WO2017222343A1 - Precipitation-hardening hot rolled steel sheet having excellent material uniformity and hole expandability, and manufacturing method therefor - Google Patents

Abstract

One aspect of the present invention relates to a precipitation-hardening hot rolled steel sheet having excellent material uniformity and hole expandability, comprising, by wt%, 0.02-0.05% of C, 0.01-0.3% of Si, 1.0-1.6% of Mn, 0.04-0.1% of Ti, 0.01-0.05% of Nb, 0.008% or less of N and the balance of Fe and other inevitable impurities, wherein the following formula (1) is satisfied, the microstructure comprises 95 area% or more of ferrite and (Ti, Nb)C composite precipitates, and the (Ti, Nb)C composite precipitates are formed such that the number of (Ti, Nb)C composite precipitates having a diameter of 10 nm or less is five times or more the number of (Ti, Nb)C composite precipitates having a diameter of more than 10 nm. Formula (1): 0.35 ≤ (Ti+Nb+V+Mo)/(C+N) ≤ 0.70 (In formula (1), each element symbol means the at.% of each element and the element, which is not contained in steel, is calculated as zero.)

Description

Precipitation-reinforced hot-rolled steel sheet with excellent material uniformity and hole expansion property

The present invention relates to a precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion property and a method of manufacturing the same.

In order to improve the hole expandability, it is necessary to suppress the formation of microcracks during processing, and the generation of such microcracks is that when the hardness difference between the hard phase and the soft phase is large, the concentration of deformation occurs at the grain boundaries of the two phases to generate microcracks. Known.

Accordingly, Patent Document 1 discloses a transformation hardening type steel sheet having a large number of bainite to prevent fine cracks and to secure hole expandability.

Furthermore, in order to maximize the hole expandability and secure high strength, research was conducted on the development of precipitation-reinforced steels that can secure high strength by precipitation strengthening effect using Ti, Nb, Mo, V, C, etc. on the ferrite column.

In terms of the patent document 2 has implemented a columnar base structure of ferrite-based soft to prevent micro-cracks caused by the hardness difference between the soft / hard phase in advance, and the Ar ₃ Nose and precipitated Nose match after the spirit rolling of ferrite from austenite The technique of inducing fine precipitation phenomenon in the mouth during transformation of to secure the strength is disclosed.

In the case of the precipitation-reinforced steels of Patent Document 2, it can be confirmed that the burring property is further increased while securing the same strength material by comparison with the transformation-reinforced steels. However, Patent Literature 2 relates to a conventional general milling process based manufacturing method of cooling a slab to room temperature and then reheating and rolling the slab, and the strength, There is a problem that material deviation exists in elongation, hole expandability, and the like.

On the other hand, the CEM (Continuous Endless Mill) process, which manufactures steel sheets in the so-called high-speed and continuous rolling method, a new steel process, which is recently attracting attention, has a good material deviation because the temperature deviation is small in the longitudinal direction of the strip. It is known to process steel.

However, as in Patent Documents 3 and 4, most of the research and development is concentrated on transformed hot rolled steel sheets such as DP steel and TRIP steel. However, the transformation-reinforced hot-rolled steel sheet has a problem in securing excellent burring property because the combination of soft / hard phase in the tissue is inevitable.

In addition, in the case of solid-solution-reinforced hot-rolled steel sheet, a large amount of solid-solution-reinforced elements such as Si, Mn, Cr, etc. should be added when manufacturing high-strength steel having a tensile strength of 590 MPa or more.

On the other hand, precipitation-reinforced steels have the advantage of low manufacturing cost and excellent burring properties.However, when manufacturing precipitation-reinforced steels by applying CEM processes of high-speed casting and continuous rolling, Problems such as edge crack generation due to precipitation elements such as Ti, Nb, and N, and relatively low finishing rolling temperature, and Nb, etc., may cause problems such as reduced mail flow due to changes in deformation resistance behavior of materials.

Accordingly, there is a demand for development of a precipitation-reinforced hot rolled steel sheet and a method of manufacturing the same, which are applicable to a CEM process and have excellent material uniformity and hole expansion property by solving the above problems.

(Prior art document)

Patent Document 1: Japanese Unexamined Patent Publication No. 194-200351

Patent Document 2: Japanese Unexamined Patent Publication No. 2003-321739

Patent Document 3: Korean Unexamined Patent Publication No. 2012-0049993

Patent Document 4: Korean Unexamined Patent Publication No. 2012-0052022

One aspect of the present invention is to provide a precipitation-reinforced hot rolled steel sheet excellent in material uniformity and hole expansion properties and a method of manufacturing the same.

In addition, the subject of this invention is not limited to the content mentioned above. The problem of the present invention will be understood from the general contents of the present specification, those skilled in the art will have no difficulty understanding the additional problem of the present invention.

One aspect of the present invention is by weight, C: 0.02 ~ 0.05%, Si: 0.01 ~ 0.3%, Mn: 1.0 ~ 1.6%, Ti: 0.04 ~ 0.1%, Nb: 0.01 ~ 0.05%, N: 0.008% or less , Including the remaining Fe and other unavoidable impurities, satisfying the following formula (1),

The microstructure comprises at least 95 area% of ferrite and (Ti, Nb) C composite precipitates, and the number of (Ti, Nb) C composite precipitates having a diameter of 10 nm or less is 5 of the number of (Ti, Nb) C composite precipitates having a diameter of more than 10 nm. The present invention relates to a precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion property.

Equation (1): 0.35 ≦ (Ti + Nb + V + Mo) / (C + N) ≦ 0.70

(In the formula (1), each element symbol means at.% Of each element, and elements not included in steel are counted as 0.)

In addition, another aspect of the present invention is a continuous casting step of manufacturing a thin slab by continuously casting molten steel satisfying the above-described alloy composition;

A rough rolling step of roughly rolling the thin slab to obtain a bar;

A heating step of heating or keeping the bar;

A finish rolling step of finishing rolling the heated bar to obtain a hot rolled steel sheet;

A cooling step of cooling the hot rolled steel sheet at a cooling rate of 30 ° C./s or more to a temperature of 600 ° C. to 700 ° C. in a runout table and then air-cooling; And

It relates to a method of manufacturing a precipitation-enhanced hot-rolled steel sheet having excellent material uniformity and hole expansion properties, including; winding step of winding the cooled hot-rolled steel sheet at 450 ~ 600 ℃.

In addition, the solution of the said subject does not enumerate all the characteristics of this invention. Various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, it is possible to provide a precipitation-reinforced hot rolled steel sheet excellent in material uniformity and hole expansion property having a tensile strength of 590 MPa or more and a width / length direction material deviation within ± 5% of an average value, and a method of manufacturing the same. There is.

1 is a photograph taken with a scanning electron microscope of the microstructure of Inventive Example 1.

2 is a photograph taken with a scanning electron microscope of the microstructure of Comparative Example 1.

3 is a photograph taken with a scanning electron microscope of the microstructure of Comparative Example 7.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

The inventors of the present invention, when manufacturing the precipitation-reinforced steel by applying the CEM process of the high-speed casting and continuous rolling method, the edge crack generation according to the casting element, Ti, Nb, N precipitate element and relatively low finishing rolling temperature, In order to solve this problem, it was recognized that there is a problem such as decrease of the flowability due to the change of the deformation resistance behavior of the material under the influence of Nb.

As a result, by precisely controlling the alloy composition and manufacturing method, it was confirmed that by applying the CEM process of the high-speed casting and the continuous rolling method, the precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion property can be provided. The invention has been completed.

Hereinafter, the precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion property according to an aspect of the present invention will be described in detail.

Precipitation-reinforced hot-rolled steel sheet having excellent material uniformity and hole expansion property according to an aspect of the present invention in weight%, C: 0.02 to 0.05%, Si: 0.01 to 0.3%, Mn: 1.0 to 1.6%, Ti: 0.04 0.1%, Nb: 0.01% to 0.05%, N: 0.008% or less, containing the remaining Fe and other unavoidable impurities, satisfying the following formula (1),

The microstructure comprises at least 95 area% of ferrite and (Ti, Nb) C composite precipitates, and the number of (Ti, Nb) C composite precipitates having a diameter of 10 nm or less is 5 of the number of (Ti, Nb) C composite precipitates having a diameter of more than 10 nm. It is formed more than twice.

Equation (1): 0.35 ≦ (Ti + Nb + V + Mo) / (C + N) ≦ 0.70

(In the formula (1), each element symbol means at.% Of each element, and elements not included in steel are counted as 0.)

First, the alloy composition of the precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion property according to an aspect of the present invention will be described in detail. Hereinafter, the unit of each element content is weight%.

C: 0.02 to 0.05%

The C is the most economical and effective element to strengthen the steel, when the content is present in the Hypo-Peritectic Composition, it causes a volume shrinkage during the liquid + ferrite → austenite transformation process during the solidification process during high-speed casting Promotes development

If the C content is more than 0.05%, surface cracks may occur during high speed performance. On the other hand, when the C content is less than 0.02%, strength and weldability may be significantly reduced.

Si: 0.01 ~ 0.3%

Si is a ferrite stabilizing element that removes oxygen in molten steel and has the effect of promoting ferrite transformation during cooling after hot rolling, so that a uniform ferrite structure can be formed and it is an element that helps to secure a high-strength material by solid solution strengthening effect.

If the Si content is less than 0.01%, the above effects are insufficient. On the other hand, when the Si content is more than 0.3%, the solid solution strengthening effect may be greater than the precipitation strengthening effect, and the red color scale due to Si may be formed on the steel sheet surface during hot rolling, thereby degrading the steel sheet surface quality. The upper limit is more preferably 0.2%, and even more preferably 0.1%.

Mn: 1.0-1.6%

Mn, like Si, is an effective element to solidify steel.

If the Mn content is less than 1.0%, it may be difficult to secure sufficient weld strength with the low C content described above. On the other hand, when the Mn content exceeds 1.6%, excessive ferrite transformation may be delayed, and sufficient precipitation effect may not be realized, and since the carbide or pearlite is formed in the tissue to decrease the burring property, the content is 1.0 in the present invention. It is desirable to limit it to -1.6%.

Ti: 0.04 ~ 0.1%

Ti is an element that can maximize the strength increase due to the micro-precipitation effect of interphase interface (Ti, Nb) C during transformation from austenite to ferrite.

When the Ti content is less than 0.04%, it is not easy to secure sufficient microprecipitation effect by N, which is inevitably included in the steelmaking process. On the other hand, when the Ti content is more than 0.1%, the microprecipitation effect is saturated, and excess Ti is present after the microprecipitation is formed, which is not economically desirable.

Nb: 0.01% to 0.05%

Nb, together with Ti, is an important element for the formation of (Ti, Nb) C microprecipitation and is also useful for finely forming ferrite grain size.

If the Nb content is less than 0.01%, it is difficult to implement the above-described effects. On the other hand, when the Nb content is more than 0.05%, the unrecrystallized zone temperature of the material is increased, which may cause a load problem during rolling.

N: 0.008% or less

N, even in the presence of extremely small amounts, has a great influence on the mechanical properties of the steel, increasing its tensile strength and yield strength. However, it is the main factor of the strain aging, which lowers the elongation, causes the clear heat brittleness, reduces the impact property of the material, and in particular causes wrinkles on the surface during sheet metal working. In addition, since nitrogen combines with other alloying elements to form nitrides, it is possible to reduce the (Ti, Nb) C microprecipitation effect, which is important in the present invention.

Therefore, in the present invention, N corresponds to an impurity, and it is preferable to strictly control the content to 0.008% or less.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification. For example, P, S, Cu, V, Ni, Al, Cr and the like may be inevitably mixed.

In the present invention, in addition to satisfying the above-described content of each element, the following formula (1) must be satisfied.

Equation (1): 0.35 ≦ (Ti + Nb + V + Mo) / (C + N) ≦ 0.70

(In the formula (1), each element symbol means at.% Of each element, and elements not included in steel are counted as 0.)

The metallurgical meaning of the formula (1) is to consider the optimal atomic weight ratio in consideration of the loss of Ti, Nb, V, Mo by the initial coarse precipitation by C, N in order to maximize the microprecipitation behavior. Considering such an equivalence ratio prior to minimizing the content of solid solutions such as Si, Cr, it is possible to produce steel with low burring properties while having low ferroalloy costs.

If the value of Equation (1) is less than 0.35, it is difficult to secure sufficient (Ti, Nb) C composite precipitates in ferrite, which may result in insufficient strength material. On the other hand, when the value of Equation (1) is greater than 0.70, the effect of surplus Ti, Nb, V, Mo, etc. remaining alone after formation of a composite precipitate through chemical bonding with C based on the equivalence ratio alone is relatively high. It is small and has a negative effect on economic feasibility.

In addition, the element content and the value of the formula (1) may be satisfied, and the value of the following formula (2) may be satisfied.

Equation (2): 0.04 ≦ (Ti + Nb + V + Mo) / (C + Mn + Si + Cr)

(In the formula (2), each element symbol means at.% Of each element, and elements not included in steel are counted as 0.)

Equation (2) takes into account the atomic ratio of the precipitation elements (Ti, Nb, Mo, V) and the solid solution elements (C, Si, Mn, Cr), in order to secure the target tensile strength by strengthening precipitation. .

When the value of Equation (2) is less than 0.04, the effect of solid solution strengthening is greater than the effect of precipitation strengthening, and in order to satisfy the tensile strength of 590 MPa or more, a large amount of solid solution strengthening elements such as Si, Mn, and Cr must be added. Therefore, there is a problem that the manufacturing cost rises.

The microstructure of the precipitation-reinforced hot-rolled steel sheet having excellent material uniformity and hole expansion property according to an aspect of the present invention includes ferrite and (Ti, Nb) C composite precipitates of more than 95 area%, and having a diameter of 10 nm or less (Ti, The number of Nb) C composite precipitates is more than five times the number of (Ti, Nb) C composite precipitates having a diameter of more than 10 nm. In the present invention, the (Ti, Nb) C composite precipitate is a concept including TiC, NbC and (Ti, Nb) C composite carbide, and the diameter refers to a circular equivalent diameter.

If the ferrite is less than 95 area%, there is a problem that it is difficult to secure burring property because a large amount of hardness difference between phases is present in comparison with ferrite such as pearlite and low temperature transformation phase.

When the number of (Ti, Nb) C composite precipitates having a diameter of 10 nm or less is less than five times the number of (Ti, Nb) C composite precipitates having a diameter of 10 nm or less, the precipitation strengthening effect may be insufficient and the tensile strength may be inferior.

The fact that the number of (Ti, Nb) C composite precipitates having a diameter of 10 nm or less is formed at least five times the number of the (Ti, Nb) C composite precipitates having a diameter of 10 nm or more means that the value of the following formula (3) is 5 or more. .

Equation (3):

In the formula (3), PN is the number of (Ti, Nb) C composite precipitate in the hot rolled steel sheet structure, d is the diameter of the composite precipitate observed by transmission microscope (TEM) and the unit is nm.

Is the number of precipitates having a diameter of more than 0 ~ 10nm, more than 10nm ~ 20nm, more than 20nm ~ 50nm, more than 50nm ~ 100nm, respectively, excluded because the precipitates exceeding 100nm is almost absent.

In this case, the distance between the (Ti, Nb) C composite precipitate may be 30 nm or less.

More specifically, Line-Spacing (LS) of interphase precipitation, which is arranged in the form of Curve or Straight, is observed during tissue observation by TEM analysis based on [001] or [110] Zone Axis. It means that it is 30 nm or less.

When the distance between the (Ti, Nb) C composite precipitates is greater than 30 nm, there is a problem in that the effect of fine precipitation on strength is significantly reduced. The lower limit is not particularly limited but may be 5 nm or more.

In addition, the (Ti, Nb) C composite precipitate may be 15000 / ㎛ ^{2 or} more.

Even if the distance between the (Ti, Nb) C composite precipitates satisfies 30 nm or less, when the (Ti, Nb) C composite precipitates are less than 15000 / μm ² , uniform and fine precipitates are not realized in the entire area of the mouth and thus strength And because it may not be easy to secure burring properties.

On the other hand, the hot rolled steel sheet according to the present invention has a tensile strength of 590MPa or more, the width / length direction material deviation may be within ± 5% of the average value. Not only material deviations such as strength and elongation, but also material deviations of hole expandability can be secured.

Hereinafter, another aspect of the present invention will be described in detail a method of manufacturing a precipitation-reinforced hot rolled steel sheet excellent in material uniformity and hole expansion properties.

Another aspect of the present invention, the precipitation-enhanced hot-rolled steel sheet having excellent material uniformity and hole expansion property includes: a continuous casting step of manufacturing a thin slab by continuously casting molten steel that satisfies the alloy composition described above; A rough rolling step of roughly rolling the thin slab to obtain a bar; A heating step of heating or keeping the bar; A finish rolling step of finishing rolling the heated bar to obtain a hot rolled steel sheet; A cooling step of cooling the hot rolled steel sheet at a cooling rate of 30 ° C./s or more to a temperature of 600 ° C. to 700 ° C. in a runout table and then air-cooling; And a winding-up step of winding the cooled hot rolled steel sheet at 450 to 600 ° C.

Continuous casting step

A thin slab is manufactured by continuously casting molten steel that satisfies the above-described alloy composition.

At this time, the thickness of the thin slab may be 30 ~ 150mm. Thin slabs are contrasted with slabs of 200 mm or more produced by conventional mill casting machines. Since a slab of 200 mm or more is completely cooled in a yard, etc., it is necessary to sufficiently reheat it to a surface temperature of 1100 ° C. or more in a reheating furnace before hot rolling. On the other hand, the thin slabs are transferred directly to the roughing furnace without going through the reheating furnace, so that the heat of play can be used as it is, thereby saving energy and greatly improving productivity.

Rough rolling  step

The thin slab is roughly rolled to obtain a bar. At this time, the rough rolling may be performed at 850 ~ 1150 ℃. If the rough rolling temperature is less than 850 ℃ cracks are likely to occur at the edge portion, if it is above 1150 ℃ cracks are likely to occur on the surface of the thin slab during rolling.

Heating stage

The bar is heated or heated. At this time, it may be heated or maintained in the temperature range of 850 ~ 1150 ℃. If the temperature is less than 850 ° C, the rolling load is greatly generated during finishing rolling, and if it exceeds 1150 ° C, not only the energy cost for the temperature rises but also the tendency of surface scale defects increases, so the heating temperature is increased to 850 ~ 1150 ° C. It is desirable to limit.

At this time, after the heating step before the finishing rolling step may further comprise the step of winding the heated bar (Bar).

Finish rolling  step

The heated bar is finish rolled to obtain a hot rolled steel sheet. At this time, the finish rolling may be performed in a temperature range of 770 ~ 1000 ℃. If the finish rolling temperature is less than 770 ℃ the ideal reverse rolling effect is stronger because it is difficult to secure the desired material due to the anisotropy of the extreme structure. On the other hand, in order to control the finish rolling temperature to more than 1000 ℃ high temperature heating at 1150 ℃ or more is required in the heating step, which causes a surface scale defect and can be economically expensive.

Cooling stage

The hot rolled steel sheet is cooled at a cooling rate of 30 ° C./s or more to a temperature of 600 to 700 ° C. in a runout table, and then air cooled. If the cooling end temperature is out of the temperature range of 600 ~ 700 ℃, it is not possible to fully implement the microprecipitation behavior that occurs when the ferrite transformation in austenite. In addition, even if the cooling end temperature is satisfied, if the cooling rate is less than 30 ℃ / s before the air-cooled section a considerable portion of the ferrite transformation is already made, the fraction of the ferrite to secure a fine precipitate is significantly reduced to secure the target strength material.

At this time, the run-out speed of the hot rolled steel sheet in the runout table is 100 ~ 200mpm, the speed difference may be less than 10%.

Winding  step

The cooled hot rolled steel sheet is wound at 450 to 600 ° C. If it is wound below 450 ℃, it is very difficult to control the uniform temperature because it enters the boiling zone. This may cause the mixing of hard phases such as martensite and bainite to significantly reduce the burring properties. On the other hand, when wound above 600 ° C., another hard phase such as cementite is formed, which reduces the burring property.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

The molten steel having the component composition shown in Table 1 below was prepared.

Except for Comparative Examples 5, 6 and 7, after manufacturing a 90 mm thick thin slab by high-speed casting by applying the CEM process, and performing rough rolling at 1000 ℃ without cooling to room temperature, the finish shown in Table 2 below Finish rolling was carried out at a rolling temperature (FDT), and cooled to 650 ° C. at a cooling rate and a sheet speed, and then air-cooled, and then wound at a winding temperature (CT) shown in Table 2 to prepare a hot rolled steel sheet. .

In Comparative Examples 5, 6, and 7, by applying the conventional milling method, a slab having a thickness of 220 mm was manufactured, cooled to room temperature, and then reheated (1250 ° C.) and rough rolling (1000 ° C.) to finish rolling temperature shown in Table 2 below. Finish rolling was carried out with (FDT), and cooled to 650 ° C. at the cooling rate and the sheet speed, shown in Table 2, followed by air-cooling, and wound at a winding temperature (CT) shown in Table 2 to produce a hot rolled steel sheet.

The number of precipitates for each size of the prepared hot rolled steel sheet, Equation (3), material uniformity, hole expandability, tensile strength (TS), fracture elongation (El) and ferrite fractions were measured or evaluated and are shown in Table 3 below. .

The tensile test was carried out with test pieces collected according to JIS-5 standard based on the 90 ° direction with respect to the rolling direction of the rolled sheet material. Ferrite phase fraction was obtained by etching the rolled plate specimens with Nital etchant and LePera etchant respectively at 500 magnification using an optical microscope and analyzing and comparing them with an image analyzer.

For hole expansion evaluation, a square specimen of 120 mm width and length was prepared, and after punching a 10 mm diameter hole in the center of the specimen through punching, the burr was raised and pushed up to the cone to crack the circumference. It is a value calculated as a percentage of the initial hole diameter (10mm) of the direct hole extended to the last time.

In order to check whether the material deviation such as strength and elongation can be secured well, the material deviation of the hole / expansion in the width / length direction was measured, and the material deviation of the hole / expansion in the width / length direction was measured. If it is within ± 5% of the mean value was marked as good, if it is greater than ± 5% it was marked as bad.

In the precipitate analysis, the precipitate in the tissue of the hot-rolled steel sheet was observed in the area of about 1,400,000 nm ² using TEM based on the Zone Axis [100]. Could.

Equation (3):

In the formula (3), PN is the number of (Ti, Nb) C composite precipitate in the hot rolled steel sheet structure, d is the diameter of the composite precipitate observed by transmission microscope (TEM) and the unit is nm.

Means a number of precipitates having a diameter of more than 0 to 10 nm, more than 10 nm to 20 nm, more than 20 nm to 50 nm, more than 50 nm to 100 nm, respectively, and shown in Table 3 as PN10, PN20, PN50, and PN100.

division	C	Si	Mn	Nb	Ti	N	Formula (1)	Formula (2)
Comparative Example 1	0.038	0.060	1.435	0.014	0.045	0.008	0.292	0.039
Comparative Example 2	0.044	0.064	1.447	0.014	0.049	0.008	0.277	0.041
Comparative Example 3	0.050	0.059	1.403	0.003	0.067	0.008	0.302	0.052
Comparative Example 4	0.040	0.088	1.429	0.005	0.055	0.007	0.314	0.041
Comparative Example 5	0.030	0.050	1.400	0.010	0.070	0.004	0.564	0.058
Comparative Example 6	0.045	0.060	1.400	0.011	0.075	0.006	0.404	0.061
Comparative Example 7	0.050	0.065	1.500	0.010	0.100	0.006	0.455	0.071
Comparative Example 8	0.030	0.050	1.400	0.010	0.070	0.004	0.564	0.058
Inventive Example 1	0.049	0.069	1.427	0.014	0.076	0.007	0.360	0.061
Inventive Example 2	0.046	0.067	1.418	0.014	0.066	0.007	0.391	0.054
Inventive Example 3	0.049	0.077	1.416	0.011	0.077	0.007	0.377	0.061
Inventive Example 4	0.043	0.080	1.553	0.013	0.081	0.006	0.457	0.059
Inventive Example 5	0.030	0.050	1.400	0.010	0.070	0.004	0.564	0.058
Inventive Example 6	0.044	0.061	1.472	0.011	0.075	0.006	0.412	0.058
Inventive Example 7	0.030	0.050	1.400	0.010	0.085	0.006	0.644	0.069

In Table 1, the unit of each element content is weight%, and formulas (1) and (2) are as follows.

Equation (1): 0.35 ≦ (Ti + Nb + V + Mo) / (C + N) ≦ 0.70

Equation (2): 0.04 ≦ (Ti + Nb + V + Mo) / (C + Mn + Si + Cr)

(In the formulas (1) and (2), each element symbol means at.% Of each element, and elements not included in the steel are counted as 0.)

division	fair	FDT (℃)	Cooling rate (℃ / s)	CT (℃)	Mail speed (mpm)
Comparative Example 1	CEM	808	23	622	190
Comparative Example 2	CEM	778	22	596	190
Comparative Example 3	CEM	769	32	506	190
Comparative Example 4	CEM	764	24	566	190
Comparative Example 5	Original wheat	880	28	601	410
Comparative Example 6	Original wheat	859	26	599	410
Comparative Example 7	Original wheat	877	27	610	410
Comparative Example 8	CEM	778	24	581	190
Inventive Example 1	CEM	770	35	481	190
Inventive Example 2	CEM	771	33	501	190
Inventive Example 3	CEM	774	36	477	190
Inventive Example 4	CEM	769	30	523	190
Inventive Example 5	CEM	771	33	499	190
Inventive Example 6	CEM	770	31	514	190
Inventive Example 7	CEM	779	32	514	190

division	PN10	PN20	PN50	PN100	Formula (3)	TS (MPa)	El (%)	Hole expandability (%)	Material uniformity (%)	Ferrite (area%)
Comparative Example 1	14520	5720	4230	30	1.45	543	28	130	4	97
Comparative Example 2	11250	2230	1940	0	2.70	511	31	140	3	98
Comparative Example 3	18010	4010	2110	0	2.94	551	27	121	5	98
Comparative Example 4	20940	4240	710	0	4.23	505	24	134	4	98
Comparative Example 5	11790	19470	670	0	0.59	699	21	93	20	98
Comparative Example 6	13920	16550	1140	0	0.79	674	21	89	16	97
Comparative Example 7	19540	14760	2120	0	1.16	601	23	91	21	97
Comparative Example 8	7643	3710	1430	0	1.48	537	25	119	6	96
Inventive Example 1	29830	1810	260	0	14.41	644	23	101	4	97
Inventive Example 2	41750	3320	220	0	11.79	598	24	99	3	97
Inventive Example 3	24510	330	70	0	61.28	609	22	97	2	96
Inventive Example 4	17270	2910	170	0	5.61	640	21	105	4	98
Inventive Example 5	35880	1180	220	0	25.63	599	24	92	3	97
Inventive Example 6	21190	1910	190	0	10.09	617	25	109	4	96
Inventive Example 7	33490	2140	240	0	14.07	607	25	120	2	96

Inventive Examples 1 to 7 satisfying all the conditions of the present invention, it can be confirmed that both the material uniformity and the hole expansion properties are excellent and the tensile strength is 590 MPa or more.

On the other hand, Comparative Examples 1 and 2 each element content is in the effective range suggested by the present invention, but did not satisfy the value of formula (1), Comparative Examples 3 and 4 did not satisfy the Nb content and formula (1) value In this case, it is excellent in hole expansion but can not realize sufficient precipitation strengthening effect, so it can be confirmed that the tensile strength is low.

In Comparative Examples 5, 6 and 7, the tensile strength was excellent in the case of applying the conventional milling method, but it was confirmed that the material uniformity was inferior.

Comparative Examples 5, 8 and Example 5 are the same alloy composition but different manufacturing conditions, Comparative Example 5 is inferior to the uniformity of the material by applying a general milling process, Comparative Example 8 has a low tensile strength due to the low cooling rate Inferior one, but Inventive Example 5 can be confirmed that both excellent material uniformity and tensile strength.

1, 2, and 3 are photographs of the microstructures of Inventive Example 1, Comparative Example 1, and Comparative Example 7, respectively, with a scanning electron microscope. Compared with FIGS. 2 and 3, in the case of Inventive Example 1, it can be seen from FIG. 1 that a fine and uniform (Ti, Nb) C composite precipitate was formed.

Although described with reference to the embodiments above, those skilled in the art will understand that the present invention can be variously modified and changed without departing from the spirit and scope of the invention as set forth in the claims below. Could be.

Claims (13)

1. By weight, C: 0.02 to 0.05%, Si: 0.01 to 0.3%, Mn: 1.0 to 1.6%, Ti: 0.04 to 0.1%, Nb: 0.01 to 0.05%, N: 0.008% or less, remaining Fe and other unavoidable It contains impurities, satisfies the following formula (1),

   The microstructure comprises at least 95 area% of ferrite and (Ti, Nb) C composite precipitates, and the number of (Ti, Nb) C composite precipitates having a diameter of 10 nm or less is 5 of the number of (Ti, Nb) C composite precipitates having a diameter of more than 10 nm. Precipitation-reinforced hot rolled steel sheet with excellent material uniformity and hole expansion properties.

   Equation (1): 0.35 ≦ (Ti + Nb + V + Mo) / (C + N) ≦ 0.70

   (In the formula (1), each element symbol means at.% Of each element, and elements not included in steel are counted as 0.)
2. The method of claim 1,

   The hot rolled steel sheet is a precipitation-reinforced hot rolled steel sheet excellent in material uniformity and hole expansion properties, characterized in that to satisfy the following formula (2).

   Equation (2): 0.04 ≦ (Ti + Nb + V + Mo) / (C + Mn + Si + Cr)

   (In the formula (2), each element symbol means at.% Of each element, and elements not included in steel are counted as 0.)
3. The method of claim 1,

   The precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion property, wherein the distance between the (Ti, Nb) C composite precipitates is 30 nm or less.
4. The method of claim 1,

   The (Ti, Nb) C composite precipitate is precipitated reinforced hot rolled steel sheet having excellent material uniformity and hole expandability, characterized in that 15000 / ㎛ ² or more.
5. The method of claim 1,

   The hot rolled steel sheet has a tensile strength of 590 MPa or more, and a material / uniformity-strengthening hot rolled steel sheet having excellent material uniformity and hole expandability, characterized in that a width / length direction material deviation is within ± 5% of an average value.
6. By weight, C: 0.02 to 0.05%, Si: 0.01 to 0.3%, Mn: 1.0 to 1.6%, Ti: 0.04 to 0.1%, Nb: 0.01 to 0.05%, N: 0.008% or less, remaining Fe and other unavoidable A continuous casting step of manufacturing a thin slab by continuously casting molten steel containing impurities and satisfying the following formula (1);

   A rough rolling step of roughly rolling the thin slab to obtain a bar;

   A heating step of heating or keeping the bar;

   A finish rolling step of finishing rolling the heated bar to obtain a hot rolled steel sheet;

   A cooling step of cooling the hot rolled steel sheet at a cooling rate of 30 ° C./s or more to a temperature of 600 ° C. to 700 ° C. in a runout table and then air-cooling; And

   Winding step of winding the cooled hot rolled steel sheet at 450 ~ 600 ℃;

   Method for producing a precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion properties.

   Equation (1): 0.35 ≦ (Ti + Nb + V + Mo) / (C + N) ≦ 0.70

   (In the formula (1), each element symbol means at.% Of each element, and elements not included in steel are counted as 0.)
7. The method of claim 6,

   The molten steel is a method of manufacturing a precipitation-reinforced hot rolled steel sheet excellent in material uniformity and hole expansion properties, characterized in that the following formula (2).

   Equation (2): 0.04 ≦ (Ti + Nb + V + Mo) / (C + Mn + Si + Cr)

   (In the formula (2), each element symbol means at.% Of each element, and elements not included in steel are counted as 0.)
8. The method of claim 6,

   The thickness of the thin slab is a method of manufacturing a precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion properties, characterized in that 30 ~ 150mm.
9. The method of claim 6,

   The sheeting speed of the hot rolled steel sheet in the runout table is 100 ~ 200mpm, the speed difference is less than 10%, characterized in that the uniformity and hole expansion properties excellent precipitation strengthening hot rolled steel sheet manufacturing method.
10. The method of claim 6,

    The method of manufacturing a precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion property, further comprising winding the heated bar between the heating step and the finishing rolling step.
11. The method of claim 6,

    The rough rolling is a method of producing a precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion properties, characterized in that performed in the temperature range of 850 ~ 1150 ℃.
12. The method of claim 6,

    The method of manufacturing a precipitation-reinforced hot-rolled steel sheet having excellent material uniformity and hole expandability, characterized in that the heating (Bar) in the heating step in the temperature range of 850 ~ 1150 ℃.
13. The method of claim 6,

    The finish rolling is a method of producing a precipitation-reinforced hot rolled steel sheet having excellent material uniformity and hole expansion properties, characterized in that performed in the temperature range of 770 ~ 1000 ℃.

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