CN112840046B

CN112840046B - Hot-rolled steel sheet and method for producing same

Info

Publication number: CN112840046B
Application number: CN202080005669.3A
Authority: CN
Inventors: 塚本绘里子; 豊田武; 林宏太郎
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2019-03-07
Filing date: 2020-02-12
Publication date: 2023-03-28
Anticipated expiration: 2040-02-12
Also published as: JP6835294B2; JPWO2020179387A1; WO2020179387A1; CN112840046A

Abstract

Disclosed are a hot-rolled steel sheet having high strength and excellent energy absorption capacity upon impact, hydrogen embrittlement resistance, and punching properties, and a method for producing the same. The hot-rolled steel sheet has a predetermined chemical composition, contains 90% or more of tempered martensite in terms of area ratio, has a cementite content of 70% or more with a major diameter of 400nm or less and an aspect ratio of 3 to 5, and contains 30V-containing carbides having a circle-equivalent diameter of 8 to 15nm at a ratio of 30V/. Mu.m ² The above number density precipitates. The manufacturing method of the present application includes the steps of: heating the slab to 1100 ℃ or higher; a hot rolling step in which the finish temperature of finish rolling is 850 to 1050 ℃; cooling the steel sheet to 350 ℃ or lower at an average cooling rate of 40 ℃/sec or higher; a step of coiling at 350 ℃ or lower; and at a tempering temperature T of more than 400 ℃ and less than 480 ℃ to satisfy 15000<(T+273)×(log(t)+20)<And a step of tempering at time t (sec) of 17000.

Description

Hot-rolled steel sheet and method for producing same

Technical Field

The present invention relates to a hot-rolled steel sheet and a method for producing the same, and more particularly, to a hot-rolled steel sheet used for structural members of automobiles and the like, having high strength with a tensile strength of 1180MPa or more, and excellent energy absorption capacity at the time of collision, hydrogen embrittlement resistance, and punching properties, and a method for producing the same.

Background

In recent years, in the automobile industry, weight reduction of a vehicle body is required from the viewpoint of improvement of fuel efficiency. On the other hand, due to the enhancement of the restrictions concerning collision safety, it becomes necessary to add a reinforcing member or the like to the vehicle body frame, resulting in an increase in weight. In order to achieve both weight reduction of a vehicle body and collision safety, it is one of effective methods to increase the strength of a steel sheet to be used.

In the automobile industry, in addition to weight reduction of a vehicle body, further improvement of collision resistance is required, and therefore, there is a demand for a high-strength steel sheet having excellent energy absorption capability at the time of collision. For example, in the case of a member having a low strength-ductility balance, there are the following problems: the amount of impact absorption energy is significantly reduced with breakage at the time of collision.

Patent document 1 describes a high-strength hot-rolled steel sheet having a composition as follows: contains C:0.10 to 0.25%, si:1.5% or less, mn:1.0 to 3.0%, P:0.10% or less, S:0.005% or less, al:0.01 to 0.5%, N:0.010% or less and V:0.10 to 1.0%, and satisfies (10Mn + V)/C.gtoreq.50, and the balance is Fe and inevitable impurities, wherein the volume fraction of the tempered martensite phase is 80% or more, and the V-containing carbide with the particle size of 20nm or less is 1000/mum ³ The above precipitates, and the average particle diameter of the V-containing carbide having a particle diameter of 20nm or less is 10nm or less. Further, patent document 1 describes that a high-strength hot-rolled steel sheet having excellent strength-ductility balance and tensile strength of 980MPa or more can be obtained by using V which has not been used so far as the high performance of the thin steel sheet for automobile structural members is improved.

Patent document 2 describes a method for producing a high-strength hot-rolled steel sheet, characterized in that a steel slab is heated to 1000 ℃ or higher, then rough-rolled into a sheet bar, then finish-rolled under the condition that the temperature on the finish-rolling exit side is 800 ℃ or higher, then cooled to a temperature range of less than 400 ℃ at a rate of 20 ℃/sec or higher at an average cooling rate within 3 seconds after completion of finish-rolling, coiled, and then treated under conditions satisfying 11000 to 3000[% V ] ≦ Tb (20 + log) ≦ 15000 to 1000[% V ] (where Tb is a tempering temperature (deg.c), t is a holding time(s), and [% V ] is a content (mass%)) of V in a temperature range of 400 to Ac1 transformation point, the steel slab being composed of: contains C:0.10 to 0.25%, si:1.5% or less, mn:1.0 to 3.0%, P:0.10% or less, S:0.005% or less, al:0.01 to 0.5%, N:0.010% or less and V: 0.10-1.0%, and satisfies (10Mn + V)/C ≥ 50, and the balance Fe and unavoidable impurities. Further, patent document 2 describes that a high-strength hot-rolled steel sheet having excellent strength-ductility balance and tensile strength of 980MPa or more can be obtained by using V, which has not been actively used in the past for improving the performance of a thin steel sheet for an automotive structural member, as in the case of patent document 1.

On the other hand, when a high-strength steel sheet having a tensile strength of 980MPa or more is used as an automobile member, it is generally necessary to solve the problem of hydrogen embrittlement cracks (also referred to as delayed fracture or the like) in the steel sheet. The hydrogen embrittlement cracking is a phenomenon in which a steel member to which a high stress acts in a use state is suddenly broken by hydrogen entering the steel from the environment. Generally, it is known that hydrogen embrittlement cracks occur more easily as the strength of a steel sheet increases. This is believed to be due to: as the strength of the steel sheet is higher, the stress remaining in the member formed from the steel sheet is increased, and hydrogen is more likely to concentrate in the concentrated portion of such residual stress.

Patent document 3 describes a high-strength cold-rolled steel sheet having a composition of components: contains C in mass% (hereinafter, the same as for the chemical components): 0.03 to 0.30%, si:3.0% or less (including 0%), mn: more than 0.1% and 2.8% or less, P:0.1% or less, S:0.005% or less, N:0.01% or less, al:0.01 to 0.50%, V:0.001 to 1.00%, the balance consisting of iron and unavoidable impurities, wherein the steel has the following structure: the tempered martensite contains 50% or more (including 100%) by area percentage, and the remainder is composed of ferrite, and the distribution state of precipitates in the tempered martensite is as follows: precipitates having an equivalent circle diameter of 1 to 10nm per 1 μm ² The tempered martensite contains 20 or more precipitates each having an equivalent circle diameter of 20nm or more and containing V in an amount of 1 μm ² The number of tempered martensite is 10 or less. Patent document 3 describes that the hydrogen embrittlement resistance can be ensured and the hydrogen embrittlement resistance can be ensured by the above-described high-strength cold-rolled steel sheet in which the area ratio of tempered martensite and the distribution state of V-containing precipitates precipitated in the tempered martensite are appropriately controlledThe stretch flange formability can be improved. Further, patent document 3 describes that elongation and stretch flangeability can be improved by controlling not only the dispersion state of precipitates containing V but also the size and number of cementite particles precipitated in martensite during tempering.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2006-183141

Patent document 2: japanese patent laid-open publication No. 2011-102434

Patent document 3: japanese laid-open patent publication No. 2010-018862

Disclosure of Invention

Problems to be solved by the invention

Patent documents 1 and 2 describe a high-strength hot-rolled steel sheet having a tensile strength of 980MPa and an improved strength-ductility balance in relation to energy absorption capacity at the time of collision, but no sufficient studies have been made from the viewpoint of improvement in hydrogen embrittlement resistance. Therefore, in patent documents 1 and 2, there is still room for improvement in the improvement of the properties of high-strength steel sheets, particularly high-strength steel sheets for use in automobile members.

Further, although the punching process by a press machine is often included in the processing of automobile parts and the like, the following problems are encountered particularly in the case of punching a high-strength steel sheet: cracks (punching cracks) are likely to occur at the punched end due to the increased strength of the steel sheet. In patent documents 1 and 2, improvement of punching properties of high-strength hot-rolled steel sheets is not studied.

Patent document 3 describes that hydrogen embrittlement resistance of a high-strength cold-rolled steel sheet can be ensured by appropriately controlling the area ratio of tempered martensite and the distribution state of V-containing precipitates precipitated in the tempered martensite, as described above, but no sufficient studies have been made in view of improving the punching formability of the high-strength cold-rolled steel sheet.

Accordingly, an object of the present invention is to provide a hot-rolled steel sheet having a high strength, particularly a high tensile strength of 1180MPa or more, and excellent energy absorption capacity at the time of collision, hydrogen embrittlement resistance, and punching properties, and a method for manufacturing the same, by a novel configuration.

Means for solving the problems

The inventors of the present invention studied the chemical composition and structure of the hot-rolled steel sheet in order to achieve the above object. As a result, the present inventors have found that: by containing tempered martensite in an amount of 90% or more by area ratio in the steel sheet and further controlling the Si content in the steel sheet within a predetermined range, high strength, specifically, tensile strength of 1180MPa or more can be achieved. In addition, in general, in order to improve the energy absorption capacity at the time of an automobile collision, it is effective to increase the yield strength or yield ratio of the steel material, whereby energy at the time of collision can be efficiently absorbed even with a small amount of deformation. Therefore, the inventors of the present invention further studied the structure of the hot-rolled steel sheet and found that: the present inventors have completed the present invention by controlling the Si/V ratio in a steel sheet within a predetermined range to precipitate V-containing carbides in the steel sheet and appropriately controlling the same, and in addition, by controlling cementite in tempered martensite to an appropriate form, the yield ratio of the obtained hot-rolled steel sheet can be increased to improve the energy absorption capacity at the time of collision, and further, the hydrogen embrittlement resistance and the punching property can be improved.

The present invention has been completed based on the above-described knowledge, and is specifically described below.

(1) A hot-rolled steel sheet characterized by having the following chemical composition:

contains by mass%:

C：0.15～0.30％、

Si：0.50～4.00％、

Mn：2.00～4.00％、

p: less than 0.100 percent,

S: less than 0.005 percent of,

Al：0.010～0.500％、

N:0.010% or less, and

V：0.20～1.00％、

a Si/V ratio of 10.0 or less, and the balance of Fe and impurities,

contains tempered martensite in an amount of 90% or more in terms of area percentage,

among the cementites contained in the tempered martensite, the cementite having a major diameter of 400nm or less and an aspect ratio of 3 to 5 is contained in an amount of 70% or more,

in the tempered martensite, 30V-containing carbides having a circle-equivalent diameter of 8 to 15nm are present per μm ² The above number density precipitates.

(2) The hot-rolled steel sheet according to the above (1), further comprising Nb:0.01 to 0.10%, ti:0.01 to 0.10%, B:0.0001 to 0.0050%, cr:0.005 to 1.000%, mo:0.005 to 0.500%, cu:0.50 to 3.00% and Ni: 0.25-1.50% of 1 or more than 2,

when 1 or 2 of Cr and Mo are contained, the contents of Cr, mo and V satisfy the relationship of (2Cr + Mo)/2V ≦ 2.0.

(3) A method for manufacturing a hot-rolled steel sheet, characterized by comprising the steps of:

heating a slab having the chemical composition described in (1) or (2) above to 1100 ℃ or higher;

a hot rolling step of subjecting the heated slab to a finish rolling at a finish temperature of 850 to 1050 ℃;

cooling the obtained steel sheet to 350 ℃ or lower at an average cooling rate of 40 ℃/sec or higher;

a step of coiling the steel sheet at a coiling temperature of 350 ℃ or lower; and a step of tempering the steel sheet at a tempering temperature T of more than 400 ℃ and less than 480 ℃ for a time T (sec) satisfying the following formula (1):

15000<(T+273)×(log(t)+20)<17000(1)。

effects of the invention

According to the present invention, a hot-rolled steel sheet having a high tensile strength of 1180MPa or more and excellent energy absorption capacity at the time of collision, hydrogen embrittlement resistance, and punching properties can be obtained.

Detailed Description

< Hot rolled Steel sheet >

A hot-rolled steel sheet according to an embodiment of the present invention has the following chemical composition: contains by mass%:

C：0.15～0.30％、

Si：0.50～4.00％、

Mn：2.00～4.00％、

p: less than 0.100 percent,

S: less than 0.005 percent,

Al：0.010～0.500％、

N:0.010% or less, and

V：0.20～1.00％、

a Si/V ratio of 10.0 or less, the remainder being Fe and impurities,

As described above, the present inventors have found that: the tensile strength of 1180MPa or more can be achieved in the obtained steel sheet, and the energy absorption capacity at the time of collision can be improved, and further the hydrogen embrittlement resistance and the punching property can be improved by containing 90% or more of tempered martensite in terms of area ratio in the steel sheet, and appropriately controlling the Si content and the Si/V ratio in the steel sheet so as to precipitate V-containing carbide in the steel sheet, and in addition, controlling the cementite in the tempered martensite to an appropriate form. In the present invention, the V-containing carbide includes not only Vanadium Carbide (VC), but also a composite carbide of V and an element such as Nb or Ti, for example, a composite carbide such as (V, ti) C.

More specifically, tempered martensite is a structure obtained by heating and holding a martensite structure at an appropriate temperature (i.e., tempering) to precipitate cementite, but the progress of the structure recovery by such tempering is generally adjusted by the following tempering parameter P using temperature and time as variables.

P＝(T+273)(log(t)+273)

Wherein T is a tempering temperature (. Degree. C.) and T is a tempering time (sec). Since the hardness after tempering can be generally expressed as a function of the tempering parameter P, the hardness and tensile strength of the tempered steel sheet can be predicted from the tempering temperature and tempering time.

Among these, as tempering progresses, the dislocation density in the structure becomes smaller, and therefore the tensile strength generally decreases. Therefore, excessive tempering may not necessarily be preferable from the viewpoint of obtaining a high-strength steel sheet. However, it is known that the progress of tempering greatly varies depending on the addition of alloy elements, and that the tensile strength of a steel sheet can be improved even at the same value of the tempering parameter P by appropriately adding a specific alloy element. In addition, by adding carbide-forming elements to appropriately precipitate carbides in tempered martensite, the yield ratio of the steel sheet can be increased to improve the energy absorption capacity at the time of collision. Among a certain number of carbides, particularly, V-containing carbides also function as hydrogen trap sites, and therefore, by using such carbides, the hydrogen embrittlement resistance of the steel sheet can be improved.

Therefore, the inventors of the present invention have studied the chemical composition and structure of the hot-rolled steel sheet with attention paid to Si as the above-mentioned alloying element and V as the above-mentioned carbide-forming element. As a result, the present inventors have found that: by setting the Si content in the hot-rolled steel sheet to 0.50% or more, tempering can be delayed, and therefore, even with the same value of the tempering parameter P, the tensile strength of the hot-rolled steel sheet can be improved. Further, the present inventors have found that: the Si content in the hot-rolled steel sheet greatly affects the precipitation driving force of V-containing carbide by the relationship with the V content. Specifically, it was found that: by setting the Si/V ratio in the hot-rolled steel sheet to 10.0 or less, a large amount of V-containing carbide can be precipitated and the V-containing carbide can be refined. Further, the present inventors have found that: since V diffusion is slowed down by tempering at a low temperature, coarsening of V-containing carbide can be suppressed, and as a result, V-containing carbide can be finely and largely dispersed in tempered martensite, the yield ratio of the hot-rolled steel sheet can be increased by particle dispersion strengthening by such fine V-containing carbide dispersion, the energy absorption capacity at the time of collision can be improved, and the hydrogen embrittlement resistance of the hot-rolled steel sheet can also be improved by utilizing the fine V-containing carbide also as a hydrogen trapping site.

Further, the present inventors have found that: as a result of tempering at a low temperature as described above, coarsening and spheroidizing of cementite in the tempered martensite can be suppressed, and as a result, by containing a relatively large amount of fine cementite having a relatively high aspect ratio in the tempered martensite, more specifically, by containing 70% or more of the cementite having a long diameter of 400nm or less and an aspect ratio of 3 to 5 in the cementite contained in the tempered martensite, micro cracks are less likely to occur during punching, and the punching property of the hot-rolled steel sheet is improved. As a result, according to the present invention, a hot-rolled steel sheet having a high tensile strength of 1180MPa or more and excellent energy absorption capacity at the time of collision, hydrogen embrittlement resistance, and punching properties can be provided. Hereinafter, a hot-rolled steel sheet and a method for manufacturing the same according to an embodiment of the present invention will be described in more detail.

First, the chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention and the slab used for manufacturing the same will be described. In the following description, the unit of the content of each element contained in the hot-rolled steel sheet and the slab, i.e., "%" means "% by mass" unless otherwise specified.

[C：0.15～0.30％]

C is an element necessary for securing the strength of the steel sheet. Since the desired high strength cannot be obtained when C is less than 0.15%, the C content is set to 0.15% or more. The C content may be 0.16% or more, 0.18% or more, or 0.20% or more. On the other hand, if it exceeds 0.30%, formability and weldability are deteriorated, so the C content is set to 0.30% or less. The C content may be 0.28% or less, 0.26% or less, or 0.25% or less.

[ Si:0.50 to 4.00%, si/V ratio: 10.0 or less ]

Si is an element having an effect of delaying tempering. Since the desired tempering-retarding effect cannot be obtained when the Si content is less than 0.50%, the Si content is set to 0.50% or more. The Si content may be 0.60% or more, 0.80% or more, 1.00% or more, or 1.50% or more. On the other hand, if the Si content exceeds 4.00%, the workability is lowered, so the Si content is set to 4.00% or less. The Si content may be 3.50% or less or 3.00% or less. Further, if the Si/V ratio exceeds 10.0, the driving force for precipitation of V-containing carbide particles decreases, so the Si/V ratio is set to 10.0 or less. The Si/V ratio may be 9.5 or less, 8.0 or less, 7.0 or less, or 6.0 or less. The lower limit of the Si/V ratio is not particularly limited, but may be, for example, 0.5, 0.8 or 1.0.

[Mn：2.00～4.00％]

Mn is a hardenability element. When Mn is less than 2.00%, the martensite content before tempering cannot be secured, bainite transformation and pearlite transformation are caused, and the coarse cementite content increases. Therefore, the Mn content may be set to 2.00% or more, or 2.10% or more, 2.20% or more, or 2.30% or more. On the other hand, if the Mn content exceeds 4.00%, the co-segregation of P, S is promoted, and the workability is significantly deteriorated, so the Mn content is set to 4.00% or less. The Mn content may be 3.50% or less, 3.00% or less, or 2.80% or less.

[ P:0.100% or less)

The lower the P content is, the more preferable it is, and if it exceeds 0.100%, the lower the fatigue characteristics, since the formability and weldability are adversely affected and the fatigue characteristics are lowered, the content is set to 0.100% or less. P is preferably 0.050% or less, and more preferably 0.030% or less. The P content may be 0%, but is preferably 0.0001% or more because excessive reduction leads to an increase in cost.

[ S:0.005% or less ]

S is an element that produces non-metallic inclusions such as MnS in steel and causes a reduction in ductility of steel components. Therefore, the S content is set to 0.005% or less, preferably 0.003% or less, and more preferably 0.002% or less. The smaller the S content, the more preferable it is, and the 0% content is, but in order to make the S content extremely low in the refining step, the time required for refining becomes long, and the cost is significantly increased. Therefore, the lower limit of S is preferably set to 0.0001% or more or 0.0005% or more.

[Al：0.010～0.500％]

Al functions as a deoxidizer and is preferably added in the deoxidation step. In order to obtain such an effect, the Al content needs to be set to 0.010% or more. The Al content may be 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, if Al exceeds 0.500%, coarse Al oxide is formed, resulting in a decrease in cold formability. Therefore, the upper limit of Al is set to 0.500% or less. The Al content may be 0.400% or less, 0.300% or less, or 0.100% or less.

[ N:0.010% or less

N is also favorable for deterioration of workability and generation of pores during welding, and is therefore less. If N exceeds 0.010%, workability deteriorates, so 0.010% is set as the upper limit. The N content may be 0.005% or less or 0.004% or less. The N content may be 0%, but is preferably 0.001% or more because excessive reduction increases the cost.

[V：0.20～1.00％]

V is an element effective for controlling the morphology of carbide. The V-containing carbide, such as Vanadium Carbide (VC), acts as a trap site for hydrogen, and contributes to the improvement of hydrogen embrittlement resistance of the steel sheet. Further, the V-containing carbide precipitated in a finely dispersed manner enhances the yield strength and yield ratio of the steel sheet by particle dispersion strengthening. In the present invention, when V is less than 0.20%, the precipitation amount of V-containing carbide particles is small, and the improvement of hydrogen embrittlement resistance, yield strength and/or yield ratio is insufficient, so the lower limit is set to 0.20% or more. The V content may be 0.25% or more, 0.30% or more, or 0.40% or more. On the other hand, since the amount of V bonded to C is determined by the stoichiometric ratio, an excessive amount of V content leads to an increase in cost and coarsening of V-containing carbide. Therefore, the upper limit of V is set to 1.00%. The V content may be 0.80% or less, 0.70% or less, or 0.60% or less.

The hot-rolled steel sheet according to the embodiment of the invention and the slab used for manufacturing the same have the basic composition as described above. Further, the hot-rolled steel sheet and slab may contain 1 or 2 or more of the following optional elements as necessary. The optional element may not be contained, and the content in this case is 0%.

[ Nb:0.01 to 0.10%, ti:0.01 to 0.10%, B: 0.0001-0.0050% of 1 or more than 2]

Nb and Ti are elements effective for controlling the form of carbide, as in V. In addition, ti has an effect of suppressing the bonding between B and N when B is added to improve hardenability by preferentially bonding to N. However, if Nb and Ti are added in too large amounts, V carbide containing Nb and/or Ti is precipitated and coarsened in the hot rolling step and the tempering step because the solubility is lower than that of VC, and there is a possibility that fine V-containing carbide cannot be obtained because the concentration of solid-solution carbon is reduced. Therefore, the Nb and Ti contents may be set to 0.10% or less, 0.08% or less, or 0.05% or less, respectively. On the other hand, when Ti and Nb are added, the respective contents may be set to exceed 0%, but the lower limit of the respective contents is preferably set to 0.01%. B is an element that suppresses ferrite transformation by segregating at the austenitic grain boundaries in the heat treatment step. The B content may be more than 0%, but from the viewpoint of obtaining higher effects, the B content is preferably 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, the ferrite transformation suppressing effect is saturated, so 0.0050% is preferably set to a substantial upper limit. The B content may be 0.0030% or less or 0.0020% or less.

[ the Cr content is in a range satisfying (2Cr + Mo)/2V ≤ 2.0: 0.005 to 1.000% and Mo: 1 or 2 of 0.005 to 0.500% ]

When 1 or 2 of Cr and Mo are contained, (2Cr + Mo)/2V exceeds 2.0, the composition of the V-containing carbide becomes rich in Mo and/or Cr, and as a result, precipitates may easily coarsen, and the strength-ductility balance and the softening properties of the weld heat-affected zone may deteriorate. The value of (2Cr + Mo)/2V may be 1.5 or less, or 1.0 or less. The value of (2Cr + Mo)/2V may be 0 or 0.01 or more. The respective contents of Cr and Mo may be more than 0%, but it is preferable to control the ratio of Cr:0.005 to 1.000% and Mo: the content of Cr and Mo is selected to be within the range of 0.005 to 0.500%.

[Cu：0.50～3.00％]

Cu precipitates alone during tempering, and contributes effectively to strength increase. In addition, cu promotes the fine precipitation of V-containing carbide. The Cu content may be more than 0%, but is preferably 0.50% or more from the viewpoint of obtaining higher effects. The Cu content may be 0.80% or more or 1.00% or more. On the other hand, if the Cu content becomes more than 3.00%, not only the above-described effects are saturated, but also the steel sheet strength is significantly increased, resulting in deterioration of formability. Therefore, the Cu content may be set to 3.00% or less, or 2.80% or less, or 2.50% or less.

[Ni：0.25～1.50％]

Ni is effective for preventing surface defects from occurring on the surface of the steel sheet when Cu is added, and may be contained as necessary when Cu is added. In this case, the Ni content depends on the Cu content, and is preferably set to about half of the Cu content, that is, about 0.25 to 1.50%. The Ni content may be more than 0%, but from the viewpoint of obtaining higher effects, it is preferably set to 0.25% or more, and more preferably set to 0.30% or more or 0.50% or more. The Ni content may be 1.40% or less or 1.20% or less.

In the hot-rolled steel sheet according to the embodiment of the invention, the remainder other than the above components is composed of Fe and impurities. The impurities mean components mixed in due to various factors in the manufacturing process, such as raw materials such as ores and scraps, in the industrial production of the hot-rolled steel sheet, and include components (so-called unavoidable impurities) which are not intentionally added to the hot-rolled steel sheet according to the embodiment of the present invention. In addition, the impurities include elements other than the above-described components, and are included in the hot-rolled steel sheet in such a level that the characteristic action and effect of the elements do not affect the properties of the hot-rolled steel sheet according to the embodiment of the present invention.

Next, the reason why the structure of the hot-rolled steel sheet according to the embodiment of the invention is limited will be described.

[ tempered martensite: over 90%)

Martensite is a microstructure in which carbon and alloy elements are dissolved in a supersaturated state and dislocations are present at a high density, and is a structure in which a large number of carbide nuclei are present in a rich state and carbides are dispersed and precipitated during tempering. In order to precipitate a large amount of cementite and V-containing carbide while recovering the structure, the lower limit of the tempered martensite may be 90% or more in terms of area ratio, and may be 95% or more, 96% or more, 97% or more, 98% or more, 99% or more, or 100% or more in terms of area ratio, for example.

The residual structure other than tempered martensite may be 0%, but when a residual structure is present, the residual structure may be bainite, ferrite, pearlite, or the like, for example. The residual structure may be, for example, 10% or less, 5% or less, 4% or less, 3% or less, 2% or less, or 1% or less in terms of area ratio.

[ the content of cementite having a major diameter of 400nm or less and an aspect ratio of 3 to 5 among cementite contained in tempered martensite ] is 70% or more ]

The finer the cementite generated in tempering, the more excellent the punchability, and the more elongated the shape, the more excellent the suppression of the progress of cracks. Therefore, the content of cementite having a long diameter of 400nm or less and an aspect ratio of 3 to 5 is set to 70% or more. Preferably 75% or more, and more preferably 80% or more. The upper limit of the content of cementite having a major axis of 400nm or less and an aspect ratio of 3 to 5 is not particularly limited, and may be 100%, but is generally 95% or less or 90% or less. In the present invention, the tempered martensite may contain cementite having an aspect ratio exceeding 5, or may contain cementite having an aspect ratio lower than 3. In the present invention, the aspect ratio is a ratio of the longest diameter (major diameter) of the cementite to the longest diameter (minor diameter) of the cementite orthogonal to the longest diameter.

[ in coming back toIn the hot martensite, 30V-containing carbides having a circle-equivalent diameter of 8 to 15nm are present per μm ² Precipitation of number density of above]

Precipitation of a large amount of fine V-containing carbides, more specifically V-containing carbides having a circle-equivalent diameter of 8 to 15nm, into tempered martensite at a ratio of 30V/μm ² The above number density precipitates, and these precipitates act as resistance to the movement of movable dislocations, and exhibit precipitation strengthening to increase the yield strength and yield ratio of the hot-rolled steel sheet, thereby improving the energy absorption capacity at the time of collision. Further, since not only the V-containing carbide such as VC but also the complex carbide of Nb and/or Ti and V act as trap sites of hydrogen, the hydrogen embrittlement resistance of the hot-rolled steel sheet can be improved by precipitating a large amount of the above-mentioned fine V-containing carbide. When the volume ratio of the V-containing carbide to the matrix phase is the same, the larger the number of the V-containing carbide, the finer the size of the V-containing carbide, the higher the yield strength and yield ratio, and the higher the hydrogen embrittlement resistance. In order to obtain these effects, the hot-rolled steel sheet according to the present embodiment is tempered martensite at 30 pieces/μm ² The above number density contains V-containing carbide particles having a circle-equivalent diameter of 8 to 15 nm. The number density is preferably 32 pieces/. Mu.m ² Above, more preferably 35 pieces/. Mu.m ² The above.

[ method of identifying tempered martensite and method of measuring tempered martensite ]

The tempered martensite is identified by the following method: the cross section in the thickness direction was etched with a nital reagent, and a range of 1/8 to 3/8 of the thickness (position 1/8 to 3/8 of the thickness from the surface of the steel sheet) centered at a position 1/4 of the thickness from the surface of the steel sheet was subjected to a field emission scanning electron microscope (FE-SEM) at a magnification of 3000 times (field area: 1370 μm) ² ) Observations were made of the location and modification (variant) of cementite contained within the interior of the tissue. Specifically, although tempered martensite is a structure in which cementite is generated inside martensite laths, the crystal orientation relationship between martensite laths and cementite is 2 or more types, and thus the generated cementite has a plurality of modifications. By detecting the characteristics of these cementitesThereby, the tempered martensite was identified and the area ratio was calculated.

[ method for identifying and measuring cementite ]

In addition, the identification of cementite can be performed by: the cross section in the thickness direction was etched with a nital, and 2-fold electron images (10000 times, field area: 123 μm) obtained by a Scanning Electron Microscope (SEM) were used in a range of 1/8 to 3/8 of the thickness from the surface of the steel sheet to a position 1/4 of the thickness ² ) And (6) carrying out observation. The area imaged with a bright contrast in the 2-time electron image is set as cementite, and the content of cementite having a long diameter of 400nm or less and an aspect ratio of 3 to 5 is measured by measuring all cementite in the field of view.

[ method of measuring circle-reduced diameter and number density of V-containing carbide ]

The circle-equivalent diameter and number density of the V-containing carbide particles were determined as follows. First, a Transmission Electron Microscope (TEM) was used to sample an extraction replica of a circular region having a diameter of 3.0mm at a position 1/4 of the surface of a steel plate at a magnification of 6 ten thousand times (field area: 4.5 μm) ² ) The 3 fields of view were observed, and for the precipitates in which V was detected by energy dispersive X-ray spectroscopy (EDX) in each field of view, the area of each precipitate was obtained using an image analyzer, and the area was converted into a circle-converted diameter. Then, the number of V-containing carbide particles having a circle-equivalent diameter of 8 to 15nm was calculated, the value obtained by dividing the number by the area of the observation field was obtained, the number density of V-containing carbide particles in each field was calculated, and the sum obtained by performing the above-mentioned operation in the 3 fields was determined as the number density of V-containing carbide particles having a circle-equivalent diameter of 8 to 15nm on average.

[ mechanical characteristics ]

According to the hot-rolled steel sheet having the above chemical composition and structure, a high tensile strength, specifically, a tensile strength of 1180MPa or more can be achieved. The tensile strength is set to 1180MPa or more in order to satisfy the requirement of weight reduction of the vehicle body in the automobile. The tensile strength is preferably 1200MPa or more, and more preferably 1300MPa or more.

< method for producing Hot rolled Steel sheet >

A method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention includes:

a step of heating the slab having the chemical composition described above to 1100 ℃ or higher;

a step of coiling the steel sheet at a coiling temperature of 350 ℃ or lower; and

tempering the steel sheet at a tempering temperature T of more than 400 ℃ and less than 480 ℃ for a time T (sec) satisfying the following formula (1):

15000<(T+273)×(log(t)+20)<17000(1)。

[ heating Process of sheet blank ]

First, a slab having the chemical composition described above is heated before hot rolling. The heating temperature of the slab is set to 1100 ℃ or higher in order to sufficiently resolubilize the V carbonitride and the like. If the heating temperature is less than 1100 ℃, V-containing carbide precipitates and coarsens during hot rolling, and fine V-containing carbide cannot be precipitated at a desired number density, and as a result, a sufficient yield ratio and hydrogen embrittlement resistance may not be obtained. Therefore, the heating temperature of the slab may be set to 1100 ℃ or higher, for example, 1150 ℃ or higher or 1200 ℃ or higher. The upper limit of the heating temperature is not particularly limited, but may be 1300 ℃ or less or 1250 ℃ or less in general. The holding time at the heating temperature is not particularly limited, but is generally preferably set to 30 minutes or more for setting the temperature to a predetermined temperature up to the center of the slab, and is preferably 180 minutes or less, more preferably 120 minutes or less for suppressing excessive scale loss. The slab to be used is preferably cast by a continuous casting method from the viewpoint of productivity, and may be produced by an ingot casting method or a thin slab casting method.

[ Hot Rolling Process ]

(Rough rolling)

In the method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the thickness of the slab. The conditions for rough rolling are not particularly limited as long as the desired sheet bar size can be secured.

(finish rolling)

The finishing temperature of the finish rolling is 850-1050 ℃. By setting the finish temperature of the finish rolling within the above range, the precipitation of the V-containing carbide immediately after the finish rolling can be suppressed. On the other hand, if the heating temperature is less than 850 ℃, V-containing carbide precipitates and coarsens in the finish rolling, and thus fine V-containing carbide cannot be precipitated at a desired number density, and as a result, a sufficient yield ratio and hydrogen embrittlement resistance may not be obtained. For example, the heating temperature may be 900 ℃ or higher and/or 1000 ℃ or lower.

[ Cooling Process ]

The hot-rolled steel sheet obtained by the finish rolling is then cooled in a cooling step. The cooling is performed at an average cooling rate of 40 ℃/sec or more to 350 ℃ or less. Preferably 45 ℃/sec or more, and more preferably 50 ℃/sec or more. By setting the average cooling rate to 40 ℃/sec or more, the precipitation of V-containing carbide can be further suppressed. On the other hand, if the average cooling rate is less than 40 ℃/sec, the steel is not sufficiently quenched, and therefore, a desired tempered martensite area ratio may not be achieved in the final structure. In addition, V-containing carbide precipitates during cooling, and there is a possibility that it coarsens during tempering. Further, since a sufficient amount of tempered martensite is not obtained, the number of nucleation sites of the V-containing carbide particles is also reduced, and as a result, there is a possibility that a desired number density of the V-containing carbide particles cannot be achieved in the final structure. The upper limit of the average cooling rate is not particularly limited, but is preferably 1000 ℃/sec or less, more preferably 200 ℃/sec or less, and even more preferably 100 ℃/sec or less, in consideration of the suppression of the occurrence of uneven cooling and the facility capacity.

[ coiling Process ]

After the cooling step, the hot-rolled steel sheet is wound. The coiling temperature is set to 350 ℃ or lower. By setting the coiling temperature to 350 ℃ or lower, V can be brought into a solid solution state, and precipitation of V-containing carbide in the coiling step can be suppressed. If the coiling temperature exceeds 350 ℃, V-containing carbide precipitates in coiling rolling and coarsens in tempering, and thus fine V-containing carbide cannot be precipitated at a desired number density, and as a result, sufficient yield ratio and hydrogen embrittlement resistance may not be obtained. Therefore, the winding temperature is set to 350 ℃ or less, preferably 300 ℃ or less, more preferably 200 ℃ or less, and still more preferably 100 ℃ or less. The lower limit of the coiling temperature is not particularly limited, but may be about room temperature (about 25 ℃) from the viewpoint of productivity.

[ tempering step ]

After the coiling step, the hot-rolled steel sheet is tempered at a temperature exceeding 400 ℃ and below 480 ℃ to form tempered martensite, and fine V-containing carbides are precipitated in the tempered martensite. By setting the tempering temperature to more than 400 ℃ and less than 480 ℃, fine V-containing carbide particles can be sufficiently precipitated, and at the same time, coarsening of cementite can be suppressed, and the yield strength, yield ratio, and hydrogen embrittlement resistance can be increased. On the other hand, if the tempering temperature is 400 ℃ or lower, the diffusion of V necessary for the precipitation of V-containing carbide does not sufficiently occur, and as a result, there is a possibility that V-containing carbide does not precipitate sufficiently or at all. Further, if the tempering temperature is 480 ℃ or higher, the cementite may be coarsened, and a desired aspect ratio may not be obtained, resulting in a decrease in punching formability. Further, since the V-containing carbide particles coarsen, there is a possibility that fine V-containing carbide particles cannot be precipitated at a desired number density. The tempering temperature may be 410 ℃ or higher or 420 ℃ or higher and/or 470 ℃ or lower or 460 ℃ or lower.

Furthermore, by setting the following tempering parameter P with the tempering temperature and the tempering time as variables to be more than 15000 and less than 17000, the nucleation and growth of V-containing carbide can be sufficiently progressed, and the coarsening thereof can be suppressed.

P＝(T+273)(log(t)+20)

Wherein T is a tempering temperature (. Degree. C.) and T is a tempering time (sec). On the other hand, if the tempering parameter is 15000 or less, precipitation and growth of V-containing carbide do not sufficiently proceed, and sufficient particle dispersion strengthening and a function as a hydrogen trap site cannot be obtained, so that there is a possibility that a desired yield ratio and hydrogen embrittlement resistance cannot be achieved. Further, if the tempering parameter is 17000 or more, the V-containing carbide coarsens and the fine V-containing carbide cannot be precipitated at a desired number density, and as a result, there is a possibility that a sufficient yield ratio and hydrogen embrittlement resistance cannot be obtained. The tempering parameter P is preferably 16000 or more, and more preferably 16200 or more.

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples at all.

Examples

In the following examples, hot-rolled steel sheets according to embodiments of the present invention were produced under various conditions, and the mechanical properties of the obtained hot-rolled steel sheets were examined.

First, slabs having chemical compositions shown in table 1 were manufactured by a continuous casting method. Then, hot-rolled steel sheets having a thickness of 2.5mm were produced from these slabs under the heating, hot-rolling, cooling, coiling and tempering conditions shown in table 2. The balance other than the components shown in table 1 was Fe and impurities. Further, the chemical composition obtained by analyzing the samples collected from the manufactured hot-rolled steel sheet was the same as the chemical composition of the slab shown in table 1.

[ Table 1]

Underlining is indicated as being outside the scope of the invention.

[ Table 2]

Underlining is indicated as being outside the scope of the invention.

[ measurement of Tensile Strength (TS), yield Strength (YS) and Yield Ratio (YR) ]

From the hot-rolled steel sheet obtained in this manner, JIS5 tensile test pieces were sampled from a direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z2241 (2011) to measure the Tensile Strength (TS) and the Yield Strength (YS) and calculate the Yield Ratio (YR). YR is a value obtained by dividing YS by TS.

[ measurement of circle-equivalent diameter and number density of V-containing carbide ]

The circle-equivalent diameter and number density of the V-containing carbide particles were determined as follows. First, a Transmission Electron Microscope (TEM) was used to examine an extraction replica sample of a circular region having a diameter of 3.0mm at a position 1/4 of the surface of a steel plate at a magnification of 6 ten thousand times (field area: 4.5 μm) ² ) The 3 fields of view were observed, and for the precipitates in which V was detected by energy dispersive X-ray spectroscopy (EDX) in each field of view, the area of each precipitate was obtained using an image analyzer, and the area was converted into a circle-converted diameter. Then, the number of V-containing carbide particles having a circle-equivalent diameter of 8 to 15nm was calculated, the value obtained by dividing the number by the area of the observation field was obtained, the number density of V-containing carbide particles in each field was calculated, and the sum obtained by performing the above-mentioned operation in the 3 fields was determined as the number density of V-containing carbide particles having a circle-equivalent diameter of 8 to 15nm on average.

[ evaluation of punchability ]

The obtained hot-rolled steel sheet sample was punched out on a disk, and the sheared surface was observed with an optical microscope to confirm the presence or absence of 2 times of sheared surface. The sample not having the 2-time shear plane was determined to be acceptable (o), and the sample having the 2-time shear plane was determined to be unacceptable (x).

[ evaluation of Hydrogen embrittlement resistance ]

In the measurement of hydrogen embrittlement resistance, each of the hot-rolled steel sheets described above was ground to a thickness of 1.4mm, a test piece having a width of 6mm × a length of 68mm was cut out, a strain corresponding to a yield stress was applied by a four-point bending test, and then the steel sheet was immersed in hydrochloric acid having a pH of 3 for 100 hours, and the hydrogen embrittlement resistance was evaluated based on the presence or absence of cracks. The hot rolled steel sheet having no cracks was judged as good (O), and the hot rolled steel sheet having cracks was judged as bad (X).

A hot-rolled steel sheet having a TS of 1180MPa or more, a YR of 87% or more, and a hydrogen embrittlement resistance and punching formability evaluated as O, which is high in strength and excellent in energy absorption capacity at the time of collision, hydrogen embrittlement resistance and punching formability. The results are shown in table 3 below.

[ Table 3]

Underlining is indicated as being outside the scope of the invention.

In comparative example 5, since the finish temperature of the finish rolling was less than 850 ℃, the V-containing carbide such as VC was precipitated and coarsened in the finish rolling, and it was not possible to precipitate the V-containing carbide having a circle-equivalent diameter of 8 to 15nm at a desired number density. Therefore, a sufficient YR and hydrogen embrittlement resistance are not obtained. In comparative example 14, since the average cooling rate after finish rolling was less than 40 ℃/sec, quenching was not sufficiently performed, and a desired tempered martensite area ratio could not be achieved in the final structure. In addition, V-containing carbide such as VC precipitates during cooling and coarsens during tempering heat treatment. Furthermore, since tempered martensite is not made into a parent phase in the heat treatment, the number of nucleation sites for V-containing carbide is small, and the desired number density of V-containing carbide in the final structure cannot be achieved. Therefore, a sufficient YR and hydrogen embrittlement resistance are not obtained. In comparative example 16, since the coiling temperature is higher than 350 ℃, bainite transformation occurs during coiling, and V-containing carbide such as VC precipitates, and coarsens during the subsequent tempering heat treatment. Therefore, the desired number density of V-containing carbide particles cannot be achieved in the final structure, and thus sufficient YR and hydrogen embrittlement resistance are not obtained. Further, since cementite generated in the winding process is large, punchability is deteriorated.

In comparative example 18, since the tempering temperature was 400 ℃ or lower, V diffusion required for precipitation of V-containing carbide such as VC did not occur, and V-containing carbide did not precipitate. Therefore, a sufficient YR and hydrogen embrittlement resistance are not obtained. In comparative example 20, since the tempering temperature was 480 ℃ or higher, the cementite coarsened and a desired aspect ratio could not be obtained, and the punching property was degraded. Further, since the V-containing carbide particles are coarsened, fine V-containing carbide particles cannot be precipitated at a desired number density, and thus the YR and the hydrogen embrittlement resistance are not sufficiently obtained. In comparative example 26, since the tempering parameter was 15000 or less, precipitation and growth of V-containing carbide such as VC did not proceed sufficiently, and the function as a hydrogen trap site was not obtained sufficiently, and therefore YR and hydrogen embrittlement resistance were reduced. In comparative example 28, since the tempering parameter was 17000 or more, the V-containing carbide such as VC was coarsened, and sufficient YR and hydrogen embrittlement resistance were not obtained. In comparative example 30, since the heating temperature before hot rolling was less than 1100 ℃, V-containing carbide precipitates during hot rolling, and the V-containing carbide coarsens in the subsequent heat treatment, and thus sufficient YR and hydrogen embrittlement resistance were not obtained.

In comparative example 37, the tensile strength TS was insufficient because the C content was low. In comparative example 38, since the Si content is low, the retardation effect of tempering is not exhibited, and TS is insufficient. In comparative example 39, since the Mn content was low, the matrix dislocation density required for fine and large-scale precipitation of V-containing carbide particles was not obtained without quenching, and sufficient YR and hydrogen embrittlement resistance were not obtained. In comparative example 40, since the Mn content was low, the matrix dislocation density required for the fine and large precipitation of V-containing carbide was not obtained, and cementite coarsened, and thus sufficient YR and hydrogen embrittlement resistance were not obtained. In comparative example 41, since the Mn content was low and the tempering temperature was 480 ℃ or higher, the cementite coarsened and the desired aspect ratio could not be obtained, and the punchability was lowered. Further, the mother phase dislocation density required for the precipitation of a large amount of V-containing carbide particles cannot be obtained, and thus the YR and hydrogen embrittlement resistance are not sufficient. In comparative example 42, since the V content was low, the precipitation amount of the V-containing carbide particles was small, and sufficient YR and hydrogen embrittlement resistance were not obtained. In comparative example 43, since the content of V was high, the V-containing carbide coarsened, and thus sufficient YR and hydrogen embrittlement resistance were not obtained. In comparative example 44, since the Si/V ratio was high, the precipitation driving force of the V-containing carbide particles was low, and the fine V-containing carbide particles could not be precipitated at a desired number density, and sufficient YR and hydrogen embrittlement resistance were not obtained.

In contrast, in all of the examples of the present invention, by appropriately controlling the chemical composition and structure of the hot-rolled steel sheet, a hot-rolled steel sheet having high strength and excellent energy absorption capability at the time of collision, hydrogen embrittlement resistance, and punching properties can be obtained.

Claims

1. A hot-rolled steel sheet characterized by having the following chemical composition:

contains by mass%:

C：0.15～0.30％、

Si：0.50～4.00％、

Mn：2.00～4.00％、

p: less than 0.100 percent,

S: less than 0.005 percent,

Al：0.010～0.500％、

N:0.010% or less, and

V：0.20～1.00％、

a Si/V ratio of 10.0 or less, the remainder being Fe and impurities,

the content of cementite having a major diameter of 400nm or less and an aspect ratio of 3 to 5 among the cementites contained in the tempered martensite is 70% or more,

in the tempered martensite, V-containing carbides having a circle-equivalent diameter of 8 to 15nm are present at 30 particles/μm ² The above number density precipitates.

2. The hot-rolled steel sheet according to claim 1, further comprising Nb:0.01 to 0.10%, ti:0.01 to 0.10%, B:0.0001 to 0.0050%, cr:0.005 to 1.000%, mo:0.005 to 0.500%, cu:0.50 to 3.00% and Ni: 0.25-1.50% of 1 or more than 2,

3. A method for manufacturing a hot-rolled steel sheet, characterized by comprising the steps of:

a step of heating a slab having the chemical composition according to claim 1 or 2 to 1100 ℃ or higher;

a step for coiling the steel sheet at a coiling temperature of 350 ℃ or lower; and

a step of tempering the steel sheet at a tempering temperature T of more than 400 ℃ and less than 480 ℃ for a time T (sec) satisfying the following formula (1):

15000<(T+273)×(log(t)+20)<17000 (1)。