JP2010174281A - 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 and El of about 9.5 to 12%; 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 containing, by mass, 0.22 to 0.29% C, 0.05 to 2.0% Si, 0.5 to 3.0% 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 martensitic phase occupied in the whole structure is 90 to 100%, and also, the average grain size of old austenitic grains is ≤8 μm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is a hot press member that can be heated at the same time as a steel plate heated in a die and punch mold, and at the same time has high strength, in particular, a tensile strength TS of 1470 to 1750 MPa and a total elongation of 9.5% 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 called die quenching) that heats a heated steel sheet in a mold and simultaneously quenches to increase the strength has attracted attention. Some parts that require TS of 1.0 to 1.5 GPa have been put to practical use. In this method, the steel sheet is heated to around 950 ° C. and then processed at a high temperature, so that the problem of workability in cold pressing is reduced, and because it is quenched with a water-cooled mold, the member is made using a transformation structure. There is an advantage that the strength of the steel can be increased and 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 guards and side members, require high ductility from the viewpoint of ensuring safety at the time of automobile 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. Furthermore, when a hot press member having a TS of 1300 MPa or more is manufactured by applying the manufacturing method of Patent Document 2, it is difficult to obtain a stable material because the range of the appropriate pre-hot press heating temperature is narrow. This tendency becomes more prominent as the strength increases, that is, as the C content increases. In particular, when TS is 1470 MPa or more, the material becomes extremely unstable.

  An object of the present invention is to provide a hot press member excellent in ductility having TS of 1470 to 1750 MPa and El of about 9.5 to 12%, a steel plate for the hot press member, and a method for producing the hot press member. . In addition, TS and El of the hot press member here are TS and El of the steel plate constituting the member after hot pressing.

  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 martensite phase in the whole structure is 90 to 100%, and the average grain size of the prior austenite grains is 8 μm or less, so that 1470 to 1750 MPa With this TS, a hot press member having an El of about 9.5 to 12% can be obtained.

ii) As a hot-press member steel plate, a hot-rolled steel plate having a microstructure of a bainite phase mainly having an average grain size of prior austenite grains of 15 μm or less, or cold-rolled having a microstructure comprising a cold-rolled structure. Steel sheet, or a cold rolled steel sheet having a microstructure with an average grain size of 15 μm or less, in the low temperature range of the austenite single phase region, that is, the temperature range of Ac ₃ transformation point to (Ac ₃ transformation point +50) ° C. It is effective to hot press after heating.

  The present invention was made based on such findings, and in mass%, C: 0.22 to 0.29%, Si: 0.05 to 2.0%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.05% Hereinafter, Al: 0.005-0.1%, N: containing 0.01% or less, the balance is composed of Fe and inevitable impurities, the area ratio of the martensite phase in the entire structure is 90-100%, A hot press member having excellent ductility, characterized by having a microstructure in which the average grain size of prior austenite grains is 8 μm or less, is provided.

  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 a composition as described above as a hot-pressed steel sheet and having a bainite-phase microstructure in which the average grain size of prior austenite grains is 15 μm or less, and a cold-rolled structure. Provided is a cold-rolled steel sheet having a microstructure 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 a hot-press member steel sheet of the present invention, which is heated at a heating rate of 10 ° C./second or more, and is in the temperature range of Ac ₃ transformation point to (Ac ₃ transformation point + 50) ° C. After holding for ˜600 seconds, it can be produced by a method of hot pressing in a temperature range of 550 ° C. or higher.

  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 about 9.5 to 12% 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 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: 0.22 ~ 0.29%
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 0.22% or more. On the other hand, if the amount of C exceeds 0.29%, it becomes difficult to set TS to 1750 MPa or less. Therefore, the C content is 0.22 to 0.29%.

Si: 0.05-2.0%
Si, like 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 0.05% or more. On the other hand, when the Si content exceeds 2.0%, the occurrence of surface defects called red scale during hot rolling is remarkably increased, the rolling load is increased, and the ductility of the hot-rolled steel sheet is deteriorated. Furthermore, if the Si content exceeds 2.0%, the plating processability may be adversely affected when a plating process for forming a plating film mainly composed of Zn or Al on the steel sheet surface is performed. Therefore, the Si content is 0.05 to 2.0%.

Mn: 0.5-3.0%
Mn is an element effective for suppressing the ferrite transformation and improving the hardenability, and is also an element effective for lowering the heating temperature before hot pressing because it lowers the Ac ₃ transformation point. . In order to exhibit such an effect, the amount needs to be 0.5% or more. On the other hand, if the amount of Mn exceeds 3.0%, segregation occurs and the uniformity of the characteristics of the raw steel plate and hot-pressed member decreases. Therefore, the Mn content is 0.5 to 3.0%.

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 exhibit 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. It is also an element effective for forming a nitride in preference to B, which will be described next, and exhibiting the effect of improving 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 and a crystallized substance, 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%
REM is an effective element for controlling the shape of inclusions, and contributes to the improvement of ductility and hydrogen embrittlement resistance. In order to exhibit such an effect, the REM content is preferably 0.0005% or more. On the other hand, if the REM amount exceeds 0.01%, the hot workability deteriorates, so the upper limit is preferably made 0.01%.

Ca: 0.0005-0.01%
Ca, like REM, is an element effective for controlling the morphology of inclusions, and contributes to improving ductility and hydrogen embrittlement resistance. In order to exhibit such an effect, the Ca content is preferably 0.0005% or more. On the other hand, if the Ca content 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 1470-1750MPa and El of about 9.5-12%, the area ratio of martensite phase in the whole structure is 90-100%, and the average grain size of prior austenite grains is 8 μm or less It is necessary to have a microstructure. When the area ratio of the martensite phase is less than 90%, TS of 1470 MPa or more cannot be secured, and when the average grain size of the prior austenite grains exceeds 8 μm, it becomes difficult to secure El of 9.5% or more. In particular, when the average grain size of the prior austenite grains is 5 μm or less, El of 11% or more is reliably achieved. For this reason, the martensite phase is 90% or more in area ratio. More preferably, it is 96% or more, and may be 100%. The average grain size of the prior austenite grains is 8 μm or less. More preferably, it is 5 μm or less.

  In addition to the 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, pearlite phase, and ferrite phase is contained in an area ratio of 10% or less. Will not be damaged.

  Here, the average particle diameter of the prior austenite grains in the present application was measured according to JIS G 0551 (2005). In addition, when the microstructure contains a phase other than the martensite phase, the area ratio of the martensite phase or other phases can be obtained by image analysis of the structure photograph for the ratio of the martensite phase or other phases. Is to be sought.

2) Hot-pressed steel sheet The hot-pressed steel sheet has the composition of the hot-pressed member described above, and has a bainite phase-based microstructure with an average austenite grain size of 15 μm or less. , Cold-rolled steel sheet having a microstructure composed of a cold-rolled structure, or an average grain size (average grain diameter of ferrite phase or, if further including a second phase, the average grain diameter of ferrite phase and second phase) ) Can be used a cold-rolled steel sheet having a microstructure of 15 μm or less. This is a hot-rolled steel sheet with an average grain size of prior austenite grains of 15 μm or less, a cold-rolled steel sheet with a cold-rolled structure, or a cold-rolled steel sheet with an average grain size of 15 μm or less, Ac ₃ transformation point to by heating to a temperature range of (Ac ₃ transformation point +50) ° C. by hot pressing, it is possible to an average particle size of prior austenite grains of the hot pressing member to 8μm or less, to ensure that 9.5% or more El It is because it is obtained. In many cases, a cold-rolled steel sheet has a cementite phase precipitated in the ferrite phase, but when determining the average particle size, ignore the cementite phase and determine the particle size by focusing only on the ferrite phase. Was calculated. In addition, in some cold-rolled steel sheets (the annealing temperature exceeds the Ac ₁ transformation point), 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 prior austenite grains) was determined.

  In the present invention, the microstructure mainly composed of the bainite phase means a microstructure having a bainite phase of 60% or more by area ratio. In addition, if it has a bainite phase in the said range, it is not necessary to specifically limit structures other than a bainite phase. Here, the average particle diameter of the prior austenite grains of the hot-rolled steel sheet and the average particle diameter of the cold-rolled steel sheet in the present application were measured according to JIS G 0551 (2005).

  Hot rolled steel sheet having a microstructure mainly composed of bainite phase in which the average grain size of prior austenite grains is 15 μm or less, as-rolled steel sheet having a microstructure composed of a cold rolled structure, or an average grain diameter of 15 μm or less About the reason why the average grain size of the prior austenite grains of the hot pressed member can be 8 μm or less by using the cold rolled steel sheet, although not necessarily clear, when austenitizing during heating before hot pressing, Since fine carbides, grain boundaries and strain accumulation in the bainite phase act as nucleation sites, a large amount of fine carbide is introduced into the steel plate before heating, the grain size of the steel plate before heating is reduced, It is considered that by introducing strain by rolling, the number of nucleation sites is increased, and austenite grains are refined during heating before hot pressing. In particular, when the heating rate is increased, nucleation sites that are difficult to work can be used effectively, so that further refinement is possible.

Here, the hot rolled steel sheet having a microstructure of bainite phase mainly having an average grain size of prior austenite grains of 15 μm or less is, for example, a finish rolling entry side temperature of 1050 ° C. or less, and a finish rolling temperature of Ar ₃ transformation point to ( Ar ₃ transformation point +30) ° C can be controlled as close to the Ar ₃ transformation point as possible, and the cooling conditions and coiling temperature can be set as usual to obtain a bainite structure.For example, cooling starts immediately after finish rolling. , And can be produced at a coiling temperature of about 400 to 550 ° C. 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 rolling reduction during cold rolling (also referred to as the cold pressing rate) is preferably 40% or more, and more preferably 60% or more, when the prior austenite grains of the hot pressed member are 8 μm or less. 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 if the second phase is included) of 15 μm or less are, for example, the annealing temperature of the continuous annealing line (Ac ₁ transformation point). -50) It is relatively easy to manufacture at a low setting below ℃. Although it is also possible manufacturing set to high (Ac ₁ transformation point -50) ° C. to Ac ₁ transformation point than the annealing temperature, the cold rolling reduction ratio before annealing case approximately (as a guide) over 65% Restrictions such as making it higher are necessary. Further, even if the annealing temperature is further raised and the Ac ₁ transformation point is exceeded, it can be produced as long as it slightly exceeds the Ac ₁ transformation point, 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 contains the second phase according to the degree exceeding the Ac ₁ transformation point. Ac ₁ when the transformation point than modest second phase becomes too much, since hard and be disadvantageous handling of the steel sheet, it is preferably not more than Ac ₁ transformation point. 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, Ni, or the like. 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-mentioned hot press member steel plate heated at a heating rate of 10 ° C./second or more, from Ac ₃ transformation point to (Ac ₃ transformation point +50) ° C. After holding for 1 to 600 seconds in the temperature range, it can be produced by a method of hot pressing at a temperature range of 550 ° C. or higher.

  When the heating rate was set to 10 ° C / second or more, if it was slower than 10 ° C / second, productivity decreased and austenite grains could not be refined during heating. After quenching, TS of 1470 to 1750MPa and 9.5 to This is because about 12% El cannot be obtained. In order to make the prior austenite grains 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 range from Ac ₃ transformation point to (Ac ₃ transformation point +50) ° C. When the temperature was below the Ac ₃ transformation point, a ferrite phase was generated after quenching and a TS of 1470 MPa or more was obtained. On the contrary, when the temperature exceeds (Ac ₃ transformation point +50) ° C., the austenite grains become coarse and El of 9.5% or more cannot be obtained. In addition, if the area ratio of the martensite phase after quenching is 90% or more, a TS of 1470 MPa or more can be obtained, and therefore it is allowed that a second phase such as a ferrite phase with an area ratio of 0 to 10% coexists. .

  If the holding time is set to 1 to 600 seconds, if less than 1 second, a sufficient amount of austenite phase is not generated during heating, so the strength cannot be increased by the martensite phase. This is because coarsening does not yield El of 9.5% or more. More preferably, it is 1 to 300 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 with a die and punch, or cool the member at a cooling rate of 3 to 400 ° C / second by combining air cooling. It is preferable to take out the member from the mold after hot pressing and cool it with liquid or gas from the viewpoint of improving productivity and securing TS of 1470 MPa or more.

Steel plate Nos. A to O 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 20. The Ac ₃ transformation point shown in Table 1 was obtained from the following equation, which is an empirical formula representing the Ac ₃ transformation point.

Ac ₃ transformation point = 881-206C + 53Si-15Mn-20Ni-1Cr-27Cu + 41Mo
However, the element symbol in a formula represents content (mass%) of each element.

  Further, the hot-rolled steel sheet shown in Table 1 as within the scope of the invention had a microstructure mainly composed of a bainite phase.

  The mold used in the hot press has a punch width of 70mm, punch shoulder R4mm, die shoulder R4mm, and molding depth is 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 of the steel sheet between the punch and the die 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. 19) 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 sheet having a galvanized film on the surface of a cold-rolled steel sheet that has been subjected to annealing and hot dip galvanizing treatment in a 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 members No. 1, 5-7, 10-16, 18-20, which are the present invention, have a TS of 1470-1750 MPa, an El of about 9.5-12%, a high strength and excellent ductility It turns out that it is a member.

Claims (15)

  In mass%, C: 0.22 to 0.29%, Si: 0.05 to 2.0%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.05% or less, Al: 0.005 to 0.1%, N: 0.01% or less Containing, the balance is composed of Fe and inevitable impurities, the area ratio of the martensite phase in the whole structure is 90-100%, and the average grain size of the prior austenite grains is 8 μm or less A hot press member having excellent ductility, characterized by comprising:   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: 0.22 to 0.29%, Si: 0.05 to 2.0%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.05% or less, Al: 0.005 to 0.1%, N: 0.01% or less It has excellent ductility characterized by being a hot-rolled steel sheet having a composition composed of Fe and unavoidable impurities in the balance, and having a microstructure of bainite phase mainly having an average grain size of prior austenite grains of 15 μm or less. Steel sheet for hot pressed members.   In mass%, C: 0.22 to 0.29%, Si: 0.05 to 2.0%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.05% or less, Al: 0.005 to 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: 0.22 to 0.29%, Si: 0.05 to 2.0%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.05% or less, Al: 0.005 to 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. The steel sheet according to any one of claims 6 to 12, is heated at a heating rate of 10 ° C / second or more, and is 1 to 600 in a temperature range of Ac ₃ transformation point to (Ac ₃ transformation point + 50) ° C. 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 after holding for 2 seconds.   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.