JP2014019941A - Hot molded steel sheet member and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot molded steel sheet member excellent in extensibility and impact property and having tensile strength of 900 MPa or more, and to provide a manufacturing method therefor.SOLUTION: A hot molded steel sheet member contains, by mass%, C:0.05% to 0.40%, Si:0.5% to 3.0%, Mn:1.2% to 8.0%, P:0.05% or less, S:0.01% or less, sol.Al:0.001% to 3.0% and N:0.01% or less, and the balance Fe with inevitable impurities, and a steel structure containing, by area%, 10 to 40% of austenite and having austenite + martensite with density of 1.0/μm, and has tensile strength of 900 MPa or more.

Description

  The present invention relates to a hot-formed steel plate member used for machine structural parts such as body structural parts and undercarriage parts of automobiles and a method for manufacturing the same. Specifically, the present invention relates to a hot-formed steel sheet member having excellent ductility and impact properties while having a tensile strength of 900 MPa or more and a method for producing the same.

  In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel used for the vehicle body and reduce the weight used. In a thin steel plate widely used for automobiles, press formability decreases with an increase in steel plate strength, and it becomes difficult to manufacture a complicated shape member. Specifically, there arises a problem that the ductility is lowered and the fracture occurs at a site where the degree of processing is high, or the spring back and the wall warp become large and the dimensional accuracy is deteriorated. Therefore, it is not easy to produce such a member by press molding using a steel plate having a high strength, particularly a tensile strength of 900 MPa class or higher. According to roll forming rather than press forming, a high-strength steel sheet can be processed, but it can be applied only to members having a uniform cross section in the longitudinal direction.

  On the other hand, as shown in Patent Document 1, in a method called hot press for press-forming a heated steel plate, the steel plate is soft and highly ductile at a high temperature. It is possible to mold well. Further, by heating the steel sheet to the austenite single phase region and quenching (quenching) in the mold, the strength of the member can be increased simultaneously by martensitic transformation. Therefore, such a hot pressing method is an excellent forming method that can simultaneously ensure the strength of the member and the formability of the steel sheet.

  Patent Document 2 discloses a pre-press quench method that achieves high strength of a member by forming it in a predetermined shape at room temperature, heating it to an austenite region, and quenching in a mold. . The pre-press quench method, which is one aspect of such a hot press, can restrain the deformation due to thermal strain by restraining the member with a mold, so that the strength of the member and high dimensional accuracy can be secured at the same time. It is an excellent molding method that can be performed.

  However, in recent years, hot-pressed steel sheet members have been required to have excellent impact absorption characteristics, that is, excellent ductility and impact characteristics at the same time, and the steel structure is substantially martensitic single phase. However, the conventional techniques represented by Patent Document 1 and Patent Document 2 have a problem that such a request cannot be met.

  Therefore, Patent Document 3 discloses that a steel sheet is heated to a two-phase temperature range of ferrite and austenite, and further pressed while maintaining a two-phase structure, quenched in a mold, and two-phase of ferrite and martensite. A member that is considered to have high strength and excellent ductility by being made into a structure is disclosed. However, with regard to ductility, the elongation is at most about 10%. In a member that requires excellent shock absorption characteristics, further excellent ductility, specifically, an elongation of 15% or more is required. The elongation is preferably 18% or more, more preferably 21% or more.

  By the way, the ductility can be remarkably increased by applying the structure control of TRIP steel or Q & P steel to the hot press method. This is because retained austenite is generated by a special heat treatment as will be described later.

  In Patent Document 4, a steel plate to which Si and Mn are positively added is heated to a two-phase temperature range of ferrite and austenite, and rapidly cooled at the same time as forming with a deep drawing apparatus, and ferrite and martensite, particularly austenite, are added. A member having high strength and excellent ductility is disclosed by using a contained multiphase structure. In order to contain austenite, 300-400 degreeC isothermal holding | maintenance, ie, an austemper, is required. Therefore, the mold of the deep drawing apparatus must be controlled to be heated to 300 to 400 ° C., and as described in the embodiment, it is necessary to hold the mold in the mold for about 60 seconds. However, since not only the tensile strength but also the elongation varies significantly depending on the holding temperature and holding time of the austemper, stable mechanical properties cannot be ensured. Furthermore, when the steel containing Si is austempered, such as the steel type targeted by the present invention, a very hard martensite is likely to be generated, resulting in a problem that impact characteristics are remarkably deteriorated.

  In Patent Document 5, a steel sheet to which Si and Mn are positively added is heated to a two-phase temperature range or an austenite single-phase range, rapidly cooled to a predetermined temperature at the same time as forming, further reheated, and martensite. A member having high strength and excellent ductility is disclosed by forming a multiphase structure containing austenite. However, the tensile strength varies significantly depending on the rapid cooling conditions, specifically, the temperature at which cooling is stopped. Furthermore, a process problem such as extremely difficult control of the cooling stop temperature is unavoidable. Further, since an extra heat treatment called reheating is required for the conventional method for producing a hot-pressed steel sheet member, productivity is significantly hindered. On the other hand, as described in the examples, since it is necessary to heat at a high temperature, the second phase such as martensite tends to be sparsely distributed, and this also causes a problem that impact characteristics are remarkably deteriorated. .

  Therefore, a hot pressing method for obtaining a steel plate member containing retained austenite has to be newly studied regardless of the structure control of TRIP steel or Q & P steel.

On the other hand, by heat-treating a low carbon steel added with Mn positively near _point A, the intensity - is significantly higher steel ductility balance obtained. For example, in Non-Patent Document 1, by hot rolling a 0.1% C-5% Mn alloy and further reheating, it contains several 10% of retained austenite and is extremely excellent in high strength and ductility. Steel is disclosed.

British Patent Publication No. 1490535 Japanese Patent Laid-Open No. 10-96031 JP 2010-65292 A Special table 2009-508692 gazette JP 2011-184758 A

Heat Treatment, Vol. 37 No. 4 (1997), p. 204

As the non-patent document 1, and optimizing the chemical composition of the alloy used in the hot press steel, further, by strictly controlled to a heat treatment temperature in the hot pressing process becomes near _point A, the residual austenite It is possible to produce steel containing. However, in the method disclosed in Non-Patent Document 1, since the influence of the heating time on the tensile strength and elongation is extremely large, heating for 30 minutes or more is necessary to suppress the fluctuation. That is, such a structure control cannot be applied to the production technology of hot press in terms of productivity and surface quality. Furthermore, since the dissolution of cementite tends to be insufficient, it is easily expected that the steel material obtained by the present technology is inferior in impact characteristics.

  As described above, mass production technology for providing a member having a tensile strength of 900 MPa or more after hot pressing and excellent in ductility and impact properties has not yet been established.

  As described above, a specific problem of the present invention is that a hot-formed steel sheet member having a tensile strength of 900 MPa or more and excellent in ductility and impact properties after hot pressing, which was conventionally impossible to mass-produce, and its manufacture Is to provide a method.

  As a result of intensive studies to improve the ductility and impact properties of a hot-formed steel sheet member having a tensile strength after hot forming of 900 MPa or more, the present inventors have determined that the chemical composition has a specific C and Mn content. On the other hand, by adding Si positively, containing a predetermined amount of austenite and making the steel structure finely present with austenite and martensite as a whole, the ductility and impact properties are remarkably improved. Obtained knowledge. In order to obtain such a steel structure, a steel sheet having the above-described chemical composition as a steel sheet to be subjected to hot forming, and containing one or two kinds selected from bainite and martensite and having a fine steel structure is used. In addition, we have obtained new knowledge that it can be achieved by optimizing the heat treatment conditions during hot forming.

This invention is made | formed based on the knowledge, The summary is as follows.
(1) By mass%, C: 0.05% to 0.40%, Si: 0.5% to 3.0%, Mn: 1.2% to 8.0%, P: 0.0. 05% or less, S: 0.01% or less, sol. Al: 0.001% or more and 3.0% or less and N: 0.01% or less, having a chemical composition composed of the balance Fe and impurities, and containing 10% or more and 40% or less austenite A hot-formed steel sheet member having a steel structure in which austenite and martensite as a whole are present at a density of 1.0 piece / μm ² or more and having a mechanical property of a tensile strength of 900 MPa or more.

  (2) The chemical composition is replaced by a part of Fe in mass%, Ti: 1.0% or less, Nb: 1.0% or less, V: 1.0% or less, Cr: 1.0% The above (1), which contains one or more selected from the group consisting of Mo: 1.0% or less, Cu: 1.0% or less, and Ni: 1.0% or less The hot-formed steel plate member described in 1.

  (3) The chemical composition is mass% in place of part of Fe: Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% The hot-formed steel sheet member according to (1) or (2) above, which contains one or more selected from the group consisting of:

  (4) In any one of the above (1) to (3), the chemical composition contains, in mass%, B: 0.01% or less instead of part of Fe The hot-formed steel sheet member described.

  (5) In any one of the above (1) to (4), the chemical composition may contain Bi: 0.01% or less in mass% instead of part of Fe The hot-formed steel sheet member described.

(6) The chemical composition according to any one of (1) to (5) above, wherein the Mn content is 2.4% by mass or more, and one or two selected from bainite and martensite A base steel plate containing a seed and having a steel structure in which the average grain size of prior austenite is 20 μm or less is held in a temperature range of 670 ° C. or more and less than 780 ° C. and less than Ac ₃ points for 2 minutes or more and 20 minutes or less. A hot-formed steel sheet member, characterized by performing hot forming on a base steel sheet heated to 600 ° C. and then cooling from 600 ° C. to 150 ° C. at an average cooling rate of 5 ° C./second or more and 500 ° C./second or less. Manufacturing method.

(7) One or two selected from bainite and martensite, having the chemical composition according to any one of (1) to (5) above, wherein the Mn content is less than 2.4% by mass. A base steel plate containing a seed and having a steel structure in which the average grain size of prior austenite is 20 μm or less is held in a temperature range of 670 ° C. or more and less than 780 ° C. and less than Ac ₃ points for 2 minutes or more and 20 minutes or less. Then, hot forming is performed on the base steel sheet heated to 600 ° C., then the average cooling rate from 5 ° C./second to 500 ° C./second from 600 ° C. to 150 ° C., and 5 ° C./second from 500 ° C. to 150 ° C. A method for producing a hot-formed steel sheet member, characterized by cooling at an average cooling rate of 20 ° C./second or less.

  According to the present invention, a technically valuable effect is achieved in that a hot-formed steel sheet member having excellent tensile properties and excellent impact properties and a tensile strength of 900 MPa or more becomes possible for the first time.

  Next, the reason why the present invention is limited to each range will be described. In the following description, hot forming will be described by taking a hot press as a specific embodiment as an example.

1. Chemical Composition First, the reason why the chemical composition of the hot-formed steel sheet member according to the present invention is defined as described above will be described. In the following description, “%” representing the content of each alloy element means “% by mass” unless otherwise specified.

(C: 0.05% or more and 0.40% or less)
C is a very important element that increases the hardenability of steel and mainly determines the strength after quenching. If the C content is less than 0.05%, it is difficult to ensure a tensile strength of 900 MPa or more as the strength after quenching. Therefore, the C content is 0.05% or more. On the other hand, when the C content exceeds 0.40%, the impact characteristics are significantly deteriorated. Therefore, the C content is set to 0.40% or less. From the viewpoint of weldability, the C content is preferably 0.25% or less.

(Si: 0.5% to 3.0%)
Si is an extremely effective element in order to stably secure the strength after quenching. If the Si content is less than 0.5%, it is difficult to obtain the above effect. Therefore, the Si content is 0.5% or more. Note that when the Si content is 1.0% or more, the ductility is further improved. Therefore, the Si content is preferably 1.0% or more. On the other hand, if the Si content exceeds 3.0%, the effect of the above action is saturated and disadvantageous economically, and the surface properties are significantly deteriorated. Therefore, the Si content is 3.0% or less.

(Mn: 1.2% to 8.0%)
Mn is an extremely effective element for enhancing the hardenability of the steel and ensuring the strength after quenching stably. However, if the Mn content is less than 1.2%, the effect cannot be sufficiently obtained, and it becomes very difficult to ensure a tensile strength of 900 MPa or more as a strength after quenching. Therefore, the Mn content is 1.2% or more. When the Mn content is 2.4% or more, the slow cooling after hot pressing as described later becomes unnecessary, and the productivity is remarkably improved. For this reason, it is preferable that Mn content shall be 2.4% or more. On the other hand, if the Mn content exceeds 8.0%, austenite is excessively generated and delayed fracture is likely to occur. Therefore, the Mn content is 8.0% or less. In addition, in the base steel plate before being subjected to hot pressing, lowering its tensile strength contributes to improvement of productivity, so that the Mn content is preferably 6.0% or less.

(P: 0.05% or less)
Generally, P is an impurity inevitably contained in steel, but it may be positively incorporated because it has the effect of increasing strength by solid solution strengthening. However, if the P content exceeds 0.05%, the weldability deteriorates remarkably. Therefore, the P content is 0.05% or less. The P content is preferably 0.02% or less. In order to obtain the above action more reliably, the P content is preferably set to 0.003% or more.

(S: 0.01% or less)
S is an impurity inevitably contained in steel, and is preferably as low as possible from the viewpoint of weldability. If the S content exceeds 0.01%, the weldability is significantly reduced. Therefore, the S content is 0.01% or less. Preferably it is 0.003% or less, More preferably, it is 0.0015% or less.

(Sol.Al: 0.001% to 3.0%)
Al is an element having an action of deoxidizing steel to make the steel material sound, and is also an element having an action of improving the yield of carbonitride forming elements such as Ti. sol. If the Al content is less than 0.001%, it is difficult to obtain the above effect. Therefore, sol. The Al content is 0.001% or more. Preferably it is 0.01% or more. On the other hand, sol. If the Al content exceeds 3.0%, the weldability is significantly lowered, the oxide inclusions are increased, and the surface properties are significantly deteriorated. Therefore, sol. The Al content is 3.0% or less. Preferably it is 1.5% or less.

(N: 0.01% or less)
N is an impurity inevitably contained in the steel, and is preferably as low as possible from the viewpoint of weldability. If the N content exceeds 0.01%, the weldability is significantly reduced. Therefore, the N content is 0.01% or less. Preferably it is 0.006% or less.

  The steel composition of the steel sheet according to the present invention is the balance Fe and impurities, but may further contain at least one element as described below as an optional component.

(Ti: 1.0% or less, Nb: 1.0% or less, V: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, and Ni : 1 type or 2 types or more selected from the group consisting of 1.0% or less)
All of these elements are effective elements for enhancing the hardenability of the steel sheet and stably securing the strength after quenching. Therefore, you may contain 1 type, or 2 or more types of these elements. However, when Ti, Nb, and V are contained in amounts exceeding 1.0%, hot rolling and cold rolling become difficult. Moreover, about Cr, Mo, Cu, and Ni, when it contains exceeding 1.0%, the effect by the said effect | action will be saturated and it will become economically disadvantageous. In order to obtain the effect of the above operation more reliably, Ti: 0.003% or more, Nb: 0.003% or more, V: 0.003% or more, Cr: 0.003% or more, Mo: 0.00. It is preferable to satisfy at least one of 003% or more, Cu: 0.003% or more, and Ni: 0.003% or more.

(Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: one or more selected from the group consisting of 0.01% or less)
All of these elements contribute to inclusion control, in particular, fine dispersion of inclusions, and have an effect of increasing low temperature toughness. Therefore, you may contain 1 type, or 2 or more types of these elements. However, when the content exceeds 0.01%, deterioration of the surface properties may become obvious. Therefore, the content of each element is as described above. In addition, in order to acquire the effect by the said action more reliably, it is preferable that content of at least 1 sort (s) of these elements shall be 0.0003% or more. Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

(B: 0.01% or less)
B is an element having an effect of increasing low temperature toughness. Therefore, B may be contained. However, if the content exceeds 0.01%, the hot workability deteriorates and hot rolling becomes difficult. Therefore, the B content is 0.01% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make B content 0.0003% or more.

(Bi: 0.01% or less)
Bi is an element having an action of suppressing cracking during deformation of the hot-pressed steel sheet member. Therefore, Bi may be included. However, when Bi is contained in an amount exceeding 0.01%, hot workability deteriorates and hot rolling becomes difficult. Therefore, the Bi content is 0.01% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable that Bi content shall be 0.0003% or more.

2. Next, the steel structure of the hot-formed steel sheet member according to the present invention will be described.

(Austenite area ratio: 10% to 40%)
By including an appropriate amount of austenite in the steel, the ductility of the hot-pressed steel sheet member is significantly improved. If the area ratio of austenite is less than 10%, it is difficult to ensure excellent ductility. Therefore, the area ratio of austenite is 10% or more. If the area ratio of austenite is 18% or more, the elongation is 21% or more, and extremely excellent ductility is exhibited. Therefore, the area ratio of austenite is preferably 18% or more. On the other hand, if the area ratio of austenite exceeds 40%, delayed fracture tends to occur. Therefore, the area ratio of austenite is 40% or less.

  A method for measuring the area ratio of austenite is well known to those skilled in the art, and can also be measured by a conventional method in the present invention. As will be shown later in the examples, the area ratio is obtained by X-ray diffraction.

(Other organizations)
As a structure other than those described above, one or more of ferrite, martensite, bainite, cementite, and pearlite may be contained.

(Distribution of austenite and martensite: steel structure in which austenite and martensite as a whole are present at 1.0 pieces / μm ² or more)
By making the hard structure fine, that is, by increasing the number density of austenite and martensite, it is possible to improve the impact characteristics of the hot pressed steel sheet member. In order to achieve excellent impact properties with a tensile strength of 900 MPa or more, a steel structure in which austenite and martensite are present at a total density of 1.0 piece / μm ² or more is used.

3. Manufacturing Method Next, a preferable manufacturing method of the hot-formed steel sheet member according to the present invention having the above characteristics will be described.

In order to ensure excellent ductility and impact properties at a tensile strength of 900 MPa or more, the structure after quenching contains austenite of 10 area% or more and 40 area% or less, and austenite and martensite as a whole. It is important to have a steel structure that exists at a density of 1.0 piece / μm ² or more.

In order to obtain such a steel structure, the base material has the above chemical composition, contains one or two selected from bainite and martensite, and has a steel structure in which the average grain size of prior austenite is 20 μm or less. A steel plate (also referred to as a hot-pressed steel plate) is hot-pressed after being held in a temperature range of 670 ° C. or more and less than 780 ° C. and less than Ac ₃ points for 2 minutes or more and 20 minutes or less, and from 600 ° C. to 150 ° C. at 5 ° C. Cooling at an average cooling rate of not less than 500 ° C / sec.

(Steel structure of hot-press steel sheet)
The steel sheet for hot pressing to be subjected to hot pressing has the above chemical composition and contains one or two selected from bainite and martensite, and the average grain size of prior austenite is 20 μm or less. A steel sheet having a steel structure, for example, a hot-rolled steel sheet, a cold-rolled steel sheet, a hot-dip galvanized cold-rolled steel sheet, or an alloyed hot-dip galvanized cold-rolled steel sheet is used. By hot pressing the steel sheet for hot pressing having the above steel structure under the heat treatment conditions described later, it has a desired steel structure, a tensile strength of 900 MPa or more, and excellent ductility and impact properties. A hot pressed steel sheet member is obtained.

  If the total area ratio of bainite and martensite in the steel sheet for hot pressing is 70% or more, the formation of the structure in the heating process during hot pressing is promoted, and the strength after quenching is stably secured. It becomes easy. Accordingly, the total area ratio of bainite and martensite in the steel sheet for hot pressing is preferably 70% or more.

Moreover, the hot-rolled steel sheet having the steel structure can be manufactured by subjecting it to finish rolling in a temperature range of 900 ° C. or less, and rapidly cooling to a temperature range of 600 ° C. or less at a cooling rate of 5 ° C./second or more. In addition, if the average grain size of the prior austenite is 20 μm or less, a steel sheet that is cold-rolled from the hot-rolled steel sheet may be used. On the other hand, a cold-rolled steel sheet, a hot-dip galvanized cold-rolled steel sheet, and an alloyed hot-dip galvanized cold-rolled steel sheet, for example, anneal the hot-rolled steel sheet at Ac ₃ points or more and 600 ° C at an average cooling rate of 5 ° C / second or more. It can be manufactured by quenching to the following temperature range.

(Heating of the steel sheet for hot pressing: 670 ° C. or more and less than 780 ° C. and Ac held at a temperature range of less than ₃ points for 2 minutes or more and 20 minutes or less)
Heating of the steel sheet used for hot pressing is maintained at a temperature range of 670 ° C. or more and less than 780 ° C. and less than ₃ points (° C.) of Ac, which becomes an austenite single phase defined by the following empirical formula (i), for 2 minutes or more and 20 minutes or less. To do.

Ac ₃ = 910−203 × (C ^0.5 ) −15.2 × Ni + 44.7 × Si
+ 104 × V + 31.5 × Mo-30 × Mn-11 × Cr
−20 × Cu + 700 × P + 400 × Al + 50 × Ti (i)
Here, the element symbol in the above formula indicates the content (unit: mass%) of each element in the chemical composition of the steel sheet.

When the holding temperature is less than 670 ° C., when a large amount of Si is contained, the area ratio of austenite before hot pressing becomes excessive, and the dimensional accuracy of the hot pressed steel sheet member is significantly deteriorated. Accordingly, the holding temperature is 670 ° C. or higher. On the other hand, when the holding temperature is 780 ° C. or higher or Ac ₃ points or higher, sufficient austenite is not contained in the structure after quenching, not only deterioration of ductility becomes remarkable, but also the hard structure does not exist finely. Therefore, the impact characteristics are also deteriorated. Accordingly, the holding temperature is less than 780 ° C. and less than Ac ₃ points. Further, if the holding time is less than 2 minutes, it is difficult to stably secure the strength after quenching. Accordingly, the holding time is 2 minutes or more. On the other hand, if the holding time exceeds 20 minutes, not only the productivity is lowered, but also the surface properties deteriorate due to the generation of scales and zinc-based oxides. Accordingly, the holding time is 20 minutes or less.

At this time, the heating rate to a temperature range of 670 ° C. or more and less than 780 ° C. and less than Ac ₃ point is not particularly limited, but the average heating rate may be 0.2 ° C./sec or more and 100 ° C./sec or less. preferable. By setting the average heating rate to 0.2 ° C./second or more, higher productivity can be secured. In addition, when the average heating rate is 100 ° C./second or less, the heating temperature can be easily controlled in the case of heating using a normal furnace. If high-frequency heating or the like is used, the heating temperature can be accurately controlled even if heating is performed at a heating rate exceeding 100 ° C./second.

(Average cooling rate from 600 ° C to 150 ° C: 5 ° C / second or more and 500 ° C / second or less)
Cooling in a temperature range of 150 ° C. or more and 600 ° C. or less is performed so that diffusion type transformation does not occur. If the average cooling rate in the said temperature range is less than 5 degree-C / sec, a soft ferrite and pearlite will produce | generate excessively and it will become difficult to ensure the tensile strength of 900 Mpa or more by the intensity | strength after hardening. Therefore, the average cooling rate in the temperature range is set to 5 ° C./second or more. On the other hand, it is difficult for ordinary equipment to set the average cooling rate in the above temperature range to more than 500 ° C./second. Therefore, the average cooling rate in the temperature range is set to 500 ° C./second or less. Preferably it is 200 degrees C / sec or less.

  In addition, when the Mn content of the steel sheet for hot pressing is 1.2% or more and less than 2.4%, in order to increase the ductility of the hot pressed steel sheet member, it is slowly cooled from 500 ° C. to 150 ° C., specifically Specifically, it is necessary to cool from 500 ° C. to 150 ° C. at an average cooling rate of 5 ° C./second or more and 20 ° C./second or less. Specifically, it is preferable to control the cooling rate as described below. .

  In the hot pressing method, cooling is usually achieved by a steel mold at room temperature or about several tens of degrees Celsius. Therefore, in order to change the cooling rate, the heat capacity may be changed by changing the die size. If the mold dimensions cannot be changed, the cooling rate can also be changed by changing the amount of cooling water using a water-cooled mold. Cooling can also be achieved by using a mold with several grooves cut in advance and flowing a cooling medium (water or gas) into the grooves during pressing, or raising the press machine during pressing and flowing gas between them. You can change the speed. Furthermore, the cooling rate can also be changed by changing the mold clearance and changing the contact area with the steel plate. For example, the following means can be considered as means for changing the cooling rate around 500 ° C.

(1) Immediately after reaching 500 ° C., it is moved to a mold having a different heat capacity or a mold heated to over 100 ° C. to change the cooling rate;
(2) In the case of a water-cooled mold, the cooling rate is changed by changing the amount of flowing water immediately after reaching 500 ° C .;
(3) Immediately after reaching 500 ° C., the press is raised, the gas is flowed, and the flow rate is changed to change the cooling rate.

  The form of molding in the hot press method in the present invention is not particularly limited, and examples thereof include bending, drawing, stretch forming, hole expansion molding, and flange molding. What is necessary is just to select suitably by the kind and shape of the target hot press steel plate member. Representative examples of hot-pressed steel sheet members include door guard bars and bumper reinforcements that are reinforcing parts for automobiles. For example, when the hot-pressed steel plate member is a bumper reinforcement, an alloyed hot-dip galvanized steel plate having a predetermined length is prepared, and a process such as bending is sequentially performed in the die.

  In the above description, hot forming has been described by taking an example of hot pressing, which is a specific embodiment, but the present invention includes a means for cooling a steel plate at the same time as or immediately after forming, as in hot pressing. It can also be applied to hot forming such as roll forming.

The product according to the present invention is characterized by excellent ductility and impact properties, and as the ductility at that time, the total elongation of the tensile test is preferably 15% or more. More preferably, the total elongation of the tensile test is 18% or more. Most preferably, the total elongation of the tensile test is 21% or more. On the other hand, as impact characteristics, it is preferable that the impact value of the Charpy test at 0 ° C. is 20 J / cm ² or more.

  After hot pressing, shot blasting is usually performed for scale removal purposes. This shot blasting has the effect of introducing a compressive stress on the surface, so that delayed fracture is suppressed and the fatigue strength is improved.

Examples of the present invention will be described below.
A steel plate having a chemical composition shown in Table 1 and a steel thickness and steel structure shown in Table 2 was used as a base steel plate.

These base steel sheets are manufactured by hot rolling a slab melted in a laboratory (referred to as a hot rolled steel sheet in Table 2), or by cold rolling and recrystallization annealing of a hot rolled steel sheet. Steel plate (denoted as cold-rolled steel plate in Table 2). In addition, by using a plating simulator, some steel plates were subjected to hot dip galvanizing treatment (plating adhesion amount per side was 60 g / m ² ), alloying hot dip galvanizing treatment (Fe content in the plating film was 15 mass). %). In Table 2, each is described as a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet. Moreover, the steel plate of cold rolling (it describes with full hard in Table 2) was also used.

  These steel plates were cut into dimensions of a width of 100 mm and a length of 200 mm, and heated and cooled under the conditions shown in Table 3. In addition, a thermocouple was attached to the steel plate, and the cooling rate was also measured. The average heating rate in Table 3 is a temperature range from room temperature to 670 ° C., a holding time is a temperature range above 670 ° C., and an average cooling rate is a value for a temperature range from 600 ° C. to 150 ° C. and from 500 ° C. to 150 ° C. . Metallographic observation, X-ray diffraction measurement, tensile test, and Charpy test were performed on the steel sheets obtained under various production conditions.

  The steel plate member produced in this example is not hot-pressed by a mold, but receives the same thermal history as the hot-pressed steel plate member, so the mechanical properties of the steel plate are the heat having the same thermal history. It is substantially the same as the pressed steel sheet member.

(Structure of the base steel sheet)
Test pieces were taken from the direction perpendicular to the rolling direction and the rolling direction of the steel sheet rolling direction cross-section, observed with an electron microscope of tissue perpendicular cross section to the rolling direction, photographed, a total of 0.01 mm ² region The steel structure was identified, and the total area ratio of bainite and martensite and the prior austenite average particle diameter were measured.

(Distribution of austenite and martensite in heat-treated steel sheet)
Test pieces were taken from the direction perpendicular to the rolling direction and the rolling direction of each steel sheet was heat-treated, the rolling direction cross-section, observed with an electron microscope of tissue perpendicular cross section to the rolling direction, photographed, a total of 800 [mu] m ² area The density of austenite and martensite was investigated.

(Area ratio of retained austenite of heat-treated steel sheet)
A test piece having a width of 25 mm and a length of 25 mm was cut out from each heat-treated steel sheet, this test piece was subjected to chemical polishing to reduce the thickness by 0.3 mm, and X-ray diffraction was performed three times on the surface of the test piece after chemical polishing. The obtained profile was analyzed, and a value obtained by averaging the area ratio of retained austenite was calculated.

(Tensile test)
From each heat-treated steel plate, a JIS No. 5 tensile test piece was sampled so that the load axis was perpendicular to the rolling direction, and TS (tensile strength) and EL (total elongation) were measured.

(Impact characteristics)
The heat-treated steel sheet was machined to have a thickness of 1.2 mm to produce a V-notch test piece. Four test pieces were stacked and screwed, and then subjected to a Charpy impact test. The direction of the V notch was the rolling direction. The impact characteristics were good when the impact value at 0 ° C. was 20 J / cm ² or more. The case where it did not reach it was regarded as defective.

(Explanation of test results)
Table 4 shows the results of tests simulating these hot presses.

  In addition, the numerical value underlined in Tables 1-4 has shown that content, conditions, or a mechanical characteristic shown by the numerical value is outside the range of this invention.

  In Table 4, the test sample No. 1-3, 7-9, 11, 13, 15, 17, 19, 21, and 22 have high tensile strength of 900 MPa or more and excellent ductility and impact characteristics.

  On the other hand, the test material No. For No. 4, the manufacturing conditions were outside the range specified in the invention, and the target tensile strength was not obtained. No. In Nos. 5 and 18, the steel structure of the base steel sheet, which is a hot-press steel sheet, deviated from the range specified in the invention, and the desired structure could not be obtained. Sample No. In Nos. 6 and 20, the chemical composition was outside the range specified in the invention, and the target tensile strength was not obtained. Specimen No. 10 had poor ductility because the manufacturing conditions were outside the range specified in the invention and the desired structure could not be obtained. Since the test materials No. 12 and No. 14 were out of the range specified in the invention and the desired structure could not be obtained, the ductility and impact properties were poor. Sample No. In No. 16, the chemical composition was outside the range specified in the invention, and the impact characteristics were poor.

  Further, among the steel plates of the present invention examples, the test material No. 1-3, 7-9, 11, 13, 15, 17, 19 and 21 have a preferable Si content, and the ductility is even better. Among them, the test material No. 2, 8, 11, 17, 19, and 21 are in a preferable range of the area ratio of austenite, and the ductility is very good.

Claims (7)

In mass%, C: 0.05% to 0.40%, Si: 0.5% to 3.0%, Mn: 1.2% to 8.0%, P: 0.05% or less , S: 0.01% or less, sol. Al: 0.001% or more and 3.0% or less and N: 0.01% or less, having a chemical composition composed of the balance Fe and impurities, and containing 10% or more and 40% or less austenite A hot-formed steel sheet member having a steel structure in which austenite and martensite as a whole are present at a density of 1.0 piece / μm ² or more and having a mechanical property of a tensile strength of 900 MPa or more.   Instead of part of Fe, the chemical composition is in mass%, Ti: 1.0% or less, Nb: 1.0% or less, V: 1.0% or less, Cr: 1.0% or less, Mo 2. The heat according to claim 1, comprising one or more selected from the group consisting of: 1.0% or less, Cu: 1.0% or less, and Ni: 1.0% or less. Inter-formed steel plate member.   The chemical composition is, in place of a part of Fe, in mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less. The hot-formed steel sheet member according to claim 1 or 2, comprising one or more selected from the group.   The hot forming according to any one of claims 1 to 3, wherein the chemical composition contains B: 0.01% or less in mass% instead of a part of Fe. Steel plate member.   The hot forming according to any one of claims 1 to 4, wherein the chemical composition contains Bi: 0.01% or less in mass% instead of part of Fe. Steel plate member. The Mn content is 2.4% by mass or more, the chemical composition according to any one of claims 1 to 5, comprising one or two selected from bainite and martensite, A steel sheet having a steel structure having an average grain size of prior austenite of 20 μm or less is maintained in a temperature range of 670 ° C. or more and less than 780 ° C. and less than ₃ points of Ac for 2 minutes or more and 20 minutes or less, and heated in this manner. A method for producing a hot-formed steel sheet member is characterized in that hot forming is performed and then cooling from 600 ° C. to 150 ° C. is performed at an average cooling rate of 5 ° C./second or more and 500 ° C./second or less. The Mn content is less than 2.4% by mass, the chemical composition according to any one of claims 1 to 5, comprising one or two selected from bainite and martensite, A steel sheet having a steel structure having an average grain size of prior austenite of 20 μm or less is maintained in a temperature range of 670 ° C. or more and less than 780 ° C. and less than ₃ points of Ac for 2 minutes or more and 20 minutes or less, and heated in this manner. Next, an average cooling rate from 5 ° C./second to 500 ° C./second from 600 ° C. to 150 ° C., and from 5 ° C./second to 20 ° C./second from 500 ° C. to 150 ° C. The manufacturing method of the hot-formed steel plate member characterized by cooling with the average cooling rate of.