TWI527909B - A hot rolled steel sheet having excellent drawability and surface hardness after processing - Google Patents

Description

Hot rolled steel sheet with excellent punching workability and surface hardness after processing

The present invention relates to a hot-rolled steel sheet which exhibits good punchability during processing and exhibits a predetermined surface hardness after processing.

In recent years, based on the viewpoint of environmental protection, in order to improve the fuel efficiency of automobiles, the weight of steel used for various parts for automobiles, such as gearbox parts such as gears and gearboxes, is high. The demand for intensity is increasing. In response to the demand for such lightweight and high-strength, as a steel commonly used in general, steel (hot forging material) obtained by hot-forging steel is used. In addition, in order to reduce the amount of CO ₂ discharged in the part manufacturing process, the requirements for the gears that have been conventionally processed by hot forging have been gradually increased by the need for cold forging.

Cold room processing (cold forging) is compared with hot or warm processing, which has the advantages of high productivity, dimensional accuracy and good steel yield. However, when using such cold room processing to manufacture parts, the problem is: if you want to use the strength of the parts after cold processing, When it is ensured that it is more than the predetermined value expected, it is inevitable that steel having a high deformation resistance is not required. However, the higher the deformation resistance of the steel material to be used, the shorter the life of the mold for cold working, and the more likely the crack is likely to occur during cold working.

Therefore, in the conventionally manufactured manufacturing method, after the steel material is forged into a predetermined shape, a heat treatment such as quench hardening or tempering is performed to produce a high-strength part capable of securing a predetermined strength (hardness). method. However, in the heat treatment after cold forging, the size of the part is inevitably changed, so it must be corrected by secondary processing, for example, by machining such as cutting. Therefore, the manufacturer is expecting to find it soon: the heat treatment can be omitted and thereafter Corrected machining solution.

In order to solve the above-described technical problems, the technique disclosed in Patent Document 1 is, for example, a low carbon steel, which uses solid solution C to suppress the progress of normal temperature aging, and obtains deformation by ensuring a predetermined amount of age hardening due to deformation aging. Wire for cold forging and strip steel excellent in aging characteristics (refer to Patent Document 1).

However, this technique only controls the deformation aging depending on the amount of solid solution C, and it is difficult to obtain a steel material having both sufficient cold workability and desired hardness and strength after processing.

Therefore, the applicant of this case focused on the fact that the solid solution C and the solid solution N contained in the steel have different effects on the deformation resistance and the static deformation aging, and various results have been reviewed, and a brand new one has been obtained. Transcend, by properly controlling the amount of these solid solution elements, can be obtained: in the processing can not only play a good cold inter-processability, in the cold After the work (cold forging), a steel material for mechanical construction having a predetermined surface hardness (strength) can be displayed, and a patent has been filed (refer to Patent Document 2).

In addition to the characteristics of the cold-workability and the high hardness (high-strength) after the processing, the steel material is also a hot-forged material similarly to the wire and strip steel described in Patent Document 1 above. Therefore, there are disadvantages of high manufacturing costs. Therefore, in order to reduce the cost of manufacturing, it is also possible to review the feasibility of replacing the conventional hot-forged forged materials by using cold-rolled steel sheets to produce automotive parts by cold-working.

For example, a hot-rolled steel sheet having a high surface hardness and a sufficient depth of hardening after nitriding treatment has been proposed (see Patent Document 3).

However, this technique has to perform nitriding treatment after cold working, and there is still a problem that sufficient cost reduction cannot be achieved.

Further, a technical proposal of a hot-rolled steel sheet having a composition of C: 0.10% or less, Si: less than 0.01%, Mn: 1.5% or less, and Al: 0.20% or less, and (Ti + Nb) / 2 is in the range of 0.05 to 0.50%, S: 0.005% or less, N: 0.005% or less, O: 0.004% or less, the total of S, N and O is 0.0100% or less, and the metal fine structure is 95% or more. The substantial fermented iron single-phase structure, this hot-rolled steel sheet, the precision of the precision punched surface is excellent, and the surface hardness of the punched surface after processing is extremely high, and it is resistant to red. The characteristics of the rust skin are also excellent (refer to Patent Document 4).

However, for such a hot-rolled steel sheet, N is a harmful element and must be limited to an extremely low content, and the technical idea of the two is completely different from that of the hot-rolled steel sheet of the present invention which is intended to actively utilize N.

In general, in the conventional control of the aggregate structure for improving the formability of the steel sheet, the punching workability of the steel sheet used for the outer panel of the automobile body is the plastic anisotropy of the material (r value (that is, the Lankford value). The larger the ratio of the plate width deformation to the plate thickness deformation in the tensile test, the higher the workability, and the more the {111} surface which is parallel to the plate surface orientation in the recrystallized aggregate structure. Developed, the practice of making the {100} face less developed is indispensable for improving deep drawing. This fact has been verified experimentally or theoretically. Please refer to Non-Patent Document 1).

Therefore, various attempts have been made to improve the workability by controlling the assembly of the steel sheets.

A technical proposal for a high-strength steel sheet has been proposed, for example, its composition, in terms of mass%, containing C: 0.01 to 0.1%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P ≦ 0.1%, S ≦0.03%, Al: 0.005~2.0%, N≦0.01%, B:0.0005~0.0030, in addition, Ti content is consistent with Ti-(48/12)C-(48/14)N-(48/32)S ≧-0.03% of the range, the rest is a steel plate composed of Fe and unavoidable impurities. The value of the hardness deviation of the steel plate is divided by the average value of the hardness, and the value is 0.2 or less, and the rolling direction is The surface strength of the {110} surface is 1.7 or less, and the punching magnification is also excellent (please refer to Patent Document 5).

Further, a technical proposal of a high-strength steel sheet having a composition of C: more than 0.0005% and less than 0.10%, Si: 1.5% or less, Mn: 0.1% or more and 3.0% or less, P: 0.080 is proposed. % or less, S: 0.03% or less, sol. Al: 0.01% or more and 0.50% or less, and N: 0.005% or less, and one or two selected from Nb: 0.20% or less and Ti: 0.20% or less. The rest is composed of Fe and unavoidable impurities, wherein the steel structure is in volume percent, more than 60% is the ferrite grain iron phase, and the density function (ODF) of the 3-dimensional crystal orientation is { 1, Φ, 2}, Φ is 0°, 1 is 0°, and 2, the intensity of ODF{0°, 0°, 45°} at 45° is 3.0 or less, and Φ is 35°. 1 is 0°, 2 is a high-strength steel sheet having a strength of ODF{0°, 35°, 45°} at 45° of 2.5 or more and 4.5 or less. It is possible to provide a crack that is less likely to occur during press forming, and a ductile side. High-strength steel sheet with little elasticity (refer to Patent Document 6).

Further, a technical proposal of a hot-rolled steel sheet having a composition of C: 0.01 to 0.05%, Si: 0.2% or less, Mn: 0.50% or less, and Al: 0.005 to 0.10%, P: 0.05% or less, S: 0.05% or less, N: 0.01% or less, O: 0.01% or less, and the rest is composed of Fe and unavoidable impurities, and the average crystal grain size is 40 to 60 μm. Phase, and the random intensity ratios of {118}<110> and {115}<110> are respectively 10 or more, and the random intensity ratios of {111}<110> and {111}<121> are respectively 1 or less. Hot-rolled steel sheet, which provides: in-plane after cold rolling-recrystallization annealing A hot-rolled steel sheet having a small anisotropy (refer to Patent Document 7).

The above examples show that various attempts have been made to improve the formability of steel sheets by controlling the assembly organization, but most of them are related to automotive exterior panels, body frames, and suspension components. s things. The components such as the gearbox parts and the gearbox, which are the target gears of the present invention, are required to have different formability from the automobile body parts, and are required to have not only the punching workability and ironing. It is a part that is required to have the surface hardness after processing as described above, such as flat workability.

In addition, in the technical field of gearbox parts, in addition to the attempt to reduce the weight and cost of parts from forged products (hot forging, cold forging, etc.), The practice of manufacturing parts has been reviewed and, therefore, is required to have further formability.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. Hei 10-306345

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2009-228125

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2007-162138

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2004-137607

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2009-24226

[Patent Document 6] Japanese Patent Laid-Open Publication No. 2012-158797

[Patent Document 7] Japanese Patent Laid-Open Publication No. 2007-291514

[Non-patent literature]

[Non-Patent Document 1] Journal of the Japan Iron and Steel Association: "Application to Recrystallization and Aggregation and Tissue Control (The Forefront of Recrystallization Research)", March 1999, p.208

The present invention has been developed in view of the above circumstances, and an object thereof is to provide a hot-rolled steel sheet which exhibits good punchability in processing and has a predetermined surface hardness after processing.

The hot-rolled steel sheet having excellent punching workability and surface hardness after processing according to the invention of claim 1 is characterized in that the sheet thickness is 2 to 15 mm, and the composition thereof is in mass% (hereinafter, for chemical components) Also in mass %), C: 0.3% or less (excluding 0%), Si: 0.5% or less (excluding 0%), Mn: 0.2 to 1%, P: 0.05% or less (excluding 0%) ), S: 0.05% or less (excluding 0%), Al: 0.01~0.1%, N: 0.008-0.025%, the rest is composed of iron and unavoidable impurities, solid solution N: 0.007% or more, and the depth is t/4 (t: plate thickness, below In the same position, the ferrite-grained iron crystal grain has an area ratio of 20% or more of the ferrite-grained crystal grain within 10° from the (123) plane of the plate surface orientation, and 10° from the (111) plane of the plate surface orientation. The area ratio of the ferrite-grained crystal grains is 5% or more, and the area ratio of the ferrite-grain crystal grains within 10° from the (001) plane of the sheet surface orientation is 20% or less, and the depth is t/ The average particle size of the ferrite-grain crystal grains at the position of 4 is 3 to 35 μm.

The invention according to claim 2 is the hot-rolled steel sheet according to claim 1, wherein the composition component further contains Cr: 2% or less (excluding 0%) and/or Mo: 2% or less (excluding 0). %).

The invention according to claim 3 is the hot-rolled steel sheet according to claim 1 or 2, wherein the composition component further contains, from Ti: 0.2% or less (excluding 0%) and Nb: 0.2% or less (excluding At least one selected from the group consisting of 0%) and V: 0.2% or less (excluding 0%).

The invention of claim 4 is the hot-rolled steel sheet according to any one of claims 1 to 3, wherein the composition component further contains B: 0.005% or less (excluding 0%).

The hot-rolled steel sheet according to any one of Claims 1 to 4, wherein the composition component further contains Cu: 5% or less (excluding 0%), Ni: at least one selected from the group consisting of 5% or less (excluding 0%) and Co: 5% or less (excluding 0%).

The hot-rolled steel sheet according to any one of Claims 1 to 5, wherein the composition component further contains Ca: 0.05% or less (excluding 0%), REM: 0.05% or less (excluding 0%), Mg: 0.02% or less (excluding 0%), Li: 0.02% or less (excluding 0%), Pb: 0.5% or less (excluding 0%), Bi: At least one selected from the group consisting of 0.5% or less (excluding 0%).

According to the present invention, it is possible to provide a hot rolled steel sheet which is In the structure mainly composed of the ferrite-grained iron having a predetermined average particle diameter, the amount of solid solution N is ensured, and the aggregate structure of the hot-rolled steel sheet is controlled to a predetermined microstructure, even if it is required to have a punching processability. In the part, the life of the mold can be prolonged by increasing the deformation energy in the process, and the steel sheet is less prone to cracking, and the parts obtained after the processing can ensure the predetermined surface hardness.

Hereinafter, the hot-rolled steel sheet of the present invention (hereinafter referred to as "the steel sheet of the present invention" or sometimes only the "steel sheet") will be described in further detail. The steel sheet according to the present invention is common to the hot-forged material disclosed in the above-mentioned Patent Document 2 in that the amount of N solid solution is ensured. However, the C content is allowed to reach a high range, and the aggregate structure of the steel sheet is controlled. And the fact that the ferrite particles are fined is different.

[Thickness of steel plate of the present invention: 2 to 15 mm]

First, the steel sheet of the present invention is intended to be a steel sheet having a thickness of 2 to 15 mm. If the thickness is less than 2 mm, the rigidity required as a structure cannot be ensured. On the other hand, if the sheet thickness exceeds 15 mm, it is difficult to achieve the aggregate structure form defined by the present invention, and the desired effect of improving the punching workability cannot be obtained. The preferred plate thickness is 3 to 14 mm.

Next, the composition of the steel sheets constituting the present invention will be explained. In the following description, the units of the chemical components are all in mass%.

[Composition of Steel Sheet of the Present Invention] <C: 0.3% or less (excluding 0%)>

C is an element that has a great influence on the formation of the steel sheet. Although the structure is the ferrite iron main body or the ferrite iron-Bora iron complex phase structure, in order to form as much as possible, the ferrite is rarely formed. Iron body organization is an element that must limit its content. If C is excessively contained, the percentage of the pulverized iron in the steel sheet structure will increase, which may cause the deformation resistance to become excessive due to the work hardening of the ferritic iron. Therefore, the C content in the steel sheet is limited to 0.3% or less, preferably 0.25% or less, more preferably 0.2% or less, and particularly preferably 0.15% or less. However, if the C content is too small, the deoxidation operation becomes difficult during the melting of the steel, and it is difficult to meet the desired strength and surface hardness after deep drawing, and therefore, it is 0.0005% or more. Preferably, it is preferably 0.0008% or more, more preferably 0.001% or more.

<Si: 0.5% or less (excluding 0%)>

Si is an element which must be reduced as much as possible by solid-solution in steel to increase the deformation resistance of the steel sheet. Therefore, the Si content in the steel sheet is limited to 0.5% or less, preferably 0.45% or less, more preferably 0.4% or less, and particularly preferably 0.3% or less, based on the consideration of suppressing an increase in deformation resistance. However, when the content of Si is extremely small, deoxidation during the melting of the steel sheet becomes difficult, and the strength and surface hardness after the deep drawing process are difficult to achieve the required degree, so it is preferably 0.005% or more, more preferably. Yes More than 0.008%, more preferably 0.01% or more.

<Mn: 0.2~1%>

Mn is an element which has a function of deoxidation and desulfurization in the steel making process. In addition, if the N content in the steel is increased, the dynamic deformation will be caused by the heat generated during the processing, and the crack will easily occur. On the other hand, Mn can improve the processing at this time. Sexuality has the effect of suppressing the occurrence of cracks. In order to effectively exert these effects, the Mn content in the steel material is preferably 0.2% or more, more preferably 0.22% or more, and still more preferably 0.25% or more. However, if the Mn content is too large, the deformation resistance becomes too large, and the structure may be heterogeneous due to segregation. Therefore, it is preferably 1% or less, more preferably 0.98% or less, and even more preferably 0.95% or less.

<P: 0.05% or less (excluding 0%)>

P is an impurity element that is inevitably contained in steel. If P is contained in the ferrite iron, it will segregate in the iron grain boundary of the fat grain, resulting in deterioration of cold workability, and it is also solid solution strengthening of the ferrite iron. Therefore, an element causing an increase in deformation resistance. Therefore, the P content, based on the cold workability, is preferably minimized. If the extreme reduction results in an increase in the cost of steelmaking, it is preferable to use 0.05% or less in consideration of engineering capability. Good is 0.03% or less.

<S: 0.05% or less (excluding 0%)>

S is an unavoidable impurity similarly to P, and is an element which precipitates in the form of FeS in the form of a film grain boundary, and thus deteriorates workability. In addition, it also has the effect of inducing thermal brittleness. Therefore, in view of the viewpoint of improving the deformation energy, the present invention sets the S content to 0.05% or less, more preferably 0.03% or less. However, it is industrially difficult to change the S content to zero. Further, since S is an effect of improving the machinability, the recommended S content is preferably 0.002% or more, more preferably 0.006% or more, from the viewpoint of improving the machinability.

<Al: 0.01~0.1%>

Al is an element effective for deoxidation during the steel making process. In order to obtain such an effect of deoxidation, the Al content in the steel material is preferably 0.01% or more, more preferably 0.015% or more, and still more preferably 0.02% or more. However, when the Al content is too large, the toughness is lowered and cracks are likely to occur. Therefore, the content is set to 0.1% or less, more preferably 0.09% or less, and still more preferably 0.08% or less.

<N: 0.008 to 0.025%>

N is an important element that is desired to obtain a predetermined strength by static deformation aging after processing. Therefore, the N content in the steel material is set to 0.008% or more, more preferably 0.0085% or more, and still more preferably 0.009% or more. However, if the N content is too large, in addition to the static deformation aging, the influence of the dynamic deformation aging during processing tends to be remarkable, and the deformation resistance may increase and is not appropriate, so it is set to be 0.025% or less, more preferably 0.023. % by Next, the better is 0.02% or less.

<Solid N: 0.007% or more>

Further, by securing the solid solution N in the steel sheet to a predetermined amount (hereinafter referred to as "solid solution N amount"), the static deformation aging can be promoted without increasing the deformation resistance. If it is desired to ensure the required strength after cold working, the amount of solid solution N must be 0.007% or more. However, if the amount of solid solution N is too large, the cold workability deteriorates, so it is preferably set to 0.03% or less. Further, since the N content in the steel material is 0.025% or less, the amount of solid solution N does not become 0.025% or more.

The amount of solid solution N in the present invention is an amount obtained by subtracting the amount of the total N compound from the total amount of N in the steel material according to the method established by Japanese Industrial Standard JIS G 1228. A practical measurement method for such a solid solution N amount is as follows.

(a) Blown gas melting method - Thermal conductivity method (measurement of total N amount)

A sample cut out from the test material was placed in a crucible, melted in a blunt gas stream to extract N, and the extract was sent to a thermal conductivity unit to measure a change in thermal conductivity to obtain a total N amount.

(b) Ammonia distillation separation indophenol blue spectrophotometry (determination of total N compound amount)

The sample cut out from the test material was dissolved in a 10% AA electrolyte solution, and fixed current electrolysis was performed to measure the total amount of N compound in the steel. Make The 10% AA electrolyte used is a non-aqueous solvent electrolyte consisting of 10% acetone, 10% tetramethylammonium chloride and the rest of methanol. It does not produce a corrosion-resistant oxide film on the steel surface. The solution.

About 0.5 g of the sample for the test material was dissolved in the 10% AA-based electrolytic solution, and the resulting insoluble residue (N compound) was filtered through a filter paper made of polycarbonate having a pore size of 0.1 μm. The obtained insoluble residue was decomposed by heating in sulfuric acid, potassium sulfate, and pure copper chips, and the decomposed product was added to the filtrate. After this solution was made alkaline with sodium hydride, steam distillation was carried out, and the distilled ammonia was absorbed by dilute sulfuric acid. Further, phenol, sodium hypochlorite and sodium pentacyano-nitrite-sodium iron (III) were added to produce a blue complex, and the absorbance was measured using an absorptiometer to determine the total amount of the N compound.

Then, the amount of solid solution N can be obtained by subtracting the total amount of N compound obtained by the method of the above (b) from the total amount of N obtained by the method (a).

The steel of the present invention basically contains the above-mentioned components, and the balance is iron and unavoidable impurities. Others are within the range not detracting from the effects of the present invention, and the following allowable components may be added.

<Cr: 2% or less (excluding 0%) and/or Mo: 2% or less (excluding 0%)>

Cr is an element which can increase the strength of the crystal grain boundary and thus has an effect of improving the deformation energy of the steel. It is preferable to contain Cr in an amount of 0.2% or more in order to effectively exhibit such an effect. However, if it contains too much Cr, it will change. The shape resistance will increase, and there is a concern that the cold workability is lowered. Therefore, the content is preferably 2% or less, more preferably 1.5% or less, and the recommended value is 1% or less.

Further, Mo is an element having an action of increasing the hardness and deformation energy of the steel material after processing, and it is preferable to contain Mo in an amount of 0.04% or more, more preferably 0.08% or more. However, if too much Mo is contained, there is a concern that the cold workability is deteriorated. Therefore, the content is preferably 2% or less, more preferably 1.5% or less, and the recommended value is preferably 1% or less.

<at least one selected from the group consisting of Ti: 0.2% or less (excluding 0%), Nb: 0.2% or less (excluding 0%), and V: 0.2% or less (excluding 0%) >

These elements have a strong affinity with N and will coexist with N to form an N compound, which has the ability to refine the crystal grains of the steel, improve the toughness of the processed product obtained after cold working, and improve the crack resistance. The element of action. However, each element is the same, and if the content exceeds the upper limit, the characteristic improvement effect cannot be obtained. The content of each element is preferably 0.2% or less, more preferably 0.001 to 0.15%, and the recommended value is 0.002 to 0.1%.

<B: 0.005% or less (excluding 0%)>

B is similar to Ti, Nb, and V described above, and has a strong affinity with N, and coexists with N to form an N compound, which is capable of crystallizing steel. The fineness of the particles enhances the toughness of the processed product obtained after the cold working, and the element which can improve the crack resistance. Therefore, if B is contained in the steel sheet of the present invention, the amount of solid solution N required can be ensured to increase the strength after cold working, so the content is preferably 0.005% or less, more preferably 0.0001 to 0.0035%, particularly The recommended value is 0.0002~0.002%.

<At least one selected from the group consisting of Cu: 5% or less (excluding 0%), Ni: 5% or less (excluding 0%), and Co: 5% or less (excluding 0%) >

These elements are all effective in aging and thus hardening the steel, and are effective for improving the strength after processing. In order to effectively perform such a function, these elements are preferably contained in an amount of 0.1% or more, more preferably 0.3% or more. However, if the content of these elements is too large, the effect of deformation and hardening of the steel material, the effect of improving the strength after processing will be saturated, and there will be a concern for promoting the occurrence of cracks, and therefore, the contents are respectively 5% or less is preferred, more preferably less than 4%, and the recommended value is below 3%.

<Ca: 0.05% or less (excluding 0%), REM (rare earth metal): 0.05% or less (excluding 0%), Mg: 0.02% or less (excluding 0%), Li: 0.02% or less (excluding 0%), Pb: at least one selected from the group consisting of 0.5% or less (excluding 0%) and Bi: 0.5% or less (excluding 0%) >

Ca is such that sulfide compound inclusions such as MnS can be turned into balls. Shaped, which can improve the deformation energy of steel and contribute to the improvement of machinability. In order to effectively exert such an effect, the Ca content is preferably 0.0005% or more, more preferably 0.001% or more. However, if it is contained excessively, the effect will be saturated, and the effect matching the content thereof will not be obtained. Therefore, the content is preferably 0.05% or less, more preferably 0.03% or less, and the recommended value is 0.01% or less.

In the same manner as Ca, the REM (rare earth metal) is an element which can make the sulfide compound-based inclusions such as MnS spheroid, improve the deformation energy of the steel, and contribute to the improvement of the machinability. In order to effectively exert such an effect, the content of REM is preferably 0.0005% or more, more preferably 0.001% or more. However, if it is contained excessively, the effect will be saturated, and the effect matching the content thereof will not be obtained. Therefore, the content is preferably 0.05% or less, more preferably 0.03% or less, and the recommended value is 0.01% or less.

Further, in the present invention, REM (rare earth metal) means lanthanoid elements (15 elements up to and from Ln) and Sc(钪) and Y(钇). Among these elements, at least one element selected from the group consisting of La, Ce, and Y is preferably contained, and more preferably contains La and/or Ce.

In the same manner as Ca, Mg can be made into a spheroidized sulfide compound-based inclusion such as MnS, which can improve the deformation energy of steel and contribute to the improvement of machinability. In order to effectively exert such an effect, the content of Mg is preferably 0.0002% or more, more preferably 0.0005% or more. However, if it is excessively contained, the effect will be full. And, the effect matching with its content cannot be obtained, so the content is preferably 0.02% or less, more preferably 0.015% or less, and the recommended value is 0.01% or less.

In the same manner as Ca, Li can make the sulfide compound-based inclusions such as MnS spheroidized, improve the deformation energy of the steel, and lower the melting point and harmlessness of the Al-based oxide to improve the machinability. Helpful element. In order to effectively exert such an effect, the content of Li is preferably 0.0002% or more, more preferably 0.0005% or more. However, if it is contained excessively, the effect will be saturated, and the effect matching the content thereof will not be obtained. Therefore, the content is preferably 0.02% or less, more preferably 0.015% or less, and particularly preferably 0.01% or less.

Pb is an effective element for improving machinability. In order to effectively exert such an effect, the content of Pb is preferably 0.005% or more, more preferably 0.01% or more. However, if it is contained excessively, it will cause a problem in the production of a rolled roll or the like. Therefore, the content thereof is preferably 0.5% or less, more preferably 0.4% or less, and particularly preferably 0.3% or less.

Bi is an effective element for improving machinability, similarly to Pb. In order to effectively exert such an effect, the content of Bi is preferably 0.005% or more, more preferably 0.01% or more. However, if it is contained excessively, the effect of improving the machinability will be saturated, so the content is preferably 0.5% or less, more preferably 0.4% or less, and the recommended value is 0.3% or less.

Next, the description will be made to make the steel sheet of the present invention characteristic. Collection organization.

[Collective organization of the steel sheet of the present invention]

As described above, the steel sheet of the present invention is mainly composed of a ferrite iron main body or a ferrite iron-wave iron composite microstructure steel as a base material, but particularly: a plate surface orientation of the ferrite iron crystal grain and Size control is characteristic in this respect within a certain range.

The aggregate structure of the steel sheet of the present invention is constituted by a multiphase structure of ferrite iron and Borne iron. If the excess of the Bora iron is present, the formability of the steel sheet will be deteriorated. Therefore, the area ratio is preferably 10% or less, more preferably 9% or less, and even more preferably 8% or less. . The rest is fat iron.

<About the ferrite-grained crystal grains present at a position having a depth of t/4 (t: plate thickness, the same applies hereinafter), the area of the ferrite-grained crystal grains within 10° from the (123) plane of the plate surface orientation Rate: 20% or more, the area ratio of the ferrite-grained crystal grains within 10° from the (111) plane of the board surface: 5% or more, and the ferrite iron within 10° from the (001) plane of the board surface orientation Area ratio of crystal grains: 20% or less>

The formation method of the aggregate structure differs depending on the processing method even when the crystal system is the same, and in the case of the rolled material, it is expressed in the rolling surface and the rolling direction. In the present invention, it is expressed by a rolled surface (○○○). Further, ○ is an integer. The method of expression for these parties is revealed in: "Collection Group" edited by Nagashima Jinichi Weaving" (Maruzon Co., Ltd.) and other books.

In the present invention, the ferrite-grained crystal grains present at a position having a depth of t/4 are controlled to have an area ratio of ferrite-grained iron crystal grains within 10 degrees from a plane orientation of (123). % or more, the area ratio of the ferrite iron crystal grains within 10° from the (111) plane is controlled to 5% or more, and the area ratio of the ferrite iron crystal grains within 10° from the (001) plane is controlled to 20%. In the following, it is possible to improve the punch formability of the hot-rolled steel sheet.

In the past, an effective method for improving deep drawability is to emphasize the orientation of the (111) plane parallel to the plate surface and to weaken the (001) plane orientation. In the process of performing the cold rolling process and the annealing process, although it is possible to perform such control of the plate surface orientation, in the hot rolling process, if it is for a plate thickness of 2 to 15 mm, it is slightly in the steel sheet. In the case of a thicker hot rolled steel sheet, it is difficult to perform such control of the orientation of the panel.

Therefore, in the present invention, the ferrite-grained iron crystal grain having the plate surface orientation of the (123) plane is newly introduced, and by this, the aggregate structure in the hot-rolled sheet can be controlled, and the formability can be improved.

As described above, the ferrite-grained iron crystal grain having the (123) plane orientation has a function of improving the formability and the deep drawability, and in order to effectively exert such an effect, in terms of area ratio, Must be above 20%. More preferably, it is above 22%, more preferably it is above 24%, and the superior is more than 26%.

(111) surface also has improved formability and deep drawing In order to make it effective, the effect of the effect is preferably 5% or more, more preferably 6% or more, and even more preferably 8% or more.

The (001) surface causes an in-plane anisotropy during the forming process to deteriorate the formability. Therefore, the area ratio is limited to 20% or less, more preferably 18% or less, and even more preferably 15% or less.

Further, the microstructure of the hot-rolled steel sheet has a texture distribution in the thickness direction, but the microstructure is determined as a representative position at a depth of 1/4 of the sheet thickness. Further, it is considered that the ferrite-grained iron crystal grains having a plate surface orientation within 10° from the above-mentioned ideal surface orientations ((123) plane, (111) plane, (001) plane)) are considered to have nearly equal functions. It is determined according to the area ratio of the ferrite-grained crystal grains having the plate surface orientation in this range.

<Average particle size of ferrite grain crystal grains present at a depth of t/4: 3 to 35 μm>

The average particle size of the ferrite-grained crystal grains constituting the ferrite-grained iron structure must conform to the surface properties after processing in order to improve the workability (punching processability, flexural workability, and press workability) of the steel sheet. Must be in the range of 3 to 35 μm. If the ferrite iron crystal grains are too fine, the deformation resistance will become too high. Therefore, the average particle diameter is selected to be 3 μm or more, more preferably 4 μm or more, and still more preferably 5 μm or more. On the other hand, when the ferrite-grain crystal grains are too coarse, the toughness, the fatigue resistance, and the like are deteriorated, and even if the crystal orientation is controlled, the flexural workability, the flange extension, and the like are formed. The shape is remarkably deteriorated, and when forming, cracks and surface roughness are likely to occur. Therefore, the average particle diameter is selected to be 35 μm or less, more preferably 30 μm or less, and even more preferably 28 μm or less. . Further, in the same manner as the above-described reason, the hot-rolled steel sheet has a distribution phenomenon in which the size of the ferrite-grained crystal grains is different in the thickness direction of the hot-rolled steel sheet, but it is regarded as a depth position of 1/4 of the sheet thickness. The average particle size of the ferrite iron crystal grains is determined by the position.

[Method for Measuring Area Ratio of Each Phase]

Regarding the area ratio of each of the above-mentioned phases, each test steel plate was first etched with a nitric acid etching solution, and then five observation fields were imaged by a scanning electron microscope (SEM; magnification: 1,000 times), and then, according to a point algorithm, The ratio of fertilized iron and ferrite.

[Method for determining the orientation of the surface of the ferrite grain crystal grain]

The plate orientation of the ferrite grains is based on scanning electron microscopy (SEM)-EBSP (Electron Back Scattering Pattern) and EBSD (Electron Back Scattering Diffraction). Determination and analysis. For the SEM device, for example, SEM (JEOL JSM5410) manufactured by JEOL Ltd. is used, and EBSP measurement analysis system is, for example, EBSP: TSL (OIM). In addition, although the size of the crystal grains may vary, the measurement range of the sample is set at 300 to 1000 μm × 300 to 1000 μm, and the interval between the measurement steps is set as an example. Such as: 1~3μm or less. From the crystal orientation of each of the fertilized iron crystal grains defined in this manner, the areas of the orientations within 10 degrees from each of the above-described ideal surface orientations are collectively collected to obtain a total area, and divided by the measurement area. In this way, the area ratio of each ideal surface orientation can be obtained.

[Method for Measuring Average Particle Size of Fermented Iron Crystal Grain]

The average particle diameter of the ferrite-grained crystal grains is obtained by using the SEM-EBSP and the measurement conditions thereof, and the maximum diameter of each of the ferrite-grain crystal grains observed in a predetermined measurement range is obtained. The average value of the maximum diameter of the ferrite iron crystal grains is taken as the average particle diameter.

Next, a description will be given of a preferred manufacturing method for producing the above-described steel sheet of the present invention as follows.

[Preferred Manufacturing Method of Steel Sheet of the Present Invention]

The method for producing a steel sheet according to the present invention may be any method as long as it can form a raw material steel having the above composition component to a desired thickness. For example, a molten steel having the above composition may be prepared by a converter according to the conditions shown below, and the molten steel may be formed into a steel preform by a block forming method or a continuous casting method, and then rolled to be formed. A hot rolled steel sheet of a desired thickness.

[Fused steel modulation]

The content of N in the molten steel can be adjusted by adding a raw material containing an N compound to the molten steel and/or controlling the atmosphere of the converter to an N ₂ atmosphere when it is melted by a converter. .

[heating]

The heating before hot rolling is performed at 1100 to 1300 °C. In this heating process, in order to prevent the formation of N compounds and to make more N solid solution, high temperature heating conditions are necessary. A lower limit of the heating temperature is 1100 ° C, and a lower limit is 1150 ° C. On the other hand, a temperature exceeding 1300 ° C has difficulty in operation.

[hot roll]

Hot rolling is performed so that the final refining rolling temperature is 870 ° C or higher. If the final refining rolling temperature is too low, the metamorphic iron will be metamorphosed at high temperatures, the precipitated carbides in the ferrite will become coarser, and the fatigue strength will deteriorate, so a certain temperature must be reached. The final refined rolling temperature above. In order to coarsen the iron grains of the Worth, the particle size of the ferrite iron is increased to a certain extent, and it is more preferable to set the final refining rolling temperature to 900 ° C or higher. Further, the upper limit of the final refining rolling temperature is set at 1000 ° C because it is difficult to secure the temperature.

[Hot Roller Schedule]

Although the thickness of the hot-rolled steel sheet of the present invention is 2 to 15 mm, in order to finely control the ferrite-grained crystal grains and control the average particle diameter within a predetermined particle diameter range, it is necessary to control not only the above-described rolling temperature but also the above-mentioned rolling temperature. Must be finalized The final rolling ratio of the tandem rolling of the refining rolling is set to 15% or more. In general, the final refining roll is a tandem roll of 5 to 7 passes, and the pass schedule is set from the viewpoint of bite control of the steel plate, and the final roll rate is only 12 ~13% degree only. The final rolling ratio described above is preferably 16% or more, more preferably 17% or more. The higher the above-mentioned final rolling ratio, the higher the degree of 20% and 30%, the more the effect of miniaturizing the crystal grains can be obtained, but the upper limit value is specified based on the viewpoint of the rolling control. At the level of 30%.

[Rolling speed of hot rolling]

In order to obtain the aggregate structure of the ferrite iron crystal grains having such a plate surface orientation as described above, and in order to be able to control so that the aggregate structure in the plate thickness direction can be as uniform as possible, it is necessary to roll for the final refining roll. Speed is controlled. Therefore, the rolling speed of the final pass is controlled to 300 to 700 meters/min. If the rolling speed is too high or too low, it is difficult to obtain a desired sheet surface orientation, and the aggregate structure in the sheet thickness direction tends to become uneven, which is not preferable. In addition, if the rolling speed is too slow, productivity will also be deteriorated. Therefore, it is preferably 350 to 650 meters per minute, more preferably 400 to 600 meters per minute.

[Quenching after hot rolling]

After the completion of the final refining rolling, the cooling is performed at a cooling rate (first rapid cooling rate) of 20 ° C /sec or more within 5 seconds. In the range of 580 ° C or more and less than 670 ° C (quick cooling stop temperature), rapid cooling is stopped. This is to obtain: the structure of ferrite-iron as the main body, that is, the ferrite-iron-borite complex phase in which the percentage of Borne iron falls within the allowable range. If the cooling rate (quick cooling speed) is less than 20 ° C / sec, it will promote the wave iron metamorphosis, or if the quenching stop temperature is less than 580 ° C, it will promote the wave iron metamorphosis or tough iron deformation. It is difficult to obtain the conditions: the ferrite-iron-iron steel with the percentage of the Borne iron falling within the allowable range cannot be controlled to the desired aggregate structure, and the workability and the flexural workability of the punching will be deteriorated. On the other hand, when the quenching stop temperature is 670 ° C or more, the carbide precipitated in the ferrite iron becomes coarse, and similarly, the punching workability and the flexural workability are deteriorated. The quenching stop temperature is preferably 600 to 650 ° C, more preferably 610 to 640 ° C.

[slow cooling after quenching stops]

After the quenching is stopped, the cooling is performed for 5 to 20 seconds at a cooling rate (slow cooling rate) of 10 ° C /sec or less by using a cooling or air cooling method. Thereby, the formation of the ferrite iron can be sufficiently performed to appropriately finely precipitate the precipitated carbide in the ferrite iron. If the cooling rate exceeds 10 ° C / sec, or the chilling time is less than 5 seconds, the amount of ferrite iron formed will be insufficient, and it will not be able to control the desired aggregate structure, and the punching processability and flexural workability will be Will deteriorate. On the other hand, if the slow cooling time exceeds 20 seconds, the precipitated carbide does not become coarse, and the fatigue strength is deteriorated.

[Quenching and retracting after slow cooling]

After the slow cooling, the cooling is again performed at a cooling rate (second rapid cooling rate) of 20 ° C /sec or more, and the coiling is performed in a temperature range exceeding 300 ° C and 450 ° C or lower. This is because it is a structure mainly composed of ferrite and iron, and by forming a desired aggregate structure, this is to ensure the punching workability and the flexural workability. If the cooling rate (second quenching speed) is less than 20 ° C / sec, or the coiling temperature exceeds 450 ° C, a lot of wave iron will be formed. On the other hand, if the coiling temperature is less than 300 ° C, The formation of the granulated iron and the residual Worthite iron will result in deterioration of the workability and flexural workability.

The present invention will be described in more detail below by way of examples, but the nature of the following examples are not intended to limit the invention, and may be appropriately made as long as they meet the scope of the invention described above and below. Modifications are made and these are also included in the technical scope of the present invention.

[Examples]

The steel of the composition shown in the following Table 1 was melted by a vacuum melting method, and cast into a billet having a thickness of 120 mm, and the billet was subjected to a hot roll according to the conditions shown in Table 2 below. It is rolled into a hot rolled steel sheet. Further, in each test, the cooling rate until the quenching stop after the completion of the final refining rolling was 20 ° C /sec or more, and the cooling conditions after the quenching was stopped were 10 ° C / sec or less. Speed, slow cooling for 5 to 20 seconds.

The hot-rolled steel sheet obtained in this manner is obtained by the respective measurement methods described in the above [Embodiment]: the amount of solid solution N, the area ratio of each phase of the structure in the steel sheet, and the ferrite iron The plane orientation and average particle size of the crystal grains.

In addition, the punching workability of the above-mentioned hot-rolled steel sheet is evaluated by a cylindrical forming test on a steel sheet having a thickness of about 4 to 10 mm. The die size is: 100mm, die shoulder is: R8mm, the die diameter is: 103mm, the die shoulder is: R8mm. In the case of the "flush ratio" of the ratio (D/d) of the billet diameter (D) with respect to the diameter (d) of the cylinder, in the case where the fracture does not occur in the single-diameter tapping process, When the maximum blank diameter that can be punched into a cylinder is set to Dmax, Dmax/d is defined as "Limiting Drawing Ratio" (LDR), which is used as a rating index. And the value of LDR is 2~2.1.

Further, the molded test piece was taken out, and the outer sides of the cylindrical portion and the flange portion were visually observed. As a result of visual observation, if a crack is observed, it is marked as ×; although no crack occurs, if a crack sufficient to visually see is seen, it is marked as Δ; although it is a small unevenness (wrinkle) but no If cracking occurs, it is marked as ○; even if the occurrence of wrinkles is invisible, it is marked as ◎. And only if it is marked as ◎ or ○ is considered qualified. In addition, the difference between the definition of "fracture" and "crack" is that the maximum width of the gap is 1 mm or more, which is called "crack"; the maximum width of the gap is less than 1 mm, which is called "turtle". crack".

Further, the hardness of the surface portion of the flange portion after the above-described punching reduction processing test was also measured, and the surface hardness after the processing was evaluated. Using a Vickers hardness tester, the load was 1000 g, and the measurement position was the center of the D/4 position of the test piece section (D is the part diameter) and the number of times of measurement was 5 times. For each processed test piece, Vickers hardness (Hv) was measured. And only the Vickers hardness of 250Hv or more is considered acceptable.

These measurement results are shown in Table 3 below. In addition, the mark of "(○○○) face (%)" in the field of "Ferrous iron crystal grain" in Table 3 means: 10° from the face orientation of the board (○○○)) The area ratio (unit is %) of the ferrite grain crystal grains.

As shown in Table 3, steel Nos. 1, 2, 7 to 14, 25 to 31 are all manufactured using the steel of the specified requirements of the composition of the present invention, and are manufactured by the hot rolling conditions recommended by the present invention. As a result, the steel of the present invention which is in accordance with the requirements of the organization of the present invention can be confirmed that it can obtain: punching workability and surface hardness after processing, all of which meet the qualification standard, and also exhibit good punching during processing. A hot-rolled steel sheet which is processable and exhibits a predetermined surface hardness (strength) after processing.

On the other hand, at least one of the comparative steels shown in Steel Nos. 3 to 6 and 15 to 24 is not in conformity with one of the conditions of the composition and the structural requirements specified in the present invention, and therefore, is in the punching process. At least one of the properties and the surface hardness after processing did not meet the qualification criteria.

For example, Steel No. 3, although the requirements of its composition are consistent, the heating temperature before hot rolling, the final refining rolling temperature during hot rolling, and the quenching stop temperature after hot rolling fall outside the recommended range, too When the concentration is low, the amount of solid solution N is insufficient, and the crystal grain of the ferrite in the (123) plane orientation is insufficient. In addition, the grain of the ferrite grain in the (001) plane orientation is excessive, and the workability after punching and the surface hardness after processing are not good.

Moreover, steel No. 4, although the components of its composition are consistent, but the thickness of the plate after hot rolling falls outside the specified range, too large, so (123) plane orientation, (111) plane orientation of ferrite iron crystal The granules are insufficient, and the ferrite iron crystal grains in the (001) plane orientation are excessive, so that the ferrite granules become coarse and coarse, at least in the aspect of the punching processability.

In addition, steel No. 5, although the components of its composition are in conformity, the rolling speed of the final pass during hot rolling falls outside the recommended range, too large Therefore, in addition to the insufficient grain crystal grains of the (123) plane orientation, the ferrite iron crystal grains in the (001) plane orientation are excessive, so the punching workability is poor.

In addition, steel No. 6, although the components of the composition are consistent, but the final rolling rate during hot rolling falls outside the recommended range, too small, so the ferrite iron crystal grains become coarse, at least in the punching processability. This aspect of performance is poor.

Further, although Steel No. 15 (steel type j) has a hot rolling condition falling within the recommended range, the N content is too low, so the amount of solid solution N is insufficient, and the surface hardness after processing is poor.

In addition, steel No. 16 (steel type k), although its hot rolling conditions fall within the recommended range, the N content is too high, so it is inferior in at least the aspect of the punching workability.

Further, although steel No. 17 (steel type 1) has a hot rolling condition falling within the recommended range, the C content is too high, so the amount of solid solution N is insufficient, and the surface hardness after processing is poor.

In addition, steel No. 18 (steel type m), although its hot rolling conditions fall within the recommended range, the Si content is too high, in addition to the (123) plane orientation of the ferrite iron crystal grain, the (001) plane orientation The ferrite-grained crystal grains are excessive, so they perform poorly at least in terms of handling properties.

In addition, steel No. 19 (steel type n), although its hot rolling conditions fall within the recommended range, the Mn content is too low, the (001) plane orientation of the ferrite iron crystal grain is excessive, the punching processability, the surface hardness after processing Very poor.

In addition, steel No. 20 (steel type o), although its hot rolling conditions It falls within the recommended range, but the Mn content is too high. In addition to the insufficient grain crystal grains of the (123) plane orientation, the ferrite iron crystal grains in the (001) plane orientation are excessive, so at least in the process of drawing. This performance is poor.

In addition, steel No. 21 (steel type p), although its hot rolling conditions fall within the recommended range, the P content is too high, except for the (123) plane orientation of the ferrite iron crystal grain, the (001) plane orientation The ferrite-grained crystal grains are excessive, so they perform poorly at least in terms of handling properties.

In addition, steel No. 22 (steel type q), although its hot rolling conditions fall within the recommended range, the S content is too high, except for the (123) plane orientation and the (111) plane orientation of the ferrite iron crystal grain. The (001) plane orientation of the ferrite iron crystal grains is excessive, so it is at least poor in terms of handling properties.

In addition, steel No. 23 (steel type r), although its hot rolling conditions fall within the recommended range, the Al content is too low, except for the (123) plane orientation of the ferrite iron crystal grain, the (001) plane orientation The ferrite-grained crystal grains are excessive, so they perform poorly at least in terms of handling properties.

On the other hand, although steel No. 24 (steel type s) has a hot rolling condition falling within the recommended range, the Al content is too high, and the (001) plane orientation is in addition to the (123) plane orientation of the ferrite iron crystal grain. The ferrite-grained iron crystal grains are excessive, so at least the performance of the punching processability is poor.

The applicability of the present invention can be confirmed by the above method.

The present invention has been described in detail above with reference to specific embodiments (embodiments), but may be omitted without departing from the invention. Under the premise of the spirit and scope, various changes or amendments are applied. This practice is easy for those skilled in the art to think about.

The present application is based on the Japanese Patent Application (Japanese Patent Application No. 2013-053564) filed on March 15, 2013, the content of which is hereby incorporated by reference.

[Industrial availability]

The hot-rolled steel sheet according to the present invention can be used for a gearbox component such as a gear of an automobile, a gearbox, etc., and can realize weight reduction and high strength of these components.

Claims (2)

A hot-rolled steel sheet having excellent punching workability and surface hardness after processing, characterized in that the sheet thickness is 2 to 15 mm, and the composition thereof is in mass% (hereinafter, the chemical composition is also in mass%) C: 0.3% or less (excluding 0%), Si: 0.5% or less (excluding 0%), Mn: 0.2 to 1%, P: 0.05% or less (excluding 0%), and S: 0.05% or less (excluding 0%), Al: 0.01~0.1%, N: 0.008-0.025%, the rest consists of iron and unavoidable impurities, solid solution N: 0.007% or more, and exists at a depth of t/4 ( t: the thickness of the plate, the following is the same as the position of the ferrite grain crystal grain, the area ratio of the ferrite grain crystal grain within 10° from the (123) plane of the plate surface orientation is 20% or more, and the orientation from the plate surface is The area ratio of the ferrite-grained crystal grains within 10° of the (111) plane is 5% or more, and the area ratio of the ferrite-grained crystal grains within 10° from the (001) plane of the sheet surface orientation is 20% or less. The flatness of the ferrite iron crystal grains present at the position of the aforementioned depth of t/4 The average particle size is 3 to 35 μm. The hot-rolled steel sheet having excellent punching workability and surface hardness after processing as described in claim 1 has at least one of the following (a) to (e): (a) Cr: at least one of 2% or less (excluding 0%) and Mo: 2% or less (excluding 0%); (b) Ti: 0.2% or less (excluding 0%), and Nb: 0.2% or less (excluding 0%) and at least one of V: 0.2% or less (excluding 0%); (c) B: 0.005% or less (excluding 0%); (d) Cu: 5% or less (excluding 0%), Ni: 5% or less (excluding 0%), and Co: at least one of 5% or less (excluding 0%); (e) Ca: 0.05% or less (excluding 0%), REM ( Rare earth metal): 0.05% or less (excluding 0%), Mg: 0.02% or less (excluding 0%), Li: 0.02% or less (excluding 0%), Pb: 0.5% or less (excluding 0%), and Bi: at least one of 0.5% or less (excluding 0%).