analysis of the potentiostatic electrolytic extraction residue that the W precipitation ratio exceeded 10% in the base material. When the segregation degree exceeds 1.5, the base metal toughness does not reach 47J.

  In the present invention, there are no particular restrictions on the steel making process. Slabs or ingots may be obtained by continuous casting, ingot casting, etc. through an integrated blast furnace integrated process such as normal blast furnace-converter-secondary refining, and scrap is produced by a reduction melting furnace using an electric furnace or other heat source. It is also possible to obtain an ingot as a raw material. The slab or ingot can be further subjected to homogeneous heating treatment after reheating to reduce segregation, and then hot rolled or hot forged to form a steel plate, steel bar, steel pipe, wire, and the present invention. Is effective as a structural material, and does not hinder the effects of the invention. Ordinary iron making processes can be applied to refractories and atmospheres used in the process up to ingot making, and it is not necessary to introduce special equipment or processes. When it is necessary to control the cleanliness or the aggregated particle size as the characteristics of the steel that is the final product, the ESR method, plasma melting, unidirectional solidification method, strip melting refining method, ie zone Melting or the like can also be applied and does not hinder the effects of the present invention. Moreover, there is no restriction | limiting also in the size or weight in intermediate products, such as an ingot or a slab, and manufacture in a unit of several hundred tons to a gram is also possible.

  However, utmost care is required to control the amount of precipitated W in steel, and the segregation degree is set to 1.5 or less, and in the cooling process after hot working such as hot rolling or forging, or normalization, Even if it is discontinuous in the temperature range of 400 to 700 ° C. in the cooling step after heat treatment such as reversion and annealing, holding for more than 30 hours may not be effective because the effects of the present invention may not be sufficiently exerted. .

  The high-tensile steel of the present invention can produce a structure with high joint toughness and strength even when welding with high heat input is used as described above. It is preferable to use for high heat input welding exceeding J / cm.

  The steel according to claim 1 to 10 through a normal blast furnace-converter-secondary refining-continuous casting-hot rolling-heat treatment process, or a steel plate having a thickness of 6-120 mm, or separately melted in an electric furnace-secondary Steel plate test pieces were similarly prepared through refining, ingot casting, hot forging, hot rolling, and heat treatment steps. The steel ingot weight was from 2 ton to 300 ton, the steel plate was 6 to 12 m long and 2 to 4 m wide and the weight was 1.8 to 12 ton. FIG. 4 is a diagram showing a sampling procedure of the test piece. From steel plate 1 as a steel plate test piece, No. 4 impact test piece 2 mmV grooved 2 specified in JIS Z 2201 from plate thickness 1 / 4t position 5 and plate thickness 1 / 2t position 4 at the center in the width direction of the steel plate. Various test pieces such as No. 4 tensile test piece 3 defined in JIS Z 2201 were collected and used as representative values of steel plate characteristics.

  Various heat treatments were applied to the prototype steel sheet. Those that are hot-rolled, those that are subjected to tempering treatment at an Ac1 point or less, those that carry out quenching and tempering treatments, those that carry out normalizing treatment, those that carry out tempering a plurality of times, tempering Further, cold working was performed later, and stress relief annealing was also evaluated.

  Evaluation was made by using the No. 4 tensile test piece specified in JIS Z 2201 collected by the above-mentioned method, the same as the No. 4 impact test piece with 2 mmV notch, and the Charpy absorption energy transition curve. It was observed after performing the structure appearing corrosion with an optical microscope in a cross section parallel to the surface. In addition, the segregation degree of W was determined by performing line analysis with an EPMA (Electron Probe Micro Analyzer) having a focused beam diameter of 10 μm in the width direction of the steel sheet at the 1/2 thickness position of the steel sheet, measuring the maximum concentration value, and adding W The ratio with the amount was calculated and evaluated. Furthermore, the holding or passing time at 400 to 700 ° C. in the heat treatment in the production process was integrated and recorded. In addition, the W compound present in the material is extracted from the residue by constant-potential electrolysis of the base material using an organic acid, the presence form is analyzed by X-ray diffraction, and the amount of W is analyzed by wet analysis. The amount of precipitation was calculated based on the ratio.

Tables 1 and 2 show the ratio of the chemical composition, tensile strength, and amount of precipitated W in the base metal to the amount of added W (base metal W precipitation ratio) of 100,000 J / cm. The ratio of the precipitation W amount in the weld heat-affected zone to the amount of added W when welding the V groove at (welding heat-affected zone W precipitation ratio), the intermetallic compound Fe ₂ W type Laves phase added as the precipitation W amount W The ratio to the amount and the W segregation degree measured by the above method are shown. Here, the welding heat-affected zone means a heat-affected zone that is exposed to at least the Ac1 point of the steel material during welding. In addition, the Fe ₂ W precipitation control parameter LP value and the absorbed energy of the base material and the weld bond at 0 ° C. are shown. When considering the steel of the present invention as structural steel, the minimum required toughness value is usually required to be 47 J or more as the absorbed energy at 0 ° C. Therefore, this was used for evaluation as a threshold value. . That is, it was judged that a material that could not exhibit absorbed energy of 47 J or less did not achieve the purpose of the steel of the present invention. As described above, the effect of W for improving the tensile strength is exhibited regardless of the material strength, but it goes without saying that the material is particularly industrially significant for a material whose strength is to be improved. Therefore, assuming the case where it is used for high-tensile steel, a threshold value is also provided for this, and the material of the present invention is a material that exhibits the above-mentioned 550 MPa or more. That is, the steel of the present invention is limited to steel having strength of 550 MPa or more and containing W and having material characteristics of absorbed energy at 0 ° C. of 47 J or more.

Tables 3 and 4 show the evaluation results of the comparative steels. In No. 50 steel, since C greatly exceeded the upper limit of 0.05, the added W was partially precipitated (W, Mo, V) C, and the toughness of the base metal and the weld heat-affected zone deteriorated. No. 53 was an example in which the steel itself was brittle and the toughness could not be secured due to the addition of Si, No. 52 was an example in which the amount of added W was small and the tensile strength at room temperature could not be ensured at 550 MPa, No. 53 The amount of added W exceeds 1.0% in steel, and the amount of precipitated W in the base metal exceeds 10.0% of the added amount, so the intermetallic compound Laves phase precipitates at the grain boundaries of the structure. However, as the material becomes brittle, the solid solution W in the vicinity of the grain boundary decreases, and the strength also decreases. In the Nos. 54 to 57 steels, Nb, V, Ti, and Zr, which are selective elements, all have upper limits. Since it was added in excess of the value, all carbonitrides were excessively precipitated and became brittle and tough. In the reduced example, No. 58 steel added excessive B, grain boundary embrittlement and coarse precipitation of borides were observed. As a result, the material became brittle, No. 59 steel is claimed in the claims of the present invention. Although there is no deviation regarding the amount of each element added from each chemical component range, the parameter that promotes precipitation of the Fe ₂ W-type Laves phase defined by the LP value exceeds 2.5, resulting in deterioration of toughness. Example, No. 60 steel is an example in which the sum of the time kept in the temperature range of 400 to 700 ° C. in the manufacturing process exceeds 30 hours, so that the precipitation of the intermetallic compound Laves phase similarly increases and the material becomes brittle. No. 61 steel is an example in which the segregation degree of W has exceeded 1.5, and therefore, a portion with a large amount of W precipitation locally occurs and the material becomes brittle.

It is a figure which shows the relationship between LP value and the amount of Laves phase precipitation. It is a figure which shows the relationship between the W precipitation rate in a welding heat affected zone, and the toughness in 0 degreeC. It is a figure which shows the relationship between the segregation degree of W in steel, and the toughness of the base material in 0 degreeC. It is a figure which shows the test piece collection point.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel plate 2 No. 4 impact test piece specified by JIS Z 2201 with 2 mm V groove 3 No. 4 tensile test piece specified by JIS Z 2201 4 Plate thickness 1/2 t position 5 Plate thickness 1/4 t position 6 Rolling direction of steel plate

Claims (8)

% By mass
C: 0.001 to 0.05%,
Si: 0.01 to 0.50%,
Mn: 0.10 to 3.0%,
W: 0.10 to 1.0%
In addition,
P ≦ 0.03%
S ≦ 0.02%,
O ≦ 0.01%
Limited to
Nb: 0.005 to 0.017%,
V: 0.005-0.045%,
Ti: 0.005 to 0.041%,
Zr: 0.005 to 0.1%
The amount of precipitation W is 1% of the amount of added W in the heat-affected zone exposed to at least the Ac1 point of the steel during welding, with the steel composition comprising one or more of the following, the balance being Fe and inevitable impurities At the same time, the amount of precipitated W in the base material other than the heat-affected zone is 10% or less of the amount of added W, and the LP value represented by the following formula is the suppression of precipitation of the intermetallic compound Laves phase containing W Therefore, a high-tensile steel excellent in weld heat-affected zone toughness having a tensile strength of 550 MPa or more, characterized by being 2.5 or less.
LP = 3 × (% Si) + (% W) + 2 × (% Cr) + 0.5 × (% Mo) Furthermore, in mass% ,
Mo: 0.01-1.0% is contained, The high strength steel excellent in the weld heat affected zone toughness of the tensile strength 550 Mpa or more of Claim 1 characterized by the above-mentioned. Furthermore, in mass%,
Ni: 0.01 to 5.0%,
Cu: 0.01~1.0%,
Cr: 0.10 to 1.0%
B: 0.0003 to 0.005%
The high-strength steel excellent in weld heat-affected zone toughness with a tensile strength of 550 MPa or more according to claim 1 or 2, characterized by containing one or more of the following. Furthermore, in mass%,
Ca: 0.0003 to 0.005%,
Mg: 0.0003 to 0.005%,
Ba: 0.0003 to 0.005%,
Y: 0.0005 to 0.10%,
Ce: 0.0005 to 0.10%,
La: 0.0005 to 0.10%
The high-tensile steel excellent in weld heat-affected zone toughness having a tensile strength of 550 MPa or more according to any one of claims 1 to 3, characterized by containing one or more of the following. Furthermore, in mass%,
Al: 0.002 to 0.20%,
Ta: 0.002 to 0.20%
The high-strength steel excellent in weld heat-affected zone toughness having a tensile strength of 550 MPa or more according to any one of claims 1 to 4, characterized by containing one or two of the following. The ratio of the intermetallic compound Fe ₂ W type Laves phase in the heat-affected zone exposed to at least the Ac1 point of the steel during welding is 1% or less as the amount of precipitated W with respect to the amount of added W. The high-tensile steel excellent in weld heat-affected zone toughness having a tensile strength of 550 MPa or more according to any one of claims 5 to 6.   The segregation degree of W obtained by dividing the maximum value of the segregated W concentration analysis value in the steel material by characteristic X-ray measurement by the amount of addition of W is 1.5 or less. A high-tensile steel excellent in weld heat-affected zone toughness having a tensile strength of 550 MPa or more according to any one of the items. A steel having the steel composition according to any one of claims 1 to 7 is subjected to a cooling step after hot working such as hot rolling or forging, or heat treatment such as normalizing, tempering, annealing, etc. In the subsequent cooling step, the retention time or the passage time in the temperature range of 400 to 700 ° C. is limited to 30 hours or less, and the precipitation W in the heat affected zone exposed to the Ac1 point or more of the steel material during welding is characterized. The amount is 1% or less of the added W amount, and at the same time, the amount of precipitated W in the base material other than the heat-affected zone is 10% or less of the added W amount, and the LP value represented by the following formula contains W. A method for producing high-strength steel excellent in weld heat-affected zone toughness with a tensile strength of 550 MPa or more, which is 2.5 or less in order to suppress precipitation of the intermetallic compound Laves phase.
LP = 3 × (% Si) + (% W) + 2 × (% Cr) + 0.5 × (% Mo)

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