JP2010065293A - Hot press member having excellent ductility, steel sheet for the hot press member, and method for producing the hot press member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot press member having excellent ductility which has TS of 1,470 to 1,750 MPa and El of ≥8%, to provide a steel sheet for the hot press member, and to provide a method for producing the hot press member. <P>SOLUTION: The hot press member having excellent ductility has a composition comprising, by mass, 0.20 to 0.40% C, 0.01 to 3.0% Si, 1.0 to 4.0% Mn, ≤0.05% P, ≤0.05% S, 0.005 to 0.1% Al and ≤0.01% N, and the balance Fe with inevitable impurities, and has a microstructure where the area ratio of a ferritic phase occupied in the whole structure is 5 to 55%, the area ratio of a martensitic phase occupied in the whole structure is 45 to 95%, and also, the average grain size of the ferritic phase and the martensitic phase is ≤7 μm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is a hot press member that is processed at the same time as a steel plate heated in a die and punch mold and is rapidly cooled to increase the strength, in particular, a tensile strength TS of 1470 to 1750 MPa and a total elongation of 8% or more. The present invention relates to a hot press member having excellent ductility having El, a steel plate for the hot press member, and a method for manufacturing the hot press member.

  Conventionally, structural members used in automobiles and the like have been manufactured by pressing a steel plate having a desired strength. In recent years, based on the demand for weight reduction of automobile bodies, as a steel plate as a material, for example, a high-strength steel plate with a thickness of about 1.0 to 4.0 mm is desired, but the higher the strength of the steel plate, the more The properties deteriorate and it becomes difficult to process the steel sheet into a desired member shape.

  Therefore, as described in Patent Document 1, a method of manufacturing a structural member called a hot press (also referred to as die quenching) that simultaneously heats a steel sheet heated in a mold and rapidly increases the strength is drawing attention. Some parts that require TS of 1.0 to 1.5 GPa have been put to practical use. In this method, the steel plate is processed at a high temperature of around 950 ° C, so the problem of workability in cold pressing is reduced. In addition, because the material is quenched by a water-cooled mold, the material is made to have high strength using a transformation structure. There is an advantage that the amount of alloying elements added to the steel sheet as the material can be reduced.

  On the other hand, some structural members used in automobiles, such as door guard bars and side members, require high ductility from the viewpoint of ensuring safety in the event of a car collision. However, the conventional hot press member described in Patent Document 1 has insufficient ductility and does not satisfy these requirements.

Recently, in Patent Document 2, hot pressing is performed in a temperature range of two phases of ferrite and austenite, and the structure after hot pressing is divided into two phases of 40 to 90% ferrite and 10 to 60% martensite in area ratio. A hot-pressed member excellent in ductility has been proposed which has a microstructure of TS of 780 to 1180 MPa and El of 10 to 20%.
GB 1490535 JP 2007-16296 A

  However, with the hot press member described in Patent Document 2, only a TS of about 1270 MPa can be obtained, and it is difficult to say that the hot press member has sufficient strength to further reduce the weight of the automobile body.

  The present invention provides a hot-press member excellent in ductility having TS of 1470 to 1750 MPa and El of 8% or more, more preferably 9% or more, a steel sheet for the hot-press member, and a method for producing the hot-press member. For the purpose.

  As a result of intensive studies to achieve the above object, the present inventors have obtained the following knowledge.

  i) By optimizing the composition, the area ratio of the ferrite phase in the entire structure is 5 to 55%, the area ratio of the martensite phase is 45 to 95%, and the average particle diameter of the ferrite phase and the martensite phase By forming a microstructure having a thickness of 7 μm or less, a hot press member having an El of 8% or more, more preferably 9% or more with a TS of 1470 to 1750 MPa can be obtained.

  ii) As a hot-press member steel plate, a hot-rolled steel plate having a microstructure in which the average grain size of old γ grains is 15 μm or less, a cold-rolled steel plate having a microstructure composed of a cold-rolled structure, or It is effective to use a cold-rolled steel sheet having a microstructure with an average particle size of 15 μm or less and hot press in a temperature range in which two phases of ferrite and austenite are formed.

  The present invention was made based on such findings, and in mass%, C: 0.20 to 0.40%, Si: 0.05 to 3.0%, Mn: 1.0 to 4.0%, P: 0.05% or less, S: 0.05% Hereafter, Al: 0.005 to 0.1%, N: 0.01% or less, the balance is composed of Fe and inevitable impurities, the area ratio of the ferrite phase in the entire structure is 5 to 55%, martensite There is provided a hot press member excellent in ductility characterized by having a microstructure in which the area ratio of the phase is 45 to 95% and the average grain size of the ferrite phase and the martensite phase is 7 μm or less.

  The hot press member of the present invention further comprises at least one selected from Ni: 0.01 to 5.0%, Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, Mo: 0.01 to 3.0% by mass%. Seeds can be included. Furthermore, at least 1 type selected from Ti: 0.005-3.0%, Nb: 0.005-3.0%, V: 0.005-3.0%, W: 0.005-3.0%, and B: 0.0005-0.05. %, REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, preferably at least one selected individually or simultaneously.

  The present invention also includes a hot-rolled steel sheet having the above-described composition as a hot-pressed steel sheet and having a microstructure in which the average grain size of old γ grains is 15 μm or less, and a microstructure composed of a cold-rolled structure. The present invention provides a cold rolled steel sheet or a cold rolled steel sheet having a microstructure with an average grain size of 15 μm or less.

The hot-press member of the present invention is the hot-press member steel sheet of the present invention, heated at a heating rate of 10 ° C./second or more, at a temperature T ° C. within the range of the following formula (1) for 1 to 600 seconds. After holding, it can be produced by a method of hot pressing in a temperature range of 550 ° C. or higher.
{(0.35475-C) (Ac3-Ac1) +0.45 (0.77-C) Ac1} / {0.45 (0.77-C)} ≦ T ≦
{(0.73225-C) (Ac3-Ac1) +0.95 (0.77-C) Ac1} / {0.95 (0.77-C)} ... (1)
However,
Ac1 = 750.8-26.6C + 17.6Si-11.6Mn-23.0Ni + 24.1Cr-22.9Cu + 22.5Mo-39.7V-5.7Ti + 232.6Nb-169.4Al-894.7B,
Ac3 = 881-206C + 53Si-15Mn-20Ni-1Cr-27Cu + 41Mo,
The element symbol in the formula represents the content (% by mass) of each element.

  At this time, during hot pressing, hold the punch at the bottom dead center for 1 to 60 seconds, cool the member at a cooling rate of 3 to 400 ° C. / second, after hot pressing, remove the member from the mold, Cooling with liquid or gas is preferred.

  According to the present invention, a hot press member having excellent ductility having TS of 1470 to 1750 MPa and El of 8% or more, more preferably 9% or more can be produced. The hot press member of the present invention is suitable for a structural member for ensuring safety at the time of a collision such as a door guard bar or a side member of an automobile.

  The present invention will be specifically described below. Note that “%” in relation to the composition means “% by mass” unless otherwise specified.

1) Hot press material
1-1) Composition
C: 0.20 ~ 0.40%
C is an element that improves the strength of steel. In order to increase the TS of a hot-pressed member to 1470 MPa or more, the amount needs to be 0.20% or more. On the other hand, when the C content exceeds 0.40%, it becomes difficult to set TS to 1750 MPa or less. Therefore, the C content is 0.20 to 0.40%, preferably 0.26 to 0.35%.

Si: 0.05-3.0%
Si, like C, is an element that improves the strength of steel, and is also a stabilizing element for the ferrite phase. Enables time savings. In addition, the Ac3 transformation point is raised and the temperature range that becomes a two-phase region during heating before hot pressing is expanded, so the manufacturing conditions in hot pressing are relaxed and stable TS and El of hot pressing materials can be secured. . In order to realize such an effect, the Si amount needs to be 0.05% or more, desirably 0.20% or more, and more desirably 0.50% or more. On the other hand, if the Si content exceeds 3.0%, the occurrence of surface defects called red scales during hot rolling is remarkably increased, the rolling load is increased, and the ductility of the hot-rolled steel sheet is deteriorated. From the above, the Si content is 0.05 to 3.0%. In addition, when applying a plating treatment to form a plating film mainly composed of Zn or Al on the surface of the steel sheet, if the Si content is too large, the plating processability may be adversely affected. % Or less is preferable.

Mn: 1.0-4.0%
Mn is an element effective for improving the hardenability. In order to increase the TS of the hot press member to 1470 MPa or more, the amount needs to be 1.0% or more. On the other hand, if the amount of Mn exceeds 4.0%, segregation occurs and the uniformity of the characteristics of the raw steel plate and hot press member decreases. Therefore, the Mn content is 1.0 to 4.0%.

P: 0.05% or less
When the amount of P exceeds 0.05%, segregation causes a reduction in the uniformity of the properties of the raw steel plate and hot press member, and a significant reduction in toughness. Therefore, the P content is 0.05% or less. In addition, since excessive de-P treatment causes high costs, the P amount is preferably 0.001% or more.

S: 0.05% or less
When the amount of S exceeds 0.05%, the toughness of the hot pressed member is lowered. Therefore, the S content is 0.05% or less.

Al: 0.005-0.1%
Al is added as a deoxidizer for steel. In order to obtain such an effect, the Al content needs to be 0.005% or more. On the other hand, when the Al content exceeds 0.1%, blanking workability and hardenability of the steel plate as a raw material are lowered. Therefore, the Al content is 0.005 to 0.1%.

N: 0.01% or less
When the N content exceeds 0.01%, nitride of AlN is formed during hot rolling or heating before hot pressing, and the blanking workability and hardenability of the raw steel sheet are reduced. Therefore, the N content is 0.01% or less.

  The balance is Fe and inevitable impurities, but at least selected from Ni: 0.01-5.0%, Cu: 0.01-5.0%, Cr: 0.01-5.0%, Mo: 0.01-3.0% for the following reasons 1 type, Ti: 0.005-3.0%, Nb: 0.005-3.0%, V: 0.005-3.0%, W: 0.005-3.0%, B: 0.0005-0.05%, It is preferable to contain at least one selected from REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and Mg: 0.0005 to 0.01% individually or simultaneously.

Ni: 0.01-5.0%
Ni is an element effective for strengthening steel and improving hardenability. In order to exhibit such an effect, the Ni content is preferably 0.01% or more. On the other hand, if the Ni content exceeds 5.0%, the cost is significantly increased. Therefore, the upper limit is preferably set to 5.0%.

Cu: 0.01-5.0%
Cu, like Ni, is an element effective for strengthening steel and improving hardenability. In order to exhibit such an effect, the Cu content is preferably 0.01% or more. On the other hand, if the amount of Cu exceeds 5.0%, the cost is significantly increased, so the upper limit is preferably 5.0%.

Cr: 0.01-5.0%
Cr, like Cu and Ni, is an element effective for strengthening steel and improving hardenability. In order to exhibit such an effect, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content exceeds 5.0%, the cost is significantly increased. Therefore, the upper limit is preferably set to 5.0%.

Mo: 0.01-3.0%
Mo, like Cu, Ni and Cr, is an element effective for strengthening steel and improving hardenability. Moreover, it has the effect of suppressing the growth of crystal grains and improving toughness by making the grains fine. In order to exhibit such effects, the Mo content is preferably 0.01% or more. On the other hand, if the amount of Mo exceeds 3.0%, the cost is significantly increased. Therefore, the upper limit is preferably set to 3.0%.

Ti: 0.005-3.0%
Ti is an element effective for strengthening steel and improving toughness by refining. In addition, it is an element effective for forming a nitride in preference to B, which will be described below, and exhibiting the effect of improving the hardenability by solid solution B. In order to exhibit such an effect, the Ti content is preferably 0.005% or more. On the other hand, if the amount of Ti exceeds 3.0%, the rolling load during hot rolling is extremely increased, and the toughness of the hot pressed member is lowered. Therefore, the upper limit is preferably set to 3.0%.

Nb: 0.005-3.0%
Nb, like Ti, is an element effective for strengthening steel and improving toughness by refining. In order to exhibit such an effect, the Nb content is preferably 0.005% or more. On the other hand, if the amount of Nb exceeds 3.0%, the precipitation of carbonitride increases and the ductility and delayed fracture resistance decrease, so the upper limit is preferably made 3.0%.

V: 0.005-3.0%
V, like Ti and Nb, is an element effective for strengthening steel and improving toughness by refining. Moreover, it precipitates as a precipitate etc., becomes a hydrogen trap site, and improves hydrogen embrittlement resistance. In order to exhibit such an effect, the V amount is preferably 0.005% or more. On the other hand, if the amount of V exceeds 3.0%, the precipitation of carbonitrides becomes remarkable and the ductility is remarkably lowered. Therefore, the upper limit is preferably set to 3.0%.

W: 0.005-3.0%
W, like V, is an element effective for strengthening steel, improving toughness, and improving hydrogen embrittlement resistance. In order to achieve such an effect, the W content is preferably 0.005% or more. On the other hand, if the W content exceeds 3.0%, the ductility is remarkably lowered, so the upper limit is preferably made 3.0%.

B: 0.0005-0.05%
B is an element effective for improving the hardenability during hot pressing and toughness after hot pressing. In order to exhibit such an effect, the B content is preferably 0.0005% or more. On the other hand, if the amount of B exceeds 0.05%, the rolling load during hot rolling is extremely increased, and the martensite phase and bainite phase are generated after hot rolling, causing cracks in the steel sheet, so the upper limit is It is preferably 0.05%.

REM: 0.0005-0.01%, Ca: 0.0005-0.01%
REM and Ca are effective elements for controlling the morphology of inclusions, and contribute to improving ductility and hydrogen embrittlement resistance. In order to exhibit such an effect, the amount of REM or Ca is preferably 0.0005% or more. On the other hand, if the amount of REM or Ca exceeds 0.01%, the hot workability deteriorates, so the upper limit is preferably made 0.01%.

Mg: 0.0005-0.01%
Mg is also an element effective for controlling the form of inclusions, and improves ductility, and forms composite precipitates and composite crystallized substances with other elements, contributing to the improvement of hydrogen embrittlement resistance. In order to exhibit such an effect, the Mg amount is preferably 0.0005% or more. On the other hand, if the Mg content exceeds 0.01%, coarse oxides and sulfides are produced and the ductility is lowered, so the upper limit is preferably made 0.01%.

1-2) Microstructure
To secure TS of 1470 to 1750 MPa and El of 8% or more, more preferably 9% or more, the area ratio of the ferrite phase in the entire structure is 5 to 55%, and the area ratio of the martensite phase is 45 to 95. And a microstructure in which the average particle size of the ferrite phase and the martensite phase is 7 μm or less is necessary. When the ferrite phase area ratio exceeds 55%, that is, when the martensite phase area ratio is less than 45%, a TS of 1470 MPa or more cannot be secured, and the ferrite phase area ratio is less than 5%, that is, the martensite phase area ratio. If it exceeds 95%, it is difficult to secure El of 8% or more. In particular, when the average particle size of the ferrite phase and the martensite phase is 7 μm or less, a TS of 1470 MPa or more is reliably achieved. More preferably, the average particle size of the ferrite phase and the martensite phase is 5 μm or less.

  In addition to the ferrite phase and martensite phase, the effect of the present invention can be achieved even if at least one of the bainite phase, retained austenite phase, cementite phase, and pearlite phase is contained in an area ratio of 10% or less. Will not be damaged.

  Here, the prior austenite average particle diameter of the hot-rolled steel sheet, the average particle diameter of the cold-rolled steel sheet, and the average particle diameter of the ferrite phase and martensite phase of the member in the present application were measured according to JIS G 0551 (2005). In particular, the member often has a mixed structure of a ferrite phase and a martensite phase, but the average particle size was calculated by determining the overall average particle size without distinguishing these two phases. As for the phase fraction of the ferrite phase and martensite phase (or other second phase), the structure photograph used for the particle size measurement is taken into a measuring instrument, and the ferrite phase and martensite phase or Furthermore, the area ratio of the other part was calculated | required.

2) Hot-press member steel sheet A hot-press member steel sheet is a hot-rolled steel sheet having the above-mentioned hot-press member composition and a microstructure in which the average grain size of old γ grains is 15 μm or less, cold rolling. Cold-rolled steel sheet having a microstructure consisting of a microstructure, or average grain size (average grain diameter of ferrite phase, or ferrite grain and second phase if the second phase is included) is 15 μm or less A cold-rolled steel sheet having a microstructure that is: This is a hot-rolled steel sheet with an average grain size of old γ grains of 15 μm or less, a cold-rolled steel sheet having a cold-rolled structure, or a cold-rolled steel sheet with an average grain size of 15 μm or less, and ferrite + austenite. This is because the average particle size of the ferrite phase and the martensite phase of the hot-pressed member can be reduced to 7 μm or less by heating to a temperature range to be a phase and hot pressing, and a TS of 1470 MPa or more can be reliably obtained. . In many cases, cold-rolled steel sheets have a cementite phase with an area ratio of 6.0% or less precipitated in the ferrite phase, but when determining the average grain size, ignore the cementite phase and focus on the ferrite phase. And the average particle size was calculated. In addition, in some cold-rolled steel sheets (thicknesses exceeding the Ac1 annealing temperature), the second phase is mixed in addition to the ferrite phase. The second phase is a martensite phase, a bainite phase, or a phase in which both are mixed during cooling during annealing. In this case, when determining the average particle size, the average particle size of the entire ferrite grains and second phase grains (corresponding to old γ grains) was determined.

  A hot-rolled steel sheet having a microstructure in which the average grain size of old γ grains is 15 μm or less, a cold-rolled steel sheet having a microstructure composed of a cold-rolled structure, or a cold-rolled steel sheet having an average grain diameter of 15 μm or less. Although the reason why the average particle size of the ferrite phase and martensite phase of the hot-pressed member can be reduced to 7 μm or less by using it is not necessarily clear, it becomes a two-phase of ferrite and austenite during heating before hot pressing. When changing, grain boundaries and strain accumulation parts work as nucleation sites, so reducing the grain size of the original steel sheet or introducing strain by cold rolling increases the number of nucleation sites. It is considered that the structure after hot pressing becomes finer. In particular, when the heating rate is increased, nucleation sites that are difficult to work can be used effectively, so that further refinement can be achieved.

  Here, the hot-rolled steel sheet having a microstructure in which the average grain size of old γ grains is 15 μm or less is, for example, as close to Ar3 as possible, with the finish rolling entry side temperature set to 1050 ° C. or less and the finish rolling temperature set to Ar3 to Ar3 + 30 ° C. It is possible to manufacture by controlling the cooling conditions and the winding temperature as usual. A steel sheet that has been cold-rolled into a microstructure composed of a cold-rolled structure can be manufactured by cold-rolling a hot-rolled steel sheet manufactured under normal hot-rolling conditions. The reduction ratio during cold rolling (also referred to as the cold reduction ratio) is preferably 40% or more, and more preferably 60% or more, in order to make the ferrite phase and martensite phase of the hot pressed member finer. The cold pressure ratio is preferably 85% or less because productivity decreases when the cold pressure ratio becomes too large. Cold-rolled steel sheets with an average grain size of ferrite phase (the average grain size of ferrite phase and second phase when the second phase is included) of 15 μm or less are, for example, as cold-rolled with a cold pressure ratio of 50% or more. It is relatively easy to manufacture by using a steel plate and setting the annealing temperature as low as Ac1-50 ° C or lower in a continuous annealing line. Manufacture is possible even if the temperature is set to Ac1 to Ac1-50 ° C, which is higher than the annealing temperature, but in that case, restrictions such as increasing the cold pressure ratio before annealing to about 65% or more are required. Become. Further, even if the annealing temperature is raised to a level exceeding Ac1, even if it exceeds Ac1, it can be produced, but the restriction becomes more severe, such as a cold pressure ratio of about 75% or more. In this case, the structure of the cold-rolled steel sheet after annealing includes the second phase depending on the degree exceeding Ac1. If it exceeds Ac1 too much, the second phase becomes too much and becomes hard, which is disadvantageous for the handling of the steel sheet. In addition, a plating film mainly composed of Zn or Al can be formed on the surface of these steel plates. A normal method can be applied to the formation of a plating film mainly composed of Zn or Al. The plating film mainly composed of Zn is a Zn-based plating film containing Al: 0.001 to 0.5% and Fe: 0.001 to 20%, and may contain Si, Mn, Cr, and Ni. Moreover, the plating film mainly composed of Al is an Al-based plating film containing Si: 1 to 15% and Mg: 0.5 to 10%, and Zn: 1 to 60% can also be added. In this way, by using a plated steel sheet mainly composed of Zn or Al, generation of scale can be suppressed during heating or hot pressing, and there is no need to provide a scale removal process such as shot blasting, thereby improving productivity. .

3) Hot press conditions The hot press member of the present invention is the above-described hot-press member steel plate heated at a heating rate of 10 ° C./second or more, more preferably 100 ° C./second or more. After being held at a temperature T within the range of 1 to 600 seconds, it can be produced by a method of hot pressing in a temperature range of 550 ° C. or higher.

  The reason for setting the heating rate to 10 ° C / second or more is that if it is slower than 10 ° C / second, the productivity will decrease and the grain size will not be reduced during heating, and a TS of 1470 MPa or more will not be obtained. is there. In order to make the structure of the member finer, it is preferable that the heating rate is higher, and therefore, more preferably 100 ° C./second or more.

  The heating temperature was set to the temperature T within the range of the above formula (1) because when the temperature falls below the lower limit temperature of the formula (1), the ferrite phase is more than the appropriate amount and the martensite phase is appropriate. If the required TS is not obtained and the upper limit temperature is exceeded, the ferrite phase is less than the appropriate amount and the martensite phase is higher than the appropriate amount. This is because El cannot get. In addition, the above formula (1) is obtained by the inventors from the Fe-C system phase diagram, which is a temperature range in which the area ratio of the ferrite phase and the martensite phase can be within an appropriate range. To be able to withstand enough. In the formula (1), Ac1 and Ac3 represent the Ac1 transformation point and Ac3 transformation point, respectively.

  If the retention time is 1 to 600 seconds, if less than 1 second, a sufficient amount of austenite phase will not be generated during heating, so the strength cannot be increased by the martensite phase. This is because the austenite grains become coarse and a TS of 1470 MPa or more cannot be obtained. More preferably, it is 1 to 240 seconds.

  The reason for setting the temperature during hot pressing to 550 ° C. or higher is that if it is lower than 550 ° C., a soft ferrite phase or bainite phase is excessively generated during the cooling process, making it difficult to secure a TS of 1470 MPa or higher.

  During hot pressing, hold the punch at the bottom dead center for 1 to 60 seconds, cool it using a die and punch, or cool the member at a cooling rate of 3 to 400 ° C / second by combining air cooling. Or, after hot pressing, it is preferable from the viewpoint of improving productivity and securing TS of 1470 MPa or more to remove the member from the mold and cool it with liquid or gas.

  Steel plate Nos. A to P having the conditions shown in Table 1 were heated, held, hot pressed, and cooled under the hot press conditions shown in Table 2 to produce hat-shaped hot press members No. 1 to 21. The mold used has a punch width of 70mm, a punch shoulder of R4mm, a die shoulder of R4mm, and a molding depth of 30mm. Heating was performed in air using either an infrared heating furnace or an atmosphere heating furnace depending on the heating rate. Cooling was performed by combining sandwiching between the punch and die of the steel sheet and air cooling on the die released from the sandwiching, and cooling from the press (start) temperature to 150 ° C. At this time, the cooling rate was adjusted by changing the time for holding the punch at the bottom dead center in the range of 1 to 60 seconds. Further, a part of the members (member No. 20) was taken out from the mold immediately after being molded by hot pressing, and forcedly cooled using air. At this time, the cooling rate in these cooling was an average cooling rate from the press temperature to 200 ° C. Steel plate No. D is a galvanized steel plate that has been annealed and hot dip galvanized in the CGL line after cold rolling. Steel plates No. C and D were composed of a ferrite phase and a cementite phase, and the average particle size of the ferrite phase was determined as the average particle size.

  And the JIS No. 5 tensile test piece was extract | collected from the position of the hat bottom part of the produced hot press member, the tensile test was done based on JIS Z 2241, and TS and El were measured. In the processing of tensile test pieces, after finishing by normal machining, the parallel part and R part are polished with # 300 to # 1500 paper, and further buffed with diamond paste, and machined Damage was removed. This is because when TS is at a super-high strength level as in the present application, normal machining alone will cause early breakage from damaged parts (such as small scratches) due to machining during tensile testing, and the original TS and El cannot be evaluated. It is. Moreover, the structure | tissue near the collection position of a tensile test piece was investigated by said method.

  The results are shown in Table 2. Hot press member No. 1, 5-7, 10-17, 19-21 of the present invention is a hot press member with TS of 1470 to 1750 MPa, El of 8% or more, high strength and excellent ductility. I know that there is.

Claims (15)

  In mass%, C: 0.20-0.40%, Si: 0.05-3.0%, Mn: 1.0-4.0%, P: 0.05% or less, S: 0.05% or less, Al: 0.005-0.1%, N: 0.01% or less And the balance is Fe and inevitable impurities, the area ratio of the ferrite phase occupying the entire structure is 5 to 55%, the area ratio of the martensite phase is 45 to 95%, and the ferrite A hot-pressed member excellent in ductility, characterized by having a microstructure in which an average particle size of a phase and a martensite phase is 7 μm or less.   Furthermore, it is characterized by containing at least one selected from Ni: 0.01-5.0%, Cu: 0.01-5.0%, Cr: 0.01-5.0%, Mo: 0.01-3.0% by mass%. 2. A hot press member having excellent ductility according to claim 1.   Furthermore, it is characterized by containing at least one selected from Ti: 0.005-3.0%, Nb: 0.005-3.0%, V: 0.005-3.0%, W: 0.005-3.0% by mass%. 3. A hot press member having excellent ductility according to claim 1.   4. The hot press member with excellent ductility according to claim 1, further comprising B: 0.0005 to 0.05% by mass%.   Furthermore, by mass%, it contains at least one selected from REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and Mg: 0.0005 to 0.01%. 2. A hot press member having excellent ductility according to 1.   In mass%, C: 0.20-0.40%, Si: 0.05-3.0%, Mn: 1.0-4.0%, P: 0.05% or less, S: 0.05% or less, Al: 0.005-0.1%, N: 0.01% or less A hot-pressed member excellent in ductility, characterized in that it is a hot-rolled steel sheet having a microstructure comprising Fe and inevitable impurities in the balance, and having an average grain size of old γ grains of 15 μm or less Steel plate.   In mass%, C: 0.20-0.40%, Si: 0.05-3.0%, Mn: 1.0-4.0%, P: 0.05% or less, S: 0.05% or less, Al: 0.005-0.1%, N: 0.01% or less A steel sheet for hot press members having excellent ductility, characterized in that it is a cold-rolled steel sheet having a composition comprising Fe and inevitable impurities, and having a microstructure comprising a cold-rolled structure.   In mass%, C: 0.20-0.40%, Si: 0.05-3.0%, Mn: 1.0-4.0%, P: 0.05% or less, S: 0.05% or less, Al: 0.005-0.1%, N: 0.01% or less A steel sheet for hot press members having excellent ductility, characterized in that it is a cold-rolled steel sheet having a composition comprising Fe and unavoidable impurities in the balance, and having an average particle size of 15 μm or less.   Furthermore, it is characterized by containing at least one selected from Ni: 0.01-5.0%, Cu: 0.01-5.0%, Cr: 0.01-5.0%, Mo: 0.01-3.0% by mass%. The steel sheet for hot press members excellent in ductility according to any one of claims 6 to 8.   Furthermore, it is characterized by containing at least one selected from Ti: 0.005-3.0%, Nb: 0.005-3.0%, V: 0.005-3.0%, W: 0.005-3.0% by mass%. 10. A steel sheet for a hot press member having excellent ductility according to any one of claims 6 to 9.   The steel sheet for hot press members having excellent ductility according to any one of claims 6 to 10, further comprising B: 0.0005 to 0.05% by mass%.   Furthermore, by mass%, it contains at least one selected from REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and Mg: 0.0005 to 0.01%. 2. A steel sheet for hot press members having excellent ductility according to 1. Heating the steel sheet according to any one of claims 6 to 12 at a heating rate of 10 ° C / second or more, and holding for 1 to 600 seconds at a temperature T ° C within the range of the following formula (1) , A method for producing a hot press member having excellent ductility, characterized by performing hot pressing in a temperature range of 550 ° C. or higher;
{(0.35475-C) (Ac3-Ac1) +0.45 (0.77-C) Ac1} / {0.45 (0.77-C)} ≦ T ≦
{(0.73225-C) (Ac3-Ac1) +0.95 (0.77-C) Ac1} / {0.95 (0.77-C)} ... (1)
However,
Ac1 = 750.8-26.6C + 17.6Si-11.6Mn-23.0Ni + 24.1Cr-22.9Cu + 22.5Mo-39.7V-5.7Ti + 232.6Nb-169.4Al-894.7B,
Ac3 = 881-206C + 53Si-15Mn-20Ni-1Cr-27Cu + 41Mo,
The element symbol in the formula represents the content (% by mass) of each element.   14. The hot duct having excellent ductility according to claim 13, wherein, during hot pressing, the punch is held at a bottom dead center for 1 to 60 seconds, and the member is cooled at a cooling rate of 3 to 400 ° C./second. The manufacturing method of a press member.   14. The method for producing a hot-pressed member with excellent ductility according to claim 13, wherein the member is taken out of the mold after the hot pressing and cooled using a liquid or a gas.