JP4867177B2 - High tensile hot rolled steel sheet excellent in bake hardenability and formability and method for producing the same - Google Patents

Description

  The present invention relates to a thin hot-rolled steel sheet suitable for use in automobile structural members, leg members, and the like, and a method for producing the same, and particularly, a high-tensile hot-rolled steel sheet having excellent bake hardenability and excellent formability, and It relates to the manufacturing method.

  In recent years, steel sheets for automobiles used for automobile structural members, leg-surrounding members, and the like have been required to have higher strength in order to improve fuel efficiency by reducing weight. However, increasing the strength of the steel sheet has the problem of making press forming difficult. Further, recently, for the purpose of ensuring the safety of passengers, it is also desired to improve the collision resistance evaluated by the amount of deformation energy at a high strain rate as in the case of a collision.

  Under such circumstances, as a technology for increasing the strength to prevent deterioration of press formability, the so-called bake hardenability (BH property), which is easy to process with relatively low strength during molding and increases strength by baking during coating. ) Is known, and cold-rolled steel sheets are widely used (for example, Patent Document 1 and Patent Document 2). However, the improvement in bake hardenability obtained by these techniques is effective in improving the dent resistance of automobile outer plates because only the yield strength increases and the tensile strength cannot be increased. It does not lead to improvement in fatigue resistance and impact resistance required for the plate.

  On the other hand, in Patent Document 3, C: 0.030 to 0.100 wt%, N: 0.0015 to 0.0150 wt%, Al: 0.025 to increase the solid solution amount of C and N in the steel sheet and improve the BH property. Steel containing 0.100wt% is heated to 1200 ° C or lower, finish rolled at a temperature of (Ar3 + 30 ° C) to 950 ° C, and rapidly cooled to 500 ° C or lower at a cooling rate of 50 ° C / sec or higher within 3 seconds after rolling. And the manufacturing method of the hot-rolled steel plate excellent in bake hardenability after the heating which is wound up at 400 to 500 degreeC is disclosed.

  In Patent Document 4, in order to obtain a hot rolled steel sheet having excellent bake hardenability and workability, C: 0.02 to 0.1 wt%, N: 0.0080 to 0.0250 wt%, Sol.Al: 0.10 wt% or less. Re-heat the contained steel to 1100 ° C or higher, perform hot rolling to finish finish rolling at a temperature of 850 to 950 ° C, and then sandwich air cooling or air cooling at a cooling rate of 15 ° C / sec or higher Without being disclosed, a manufacturing method of winding after cooling to 350 ° C. or lower is disclosed.

In Patent Document 5, C: 0.010 to 0.02 wt%, N: 0.0015 to 0.0030 wt%, Nb: 0.01 to 0.05 wt%
Sol.Al: 0.008 wt% or less, and by controlling the coiling temperature after hot rolling, disclosed is a bake-hardening hot-rolled steel sheet in which an appropriate amount of solid solution C and solid solution N remains, and its effect It is described that the fatigue limit increases after the processing-paint baking process.

In Patent Document 6, the amount of BH (yield by baking treatment) is controlled by controlling the cooling rate and coiling temperature after hot rolling in steel containing C: 0.01 to 0.08 wt% and N: 0.001 to 0.01 wt%. A technique for increasing the amount of increase in strength) is disclosed.
JP-A-6-73498 JP-A-7-268544 Japanese Unexamined Patent Publication No. 1-180917 Japanese Unexamined Patent Publication No. 4-74824 Japanese Unexamined Patent Publication No. 63-96248 Japanese Patent Laid-Open No. 10-310824

  However, in a hot-rolled steel sheet manufactured by the technique described in Patent Document 3, the normal temperature aging resistance deteriorates. Moreover, although the yield strength after baking is increased, the tensile strength is not taken into account and is insufficient. Furthermore, since the tensile strength is insufficient, a remarkable improvement in fatigue resistance and impact resistance cannot be expected.

  The hot-rolled steel sheet manufactured by the technique described in Patent Document 4 is a composite structure mainly composed of ferrite and martensite, and the tensile strength after the processing-paint baking process is increased, but the structure including martensite. Therefore, there is a problem that the hole expansion rate, which is an evaluation index of stretch flangeability, is low, and the parts and parts where the application is applied are limited.

  In the steel sheet described in Patent Document 5, the increase in fatigue limit is small compared to the increase in yield strength, and the amount of increase is only about 25 Mpa, and the improvement in fatigue resistance is insufficient.

  In the hot-rolled steel sheet manufactured by the technique described in Patent Document 6, it is mainly intended to increase the yield strength after the processing-paint baking process, and the optimum condition is not always found for the formability. Not enough.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a high-tensile hot-rolled steel sheet excellent in both bake hardenability and formability and a method for producing the same.

  Conventionally, in order to achieve high bake-hardening characteristics, a structure containing a low-temperature transformation phase (for example, bainite or bainitic ferrite) is used to dissolve C and N in a supersaturated state, and the formation of a pearlite phase is suppressed as much as possible. It was. However, since the low temperature transformation phase has low ductility, the moldability tends to be inferior. The present inventors have paid attention to the above points, and have made extensive studies to solve the above problems and obtain a steel sheet having high bake hardenability but also formability (ductility). As a result, paying attention to the presence of solid solution N in the steel sheet, that is, the form of solid solution N, the main phase of the structure is ferrite and the crystal grain is refined to increase the grain boundary. It was found that a hot-rolled steel sheet having good bake hardenability and formability (ductility) can be obtained by appropriately including a pearlite phase as a phase.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

  [1] In mass%, C: 0.01 to 0.12%, Si: 0.2% or less, Mn: 0.01 to 1.7%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.004 to 0.020%, The balance consists of Fe and inevitable impurities, the ferrite phase is the main phase, the second phase has a pearlite phase of 10 to 25%, and the average grain size of the ferrite phase is 10 μm or less. Further, a high-tensile hot-rolled steel sheet excellent in bake hardenability and formability, characterized in that the amount of solute N is 0.004 to 0.010%.

  [2] In the above [1], Nb: 0.001-0.05%, Ti: 0.005-0.01%, V: 0.001-0.05%, Cr: 0.01-0.1%, B: 0.0001-0.001% A high-tensile hot-rolled steel sheet excellent in bake hardenability and formability, characterized by containing one or more.

  [3] A high-tensile hot-rolled steel sheet excellent in bake hardenability and formability, wherein a plating layer is formed on the surface of the high-tensile hot-rolled steel sheet in [1] or [2].

  [4] In mass%, C: 0.01 to 0.12%, Si: 0.2% or less, Mn: 0.01 to 1.7%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.004 to 0.020%, The remainder is steel consisting of Fe and inevitable impurities, heated to a temperature of 1000 ° C or higher and 1300 ° C or lower, and after rough rolling, with a final stand rolling reduction of 10% or higher and a final finish rolling finish temperature FDT of (Ar3 + 10 ° C) or higher. After finishing rolling, and then cooling to 650 ° C to 750 ° C at a cooling rate of 100 ° C / sec to 200 ° C / sec, 450 ° C to a cooling rate of 10 ° C / sec to 25 ° C / sec A method for producing a high-tensile hot-rolled steel sheet excellent in bake hardenability and formability, characterized by cooling to a coiling temperature of 600 ° C. or lower and winding at the coiling temperature.

  [5] By mass%, C: 0.01 to 0.12%, Si: 0.2% or less, Mn: 0.01 to 1.7%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.004 to 0.020%, Further, Nb: 0.001 to 0.05%, Ti: 0.005 to 0.01%, V: 0.001 to 0.05%, Cr: 0.01 to 0.1%, B: 0.0001 to 0.001%, or one or more types, with the balance being Fe And the steel consisting of inevitable impurities is heated to a temperature range of 1070 ° C to 1300 ° C, and after rough rolling, finish rolling is performed at a final stand rolling reduction rate of 10% or more and final finish rolling finish temperature FDT of (Ar3 + 10 ° C) or more Then, after cooling at a cooling rate of 100 ° C./sec to 200 ° C./sec to 650 ° C. to 750 ° C., cooling at a cooling rate of 10 ° C./sec to 25 ° C./sec, 450 ° C. to 600 ° C. A method for producing a high-tensile hot-rolled steel sheet having excellent bake hardenability and formability, wherein the steel sheet is cooled to the following winding temperature and wound at the winding temperature.

  In the present specification, “%” indicating the component of steel is “% by mass”.

  Further, in the present invention, the high-tensile hot-rolled steel sheet is a hot-rolled steel sheet having a tensile strength suitable for automobile interior materials exceeding 400 MPa.

  Further, in the present invention, the improvement in bake hardenability means an increase in tensile strength as well as an increase in yield strength after processing-baking coating treatment.

  Furthermore, the automotive interior material that is the object of the present invention is accompanied by press working at the time of component molding, and elongation (ductility) is used as an index of this press formability. Therefore, the formability in the present invention means the elongation of the hot-rolled steel sheet.

  According to the present invention, a high-tensile hot-rolled steel sheet having excellent bake hardenability and formability can be stably obtained. In addition, the high-tensile hot-rolled steel sheet obtained by the present invention is excellent in fatigue resistance and impact resistance because the tensile strength is sufficiently improved, and further does not require excessive addition of solid solution elements. It is suitable as an inner plate part of an automobile and has a remarkable industrial effect.

  The high-tensile hot-rolled steel sheet of the present invention has the following components, the ferrite phase is the main phase, the second phase has a pearlite phase of 10 to 25%, and the average crystal of the ferrite phase It is a mixed tissue form having a particle size of 10 μm or less, and further has a solid solution N content of 0.004 to 0.010%. These are the most important requirements in the present invention. Thus, a high-tensile hot-rolled steel sheet excellent in bake hardenability and formability can be obtained by defining the components, the structure, and the solute N amount. The high-tensile hot-rolled steel sheet controls the hot-rolling conditions. Specifically, it is heated at a heating temperature of 1000 ° C or higher and 1300 ° C or lower, and after rough rolling, the final stand rolling reduction: 10% or higher, final finish rolling. Finishing temperature FDT: Finish rolling at (Ar3 + 10 ° C) or higher, and after rolling, after cooling to 650 ° C or higher and 750 ° C or lower at a cooling rate of 100 ° C / sec or higher and 200 ° C / sec or lower, further 10 ° C / sec It can be manufactured by cooling to a winding temperature of 450 ° C. or more and 600 ° C. or less at a cooling rate of 25 ° C./sec or less and winding at the winding temperature.

  Hereinafter, the present invention will be described in detail.

  First, the reasons for limiting the chemical components of steel in the present invention will be described.

C: 0.01-0.12%
C is an element that increases the strength of steel, and is 0.01% or more, preferably 0.02% or more in order to obtain a structure containing a pearlite phase constituting the present invention. On the other hand, if it exceeds 0.12%, weldability deteriorates. Accordingly, C is 0.01% or more and 0.12% or less, preferably 0.02% or more and 0.12% or less.

Si: 0.2% or less
Si is an element that increases the strength of the steel by solid solution strengthening, and the content is adjusted according to the desired strength. However, if it exceeds 0.2%, workability deteriorates. From the above, Si is 0.2% or less. From the viewpoint of securing strength, Si is preferably contained in an amount of 0.003% or more.

Mn: 0.01-1.7%
Mn is an element that increases the strength of the steel and prevents hot brittleness due to S, and is actively contained in the present invention. Thus, in order to ensure desired intensity | strength and to prevent hot brittleness, containing 0.01% or more is required. However, if the content exceeds 1.7%, the workability deteriorates. Therefore, Mn is set to 0.01% or more and 1.7% or less.

P: 0.2% or less
P is an element that increases the strength of the steel, and is desirably contained in an amount of 0.005% or more in order to ensure a desired strength. However, if the content exceeds 0.2%, the weldability deteriorates, and P segregates at the grain boundaries, which may cause grain boundary cracks. Therefore, P is 0.2% or less.

Al: 0.001 to 0.1%
Al acts as a deoxidizer and requires 0.001% or more for deoxidation of steel. On the other hand, if the content exceeds 0.1%, the surface properties deteriorate. From the above, Al is made 0.001% or more and 0.1% or less. Furthermore, since Al is also an element forming a nitride, it is preferably 0.07% or less from the viewpoint of securing the amount of dissolved N.

N: 0.004 to 0.020%, solid solution N amount: 0.004 to 0.010%
N is an important element in the present invention, and effectively acts to increase the yield strength and tensile strength after processing-paint baking treatment, that is, to improve bake hardenability, by dissolving in the steel sheet. In order to obtain this effect, it is necessary to leave 0.004% or more, preferably 0.005% or more as solid solution N in the steel sheet. On the other hand, even if solid solution N is present in an amount exceeding 0.010%, a marked improvement in ΔTS cannot be obtained, and the aging resistance is lowered. For this reason, the upper limit of solute N is made 0.010% or less. And the amount of addition N for obtaining the solid solution N in this range shall be 0.004% or more. On the other hand, even if the added N amount exceeds 0.020%, it does not become solid solution N that contributes to the improvement of ΔTS, and the formability deteriorates due to the formation of excess nitride and the like. From the above, the added N amount N is set to 0.004% or more and 0.020% or less.

  The steel of the present invention can achieve the desired characteristics with the above-mentioned essential additive elements, but in addition to the above-mentioned essential additive elements, Nb, Ti, V, Cr, B may be used alone or as needed for increasing the strength. Two or more kinds may be added. In that case, the effect of addition is not sufficient if the respective addition amounts are less than 0.001%, 0.005%, 0.001%, 0.01%, and 0.0001%. On the other hand, if Nb, Ti, V, and B exceed 0.05%, 0.01%, 0.05%, and 0.001%, respectively, the amount of reinforcement becomes too large due to the formation of carbides or nitrides. In addition, the formation of precipitates deteriorates mechanical properties (decreases elongation) and decreases the amount of C and N contributing to bake hardenability, thereby reducing the bake hardenability targeted by the present invention. On the other hand, if the Cr content exceeds 0.1%, chemical conversion properties and spot weldability are deteriorated. Therefore, when these elements are added, Nb is 0.001% to 0.05%, Ti is 0.005% to 0.01%, V is 0.001% to 0.05%, Cr is 0.01% to 0.1%, and B is 0.0001. More than 0.001%.

  The remainder other than the above is Fe and inevitable impurities. As unavoidable impurities, for example, S and O form non-metallic inclusions and adversely affect the quality. Therefore, it is desirable to reduce S to 0.05% or less and O to 0.01% or less.

  Next, in order to investigate the relationship between the structure and the mechanical properties of the hot-rolled steel sheet adjusted to the above chemical component range, the following experiment was performed.

  Contains C: 0.08%, Si: 0.01%, Mn: 1.25%, P: 0.013%, Al: 0.025%, N: 0.009%, the balance being made of steel consisting of Fe and inevitable impurities. A hot-rolled steel sheet was manufactured by performing hot working under conditions to change the ferrite grain size as the main phase structure and the pearlite fraction of the second phase.

Next, specimens for JIS No. 5 tensile test were collected from these hot-rolled steel sheets, 1) normal tensile test, 2) 10% tensile pre-strain and then temporarily stopped, heat treatment at 170 ° C x 20 min. After applying (corresponding to the paint baking process), two types of tensile tests were performed, in which the samples were pulled again until breakage, and the respective tensile strengths were measured. The difference between the tensile strength TS _BH after the processing-paint bake treatment obtained in 2) and the hot-stretched tensile strength TS obtained in 1) is obtained from the obtained measurement results, and ΔTS is obtained. It was. Furthermore, as the mechanical properties of the steel sheet increase in strength, the elongation decreases. Therefore, TS × EL was calculated and used as a material evaluation index. FIG. 1 shows the relationship between the pearlite fraction in the tissue and TS × EL. FIG. 2 shows the relationship between the pearlite fraction in the tissue and ΔTS. The tensile strength (TS) of the various hot-rolled steel sheets used in the above test was 450 to 500 MPa, and the solute N amount was 0.0089%.

  From FIG. 1, it can be seen that TS × El is 17500 or more when the structure is composed of ferrite and pearlite of 10% to 25%. Here, TS × EL is about 17000 in the hot-rolled steel sheet having a tensile strength of around 450 MPa currently used. If it is 17500 or more, the elongation is improved by 1% or more when compared at the same strength level. Therefore, in the present invention, TS × El is 17500 or more as a criterion for determining good moldability.

Further, FIG. 2 shows that in the case of the pearlite fraction (10% or more and 25% or less), the ferrite particle diameter is 10 μm or less and ΔTS is 50 Mpa or more. Here, as ΔTS increases, it is possible to reduce the thickness of the steel sheet to be used (weight reduction). For example, when the steel sheet of the present invention is applied to a structural member of an automobile, the thickness is reduced by 5% or more. 50Mpa or more is enough to make it happen. Therefore, in the present invention, ΔTS is 50 Mpa or more as a criterion for determining good bake hardenability.
Although a high ΔTS can be obtained even when the pearlite fraction is 10% or less, as shown in FIG. 1, TS × EL is greatly reduced, and both the moldability and the tensile strength after processing-paint baking treatment are achieved. Not.

  Furthermore, in order to obtain a significant increase in tensile strength after processing and paint baking (significant improvement in bake hardenability), the structure has ferrite as the main phase, and further refines the main phase ferrite to form a solid solution. It is important to increase the grain boundary as the location of N. In order for all the solute N to exist (segregate) at the crystal grain boundaries, the average crystal grain size of ferrite needs to be 10 μm or less. And the increase in grain interfacial area due to the refinement of ferrite crystal grains increases the ratio of solid solution N present in the grain boundary, and the amount of solid solution N in the grain decreases, so that aging deterioration at room temperature decreases. It is suppressed. Further, when 10% to 25% pearlite is present, the elongation of the steel sheet is greatly improved. However, if it exceeds 25%, TS × EL decreases.

From the above results, the hot-rolled steel sheet of the present invention has a mixed structure in which a ferrite phase is a main phase, a second phase has a pearlite phase of 10 to 25%, and an average crystal grain size of the ferrite phase is 10 μm or less. And These are the most important requirements in the present invention.
In the present invention, the “main phase” is a phase with a volume fraction exceeding 60%.

  Furthermore, in the structure of the hot-rolled steel sheet of the present invention, in addition to the above, as a phase other than ferrite and second-phase pearlite (third phase), one or two of bainite, martensite, and retained austenite You may contain seed | species or more with less than the fraction of a pearlite phase. By including and present these third phases, it is possible to increase the strength without adding a large amount of expensive additive elements, and the impact resistance and fatigue resistance are improved. From the viewpoint of improving moldability (TS × EL), the volume ratio of the third phase is preferably 10% or less, and from the viewpoint of improving hole expansibility, it is further preferably 5% or less.

  Next, a method for producing a high-tensile hot-rolled steel sheet having excellent bake hardenability and formability according to the present invention will be described.

  The high-tensile hot-rolled steel sheet according to the present invention is a steel adjusted to the above chemical composition range at a heating temperature of 1000 ° C. (1070 ° C. when Nb, Ti, V, Cr, and B are added) to 1300 ° C. After heating and rough rolling, finish rolling is performed at a final stand rolling reduction of 10% or more, final finish rolling finish temperature FDT: (Ar3 + 10 ° C) or more, and then a cooling rate: 100 ° C / sec or more and 200 ° C / sec. After cooling to 650 ° C. or more and 750 ° C. or less below, cool to a coiling temperature of 450 ° C. or more and 600 ° C. or less at a cooling rate of 10 ° C./sec or more and 25 ° C./sec. Can be obtained.

  In order to secure the solid solution N amount of the present invention in the hot-rolled sheet, it is necessary to dissolve the nitride during heating, and in order to refine the structure of the hot-rolled sheet, the heating temperature is lowered. Thus, it is necessary to make the austenite grains as fine as possible during heating. From the above, the heating temperature is 1000 ° C. or higher and 1300 ° C. or lower, more preferably 1100 ° C. or higher and 1180 ° C. or lower when nitride-forming elements (Nb, Ti, V, Cr 2, B) are not included. Further, when adding one or more of Nb, Ti, V, Cr, and B as required, in order to fully utilize the strengthening ability of these elements, it is 1070 ° C or higher and 1300 ° C or lower, more preferably 1100 ° C. Above 1180 ° C. If it is less than 1000 ° C. (less than 1070 ° C.), precipitation of N proceeds, making it difficult to leave N in a solid solution state in the hot-rolled sheet. If it exceeds 1300 ° C., it becomes difficult to make the average crystal grain size of ferrite 10 μm or less.

  The heating at this time can be performed by a known apparatus such as a heating furnace. Furthermore, as the steel at this time, it is preferable that the molten steel melted by a known melting method is cast and solidified by a known continuous casting method or ingot forming method to form a slab or the like.

  After heating, it is hot rolled. Hot rolling consists of rough rolling and finish rolling, and the steel adjusted to an appropriate thickness by rough rolling is then subjected to finish rolling. At this time, the final rolling reduction rate is 10% or more and the final finishing rolling temperature FDT (hereinafter referred to as FDT) is (Ar3 + 10 ° C) or higher in order to refine the crystal diameter of the microstructure after hot rolling. It shall be done in If the final stand rolling reduction is less than 10% and the FDT is less than (Ar 3 + 10 ° C.), the strain distribution in the thickness direction before transformation becomes non-uniform, and the average crystal grain size of ferrite cannot be refined. In addition, when cooling the final stand after rolling (after finish rolling), if the FDT, which is the cooling start temperature, is high, the accumulation of strain before ferrite transformation is not sufficient, resulting in refinement of crystal grains and the form of solute N Insufficient control. For the above reasons, the rolling reduction of the final stand rolling is preferably 20% or more, and the FDT is preferably (Ar3 + 100 ° C.) to (Ar 3 + 10 ° C.).

  After finishing rolling, after cooling to 650 ° C or higher and 750 ° C or lower (primary) at a cooling rate of 100 ° C / sec to 200 ° C / sec, the coiling temperature is set to 10 ° C / sec or higher and 25 ° C / sec or lower. Cool to (secondary).

  As described above, in the present invention, in order to increase the degree of supercooling in a state where strain is accumulated, cooling is performed at a cooling rate of 100 ° C./sec or more after rolling. As a result, more ferrite nuclei are generated, crystal grains are refined, and solid solution N can be prevented from diffusing into the ferrite grains, so that the amount of solid solution N existing at the grain boundaries increases. On the other hand, when the cooling rate exceeds 200 ° C./sec, a hard low-temperature transformation phase such as a martensite phase is generated in the surface structure of the steel sheet, thereby reducing ductility. Therefore, the (primary) cooling rate is set to 100 ° C./sec or more and 200 ° C./sec or less.

  Then, after cooling to 650 ° C. or higher and 750 ° C. or lower at the above cooling rate so that an appropriate pearlite phase is formed, the cooling rate is changed to 10 ° C./sec or higher and 25 ° C. or lower, and winding is performed at 450 ° C. or higher and 600 ° C. or lower. Cool to temperature. When the cooling stop temperature of the primary cooling exceeds 750 ° C., a large amount of pearlite nuclei are generated, and when the secondary cooling rate is less than 10 ° C./sec, the formation of pearlite exceeds an appropriate amount, thereby reducing the moldability. When the cooling stop temperature of the primary cooling is less than 650 ° C. or the cooling rate is more than 25 ° C./sec, the pearlite nuclei are not sufficiently generated and grown, and the moldability is also lowered. In order to obtain sufficient solute N from the viewpoint of improving ΔTS, the secondary cooling rate is preferably 15 ° C. or higher.

  After cooling, it is wound at the winding temperature (450 ° C. or higher and 600 ° C. or lower). When the winding temperature exceeds 600 ° C., solid solution N precipitates after winding, and the amount of solid solution N necessary for bake hardening cannot be set to a predetermined value or more. On the other hand, when the coiling temperature is less than 450 ° C, low-temperature transformation ferrite (bainitic ferrite) and bainite are generated, and the operational problems such as deterioration of formability, plate shape, and deterioration of sheet passability. Will occur. Therefore, the coiling temperature is set to 450 ° C. or higher and 600 ° C. or lower. In order to obtain sufficient solute N from the viewpoint of improving ΔTS, the coiling temperature is preferably 550 ° C. or lower.

  In addition, the high-tensile hot-rolled steel sheet excellent in bake hardenability and formability of the present invention obtained as described above is suitable as various plating hot metal sheets, various plating layers are formed on the surface, and used as various plating steel sheets. Also good. Examples of the type of plating include electrogalvanizing, hot dip galvanizing, electrotin plating, and electrochrome plating, and any of them is suitable as a plating layer formed on the surface of the hot-rolled steel sheet of the present invention.

  Thickness containing the elements shown in Table 1, with the balance being steel made of Fe and inevitable impurities, hot-rolled under various production conditions shown in Table 2, and changing the volume fraction of the pearlite phase A 2.3 mm hot-rolled steel sheet was produced.

Next, JIS No. 5 tensile test specimens were collected from these hot-rolled steel sheets, 1) normal tensile test, and 2) 10% tensile pre-strain and then temporarily stopped, and heat treatment (170 ° C x 20 min) After applying the paint baking process, the tensile strength was measured again, and the tensile strength was measured. From the measurement results obtained, the difference between the tensile strength TS _BH after the processing-paint baking process obtained in 2) and the tensile strength TS as-hot-rolled obtained in 1), ΔTS (BHT ) Was calculated.

  Furthermore, in order to evaluate the formability required for the steel sheet, the hole expandability λ, which is an index of stretch flangeability, was measured in addition to the ductility. The test conditions were measured in accordance with the Japan Iron and Steel Federation standard hole expansion test method (T1001-1996).

  The microstructure of the hot-rolled steel sheet was determined by observing and photographing using a metal microscope or a scanning electron microscope after corroding the L cross section of each hot-rolled sheet. The tissue fraction was converted to a volume fraction using a photograph after photographing.

  The results obtained as described above are shown in Table 3. Further, since the microstructure and the tensile strength greatly change depending on the hot rolling conditions, Table 3 also shows TS × EL used as an index of formability of an automotive steel sheet as an index of the present invention. The pearlite fraction, ferrite crystal grain size, and solid solution N content are also shown.

  From Table 3, it can be seen that in the examples of the present invention, pearlite is present at 10% or more and 25% or less, TS × El shows a value of 17500 or more, and the moldability is sufficient.

  On the other hand, sample symbols A, E1 and L, which are comparative examples, have pearlite exceeding 25%, F1, G1 and K have pearlite less than 10%, TS × EL does not reach 17500, and formability It turns out that it is inferior to. That is, it can be seen that in order to improve the formability of a steel sheet containing a pearlite phase, there is an appropriate region of hot rolling conditions, and it is necessary to determine the hot rolling conditions so as to be within that range.

  The hole expansion ratio of a hot rolled steel sheet with a tensile strength of 440 MPa is usually about 50%, and 80% or more for high hole expansion. On the other hand, it can be seen that all of the examples of the present invention can obtain a performance close to 70% or more, which is similar to a steel sheet for high hole expansion. On the other hand, it can be seen from Comparative Examples A and E1 that when the fraction of the pearlite phase is higher than 25%, a stable hole expansion rate is not obtained.

  In addition, paying attention to ΔTS (BHT), which is another target of the characteristics of the present invention, when the crystal grain size of ferrite is 10 μm or less and the amount of solute N exceeds 0.004%, it shows a good value of 50 MPa or more. I understand that.

  On the other hand, sample numbers A and L, which are comparative examples, have a particle size of 10 μm or more, and it can be seen that the crystal grains are coarser than other samples. This is because solid solution C and N segregates also at the lattice defects such as dislocations and in the vicinity and at the crystal grain interface, and when the crystal grains become finer, the grain interface area increases. Increased and considered to work effectively during strain aging, but with Sample Nos. A and L, grain interfacial area decreases due to the coarsening of crystal grains and is necessary for strength increase after strain aging (ΔTS) This is thought to be due to insufficient solid solution N.

  In order to reduce the ferrite grain size to 10 μm or less, the primary cooling stop temperature is important after slab heating to hot rolling, and for sample symbol A with a grain size of 10 μm or more, the primary cooling temperature is the scope of this claim. Therefore, in the sample symbol L, the final stand rolling reduction is low beyond the range, and the coarsening of the ferride is proceeding.

  Further, even when the ferrite grains are 10 μm or less, J having a low slab heating temperature cannot sufficiently obtain a solid solution N because the nitride is not sufficiently dissolved.

  Sample symbols B to D2, E2, I, and M, in which the hot rolling conditions from the steel composition and hot rolling final finish rolling to coil winding are within the scope of the present invention, a sufficient amount of solute N is obtained and the pearlite phase Is present within the range of the present invention, TS × EL is 17500 or more, which is excellent in moldability, and it shows a good value of 50 MPa or more, and it is understood that bake hardenability is also excellent.

  In other comparative examples, it can be seen that one or more of moldability or bake hardenability is inferior when TS × EL is less than 17500 or ΔTS (BHT) is less than 50 Mpa.

  Moreover, in the example of this invention, since the tensile strength is also improving sufficiently, it is excellent also in fatigue resistance and impact resistance.

The high-tensile hot-rolled steel sheet of the present invention is suitable not only for structural members of automobiles but also in fields where excellent fatigue resistance, impact resistance, bake hardenability and formability are required.

It is a figure which shows the relationship between a pearlite fraction and TSxEL. It is a figure which shows the relationship between a pearlite fraction and (DELTA) TS.

Claims (5)

In mass%, C: 0.01-0.12%, Si: 0.2% or less, Mn: 0.01-1.7%, P: 0.2% or less, Al: 0.001-0.1%, N: 0.004-0.020%, the balance being Fe And inevitable impurities,
A mixed structure in which a ferrite phase of more than 60% is a main phase, a second phase has a pearlite phase of 10 to 25%, and an average crystal grain size of the ferrite phase is 10 μm or less;
Furthermore, a high-tensile hot-rolled steel sheet excellent in bake hardenability and formability, characterized in that the amount of solute N is 0.004 to 0.010%.   Furthermore, it contains one or more of Nb: 0.001 to 0.05%, Ti: 0.005 to 0.01%, V: 0.001 to 0.05%, Cr: 0.01 to 0.1%, B: 0.0001 to 0.001% by mass%. The high-tensile hot-rolled steel sheet having excellent bake hardenability and formability according to claim 1.   The high-tensile hot-rolled steel sheet having excellent bake hardenability and formability according to claim 1 or 2, wherein a plating layer is formed on the surface of the high-tensile hot-rolled steel sheet. In mass%, C: 0.01-0.12%, Si: 0.2% or less, Mn: 0.01-1.7%, P: 0.2% or less, Al: 0.001-0.1%, N: 0.004-0.020%, the balance being Fe And the steel consisting of inevitable impurities is heated to a temperature of 1000 ° C or higher and 1300 ° C or lower, and after rough rolling,
Finish rolling at a final stand rolling reduction rate of 10% or more, final finish rolling finish temperature FDT of (Ar3 + 10 ℃) or more,
Next, after cooling to 650 ° C. or more and 750 ° C. or less at a cooling rate of 100 ° C./sec or more and 200 ° C./sec or less,
Cool to a coiling temperature of 450 ° C or more and 600 ° C or less at a cooling rate of 10 ° C / sec or more and 25 ° C / sec or less,
A method for producing a high-tensile hot-rolled steel sheet having excellent bake hardenability and formability, wherein the steel sheet is wound at the winding temperature. In mass%, C: 0.01 to 0.12%, Si: 0.2% or less, Mn: 0.01 to 1.7%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.004 to 0.020%, Nb : 0.001 to 0.05%, Ti: 0.005 to 0.01%, V: 0.001 to 0.05%, Cr: 0.01 to 0.1%, B: 0.0001 to 0.001%, 2 or more types, the balance being Fe and inevitable Steel made of impurities is heated to a temperature range of 1070 ° C to 1300 ° C, and after rough rolling,
Finish rolling at a final stand rolling reduction rate of 10% or more, final finish rolling finish temperature FDT of (Ar3 + 10 ℃) or more,
Next, after cooling to 650 ° C. or more and 750 ° C. or less at a cooling rate of 100 ° C./sec or more and 200 ° C./sec or less,
Cool to a coiling temperature of 450 ° C or more and 600 ° C or less at a cooling rate of 10 ° C / sec or more and 25 ° C / sec or less,
A method for producing a high-tensile hot-rolled steel sheet having excellent bake hardenability and formability, wherein the steel sheet is wound at the winding temperature.

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