JP4959402B2 - High strength welded structural steel with excellent surface cracking resistance and its manufacturing method - Google Patents

Description

  The present invention relates to a steel for welded structures having excellent surface cracking resistance and a method for producing the same, which can be widely used as welded structures such as shipbuilding, offshore structures, bridges, buildings, construction machines, tanks, and line pipes. It is.

  As is well known, Cu has often been used as an effective additive element from the viewpoint of increasing the strength and toughness of welded structures such as shipbuilding, offshore structures, bridges, and buildings. Usually, Cu-added thick steel sheets may cause cracks in the slab surface in the continuous casting stage and cracks in the steel plate surface part in the rolling stage. Ni is added. However, these methods cause new problems such as a decrease in productivity and an increase in cost, and further improvement measures are required from a practical viewpoint.

  The cause of surface cracking due to Cu is not necessarily fully elucidated, but when a scale is formed on the slab surface or the steel plate surface, a concentrated portion of Cu is formed at the scale / steel plate interface, and Cu having a low melting point is crystallized on the surface of the steel plate. The established theory is that grain boundary embrittlement is caused by intrusion into the steel sheet in a molten state along the grain boundary. As can be seen from this cause, it is easily assumed that it is effective to reduce the amount of intergranular immersion of molten Cu as a Cu crack suppression technique. As a technique for suppressing the amount of intergranular infiltration, it is conceivable to make the surface portion finer in the slab stage (reduction of the amount of Cu intrusion due to an increase in the grain boundary area). Conventionally, from such a point of view, there are few known techniques for manufacturing thick steel plates focusing on the refinement of the steel plate surface. Moreover, utilization of the element which has the effect which suppresses the grain boundary diffusion of Cu melt is also considered.

  With regard to the fine graining technology, for example, as a technology for ensuring the toughness of the base metal, a technology for reducing the final ferrite grain size has been developed so far, depending on the required toughness level, a normal rolling method, a controlled rolling method, and further Controlled rolling + accelerated cooling methods have been used. The basics are that high-temperature stable nitrides such as AlN and TiN are used as pinning particles, the heated austenite (γ) grain size of the base material is refined, and further, the ferrite nucleation sites are formed in the austenite by rolling. A large number of them are introduced to make the final ferrite grain size fine.

  However, the technique used in the production of such a base material is just fine graining after hot rolling, and does not exert a great effect on fine graining at the slab stage, which is currently a problem. In addition, it is often necessary to change the heating temperature depending on the type of nitride even in the reheating stage, or sufficient miniaturization cannot be achieved. That is, with these nitrides, it is difficult to achieve fine graining at the slab stage, and the grain size is on the order of several thousand μm (several mm).

On the other hand, in recent years, it has been widely used as a method for refining the heating γ during super adult heat welding. As an excellent high-strength welded structural steel, in mass%, C: 0.01 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.3 to 2.0%, P: ≦ 0.03%, S: 0.0001 to 0.030%, Al: 0.0005 to 0.050%, Ti: 0.003 to 0.050%, Mg: 0.0001 to 0.005%, Ca : 0.0001 to 0.005%, the balance being iron and inevitable impurities, Cr: 0.005 to 0.30%, Nb: 0.001 to 0.20%, Mo: 0.00. High toughness for high-strength welded structures characterized by containing one or more of 005 to 0.30% Steel (for example, refer to Patent Document 1) and high strength welded structural steel that improves dispersion of both base metal toughness and welded portion HAZ toughness by controlling dispersion of oxide containing REM, in mass%, C : 0.01-0.2%, Si: 0.02-0.5%, Mn: 0.3-2%, P: 0.03% or less, S: 0.0001-0.03%, Al : 0.0005 to 0.05%, Ti: 0.003 to 0.05%, Mg: 0.0001 to 0.01%, Ca: 0.0001 to 0.01%, REM: 1 type or 2 types or more out of 0.0001-0.05%, the balance consists of iron and inevitable impurities, and 1 type or 2 types or more of Mg, Ca, REM, and O, S One or both of the particles having a particle diameter of 0.005 to 0.5 μm is 1000 per 1 mm ^2. There has been proposed a high-strength welded structural steel (see, for example, Patent Document 2) excellent in base metal toughness and welded portion HAZ toughness characterized by being dispersed by zero or more.

  These proposed techniques are not related to fine graining at the slab stage. As the fine graining at the slab stage, the use of oxides and sulfides that are difficult to dissolve even at high temperatures is conceivable. For example, various methods such as a composite deoxidation method have been used for introducing oxides, In a known method, a technique for refining the steel plate surface portion at the slab stage to about 1200 μm or less has not been established at present.

JP 2006-28627 A JP 2003-49237 A

  It is an object of the present invention to provide a high-strength welded structural steel excellent in surface cracking resistance by a crystal grain refinement technique at a slab stage by fine dispersion of oxide particles (or sulfide particles) and a method for producing the same. did.

  The present inventors pay attention to the strong deoxidizer of REM or the strong sulfide generating ability, and by combining with TiN that has been widely used in the past, the refinement of crystal grains at the slab stage can be achieved. Has found that a high-strength welded structural steel having excellent surface cracking resistance can be obtained, thereby completing the present invention.

  The gist of the present invention is as follows.

(1) In mass%,
C: 0.01-0.20%
Si: 0.02 to 0.50%,
Mn: 0.3 to 2.5%
P: ≦ 0.03%,
S: 0.0015 to 0.03%,
Cu: 0.01 to 1.0%,
Nb: 0.005 to 0.05%,
Al: 0.001 to 0.05%,
Ti: 0.005 to 0.05%,
N: 0.001 to 0.01%,
REM: 0.0005 to 0.01%,
Ca: 0.0001 to 0.003%,
A high-strength welded structural steel with excellent surface cracking resistance, characterized in that the balance is made of iron and inevitable impurities.

(2) Furthermore, in mass%,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 0.5%
Mo: 0.01 to 0.5%,
V: 0.01-0.5%
Zr: 0.005 to 0.05%,
Ta: 0.005 to 0.05 %,
1 type or 2 types or more of these, The steel for high-strength welded structures excellent in the surface crack-proof characteristic of the said (1) description characterized by the above-mentioned.

  (3) The high strength welded structural steel having excellent surface cracking resistance as described in (1) or (2) above, wherein the surface layer γ grain size of the slab is 1200 μm or less.

  (4) After heating the steel slab having the same component as the steel described in (1) or (2) above to AC3 point or higher and 1350 ° C or lower, hot rolling in the recrystallization temperature range, and then naturally cooling. A method for producing high strength welded structural steel with excellent surface cracking resistance.

  (5) A steel slab having the same composition as the steel described in (1) or (2) is heated to AC3 or higher and 1350 ° C or lower, and then hot-rolled in a recrystallization temperature range, and further in an unrecrystallization temperature range. A method for producing high strength welded structural steel having excellent surface cracking resistance, characterized by subjecting to natural rolling cooling after hot rolling at a cumulative rolling reduction of 40 to 90%.

  (6) A steel slab having the same component as the steel described in (1) or (2) above is heated to AC3 or higher and 1350 ° C or lower, hot-rolled in a recrystallization temperature range, and further in an unrecrystallization temperature range. For high-strength welded structures with excellent surface cracking resistance, characterized by hot rolling at a cumulative rolling reduction of 40 to 90% and then cooling to 0 to 600 ° C. at a cooling rate of 1 to 60 ° C./sec. Steel manufacturing method.

(7) A steel slab having the same composition as the steel described in (1) or (2) is heated to AC3 or higher and 1350 ° C or lower, and then hot-rolled in a recrystallization temperature range, and further in an unrecrystallization temperature range. After hot rolling with a cumulative rolling reduction of 40 to 90%, cooling to 0 to 600 ° C. at a cooling rate of 1 to 60 ° C./sec, followed by heating to 300 ° C. to AC 1 point for tempering heat treatment. A method for producing high strength welded structural steel with excellent surface cracking resistance.

  By limiting to the chemical components and the production method of the present invention and adding REM amount, Ca amount, Ti amount, and N amount appropriately, the γ particle size at the slab stage can be refined, thereby making Cu It is possible to dramatically improve the surface cracking characteristics resulting from the above, and it becomes possible to produce a high strength welded structural steel with excellent surface cracking resistance that has never been obtained. As a result, the production amount of Cu-containing steel that can be used for steel structures such as shipbuilding, offshore structures, bridges, buildings, construction machinery, tanks, and line pipes is greatly improved, and expensive elements such as Ni are added. Therefore, the industrial effect is remarkably great.

  REM is known to improve the toughness of the weld heat affected zone (HAZ) by increasing the cleanliness of steel as a strong deoxidizer and desulfurizer. Patent Document 2 describes an example in which the dispersion of an oxide containing REM is controlled to improve both the base material toughness and the welded portion HAZ toughness.

  The present inventors pay attention to such a strong deoxidizer of REM or a strong sulfide generating ability, and use it for refining crystal grains at the slab stage by combining with TiN which has been widely used conventionally. I thought there was room for it.

  Hereinafter, the present invention will be described in detail.

The present inventors systematically examined the state of crystal grain refinement at the slab stage when REM was added. As a result, after deoxidation with Si and Mn, in the molten steel to which Ti and Al are added, when Ca is first added and REM is further added, REM oxide or sulfide (REM (O, S)) Has been found to be produced very finely and with high density. The particle diameter was 0.003 to 0.3 μm, and the number of particles was 5000 or more per 1 mm ² in steel, and it was confirmed that these existences have a strong pinning force even in the slab stage. In particular, it was also found that TiN was remarkably finely dispersed when used in combination with TiN compared to REM-free steel. As a result, it has been found that due to the presence of REM oxide (or sulfide), the γ particle size at the slab stage is 1200 μm at the maximum.

  The present invention relates to a steel material having excellent surface cracking resistance achieved by refining γ grains at the slab stage as described above, and the amount of Cu melt intrusion at the γ grain boundary by refining the γ grain size. Is an epoch-making technology that suppresses as much as possible. That is, a feature of the present invention is that a high-strength welded structural steel having excellent surface cracking resistance can be provided through a slab surface refinement technique by adding REM.

  Although it is the addition method of REM in this invention, as already stated, after adding Si and Mn first, after adding Ti and A1, first, it adds Ca further. REM is then added. At this time, Ca has an effect of reducing the amount of free oxygen before the addition of REM, and contributes to miniaturization of REM (O, S). The optimum addition amount of REM is 0.0005 to 0.01%, and depends on the dissolved oxygen amount after addition of Ti. According to the experimental data, the minimum 0.0005% is the minimum amount of fine REM oxide (or REM sulfide), and if it exceeds 0.01%, coarse REM oxide can be formed. This was limited because the adverse effects such as reduction of HAZ toughness increased. Since REM is easily consumed as a coarse oxide, it is desirable that FeO in the slag is 10% or less. In addition, an appropriate amount of oxygen (TO) in the REM-added product is about 10 to 50 ppm.

  Hereinafter, the reasons for limiting the components of the present invention will be described.

  C: C is an element indispensable as a basic element for improving the strength of the base metal in steel, and as an effective lower limit, addition of 0.01% or more is necessary, but an excess exceeding 0.20% Addition causes a decrease in the weldability and toughness of the steel material, so the upper limit was made 0.20%.

  Si: Si is an element necessary as a deoxidizing element in steelmaking, and 0.02% or more is necessary to be added to the steel. However, if it exceeds 0.5%, the HAZ toughness is lowered, so that is the upper limit. .

  Mn: Mn is an element necessary for ensuring the strength and toughness of the base material. However, if it exceeds 2.5%, it significantly inhibits the HAZ toughness. Conversely, if it is less than 0.3%, the strength of the base material is secured. Therefore, the range is made 0.3 to 2.5%. In the present invention in which Ca and REM are added in combination, the crystal grains are remarkably refined, so the adverse effect on the HAZ toughness is small, and it is possible to add up to 2.5%.

  P: P is contained as an impurity in steel and is an element that affects the toughness of steel. If it exceeds 0.03%, it is contained because it significantly inhibits the brittleness of HAZ as well as the base material of steel. The upper limit is set to 0.03%.

S: When S is added in excess of 0.030%, coarse sulfides are generated and toughness is inhibited. However, when the content is less than 0.001%, fine γ grains The amount of sulfides such as REM (O, S) and MnS, which are effective for crystallization and the formation of intragranular ferrite, is significantly reduced . S has a lower limit of 0.0015% of S shown in the Examples as a lower limit, and a range of 0.0015 to 0.030%.

  Cu: Cu is an element effective for increasing the strength without reducing brittleness, but if it is less than 0.01%, there is no effect, and if it exceeds 1.0%, the steel piece is heated or welded even when REM is added. Sometimes prone to cracking. Therefore, the content is made 0.01 to 1.0% or less.

  Nb: Nb is an element that forms carbides and nitrides and is effective in improving the strength of the base material. In the present invention, Nb is an essential element for manufacturing the base material. Nb has no effect when added in an amount of 0.005% or less, and when added over 0.05%, both the base metal toughness and the HAZ brittleness decrease, so the range is 0.005 to 0.05% or less. And Note that Nb, like Mn, has a smaller adverse effect on brittleness due to the grain refinement effect of the present invention than in the case where Ca is not added.

  Al: Al is usually added as a deoxidizer, but in the present invention, if added over 0.05%, the effect of the addition of REM is hindered. Further, 0.001% is necessary to stably generate the REM oxide, and this is set as the lower limit.

  Ti: Ti is an element that exerts an effect on the refinement of crystal grains as a deoxidizer and further as a nitride-forming element. However, a large amount of addition causes a significant decrease in toughness due to the formation of carbides. Although the upper limit needs to be 0.05%, in order to acquire a predetermined effect, addition of 0.005% or more is required, and the range shall be 0.005-0.05%.

  N: N should be handled as an impurity, but in the case where this amount is in an appropriate range, an extremely strong pinning effect of TiN appears, so the content was made 0.001 to 0.01%. The lower limit value is the minimum amount for exhibiting the pinning effect, and the upper limit value is specified to inhibit the HAZ toughness.

  REM: REM suppresses the generation of elongated MnS by generating sulfides, and improves the properties in the thickness direction of the steel material, particularly the lamellar resistance. Furthermore, since it has a strong pinning effect through the inclusion of REM, it is an important element of the present invention. The optimum amount of REM is as described above, and if it is less than 0.0005%, a sufficient effect cannot be obtained, and if it exceeds 0.01%, the number of coarse oxides in REM increases, resulting in an ultrafine oxide. Or since the number of sulfides falls, the upper limit is made 0.01%.

  Ca: Ca suppresses the generation of stretched MnS by generating sulfides, and improves the properties in the thickness direction of the steel material, particularly the lamellar resistance. Furthermore, Ca is an important element of the present invention because it has the same effect as REM. The range of Ca is limited to the range of 0.0001% to 0.003%. If the content is less than 0.0001%, a sufficient effect is desired to be obtained. If the content exceeds 0.003%, the number of coarse oxides of Ca increases, and the base metal toughness and the HAZ toughness are lowered. Set to 003%.

In the present invention, one or more elements among Ni, Cr, Mo, V, Zr, and Ta can be added as elements for improving strength and toughness.

  Ni: Ni is an element effective for improving toughness and strength. To obtain the effect, addition of 0.01% or more is necessary, but addition of 1.0% or more lowers weldability. Therefore, the upper limit is made 1.0%.

  Cr: Cr is effective to improve the strength of steel by precipitation strengthening, but addition of 0.01% or more is effective, but if added in a large amount, the hardenability is increased, the bainite structure is generated, and the toughness is decreased. . Therefore, the upper limit is made 0.5%.

  Mo: Mo is an element that improves hardenability and at the same time forms carbonitride to improve strength. To obtain the effect, addition of 0.01% or more is necessary. Addition of a large amount exceeding 5% causes excessive reduction of toughness as well as unnecessary strengthening, so the range is made 0.01 to 0.5% or less.

  V: V is an element that forms carbides and nitrides and is effective in improving the strength. However, the addition of 0.01% or less has no effect, and the addition of more than 0.5% conversely exhibits toughness. In order to cause a decrease, the range is set to 0.01 to 0.5%.

  Zr, Ta: Zr and Ta are elements that form carbides and nitrides as well as Nb and are effective in improving the strength. However, addition of 0.005% or less has no effect and exceeds 0.05%. Addition, on the contrary, causes a decrease in toughness, so the range is made 0.005 to 0.05%.

  The steel containing the above components is subjected to reheating, rolling, and cooling through continuous casting after melting in the steelmaking process. In this case, the following points were limited.

  In both hot rolling and controlled rolling, it is necessary to heat the steel slab to a temperature of 3 or more AC points to austenite. However, if heating exceeds 1350 ° C., the heat source cost increases, so the heating temperature is set to 1350 ° C. or lower.

  Next, it is important to reduce the austenite grain size by rolling in the recrystallization temperature range in both the hot rolling and the controlled rolling utilized during the production of the base material. In addition, when using controlled rolling to increase strength and improve toughness, it is effective to introduce a deformation band into the austenite grains by rolling in the non-recrystallization temperature range and introduce ferrite transformation nuclei. is there. If the cumulative reduction rate in the non-recrystallized region is less than 40%, the deformation band is not sufficiently formed. Therefore, the lower limit value of the cumulative reduction rate in the non-recrystallized region is set to 40%. However, if the cumulative rolling reduction exceeds 90%, the absorbed energy in the base metal Charpy test is significantly reduced, so the upper limit was made 90%.

  After hot rolling, natural cooling is usually sufficient. However, accelerated cooling is necessary to increase the strength further than natural cooling. However, if the cooling rate is less than 1 ° C./sec, sufficient strength cannot be obtained. On the contrary, when the cooling rate exceeds 60 ° C./sec, the toughness of the base material decreases because the microstructure is mainly bainite or martensite.

  Therefore, the cooling rate was limited to 1-60 ° C./sec. In this case, it is necessary to continue accelerated cooling until the transformation is completed in order to obtain the strength of the base material. For this reason, the upper limit of the cooling stop temperature was set to 600 ° C. Since the transformation does not end at a stop temperature exceeding 600 ° C., sufficient strength cannot be obtained. Usually, accelerated cooling uses water as a cooling medium. Therefore, since the lower limit of the actual cooling stop temperature is 0 ° C., the lower limit is set to 0 ° C.

  Since the tempering heat treatment after accelerated cooling is intended to improve the toughness of the base metal structure by recovery, the heating temperature must be AC1 point or less, which is a temperature range in which reverse transformation does not occur. Recovery reduces the lattice defect density by the disappearance and coalescence of dislocations. In order to realize this, heating to 300 ° C. or higher is necessary. For this reason, the minimum of heating temperature was 300 degreeC. Since the upper limit is below the transformation point, AC1 was set as the upper limit.

  Next, examples of the present invention will be described.

  A block having a length of 50 mm was cut out from the surface portion of the cast slab having the chemical components shown in Table 1, and the γ particle size and refinement of the slab were evaluated. Here, the length direction was parallel to the slab width direction. The γ particle size was determined by a cutting method from a microstructure photograph. Moreover, the size of the γ particle size was evaluated as fine particles of 1200 μm or less. Subsequently, the surface crack resistance characteristic which is an important characteristic of the present invention was evaluated. Here, the number of cracks includes the surface of the slab, cuts out a sample of at least 50 mm in the width direction of the slab, observes a cross section perpendicular to the casting direction with an optical microscope, and cracks with a depth of 0.1 mm or more As measured. Finally, after hot rolling and heat treatment shown in Table 2 to obtain a steel sheet, the base metal toughness was evaluated as a basic characteristic of high strength welded structural steel. The test was conducted by a Charpy impact test at −40 ° C. and evaluated by Charpy absorbed energy.

Steels 1 , 3, 4, 6-8, and 10-14 are examples of the present invention. As is apparent from Table 2, the steel sheet of the present invention satisfies the requirements of chemical composition and production conditions, and exhibits a microstructure with a slab γ grain size of 1200 μm or less, reflecting this and surface cracking characteristics. It is understood that is very excellent. In Table 2, 0.2 / mm is used as an index, which is compared with a practical standard of whether or not the slab surface is maintained. Furthermore, it can be seen from Table 2 that each base metal toughness also shows a high absorbed energy of 20 kgf · m or more, and all have high toughness.

In contrast, the steel 16-2 8 shows a comparative example in which depart the onset bright or al. That is, steels 16 to 24, 26, and 28 are examples in which any one of the basic components or selected elements is added in excess of the requirements of the invention, and REM amount, Ca, which is an important element of the present invention. Even when the amount, Ti amount, and N amount are appropriate, the deterioration of the base material toughness is promoted by the excessive addition of the elements that cause the base material toughness deterioration.

  Further, in Steels 16, 18, 20, 22, 25, 27, and 29, this corresponds to the case where the REM amount, Ti amount, and N amount are smaller than the lower limit values, and the γ particle size of the slab becomes coarse and surface cracks occur. . Of these, steels 25, 27, and 28 have no problem in the base material toughness but have poor surface crack resistance. Steel 29 has a REM amount equal to or lower than the lower limit value and a Ca content equal to or higher than the upper limit value. Therefore, neither the surface cracking resistance nor the base material toughness is good.

Claims (7)

% By mass
C: 0.01-0.20%
Si: 0.02 to 0.50%,
Mn: 0.3 to 2.5%
P: ≦ 0.03%,
S: 0.0015 to 0.03%,
Cu: 0.01 to 1.0%,
Nb: 0.005 to 0.05%,
Al: 0.001 to 0.05%,
Ti: 0.005 to 0.05%,
N: 0.001 to 0.01%,
REM: 0.0005 to 0.01%,
Ca: 0.0001 to 0.003%,
A high-strength welded structural steel with excellent surface cracking resistance, characterized in that the balance is made of iron and inevitable impurities. Furthermore, in mass%,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 0.5%
Mo: 0.01 to 0.5%,
V: 0.01-0.5%
Zr: 0.005 to 0.05%,
Ta: 0.005 to 0.05 %,
The high strength welded structural steel having excellent surface cracking resistance according to claim 1, comprising one or more of them.   The high strength welded structural steel excellent in surface crack resistance according to claim 1 or 2, wherein the slab has a surface γ particle size of 1200 µm or less.   A steel slab having the same composition as that of the steel according to claim 1 or claim 2 is heated to AC3 point or higher and 1350 ° C or lower, hot-rolled in a recrystallization temperature range, and then naturally cooled. A method for producing high strength welded structural steel with excellent surface cracking characteristics.   A steel slab having the same composition as that of the steel according to claim 1 or claim 2 is heated to AC3 point or more and 1350 ° C or less, then hot-rolled in a recrystallization temperature range, and further at a cumulative reduction rate in an unrecrystallization temperature range. A method for producing high strength welded structural steel having excellent surface cracking resistance, characterized by performing natural rolling after 40 to 90% hot rolling.   A steel slab having the same composition as that of the steel according to claim 1 or claim 2 is heated to AC3 point or more and 1350 ° C or less, then hot-rolled in a recrystallization temperature range, and further at a cumulative reduction rate in an unrecrystallization temperature range. 40% to 90% hot rolling and then cooling to 0 to 600 ° C. at a cooling rate of 1 to 60 ° C./sec. . Claim 1 or claim 2 wherein the steel a steel slab having the same components as AC3 or more points, after heating to 1350 ° C. or less, and hot-rolled at the recrystallization temperature region, further cumulative rolling reduction in the non-recrystallization temperature region 40% to 90% hot rolling, then cooled to 0 to 600 ° C. at a cooling rate of 1 to 60 ° C./sec, and subsequently subjected to tempering heat treatment by heating from 300 ° C. to AC 1 point. A method for producing high strength welded structural steel with excellent surface cracking characteristics.