JP2013122076A - Hot stamping molding excellent in balance of strength and toughness, method for manufacturing the same, and method for manufacturing steel plate for hot stamping molding - Google Patents

Abstract

The present invention provides a hot stamping molded body that uses a steel plate having a thickness of 4 mm or more, has high strength after quenching, has good toughness, and can be used for a frame member such as a track.
SOLUTION: A hot stamping molded body has a C: 0.28 to 0.35%, Mn + Cr: 1 to 3%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, REM: 0.005 to 0.03%, O: 0.003. Made of steel containing ~ 0.007%, the ratio of rolling direction to sheet thickness direction is 1.3 or more and the average grain size is 6μm or less in the prior austenite grain size after hot stamping and contains 90% or more martensite It shall have a tensile strength of 1370MPa or more, and the steel plate for the hot stamped molded body shall be heated to a temperature range of 1250 ° C or lower after the slab consisting of the above components, and then the final stand and two before the temperature range of 800-900 ° C. Finish rolling is finished by setting the total rolling amount at the stand from 60% or more, cooling is started within 1 second, and winding is performed at a temperature of 600 ° C. or less.
[Selection] Figure 1

Description

  The present invention relates to the application of hot stamping in the manufacture of parts having a thickness of 4 mm or more, and the molded body after hot stamping has a strength of 1370 MPa or more and imparts energy absorption capability during deformation. The present invention relates to a molded body having toughness and its production.

  In recent years, the weight reduction of automobile bodies has been an urgent issue from the viewpoint of protecting the global environment, and studies on applying high-strength steel sheets have been actively conducted in response thereto, and the strength of steel materials has been increasing. However, as the steel plate strength increases, workability deteriorates and consideration for shape freezing property is required. On the other hand, in the press work that is usually used, the molding load increases more and more, and the press capability is a major issue for practical use.

  In the hot stamping technique, the steel sheet is heated to a high temperature in the austenite region and then press forming is performed, and the forming load is greatly reduced as compared with normal press working performed at room temperature. In addition, since a so-called quenching process is performed in the mold at the same time as pressing, strength according to the amount of C contained in the steel can be obtained, and as a technology that achieves both shape freezeability and strength securing Attention has been paid.

  Patent Documents 1 to 5 are disclosed as methods for obtaining a strength of 1000 to 2000 MPa by such hot stamping technology.

Patent Document 1 defines the average grain size of prior austenite grains and the amount of martensite after hot stamping, and relates to a steel sheet for hot stamping having excellent strength and ductility of 1770 to 1940 MPa. However, consideration is given to toughness. Not.
Further, Patent Document 2 discloses a technique for prescribing the cleanliness and the segregation degree of P and S to greatly improve the toughness after hot stamping. However, there is no mention of the relationship with the average grain size of prior austenite grains.

  Furthermore, Patent Document 3 discloses a technique for improving toughness by utilizing the regulation for the average grain size of prior austenite grains and automatic tempered martensite. However, in the automatic tempering, carbides are formed in martensite, and depending on the conditions, precipitation may occur in the prior austenite grain boundaries, and the toughness may not be improved. Not only that, but the structure formed after hot stamping is not only the size of the prior austenite grain size, but also a book that further increases the balance between strength and toughness by controlling the ratio between the rolling direction and the plate thickness direction. It is completely different from the invention.

On the other hand, Patent Document 4 discloses a method for producing a high-strength hot-rolled steel sheet excellent in low-temperature toughness by defining the aspect ratio of prior austenite grain size. However, the present invention is completely different from the present invention because it is not only different from the scope of the present invention but is not related to the structure after hot stamping.
Patent Document 5 discloses a method of manufacturing a hot-rolled steel sheet for hot pressing intended for a hot-rolled steel sheet of 1.6 to 6.0 mm and a method of manufacturing a formed member using the steel sheet. However, the prior austenite grain size and shape after hot stamping are completely different from the present invention that defines the cooling rate when hot stamping is performed.

JP 2010-174282 A JP 2007-314817 A JP 2006-152427 A JP 2011-52321 A JP 2006-265583 A

As described above, the application of a steel material having a strength of 1370 MPa or more is being considered for a vehicle part, particularly a frame used for a large vehicle such as a truck, in order to significantly reduce the weight of the vehicle body. In cold press forming, application of a hot stamp material is effective because the press functional force is insufficient or the shape freezing property deteriorates. However, since the plate thickness of the applied member is 4 mm or more, it is hot. There is concern that the hardenability and toughness at the time of stamping will be insufficient.
An object of the present invention is to provide a hot stamping molded body having high strength after quenching and good toughness even with a plate thickness of 4 mm or more.

  In view of such circumstances, the present inventors have completed the present invention for the purpose of manufacturing a part having a strength of 1180 MPa or more by a hot-rolled steel sheet having a thickness of 4 mm or more by hot stamping.

The summary is as follows. That is,
(1) The component is mass%, C: 0.20 to 0.40%, Si: 0.1 to 0.5%, Mn + Cr: 1 to 3%, P: 0.001 to 0.015%, S: 0.001-0.01%, Al: 0.005-0.05%, Ti: 0.005-0.1%, Nb: 0.005-0.1%, O: 0.003- 0.007%, N: 0.001 to 0.003%, which consists of the balance Fe and inevitable impurities, and is a hot stamped steel sheet having a thickness of 4 mm or more. About 90% or more of martensite, and the former austenite grains have a ratio of the grain size rolling direction to the plate thickness direction of 1.3 or more, an average grain size of 6 μm or less, and a tensile strength of 1370 MPa or more. Hot stamp with excellent balance between strength and toughness Form.
(2) The steel sheet is further mass%, V: 0.005 to 0.1%, Mo: 0.05 to 0.5%, Ca: 0.005 to 0.03%, REM: 0.005. -0.03%, Cu: 0.01-0.5%, Sn: 0.005-0.1%, Ni: 0.01-0.5%, W: 0.01-0.5% A hot stamping molded article having an excellent balance between strength and toughness according to (1), comprising at least one kind.
(3) After heating the slab composed of the components described in (1) or (2) to a temperature range of 1250 ° C. or lower, the total amount of reduction in the final stand and the two previous stands in the temperature range of 800 to 900 ° C. The hot stamping body is excellent in the balance between strength and toughness after hot stamping, characterized in that finish rolling is finished at a temperature of 60% or more, cooling is started within 1 second, and winding is performed at a temperature of 600 ° C. or less. Steel plate manufacturing method.
(4) A method for producing a steel sheet for a hot stamped article having an excellent balance between strength and toughness after hot stamping, characterized by further performing hot dipping after the production process described in (3).
(5) When manufacturing a hot stamping molded body using the steel plate obtained in (3) or (4), the steel plate is heated at a rate of 1 ° C./s or higher, and is in a temperature range of Ac 3 point or higher and 950 ° C. or lower. After heating, the temperature range of Ar3 to 300 ° C. is cooled at a cooling rate of 20 ° C./s or more, and the balance between strength and toughness after hot stamping according to (1) or (2) is excellent A method for producing a hot stamping molded body.

  According to the present invention, a hot stamping molded article having a thickness of 4 mm or more excellent in balance between strength and toughness by appropriately controlling the prior austenite grain size after hot stamping and its shape while having strength of 1370 MPa or more. Is obtained. Moreover, the hot-rolled steel plate applied to it can be manufactured. This contributes to a significant weight reduction of the body weight of large vehicles such as automobiles, particularly trucks.

It is a figure which shows the intensity | strength change after hardening accompanying the change of C amount. It is a figure which shows the relationship between the old austenite particle size after a hot stamp, and the energy absorption amount at the time of implementing a Charpy test at -40 degreeC about a hot rolled sheet. It is a figure which shows the relationship between the ratio of the rolling direction and plate | board thickness direction in the prior austenite particle size after a hot stamp, and the energy absorption amount when a Charpy test is implemented at -40 degreeC about a hot rolled sheet. It is a figure which shows the relationship between the finishing temperature at the time of hot rolling, and the prior austenite particle size after a hot stamp. It is a figure which shows the relationship between the finishing temperature at the time of hot rolling, and the ratio of the rolling direction and the plate thickness direction in the prior austenite grain size after hot stamping. It is a figure which shows the relationship between the cooling start time after the finish rolling at the time of hot rolling, and the prior austenite grain size after a hot stamp. It is a figure which shows the relationship between the cooling start time after the finish rolling at the time of hot rolling, and the ratio of the rolling direction and the plate thickness direction in the prior austenite grain size after hot stamping. It is a figure which shows the test piece with a V notch used by delayed fracture characteristic evaluation.

First, the experiment that led to the completion of the present invention will be described.
The inventors melted the steels having the components shown in Table 1 on a laboratory scale, and after heating the obtained steel ingot at 1250 ° C., the total rolling reduction ratio in the final rolling and the previous rolling: 60%, Hot rolling was finished at a finishing temperature of 880 ° C. and a plate thickness of 6 mm, and after 1 second, cooling was started at a cooling rate of 30 ° C./s or less, and winding was performed at 600 ° C. The obtained hot-rolled sheet was pickled, heated to 900 ° C. at a heating rate of 1 ° C./s, kept for 20 seconds, and immediately cooled to room temperature at a cooling rate of 20 ° C./s. In this case, a steel sheet containing 90% or more martensite as a steel sheet structure was obtained.

Then, it processed into the No. 5 test piece as described in JISZ2201, and the tension test implemented according to the test method as described in JISZ2241 was done. The result of the obtained tensile test is shown in FIG.
From FIG. 1, it was found that to obtain a tensile strength of 1370 MPa or more after hot stamping, which is the subject of the present invention, it is necessary to add 0.20% or more of C by weight%.

Furthermore, No. 1 in Table 1 Using the steel No. 3, hot rolling was performed under various conditions to produce a 4 mm hot rolled sheet. This hot-rolled sheet is subjected to a heat treatment that is heated to 950 ° C. at a heating rate of 2 ° C./s and then cooled to room temperature at 40 ° C./sec. Tensile strength and toughness were investigated.
In any case, steel sheets containing 90% or more martensite as the steel sheet structure were obtained, but it was observed that some cementite was precipitated inside these martensites.

As a result of conducting a tensile test on the steel plate after the heat treatment according to the same test method as described above, a strength of 1370 MPa or more was obtained under any conditions. For toughness, V-notch specimens (width: 10 mm) were prepared, Charpy impact test was conducted, energy absorption at −40 ° C. (plate thickness: 10 mm equivalent) was evaluated, and old austenite grains after hot stamping were evaluated. The relationship between the particle size (average value) and the ratio of the rolling direction and the plate thickness direction was investigated. The obtained results are shown in FIGS.
2 and 3, from the viewpoint of securing toughness after hot stamping, the steel sheet after hot stamping has an average grain size of prior austenite grains of 6 μm or less, and the ratio of grain size in the rolling direction and thickness direction is 1. It was found that it is important to set the ratio to 3 or more.

  Furthermore, in the steel sheet after hot rolling, the inventor appropriately controlled the hot rolling conditions when the prior austenite grain size was 6 μm or less on average and the ratio of the rolling direction and the plate thickness direction was 1.3 or more. It was found that the steel sheet had a prior austenite grain size of 6 μm or less even after hot stamping performed under appropriate heating conditions, and the ratio between the rolling direction and the plate thickness direction was 1.3 or more.

This is because after the hot austenite is properly controlled, the prior austenite grain size is as fine as 6 μm or less, and the ratio of the rolling direction and the thickness direction is 1.3 or more, cooling after hot rolling. In the winding process, the rate of change from austenite to ferrite + cementite is as high as almost 100%, and even in the heating before hot stamping, the rate of change from ferrite + cementite to austenite is almost 100%. And higher.
Therefore, even if the transformation from austenite to ferrite + cementite, and further from ferrite + cementite to austenite is repeated, the grain size is 6 μm or less, and the ratio of rolling direction and thickness direction is 1.3 or more. This is because the diameter can be secured even after hot stamping.

In order to make the prior austenite grain size so, the finishing temperature in hot rolling, the finishing pass schedule and the cooling start time after finishing rolling are important.
As a result of the study, the knowledge shown in FIGS. 4, 5, 6 and 7 was obtained regarding the finishing temperature, the finishing pass schedule, the cooling start time after finishing rolling, and the prior austenite grain size.
That is, the hot rolling is finished at 900 ° C. or less, the total reduction amount at the last stand and the two previous stands is set to 60% or more, and cooling after rolling is started within 1 second after the completion of rolling. Is effective.

  Furthermore, as a thermal history control at this time, an examination was also made focusing on the cooling rate after the end of hot rolling. Although the present invention is directed to a molded body having a plate thickness of 4 mm or more, since the plate thickness is large, a general manufacturing apparatus cannot increase the cooling rate after hot rolling. For this reason, in the experiment leading to the present invention, the cooling rate until the coiling after hot rolling is mainly 100 ° C./s or less, but even if this cooling rate exceeds 100 ° C./s, the prior austenite grains after hot pressing It has been confirmed that the diameter of 6 μm or less and the ratio of the rolling direction and the plate thickness direction can be secured to 1.3 or more, and the effects of the present application are exhibited.

  Here, the prior austenite particle size is measured by etching using an aqueous solution containing sodium dodecylbenzenesulfonate, picric acid, oxalic acid and hydrochloric acid, and the measurement is 1/8 t part (or 7/8 t part) of the plate thickness. ).

  The present invention has been completed on the basis of such experimental facts, and the reasons for limitation of the present invention will be sequentially described below.

  In the present invention, by mass, C: 0.20 to 0.40%, Si: 0.1 to 0.5%, Mn + Cr: 1 to 3%, P: 0.001 to 0.015%, S: 0.001-0.01%, Al: 0.005-0.05%, Ti: 0.005-0.1%, Nb: 0.005-0.1%, O: 0.003-0. Steel containing 007%, N: 0.001 to 0.003%, the balance being Fe and inevitable impurities, or V: 0.005 to 0.1%, Mo: 0.05 to 0.5 %, Ca: 0.005-0.03%, REM: 0.005-0.03%, Cu: 0.01-0.5%, Sn: 0.005-0.1%, Ni: 0.00. Steels containing one or more of 01 to 0.5% and W: 0.01 to 0.5% as necessary are used. The reasons for limiting the individual components are as follows.

  C is an element that plays an important role in the present invention, and has a great influence on the strength after quenching. Therefore, 0.20% or more of addition is necessary to obtain a strength of 1370 MPa or more. On the other hand, if it exceeds 0.40%, breakage is likely to occur at the time of impact deformation, and weldability is deteriorated and the strength of the welded portion is reduced.

  Since Si is a solid solution strengthening element and an element that suppresses precipitation of cementite, it is preferably added in an amount of 0.1% or more. On the other hand, if excessively added, surface defects due to scale generated during heating during hot stamping are likely to occur, and when plating is performed, the plating properties deteriorate, so 0.5% is made the upper limit.

  Mn and Cr are one of the important elements for securing the hardenability. When hot stamping is performed in the present invention, addition of 1% or more in total of these elements is necessary. On the other hand, if added over 3%, the hardenability becomes high and the strength of the hot-rolled sheet becomes too high. Therefore, when cold rolling is performed, the load becomes too large, so this is made the upper limit.

  Ti and Nb are also important elements in the present invention. In the prior austenite grain size after hot stamping, the ratio of the rolling direction to the sheet thickness direction is 1.3 or more, and the average value is 6 μm or less. In order to achieve this, it is necessary to add 0.005% or more. On the other hand, even if added over 0.1%, the effect is saturated, so this is the upper limit.

  V is an element that may be added to refine the structure from the viewpoint of securing toughness. That is, when the steel sheet is heated to Ac3 point or higher, the formation of fine carbides suppresses recrystallization and grain growth and makes the austenite grains fine, so that there is an effect of improving toughness. If it is less than 0.005%, the effect cannot be obtained. On the other hand, even if added over 0.1%, the effect is saturated and the cost is increased, so this is the upper limit.

  Mo, like Ti, Nb, and V, improves the toughness when the steel sheet is heated to Ac3 point or higher, and the formation of fine carbides suppresses recrystallization and grain growth and makes austenite grains fine. There is an effect to. Therefore, when adding, 0.05% is made the lower limit. On the other hand, adding over 0.5% not only saturates the effect but also increases the cost, so this is the upper limit.

  P is a solid solution strengthening element, and can increase the strength of the steel sheet relatively inexpensively, but segregates at the grain boundaries, and when the strength is high, low temperature embrittlement becomes a problem, so 0.015% Is the upper limit. On the other hand, since lower than 0.001% is not preferable because the removal P cost is extremely increased, this is preferably set as the lower limit.

  S is an element that affects the hot brittleness of steel, and is not only an element that deteriorates hot workability, but also lowers workability. Therefore, the upper limit is 0.01%. However, when the content is less than 0.001%, the desulfurization cost is extremely increased, so this is preferably set as the lower limit.

  Al is added for deoxidation. If it is less than 0.005%, deoxidation becomes insufficient, and a large amount of oxide remains in the steel, and in particular, the local deformability deteriorates and the characteristic variation also increases. On the other hand, if the content exceeds 0.05%, a large amount of oxide mainly composed of alumina remains in the steel, which also causes deterioration of local deformability, which is not preferable.

  Lowering N too much is not preferable because it increases costs, so 0.001% is set as the lower limit. On the other hand, if the content exceeds 0.003%, inclusions are formed and the toughness after quenching deteriorates, so this is the upper limit.

Since REM containing Ca, Ce or the like as a deoxidizing element is an element that forms a fine oxide, it is an effective element for reducing the grain size of prior austenite. Therefore, 0.005% or more may be added. However, even if added over 0.03%, the effect is saturated, so this is the upper limit.
O is also an element necessary for forming an oxide, and if it is less than 0.003%, there are few fine oxides, so that a prior austenite grain size of 6 μm or less cannot be obtained. on the other hand. If the amount exceeds 0.007%, the amount of oxide formed is too large, so that workability and toughness deteriorate.

  W may be added to form martensite stably in the hot stamping process. If less than 0.01%, the effect is insufficient, and even if added over 0.5%, the effect is saturated, so this is the upper limit.

  In addition, when adjusting to the steel component shown above, elements, such as Cu, Sn, Ni by using a scrap in a steelmaking stage, may be contained. Even in such a case, the effect in the present invention is not changed, but Cu: 0.01 to 0.5%, Sn: 0.005 to 0.1%, Ni: 0.01 to 0.5 %. When these elements are added excessively, cracks in hot rolling occur, so the upper limit is specified respectively.

  The molten steel adjusted to the above components is continuously cast into a slab, which is hot-rolled to obtain a hot stamping steel plate. The method of continuous casting is not particularly specified, and a normal continuous casting method can be used, but the effect in the present invention is not changed by the thin slab method having a slab thickness of 100 mm or less.

  In the present invention, the hot rolling conditions are very important especially for toughness after hot stamping. That is, for the prior austenite grain size after hot stamping, the heating temperature during hot rolling is preferably low so that the ratio of the rolling direction to the sheet thickness direction is 1.3 or more and 6 μm or less. Therefore, heating temperature shall be 1250 degrees C or less.

  The finishing temperature is preferably as low as possible, but 800 ° C. or higher is ensured in consideration of rollability. On the other hand, when the temperature exceeds 900 ° C., the ratio of the rolling direction and the plate thickness direction of the particle size becomes smaller than 1.3 and the toughness deteriorates, so this is the upper limit. At that time, the total reduction amount at the final stand and the previous two stands is set to 60% or more, and further, cooling is started immediately after finishing rolling, specifically, cooling within 1 second after finishing rolling. To start.

  The cooling rate from the hot rolling to the winding does not affect the manifestation of the effect of the present invention, and may be 100 ° C./s or less which can be controlled by a general manufacturing apparatus. It may exceed 100 ° C / s.

Thereafter, by performing winding in a temperature range of 600 ° C. or less, with respect to the prior austenite grain size after hot stamping, the ratio of the rolling direction to the plate thickness direction is set to 1.3 or more and the average value is set to 6 μm or less. be able to.
When the coiling temperature exceeds 600 ° C., or when the total rolling reduction is less than 60% or the cooling after finish rolling is performed for more than 1 second, as the prior austenite grain size after hot stamping, the rolling direction and The ratio to the plate thickness direction cannot be made 1.3 or more and the average value cannot be made 6 μm or less.

In addition, since hot-rolled sheet strength will become high too much when winding at the temperature of 400 degrees C or less, this is made into a minimum, but in order to set it as the structure | tissue containing a ferrite + pearlite, 500 degrees C or more is preferable. On the other hand, after winding at a temperature of 400 ° C. or lower, reheating treatment for softening may be performed.
Here, regarding the prior austenite grain size after hot stamping, if the ratio of the rolling direction to the sheet thickness direction exceeds 2, the anisotropy becomes too large, and there is a concern that the toughness is deteriorated. Therefore, this value is preferably the upper limit.

  Plating is performed as necessary after hot rolling is completed, but the heating conditions and cooling conditions during plating are not particularly specified. Further, Zn or Al is also plated as a plating type, but there is no particular limitation on whether or not alloying is performed with respect to Zn plating. Furthermore, Al and Mg may be contained in Zn. On the other hand, Al plating does not affect the present invention even if Si is included in the plating.

  Although temper rolling (skin pass) for appropriately adjusting the shape can be performed after hot rolling or plating, the method is not particularly specified.

Hot stamping is performed using a hot-rolled steel sheet or a plated steel sheet manufactured under the above conditions. The heat treatment conditions at that time are controlled as follows.
The heating rate of the steel sheet is set to 1 ° C./s or higher, and the heating temperature is set to Ac 3 points or higher and 950 ° C. or lower. When the heating rate is less than 1 ° C./s or when the heating rate is higher than 950 ° C., not only a prior austenite grain size of 6 μm or less cannot be obtained after hot stamping, but also the rolling direction of the former austenite grain size. And the plate thickness direction ratio is less than 1.3, and the effect of the present invention is lost. On the other hand, when the heating temperature is lower than the Ac3 point, a region that does not partially austenite is formed. Therefore, sufficient martensite cannot be obtained in this portion because martensite is not formed during cooling.
The heat retention time at the heating temperature is preferably 180 seconds or less because it is preferable that the heat retention time be short.

Moreover, the cooling rate at the time of cooling the temperature range of Ar3 point -300 degreeC shall be 20 degrees C / s or more. When the cooling rate is less than 20 ° C./s, not only does the strength easily change within the member, but also carbides are precipitated at the grain boundaries, so there is a concern about deterioration of toughness.
The effect of the present invention does not change even if cementite is precipitated by autotempering during or after cooling in the hot stamp.

After the steels having the components shown in Table 2 were produced in a converter and made into slabs, the hot rolling conditions (heating temperature: 1220 ° C., finishing temperature: 870 ° C., final stand and two previous stages) The total rolling reduction from the stand: 65%, the time after finishing rolling was finished, the time until the start of cooling: 0.5 seconds, the winding temperature: 600 ° C.) was hot rolled to obtain a 6 mm hot rolled sheet. After this hot-rolled sheet was pickled, a skin pass was performed under the conditions shown in Table 3. Some hot-rolled sheets were plated between pickling and skin pass.
The hot-rolled steel sheet was plated under the conditions shown in Table 3. Plating treatment at that time includes hot dip galvanizing [GI (no alloying treatment) / GA (with alloying treatment), DZ (including Mg), SD (including Al and Mg)], or 10% Si. Molten aluminum plating (Al) was applied.

  These steel plates are heated to 900 ° C. at a heating rate of 1 ° C./sec in a laboratory heating furnace, and after holding for 60 seconds, a mold having a water supply port from which water is spouted and a drain port from which the water is sucked. The thermal history of the hot stamp was simulated by cooling to room temperature by spraying water and spraying water. The cooling rate from Ar3 point to 300 ° C at this time was 20 ° C / s.

The strength of the material before the heat treatment corresponding to the hot stamp was evaluated, and the strength, elongation, plating property, delayed fracture property, low temperature toughness and old austenitic structure were evaluated for the material after the heat treatment.
For the evaluation of strength and elongation, a No. 5 test piece described in JIS Z 2201 was processed, and a tensile test was performed according to the test method described in JIS Z 2241. The plating property was evaluated by visually observing the coating state of the plating (no plating: good / ◯, occurrence of non-plating: defective / x).

For the delayed fracture characteristics, a test piece with a V-notch as shown in FIG. 8 was used, and the specimen was immersed in an aqueous solution of 3 g / l of ammonium thiocyanate in 3% saline at room temperature for 24 hours. Judgment was made (no break: ○, with break: x). For low-temperature brittleness, a Charpy test is performed at −40 ° C., plate thickness: evaluated in terms of 10 mm, an energy absorption amount of 100 to 150 J / cm ² is shown, and a ductile fracture surface ratio of 50% or more is obtained. The case where it passed ((circle)) and was not obtained was made disqualified (x).

  The prior austenite particle size is measured by etching using an aqueous solution containing sodium dodecylbenzenesulfonate, picric acid, oxalic acid and hydrochloric acid, and the measurement is carried out at 1/8 t part and 7/8 t part in thickness. The average value was evaluated.

The obtained results are also shown in Table 3. In the steels according to the present invention (A steel to L steel, V steel to Z steel), TS shows strength of 1370 MPa or more, and there is no problem in delayed fracture characteristics and low temperature toughness.
On the other hand, the M steel with the C content deviating low does not reach the strength of 1370 MPa. Further, N steel with a high C content is 2120 MPa in strength, and there is concern about delayed fracture characteristics and deterioration of low temperature brittleness. Furthermore, in the O, P, S, T, and U steels to which no Ti or Nb is added, the ratio of the rolling direction to the plate thickness direction in the prior austenite grain size is less than 1.3 after the thermal history in hot stamping. In addition, since the average particle size is larger than 6 μm, the toughness is low. On the other hand, in the case of Q steel in which Si deviated from the range of the present invention, the plating property was poor, and surface flaws due to the scale generated in the non-plated part were also observed. Further, in the R steel in which O deviates from the range of the present invention, since the prior austenite grain size of 6 μm or less is not obtained, delayed fracture characteristics are poor.

For the E, I and L steels in Table 2, the hot rolling conditions (heating temperature: 1250 ° C., finishing temperature: 880 ° C., total rolling reduction at the last stand and the last two stands: 60% After completion of finish rolling, the time until the start of cooling was 0.8 seconds, the coiling temperature was 550 ° C.), and the sheet thickness was 4 mm.
Subsequently, the hot-rolled sheet is pickled, subjected to skin pass under the conditions shown in Table 4, and those not plated are heated as they are at 880 ° C. in a heating furnace and kept for 120 seconds, and then water is ejected from the surface. The product was sandwiched between molds having a water supply port and a drain port for sucking the water, and cooled to room temperature by spraying water. For plating, the hot-rolled sheet is pickled and then subjected to molten aluminum plating (Al) containing 10% of zinc (GI, GA, DZ, SD) or Si, and then skin pass under the conditions shown in Table 4. The same heating-cooling treatment as that without plating was performed.

  About the obtained steel plate, the material characteristic similar to Example 1 was evaluated, and the obtained result is shown in Table 4. Since both are steels according to the present invention, there are no problems with delayed fracture characteristics and low temperature toughness, and the characteristics according to the present invention are obtained.

  For steel I in Table 2, hot rolling is performed under the hot rolling conditions shown in Table 5, and after heating to 850 ° C. at the heating rate shown in the same table, a water supply port from which water is spouted and a drain port from which the water is sucked Was cooled to room temperature by spraying water. About the obtained steel plate, the material characteristic similar to Example 1 was evaluated, and the obtained result is shown in the same table. For toughness, a test was conducted at −120 ° C., the plate thickness was evaluated in terms of 10 mm, and an energy absorption value of 85 J / cm 2 or more was obtained as a pass (◯), which was not obtained. Was rejected (x).

  No. according to the present invention. In Nos. 1 to 5, the strength of TS: 1770 MPa level, delayed fracture characteristics and toughness targeted by the present invention are obtained. On the other hand, No. whose heating temperature is higher than the range of the present invention. No. 6 in which the total amount of rolling reduction at the final stand and the last two stands was out of the scope of the present invention. No. 7 has poor toughness because the grain size of the prior austenite after the heat history in the hot stamping becomes coarse and the ratio between the rolling direction and the plate thickness direction is also smaller than 1.3. Furthermore, the heating rate when carrying out hot stamping was lower than 1 ° C./s. No. 10 in which the finish rolling temperature in the hot rolling process deviates from the scope of the present invention. In No. 8, the ratio of the rolling direction to the plate thickness direction in the prior austenite grain size is out of the scope of the invention after the heat history in the hot stamp, and the delayed fracture characteristics and toughness deteriorate. In addition, the winding temperature deviated higher than the range of the present invention. No. 9 was excluded from the invention because the hot-rolled sheet strength was too high, and the shape was poor, so the skin pass rollability was poor, causing problems in commercial value and productivity.

  According to the present invention, it is possible to impart a strength and ductility of 1370 MPa or more to a steel material that has undergone a thermal history in hot stamping, and it is possible to manufacture a hot stamping molded article having an excellent balance between strength and toughness.

Claims (5)

  Ingredients are mass%, C: 0.20-0.40%, Si: 0.1-0.5%, Mn + Cr: 1-3%, P: 0.001-0.015%, S: 0 0.001-0.01%, Al: 0.005-0.05%, Ti: 0.005-0.1%, Nb: 0.005-0.1%, O: 0.003-0.007 %, N: 0.001 to 0.003%, the balance is Fe and inevitable impurities, and a hot stamped steel sheet having a thickness of 4 mm or more is 90% or more after hot stamping. And the former austenite grains have a ratio of the grain size rolling direction to the sheet thickness direction of 1.3 or more, an average grain size of 6 μm or less, and a tensile strength of 1370 MPa or more. Hot stamping molded product with excellent balance between strength and toughness   The said steel plate is further mass%, V: 0.005-0.1%, Mo: 0.05-0.5%, Ca: 0.005-0.03%, REM: 0.005-0. One or more of 03%, Cu: 0.01 to 0.5%, Sn: 0.005 to 0.1%, Ni: 0.01 to 0.5%, W: 0.01 to 0.5% The hot stamping molded article having an excellent balance between strength and toughness according to claim 1.   After heating the slab comprising the component according to claim 1 or 2 to a temperature range of 1250 ° C. or less, the total reduction amount at the final stand and the two previous stands is set to 60% or more in the temperature range of 800 to 900 ° C. Finishing finish rolling, starting cooling within 1 second, and winding up at a temperature of 600 ° C. or less, and a method for producing a steel sheet for hot stamped compacts having an excellent balance between strength and toughness after hot stamping .   A method for producing a steel sheet for a hot stamped article excellent in the balance between strength and toughness after hot stamping, characterized in that after the production process according to claim 3 is completed, further hot dipping is performed.   When manufacturing a hot stamping molded body using the steel sheet obtained in claim 3 or 4, the steel sheet is heated to a temperature range of Ac3 point or more and 950 ° C or less at a heating rate of 1 ° C / s or more, and then Ar3 The method for producing a hot stamping molded article having an excellent balance between strength and toughness according to claim 1, wherein the temperature range of ˜300 ° C. is cooled at a cooling rate of 20 ° C./s or more.

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