ES2707893T3 - Sheet steel - Google Patents

Abstract

A steel plate having a chemical composition consisting, in mass percentage, in: C: from 0.18% to 0.275%; Yes: from 0.02% to 0.15%; Mn: from 1.85% to 2.75%; AI sol .: from 0.0002% to 0.5%; Cr: from 0.05% to 1.00%; B: from 0.0005% to 0.01%; P: 0.0002% to 0.1%; S: from 0.0002% to 0.0035%; N: from 0.0002% to 0.01%; Ni: from 0% to 0.15%; Cu: from 0% to 0.05%; Ti: from 0% to 0.1%; Nb: from 0% to 0.2%; and including the rest iron and impurities; wherein a cleaning of a metal structure is 0.003% to 0.08%, cleaning being defined as the sum of the quantities of inclusions of the series A, series B and series C contained in the steel sheet, which are obtained by an arithmetic calculation specified in JIS G 0555, α, which is a degree of segregation of Mn expressed by the following expression 1 is from 1.03 to 1.6, and in the hot conformation, a difference ΔHv in an average hardness after hot forming between a portion formed of low deformation that undergoes a plastic deformation of 5% or less and a portion formed of high deformation that undergoes a plastic deformation of 20% or more is 40 or less; α = (a maximum concentration of Mn, in percentage by mass, in a central portion of thickness of the steel sheet) / (an average concentration of Mn, in mass percentage, in a position at a depth of 1/4 of the thickness of a sheet measured from a surface of the steel sheet) ... expression 1.

Description

DESCRIPTION

Sheet steel

[Technical field of the invention]

The present invention relates to a steel plate (steel plate for hot forming) which is suitable for applications in which the instant cooling is carried out at the same time as the hot forming or immediately after the hot forming , such as in a hot pressing. More specifically, the present invention relates to a sheet steel for hot forming in which, for example, even in the case where a forming process accompanied by a conformation of high deformation is carried out, which is a process of conformation in which a shaped portion receives a plastic deformation of 20% or more, the ferritic transformation, induced by the deformation in the shaped portion, is suppressed, and therefore the hardness after the hot forming is uniform, resulting in a Excellent tenacity and low anisotropy of toughness after hot forming.

[Related technique]

In recent times, in the field of steel plates used for vehicles, in order to improve the fuel efficiency or the resistance of a vehicle to impacts, the applications of a steel sheet of high mechanical strength that has a high tensile strength have increased. In general, as the strength of the steel sheet increases, the formability by pressing decreases. Therefore, and depending on the application envisaged for the high strength steel sheet, it is difficult to manufacture a product having a complex shape. Specifically, since the ductility decreases as the strength of the steel sheet increases, a break can occur from a heavily worked region, or a degree of backward jump or wall sag increases as the thickness increases. mechanical strength of the steel sheet. As a result, there is a problem of deterioration in the dimensional accuracy of a worked member, and the like. Accordingly, it is not easy to manufacture a material having a complex shape by press-forming using a steel plate of high mechanical strength, having, in particular, a tensile strength of 780 MPa or higher.

When the roll forming is carried out as a shaping rather than a press forming, it is possible to make a steel sheet of high mechanical strength to a certain extent. However, laminating has limitations due to the fact that it can be applied only to a member provided with a uniform cross section in the longitudinal direction, whereby the degree of freedom for the configuration of a member presents a significant limitation.

Here, as a technique for press forming a hard-to-shape material by pressing such as a steel plate of high mechanical strength, for example, in Patent Document 1, a hot forming technique is disclosed to carry out the shaping after heating a material to be shaped (eg, by hot pressing). This technique is a technique for carrying out instantaneous cooling simultaneously or immediately after forming a sheet of steel which is soft before forming so as to obtain a shaped member provided with a high mechanical strength thanks to the instantaneous cooling brought to after conformation, while obtaining good formability during shaping. According to this technique, a structure including mainly martensite can be obtained after instantaneous cooling, and therefore a shaped member provided with excellent local deformability and toughness can be obtained as compared to the case in which a steel plate of High mechanical strength that has a dual phase structure.

At present, hot pressing, described above, is applied to a member provided in a relatively simple manner, and in the future, hot pressing is envisaged in which a more difficult forming is foreseen. as a ridge formation. However, when the hot pressing is applied to a member on which a more difficult shaping is carried out, there is a concern that a ferritic transformation, induced by deformation, may occur in a shaped portion of high deformation. and that, therefore, it is possible that limb hardness is reduced locally after hot forming.

To suppress the ferritic transformation induced by the deformation, it is possible to carry out the hot forming in a higher temperature range. However, an increase in the hot forming temperature causes a reduction in productivity, an increase in manufacturing costs, deterioration of surface properties, and the like, so it is not applicable easy to mass production technology. For example, in Patent Document 1, a technique for carrying out a work by pressing at 850 ° C or higher is described. However, in the current hot pressing, there may be a case in which the temperature of the steel plate decreases to 850 ° C or less while the steel plate being heated to about 900 ° C in a heating furnace It is extracted from the heating furnace and is then transported and inserted in a pressing machine. In this case, it is difficult to suppress the ferritic transformation induced by the deformations during shaping.

From the point of view of increasing the productivity of hot pressing and increasing the stability of the material in a member after forming, Patent Document 2 discloses a method for manufacturing a high strength steel member, pressed in hot, provided with excellent productivity, in which a process of cooling a material by dissipating heat from a pressing die can be omitted. The method disclosed in Patent Document 2 is excellent; however, it is necessary that there be a large number of elements with a reinforcing action of the hardenability such as Mn, Cr, Cu and Ni, contained in the steel. Therefore, the technique disclosed in Patent Document 2 presents the problem of increasing costs. In addition, in the member manufactured by using the technique disclosed in Patent Document 2, there is a concern that tenacity deterioration will occur due to various inclusions present and the anisotropy of the toughness caused by the inclusions (mainly, MnS) that stretch in the direction of the laminate. The actual performance of the member is restricted by the properties on the low tenacity side, so that the original basic properties of the metal can not manifest to a sufficient degree when there is an anisotropy in tenacity. The anisotropy of the tenacity can be reduced by carrying out a morphology control on the stretched inclusions by a calcium treatment described, for example, in Patent Document 3. However, in this case, the value of the toughness is reinforced in a direction where tenacity is the lowest. However, the number of inclusions in the member increases, so there is a problem in the fact that the values of tenacity in the other directions are reduced.

Patent Document 4 refers to a sheet steel having a chemical composition comprising,% by mass, 0.09-0.60% C, 2.0% or less Si, 0.5-3 , 5% Mn, 0.10% or less of P, 0.05% or less of S, 0.005-2.0% of Al, 0.01% or less of N, plus 0.2% or less of Ti, 0.2% or less of Nb, 1.0% or less of V, 1.0% or less of Cr, 1.0% or less of Mo, 1.0% or less of Cu, 1.0 % or less of Ni, 0.01% or less of B, 0.01% or less of Ca, 0.01% or less of Mg, 0.1% or less of REM, as the case may be, and the rest consists of in iron with impurities, it has a cleaning factor of 0.08% or less; and has a segregation factor a of P = 1.6 or less, which is defined by the following expression (1): a = [maximum concentration of P (% by mass) in the central part of the thickness of the sheet / concentration of P average (% by mass) in the position 1/4 of the depth of the thickness of the sheet with respect to the surface (1), and a degree of segregation p of S = 1.6 or less, defined by the following expression (2): p = [concentration of S maximum% by mass) in the central part of the thickness of the sheet] / [concentration of S mean (% by mass) in the position 1/4 of the depth of the thickness of the layer with respect to the surface] (2).

Patent Document 5 refers to a method for hot forming comprising heating a sheet steel consisting essentially, in mass percentage, in: C: 0.15-0.45%; Mn: 0.5-3.0%; Cr: 0.1 -0.5%; Ti: 0.01 - 0.1%; B: 0.0002 - 0.004%; Yes: at most 0.5%; P: at most 0.05%; S: at most 0.05%; Al: at most 1%; N: at most 0.01%; one or more of Ni: at most 2%, Cu: at most 1%, Mo: at most 1%, V: at most 1%, and Nb: at most 1%; including the rest iron and the unavoidable impurities at a temperature of the point Ac ³ or higher, keeping it at said temperature, after which the hot steel sheet is shaped to the finished form of the member, wherein the shaped member is suddenly cooled by cooling from the shaping temperature during shaping or after shaping in such a way that the cooling gradient to the Ms point is at least the critical cooling gradient and that the average cooling coefficient from the Ms point to 200 ° C is in the range of 25-150 ° C / s.

[Prior art documents]

[Patent Document]

[Patent Document 1] Unexamined Japanese Patent Application, First Publication No. 2002-102980 [Patent Document 2] Unexamined Japanese Patent Application, First Publication No. 2006-213959 [Patent Document 3] Application for Unexamined Japanese Patent, First Publication No. 2009-242910 [Patent Document 4] Unexamined Japanese Patent Application, First Publication No. 2007-314817 [Patent Document] Ep 1642991 A1

[Exhibition of the Invention]

[Problems to be solved by the Invention]

As described in the foregoing, in the related art, hot pressing applications are limited to members that have a relatively simple shape. Therefore, the technical problems of a local reduction of the hardness, the anisotropy of the tenacity, and a reduction of the value of the tenacity of the member after the hot forming (a sheet of steel subjected to the process of hot forming) caused the ferritic transformation induced by the deformation in a portion subject to high deformation, which takes place in a case in which a more difficult conformation is carried out, such as a formation of ridges, have not been studied.

An object of the present invention is to provide a steel plate for hot forming in which, even in the case of the above-described problems, that is, even in the case where a forming is carried out hot, accompanied by a high deformation conformation, the transformation is suppressed ferrite induced by deformation in a shaped portion, so that the hardness after hot forming is uniform (the possible differences in hardness are small), resulting in excellent toughness and low anisotropy in the toughness after forming in hot.

[Means to solve the problem]

The inventors carried out a diligent investigation to solve the problems described above.

As a result, it was discovered that by controlling the chemical composition of a steel sheet, the amount of inclusions and the central segregation, it is possible to obtain a steel sheet for hot forming in which, even in the case in which a hot forming is carried out accompanied by a high deformation conformation, the ferritic transformation induced by the deformation is suppressed, and, therefore, the hardness after the hot forming is uniform, resulting in a excellent tenacity and low anisotropy in tenacity after hot forming. In addition, in the following specification, there may be a case in which the expression "uniform hardness" can be considered equivalent to a "stable hardness distribution".

The present invention, based on the new discoveries, is defined in the claims. In addition, the following is disclosed:

(1) A steel sheet according to one aspect of the present invention includes as chemical composition, in mass percentage: C: from 0.18% to 0.275%; Yes: from 0.02% to 0.15%; Mn: from 1.85% to 2.75%; To the solid: from 0.0002% to 0.5%; Cr: from 0.05% to 1.00%; B: from 0.0005% to 0.01%; P: 0.1% or less; S: 0.0035% or less; N: 0.01% or less; Ni: from 0% to 0.15%; Cu: from 0% to 0.05%; Ti: from 0% to 0.1%; Nb: from 0% to 0.2%; including the rest iron and impurities, wherein a cleaning of a metal structure is 0.08% or less, which is a degree of segregation of Mn expressed by the following expression a is 1.6 or less, and in one conformation in hot, a difference AHv in an average hardness after the hot forming between a shaped portion of low deformation that undergoes a plastic deformation of 5% or less and a shaped portion of high deformation that undergoes a classical deformation of 20% or more It is 40 or less.

a = (a maximum concentration of Mn in% by mass in a central portion of thickness of the steel sheet) / (an average concentration of Mn in% by mass in a position at a depth of% of a sheet thickness from of a surface of the steel sheet): expression to

(2) In the steel sheet described in (1), the chemical composition may further include, instead of a Fe portion, in% by mass, one or two selected from the group consisting of Ni: 0.02. % to 0.5%, and Cu: from 0.003% to 0.05%.

(3) In the steel sheet described in (1) or (2), the chemical composition may further include, instead of a Fe portion, in mass percentage, one or two selected from the group consisting of Ti: 0.005% to 0.1%, and Nb: from 0.005% to 0.2%.

(4) In the steel sheet according to any of (1) to (3), the surface of the steel sheet may further include a layer applied as a coating.

[Effects of the Invention]

According to one aspect of the present invention, even in the case where the hot forming is carried out accompanied by a high deformation conformation such as a ridge formation, the ferritic transformation, induced by the deformations, in the The shaped portion is suppressed, and therefore it is possible to obtain a steel plate provided with a stable distribution of hardness after hot forming, and with excellent toughness and low anisotropy in terms of toughness, after hot forming . The steel sheet is suitable, for example, for the material of a mechanical structural member including a chassis structural member, a lower chassis member, and the like of a vehicle, whereby the present invention is very useful in industrial fields.

In addition, the hot forming can be carried out following a routine method. For example, it is possible to heat a sheet of steel at a temperature of one point Ac ³ or higher (approximately 800 ° C) and the point Ac ³ + 200 ° C or less, can be maintained for 0 seconds or longer for 600 seconds or less, it can be transported to a pressing machine after which it is shaped by pressing, and can be maintained for 5 seconds or more in the lower dead center of the pressing machine. At this time, a heating method can be suitably selected, and in the case of rapid heating, electrical or high frequency heating can be carried out. In addition, a furnace heating adjusted to a heating temperature, or the like, can be used as a typical heating. Air cooling is carried out during transport to the pressing machine, so that there is a possibility that, when the transport time is prolonged, a ferritic transformation may take place until the pressing is started and a softening Therefore, the transport time is preferably 15 seconds or less. To prevent an increase in temperature, a cooling of a matrix can be carried out. In this case, as a cooling method, suitable cooling can be carried out, such as a cooling method consisting in installing a refrigeration pipe in a matrix and supplying a refrigerant to flow through said pipe.

[Realization of the Invention]

Next, a steel plate according to an embodiment of the present invention will be described in more detail (in some cases, with the designation "steel plate according to this embodiment"). In the following specification, the percentages referred to the chemical composition of the steel sheet consist of percentages by mass.

1. Chemical composition

(1) C: from 0.18% to 0.275%

C is an important element to increase the ability of the steel to harden, by determining its mechanical strength after hardening, in addition to controlling the local ductility and its tenacity after its hot forming. In addition, C is an austenite former, so it has an action of suppressing ferritic transformation, induced by deformation, during forming under high deformation, which facilitates obtaining a stable distribution of the hardness of a member after the hot forming. However, when the content of C is less than 0.18%, it is difficult to ensure a tensile strength of 1100 MPa or more, which is a preferable tensile strength after tempering, and it is possible to obtain a yield effect of a stable distribution of hardness due to the action described above. On the other hand, when the C content is higher than 0.275%, local ductility and toughness are reduced. Therefore, the content of C is 0.18% to 0.275%. The preferable upper limit of the C content is 0.26%, and its most preferable upper limit is 0.24%.

(2) Yes: from 0.02% to 0.15%

If it is an element that increases hardenability and reinforces the adhesion of the lamellae after hot forming. However, when the content of Si is less than 0.02%, there may be a case in which the effect described above can not be obtained to a sufficient degree. Therefore, the lower limit of the Si content is 0.02%. The preferable lower limit of silicon is 0.03%. On the other hand, when the Si content is higher than 0.15%, a heating temperature necessary for the austenitic transformation during hot forming is significantly high. Therefore, the case may arise in which the cost necessary for a heat treatment increases or in which the tempering is carried out insufficiently, due to insufficient heating. In addition, silicon is a ferrite-forming element. Therefore, when the Si content is excessive, the ferritic transformation induced by the deformation will probably take place during forming under high deformation. Thus, the case may arise where the hardness of a member after hot forming is reduced locally, and therefore, a stable distribution of hardness is not obtained. In addition, there may be a case in which a large amount of Si contained causes a reduction in wettability in the case of hot dip coating treatment, resulting in uncoated parts. Therefore, the upper limit of the Si content is 0.15%.

(3) Mn: from 1.85% to 2.75%

Mn is an effective element to increase the hardening of the steel and the stable assurance of the mechanical strength of the steel after its hardening. In addition, the Mn is an austenite former, and therefore suppresses the ferritic transformation induced by the deformations during forming under high deformation, thereby facilitating the obtaining of a stable distribution of the hardness of a member after forming in hot. However, when the content of Mn is less than 1.85%, there may be a case where the effect mentioned above can not be obtained to a sufficient degree. Therefore, the lower limit of the content of Mn is 1.85%. On the other hand, when the content of Mn is higher than 2.75%, the effect described above becomes saturated, and a deterioration in toughness is caused after tempering. Therefore, the upper limit of the content of Mn is 2.75%. The preferable upper limit of Mn is 2.5%.

(4) At sun: from 0.0002% to 0.5%

Al is an element that deoxidizes the molten steel and, therefore, improves the robustness of the steel. When the content of Al sol. is less than 0.0002%, deoxidation is carried out in an insufficient manner. Therefore, the lower limit of the content of the sun. it is 0.0002%. In addition, Al is also an effective element to increase the hardenability of a steel sheet and to stably secure its mechanical strength after its tempering, so that it can be actively contained. However, even when the content of Al is higher than 0.5%, the effect is saturated, and an increase in costs is caused. Therefore, the upper limit of the Al content is 0.5%. To the sun. indicates Al soluble in acid, and the content of Al sol. it does not include the content of Al content in A ^ O ³ and similar that does not dissolve in an acid.

(5) Cr: from 0.05% to 1.00%

Cr is an element that increases the hardenability of steel. In addition, Cr is an austenite former, by which it suppresses the ferritic transformation induced by the deformation during forming under high deformation, thereby facilitating the obtaining of a stable distribution of the hardness of a member after hot forming. However, when the Cr content is less than 0.05%, there may be a case where it is not It is possible to obtain the effect described above sufficiently. Therefore, the lower limit of the Cr content is 0.05%. The preferable lower limit of the chromium content is 0.1%, and the most preferable lower limit is 0.2%. On the other hand, when the Cr content is higher than 1.00%, Cr concentrates on carbides in the steel. As a result, when the steel is provided in the hot forming, the solubilization of the carbides during a heating process is delayed, and the hardenability is reduced. Therefore, the upper limit of the Cr content is 1.00%. The preferable upper limit of the Cr content is 0.8%.

(6) B: from 0.0005% to 0.01%

B is an effective element to increase the hardenability of the steel and to stably ensure its mechanical strength after its hardening. When the boron content is less than 0.0005%, it may be the case that the aforementioned effect is not obtained to a sufficient degree. Therefore, the lower limit of the content of B is 0.0005%. On the other hand, when the boron content is higher than 0.01%, the effect becomes saturated, and a deterioration in the toughness of a tempered portion is caused. Therefore, the preferable upper limit of the content of B is 0.005%.

(7) P: 0.0002 at 0.1%

P is an element that is generally contained as an impurity. However, the P has the action of increasing the hardening capacity of the steel and of stably ensuring the strength of the steel after hardening, and therefore it can be actively contained. However, when the content of P is greater than 0.1%, the toughness deteriorates significantly. Consequently, the content of P is limited to 0.1%. The preferable upper limit of the content of P is 0.05%. It is not necessary that the lower limit of P has a particular limitation, but an excessive reduction in the content of P is a cause of a significant increase in cost. Therefore, the lower limit of the content of P is 0.0002%.

(8) S: from 0.0002% to 0.0035%

S is an element that is contained as an impurity. In addition, in particular, the S form MnS and, therefore, is a major factor in the reduction of tenacity and tenacity anisotropy. When the content of S is greater than 0.0035%, the deterioration of toughness becomes significant and, therefore, the content of S is limited to 0.0035%. The lower limit of the content of S does not need to be particularly limited, but an excessive reduction in the content of S is the cause of a significant increase in cost. Therefore, the lower limit of the content of S is 0.0002%.

(9) N: from 0.0002% to 0.01%

N is an element that is contained as an impurity. When the N content is greater than 0.01%, coarse nitrides are formed in the steel and local deformability and toughness deteriorate significantly. Consequently, the content of N is limited to 0.01%. The lower limit of the content of N does not have a particular imitation, but an excessive reduction in the content of N causes a significant increase in cost. Therefore, the lower limit of the content of N is 0.0002%. The preferable lower limit of the content of N is 0.0008% or higher.

In addition to the elements mentioned in the foregoing, the steel sheet according to this embodiment may contain arbitrary elements described below. Such elements are not necessarily contained in the sheet. Therefore, the lower limits of the quantities thereof are not particularly limited, and the lower limits thereof are 0%.

(10) Ni: 0.15% or less; Cu: 0.05% or less

Ni and Cu are effective elements to increase the hardening capacity of the steel and to ensure resistance in a stable manner after hardening. Therefore, one or two of these elements can be contained. However, even when a quantity of any of them is contained at a level above the upper limit, the effect described above is saturated which is disadvantageous in terms of cost. Consequently, the amount of each of the elements is established as described in the preceding. It is preferable that the Ni content is 0.10% or less, and that the Cu content is 0.03% or less. To more reliably obtain the effect described above, it is preferable that one or two selected from the group consisting of Ni: 0.02% or more and Cu: 0.003% or more are contained.

(11) Ti: 0.1% or less; Nb: 0.2 or less

Ti and Nb are elements that suppress recrystallization and further suppress the growth of the grains by the formation of fine carbides, thus forming fine austenite grains when a steel plate is heated to an Ac ³ point or higher and is provided for the hot forming. When fine austenite grains are formed, the tenacity of a hot formed member is significantly improved. In addition, Ti binds mainly to N in the steel to generate TiN and, therefore, the consumption of B is suppressed due to the precipitation of BN. As a result, when Ti is included, the hardening capacity can be increased through B. To obtain the effect described above, one or two of the elements can be contained. When an amount of any of the elements above the upper limit is contained, the amount of precipitation of TiC or NbC increases and, therefore, C is consumed, therefore, there may be a case in which the resistance is reduced after of the tempered. Therefore, the quantity of each of the elements is established as described previously. It is preferable that the upper limit of the Ti content is 0.08%, and that the upper limit of the Nb content is 0.15%. In addition, in order to more reliably obtain the effect described above, it is preferable that one or two selected from the group consisting of Ti be contained: 0.005% or more and Nb: 0.005% or more.

The rest that excludes the components described above includes Fe and an impurity. The impurity indicates a raw material, such as ore or scrap, or a material incorporated from a manufacturing environment.

The steel sheet according to the present invention can be any of a hot-rolled steel sheet and a cold-rolled steel sheet, and it can be annealed hot-rolled steel sheet or annealed cold-rolled steel sheet that It is obtained by annealing on hot-rolled steel sheet or on cold-rolled steel sheet.

2. Metallic structure

(1) Cleaning: from 0.003% to 0.08%

The cleaning in this embodiment is defined as the sum of the amounts of inclusions of the series A, series B and series C contained in a steel sheet, which are obtained by an arithmetic calculation specified in JIS G 0555. When the amounts of inclusions increase, the propagation of cracks occurs easily, which results in the deterioration of the tenacity and in an increase in the degree of anisotropy in the tenacity. Therefore, the upper limit of cleaning is 0.08%. The preferable upper limit thereof is 0.04%. In the steel sheet according to this embodiment, MnS, which is the inclusion of the series A, is a major factor of deterioration of the degree of anisotropy in toughness. Therefore, in particular, the inclusion amount of the series A is preferably 0.06% or less. It is more preferable that the inclusion amount of series A be 0.03% or less.

In addition, it is preferable that the cleaning is as low as possible. However, from the point of view of costs, the lower limit thereof is 0.003% or preferably 0.005%.

(2) Degree of segregation a of Mn: from 1.03 to 1.6

It is likely that Mn will separate in the vicinity of a central part of the thickness of a steel plate during casting. In case central segregation occurs significantly, inclusions, such as MnS, are concentrated in a segregated portion, resulting in a reduction in tenacity and an increase in the degree of anisotropy in tenacity. In addition, the martensite generated in the portion segregated during tempering is hard and, therefore, the toughness deteriorates. In addition, due to the interaction between Mn and P, the segregation of P also increases in degree in the segregated portion of Mn, which also causes the deterioration of tenacity. Therefore, a degree of segregation of Mn a, expressed by the following expression 1, is 1.6 or less. The degree of segregation of Mn a is preferably about 1.0 (ie segregation does not occur). However, from the point of view of costs, the lower limit thereof is 1.03 or preferably 1.05.

a = (the maximum concentration of Mn in% by mass in a central portion of thickness of the steel sheet) / (an average concentration of Mn (in% by mass) in a position at a depth of% of a sheet thickness from a surface of the steel plate) ... (expression 1)

(3) Coating layer

A coating layer can be formed on the surface of the hot-forming steel sheet according to the present invention in order to improve the corrosion resistance and the like, and obtain a steel sheet treated on the surface. Even when the coated layer is provided, the effect of this embodiment is not reduced. The coated layer can be a layer applied by electrocoating, or it can be a layer applied by hot coating. As the electrical coating layer, an electrolytic zinc coated layer, a layer coated with electrolytic Zn-Ni alloy, and the like can be exemplified. As a hot-dip coated layer, a hot-dip galvanized layer, a galvanized layer, a hot-dip aluminum-coated layer, a Zn-AI alloy hot-dip coated layer, a hot-dip alloy Zn -AI-Mg the coated layer, a layer coated with Zn-AI-Mg-Si alloy by hot dip, and the like, can be exemplified. A coated amount is not particularly limited, and may be in a general range.

4. Manufacturing method

Next, a representative method for manufacturing the steel sheet for hot forming according to the present invention is described. By using the manufacturing method that includes the following processes, it is possible to easily obtain the steel sheet according to this embodiment.

(1) Continuous casting process (S1)

An amount of molten steel having the above-described chemical composition is cast so as to obtain a slab by continuous casting. In this continuous casting process, it is preferable that the temperature of the molten steel be greater than a liquid temperature by 5 ° C or more, with the amount of steel poured per unit time of 6 ton / min or less, and out a segregation reduction treatment of center before a cast piece solidifies completely.

When the amount of molten steel being poured per unit time (pour rate) of the molten steel during continuous casting is greater than 6 ton / min, the molten steel in a mold flows rapidly, and therefore the inclusions are easily trapped and increases the amount of inclusions in the slab. When the temperature of the molten steel is higher than the temperature of the liquid at less than 5 ° C, the viscosity increases, and therefore the inclusions are less likely to float. Therefore, the amount of inclusions in the steel increases, and the cleaning deteriorates (the value of cleaning increases). When the molten steel is cast continuously, it is more preferable that the temperature of the molten steel be higher than the liquid temperature by 8 ° C or more, and that the poured amount be 5 ton / min or less.

As a treatment for the reduction of center segregation, for example, by carrying out an electromagnetic stirring or a reduction of the non-solidified layer on a non-solidified layer before the casting solidifies completely, it is possible to effect a relief or extraction of a concentrated portion.

(2) Treatment process for the homogenization of the slab (S2)

As a treatment for the reduction of the segregation after the slab has solidified completely, it is possible to carry out additionally a slab homogenization treatment consisting of heating the slab from 1150 ° C to 1350 ° C and maintaining the resultant during from 10 hours to 50 hours. By carrying out the homogenization treatment of the slab in the aforementioned conditions, it is possible to further reduce the degree of segregation. In addition, the preferable upper limit of the heating temperature is 1300 ° C, and the preferable upper limit of the retention time is 30 hours.

(3) Hot rolling process (S3), cooling process (S4), and winding process (S5)

The slab obtained by carrying out the continuous casting process described above and the treatment process for the slab homogenization as necessary, is heated from 1050 ° C to 1350 ° C, and then it is laminated obtaining a sheet of steel. The hot-rolled steel sheet is maintained in the aforementioned temperature range for 5 seconds to 20 seconds. After being maintained, the steel plate is cooled in a temperature range of 400 ° C to 700 ° C by cooling with water. Then the cooled steel plate is rolled up.

There may be a case in which the slab contains non-metallic inclusions which are a cause of deterioration of the tenacity and local deformability of a member after the annealing on the steel sheet has been carried out. Therefore, when the slab is provided for hot rolling, it is preferable that such non-metallic inclusions are sufficiently solubilized. As for the slab having the chemical composition described above, by heating the slab at 1050 ° C or at a higher temperature to be proportioned to the hot rolling, the solubilization of the non-metallic inclusions is accelerated. Therefore, it is preferable that the temperature of the slab provided for its hot rolling is 1050 ° C or higher. In addition, the temperature of the slab provided for its hot rolling can be 1050 ° C or higher, and the slab having a temperature lower than 1050 ° C can be heated to 1050 ° C or higher. In case the transformation obtained from the processed austenite is allowed after the lamination is finished, a laminated texture remains, which causes an anisotropy of the final product. Therefore, in order to allow the transformation to take place from the recrystallized austenite, it is preferable that the steel sheet, after having been rolled, is maintained for 5 seconds or more in the aforementioned temperature range. To maintain the steel sheet for five seconds or more in the manufacturing line, it is possible to transport the steel sheet without being cooled by water to a cooling zone once the rolling is finished.

By adjusting a winding temperature to 400 ° C or higher, it is possible to increase a ratio of ferrite areas in the metal structure. When the ratio between the ferrite areas is high, the strength of the hot-rolled steel sheet is suppressed, and therefore the control of the load, the control of the flattening of the steel sheet, and the control of the thickness of the the sheet, they are facilitated during cold rolling in a subsequent process, resulting in an increase in manufacturing efficiency. Therefore, it is preferable that the winding temperature is 400 ° C or higher.

On the other hand, by adjusting the winding temperature to 700 ° C or less, the development of the lamella after winding is suppressed, and therefore the generation of lamella defects is suppressed. In addition, the deformation of a winding due to its weight after winding is suppressed, and the generation of scratches on the winding surface due to deformation can be suppressed. Therefore, the winding temperature is preferably 700 ° C or less. The deformation is caused by the volumetric expansion due to the ferritic transformation and subsequent thermal contraction, and the disappearance of the winding tension in the winding in a case where untransformed austenite remains after the winding and the non-transformed austenite is transformed into ferrite after the winding.

(4) Pickling process (S6)

The pickling can be carried out on the sheet after the winding process. The pickling can be carried out according to a routine method. Before or after pickling, in order to ensure correctness of the flatness quality or the exfoliation of the lamellae, a skin pass rolling may be carried out, and this does not influence the effect of this embodiment. An elongation coefficient in a case of carrying out a skin pass rolling must not have particular limitations, and may, for example, be greater than or less than 3.0%.

(5) Cold rolling process (S7)

The cold rolling can be carried out on the pickled steel sheet obtained by the pickling process, according to the need.

It is possible to carry out a method of cold rolling according to a routine method. The lamination reduction of cold rolling can be in a typical range, and is usually 30 to 80%

(6) Annealing process (S8)

The annealing at a temperature of 700 to 950 ° C can be carried out on the hot-rolled steel sheet obtained by the winding process (S5) or by the cold-rolled steel sheet obtained by the cold rolling process (S5). S7), depending on the need.

By carrying out the annealing in the maintenance of hot-rolled steel sheet and cold-rolled steel sheet within a temperature range of 700 ° C or higher, it is possible to reduce the effect of the conditions of hot rolling, and, therefore, it is possible to achieve a greater stabilization of the properties after tempering. Furthermore, as regards the cold-rolled steel sheet, the steel sheet can be softened by recrystallization, and, therefore, it is possible to improve the processability of the hot forming. Therefore, in the case of carrying out the annealing on the hot-rolled steel sheet or on the cold-rolled steel sheet, it is preferable that the steel sheet is maintained within a temperature range of 700 ° C or higher.

On the other hand, by adjusting the annealing temperature to a value of 950 ° C or less, it is possible to suppress the necessary cost for annealing, and high productivity can be ensured. In addition, since the thickening of the structure can be suppressed, it is possible to achieve a better tenacity after hardening. Therefore, in the case of carrying out the annealing on the hot-rolled steel sheet or on the cold-rolled steel sheet, it is preferable that the steel sheet is maintained within a temperature range of 950 ° C or less.

The cooling to 550 ° C after annealing in the case of enhancing the annealing is preferably carried out with an average cooling gradient of 3 ° C / s at 20 ° C / s. By adjusting the average cooling gradient to 3 ° C / s or higher, it is possible to suppress the generation of coarse perlite or coarse cementite, and, therefore, it is possible to improve the properties after annealing. In addition, by adjusting the average diameter gradient to 20 ° C / s or less, the material is easily stabilized.

(7) Coating process (S9)

In a case where a coating layer is formed on the surface of the steel sheet to obtain a coated steel sheet, it is possible to carry out an electrical coating or a hot dip coating according to a routine method. In the case of hot dip galvanization, an industrial hot-dip galvanizing continuous installation can be used and the annealing process can be carried out and a subsequent coating treatment can be carried out in the industrial installation. Otherwise, the coating treatment can be carried out independently with respect to the annealing process. It is also possible to carry out an alloy treatment in addition to hot dip galvanization for galvanization. In case of alloying treatment, it is preferable that the temperature for the alloy treatment is 480 ° C to 600 ° C. By adjusting the treatment temperature with the alloy to a value of 480 ° C or higher, it is possible to suppress the lack of uniformity in the treatment with the alloy. By adjusting the temperature of the treatment with the alloy to a value of 600 ° C or less, it is possible to suppress manufacturing costs, and high productivity can be ensured. If necessary, after the hot dip galvanization, it is possible to carry out the lamination by skin pass to correct the flat condition. The elongation coefficient of the skin pass lamination can follow a routine method.

The amount of inclusions and the degree of segregation in the steel sheet are mainly determined by the processes involved in the hot rolling and do not change substantially before or after the hot forming. Therefore, when the chemical composition, the amount of inclusions (degree of cleaning), and the degree of segregation of the steel sheet prior to hot forming meet the ranges of this embodiment, a hot-pressed member manufactured by the Hot pressing carried out below also satisfies the ranges of this embodiment.

Examples

Steels having the chemical compositions shown in Table 1 were melted in a converter for a test, and continuous casting was carried out in a continuous casting machine for a test. As shown in Table 2, in the continuous casting process, the pouring velocity and the heating temperature difference of the molten steel (molten steel temperature - liquid temperature) varied variously during casting. In addition, in a slab solidification process, an electromagnetic stirring was performed. In addition, in a final solidified slab portion, the extraction of a segregated central portion was performed by a non-consolidated layer reduction (extrusion) in which the interval between a pair of upper and lower rollers was narrowed in the continuous casting machine . For comparison, partially slabs were produced on which electromagnetic agitation and / or extrusion was not performed (reduction treatment of the central segregation). Subsequently, a plate homogenization treatment was carried out at 1300 ° C for 20 hours. Slab homogenization treatment was omitted for some slabs. When the slabs produced as described above were used, hot rolling was carried out, and then the resultants were cooled and rolled to obtain hot-rolled steel sheets with a sheet thickness of 5.0 mm or 2.9 mm. As for the hot rolling conditions at this time, the heating temperature of the slabs was 1250 ° C, the start temperature of the rolling was 1150 ° C, the rolling finish temperature was 900 ° C and the winding temperature was 650 ° C. The hot rolling was carried out through a lamination of several passes and the retention for 10 seconds was carried out after finishing the rolling. Cooling after hot rolling was carried out by cooling with water. For comparison, parts of them were not subject to retention.

Furthermore, as for the pour coefficients, a quantity of a real production installation is different from that of the continuous casting machine for a test used in this example. Therefore, in Table 2, taking into account the size factors, a value that is converted into the casting speed in the actual production installation is described. Further, the temperature difference of the heating of the molten steel in Table 2 is a value obtained by subtracting a liquid temperature from a temperature of the molten steel.

The obtained hot-rolled steel sheets were subjected to a pickling treatment according to a routine method in order to obtain pickled steel sheets. The pickled steel sheets having a sheet thickness of 5.0 mm were subjected to cold rolling in order to obtain cold-rolled steel sheets with a sheet thickness of 2.9 mm. Parts of the hot rolled steel sheets were subjected to an electrocoating. Parts of the cold rolled steel sheets were subjected to an annealing by recrystallization (at an annealing temperature of 800 ° C for a recognized time of 60 seconds) in an installation of recognized continuous, and parts of the parts were then subjected to an electrolytic coating of zinc. In addition, parts of the hot-rolled steel sheets and the cold-rolled steel sheets were subjected to annealing (at an annealing temperature of 800 ° C for an acknowledged time of 60 seconds) and a hot dip galvanization. in a continuous hot dip galvanizing installation. The hot dip galvanization bath temperature was 460 ° C, and parts of them were subjected to an alloy treatment at 540 ° C for 20 seconds, which resulted in hot dipped galvanized steel plates and sheets galvanized steel

A hot pressing shaping was carried out on the steel sheets manufactured as samples, for which a hot pressing test apparatus was used. The steel sheets on which a punching was made with a size of 150 square mm and a hole with a through hole of 36 mm in diameter (set: 10%) were heated in a heating furnace until the surface of the sheet steel had reached a temperature of 900 ° C, and were kept at said temperature for 4 minutes, and were removed from the heating furnace. Next, the steel plates were cooled to 750 ° C by air cooling, subjected to hot rim formation at the time when the temperature had reached 750 ° C, maintained for 1 min at the bottom dead center of the pressing machine. The conditions of formation of the flanges are as follows:

Punching shape: conical

Punching diameter: 60 mm

Speed of the press: 40 mm / s,

Cooling after shaping was carried out by cooling the die, so that the steel sheet was maintained for one minute in the lower dead center.

In the cross-section of the hot-pressed steel sheet which is parallel to the direction of the rolling, the hardness of a flange-forming portion (a portion formed with high deformation which had undergone a deformation) was measured by a Vickers hardness tester. plastic deformation of 20% or more) and a flange portion (a portion formed of low deformation that had experienced a plastic deformation amount of 5% or less) at the positions at% sheet thickness in the cross section. The measurement load was 98 kN. A measurement method was based on JIS Z 2244. The hardness measurement was carried out a total of five times while moving with a pitch of 200 pm in the same thickness position. The average value of the five Vickers hardness values obtained from each of the members was obtained as an average hardness (Hv). The difference between the average hardness of the flange formation portion and the average hardness of the flange portion (AHv = (flange portion Hv) - (flange forming process Hv)) was determined and an acceptable case in which AHv was 40 or less. The results of the hardness test are shown in Table 3.

In addition, the magnitude of the deformation was obtained, for which the thickness of the sheet in each of the positions of the worked steel sheet was measured and an amount of a reduction in the thickness of the sheet was calculated after the work done on the sheet. from the thickness of the sheet before work.

On the other hand, on the steel sheets manufactured as samples, an examination of a tenacity value (absolute value of tenacity) and of the anisotropy of the tenacity was carried out.

The examination was carried out in the following manner. First the steel sheet, which had a thick sheet of 2.9 mm, was heated until the surface temperature of the steel sheet had reached 900 ° C in the heating furnace, was kept for 4 minutes at the temperature, and then removed from the heating oven. The steel plate was then cooled to 750 ° C by air cooling, interposed between the matrices of the flat plate at the time when the temperature had reached 750 ° C and was maintained for one minute. The front and rear surfaces of the samples were then ground to a thickness of 2.5 mm. Samples of the Charpy impact test were collected in such a way that the longitudinal direction of the samples was the rolling direction and a direction perpendicular to the rolling. At this time, a notch was a "V" notch at a depth of 2 mm. The impact test was carried out on the basis of JIS Z 2242 at room temperature as the test temperature. The relationship between an impact value in the direction of the lamination (energy absorbed / area in cross section) and a value of the impact in the perpendicular direction with respect to the lamination was used as an index of the anisotropy.

The results are shown in Table 3. As a result of the test, when the value of the impact in the longitudinal rolling direction was 70 J / cm2 or more, and the relationship between the impact values was 0.65 or higher, Good properties were determined.

The cleanliness of the steel sheet was examined on the basis of document JIS G 0555. Samples were cut from the steel sheet of each of the test numbers at five points, and the cleanliness of each of the positions was examined. in 1/8, 1/4, 1/2, 3/4 and 7/8 of the thickness of the sheet, by a method for counting points. Between the results in each of the positions of the sheet thickness, a value provided with a greater cleaning was adopted as the sample cleaning. Between the results in each of the thickness portions of the layer, a value provided with the greatest cleanliness as the sample cleaning was adopted. The cleaning was the sum of the inclusions of the series A, series B and series C.

The degree of segregation of Mn was obtained by carrying out the analysis of the surface of the Mn components for which an EPMA was used. Samples were cut from the steel sheet of each of the test numbers in five points; 10 visual fields were measured in each of the positions in 1/4 and 1/2 of the sheet thicknesses with a magnification of 500 times, and the average value of the degrees of Mn segregation of each of the visual fields was used.

[Table 3]

In all of the test numbers 16 to 19, 21 and 22, the average duration of the flange-forming portion that was the deformed portion of high deformation was significantly reduced compared to the average hardness of the flange portion that was the deformed portion of low deformation, and the AHv values were increased from 41 to 99. This is due to the fact that the portion of ridge formation was softened by the ferritic transformation, induced by the deformation, caused by the ridge formation work. . In this case, the manufactured hot-formed product, the hardness was locally different, so that the mechanical strength of the shaped product was not uniform but was partially reduced. Therefore, its reliability as a product was reduced.

In addition, in the test numbers 4, 8, 10, 12, 15, 18, 20, 23, and 24, the chemical compositions, the cleaning or the degree of segregation were outside the ranges of the present invention, so that the impact value in the rolling direction and / or the impact value ratio were insufficient.

Contrary to this, in all of the steel sheets having the chemical composition of the present invention, regardless of the presence or absence of the cold rolling and annealing process, or the type of coating, the AHv was de - 4 to 24, the difference between the average hardness of the flange portion and the average hardness of the flange formation portion was small, and the stability of hardness and mechanical strength during forming under high formation was excellent.

In addition, tenacity after hot rolling and anisotropy of toughness showed sufficient values.

[Industrial applicability]

In the steel sheet of the present invention, even in the case where a hot forming is carried out accompanied by a high deformation conformation, the ferritic transformation induced by the deformation ^{in the formed portion is suppressed. Therefore, it is possible to obtain a steel plate provided with a} stable ^distribution of hardness after hot forming, excellent toughness and low anisotropy in toughness after hot forming. The steel sheet is suitable for, for example, a material of a mechanical structural member includes a structural member of a chassis, a lower chassis member, and the like of a vehicle, whereby the present invention is very useful in the fields industrial

Claims (4)

1. A steel plate that has a chemical composition that consists, in mass percentage, of: C: 0.18% to 0.275%; Yes: from 0.02% to 0.15%; Mn: from 1.85% to 2.75%; AI sol .: from 0.0002% to 0.5%; Cr: from 0.05% to 1.00%; B: from 0.0005% to 0.01%; P: 0.0002% to 0.1%; S: from 0.0002% to 0.0035%; N: from 0.0002% to 0.01%; Ni: from 0% to 0.15%; Cu: from 0% to 0.05%; Ti: from 0% to 0.1%; Nb: from 0% to 0.2%; Y including the rest iron and impurities; wherein a cleaning of a metal structure is 0.003% to 0.08%, cleaning being defined as the sum of the quantities of inclusions of the series A, series B and series C contained in the steel sheet, which are obtained by means of an arithmetic calculation specified in document JIS G 0555, a, which is a degree of segregation of Mn expressed by the following expression 1 is from 1.03 to 1.6, and in the hot forming, an AHv of difference in an average hardness after hot forming between a portion formed of low deformation that undergoes a plastic deformation of 5% or less and a portion formed of high deformation that undergoes a plastic deformation of the 20% or more is 40 or less; a = (a maximum concentration of Mn, in percentage by mass, in a central portion of thickness of the steel sheet) / (an average concentration of Mn, in mass percentage, in a position at a depth of 1/4 of the thickness of a sheet measured from a surface of the steel sheet) ... expression 1. The steel sheet according to claim 1, wherein the chemical composition further includes, instead of a Fe portion, in mass percentage, one or two selected from the group consisting of Ni: from 0.02% to 0 , 15%, and Cu: from 0.003% to 0.05%. 3. The steel sheet according to claim 1 or 2, wherein the chemical composition further includes, instead of a Fe portion, in mass percentage, one or two selected from the group consisting of Ti: from 0.005% to 0 , 1%, and Nb: from 0.005% to 0.2%. 4. The steel sheet according to any of claims 1 to 3, wherein the surface of the steel sheet further includes a coating layer.