WO2020080339A1 - 薄鋼板およびその製造方法 - Google Patents
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Definitions
- the present invention relates to a thin steel plate and a manufacturing method thereof.
- the thin steel sheet of the present invention has a tensile strength (TS) of 980 MPa or more and has excellent workability. Therefore, the thin steel sheet of the present invention is suitable as a raw material for automobile seat parts.
- TS tensile strength
- ferrite has an average crystal grain size of 3 ⁇ m or less and a volume fraction of 5% or less
- retained austenite has a volume fraction of 10 to 20%
- martensite has an average crystal grain size of 4 ⁇ m or less and a volume fraction of 20% or less.
- the ferrite content is 5% or less or the ferrite content is 5% or more and 50% or less and the retained austenite amount is 10% or more, and a composite structure of retained austenite and martensite is obtained. It is said that a steel sheet excellent in elongation, hole expandability and deep drawability can be obtained by refining MA and increasing the amount of retained austenite having a size of 1.5 ⁇ m or more.
- Patent Document 1 According to the technology proposed in Patent Document 1, the hardness of tempered martensite and bainite becomes high and the stretch-flangeability deteriorates unless cementite is precipitated. That is, the steel plate strength and formability inevitably change depending on the precipitation state of cementite. Therefore, the technique proposed in Patent Document 1 cannot provide a steel sheet having stable mechanical properties.
- the main object of the present invention is a sheet component, but extremely high bendability is required. At this time, since there is an effect of bending back until the final process, it is necessary to suppress the reduction of the plate thickness of the processed part under the condition of bending-bending back. In addition to the usual bendability, high uniform elongation and work hardening amount It is also necessary to combine them. In order to realize this, it was found that it is effective to contain BCC iron with a small disorder of the crystal structure in a certain fraction or more. Furthermore, it was found that the size of the hard phase needs to be reduced in order to prevent the occurrence of voids when tension and compression are repeated.
- the plate thickness of the thin steel plate targeted by the present invention is 0.4 mm or more and 2.6 mm or less.
- the present invention has been completed by earnestly investigating the steel plate components and the manufacturing conditions of the steel plate structure that satisfy the above requirements.
- the summary is as follows. [1]% by mass, C: 0.10% or more and 0.23% or less, Si: 1.30% or more and 2.20% or less, Mn: 2.0% or more and 3.2% or less, P: 0. 05% or less, S: 0.005% or less, Al: 0.005% or more and 0.100% or less, N: 0.0060% or less, the balance being Fe and inevitable impurities, and the ferrite area ratio.
- the area ratio of BCC iron surrounding the retained austenite having a circle equivalent diameter of 1 ⁇ m or less and having an orientation difference of 1 ° or less is 5% or more and 50% or less, and the area ratio of BCC iron having an orientation difference of 1 ° or more is 25% or more and 85% or more.
- the following steel structures are Thin steel sheet that. [3] The thin steel sheet according to [1] or [2], wherein the component composition further contains, in mass%, Sb: 0.001% or more and 0.050% or less. [4] The composition of the components is, in mass%, Ti: 0.001% or more and 0.1% or less, Nb: 0.001% or more and 0.1% or less, V: 0.001% or more and 0.3.
- the thin steel sheet according to any one of [1] to [3].
- the component composition is, in mass%, Cu: 0.01% or more and 0.2% or less, Mo: 0.01% or more and 1.0% or less, REM: 0.0002% or more and 0.050.
- Mg 0.0002% or more and 0.050% or less
- Ca 0.0002% or more and 0.050% or less, and any one or more of [1] to [4] is contained.
- the present invention has a high tensile strength (TS) of 980 MPa or more and excellent moldability. If the thin steel sheet of the present invention is applied to automobile parts, the weight of automobile parts can be further reduced.
- TS tensile strength
- 1 (a) to 1 (c) are schematic diagrams for explaining the definition of BCC iron with a misorientation of 1 ° or less surrounding a retained austenite having a circle equivalent diameter of 1 ⁇ m or less in the present invention.
- C 0.10% or more and 0.23% or less C contributes to the strengthening of the steel sheet and also has the effect of increasing the workability by promoting the formation of retained austenite.
- the C content needs to be 0.10% or more. It is preferably 0.11% or more.
- the C content exceeds 0.23%, BCC iron having a small crystal disorder and fine retained austenite cannot be obtained, so that the workability deteriorates. From the above, the C content was set to 0.23% or less. It is preferably 0.22% or less.
- the Si content is set to 1.30% or more and 2.20 or less Si increases the elongation of the steel sheet. Therefore, the Si content is set to 1.30% or more. It is preferably 1.35% or more. On the other hand, if Si is excessively added, the chemical conversion treatability is deteriorated and it becomes unsuitable as a member for automobiles. From such a viewpoint, the Si content is set to 2.20% or less. It is preferably 2.10% or less.
- Mn 2.0% or more and 3.2% or less
- Mn is an austenite stabilizing element and is an element necessary for suppressing the residual ferrite phase and obtaining the retained austenite area ratio. Therefore, the Mn content is set to 2.0% or more. It is preferably at least 2.1%. On the other hand, when the Mn content becomes excessive, the above effects are saturated and there are problems in castability and rollability. From the above, the Mn content was set to 3.2% or less. It is preferably 3.0% or less.
- P 0.05% or less
- P is a harmful element that reduces weldability. Therefore, it is preferable to reduce the P content as much as possible.
- the P content is acceptable up to 0.05%. It is preferably 0.02% or less. For use under more severe welding conditions, it is more preferable to suppress the content to 0.01% or less. On the other hand, in manufacturing, 0.002% may be unavoidably mixed.
- S 0.005% or less S forms coarse sulfides in steel, which extend during hot rolling to form wedge-shaped inclusions, which adversely affects weldability. Therefore, since S is also a harmful element, it is preferable to reduce S as much as possible.
- the S content is set to 0.005% or less. The amount is preferably 0.003% or less, but more preferably 0.001% or less for use under more severe welding conditions. In manufacturing, 0.0002% may be unavoidably mixed.
- Al 0.005% or more and 0.100% or less Al is added as a deoxidizer at the stage of steelmaking.
- the Al content is 0.005% or more.
- the Al content is set to 0.100% or less. It is preferably 0.085% or less.
- N 0.0060% or less
- N is a harmful element that adversely affects the formability because it deteriorates the room temperature aging property and causes unexpected cracks. Therefore, it is desirable to reduce N as much as possible. In the present invention, up to 0.0060% is acceptable. It is preferably 0.0050% or less. Although it is desirable to reduce the N content as much as possible, 0.0005% may inevitably be mixed in during production.
- the thin steel sheet of the present invention contains the above-mentioned basic components, and the balance other than the above-mentioned basic components has a component composition containing Fe (iron) and inevitable impurities.
- the thin steel sheet of the invention contains the above-mentioned basic components, and the balance has a composition of Fe and inevitable impurities.
- the component composition of the present invention may include the following elements as arbitrary elements.
- Mass% may include Sb: 0.001% or more and 0.050% or less.
- Sb is an element that suppresses decarburization of the steel sheet surface during annealing at high temperature and helps ensure stable mechanical properties. In order to obtain such effects, it is necessary to contain Sb: 0.001% or more. On the other hand, when Sb exceeds 0.050%, the above effect is saturated. Therefore, the Sb content is 0.050% or less.
- Ti 0.001% or more and 0.1% or less
- Nb 0.001% or more and 0.1% or less
- V 0.001% or more and 0.3% or less
- Ni 0.01 % Or more and 0.1% or less
- Cr 0.01% or more and 1.0% or less
- B 0.0002% or more and 0.0050% or less, or two or more of them
- Ti and Nb are elements that contribute to strengthening. On the other hand, if it is contained excessively, the pinning effect hinders the production of BCC with less disorder of the crystal structure. Therefore, the Ti and Nb contents are preferably 0.001% or more and 0.1% or less and 0.001% or more and 0.1% or less, respectively.
- V has a high solubility in steel, it can be melted to some extent by the high temperature annealing aimed at by the present invention.
- the V content is preferably 0.001% or more and 0.3% or less. More preferably, the lower limit of the total content of Ti, Nb and V is 0.005% or more, and more preferably the total content of Ti and Nb is 0.1% or less.
- Ni, Cr, and B By increasing the hardenability of Ni, Cr, and B, it becomes easier to obtain BCC iron with a misorientation of 1 ° or less that surrounds retained austenite having a circle equivalent diameter of 1 ⁇ m or less, which will be described later. On the other hand, if these elements are excessively contained, fine retained austenite cannot be obtained or the effect of hardenability is saturated. Therefore, Ni: 0.01% or more and 0.1% or less, Cr: 0.01% or more and 1.0% or less, and B: 0.0002% or more and 0.0050% or less are preferable.
- Cu 0.01% or more and 0.2% or less
- Mo 0.01% or more and 1.0% or less
- REM 0.0002% or more and 0.050% or less
- Mg 0.0002 % Or more and 0.050% or less
- Ca 0.0002% or more and 0.050% or less
- Ferrite area ratio 4% or less (including 0%)
- BCC iron containing fine residual austenite and having a small crystal disorder is generated at an appropriate fraction by holding at around 450 ° C. Quenches to form a fine low temperature transformation phase. For this reason, if the ferrite phase is excessively generated, the desired steel structure formation in the holding process is delayed. Furthermore, since the ferrite generated during annealing is soft, voids are easily generated at the interface with the adjacent hard phase, and bendability is reduced. Since the allowable range that can suppress such an influence is 4%, the ferrite area ratio is set to 4% or less. It is preferably 3% or less.
- the ferrite of the present invention is polygonal ferrite and is intended for a structure that does not include a corrosion mark or a second phase structure in the grain.
- Area ratio of martensite as-quenched is 10% or less (including 0%), Since as-quenched martensite is extremely hard, the grain boundary becomes a starting point of crack formation in the vicinity of the surface during bending, and the bendability is significantly reduced. To obtain the bendability required in the present invention, the area ratio of as-quenched martensite must be 5% or less. It is preferably 3% or less. The as-quenched martensite preferably has a smaller area ratio, and may be 0%.
- Retained austenite 7% or more and 20% or less Retained austenite improves the formability, and it is necessary to generate 7% or more of retained austenite in order to obtain the tensile properties required in the present invention. Therefore, the area ratio of retained austenite is set to 7% or more. It is preferably at least 8%. On the other hand, since excessive retained austenite deteriorates the delayed fracture property, the retained austenite is set to 20% or less. It is preferably 17% or less.
- the upper bainite, the lower bainite, and the tempered martensite are more than 71% and less than 93% in total.
- the region other than the above-mentioned structure is preferably mainly composed of the upper bainite, the lower bainite, and the tempered martensite. Since the base material of the steel sheet is mainly composed of these low-temperature transformation microstructures, it becomes easy to obtain the desired strength, and the distribution of hardness within the steel microstructure is narrowed, and local stress concentration during bending is relaxed. And improve bendability. In order to effectively exhibit such effects, the total amount is set to more than 71% and less than 93%.
- BCC iron that surrounds retained austenite with a circle equivalent diameter of 1 ⁇ m or less and has a misorientation of 1 ° or less: 4% or more and 50% or less BCC iron with less crystal disorder is rich in ductility and increases the amount of dislocation strengthening accompanying deformation. , Increase work hardening amount and uniform elongation.
- the BCC iron surrounds retained austenite having a circle-equivalent diameter of 1 ⁇ m or less, that is, BCC iron containing fine retained austenite and having a small disorder of crystals is generated.
- “surrounding” means enclosing 90% or more of the outer circumference of the retained austenite having a circle-equivalent diameter of 1 ⁇ m or less when confirmed by the method described in the examples.
- BCC iron with small crystal disorder is deformed preferentially in deformation with a small amount of strain, BCC iron is hardened when dislocations accumulate, and residual austenite undergoes plastic-induced transformation to cause strain.
- a high work hardening amount can be obtained in a high deformation amount region, and a property of high resistance to bending-bending back can be obtained.
- BCC iron having a small disorder of crystals surrounding the retained austenite relaxes local stress concentration due to hardness difference between different phases and improves bendability. It has been found that if the area ratio of the BCC iron surrounding the fine retained austenite is 4%, local stress concentration due to the hardness difference between different phases can be relaxed and good bendability can be secured. Therefore, in order to obtain such characteristics, the area ratio of BCC iron surrounding fine retained austenite needs to be 4% or more. It is preferably 5% or more, more preferably 7% or more, and further preferably 10% or more. On the other hand, if the area ratio exceeds 50%, the desired steel plate strength cannot be obtained.
- the area ratio of BCC iron surrounding the fine retained austenite and having an orientation difference of 1 ° or less is set to 50% or less. It is preferably 45% or less. Further, if the circle-equivalent diameter of the fine retained austenite exceeds 1 ⁇ m, the retained austenite undergoes plastic-induced transformation with a comparatively low strain amount, and thus desired work hardening characteristics cannot be obtained. Therefore, the equivalent circle diameter of the retained austenite surrounded by the BCC iron is set to 1 ⁇ m or less. By satisfying the steel structure of the present invention, generation of BCC iron surrounding the retained austenite having a circle equivalent diameter of more than 1 ⁇ m is suppressed, and a desired effect is obtained.
- Area ratio of BCC iron having a misorientation of more than 1 °: 25% or more and 85% or less Structures having a misorientation of more than 1 ° include lower bainite, martensite and tempered martensite, which contribute to strengthening the steel sheet. Not only that, by further developing the substructure in the crystal grains, the microscopic interface becomes an obstacle to the propagation of cracks generated in bending. This has the effect of improving the bendability in a synergistic manner, in addition to the effect of forming a hard and uniform structure described above. In order to sufficiently obtain such an effect, the area ratio of BCC iron having an orientation difference of more than 1 ° must exceed 25%.
- the area ratio of BCC iron having an orientation difference of more than 1 ° is set to 25% or more and 85% or less.
- a preferred range is 35% or more and 75% or less.
- the method for manufacturing a thin steel sheet of the present invention includes a hot rolling step, a cold rolling step, and an annealing step. Hereinafter, each step will be described.
- the hot rolling process is a process of hot rolling a steel material having the above composition.
- the melting method for manufacturing the steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be adopted. Further, secondary refining may be performed in a vacuum degassing furnace. After that, it is preferable to form a slab (steel material) by a continuous casting method from the viewpoint of productivity and quality. Further, a slab may be formed by a known casting method such as an ingot-slump rolling method or a thin slab continuous casting method.
- the hot rolling conditions are not particularly limited and may be set appropriately.
- the coiling temperature after hot rolling may be 580 ° C. or lower, more preferably 530 ° C. or lower from the viewpoint of the shape of the coil for cold rolling.
- the cold rolling process is a process of pickling and cold rolling after the hot rolling process.
- the nucleation of the reverse transformation in the subsequent heating process is distributed at a high density, and the reverse transformation to austenite is promoted. Therefore, the cold rolling rate needs to be 46% or more. It is preferably 50% or more. Although there is no upper limit, it is substantially 75% or less due to the cold rolling load.
- the conditions of pickling are not particularly limited, and the conditions may be set by an ordinary method.
- a heat treatment step of heating to 480 ° C. or higher and 650 ° C. or lower and allowing it to stay in this temperature range for 1 hour or longer.
- fine cementite is precipitated, and the reverse transformation proceeds further using this as a nucleus, whereby a desired structure is easily obtained.
- the annealing step is, after the cold rolling step, heated and retained at 815 ° C. or higher for 130 seconds or more, and then cooled to 420 ° C. or higher and 520 ° C. or lower at an average cooling rate from 800 ° C. to 520 ° C. of 8 ° C./s or higher. Then, it is allowed to stay at 420 ° C. or higher and 520 ° C. or lower for 12 seconds or longer and 60 seconds or shorter, and is cooled to a cooling stop temperature of 200 ° C. or higher and 350 ° C. or lower at an average cooling rate of 420 ° C. to 300 ° C.
- Heating temperature 815 ° C. or more Residence time: 130 seconds or more
- the reverse transformation to austenite is sufficiently progressed, so that the BCC iron surrounding the retained austenite within 1 ° and the orientation difference between 1 ° and Create a substrate to form a better balance of BCC iron in an appropriate balance.
- the reverse transformation to austenite does not proceed sufficiently, the BCC iron surrounding the retained austenite with an orientation difference of 1 ° or less is insufficiently generated, and the fraction of BCC iron exceeding the orientation difference of 1 ° also decreases. , Bending-back bending resistance deteriorates.
- the heating temperature is not particularly limited, but 900 ° C. or lower is preferable for the reason of heat damage to the heating furnace.
- the upper limit of the residence time is not particularly limited, but 350 seconds or less is preferable from the viewpoint of productivity.
- the cooling stop temperature is set to 420 ° C. or higher. It is preferably 450 ° C. or higher. If it exceeds 520 ° C, fine residual austenite cannot be obtained due to the influence of polygonal ferrite formation. Therefore, the cooling stop temperature is set to 520 ° C. or lower.
- Residence time in the temperature range of 420 ° C. or higher and 520 ° C. or lower 12 seconds or more and 60 seconds or less
- the disorder of the crystal structure surrounding fine retained austenite Produces small BCC iron. If the retention temperature is lower than 420 ° C. or the retention time of 420 ° C. or higher and 520 ° C. or lower is shorter than 12 seconds, it is not possible to sufficiently obtain BCC iron with small disorder of crystals surrounding fine retained austenite. It is preferably 15 seconds or more. On the other hand, if the temperature exceeds 520 ° C, the desired retained austenite cannot be obtained.
- a suitable range is to allow the liquid to stay at 430 ° C. or more and 505 ° C. or less for 20 seconds or more and 55 seconds or less. In this retention, the temperature may change as long as it is within the above temperature range, or may be kept isothermal.
- Cooling stop temperature 200 ° C or more and 350 ° C or less
- the upper limit of the average cooling rate is not particularly limited.
- stop cooling in the temperature range of 200 ° C to 350 ° C. It is preferably 230 ° C. or higher and 330 ° C. or lower.
- the cooling stop temperature is lower than 200 ° C., the austenite existing in the steel sheet undergoes martensitic transformation, so that a desired amount of retained austenite cannot be obtained.
- the lower bainite transformation progresses insufficiently and the desired effect cannot be obtained, and if it exceeds 25 seconds, the effect is not only saturated, but also the reheating effect of the next step fluctuates, and the material, especially the strength fluctuation. Grows larger. It is preferably 3 seconds or more and 20 seconds or less.
- Heating temperature 300 ° C or more and 500 ° C or less
- Residence time in the temperature range of 300 ° C or more and 500 ° C or less 480 seconds or more and 1800 seconds or less
- C is concentrated in the retained austenite. The purpose is to make it remain as retained austenite when it is cooled to room temperature and to temper the part that has undergone martensitic transformation at the time of heating. If the residence temperature is lower than 300 ° C. or the residence time is shorter than 480 seconds, the retained austenite is not concentrated, and thermally unstable austenite undergoes martensite transformation when cooled to room temperature. The amount of austenite cannot be obtained.
- tempering of martensite does not proceed sufficiently with hard quenching.
- the residence temperature exceeds 500 ° C. or the residence time exceeds 1800 seconds
- cementite precipitates in austenite and decomposes, so that a desired amount of retained austenite cannot be obtained.
- tempering proceeds excessively, a desired strength can be obtained. Therefore, in the reheating after cooling from 200 ° C. to 350 ° C., it was made to stay in the range of 300 ° C. or more and 500 ° C. or more for 480 seconds or more and 1800 seconds or less.
- a 250 mm thick steel material having the composition shown in Table 1 was hot-rolled, pickled, and cold-rolled, and then annealed in a continuous annealing furnace under the conditions shown in Table 2 to obtain an elongation of 0.2. % To 0.4% was temper-rolled to produce a steel sheet for evaluation. Partly, the heat treatment step was performed in a box-type annealing furnace before cold rolling or before the annealing step. And the obtained steel plate was evaluated by the following methods.
- Ferrite is a structure in which corrosion marks and second phase structures are not observed in the grains.
- the upper bainite is a structure in which corrosion marks and a second phase structure are recognized in the grains
- the tempered martensite and the lower bainite are structures in which a lath structure and a fine second phase structure are observed in the grains.
- the total of the structures of the upper bainite, the lower bainite, and the tempered martensite was obtained as the total of the above area ratios.
- BCC iron surrounding the retained austenite having a circle-equivalent diameter of 1 ⁇ m or less was used for the same cross section as the SEM observation. Specifically, a region of 1 ⁇ 10 3 ⁇ m 2 or more in the plate thickness 1 ⁇ 4t portion was measured at a measurement step of 0.1 ⁇ m.
- BCC iron having a KAM value of 1 ° or less was determined by the KAM (Kernel Average Misorientation) method, and the retained austenite was identified by the phase map.
- a cutting method was adopted for both the SEM image and the EBSD image, and 20 horizontal lines and 20 vertical lines each having an actual length of 30 ⁇ m were drawn so as to form a grid pattern on the obtained photograph, The tissue was identified, and the ratio of the number of intersections of each tissue to all the intersections was defined as the area ratio of each tissue.
- it does not cross over a large angle grain boundary with an orientation difference of 15 ° or more, does not cross over a BCC iron having a KAM value of more than 1 °, and has a circle equivalent diameter of 1 ⁇ m or less around retained austenite.
- BCC iron having a KAM value of 1 ° or less that surrounds or is in contact with 90% or more of the peripheral length of retained austenite was identified as BCC iron having a misorientation of 1 ° or less that surrounds retained austenite having a circle equivalent diameter of 1 ⁇ m or less. According to the above definition, those that meet the following (a) and (b) are outside the range of BCC iron with a misorientation within 1 ° surrounding the retained austenite having a circle equivalent diameter of 1 ⁇ m or less, which is defined above, Only BCC iron that meets the following (c) is within the above definition.
- A Retained austenite having a circle equivalent diameter of 1 ⁇ m or less is in contact with two BCC iron crystal grains across a large-angle grain boundary with a misorientation of 15 ° or more, and BCC iron in two regions and a circle equivalent diameter of 1 ⁇ m or less remain BCC iron having a circle equivalent diameter of 1 ⁇ m or less and having a circle equivalent diameter of 1 ⁇ m or less and exceeding 10% of the entire circumference of the retained austenite (b) BCC iron having a KAM value of 1 ° or more adjacent to the retained austenite having a circle equivalent diameter of 1 ⁇ m or less BCC iron (c) in which the crystal grains are inherently austenite with a circle equivalent diameter of 1 ⁇ m or less is in contact with two BCC iron crystal grains across a large angle grain boundary with a misorientation of 15 ° or more.
- the product of uniform elongation and tensile strength is 12000 MPa ⁇ % or more
- the condition for suppressing necking and cracking is a material that can withstand severe processing such as bending and unbending such as roll forming.
- tensile strength TS was 980 MPa or more, and it was found that good moldability was obtained. Further, in the present invention example in which the area ratio of BCC iron surrounding fine retained austenite is 4% or more, good uniform elongation (U-El), total elongation (El), even at tensile strength TS: 980 MPa or more, The amount of work hardening and bendability were shown. On the other hand, in Comparative Examples outside the scope of the present invention, the tensile strength did not reach 980 MPa, or the work hardening amount and bendability required in the present invention were not obtained.
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Abstract
Description
[1]質量%で、C:0.10%以上0.23%以下、Si:1.30%以上2.20%以下、Mn:2.0%以上3.2%以下、P:0.05%以下、S:0.005%以下、Al:0.005%以上0.100%以下、N:0.0060%以下、残部がFeおよび不可避的不純物からなる成分組成と、フェライト面積率が4%以下(0%を含む)、焼入ままマルテンサイトの面積率が10%以下(0%を含む)、残留オーステナイトが7%以上20%以下、上部ベイナイト、下部ベイナイト、焼戻しマルテンサイトが合計で71%超え93%未満を含み、さらに円相当直径1μm以下の残留オーステナイトを囲む方位差1°以内のBCC鉄の面積率が4%以上50%以下、方位差1°を上回るBCC鉄の面積率が25%以上85%以下である鋼組織と、を有する薄鋼板。
[2]質量%で、
C:0.10%以上0.23%以下、Si:1.30%以上2.20%以下、Mn:2.0%以上3.2%以下、P:0.05%以下、S:0.005%以下、Al:0.005%以上0.100%以下、N:0.0060%以下、残部がFeおよび不可避的不純物からなる成分組成と、フェライト面積率が4%以下(0%を含む)、焼入ままマルテンサイトの面積率が10%以下(0%を含む)、残留オーステナイトが7%以上20%以下、上部ベイナイト、下部ベイナイト、焼戻しマルテンサイトが合計で71%超え93%未満を含み、さらに円相当直径1μm以下の残留オーステナイトを囲む方位差1°以内のBCC鉄の面積率が5%以上50%以下、方位差1°を上回るBCC鉄の面積率が25%以上85%以下である鋼組織と、を有する薄鋼板。
[3]前記成分組成は、さらに、質量%で、Sb:0.001%以上0.050%以下を含有する[1]または[2]に記載の薄鋼板。
[4]前記成分組成は、さらに、質量%で、Ti:0.001%以上0.1%以下、Nb:0.001%以上0.1%以下、V:0.001%以上0.3%以下、Ni:0.01%以上0.1%以下、Cr:0.01%以上1.0%以下およびB:0.0002%以上0.0050%以下の1種または2種以上を含有する[1]~[3]のいずれかに記載の薄鋼板。
[5]前記成分組成は、さらに、質量%で、Cu:0.01%以上0.2%以下、Mo:0.01%以上1.0%以下、REM:0.0002%以上0.050%以下、Mg:0.0002%以上0.050%以下およびCa:0.0002%以上0.050%以下の1種または2種以上を含有する[1]~[4]のいずれかに記載の薄鋼板。
[6][1]~[5]のいずれかに記載の成分組成を有し、熱延鋼板に46%以上の冷間圧延率で冷間圧延する冷延工程と、前記冷延工程後、加熱し、815℃以上で130秒以上滞留させた後、800℃から520℃までの平均冷却速度が8℃/s以上で420℃以上520℃以下の温度域まで冷却し、該温度域で12秒以上60秒以下滞留させ、420℃から300℃までの温度区間で平均冷却速度が8℃/s以上で200℃以上350℃以下の冷却停止温度まで冷却し、該冷却停止温度から±50℃の温度域に2秒以上25秒以下滞留したのち、300℃以上500℃以下の温度まで加熱した後、該温度範囲で480秒以上1800秒以下滞留させる焼鈍工程と、を有する薄鋼板の製造方法。
Cは、鋼板の高強度化に寄与するうえ、残留オーステナイトの生成を促進することで加工性を上昇させる効果がある。本発明で求める引張強さ:980MPa以上かつ、所望の溶融金属部の硬度を得るには、少なくともC含有量は0.10%以上とする必要がある。好ましくは0.11%以上である。一方、C含有量が0.23%を上回ると、結晶の乱れの小さいBCC鉄や微細な残留オーステナイトが得られなくなるため、加工性が劣化する。以上から、C含有量を0.23%以下とした。好ましくは0.22%以下である。
Siは鋼板の伸びを上昇させる。そこで、Si含有量は1.30%以上とする。好ましくは1.35%以上である。一方、Siを過度に添加すると化成処理性が悪化し、自動車用部材として適さなくなる。このような観点から、Si含有量は2.20%以下とした。好ましくは2.10%以下である。
Mnはオーステナイト安定化元素であり、フェライト相の残存を抑制し残留オーステナイト面積率を得るために必要な元素である。そこで、Mn含有量は2.0%以上とする。好ましくは2.1%以上である。一方、Mn含有量が過度になると、上記効果が飽和するうえ鋳造性や圧延性に問題が生じる。以上から、Mn含有量は3.2%以下とした。好ましくは3.0%以下である。
Pは、溶接性を低下させる有害元素である。このため、P含有量は極力低減することが好ましい。本発明では、P含有量は0.05%まで許容できる。好ましくは0.02%以下である。より厳しい溶接条件下で使用するには、0.01%以下まで抑制することがより好ましい。一方、製造上、0.002%は不可避的に混入する場合がある。
Sは、鋼中で粗大な硫化物を形成し、これが熱間圧延時に伸展し楔状の介在物となることで、溶接性に悪影響をもたらす。そのため、Sも有害元素であるため極力低減することが好ましい。本発明では、0.005%まで許容できるため、S含有量を0.005%以下とした。好ましくは、0.003%以下であるが、より厳しい溶接条件下で使用するには、0.001%以下まで抑制することがより好ましい。製造上、0.0002%は不可避的に混入する場合がある。
Alを製鋼の段階で脱酸剤として添加する。この添加目的からAl含有量を0.005%以上とする。一方、Alが0.100%を上回ると脱酸剤としての効果が飽和するうえ、鋳造性劣化を招く。このような観点から、Al含有量は0.100%以下とした。好ましくは0.085%以下である。
Nは、常温時効性を悪化させ予期せぬ割れを発生させるため、成形性に対して悪影響をもたらす有害元素である。そのため、Nは出来る限り低減することが望ましい。本発明では0.0060%まで許容できる。好ましくは0.0050%以下である。N含有量は極力低減する方が望ましいが、製造上、0.0005%は不可避的に混入する場合がある。
本発明では焼鈍中に充分にオーステナイトへの逆変態を進行させた後、450℃近傍での保持により微細な残留オーステナイトを内包する結晶の乱れの小さいBCC鉄を適正な分率で生成させたのち急冷して微細な低温変態相を生成する。このため、フェライト相が過度に生成した状態であると保持過程での所望の鋼組織生成が遅延する。さらには、焼鈍中に生成するフェライトは軟質であるため、隣接する硬質相との界面でボイドが生成し易くなり曲げ性を低下させる。このような影響を抑制できる許容範囲は4%であるため、フェライト面積率を4%以下とした。好ましくは3%以下である。本発明のフェライトはポリゴナルフェライトであり、粒内に腐食痕や第二相組織が含まれない組織を対象とする。
焼入ままマルテンサイトは非常に硬質であるため、曲げ加工時に表面近傍で粒界が亀裂の発生起点となり曲げ性を著しく低下させる。本発明で求める曲げ性を得るには、焼入ままマルテンサイトの面積率は5%以下とする必要がある。好ましくは3%以下である。焼入ままマルテンサイトの面積率は少ないほど好ましく、0%であっても良い。
残留オーステナイトは成形性を改善し、本発明で求める引張特性を得るには7%以上の残留オーステナイトを生成させる必要がある。そこで、残留オーステナイトの面積率は7%以上とした。好ましくは8%以上である。一方、過剰の残留オーステナイトは遅れ破壊特性を悪化させるため、残留オーステナイトは20%以下とした。好ましくは17%以下である。
上記した組織以外の領域は、主に上部ベイナイト、下部ベイナイト、焼戻しマルテンサイトから構成されるのが望ましい。鋼板の素地が主としてこれらの低温変態組織より構成されることで、所望の強度を得やすくなるとともに、鋼組織内での硬さの分布が狭小化して曲げ加工時の局部的な応力集中を緩和し曲げ性を改善する。このような効果を有効に発現するには、その合計が71%超え93%未満とした。
結晶の乱れの少ないBCC鉄は延性に富み、変形にともなう転位強化量を上昇させるため、加工硬化量および均一伸びを上昇させる。本発明では、該BCC鉄が円相当直径1μm以下の残留オーステナイトを囲むこと、すなわち微細な残留オーステナイトを内包する結晶の乱れの小さいBCC鉄を生成させることが特徴のひとつである。ここで、「囲む」とは、実施例に記載の方法で確認したときに、円相当直径が1μm以下の残留オーステナイトの外周の90%以上を囲うことを指す。このような鋼組織とすることで、ひずみ量が小さい変形では結晶の乱れが小さいBCC鉄が優先的に変形し、転位が蓄積するとBCC鉄が硬化し、残留オーステナイトが塑性誘起変態することでひずみ量が高い変形領域で高い加工硬化量が得られ、曲げ-曲げ戻しへの耐性が高い特性が得られる。さらに、残留オーステナイトがマルテンサイトに変態して硬質化するにあたり、これを囲む結晶の乱れが小さいBCC鉄が異相間の硬度差にともなう局所的な応力集中を緩和し曲げ性を向上する。微細な残留オーステナイトを囲むBCC鉄の面積率が4%あれば、異相間の硬度差にともなう局所的な応力集中を緩和でき、良好な曲げ性を担保できることを知見した。したがって、このような特性を得るには微細な残留オーステナイトを囲むBCC鉄の面積率は4%以上必要である。好ましくは5%以上、より好ましくは7%以上で、さらに好ましくは10%以上である。一方、該面積率が50%を超えると所望の鋼板強度が得られなくなる。そのため、微細な残留オーステナイトを囲む方位差1°以内のBCC鉄の面積率は50%以下とした。好ましくは45%以下である。また、この微細な残留オーステナイトの円相当直径が1μmを上回ると比較的低いひずみ量で残留オーステナイトが塑性誘起変態してしまうため、所望の加工硬化特性が得られない。したがって、上記BCC鉄に囲まれる残留オーステナイトの円相当直径は1μm以下とした。なお、本発明の鋼組織を満たすことで、円相当直径が1μmを上回る残留オーステナイトを囲むBCC鉄の生成が抑制され、所望の効果が得られる。
方位差1°を上回る組織は下部ベイナイト、マルテンサイトおよび焼き戻しマルテンサイトなどが挙げられるが、鋼板の高強度化に寄与するのみならず、さらに結晶粒内に微細に下部組織を発達させることで、その微視的な界面が曲げ加工に発生した亀裂が伝播する障害となる。これにより上記した硬質かつ均一な組織形成による効果に加え、相乗して曲げ性を向上する作用がある。このような効果を充分に得るためには、方位差1°を上回るBCC鉄の面積率が25%を超える必要がある。一方、このような組織は塑性変形能に劣るため、該面積率が85%を上回ると、所望の成形性が得られなくなる。そのため、方位差1°を上回るBCC鉄の面積率は25%以上85%以下とした。好ましい範囲は35%以上75%以下である。
滞留時間:130秒以上
この加熱及び滞留では、オーステナイトへの逆変態を充分に進行させることで、残留オーステナイトを囲む方位差1°以内のBCC鉄と、方位差1°を上回るBCC鉄を適切なバランスで形成させる素地を作る。このときオーステナイトへの逆変態が十分に進行していないと、残留オーステナイトを囲む方位差1°以内のBCC鉄の生成が不十分となり、方位差1°を上回るBCC鉄の分率も低下するため、曲げ-曲げ戻しへの耐性が悪化する。所望のオーステナイトを得るには、815℃以上で130秒以上滞留させる必要がある。好ましくは、830℃以上で130秒以上滞留させ、さらに好ましくは850℃以上で140秒以上滞留させる。加熱温度の上限は特に限定されないが加熱炉の熱損傷の理由から900℃以下が好ましい。また、滞留時間の上限は特に限定されないが生産性の観点から350秒以下が好ましい。
冷却停止温度:420℃以上520℃以下
加熱後はポリゴナルフェライトの生成を抑制する必要がある。この間にポリゴナルフェライトが生成すると微細な残留オーステナイトを含む結晶の乱れが小さいBCC鉄が得られなくなり、所望の鋼板特性が得られなくなる。この観点から、ポリゴナルフェライト生成域である800℃から520℃までの平均冷却速度を8℃/s以上とした。好ましくは10℃/s以上である。上記平均冷却速度の上限は特に定めないが、実質的に150℃/s以下である。
420℃以上520℃以下の温度域での12秒以上60秒以下の滞留で、微細な残留オーステナイトを取り囲む結晶構造の乱れが小さいBCC鉄を生成させる。滞留温度が420℃を下回ったり、420℃以上520℃以下の滞留時間が12秒を下回ったりすると微細な残留オーステナイトを取り囲む結晶の乱れが小さいBCC鉄が十分に得られなくなる。好ましくは15秒以上である。一方、520℃を上回ると所望の残留オーステナイトが得られなくなる。420℃以上520℃以下の滞留時間が60秒を上回ると、結晶の乱れが小さいBCC鉄が過度に生成して、所望の引張強さ:980MPaが得られない。好適範囲は、430℃以上505℃以下で20秒以上55秒以下滞留させることである。また、この滞留においては、上記温度範囲内にあれば温度変化があってもよいし、等温保持でもよい。
冷却停止温度:200℃以上350℃以下
冷却過程で生成する鋼組織を微細化し、方位差1°超のBCC鉄の生成を促進ため、420℃から300℃まで平均冷却速度8℃/s以上で冷却する必要がある。8℃/sを下回ると下部組織の微細化が抑制され、方位差1°超のBCC鉄の生成が不十分になる。好ましくは10℃/s以上である。平均冷却速度の上限は特に限定されない。
また、冷却停止温度から50℃低い温度までの温度域においては下部ベイナイト変態が進行する。この下部ベイナイト変態進行によって未変態オーステナイトの量が低下して、最終的な焼入ままマルテンサイト量が減少し曲げ性が改善する。この効果を得るには、200℃以上350℃以下の冷却停止温度まで冷却した時点から再加熱までの区間、すなわち、冷却停止温度から±50℃の温度域で2秒以上25秒以下滞留させる必要がある。2秒未満では下部ベイナイト変態進行が不十分で所望の効果が得られず、25秒を超えると、その効果は飽和するばかりか、次工程の再加熱効果に変動が生じ、材質、特に強度変動が大きくなる。好ましくは3秒以上20秒以下である。
300℃以上500℃以下の温度域での滞留時間:480秒以上1800秒以下
300℃以上500℃以下の温度域での滞留では、残留オーステナイト中にCを濃化させ、室温まで冷却した際に残留オーステナイトとして残存させるとともに、加熱時点でマルテンサイト変態した部分を焼き戻すことが目的である。滞留温度が300℃を下回る、あるいは滞留時間が480秒を下回ると、残留オーステナイト中が濃化されず、熱的に不安定なオーステナイトは室温まで冷却した際にマルテンサイト変態するため、所望の残留オーステナイト量が得られない。さらに、硬質な焼入れままマルテンサイトの焼き戻しが充分に進行しない。一方、滞留温度が500℃を上回る、あるいは滞留時間が1800秒を上回ると、オーステナイト中にセメンタイトが析出して分解するため、所望の残留オーステナイト量が得られない。さらには、過度に焼き戻しが進行した場合には所望の強度が得られる。そこで、200℃から350℃まで冷却した後の再加熱では300℃以上500℃以上の範囲で480秒以上1800秒以下滞留させることとした。
鋼板から、圧延方向に平行な板厚断面が観察面となるよう切り出し、板厚中心部を1%ナイタールで腐食現出し、走査電子顕微鏡で2000倍に拡大して鋼板表面から板厚の1/4の深さ位置(以下、単に板厚1/4t部という。)を10視野分撮影した。フェライトは粒内に腐食痕や第二相組織が観察されない組織である。上部ベイナイトは粒内に腐食痕や第二相組織が認められる組織であり、焼き戻しマルテンサイトおよび下部ベイナイトは粒内にラス構造や微細第二相組織が観察される組織である。上部ベイナイト、下部ベイナイトおよび焼き戻しマルテンサイトの組織の合計は上記全ての面積率合計として求めた。
(a)円相当直径が1μm以下の残留オーステナイトが方位差15°以上の大角粒界を跨いで2つのBCC鉄の結晶粒に接し、2つの領域のBCC鉄と円相当直径が1μm以下の残留オーステナイトとの境界の長さがともに円相当直径が1μm以下の残留オーステナイト周全長の10%を超えるBCC鉄
(b)円相当直径が1μm以下の残留オーステナイトと隣接するKAM値1°以上のBCC鉄の結晶粒が内存するBCC鉄
(c)円相当直径が1μm以下の残留オーステナイトが方位差15°以上の大角粒界を跨いで2つのBCC鉄の結晶粒に接しているが、2つの領域のBCC鉄と円相当直径が1μm以下の残留オーステナイトとの境界の長さのどちらかが円相当直径が1μm以下の残留オーステナイト周全長の10%を超えないBCC鉄
図1に上記(a)~(c)の模式図を示す。なお、円相当直径1μm以下の残留オーステナイトを囲む方位差1°を上回るBCC鉄の面積率の算出に当たっては、100%-(円相当直径1μm以下の残留オーステナイトを囲む方位差1°以内のBCC鉄の面積率+円相当直径1μm以上の残留オーステナイトを囲むブロックの面積率+残留オーステナイトの面積率あるいはXRDで求めた体積率)を計算すればよい。
鋼板を板厚1/4位置まで研磨後、化学研磨により更に0.1mm研磨した面について、X線回折装置でMoのKα線を用い、FCC鉄(オーステナイト)の(200)面、(220)面、(311)面と、BCC鉄(フェライト)の(200)面、(211)面、(220)面の積分反射強度を測定し、BCC鉄(フェライト)各面からの積分反射強度に対するFCC鉄(オーステナイト)各面からの積分反射強度の強度比から求めたオーステナイトの割合を残留オーステナイト分率とした。
得られた鋼板から圧延方向に対して垂直方向にJIS5号引張試験片を作製し、JIS Z 2241(2011)の規定に準拠した引張試験を5回行い、平均の、引張強さ(TS)、均一伸び(U-El)、全伸び(El)を求めた。引張試験のクロスヘッドスピードは10mm/minとした。表3において、引張強さ:980MPa以上、かつTSとU-Elとの積が12000MPa・%以上を本発明鋼で求める鋼板の機械的性質とした。
曲げ性を調査するため、幅100mm、長さ35mmの短冊状サンプルを切り出し、JIS Z 2248に準拠した頂角90°のVブロック法にて曲げ試験を実施し、割れが発生しない最小のダイス径(R)を求め、板厚(t)で除することで限界曲げ半径(R/t)を求め、この好適範囲を1.5以下とした。
Claims (6)
- 質量%で、
C:0.10%以上0.23%以下、
Si:1.30%以上2.20%以下、
Mn:2.0%以上3.2%以下、
P:0.05%以下、
S:0.005%以下、
Al:0.005%以上0.100%以下、
N:0.0060%以下、残部がFeおよび不可避的不純物からなる成分組成と、
フェライト面積率が4%以下(0%を含む)、焼入ままマルテンサイトの面積率が10%以下(0%を含む)、残留オーステナイトが7%以上20%以下、上部ベイナイト、下部ベイナイト、焼戻しマルテンサイトが合計で71%超え93%未満を含み、さらに円相当直径1μm以下の残留オーステナイトを囲む方位差1°以内のBCC鉄の面積率が4%以上50%以下、方位差1°を上回るBCC鉄の面積率が25%以上85%以下である鋼組織と、を有する薄鋼板。 - 質量%で、
C:0.10%以上0.23%以下、
Si:1.30%以上2.20%以下、
Mn:2.0%以上3.2%以下、
P:0.05%以下、
S:0.005%以下、
Al:0.005%以上0.100%以下、
N:0.0060%以下、残部がFeおよび不可避的不純物からなる成分組成と、
フェライト面積率が4%以下(0%を含む)、焼入ままマルテンサイトの面積率が10%以下(0%を含む)、残留オーステナイトが7%以上20%以下、上部ベイナイト、下部ベイナイト、焼戻しマルテンサイトが合計で71%超え93%未満を含み、さらに円相当直径1μm以下の残留オーステナイトを囲む方位差1°以内のBCC鉄の面積率が5%以上50%以下、方位差1°を上回るBCC鉄の面積率が25%以上85%以下である鋼組織と、を有する薄鋼板。 - 前記成分組成は、さらに、質量%で、Sb:0.001%以上0.050%以下を含有する請求項1または2に記載の薄鋼板。
- 前記成分組成は、さらに、質量%で、
Ti:0.001%以上0.1%以下、
Nb:0.001%以上0.1%以下、
V:0.001%以上0.3%以下、
Ni:0.01%以上0.1%以下、
Cr:0.01%以上1.0%以下および
B:0.0002%以上0.0050%以下の1種または2種以上を含有する請求項1~3のいずれかに記載の薄鋼板。 - 前記成分組成は、さらに、質量%で、
Cu:0.01%以上0.2%以下、
Mo:0.01%以上1.0%以下、
REM:0.0002%以上0.050%以下、
Mg:0.0002%以上0.050%以下および
Ca:0.0002%以上0.050%以下の1種または2種以上を含有する請求項1~4のいずれかに記載の薄鋼板。 - 請求項1~5のいずれかに記載の成分組成を有し、熱延鋼板に46%以上の冷間圧延率で冷間圧延する冷延工程と、
前記冷延工程後、加熱し、815℃以上で130秒以上滞留させた後、800℃から520℃までの平均冷却速度が8℃/s以上で420℃以上520℃以下の温度域まで冷却し、該温度域で12秒以上60秒以下滞留させ、420℃から300℃までの温度区間で平均冷却速度が8℃/s以上となるよう200℃以上350℃以下の冷却停止温度まで冷却し、該冷却停止温度から±50℃の温度域に2秒以上25秒以下滞留したのち、300℃以上500℃以下の温度まで加熱した後、該温度範囲で480秒以上1800秒以下滞留させる焼鈍工程と、を有する薄鋼板の製造方法。
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EP4151762A4 (en) * | 2020-05-11 | 2023-10-04 | JFE Steel Corporation | STEEL SHEET, ELEMENT, AND THEIR MANUFACTURING METHOD |
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US20210381077A1 (en) | 2021-12-09 |
MX2021004416A (es) | 2021-07-06 |
EP3868909A4 (en) | 2021-08-25 |
JPWO2020080339A1 (ja) | 2021-02-15 |
EP3868909A1 (en) | 2021-08-25 |
KR102517187B1 (ko) | 2023-04-03 |
CN112840055B (zh) | 2022-07-22 |
CN112840055A (zh) | 2021-05-25 |
JP6737419B1 (ja) | 2020-08-12 |
KR20210059746A (ko) | 2021-05-25 |
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