STEEL PLATE HAVING EXCELLENT HEAT AFFECTED ZONE TOUGHNESS AND METHOD FOR MANUFACTURING SAME

Abstract

The present invention relates to a steel plate for use in ships and the like, which has excellent toughness in a heat-affected zone (HAZ), even when a steel material having high strength and high ductility is welded with a certain amount of heat input or more, and to a method for manufacturing same.

Claims

1. A steel material having excellent weld heat-affected zone toughness, comprising by weight: 0.04 to 0.07% of carbon (C), 1.5 to 1.7% of manganese (Mn), 0.1 to 0.3% of silicon (Si), 0.01 to 0.04% of aluminum (Al), 0.7 to 1.0% of nickel (Ni), 0.05 to 0.30% of molybdenum (Mo), 0.010 to 0.018% of titanium (Ti), 0.01 to 0.03% of niobium (Nb), 0.003 to 0.006% of nitrogen (N), 0.007% or less of phosphorus (P), 0.002% or less of sulfur (S), a balance of Fe, and unavoidable impurities, wherein a weld heat-affected zone (HAZ) welded with a heat input of 100 to 200 KJ/cm includes a bainite phase of an area fraction of 90% or more in a region from a fusion line (FL) to FL+3 mm.

2. The steel material having excellent weld heat-affected zone toughness of claim 1, wherein an average grain size of prior austenite in the region from the fusion line (FL) to FL+3 mm is 100 ?m or less.

3. The steel material having excellent weld heat-affected zone toughness of claim 1, wherein the region from the fusion line (FL) to FL+3 mm has a tensile strength of 610 MPa or more and an impact toughness of 33 J or more at ?20? C.

4. The steel material having excellent weld heat-affected zone toughness of claim 1, wherein the steel material comprises bainite in an area fraction of 90% or more (including 100%) and residual acicular ferrite as a microstructure.

5. A method for manufacturing a steel material having excellent weld heat-affected zone toughness, comprising: reheating a steel slab including by weight: 0.04 to 0.07% of carbon (C), 1.5 to 1.7% of manganese (Mn), 0.1 to 0.3% of silicon (Si), 0.01 to 0.04% of aluminum (Al), 0.7 to 1.0% of nickel (Ni), 0.05 to 0.30% of molybdenum (Mo), 0.010 to 0.018% of titanium (Ti), 0.01 to 0.03% of niobium (Nb), 0.003 to 0.006% of nitrogen (N), 0.007% or less of phosphorus (P), 0.002% or less of sulfur (S), a balance of Fe, and unavoidable impurities, in a temperature range of 1100 to 1180? C.; rough rolling the heated steel slab in a temperature range of 900? C. or higher; finishing rolling the steel slab in a temperature range of 800? C. or higher after the rough rolling to prepare a hot-rolled steel material; and cooling the hot-rolled steel material until the temperature at t/4 point of the thickness (t, unit of mm) of the hot-rolled steel material is 600? C. or lower at a cooling rate of 10? C./s or more.

6. The method for manufacturing a steel material having excellent weld heat-affected zone toughness of claim 5, wherein the rough rolling is performed at a reduction ratio of 40% or more.

7. The method for manufacturing a steel material having excellent weld heat-affected zone toughness of claim 5, wherein the finish rolling is performed at a cumulative reduction ratio of 50% or more.

8. The method for manufacturing a steel material having excellent weld heat-affected zone toughness of claim 5, further comprising: welding the hot-rolled steel material after the cooling, wherein the welding is performed with a heat input of 100 to 200 KJ/cm.

Description

BEST MODE FOR INVENTION

[0017] Terms used in the present specification are for explaining specific exemplary embodiments rather than limiting the present disclosure. In addition, a singular form used in the present specification includes a plural form also, unless the relevant definition has a clearly opposite meaning thereto.

[0018] The meaning of comprising used in the specification is to embody the configuration and is not to exclude the presence or addition of other configurations.

[0019] Unless otherwise defined, all terms including technical terms and scientific terms used in the present specification have the same meaning as would be commonly understood by a person with ordinary skill in the art to which the present disclosure pertains. Pre-defined terms are interpreted as being consistent with the relevant technical literature and the disclosure herein.

[0020] The present inventor of the present disclosure has studied in detail a technology for improving toughness formed in a weld heat-affected zone, particularly, low-temperature toughness, when welding with a medium heat input of about 100 to 200 kJ/cm is performed on the steel material having high strength and toughness by increasing a welding heat input.

[0021] As a result thereof, the present inventor has confirmed that the technical purpose of the present disclosure may be achieved by controlling a grain size while securing a structure near a fusion line (FL) of the weld heat-affected zone as a hard state by optimizing an alloy composition and manufacturing conditions, thereby resulting in completion of the present disclosure.

[0022] Hereinafter, the present disclosure will be described in detail.

[0023] The steel material having excellent weld heat-affected zone toughness according to the present disclosure may be configured to have an alloy composition by weight: 0.04 to 0.07% of carbon (C), 1.5 to 1.7% of manganese (Mn), 0.1 to 0.3% of silicon (Si), 0.01 to 0.04% of aluminum (Al), 0.7 to 1.0% of nickel (Ni), 0.05 to 0.30% of molybdenum (Mo), 0.010 to 0.018% of titanium (Ti), 0.01 to 0.03% of niobium (Nb), 0.003 to 0.006% of nitrogen (N), 0.007% or less of phosphorus (P), and 0.002% or less of sulfur (S).

[0024] Hereinafter, a reason for limiting an alloy composition of the steel material provided in the present disclosure as described above will be described in detail.

[0025] Meanwhile, in the present disclosure, unless specifically stated, a content of each element is based on weight, and a ratio of a structure is based on area.

[0026] Carbon (C): 0.04 to 0.07%

[0027] Carbon (C) is the most important element in securing strength of not only a base material but also of a weld heat-affected zone, so it needs to be contained in steel within an appropriate range.

[0028] If the C content exceeds 0.07%, hardenability improves, the strength increases excessively, and there is a problem in that toughness of the weld heat-affected zone decreases due to precipitation of fine hard phases. On the other hand, if the C content is less than 0.04%, it is preferable because it causes a decrease in strength.

[0029] Therefore, C may be included in an amount of 0.04 to 0.07%. More advantageously, C may be included in an amount of 0.045% or more and 0.065% or less.

[0030] Manganese (Mn): 1.5 to 1.7%

[0031] Manganese (Mn) is a useful element for improving strength through solid solution strengthening and increasing hardenability to form a low-temperature transformation phase.

[0032] In the present disclosure, since the strength of the weld heat-affected zone is intended to be secured to 610 MPa or more, to this end, Mn is preferably included in an amount of 1.5% or more. However, if the Mn content exceeds 1.7%, the hardenability increases excessively and there is a risk that the toughness of the weld heat-affected zone will be greatly reduced.

[0033] Therefore, in the present disclosure, Mn may be included in an amount of 1.5 to 1.7%.

[0034] Silicon (Si): 0.1 to 0.3% and Aluminum (Al): 0.01 to 0.04%

[0035] Silicon (Si) and Aluminum (Al) are essential elements for deoxidation by precipitating dissolved oxygen in molten steel in a form of slag during a steelmaking and continuous casting process. When manufacturing a steel material using a converter, Si is preferably included in an amount of 0.1% or more, and Al is preferably included in an amount of 0.01% or more. However, if the content of Si and Al is excessive, since there is a risk that a coarse composite oxide of Si and Al may be generated or a large amount of fine hard phases may be generated in a microstructure of the weld heat-affected zone, Si is preferably included in an amount of 0.3% or less and Al is preferably included in an amount of 0.04% or less.

[0036] Nickel (Ni): 0.7 to 1.0%

[0037] Nickel (Ni) is an important element improving impact toughness by facilitating cross slip of dislocations at low temperatures and increasing strength by increasing hardenability.

[0038] The present disclosure intends to form a hard phase (preferably a bainite phase) near a fusion line in a weld heat-affected zone, and Ni is preferably included in an amount of 0.7% or more in order to improve the impact toughness in this hard phase structure.

[0039] However, if the Ni content exceeds 1.0%, there is a problem that the hardenability is excessive and the toughness is lowered, and manufacturing costs are also greatly increased. Therefore, in the present disclosure, Ni may be included in an amount of 0.7 to 1.0%.

[0040] Molybdenum (Mo): 0.05 to 0.30%

[0041] Molybdenum (Mo) is an element useful for increasing strength by improving hardenability of steel, and Mo is preferably included in an amount of 0.05% or more to secure the strength targeted in the present disclosure. However, if the Mo content is excessive, the strength may increase excessively and there is a risk that toughness may decrease, so Mo is preferably included not to exceed 0.30%.

[0042] Titanium (Ti): 0.010 to 0.018%

[0043] Titanium (Ti) precipitates as TiN when reheated and suppresses grain growth in a base material and weld heat-affected zone, thereby significantly improving toughness. In order to effectively precipitate TiN, Ti is preferably included in an amount of 0.010% or more. However, if the Ti content exceeds 0.018%, there is a problem in that low-temperature toughness deteriorates due to clogging of a casting nozzle or crystallization in a center portion, and a content ratio of Ti and N(N/Ti) decreases, and TiN precipitates become coarse, there is a risk that the toughness of the weld heat-affected zone may decrease.

[0044] Therefore, in the present disclosure, Ti may be included in an amount of 0.010 to 0.018%.

[0045] Niobium (Nb): 0.01 to 0.03%

[0046] Niobium (Nb) is precipitated into a form of NbC or NbCN to improve strength of a base material. In addition, Nb dissolved in solid solution when reheated to a high temperature precipitates very finely in a form of NbC during rolling, which has an effect of suppressing recrystallization of austenite and refining the structure.

[0047] In order to sufficiently obtain the above-described effect, Nb is preferably included in an amount of 0.01% or more. However, if the Nb content exceeds 0.03%, there is a high possibility that brittle cracks may be caused at an edge of the steel material, and a large amount of fine hard phases is generated, so that there is a risk that toughness may decrease.

[0048] Therefore, in the present disclosure, Nb may be included in an amount of 0.01 to 0.03%.

[0049] Nitrogen (N): 0.003 to 0.006%

[0050] Nitrogen (N) combines with Ti and precipitates as TiN, thereby suppressing growth of prior austenite grains, thereby exhibiting an effect of refining a grain size. As described above, in order to form fine TiN precipitates, N is preferably included in an amount of 0.003% or more. However, if the N content exceeds 0.006%, not only does toughness deteriorate due to generation of free nitrogen (free N), but also there is a problem that AlN precipitates and causes slab cracks.

[0051] Accordingly, N may be included in an amount of 0.003 to 0.006%, and more advantageously, N may be included in an amount of 0.004% or more and 0.005% or less.

[0052] Phosphorus (P): 0.007% or less and Sulfur (S): 0.002% or less

[0053] Phosphorus (P) and Sulfur (S) are elements causing brittleness of grain boundaries or by forming coarse inclusions to cause brittleness. For the purpose of improving resistance to brittle crack propagation of a steel material, P may be limited to 0.007% or less, and S may be limited to 0.002% or less.

[0054] It is preferable to have 0% of these elements, but considering that these elements may inevitably be added, 0% may be excluded.

[0055] A remainder of the present disclosure may be iron (Fe). However, in a general manufacturing process, inevitable impurities may be inevitably added from raw materials or an ambient environment, and thus, impurities may not be excluded. A person skilled in the art of a general manufacturing process may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.

[0056] The steel material of the present disclosure having the above-described alloy composition may be composed of a single phase of bainite, bainite in an area fraction of 90% or more, and residual acicular ferrite, as a microstructure, and therefrom, the steel material has a tensile strength of 610 to 770 MPa, and impact toughness of 33 J or more at ?20? C.

[0057] Meanwhile, it is preferable that the weld heat-affected zone formed by welding the steel material of the present disclosure with a medium heat input (about 100 to 200 kJ/cm) includes a bainite phase in an area fraction of 90% or more in a region from a fusion line (FL) to FL+3 mm.

[0058] If the bainite phase in the region of the fusion line is less than 90%, a target level of strength may not be secured.

[0059] The region of the fusion line may be a single-phase of bainite, but may also include an acicular ferrite phase in addition to bainite. In addition, a trace amount of the MA phase may be included in the bainite phase and the acicular ferrite structure, and it should be noted that the MA phase contained in this case is at a level that does not impair physical properties of the fusion line.

[0060] In addition, it is preferable that an average grain size of prior austenite from the fusion line (FL) to FL+3 mm region is 100 ?m or less. As described above, by refining the grain size in the fusion line region, the target level of strength and toughness may be advantageously secured.

[0061] Specifically, the region from the fusion Line (FL) to FL+3 mm has a tensile strength of 610 MPa or more and impact toughness of 33 J or more at ?20? C., illustrating excellent strength and low-temperature toughness.

[0062] Hereinafter, a method for manufacturing a steel material with excellent weld heat-affected zone toughness according to another aspect of the present disclosure will be described in detail.

[0063] The steel material of the present disclosure may be manufactured through a process of reheating a steel slab satisfying the above-described composition, performing rough rolling and finishing rolling the steel slab, and then cooling the steel slab. Hereinafter, each process is described in detail.

[0064] Reheating slab: 1100 to 1180? C.

[0065] It is preferable to reheat the steel slab satisfying the above-described alloy composition to a temperature in a range of 1100 to 1180? C. It is preferable to set the reheating temperature to 1100? C. or higher to sufficiently dissolve carbonitrides of Ti and/or Nb formed during continuous casting. However, when reheated to an excessively high temperature, there is a risk that austenite may coarsen, so the reheating temperature is preferably 1180? C. or lower.

[0066] Rough rolling: 900? C. or higher

[0067] The reheated steel slab is subjected to rough rolling to adjust a shape of the steel slab. The rough rolling temperature is preferably higher than a temperature (Tnr) at which austenite recrystallization stops, and accordingly, the rough rolling is preferably performed in a temperature range of 900? C. or higher. An effect of reducing the grain size may also be achieved through recrystallization of coarse austenite along with destruction of a casting structure such as dendrites, or the like, formed during casting by rolling.

[0068] In order to cause sufficient recrystallization and refine the structure during rough rolling at the above-described temperature, it is preferable that a total cumulative reduction ratio during rough rolling is 40% or more.

[0069] Finishing rolling: 800? C. or higher

[0070] In order to introduce an austenite structure of the rough-rolled steel sheet into a non-uniform microstructure, finishing rolling is performed to manufacture a hot-rolled steel material. Specifically, it is preferable that the finishing rolling is performed in a temperature range of 800? C. or higher in order to provide maximum deformation in the structure. If the finishing rolling temperature is less than 800? C., ferrite precipitates during cooling after completion of rolling and the strength decreases, so finishing rolling is preferably performed in a temperature range of 800? C. or higher.

[0071] In order to create a fine structure as possible during finishing rolling at the above-described temperature, the cumulative reduction ratio of the finishing rolling is preferably 50% or more.

[0072] Cooling after rolling: cooling is performed until the temperature at t/4 point of the thickness (t, unit of mm) of the hot-rolled steel material is 600? C. or lower at a cooling rate of 10? C./s or more.

[0073] When cooling the hot-rolled steel material prepared as above, when the cooling rate is less than 10? C./s or a cooling end temperature exceeds 600? C., there is a risk that a microstructure of a base material affecting the microstructure of the weld heat-affected zone formed by subsequent welding may become coarse. An upper limit of the cooling rate is not specifically limited in the present disclosure, but in the technical field to which the present disclosure pertains, the cooling rate may be 100? C./s or more, so as a preferred example, the cooling rate is preferably 200? C./s or less. In the present disclosure, the cooling end temperature is not particularly limited, and it should be noted that the cooling is performed to room temperature.

[0074] The cooled hot-rolled steel material may be welded, and the welding may be performed with a medium heat input, for example, may be performed with a heat input of 100 to 200 KJ/cm. In this case, any welding method may be used as long as it is a welding method that the above-described heat input may be applied. A non-limiting example may be EGW.

[0075] When the hot-rolled steel material according to the present disclosure is welded with a medium heat input, the formed weld heat-affected zone has a fine structure in which the region of the fusion line is mainly composed of a bainite phase, and thus the hot-rolled steel material has excellent low-temperature toughness as well as strength.

[0076] Hereinafter, the present disclosure will be described in more detail through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present disclosure. The scope of the present disclosure may be determined by matters described in the claims and matters able to be reasonably inferred therefrom.

MODE FOR INVENTION

[0077] A steel slab having a thickness of 300 mm having the composition shown in Table 1 (a remainder of Fe and unavoidable impurities) was reheated to a temperature of 1140? C., and was then continuously subjected to rough rolling in a temperature range of 980? C., and was finished with finishing rolling in a temperature range of 880? C. Thereafter, the steel slab was cooled until the temperature at a ?t point reaches a temperature in a range of 470 to 580? C. at a cooling rate of 16 to 35? C./s, to prepare a steel material. In this case, the rough rolling was performed at a reduction rate of 40% or more, and the finishing rolling was performed at a cumulative reduction rate of 50% or more.

[0078] For the steel material prepared as above, a microstructure thereof was measured and the results thereof were shown in Table 2.

[0079] In addition, for the steel material prepared according to the above, welding was performed with a heat input of 100 to 200 KJ/cm, where a microstructure of a region from a fusion line (FL) to FL+3 mm of a weld heat-affected zone and mechanical properties (tensile strength, low-temperature impact toughness) were analyzed and the results were shown in Table 2.

[0080] The microstructure of the steel material(base material) and the fusion line was observed using an optical microscope, and then classified using EBSD equipment, and a fraction thereof was measured.

[0081] The tensile strength of the steel material(base material) and the fusion line was measured using a universal tensile machine, and the low-temperature impact toughness was measured through a Charpy impact absorption energy (CVN) value at ?20? C. using a Charpy impact tester.

TABLE-US-00001 TABLE 1 Alloy composition (weight %) Steel type C Mn Si Al Ni Mo Nb Ti N* P* S* Inventive 0.049 1.64 0.16 0.02 0.89 0.13 0.012 0.013 58 65 9 Steel 1 Inventive 0.043 1.58 0.21 0.03 0.92 0.08 0.019 0.017 48 52 13 Steel 2 Inventive 0.056 1.68 0.11 0.02 0.76 0.16 0.023 0.013 39 47 12 Steel 3 Inventive 0.062 1.52 0.24 0.03 0.83 0.22 0.019 0.011 52 49 15 Steel 4 Comparative 0.093 1.66 0.16 0.03 0.91 0.32 0.016 0.015 49 68 12 Steel 1 Comparative 0.047 1.42 0.19 0.03 0.71 0.17 0.014 0.012 51 61 9 Steel 2 Comparative 0.061 1.54 0.23 0.02 0.58 0.12 0.017 0.014 49 53 8 Steel 3 Comparative 0.056 1.66 0.13 0.02 0.85 0.13 0.007 0.021 22 53 11 Steel 4 N*, P* and S* are expressed in ppm.

TABLE-US-00002 TABLE 2 FL to + FL to + FL to + Base material Welding FL to FL + 3 mm FL + 3 mm FL + 3 mm FL + 3 mm microstructure heat microstructure average tensile impact (fraction %) input (fraction %) AGS strength toughness Steel type B AF (KJ/cm) B remainder (?m) (MPa) (?20? C., J) Inventive 98 2 135 97 AF 91 635 59 Steel 1 Inventive 100 168 93 AF 88 623 43 Steel 2 Inventive 100 187 96 AF 97 619 59 Steel 3 Inventive 95 5 156 92 AF 92 621 82 Steel 4 Comparative 100 163 100 87 679 17 Steel 1 Comparative 78 22 175 67 AF + PF 97 572 63 Steel 2 Comparative 74 26 166 85 AF 89 598 13 Steel 3 Comparative 98 2 171 96 AF 129 612 29 Steel 4 B is bainite, AF is acicular ferrite, and PF is polygonal ferrite.

[0082] As illustrated in Tables 1 to 3, in Inventive Examples 1 to 4, satisfying all of the alloy composition and manufacturing conditions limited by the present disclosure, it can be confirmed that Inventive Steels 1 to 4 have excellent strength and low-temperature toughness of a weld heat-affected zone. In particular, it can be seen that the weld heat-affected zone of Inventive steels described above has a tensile strength of 610 MPa or more and an impact toughness of 33 J or more at ?20? C.

[0083] On the other hand, in Comparative Example 1 having an excessive content of C and Mo among the alloy compositions limited by the present disclosure had inferior impact toughness as the strength of the weld heat-affected zone is excessively higher as compared to that of Inventive Steel.

[0084] Comparative Example 2 illustrates a case in which a Mn content was insufficient, and in Comparative Example 2, the strength of the weld heat-affected zone was inferior as a bainite phase was not sufficiently formed of the weld heat-affected zone.

[0085] In Comparative Example 3, the low-temperature toughness of the weld heat-affected zone was significantly inferior due to an insufficient Ni content, and specifically, the low-temperature toughness thereof was 13J at ?20? C.

[0086] Comparative Example 4 illustrates a case in which a Ti content is excessive, but N and Nb contents are insufficient, and as a result, in Comparative Example 4, coarse TiN was precipitated in a weld zone, and a particle refinement effect due to NbC was insufficient, so that a grain size of prior austenite was coarse, and as a result thereof, low-temperature toughness was inferior.