Steel

Abstract

To provide a steel material that has a good HAZ toughness even after welding.SOLUTION: A steel material that has a steel composition containing C, Si, Mn, P, S, Al, N, O, Ti, B, Mg, Ca, REM, and the balance consisting of Fe and impurity elements, contains, in the steel, a MgMn-based sulfide in which the ratio of Mg to the total of Mg and Mn is 70% or more and 90% or less in atom%, the MgMn-based sulfide contains nano sulfide having a circle-equivalent diameter of 0.005 μm or more and less than 0.5 μm, and micro sulfide having a circle-equivalent diameter of 0.5 μm or more and 5.0 μm or less, the number density of nano sulfide is 1.0×104 to 30.0×104 pieces / mm2, among the micro sulfide, 80% or more in number percentage of the micro sulfide is a composite inclusion containing N by 1 atom% or more, and the number density of composite inclusion is 20 to 300 pieces / mm2.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention has excellent toughness in the heat affected zone (HAZ) of welding. Steel Regarding. [Background technology]

[0002] High-tensile steel plates with a yield stress of about 260 to 700 MPa are used in various welded steel structures such as buildings, bridges, ships, line pipes, construction machinery, marine structures, and tanks. In particular, steel plates used in the inner walls of tanks for liquefied gas are required to have excellent low-temperature toughness at about -50 to -60°C. Also, to improve the efficiency of welding work, welding must be performed in one pass. To meet this demand, it is necessary to suppress the coarsening of the structure of the heat affected zone (HAZ) of the weld formed by one pass welding.

[0003] In the HAZ, the closer to the fusion line the heating temperature during welding becomes higher, and especially in the area near the fusion line that is heated to over 1400°C, the austenite (γ) becomes significantly coarsened, and the HAZ structure after cooling becomes coarse and the toughness deteriorates. This tendency becomes more pronounced as the welding heat input increases.

[0004] Japanese Patent Laid-Open Publication No. 2002-3986 (Patent Document 1) and International Publication No. 2014 / 091604 (Patent Document 2) describe a steel material in which fine sulfide particles containing Mg and Mn are dispersed in steel and the pinning effect of the sulfide particles suppresses gamma grain growth during welding, thereby making it possible to improve HAZ toughness at -5°C or -20°C.

[0005] Furthermore, JP 2016-164289 A (Patent Document 3) describes a steel material in which the coarsening of oxides and nitrides is suppressed by regulating the amounts of O, Ti, and N, the formation of fine bainite is promoted by adding B and controlling the hardenability index DI value, and the formation of island martensite between laths is suppressed by controlling the M value, thereby improving the toughness of an ultra-high heat input welded HAZ at -5°C. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2002-3986 A [Patent Document 2] International Publication No. 2014 / 091604 [Patent Document 3] JP 2016-164289 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, as described above, there has been a demand for improved HAZ toughness at lower temperatures in recent years, and it is necessary to further refine the HAZ structure. An object of the present invention is to provide a steel material that has good HAZ toughness even after welding. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention employs the following configuration. [1] In weight percent: C: 0.030~0.250%, Si: 0.02 to 0.50%, Mn: 0.10-2.00%, P: 0.020% or less, S: 0.001 to 0.020%, Al: 0.010 to 0.200%, N: 0.0020~0.0050%, O: 0.0007 to 0.0020%, Ti: 0.004 to 0.025%, B: 0.0005~0.0050%, Mg: 0.0005~0.0050%, Ca: 0.0005% or less, REM: 0.0005% or less, The balance is a steel composition including Fe and impurities, The steel contains MgMn-based sulfides in which the ratio of Mg to the total of Mg and Mn is 70% or more and 90% or less in atomic percent, The MgMn-based sulfides include nanosulfides having a circle equivalent diameter of 0.005 μm or more and less than 0.5 μm, and microsulfides having a circle equivalent diameter of 0.5 μm or more and 5.0 μm or less, The number density of the nanosulfides is 1.0×10 4 ~30.0×10 4 pieces / mm 2 and Among the microsulfides, 80% or more of the microsulfides in terms of number ratio are composite inclusions containing 1 atomic % or more of N, The number density of the complex inclusions is 46 ~300 pieces / mm 2 That is, steel. [2] The steel composition contains, in mass%, Cu: 1.50% or less, instead of a part of the Fe, Ni: 2.00% or less, Cr: 1.00% or less, Mo: 1.00% or less Nb: 0.050% or less, V: 0.100% or less The steel material according to the above [1], characterized in that it contains one or more selected from the group consisting of: [3] The steel composition contains, in mass%, W: 1.00% or less in place of a portion of the Fe, Sn: 0.50% or less The steel material according to the above [1] or [2], characterized in that it contains one or two kinds selected from the group consisting of: 。 Effect of the Invention

[0009] According to the present invention, a steel material having good HAZ toughness even after welding can be provided. In particular, the HAZ formed by welding the steel material of the present invention has excellent low-temperature toughness at an extremely low temperature of -60°C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The steel material according to the embodiment of the present invention is premised on being a steel material manufactured by a manufacturing method including Al deoxidation, which is an excellent mass production process that has a proven track record of mass production. The present inventors conducted detailed investigations and research into the relationship between the structure and toughness of the HAZ obtained by welding with a heat input of about 5 to 30 kJ / mm, on the premise of a chemical composition satisfying the following requirements: plate thickness of 40 mm or less, yield stress of 265 MPa or more, and tensile strength of 420 to 560 MPa. As a result, they reached the conclusion that the low-temperature toughness of the HAZ is limited even if the means for controlling the structure or improving toughness of the HAZ obtained by conventional welding is directly applied.

[0011] In order to improve the toughness of the HAZ caused by welding, it is necessary to significantly refine the austenite grains, and the pinning effect of particles in the steel is effective in refining the austenite grains. The inventors have investigated various particles on the premise of Al-deoxidized steel, and have found that it is effective to finely disperse a large amount of fine MgMn-based sulfides with a circle equivalent diameter of less than 0.5 μm in the steel, which are effective in suppressing the growth of austenite grains in the HAZ, by controlling the Mn, Mg, S, Al contents, etc.

[0012] However, the refinement of austenite grains by MgMn sulfides with a circle equivalent diameter of less than 0.5 μm alone is not sufficient to improve HAZ toughness at cryogenic temperatures of -60°C. In other words, it was found that good values ​​could not be obtained when evaluating the average and minimum values ​​of three Charpy tests at -60°C.

[0013] Therefore, the inventors conducted further studies and focused on MgMn-based sulfides with an equivalent circle diameter of 0.5 to 5.0 μm, which were previously thought to have little effect on improving toughness. They found that the presence of a predetermined amount of composite inclusions containing these relatively coarse MgMn-based sulfides and nitrides promotes the formation of intragranular ferrite in the HAZ structure and refines the HAZ structure, thereby improving the low-temperature toughness at -60°C.

[0014] Hereinafter, a steel material according to an embodiment of the present invention will be described. The steel material of this embodiment contains, in mass%, C: 0.030 to 0.250%, Si: 0.02 to 0.50%, Mn: 0.10 to 2.00%, P: 0.020% or less, S: 0.001 to 0.020%, Al: 0.010 to 0.200%, N: 0.0020 to 0.0050%, O: 0.0007 to 0.0020%, Ti: 0.004 to 0.025%, B: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, Ca MgMn-based sulfides are included in the steel, the ratio of Mg to the total of Mg and Mn being 70% or more and 90% or less in atomic %, and the MgMn-based sulfides include nanosulfides having a circle equivalent diameter of 0.005 μm or more and less than 0.5 μm, and microsulfides having a circle equivalent diameter of 0.5 μm or more and 5.0 μm or less, and the number density of the nanosulfides is 1.0 × 10 4 ~30.0×10 4 pieces / mm 2 Among the microsulfides, 80% or more in terms of number ratio are composite inclusions containing 1 atomic % or more of N, and the number density of the composite inclusions is 20 to 300 pieces / mm 2 It is. In addition, the steel material of this embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of, by mass%, Cu: 1.50% or less, Ni: 2.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, Nb: 0.050% or less, and V: 0.100% or less. Furthermore, the steel material of the present embodiment may contain, in mass %, one or two selected from the group consisting of W: 1.00% or less and Sn: 0.50% or less, instead of a portion of Fe.

[0015] First, the chemical composition of the steel material according to this embodiment will be described. In the following description of the chemical composition, mass% is expressed as %. In addition, in the following description, when the upper and lower limit values ​​of the element content are connected with "~" to indicate a range, it means a range including the upper and lower limit values, unless otherwise noted. Therefore, when 0.01-0.20% is expressed as mass%, the range means a range of 0.01 mass% or more and 0.20 mass% or less.

[0016] C: 0.030~0.250% C is an element that increases the strength of steel (base material). If the C content is less than 0.030%, the effect of improving the strength of the base material is small, so the C content is set to 0.030% or more. A more preferable C content is 0.060% or more. On the other hand, if the C content exceeds 0.250%, the island martensite and cementite that are the starting points of brittle fracture will increase significantly, resulting in a decrease in HAZ toughness. Therefore, the C content is set to 0.250% or less. A more preferable C content is 0.200% or less, and an even more preferable C content is 0.150% or less.

[0017] Silicon: 0.02 to 0.50% Silicon is an element that is effective in improving hardenability and increasing the strength of the base material. If the Si content is less than 0.02%, it is difficult to ensure the desired strength. Therefore, the Si content is set to 0.02% or more. A more preferable Si content is 0.05% or more. On the other hand, if the Si content exceeds 0.50%, the HAZ toughness decreases due to island martensite caused by excess dissolved Si. Therefore, the Si content is set to 0.50% or less. A more preferable Si content is 0.40% or less, or 0.35% or less.

[0018] Mn: 0.10-2.00% Mn is an element that constitutes MgMn-based sulfides and is an essential element. In order to obtain sufficient MgMn-based sulfides, the Mn content must be 0.10% or more. If the Mn content is less than 0.10%, it is disadvantageous in terms of ensuring strength and HAZ toughness. In order to improve HAZ toughness, the Mn content may be 0.30% or more or 0.60% or more. On the other hand, if the Mn content exceeds 2.00%, the MgMn-based sulfides tend to coarsen and the HAZ toughness decreases, so the Mn content is set to 2.00% or less. In order to improve HAZ toughness, the Mn content may be 1.80% or less or 1.60% or less.

[0019] P:0.020% or less P is an element that causes grain boundary embrittlement and is harmful to toughness, so a lower content is desirable. If the P content exceeds 0.020%, even if the austenite grains of the HAZ structure are refined by MgMn-based sulfides, the toughness is significantly reduced, so the P content is set to 0.020% or less. It is preferably set to 0.010% or less, and more preferably set to 0.008% or less. There is no need to particularly limit the lower limit of the P content, but since it is technically not easy to set the P content to 0%, the P content may be set to more than 0% or to 0.001% or more.

[0020] S: 0.001 to 0.020% S is an essential element for generating MgMn-based sulfides. If the S content is less than 0.001%, the amount of MgMn-based sulfides precipitated will be insufficient, so the S content must be 0.001% or more. To generate a larger amount of MgMn-based sulfides, the S content should be 0.003% or more. On the other hand, if the S content exceeds 0.020%, coarse MgMn-based sulfides will be generated and the effect of refining the gamma grains of the HAZ structure will not be obtained, so the S content must be 0.020% or less.

[0021] Al: 0.010~0.200% Al is an essential element for suppressing the formation of coarse oxides by Mg and for forming fine MgMn-based sulfides by Mg. The Al content is also essential for Al-killed steel, and for this reason the Al content is set to 0.010% or more. In order to form a larger amount of fine MgMn-based sulfides, the Al content is more preferably 0.025% or more or 0.030% or more. On the other hand, if the Al content exceeds 0.200%, the HAZ toughness decreases due to the formation of island martensite caused by the excess solid solution Al. Therefore, the Al content is set to 0.200% or less. The more preferable Al content is 0.150% or less. To improve the HAZ toughness, the Al content may be set to 0.120% or less or 0.100% or less.

[0022] N: 0.0020~0.0050% N is an element that forms nitrides and carbonitrides, and excessive N content easily generates coarse TiN particles and (Ti, Nb)(C, N) particles. These particles become the starting point of brittle fracture. In a steel material with a plate thickness of 40 mm or less, a yield stress of 265 MPa or more, and a tensile strength of 420 to 560 MPa, a toughness evaluation of the HAZ at -60°C showed that TiN particles and (Ti, Nb)(C, N) particles of several μm can become the starting point of brittle fracture, leading to a decrease in HAZ toughness. Therefore, the N content must be strictly controlled. In addition, if the amount of solute N is large, excessive BN particles are generated and the amount of solute B is reduced, which is not preferable. If the amount of solute B is reduced, the effect of solute B delaying ferrite transformation, refining the HAZ structure, and improving the strength of the base material are reduced.

[0023] In particular, in the steel material according to the present embodiment, the Ti content is set to 0.025% or less so as not to generate coarse TiN particles, and therefore the amount of dissolved N that is not fixed to Ti as TiN particles is likely to increase. Therefore, it is necessary to strictly limit the N content from the molten steel stage. For this reason, the N content is set to 0.0050% or less. More preferably, the N content is 0.0045% or less, or 0.0040% or less, and even more preferably, 0.0030% or less. On the other hand, if the N content is too small, the number density of the complex inclusions decreases, and the low-temperature toughness at -60°C decreases. Therefore, the N content is set to 0.0020% or more. The N content may be set to 0.0023% or more, or 0.0026% or more.

[0024] O: 0.0007 to 0.0020% If the O content is high, coarse oxides are easily generated. The coarse oxides become the starting point of brittle fracture and reduce HAZ toughness. Even if the Al content prior to the inclusion of Mg is 0.010% or more, if the O content exceeds 0.0020% due to atmospheric contamination of molten steel caused by special factors such as equipment or operational malfunctions, the amount of Mg consumed by the coarse oxides increases. As a result, the Mg ratio in the MgMn sulfides decreases, and the number of MgMn sulfides decreases, which may reduce the HAZ toughness. For this reason, the O content is set to 0.0020% or less. More preferably, the O content is set to 0.0018% or less, or 0.0016% or less. Although it is desirable to have a low O content, reducing it to less than 0.0007% may result in an increase in cost, so the O content is set to 0.0007% or more. To avoid an increase in cost, the O content may be set to 0.0009% or more, or 0.0011% or more.

[0025] Ti: 0.004 to 0.025% Ti mainly enhances the hardenability improving effect of B, so it is effective in increasing the strength of the base material, and is also effective in improving HAZ toughness by refining the HAZ structure. Ensuring the amount of dissolved B is important for refining the HAZ structure, and the inclusion of Ti ensures the amount of dissolved B by fixing the dissolved N as TiN particles and suppressing the generation of BN particles. It is also effective in refining the structure (grain refinement) of the base material by suppressing the grain growth of austenite grains with TiN particles, and in refining the HAZ structure when heated to 1350℃ or less.

[0026] However, if the Ti content is less than 0.004%, these effects cannot be obtained, so the Ti content is set to 0.004% or more. In order to ensure the effect of Ti inclusion, the Ti content may be set to 0.005% or more, or 0.006% or more. On the other hand, if the Ti content exceeds 0.025%, coarse TiN particles are generated, which become the starting point of fracture, and the HAZ toughness decreases. Therefore, the Ti content is set to 0.025% or less. More preferably, the Ti content is 0.020% or less or 0.015% or less, and even more preferably, the Ti content is 0.018% or less.

[0027] B: 0.0005~0.0050% B segregates at grain boundaries and exerts a remarkable effect of increasing strength, and is an effective element for increasing the strength of the base material. In addition, in the HAZ, solute B retards ferrite transformation, so it is an essential element for refining the microstructure and improving HAZ toughness. However, with a B content of less than 0.0005%, the strength increasing effect and the HAZ toughness improving effect cannot be obtained, so the B content is set to 0.0005% or more. In order to more reliably exert these effects of B inclusion, the B content may be set to 0.0007% or more, or 0.0008% or more. On the other hand, if B is contained in excess of 0.0050%, coarse B nitrides and borocarbides precipitate, so that the solute B is insufficient and the strength decreases, or the precipitates become the starting point of fracture, decreasing the HAZ toughness. Therefore, the B content is set to 0.0050% or less. The B content is more preferably 0.0040% or less, and even more preferably 0.0035% or less or 0.0030% or less.

[0028] Mg: 0.0005~0.0050% Mg is an essential element for the formation of MgMn-based sulfides. If the Mg content is less than 0.0005%, the required number of MgMn-based sulfides cannot be obtained. Therefore, the Mg content is set to 0.0005% or more. In order to form a larger amount of MgMn-based sulfides, the Mg content is more preferably set to 0.0015% or more. On the other hand, if the Mg content exceeds 0.0050%, Mg forms oxides, so the amount of precipitated MgMn-based sulfides saturates, the effect of improving HAZ toughness also saturates, and economic efficiency is impaired, so the Mg content is set to 0.0050% or less.

[0029] Ca: 0.0005% or less, REM: 0.0005% or less In this embodiment, it is necessary to generate MgMn-based sulfides, and for this purpose, it is desirable to reduce the content of sulfide-forming elements other than Mg and Mn as much as possible. Representative sulfide-forming elements are Ca and REM, and in order to control the size and number of MgMn-based sulfides, these are set to 0.0005% or less. More preferably, each is set to 0.0003% or less. There is no need to particularly restrict the lower limits of these elements, and the lower limits of these elements are 0%. REM refers to a total of 17 elements, including Sc, Y, and lanthanoids. The REM content refers to the total content of the above elements.

[0030] The balance of the chemical components of the steel material according to this embodiment is iron (Fe) and impurities. The impurities refer to components that are mixed in due to raw materials such as ores and scraps and other factors during industrial production of the steel material, and are permissible within a range that does not adversely affect the steel material according to this embodiment. However, among the impurities, the upper limits of P, O, and N must be restricted as described above.

[0031] Furthermore, the steel material according to this embodiment basically contains the above chemical components, but in order to improve the mechanical properties and HAZ toughness of the steel material (base material), one or more of the following may be contained in place of a portion of Fe, as necessary: ​​Cu: 1.50% or less, Ni: 2.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, Nb: 0.050% or less, and V: 0.100% or less.

[0032] Cu:1.50% or less Cu is an element that is effective in increasing the strength of the base material, and in particular, a significant increase in strength can be obtained by precipitating a fine Cu phase through aging heat treatment. In order to obtain the effect of increasing the strength more reliably, the Cu content is preferably 0.05% or more. On the other hand, if the Cu content exceeds 1.50%, embrittlement of the base material and HAZ becomes significant, so the Cu content is set to 1.50% or less.

[0033] Ni: 2.00% or less Ni has the effect of increasing the strength of the base material by increasing the hardenability, and furthermore, improves the toughness. In order to obtain these effects more reliably, the Ni content is preferably 0.05% or more. On the other hand, since Ni is an expensive element, if it is contained at more than 2.00%, it is economically disadvantageous, so the Ni content is set to 2.00% or less.

[0034] Cr:1.00% or less Cr has an effect of increasing the strength of the base metal. To obtain this effect more reliably, the Cr content is preferably 0.02% or more. On the other hand, if the Cr content exceeds 1.00%, a hardened structure is formed in the HAZ, and even if the austenite grains in the HAZ are refined by MgMn-based sulfides, a significant effect of improving the HAZ toughness cannot be obtained. Therefore, the Cr content is set to 1.00% or less.

[0035] Mo: 1.00% or less Mo is effective in increasing the strength of the base metal. To obtain this effect more reliably, the Mo content is preferably 0.02% or more. On the other hand, if the Mo content exceeds 1.00%, a hardened structure is formed in the HAZ, and even if the austenite grains in the HAZ are refined by MgMn-based sulfides, a significant effect of improving the HAZ toughness cannot be obtained. Therefore, the Mo content is set to 1.00% or less.

[0036] Nb: 0.050% or less Nb is an element effective in increasing the strength of the base material and refining the grains. To obtain these effects more reliably, the Nb content is preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.050%, the precipitation of Nb carbonitrides in the HAZ becomes significant, and even if the austenite grains in the HAZ are refined by MgMn-based sulfides, no significant effect of improving the HAZ toughness can be obtained. Therefore, the Nb content is set to 0.050% or less.

[0037] V:0.100% or less V is an element effective in increasing the strength and refining the grains of the base material. To obtain these effects more reliably, the V content is preferably 0.005% or more. On the other hand, if the V content exceeds 0.100%, the precipitation of carbonitrides in the HAZ becomes significant, and even if the austenite grains in the HAZ are refined by MgMn-based sulfides, no significant improvement in HAZ toughness can be obtained. Therefore, the V content is set to 0.100% or less.

[0038] Furthermore, the steel material according to this embodiment may contain one or both of W: 1.00% or less and Sn: 0.50% or less in place of a portion of Fe, as necessary.

[0039] W: 1.00% or less W dissolves and forms oxygen acid ions WO4 - W is an element that adsorbs to rust in the form of , inhibits the permeation of chloride ions in the rust layer, and improves corrosion resistance, so it may be contained as necessary. However, even if W is contained in excess, not only the above effect is saturated, but low-temperature toughness may also decrease. Therefore, the W content is 1.00% or less, preferably 0.75% or less. To obtain the above effect, the W content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0040] Sn: 0.50% or less Sn is Sn 2+ Sn is an element that dissolves as a cation and has the effect of suppressing corrosion by its inhibitor action in an acidic chloride solution. Sn also has the effect of suppressing the anodic dissolution reaction of steel and improving corrosion resistance. Therefore, Sn may be contained as necessary. However, even if Sn is contained in excess, not only the above effect is saturated, but also rolling cracks in the steel sheet are likely to occur. Therefore, the Sn content is 0.50% or less, preferably 0.30% or less. To obtain the above effect, the Sn content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0041] From the viewpoint of HAZ toughness, the chemical composition of the steel material according to this embodiment preferably has a carbon equivalent Ceq represented by the following formula in the range of 0.25 to 0.50. If Ceq is 0.30 or more, the steel material will have better HAZ toughness. If Ceq is 0.45 or less, the generation of MA is suppressed and HAZ toughness is improved, so this is more preferable. It is even more preferable that Ceq is 0.40 or less.

[0042] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15

[0043] In the above formula, [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] are the contents (mass%) of C, Mn, Cr, Mo, V, Cu, and Ni, respectively, and 0 is substituted if not contained.

[0044] Next, the MgMn-based sulfides contained in the steel will be described. The steel material according to this embodiment contains MgMn-based sulfides in which the ratio of Mg to the total of Mg and Mn is 70% or more and 90% or less in atomic %. The MgMn-based sulfides include (Mg,Mn) nanosulfides with a circle equivalent diameter of 0.005 μm or more and less than 0.5 μm, and (Mg,Mn) microsulfides with a circle equivalent diameter of 0.5 μm or more and 5.0 μm or less. In the following description, (Mg,Mn) nanosulfides may be referred to as "nanosulfides". Also, (Mg,Mn) microsulfides may be referred to as "microsulfides".

[0045] Regarding the ratio of Mg and Mn in MgMn-based sulfides, the more the ratio of Mg increases, the more stable the particles become at high temperatures, and the stronger the austenite grain growth suppression effect is. The MgMn-based sulfides of this embodiment are sulfides in which the ratio of Mg to the total of Mg and Mn is 70%≦Mg≦90% in atomic %. If the amount of Mg is insufficient relative to the amount of S, MnS is likely to be generated, and the ratio of Mg is relatively reduced. Also, if the amount of Al is insufficient or if O is contained in excess, Mg is likely to generate oxides, and the ratio of Mg is reduced. Also, if Ca and REM are contained in excess, Ca and REM form sulfides, so that the amount of MgMn-based sulfides decreases and the ratio of Mg in the MgMn-based sulfides decreases.

[0046] In this embodiment, the equivalent circle diameter of the nanosulfides is set to 0.005 μm or more and less than 0.5 μm. If it is less than 0.005 μm, the austenite grain growth suppression effect is small. On the other hand, if the sulfides are 0.5 μm or more, it becomes difficult to obtain the effect of suppressing the austenite grain growth by the pinning effect, so the upper limit is set to less than 0.5 μm.

[0047] The number density of nanosulfides is 1.0×10 4 ~30.0×10 4 pieces / mm 2 The density is 1.0×10 4 pieces / mm 2 In the above cases, the effect of suppressing the growth of austenite grains becomes significant. A more preferable number density is 3.0×10 4 pieces / mm 2 More preferably, the number density is 4.0×10 4 pieces / mm 2 On the other hand, if the number density is 30.0×10 4 pieces / mm 2 In order to increase the number of nanosulfides beyond this, excessive Mg content would be required, which would impair economic feasibility. Therefore, the upper limit of the number of nanosulfides is set at 30.0×10 4 pieces / mm 2 The more preferable number density is 20.0×10 4 pieces / mm 2When nano-sulfides are contained within a predetermined number density range, the nano-sulfides exert a pinning effect in the HAZ structure, suppressing the coarsening of austenite grains in the structure and refining the austenite grains.

[0048] The circle equivalent diameter of the microsulfides is set to 0.5 μm or more and 5.0 μm or less. If it is less than 0.5 μm, it becomes difficult to obtain composite inclusions, the number density of the composite inclusions becomes small, and it becomes difficult to obtain the effect of forming intragranular ferrite, so the circle equivalent diameter of the microsulfides is set to 0.5 μm or more. On the other hand, if the number of sulfides with a circle equivalent diameter of more than 5.0 μm increases, the number of fine particles will be significantly reduced due to the limited amount of Mg in the steel, and the effect of suppressing austenite grain growth will be reduced.

[0049] Among the microsulfides, 80% or more in number ratio are composite inclusions containing 1 atomic % or more of N. The microsulfides are inclusions that further contain nitrides. An example of the form of the composite inclusions is a form in which nitrides are present on the surface of the microsulfides. Examples of nitrides include TiN and BN. When the number ratio of the composite inclusions is less than 80%, the effect of forming intragranular ferrite becomes insufficient, and the effect of refining the HAZ structure becomes small. When the number ratio of the microsulfides that contain less than 1 atomic % of N is large compared to the inclusions that contain 1 atomic % or more of N, the minimum value when three Charpy tests are performed is low, and the variation in toughness becomes large. For this reason, the number ratio of the composite inclusions in the microsulfides is set to 80% or more.

[0050] The complex inclusions are contained within a certain range of number density. When welding is performed, the presence of these complex inclusions in the HAZ structure causes intragranular ferrite to precipitate from these complex inclusions, which further refines the HAZ structure.

[0051] The density of the composite inclusions is 20 to 300 pieces / mm 2 The density is too low, at 20 pieces / mm 2If it is less than 300 particles / mm, the effect of forming intragranular ferrite is insufficient, and the effect of refining the HAZ structure is small. 2 If it is over 100%, it becomes a fracture initiation point when an impact is applied, and low-temperature toughness is reduced, which is undesirable.

[0052] The number density of nanosulfides, microsulfides and composite inclusions can be measured by preparing an extracted replica using a test piece taken from a position 1 / 4t of the steel plate thickness t, and measuring it with a transmission electron microscope (TEM) equipped with a characteristic X-ray detector (EDX). 2 The area above is measured, and the value converted to the number per unit area is the number density. For example, when observing one visual field of 100 mm × 80 mm at a magnification of 20,000 times, the observation area per visual field is 20 μm 2 Therefore, observation is performed for at least 50 fields of view. The number of particles with a size of 0.005 to 5.0 μm is 50 fields of view (1000 μm 2 ) is 100, the number of particles is 1 × 10 per square mm 5 It can be converted into pieces.

[0053] Next, the number of particles counted is measured to determine how many MgMn-based sulfides are present. Since the number of particles can be as many as 1,000 or more, identifying all particles one by one is a difficult task. For this reason, it is sufficient to identify whether or not at least 20 particles are MgMn-based sulfides under the conditions below, determine their abundance ratio, and then multiply the number of particles previously determined by the abundance ratio of MgMn-based sulfides to determine the number of MgMn-based sulfides. For example, if the number of particles mentioned above is 1 x 10 per square mm, 5 If the proportion of MgMn sulfides is 90%, the number of MgMn sulfides per square mm is 9 × 10 4 Let us assume that there are individuals.

[0054] Next, we will explain how to identify MgMn sulfides. MgMn sulfides are defined as Mg·Mn-containing sulfides in which the ratio of Mg to the total of Mg and Mn is 70%≦Mg≦90% in atomic %. In other words, the ratios of Mg and Mn to the total of Mg and Mn in MgMn sulfides are 70%≦Mg≦90% and 10%≦Mn≦30% in atomic %. Sulfides mainly composed of Mg and Mn have the effect of refining austenite grains, so it is acceptable for elements other than Mg and Mn to be detected. However, if the amount of elements other than Mg and Mn is greater than the total of Mg and Mn, it is not considered to be MgMn sulfides.

[0055] Also, although trace amounts of O may be detected in the particles, if the ratio of S to the total of S and O is S ≥ 95% in atomic %, and the O content is less than 5%, it is considered to be an MgMn-based sulfide. However, even if the ratio of S to the total of S and O is S ≥ 95% in atomic %, and the O content is less than 5%, if the particles are spherical and can be clearly identified as a composite of MnS and MgO, it is not considered to be an MgMn-based sulfide.

[0056] The ratios of Mg and Mn and the ratios of S and O are quantitatively determined by EDX. The diameter of the electron beam used in this quantification is 0.001 to 0.02 μm, and the TEM observation magnification is 50,000 to 1,000,000 times, and the quantification is performed at any position within the fine MgMn-based sulfides.

[0057] Among the MgMn-based sulfides identified as above, those with an equivalent circle diameter of 0.005 μm or more and less than 0.5 μm are defined as nanosulfides, and those with an equivalent circle diameter of 0.5 μm or more and 5.0 μm or less are defined as microsulfides. Furthermore, among the microsulfides, those containing 1 atomic % or more of N are defined as composite inclusions. Then, the number density of the nanosulfides, the number ratio of the composite inclusions in the microsulfides, and the number density of the composite inclusions are calculated.

[0058] When making an extraction replica from a steel material, if a large number of precipitates other than MgMn-based sulfides, such as cementite and alloy carbonitrides, are formed and it is difficult to measure the number of MgMn-based sulfides, it is also possible to hold the sample at 1400°C for 100 seconds to dissolve particles other than MgMn-based sulfides, and then rapidly cool the sample or apply a thermal cycle that forms ferrite during the rapid cooling process to create a sample with less cementite and alloy carbonitrides, and then use this to make an extraction replica. MgMn-based sulfides are stable at high temperatures, so applying the above thermal cycle will not change the results.

[0059] The thickness of the steel material of this embodiment is not particularly limited, but is preferably in the range of 10 to 40 mm.

[0060] In addition, the steel material of this embodiment preferably has a yield stress YP of 265 MPa or more and a tensile strength TS of 420 to 560 MPa. The tensile strength TS and the yield stress YP are evaluated in accordance with JIS Z 2241:2011. The test piece is a No. 1B test piece. The test method is a permanent elongation method.

[0061] The steel material of the present invention is not particularly limited, and even if the steel material has a plate thickness and strength level other than those described above, it can improve the low temperature toughness of the HAZ in the same manner as in the present invention as long as it satisfies the range of the present invention.

[0062] The steel material of this embodiment has excellent toughness in the heat-affected zone (HAZ) when welded under the condition of a welding heat input of 5 to 30 kJ / mm. In particular, the Charpy absorbed energy at -60°C can be improved.

[0063] More specifically, a sample taken from the steel material of this embodiment is subjected to a simulated heat cycle test simulating a large heat input welding, assuming the application of electrogas welding. As a specific simulated heat cycle condition, a heat input of 20 kJ / mm is assumed, simulating welding of a plate thickness of 10 to 50 mm in one pass by electrogas welding, heating from room temperature to 1400°C, holding for 10 seconds, and then cooling at a controlled rate of 3.0°C / sec in the temperature range of 800°C to 500°C, which is the temperature range related to intragranular transformation. After applying a heat cycle to the thick steel plate, it is processed into a V-notch test piece, and a Charpy impact test is performed at a test temperature of -60°C for three pieces of each steel material, and the absorbed energy is measured. If the average absorbed energy of the three test pieces is 100J or more and the minimum absorbed energy of the three test pieces is 50J or more, it can be said that the toughness of the welded heat affected zone is excellent. The V-notch test specimen shall be prepared in accordance with the V-notch test specimen described in JIS Z 2242: 2005. The Charpy impact test shall be performed in accordance with JIS Z 2242: 2005.

[0064] Next, a method for manufacturing the steel material according to this embodiment will be described. In the method for producing a steel material according to the present embodiment, molten steel is subjected to vacuum degassing, and after the dissolved oxygen concentration of the molten steel becomes 0.0040 mass% or less, Mg is added at a rate of 30 to 300 kg / min, and within 10 minutes after the Mg addition, the nitrogen gas flow rate is increased to 1.0 Nm 3 / min or more and reflux is continued for 1.0 minute or more; in the continuous casting process, when the molten steel after the refining process is continuously cast into a cast slab, the average cooling rate is set to 0.2°C / sec or more until the surface temperature of the cast slab is reduced from 1200°C to 900°C; and in the hot rolling process, the cast slab after continuous casting is hot rolled into a steel material.

[0065] In the manufacturing method of this embodiment, in the refining process, Mg is added at a predetermined rate to form MgMn-based sulfides, and then nitrogen gas is introduced and refluxed to cause nitrides to adhere to the microsulfides to form composite inclusions. In addition, excessive grain growth of the MgMn-based sulfides is suppressed by controlling the cooling rate in the continuous casting process. The details of the manufacturing method are described below.

[0066] In this embodiment, the molten steel is tapped from the steelmaking furnace into a ladle, and then decompressed in a vacuum degassing device. After being tapped into the ladle, the molten steel may be added with an alloy or the like to adjust the composition while being transported to the vacuum degassing device.

[0067] Next, in the refining process, the molten steel is vacuum degassed, and once the dissolved oxygen concentration of the molten steel has reached 0.0040% by mass or less, Mg is added at a rate of 30 to 300 kg / min. If the addition of Mg is started when the dissolved oxygen concentration of the molten steel is high, Mg will preferentially combine with the dissolved oxygen over S in the molten steel, making it impossible to form sufficient MgMn-based sulfides, so the dissolved oxygen concentration at the start of Mg addition is set to 0.0040% by mass or less.

[0068] The rate of Mg addition is extremely important for controlling the particle size of MgMn-based sulfides. If the rate of Mg addition is slow, the MgMn-based sulfides will aggregate and the amount of nano-sulfides will decrease. If the rate of Mg addition is too fast, micro-sulfides cannot be sufficiently formed. Therefore, Mg is added at a rate of 30 to 300 kg / min. Mg may be added, for example, in the form of granules or wires.

[0069] Next, within 10 minutes after adding Mg, the nitrogen gas flow rate was increased to 1.0 Nm 3 / min or more, and begin reflux for at least 1.0 minute. If the time between adding Mg and introducing nitrogen gas is too long, the MgMn sulfides will aggregate and the amount of nano-sulfides will decrease, so it is necessary to begin reflux within 10 minutes after adding Mg. Also, if the flow rate of nitrogen gas is insufficient, the amount of nitrides produced will be insufficient and the number density of compound inclusions will decrease, so the nitrogen gas flow rate should be set to 1.0 Nm 3 / min or more. In addition, since the slab is prone to cracking during continuous casting, the nitrogen gas flow rate should be 5.0 Nm 3 It is preferable to set the reflux time to 5.0 minutes or less. While blowing in nitrogen gas, start reflux for 1.0 minute or more. If the reflux time is too short, the amount of nitrides produced will be insufficient, and the number density of the compound inclusions will decrease. It is also preferable to set the reflux time to 5.0 minutes or less. If the reflux time is too long, the slab will be more likely to crack during continuous casting.

[0070] Next, the molten steel after the refining process is subjected to continuous casting. In the continuous casting, the average cooling rate is set to 0.2°C / sec or more until the surface temperature of the slab is reduced from 1200°C to 900°C. If the average cooling rate is lower than 0.2°C / sec, the MgMn-based sulfides formed in the refining process will grow into grains, and the number density of nano-sulfides in particular will decrease, so the average cooling rate is set to 0.2°C / sec or more. Note that the slab is prone to cracking during continuous casting, so the average cooling rate is preferably set to 0.5°C / sec or less.

[0071] The hot rolling and heat treatment conditions after casting can be selected appropriately according to the target mechanical properties of the steel, such as controlled rolling and controlled cooling, direct quenching and tempering after rolling, or cooling after rolling and then quenching and tempering.

[0072] According to this embodiment, a steel material having good HAZ toughness even after welding can be provided. In particular, the HAZ formed by welding the steel material of this embodiment has excellent low-temperature toughness at an extremely low temperature of -60°C. EXAMPLES

[0073] Hereinafter, the steel material according to the present embodiment will be specifically described with reference to examples. However, the conditions in the following examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to the following examples.

[0074] Steel is produced through a refining process, and then continuously cast into slabs, which are then hot-rolled into steel materials. In the refining process, the molten steel is vacuum degassed, and once the dissolved oxygen concentration of the molten steel has reached 0.0040 mass% or less, Mg is added at a rate of 30 to 300 kg / min. Within 10 minutes of adding Mg, the nitrogen gas flow rate is increased to 1.0 Nm 3 / min or more and reflux was started for 1.0 min or more.

[0075] However, for Steel AO, Mg addition was started when the dissolved oxygen concentration of the molten steel was 0.0045%. For Steel AP, the Mg addition rate was 330 kg / min, and for Steel AQ, the Mg addition rate was 27 kg / min. For Steel AR, the time from Mg addition to the start of reflux was 12 minutes. For Steel AS, the nitrogen gas flow rate was 0.8 Nm 3 For steel AT, the reflux time was 0.6 minutes.

[0076] In the continuous casting process, when the molten steel after the refining process was continuously cast into a cast piece, the average cooling rate for the surface temperature of the cast piece from 1200°C to 900°C was set to 0.2°C / sec or more. However, for Steel AU, the average cooling rate was set to 0.1°C / sec.

[0077] Furthermore, the slabs after continuous casting were subjected to hot rolling to obtain steel materials. Some of the steel materials were further subjected to heat treatment.

[0078] For the obtained steel material, the number density of nanosulfides, the number ratio of composite inclusions in microsulfides, and the number density of composite inclusions were determined by the following procedures.

[0079] First, an extraction replica was prepared using a test piece taken from a position 1 / 4t of the steel plate thickness t, and was measured using a transmission electron microscope (TEM) equipped with an EDX detector. The number of particles with sizes between 0.005 and 5.0 μm was measured, and the value converted to the number per unit area was taken as the number density. The observation field was 100 mm × 80 mm at a magnification of 20,000 times. In this case, the observation area per field was 20 μm 2 And, observations were made in 50 fields of view.

[0080] When preparing the extraction replica from the steel material, in order to dissolve precipitates other than the MgMn-based sulfides, the steel material was held at 1400°C for 100 seconds and then rapidly cooled to prepare the extraction replica.

[0081] Next, the amount of MgMn-based sulfides present among the particles whose number was measured was measured. At least 20 or more particles were identified as MgMn-based sulfides under the following conditions, and their abundance ratio was calculated. The number of MgMn-based sulfides was calculated by multiplying the number of particles previously calculated by the abundance ratio of MgMn-based sulfides.

[0082] Next, the MgMn-based sulfides were identified as follows. MgMn-based sulfides were defined as those in which the ratio of Mg to the total of Mg and Mn was 70%≦Mg≦90% in atomic percent. However, when the amount of elements other than Mg and Mn was greater than the total of Mg and Mn, the sulfides were not considered to be MgMn-based sulfides.

[0083] Furthermore, when a trace amount of O was detected in a particle, it was deemed to be an MgMn-based sulfide if the ratio of S to the total of S and O was S ≥ 95% in atomic % and the O content was less than 5% and the amount was so small. However, even if the ratio of S to the total of S and O was S ≥ 95% in atomic % and the O content was less than 5%, if the particle was spherical and clearly identified as a composite of MnS and MgO, it was not deemed to be an MgMn-based sulfide.

[0084] The ratios of Mg and Mn and the ratios of S and O were quantitatively determined by EDX. The diameter of the electron beam used for this quantification was 0.001 μm, and the TEM observation magnification was 50,000 times, and the quantification was performed at any position within the fine MgMn-based sulfides.

[0085] Among the MgMn-based sulfides identified as above, those with a circle equivalent diameter of 0.005 μm or more and less than 0.5 μm were defined as nanosulfides, and those with a circle equivalent diameter of 0.5 μm or more and 5.0 μm or less were defined as microsulfides. Furthermore, among the microsulfides, those containing 1 atomic % or more of N were defined as composite inclusions. Then, the number density of the nanosulfides, the number ratio of the composite inclusions in the microsulfides, and the number density of the composite inclusions were obtained.

[0086] The tensile strength TS and yield stress YP were evaluated in accordance with JIS Z 2241:2011. The test pieces were No. 1B test pieces. The test method was the permanent elongation method. Those with a yield stress YP of 265 MPa or more and a tensile strength TS of 420 to 560 MPa were considered to have passed the test.

[0087] Next, test pieces for thermal cycle testing were taken from the steel material. A thermal cycle simulating welding with a heat input of 10 kJ / mm was applied to these test pieces. The specific thermal cycle conditions were heating from room temperature to 1400°C, holding the temperature for 10 seconds, and then cooling at an average cooling rate of 3.0°C / s in the temperature range of 800°C to 500°C, which is the temperature range related to intragranular transformation. Three V-notch test pieces were taken from each steel material after the thermal cycle was applied, and a Charpy impact test was performed at -60°C. The absorbed energy (vE -60 ) was measured. The V-notch test piece was prepared in accordance with the V-notch test piece described in JIS Z 2242:2005. The Charpy impact test was performed in accordance with JIS Z 2242:2005. The absorbed energy (vE -60 ) is 100J or more, and the average absorbed energy (vE -60 ) was 50 J or more, it was considered to have passed.

[0088] In addition, test pieces for measuring the austenite grain size were taken from the steel material, and the test pieces were heated to a peak temperature of 1400°C, held at that temperature for 10 seconds, and then subjected to a thermal cycle in which the samples were cooled at an average cooling rate of 3.0°C / second in the temperature range from 800°C to 500°C. The austenite grain size of the samples was measured.

[0089] The results are shown in Tables 1A to 1D and 2A to 2D.

[0090] As shown in Tables 1A, 1C, 2A and 2C, all of Steels A to U had excellent low-temperature toughness and also had excellent mechanical properties.

[0091] On the other hand, as shown in Tables 1B, 1D, 2B and 2D, the chemical compositions of Steels V to AN were outside the range specified by the present invention, and therefore the toughness was deteriorated.

[0092] In addition, although the chemical compositions of Steels AO to AU were within the ranges of the present invention, the manufacturing conditions did not satisfy the conditions of the present invention, and therefore Steels AO to AU did not satisfy the number density of nanosulfides, the number ratio of composite inclusions in microsulfides, or the number density of composite inclusions, and thus the toughness was deteriorated.

[0093] [Table 1A]

[0094] [Table 1B]

[0095] [Table 1C]

[0096] [Table 1D]

[0097] [Table 2A]

[0098] [Table 2B]

[0099]

Table 2C

[0100]

Table 2D

Claims

1. In mass percent, C: 0.030-0.250%, Si: 0.02-0.50%, Mn: 0.10-2.00%, P: 0.020% or less, S: 0.001-0.020%, Al: 0.010-0.200%, N: 0.0020 to 0.0050%, O: 0.0007 to 0.0020%, Ti: 0.004-0.025%, B: 0.0005 to 0.0050%, Mg: 0.0005-0.0050%, Ca: 0.0005% or less, REM: 0.0005% or less, The balance is a steel composition consisting of Fe and impurities, The steel contains MgMn-based sulfides in which the ratio of Mg to the total of Mg and Mn is 70% or more and 90% or less in atomic percent, The MgMn-based sulfides include nanosulfides having a circle equivalent diameter of 0.005 μm or more and less than 0.5 μm, and microsulfides having a circle equivalent diameter of 0.5 μm or more and 5.0 μm or less, The number density of the nanosulfides is 1.0×10 ４ ~30.0 x 10 ４ pieces / mm ２ and Among the microsulfides, 80% or more of the microsulfides in terms of number ratio are composite inclusions containing 1 atomic % or more of N, The number density of the composite inclusions is 46 to 300 pieces / mm ２ That is, steel.

2. The steel composition contains, in mass%, replacing a part of the Fe, Cu: 1.50% or less, Ni: 2.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, Nb: 0.050% or less, V: 0.100% or less The steel material according to claim 1, characterized in that it contains one or more selected from the group consisting of:

3. The steel composition contains, in mass%, replacing a part of the Fe, W: 1.00% or less, Sn: 0.50% or less The steel material according to claim 1 or 2, characterized in that it contains one or two selected from the group consisting of:

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