CN104583445A

CN104583445A - Steel plate

Info

Publication number: CN104583445A
Application number: CN201380044652.9A
Authority: CN
Inventors: 匹田和夫; 玉城朱里; 小嵨启达; 高桥克
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2012-08-28
Filing date: 2013-08-28
Publication date: 2015-04-29
Anticipated expiration: 2033-08-28
Also published as: EP2891727A1; KR20150038303A; TW201420776A; KR101683406B1; BR112015004191A2; JPWO2014034714A1; BR112015004191B1; CN104583445B; TWI481730B; JP5541428B1; PL2891727T3; MX2015002530A; WO2014034714A1; IN2015DN01523A; ES2707893T3; EP2891727A4; US20150225821A1; EP2891727B1

Abstract

In this steel plate, the cleanliness of the metal structure is no greater than 0.08%, the degree of segregation (alpha) of Mn is no greater than 1.6, and during hot forming, the post-hot-forming average hardness difference (DeltaHv) between a low-strain formed section that has incurred a plastic strain of no greater than 5% and a high-strain formed section that has incurred a plastic strain of at least 20% is no greater than 40.

Description

Steel plate

Technical field

The present invention relates to such as hot pressing with thermoforming simultaneously or to implement in the purposes of quench preferably steel plate (thermoforming steel plate) after thermoforming immediately.In more detail, even if the present invention relates to such as when implementing the thermoforming being attended by Large strain shaping (being that forming part is subject to the shaping of the plastix strain of more than 20%), also can suppress strain induced ferrite phase transformation, after thermoforming hardness in forming part evenly, good-toughness and the few thermoforming steel plate of the anisotropy of toughness after thermoforming.

The application at the Japanese Patent Application 2012-187959 CLAIM OF PRIORITY of Japanese publication, quoted its content at this based on 08 28th, 2012.

Background technology

In recent years, in the field of automotive sheet, due to the raising of the fuel consumption charge of automobile or the raising of impact resistant characteristic, the range of application therefore with high-tensile high tensile steel plate expands.In general, during steel plate generation high strength, compression moulding can reduce.Therefore, along with the application of high tensile steel plate, become the goods being difficult to manufacture complicated shape.Specifically, due to the high strength along with steel plate, ductility reduces, and therefore ruptures at the position that degree of finish is high, or along with the high strength of steel plate, resilience or the increase of sidewall warpage.Consequently, the dimensional precision that the component processed occurs produces the problems such as deterioration.Therefore, use high strength, particularly there is the steel plate of the tensile strength of more than 780MPa, utilize compression moulding to manufacture the goods with complicated shape to be very difficult.

If not by compression moulding but by roll shaping carry out shaping, then for the processing that the steel plate of high strength also can carry out to a certain degree.But in roll is shaping, have the restriction of the processing that only can be applied to the component in the longitudinal direction with same cross-sectional, the degree of freedom of Components Shape is restricted significantly.

Therefore, as the technology be pressed to difficult compression moulding materials such as high tensile steel plates, such as patent documentation 1 discloses and carries out shaping thermoforming (such as hot pressing) technology to being supplied to after shaping material heats.This technology is to before the forming for soft steel plate is with the while of shaping or quench immediately after formation, thus when shaping, guarantees good formability, carry out on this basis shaping after, be there is by quenching acquisition the technology of the shaped component of high strength.According to this technology, the tissue formed primarily of martensite after quenching, can be obtained, compared with the situation that there is the high tensile steel plate of the tissue be made up of polyphase structure with use, the shaped component of local deformaton ability or good-toughness can be obtained.

At present, above-mentioned hot pressing facilitates the application to the component with more simple shape, expects from now on implementing the application in the more strict shaping component of the shaping grade of deburring.But, when being applied to the more strict shaping component of enforcement, strain induced ferrite phase transformation can being there is in Large strain forming part, in component after thermoforming, having the possibility that hardness can reduce partly.

In order to suppress this strain induced ferrite phase transformation, in region at higher temperature, carry out thermoforming.But the high temperatureization of hot-forming temperature can cause the reduction of productivity, the increase of manufacturing cost, the deterioration etc. of surface texture, is therefore difficult to be applied to mass-produced technology.Such as, patent documentation 1 describes the content implementing press process more than 850 DEG C, but in the hot pressing of reality, there is following situation: being extracted out from process furnace by the steel plate being heated to about 900 DEG C in process furnace etc., transport and during being fed into press, occurrence temperature can be reduced to less than 850 DEG C.This situation be difficult to suppress shaping in strain induced ferrite phase transformation.

From the view point of improve hot pressing productivity, simultaneously improve shaping after component in stable material quality, Patent Document 2 discloses the manufacture method of the excellent hot pressing high strength steel component of the productivity of the refrigerating work procedure that can omit the material utilizing the heat extraction in compacting tool set to carry out.Method disclosed in patent documentation 2 is very excellent invention, but needs the element containing Mn, Cr, Cu, Ni etc. in large quantities in steel with the effect improving hardenability.Therefore, the technology of patent documentation 2 has the problem that cost increases.In addition, in the component using the technology of patent documentation 2 to manufacture, there is following possibility: the anisotropy of the toughness toughness deterioration that causes because of existing various inclusiones occurring and causes because of the inclusion (mainly MnS) extended in the rolling direction.Due to the component performance of reality dominate by the characteristic of low toughness side, therefore when the anisotropy of flexible, mother metal characteristic originally cannot be played fully.The morphology Control by utilizing Ca process to carry out extended inclusion such as described in patent documentation 3, can reduce the anisotropy of toughness.But, now there is following problem: although the inclusion amount itself that the toughness value on the direction that reduces the most of toughness improves in component increases, therefore other directivity toughness value reduction.

Prior art document

Patent documentation

Patent documentation 1: Japanese Unexamined Patent Publication 2002-102980 publication

Patent documentation 2: Japanese Unexamined Patent Publication 2006-213959 publication

Patent documentation 3: Japanese Unexamined Patent Publication 2009-242910 publication

Summary of the invention

The problem that invention will solve

As mentioned above, hot pressing in the prior art rests on the component being applied to and having more simple shape.Therefore, for consider be applied to implement deburring shaping grade more strict shaping component time thermoforming that is contingent, that cause because of the strain induced ferrite phase transformation of Large strain forming part after the hardness of local of component (steel plate through thermoforming process) reduce, the technical task of the anisotropy of toughness and the reduction of toughness value, do not inquire into before this.

Even if the object of the present invention is to provide above-mentioned problem, namely implement be attended by Large strain shaping thermoforming time, also can suppress the strain induced ferrite phase transformation in forming part, the hardness after thermoforming evenly after (difference of hardness is little), thermoforming good-toughness and the little thermoforming steel plate of the anisotropy of toughness.

For solving the means of problem

The present inventors conducts in-depth research to solve above-mentioned problem.

Result has new discovery: by controlling the chemical constitution of steel plate, inclusion amount and center segregation, even if implement be attended by Large strain shaping thermoforming time, also can suppress strain induced ferrite phase transformation, can obtain hardness after thermoforming evenly, good-toughness and the little thermoforming steel plate of the anisotropy of toughness after thermoforming.In addition, sometimes hardness is evenly called in the following description that Hardness Distribution is stablized.

As follows based on above-mentioned newfound main idea of the present invention.

(1) steel plate of a mode of the present invention relates to a kind of steel plate, it is characterized in that, chemical constitution is in mass % containing C:0.18% ~ 0.275%, Si:0.02% ~ 0.15%, Mn:1.85% ~ 2.75%, sol.Al:0.0002% ~ 0.5%, Cr:0.05% ~ 1.00%, B:0.0005% ~ 0.01%, below P:0.1%, below S:0.0035%, below N:0.01%, Ni:0 ~ 0.15%, Cu:0 ~ 0.05%, Ti:0 ~ 0.1%, Nb:0 ~ 0.2%, and remainder is Fe and impurity; The purity of metal structure is less than 0.08%; The α as Mn degree of segregation shown in following formula a is less than 1.6; The difference Δ Hv of the average hardness after the above-mentioned thermoforming of the Large strain forming part of the low strain dynamic forming part being subject to the plastix strain of less than 5% in thermoforming and the plastix strain being subject to more than 20% is less than 40.

α=(unit of the thickness of slab central part of above-mentioned steel plate is the maximum Mn concentration of quality %)/(being the average Mn concentration of quality % apart from the unit of 1/4 depth location that above-mentioned surface of steel plate is thickness of slab)

Formula a

(2) in the steel plate described in above-mentioned (1), above-mentioned chemical constitution can replace a part of above-mentioned Fe further, in mass % containing a kind or 2 kinds that is selected from the group that is made up of Ni:0.02% ~ 0.15% and Cu:0.003% ~ 0.05%.

(3) in above-mentioned (1) or the steel plate described in (2), above-mentioned chemical constitution can replace a part of above-mentioned Fe further, in mass % containing a kind or 2 kinds that is selected from the group that is made up of Ti:0.005% ~ 0.1% and Nb:0.005% ~ 0.2%.

(4), in the steel plate according to any one of above-mentioned (1) ~ (3), the surface of above-mentioned steel plate can have coating further.

Invention effect

According to aforesaid way of the present invention, even if when implementing that deburring is shaping etc. is attended by the shaping thermoforming of Large strain, also can suppress the strain induced ferrite phase transformation of forming part, therefore can obtain the good-toughness after there is stable Hardness Distribution, thermoforming after thermoforming and the low steel plate of the anisotropy of toughness.The raw material of this steel plate such as the physical structure component headed by the car shell structure component, vehicle to run component etc. of automobile etc. is preferred, and therefore the present invention is industrially extremely useful.

In addition, thermoforming is carried out according to conventional methods.Such as raw steel can be heated to Ac ₃put above (about 800 DEG C) and Ac ₃the temperature of point less than+200 DEG C, carries out more than 0 second and the maintenance of less than 600 seconds, transports and be pressed to press, carry out the maintenance of more than 5 seconds in lower dead center.Now, type of heating can suitably be selected, and when instant heating, can carry out electrified regulation or high frequency heating.In addition, for heating usually, the stove heating etc. being set to Heating temperature can be used.During press, carry out air cooling in conveyance, therefore when the time of conveyance is elongated, have and before compacting starts, to cause ferrite transformation, softening possibility occurs.Therefore, the conveyance time is preferably less than 15 seconds.In order to prevent the rising of die temperature, the cooling of mould can be carried out.Now, as long as method of cooling carries out carrying out the type of cooling as required such as cooling tubing, flow system cryogen in mould.

Embodiment

Below the steel plate (being sometimes called the steel plate of present embodiment) of an embodiment of the invention is illustrated in greater detail.% relevant with the chemical constitution of steel plate in the following description is all quality %.

1. chemical constitution

(1)C：0.18％～0.275％

C is hardenability for improving steel, determine quenching after intensity so that the important element of local ductility after controlling thermoforming and toughness.In addition, C is austenite generting element, therefore has following effects: strain induced ferrite phase transformation when suppressing Large strain shaping, is easy to obtain stable Hardness Distribution in component after thermoforming.But, when C content is less than 0.18%, be difficult to the tensile strength of more than the 1100MPa guaranteed as preferred intensity after quenching, and, the effect obtaining the stable Hardness Distribution brought by above-mentioned effect cannot be obtained.On the other hand, when C content is more than 0.275%, local ductility and toughness reduce.Therefore, C content is made to be 0.18% ~ 0.275%.The preferred upper limit of C content is 0.26%, the preferred upper limit is 0.24%.

(2)Si：0.02％～0.15％

Si is the element of the oxide skin adaptation improve thermoforming improving hardenability while after.But, when Si content is less than 0.02%, have the situation that cannot obtain above-mentioned effect fully.Therefore, the lower limit of Si content is made to be 0.02%.Preferred lower limit is 0.03%.On the other hand, when Si content is more than 0.15%, the Heating temperature required for austenite phase transformation during thermoforming, is made to become high temperature significantly.Therefore, have and cause the cost required for thermal treatment improve or quench because of underheating and become insufficient situation.In addition, Si is ferrite generating elements, therefore when Si too high levels, then has following situation: be easy to occur hardness in strain induced ferrite phase transformation, component after thermoforming when Large strain is shaping and reduce partly, cannot obtain stable Hardness Distribution.And then, for containing of a large amount of Si, have because wettability when implementing hot dip process process reduces and the situation of not plating occurs.Therefore, the upper limit of Si content is 0.15%.

(3)Mn：1.85％～2.75％

Mn is for the hardenability improving steel and stably guarantees the intensity of the steel after quenching and effective element.In addition, Mn is austenite generting element, is easy to obtain stable Hardness Distribution in strain induced ferrite phase transformation when Large strain therefore can be suppressed shaping, component after thermoforming.But, when Mn content is less than 1.85%, have the situation that fully cannot obtain above-mentioned effect.Therefore, the lower limit of Mn content is made to be 1.85%.On the other hand, when Mn content is more than 2.75%, above-mentioned effect is saturated, cause the toughness deterioration after quenching on the contrary.Therefore, the upper limit of Mn content is made to be 2.75%.The preferred upper limit of Mn content is 2.5%.

(4)sol.Al：0.0002％～0.5％

Al is the element carrying out deoxidation to molten steel, make steel completes.When sol.Al content is less than 0.0002%, deoxidation is insufficient.Therefore, the lower limit of sol.Al content is made to be 0.0002%.And then Al is also for the hardenability improving steel plate and stably guarantees the intensity after quenching and effective element, therefore can contain energetically.But even if contain more than 0.5%, not only its effect is saturated but also cause the increase of cost.Therefore, the upper limit of Al content is made to be 0.5%.

In addition, sol.Al represents acid-solubility Al, does not comprise the Al being not dissolved in acid in its content ₂o ₃deng in contained Al amount.

(5)Cr：0.05％～1.00％

Cr is the element of the hardenability improving steel.In addition, Cr is austenite generting element, is easy to obtain stable Hardness Distribution in strain induced ferrite phase transformation when Large strain therefore can be suppressed shaping, component after thermoforming.But, when Cr content is less than 0.05%, have the situation that cannot obtain above-mentioned effect fully.Therefore, the lower limit of Cr content is made to be 0.05%.Preferred lower limit is 0.1%, preferred lower limit is 0.2%.On the other hand, when Cr content is more than 1.00%, Cr is to the carbide generation enrichment in steel.As a result, the solid solution of carbide in heating process when being supplied to thermoforming postpones, hardenability reduces.Therefore, the upper limit of Cr content is made to be 1.00%.The preferred upper limit of Cr content is 0.8%.

(6)B：0.0005％～0.01％

B is for the hardenability improving steel and stably guarantees the intensity after quenching and effective element.But, when B content is less than 0.0005%, have the situation that cannot obtain above-mentioned effect fully.Therefore, the lower limit of B content is made to be 0.0005%.On the other hand, when B content is more than 0.01%, above-mentioned effect is saturated and then cause the toughness of quench section deteriorated.Therefore, the upper limit of B content is made to be 0.01%.The preferred upper limit of B content is 0.005%.

(7) below P:0.1%

In general P is the element contained as impurity.But, owing to having the effect of the hardenability improving steel and then the intensity stably guaranteeing the steel after quenching, therefore can contain energetically.But when P content is more than 0.1%, there is deterioration in toughness significantly.Therefore, P content is restricted to 0.1%.The preferred upper limit of P content is 0.05%.The lower limit of P content does not need to limit especially, but the excessive minimizing of P content can cause significant cost to improve.Therefore, the lower limit of P content can be made to be 0.0002%.

(8) below S:0.0035%

S is the element contained as impurity.In addition, particularly form MnS, become the anisotropic major cause causing toughness reduction and toughness.When S content is more than 0.0035%, the deterioration of toughness becomes remarkable, therefore S content is restricted to 0.0035%.The lower limit of S content does not need to limit especially, but the excessive minimizing of S content can cause significant cost to improve, and the lower limit of S content therefore can be made to be 0.0002%.

(9) below N:0.01%

N is the element contained as impurity.When N content is more than 0.01%, in steel, forms thick nitride, make local deformaton ability and toughness deteriorated significantly.Therefore, N content is restricted to 0.01%.The lower limit of N content does not need to limit especially, but the excessive minimizing of N content can cause significant cost to improve.Therefore, the lower limit of N content can be 0.0002%.The preferred lower limit of N content is more than 0.0008%.

Except above element, the steel plate of present embodiment also can containing following arbitrary element.These elements do not need necessarily to contain.Therefore, do not need the lower limit of limited content especially, their lower limit is 0%.

(10) below Ni:0.15%, below Cu:0.05%

Ni and Cu is for the hardenability improving steel and stably guarantees the intensity after quenching and effective element.Therefore, a kind or 2 kinds of these elements can be contained.But even if exceed above-mentioned higher limit and containing any one element, above-mentioned effect is also saturated and also become unfavorable on cost.Therefore, the content of each element is described above.Preferred Ni content is less than 0.10%, Cu content is less than 0.03%.In order to more reliably obtain above-mentioned effect, preferably containing being selected from by 1 in the group that more than Ni:0.02% and more than Cu:0.003% is formed kind or 2 kinds.

(11) below Ti:0.1%, below Nb:0.2%

Steel plate is being heated to Ac by Ti and Nb ₃point is above, suppress recrystallization when being supplied to thermoforming and then form fine carbide, suppress grain to grow up, make austenite grain become the element of particulate.When austenite grain becomes particulate, the toughness of thermoforming component is improved widely.In addition, for Ti, by with the preferentially bonding and generate TiN, the consumption of the B suppressing the precipitation because of BN to cause of the N in steel.As consequently, by can improve the hardenability produced by B containing Ti.In order to obtain above-mentioned effect, a kind or 2 kinds of these elements can be contained.But, during when exceeding above-mentioned higher limit containing any one element, have that the amount of precipitation of TiC or NbC increases, C is consumed, quench after the situation of intensity reduction.Therefore, the content of each element is described above.The upper limit of preferred Ti content is 0.08%, the upper limit of Nb content is 0.15%.In addition, in order to obtain above-mentioned effect more reliably, preferably containing being selected from by 1 in the group that more than Ti:0.005% and more than Nb:0.005% is formed kind or 2 kinds.

Remainder other than the above is Fe and impurity.Impurity refers to from the starting material such as ore or slag or the material that is mixed into from manufacturing environment.

Steel plate of the present invention can be hot-rolled steel sheet, cold-rolled steel sheet any one, also can be to hot-rolled steel sheet or cold-rolled steel sheet implement annealing annealing hot-rolled steel sheet or annealing cold-rolled steel sheet.

2. metal structure

(1) purity: less than 0.08%

The summation of the algorithm calculations of A system contained in the steel plate that the purity JIS G0555 of present embodiment specifies, B system and C system inclusion amount defines.When inclusion amount increases, crevasse crack propagation becomes easily, cause the anisotropy of toughness deterioration and toughness to increase.Therefore, the upper limit of purity is made to be 0.08%.The preferred upper limit is 0.04%.In the steel plate of present embodiment, the MnS as A system inclusion becomes the main anisotropic major cause of reduction toughness.Therefore, particularly preferably A system inclusion is less than 0.06%.More preferably A system inclusion is less than 0.03%.

Wherein, the low person of preferred purity, but its lower limit can be made to be 0.003% or 0.005% from the viewpoint of cost.

(2) Mn degree of segregation less than α: 1.6

Mn is easy to, near the thickness of slab central part of steel plate, segregation occurs when casting.When this center segregation is large, the inclusiones such as MnS concentrate on segregation portion, cause the anisotropic increase of the reduction of toughness and toughness.And then because the martensite generated in segregation portion during quenching is hard, therefore there is deterioration in toughness.In addition, by the interaction of Mn and P, in Mn segregation portion, P segregation also increases, and also can cause toughness deterioration thus.Therefore, the Mn degree of segregation α shown in following formula 1 is made to be less than 1.6.Mn degree of segregation α preferably close to 1.0 (i.e. not segregations), but from the view point of cost, can make its lower limit be 1.03 or 1.05.

α=[the maximum Mn concentration (quality %) of thickness of slab central part]/[distance surface is the average Mn concentration (quality %) of 1/4 depth location of thickness of slab] (formula 1)

3. coating

To improve for the purpose of erosion resistance etc., can on the surface of thermoforming steel plate of the present invention, form coating and make surface treated steel plate.Even if there is coating, the effect of present embodiment also can not be impaired.Coating can be electrolytic coating, also can be hot-dip coated.As electrolytic coating, electro-galvanized layer, electroplating Zn-Ni alloy layer etc. can be exemplified.As hot-dip coated, dip galvanized, alloyed hot-dip zinc-coated layer, hot dip alumin(i)um coating, hot dip galvanizing-Al alloy layer, hot dip galvanizing-Al-Mg alloy layer, hot dip galvanizing-Al-Mg-Si alloy layer etc. can be exemplified.Plating adhesion amount there is no and limits especially, can in general scope.

4. manufacture method

Then, the representational manufacture method of thermoforming steel plate of the present invention is described.By using the manufacture method comprising following operation, the steel plate of present embodiment easily can be obtained.

(1) operation (S1) is cast continuously

Utilize Continuous casting process that the molten steel with above-mentioned chemical constitution is made steel disc (slab).In this continuous casting operation preferably: make liquid steel temperature be the temperature of distance high more than 5 DEG C of liquidus temperature and make the molten steel casting amount of time per unit be less than 6ton/ minute and then before slab is solidified completely, implement center segregation reduction process.

When casting continuously, the casting amount (casting rate) of the time per unit of molten steel is more than 6ton/ minute, the MOLTEN STEEL FLOW in casting mold is fast, therefore becomes the inclusion be easy in supplementary inclusion, increase slab.In addition, when liquid steel temperature is less than 5 DEG C for distance liquidus temperature, viscosity rises, inclusion become be difficult to float, inclusion amount in steel increases, purity worsens (value increase).When casting continuously molten steel, more preferably make that the temperature of molten steel is higher than liquidus temperature more than 8 DEG C, casting amount is less than 5ton/ minute.

Reduce process as center segregation, such as, carry out by the non-solidification layer before solidifying completely slab mitigation or the discharge that enrichment portion is carried out in induction stirring or non-solidification layer pressure etc.

(2) slab homogenizes treatment process (S2)

Segregation after solidifying completely as slab reduces process, heating of plate blank to 1150 DEG C ~ 1350 DEG C, the slab that keeps 10 hours ~ 50 hours can be carried out to homogenize process further.To homogenize process by carrying out slab under these conditions, degree of segregation can be reduced further.In addition, the preferred upper limit of Heating temperature is 1300 DEG C, the upper limit of preferred hold-time is 30 hours.

(3) hot-rolled process (S3) ~ refrigerating work procedure (S4) ~ rolling step (S5)

By by having carried out above-mentioned continuous casting operation and carried out slab as required homogenizing treatment process and the steel disc that obtains implements hot rolling after being heated to 1050 DEG C ~ 1350 DEG C, make steel plate.The steel plate finishing hot rolling is kept 5 seconds ~ 20 seconds in this temperature province.By water-cooled, steel plate is cooled to the temperature province of 400 DEG C ~ 700 DEG C after maintenance.Then, the steel plate through cooling is reeled.

Steel disc has the situation containing non-metallic inclusion, and this non-metallic inclusion makes to have carried out the toughness of the component after quenching and the reason of local deformability deterioration to steel plate.Therefore, when steel disc is supplied to hot rolling, preferably make the solid solution fully of these non-metallic inclusions.For the steel disc of above-mentioned chemical constitution, by reaching more than 1050 DEG C to promote when being supplied to hot rolling the solid solution of above-mentioned non-metallic inclusion.Therefore, the temperature being preferably supplied to the steel disc of hot rolling is more than 1050 DEG C.In addition, the temperature being supplied to the steel disc of hot rolling is more than 1050 DEG C, heats, reaches more than 1050 DEG C to the steel disc lower than 1050 DEG C.

When making its phase transformation in the austenitic situation of processing after finish rolling, rolling set tissue residue, becomes in the final article and anisotropic major cause occurs.Therefore, in order to become from the austenitic phase transformation of recrystallization, after the rolling of steel plate terminates, in its temperature province, preferably carry out the maintenance of more than 5 seconds.Such as, in order to carry out, in the maintenance manufacturing in circuit more than 5 seconds, not carrying out water cooling in the cooling zone after finish rolling and carry out transporting.

By making coiling temperature be more than 400 DEG C, the ferrite area occupation ratio in metal structure can be improved.When ferrite area occupation ratio height, the intensity of hot-rolled steel sheet can be suppressed, when carrying out cold rolling in rear operation, load control or steel plate smooth/strip thickness control becomes easy, manufacture efficiency and improve.Therefore, preferred coiling temperature is more than 400 DEG C.

On the other hand, by making coiling temperature be less than 700 DEG C, the generation of the oxide skin growth after reeling, meeting inhibited oxidation skin defect can be suppressed.In addition, also can suppress the distortion caused because of the deadweight of coiled material after reeling, the generation of the scratch of the coiled material surface caused because of this distortion can be suppressed.Therefore, preferred coiling temperature is less than 700 DEG C.In addition, above-mentioned distortion is produced by following reason: when ferrite transformation occurs non-transformed austenite remaining, this non-transformed austenite after the coiling after the coiling, the volumetric expansion that ferrite transformation causes and thermal contraction afterwards, thus can lose the winding tension of coiled material.

(4) pickling process (S6)

For the steel plate after above-mentioned rolling step, pickling can be carried out.Pickling can be carried out according to conventional methods.Before pickling or after pickling, in order to smooth rectification or accelerating oxidation skin are peeled off, can skin-pass be implemented, the effect of present embodiment can not be affected.Extensibility when implementing skin-pass does not need to specify especially, such as, be more than 0.3% and be less than 3.0%.

(5) cold rolling process (S7)

Also can implement cold rolling as required to the pickled plate obtained by above-mentioned pickling process.For cold rolling process, carry out according to conventional methods.Cold rolling draft, in common scope, is in general 30% ~ 80%.

(6) annealing operation (S8)

For the cold-rolled steel sheet obtained in the hot-rolled steel sheet obtained in above-mentioned rolling step (S5) or above-mentioned cold rolling process (S7), the annealing of 700 DEG C ~ 950 DEG C can be implemented as required.

By implementing to hot-rolled steel sheet and cold-rolled steel sheet the annealing remaining on the temperature province of more than 700 DEG C, the impact of hot-rolled condition can be reduced, the further stabilization of the characteristic after quenching can be realized.In addition, for cold-rolled steel sheet, can recrystallization be passed through thus steel plate generation softening, improve the processibility before thermoforming.Therefore, when annealing is implemented to hot-rolled steel sheet or cold-rolled steel sheet, the temperature province of more than 700 DEG C is preferably remained on.

On the other hand, by making annealing temperature be less than 950 DEG C, high productivity can be guaranteed while the required cost of suppression annealing.In addition, due to the coarse of tissue can be suppressed, therefore more good toughness can be guaranteed after quenching.Thus, when implementing annealing to hot-rolled steel sheet or cold-rolled steel sheet, the temperature province of less than 950 DEG C is preferably remained on.

Annealing when implementing annealing cools afterwards and is preferably cooled to 550 DEG C with the average cooling rate of 3 DEG C/sec ~ 20 DEG C/sec.By making above-mentioned average cooling rate be more than 3 DEG C/sec, the generation of thick perlite or thick cementite can be suppressed, quenching can be improved after characteristic.In addition, by making above-mentioned average cooling rate be less than 20 DEG C/sec, the stabilization realizing material is easy to.

(7) plating process (S9)

When surface of steel plate forming coating and makes coated steel sheet, plating and hot dip process are all carry out according to conventional methods.When galvanizing, continuous hot-dipping galvanizing equipment can be used, in equipment, implement above-mentioned annealing operation and plating then, also can implement plating independently with above-mentioned annealing operation in addition.Galvanizing can be implemented Alloying Treatment further and carry out alloyed hot-dip zinc-coated.When implementing Alloying Treatment, alloying treatment temperature is preferably made to be 480 DEG C ~ 600 DEG C.By making alloying treatment temperature be more than 480 DEG C, Alloying Treatment can be suppressed uneven.In addition, by making alloying treatment temperature be less than 600 DEG C, high productivity can be guaranteed while suppression manufacturing cost.After galvanizing, in order to carry out smooth rectification, skin-pass can be implemented as required.For the extensibility of skin-pass, carry out according to conventional methods.

Substantially determined in inclusion amount in this steel plate and the operation of degree of segregation before hot rolling, do not changed in fact before and after thermoforming.Therefore, as long as the chemical constitution of the steel plate before thermoforming, inclusion amount (purity) and degree of segregation meet the scope of present embodiment, then the hot pressing component manufactured by hot pressing after meets the scope of present embodiment similarly.

Embodiment

In test converter, carry out melting to the steel with the chemical constitution shown in table 1, use-testing continuous casting machine implements continuous casting.As shown in table 2, in continuous casting operation, various change is carried out to casting rate during casting and molten steel Heating temperature difference (liquid steel temperature-liquidus temperature).In addition, in Solidification Process in Continuous Slab Casting, carry out induction stirring.And then, finally solidify in portion at slab, by the roll shop building right up and down in constriction continuous casting machine every non-solidification layer pressure (extruding), carried out the discharge in center segregation portion.As a comparison, a part has also made the slab not carrying out induction stirring and/or extrude (center segregation reduces process).Afterwards, the slab carried out at 1300 DEG C 20 hours homogenizes process.A part eliminates slab and to homogenize process.Use the slab so made to carry out hot rolling, carry out afterwards cooling, reeling, obtain the hot-rolled steel sheet that thickness of slab is 5.0mm or 2.9mm.Hot-rolled condition is now that the Heating temperature of slab is 1250 DEG C, rolling starts that temperature is 1150 DEG C, rolling end temp is 900 DEG C, coiling temperature is 650 DEG C.Hot rolling is implemented by multi-pass rolling and carried out the maintenance of 10 seconds after the end of rolling.The water-cooled that is cooled through after hot rolling is implemented.In order to compare, a part keeps.

In addition, in the test continuous casting machine that uses in actual production equipment and the present embodiment of casting rate, the size of equipment is different.Therefore, describe in table 2 consider size factor, the value of the casting rate that is converted into actual production equipment.In addition, in table 2 molten steel Heating temperature difference refer to liquid steel temperature deduct liquidus temperature after value.

According to conventional methods cleanup acid treatment implemented to the hot-rolled steel sheet of gained and made pickled plate.Be the pickled plate of 5.0mm for thickness of slab, cold rolling by implementing, make the cold-rolled steel sheet of 2.9mm.Plating is implemented to the hot-rolled steel sheet of a part.Full annealed (annealing temperature be 800 DEG C, annealing time be 60 seconds) is implemented in continuous annealing apparatus to the cold-rolled steel sheet of a part, and then afterwards electro-galvanizing is being implemented to a part wherein.Further, annealing (annealing temperature be 800 DEG C, annealing time be 60 seconds) and galvanizing are implemented in continuous hot-dipping galvanizing equipment to a part for hot-rolled steel sheet and cold-rolled steel sheet.The temperature of bathing by making galvanizing is 460 DEG C, part enforcement Alloying Treatment of 20 seconds at 540 DEG C, thus obtains hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel plate.

Using the steel plate manufactured as trying material, hot pressing testing apparatus is used to implement hot pressing shaping.By with billet size: the steel plate that 150mm is square, punching aperture: 36mm (gap 10%) punch is heated to surface of steel plate temperature to 900 DEG C in process furnace, at this temperature after maintenance 4 minutes, take out from process furnace.Afterwards, being cooled to 750 DEG C by letting cool, implementing hot deburring at the time point of arrival 750 DEG C shaping, maintaining 1 minute in lower dead center.Hot deburring condition of molding is as described below.

Tapping machine shape: circular cone,

Tapping machine diameter: 60mm,

Pressing speed: 40mm/ second,

Cooling after shaping is by keeping the mold cools down of 1 minute to carry out in lower dead center.

For the cross section parallel with the rolling direction of the steel plate through hot pressing, Vickers hardness tester is utilized to determine the hardness of 1/4 depth location of the thickness of slab in the cross section of deburring portion (being subject to the Large strain forming part of the plastix strain of more than 20%) and flange part (plastix strain amount is the low strain dynamic forming part of less than 5%).Mensuration load is 98kN.Measuring method with JIS Z2244 for benchmark.Move with 200 μm of intervals in identical thickness of slab position while implement this measurement of hardness amounting to 5 times.Try to achieve the mean value to 5 vickers hardness numbers that each component obtains, as average hardness (Hv).Trying to achieve the difference (Δ Hv=[flange part Hv]-[deburring portion Hv]) of the average hardness in deburring portion and the average hardness of flange part, is that the situation of less than 40 is judged to be that hardness is qualified by Δ Hv.The investigation result of hardness is shown in table 3.

Wherein, dependent variable be each position of the steel plate measured through processing thickness of slab, tried to achieve by the reduction of the thickness of slab after processing relative to the thickness of slab before processing.

In addition, using manufactured steel plate as examination material, toughness value (absolute value of toughness) and the anisotropy of toughness are investigated.

Investigation is carried out according to following will getting.First, in process furnace, the steel plate of above-mentioned 2.9mm is heated to surface of steel plate temperature and reaches 900 DEG C, after keeping 4 minutes at this temperature, take out from process furnace.Then, being cooled to 750 DEG C by letting cool, using flat plate mold to clamp from upper and lower at the time point of arrival 750 DEG C, maintaining 1 minute.Afterwards, to confessing that the table back side of examination material is ground, made the thickness of 2.5mm.The mode of rolling right angle orientation is become to acquire Charpy impact test sample according to the length direction of sample with rolling direction.Now, breach is the V breach of the 2mm degree of depth.Test temperature is room temperature, with JISZ 2242 for benchmark has carried out shock test.Using the ratio of the impact value of rolling direction (absorbing energy/sectional area) and the impact value of rolling right angle orientation as anisotropic index.

Show the result in table 3.The result of test is if when the impact value of length rolling direction is 70J/cm ²above and impact value ratio is more than 0.65, be then judged to be that characteristic is good.

The purity of steel plate with JIS G0555 for benchmark is investigated.For the steel plate of each test number, cut out for examination material from 5, in the position of 1/8,1/4,1/2,3/4 and 7/8 of thickness of slab, use point count to investigate purity.In the result of each thickness of slab position, numerical value maximum for Reinheitszahl is supplied as this purity trying material.Purity is the summation of A system, B system and C system inclusion.

The composition surface analysis that Mn degree of segregation utilizes EPMA to pass through to carry out Mn is tried to achieve.To the steel plate of each test number, cut out for examination material from 5 places, 1/4 of thickness of slab, the position of 1/2 measures 10 visual fields with the multiplying power of 500 times, have employed the mean value of the Mn degree of segregation of each visual field.

In test number 16 ~ 19,21,22, be all compared with the average hardness of the flange part as low strain dynamic variant part, the average hardness as the deburring portion of Large strain variant part reduces significantly, and the value of Δ Hv is up to 41 ~ 99.Its reason is, the strain induced ferrite phase transformation caused because of deburring machining, deburring portion there occurs softening.Now, the hardness of manufactured heat product is different partly, and the intensity of products formed can not become identical, partly become low strength, therefore damages the reliability as goods.

In addition, in test number 4,8,10,12,15,18,20,23,24, chemical constitution, purity or degree of segregation have departed from scope of the present invention, and therefore rolling direction impact value and/or impact value are than insufficient.

On the other hand, the steel plate with chemical constitution of the present invention is regardless of being with or without cold rolling process, with or without annealing operation and plating kind, Δ Hv is-4 ~ 24, the hardness when difference of the average hardness of flange part and the average degree in deburring portion is little, Large strain is shaping and the excellent in stability of intensity.

In addition, the anisotropy for the toughness after hot rolling and toughness also shows sufficient value.

Operational feasibility in industry

Even if when steel plate of the present invention implements that deburring is shaping etc. is attended by Large strain shaping thermoforming, also can suppress the strain induced ferrite phase transformation in forming part, therefore can obtain the low steel plate of anisotropy of the good-toughness after there is stable Hardness Distribution, thermoforming after thermoforming, toughness.This steel plate is such as preferred as raw materials such as the physical structure components headed by the car shell structure component, vehicle to run component etc. of automobile, and therefore the present invention is industrially very useful.

Claims

1. a steel plate, is characterized in that,

Chemical constitution contains in mass %

C：0.18％～0.275％、

Si：0.02％～0.15％、

Mn：1.85％～2.75％、

sol.Al：0.0002％～0.5％、

Cr：0.05％～1.00％、

B：0.0005％～0.01％、

Below P:0.1%,

Below S:0.0035%,

Below N:0.01%,

Ni：0～0.15％、

Cu：0～0.05％、

Ti：0～0.1％、

Nb：0～0.2％，

Remainder is Fe and impurity;

The purity of metal structure is less than 0.08%;

The α as Mn degree of segregation shown in following formula 1 is less than 1.6;

The difference Δ Hv of the average hardness after the described thermoforming of the Large strain forming part of the low strain dynamic forming part being subject to the plastix strain of less than 5% in thermoforming and the plastix strain being subject to more than 20% is less than 40,

α=(unit of the thickness of slab central part of described steel plate is the maximum Mn concentration of quality %)/(being the average Mn concentration of quality % apart from the unit of 1/4 depth location that described surface of steel plate is thickness of slab) formula 1.

2. steel plate according to claim 1, is characterized in that, described chemical constitution replaces a part of described Fe further, in mass % containing a kind or 2 kinds that is selected from the group that is made up of Ni:0.02% ~ 0.15% and Cu:0.003% ~ 0.05%.

3. according to steel plate according to claim 1 or claim 2, it is characterized in that, described chemical constitution replaces a part of described Fe further, in mass % containing a kind or 2 kinds that is selected from the group that is made up of Ti:0.005% ~ 0.1% and Nb:0.005% ~ 0.2%.

4. the steel plate according to any one of claims 1 to 3, is characterized in that, the surface of described steel plate has coating further.