JP2015113501A - Steel sheet for hot press - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet for hot press capable of securing sufficient hardenability even with quick and brief heating when hot-pressed although the strength of the steel sheet before hot-pressed is reduced.SOLUTION: The steel sheet for hot press is provided which has a component composition containing, by mass%, C:0.10 to 0.30%, Si:2.0% or less (including 0%), Mn:1.0 to 3.0%, Cr:1.0% or less (excluding 0%), Ti:0.01 to 0.10%, B:0.0003 to 0.0100% and the balance iron with inevitable impurities. The steel sheet for hot press has steel structure which is composed of ferrite and cementite, and in which an area ratio of ferrite including cementite is 50% or more and an average aspect ratio of the cementite included in the ferrite is 2 or less, and Mn/Mn≤1.3 and Si/Si≥0.8 are satisfied where Siand Mnare respectively Si concentration and Mn concentration of the cementite included in the ferrite, and Mn and Si are Mn content and Si content respectively in whole steel sheet.

Description

  The present invention relates to a steel sheet for hot pressing that exhibits excellent hardenability even during high-speed and short-time heating during hot pressing.

  In recent years, there has been a demand for higher strength of steel sheets in order to improve the fuel efficiency of automobiles. For example, a high-tensile material having a strength of 600 MPa or more even though it is thin is widely used because it can achieve both weight reduction of the vehicle body and stability at the time of collision. Recently, the use of 1000 MPa class or 1500 MPa class ultra-high tensile materials has been studied in order to further increase the strength of the side collision body. However, the super-high tensile material has a problem that it is inferior in workability because of its very high strength.

  Hot press (also called hot stamping, hot pressing, die quenching, etc.) has been attracting attention as a technique for obtaining high-strength processed parts without using ultra-high tensile materials. Hot pressing is a method in which a steel plate (blank) is softened by heating to a temperature in the austenite region, and then quenched and quenched while being processed with a mold, whereby high strength with a tensile strength of 1300 MPa or more is achieved. In addition, a processed part (hot press-molded part) having excellent shape freezing property can be obtained. In addition, a processed part manufactured by a hot press has an advantage that it has excellent delayed fracture resistance as compared with a processed part of the same strength manufactured by a conventional cold press.

  As described above, hot pressing is an excellent technique, but has the following problems.

  That is, in order to ensure a tensile strength of 1300 MPa or more in the processed part after hot pressing, a large amount of C is added to the hot pressing steel plate to be used for hot pressing in order to ensure strength after quenching, It is necessary to add a large amount of alloying elements such as adding a large amount of Mn or Cr to ensure hardenability and adding B to ensure toughness. Furthermore, addition of Ti or the like is also necessary in order to neutralize adverse effects such as N existing as impurities. Addition of a large amount of these alloy elements causes problems such as an increase in the strength of the hot-pressed steel sheet used for hot pressing and increased wear of the shear blade during blanking.

  Moreover, although the hot pressing method disclosed in Patent Document 1 is an excellent technique, in order to reduce the strength of the steel plate before hot pressing, the quenching element of the hot pressing steel plate, particularly the content of Mn, is reduced. It is low. A decrease in the content of the quenching element deteriorates the hardenability and requires enhancement of the cooling equipment. In addition, when it becomes difficult to obtain stable hardenability, a variation in hardness occurs in the hot press-formed part obtained by hot pressing, and this variation in hardness reduces the toughness of the hot-press formed part. There is also. Therefore, in consideration of productivity and quality stability, it is necessary to take measures not to reduce the content of quenching elements such as Mn in order to improve hardenability.

  Therefore, in Patent Document 2, as a steel sheet for hot pressing, which is a material of a hot press-formed part having a tensile strength of 1300 MPa or more, particularly in a state before being subjected to hot pressing, good workability and flatness correction of the steel sheet 60% by area of cementite in ferrite without reducing the Mn content for the purpose of providing a steel sheet for hot pressing that has excellent hardenability at the time of hot pressing while having excellent properties and blank workability A steel sheet for hot pressing in which the above is spheroidized cementite has been proposed.

  On the other hand, as a problem of hot press itself, heating of the steel sheet used for hot press takes time, and the long holding time (heating holding time) after reaching the heating temperature limits the productivity. Improvement and shortening of the heating and holding time have been studied (for example, see Non-Patent Document 1).

  For the purpose of improving the productivity of the hot press, using the steel sheet for hot press described in Patent Document 2 above, when trying to improve the heating rate and shorten the heating holding time, cementite, which is a carbide, is spheroidized. Therefore, the dissolution is delayed, the hardenability is deteriorated, and there is a problem that the strength of the part formed by hot pressing cannot be sufficiently secured.

Japanese Patent Laid-Open No. 10-96031 JP 2010-47786 A

Seijiro Maki et al .: “Partial quenching of steel using electric heating (plastic processing and related technologies (1), fusion of basic research and advanced technology in manufacturing)”, Proceedings of Lectures on Mechanical Materials and Materials Processing Technology, Japan The Society of Mechanical Engineers, November 27, 2010, 2010, No. 18, p. 305-1 to 305-5

  The present invention has been made paying attention to the above circumstances, and its purpose is to ensure sufficient hardenability even during rapid and short-time heating in hot pressing while reducing the strength of the steel plate before hot pressing. An object of the present invention is to provide a hot-press steel plate that can be used.

The invention described in claim 1
Ingredient composition is in mass% (hereinafter the same for chemical ingredients)
C: 0.10 to 0.30%,
Si: 2.0% or less (excluding 0%)
Mn: 0.5 to 3.0%
Cr: 1.0% or less (excluding 0%),
Ti: 0.01-0.10%,
B: 0.0003 to 0.0100%,
Each of which comprises iron and inevitable impurities,
Steel structure
Made of ferrite and cementite,
Ferrite containing cementite is present in an area ratio of 50% or more,
The average aspect ratio of cementite encapsulated in the ferrite is 2 or less,
When the Si concentration of cementite contained in the ferrite is Si _θ , the Mn concentration is Mn _θ , the Mn content of the whole steel sheet is Mn, and the Si content is Si, Mn _θ /Mn≦1.3, Si _θ It is a steel sheet for hot pressing characterized by satisfying /Si≧0.8.

The invention described in claim 2
Ingredient composition further
Cu: 0.50% or less (excluding 0%),
Ni: 0.50% or less (excluding 0%),
The steel sheet for hot pressing according to claim 1, comprising at least one of Mo: 0.50% or less (not including 0%).

The invention according to claim 3
Ingredient composition further
V: 0.10% or less (not including 0%) and / or Nb: 0.10% or less (not including 0%)
It is a steel plate for hot presses according to claim 1 or 2.

  According to the present invention, the cementite encapsulated in ferrite is spheroidized to reduce the strength of the steel sheet before hot pressing, while limiting the Mn concentration of the spheroidized cementite and securing the Si concentration. By increasing the ease of dissolving cementite, which is a carbide, the above-mentioned spheroidized cementite can be easily and sufficiently dissolved even in high-speed and short-time heating when subjected to hot pressing, and it is possible to ensure excellent hardenability. It is now possible to provide steel plates for pressing.

  Hereinafter, the steel sheet for hot pressing according to the present invention (hereinafter also referred to as “the steel sheet of the present invention”) will be described in more detail.

  First, the steel structure which comprises this invention steel plate is demonstrated.

[Steel structure of the steel sheet of the present invention]
The steel structure of the steel sheet for hot pressing according to the present invention is composed of ferrite and cementite, ferrite containing cementite is present in an area ratio of 50% or more, and the average aspect ratio of cementite included in the ferrite is 2 When the Si concentration of cementite included in the ferrite is Si _θ , the Mn concentration is Mn _θ , the Mn content of the whole steel sheet is Mn, and the Si content is Si, Mn ≦ 1.3, Si _{It is} assumed that _{θ /} Si ≧ 0.8.

<Ferrite and cementite, ferrite containing cementite is 50% or more in area ratio>
This is to ensure the cold workability of the steel sheet for hot pressing. If the steel structure contains a phase or structure other than ferrite and cementite, or the ratio of ferrite encapsulating spheroidized cementite (described later) is less than 50% in area ratio, the steel sheet for hot pressing will be hardened. .

<Average aspect ratio of cementite encapsulated in the ferrite is 2 or less>
This is to improve the cold workability of the hot-press steel sheet by spheroidizing the cementite encapsulated in the ferrite. If the average aspect ratio of cementite exceeds 2, that is, if the cementite is not sufficiently spheroidized, the steel sheet for hot pressing becomes hard as described above.

<The Si concentration of the cementite that is included in said ferrite Si _theta, the Mn concentration Mn _theta, when the Mn content of the entire steel sheet Mn, the Si content and _{Si, Mn ≦ 1.3, Si θ} / Si ≧ 0.8>
By limiting the Mn concentration of the spheroidized cementite and securing the Si concentration, the ability to dissolve cementite, which is a carbide, is enhanced, so that the spheroidized cementite can be heated even during high-speed and short-time heating during hot pressing. This is because it dissolves easily and sufficiently and ensures excellent hardenability. When Mn _θ / Mn exceeds 1.3 (that is, when Mn is excessively concentrated in spheroidized cementite) or when Si _θ / Si is less than 0.8 (that is, from spheroidized cementite to Si Is excessively discharged), the high-speed and short-time heating during the hot pressing does not sufficiently dissolve the spheroidized cementite, and the desired hardenability cannot be ensured.

  Here, a method for measuring the area ratio of ferrite enclosing cementite, the average aspect ratio of cementite encapsulating in the ferrite, and the Si concentration and Mn concentration will be described below.

[Method for measuring the area ratio of ferrite containing cementite]
After the steel plate is corroded with a repeller, the structure of each phase (martensite, bainite, etc.) including ferrite is identified by observation with a transmission electron microscope (TEM; magnification: 1500 times), and then observed with an optical microscope (magnification: 1000 times). The total area of ferrite distributed by dividing cementite in the grains was measured, and the fraction (area ratio) with respect to the area of the entire structure was obtained. The total area of the ferrite includes the area of encapsulated cementite.

[Measuring method of average aspect ratio of cementite encapsulated in ferrite]
For each cementite encapsulated in the ferrite, the major axis is obtained, the diameter in the direction perpendicular to the major axis direction is defined as the minor axis, and the ratio of major axis / minor axis is defined as the aspect ratio. The average aspect ratio was obtained by arithmetically averaging the aspect ratios of cementite.

[Method for measuring Si concentration and Mn concentration of cementite encapsulated in ferrite]
An extracted replica sample is prepared, and the composition of cementite in the ferrite is analyzed by energy dispersive X-ray analysis using a transmission electron microscope. At this time, since an analysis result including the components of the thin film of the extracted replica is obtained, in order to correct the analysis value, the ratio (Si _θ / Fe _θ ) between the Fe concentration and the Si concentration in cementite obtained from the analysis result and the cementite The Si concentration Si _θ and Mn concentration Mn _θ in the cementite were determined by the following formulas (1) and (2), respectively, using the ratio of the Fe concentration to the Mn concentration (Mn _θ / Fe _θ ).

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
C: 0.10 to 0.30%
One of the features of hot pressing is that it heats and softens the steel sheet for hot pressing, making it easy to perform press forming. In addition, the steel is quenched by quenching with a press die. In addition, it is also characterized by obtaining a hot-pressed part with higher strength. Since the strength after quenching of the steel is mainly determined by the C content, it is necessary to set the C content according to the strength required for the hot press-formed parts. To obtain a tensile strength of 1300 MPa or more of the hot press-formed part targeted by the present invention, the C content is made 0.10% or more. However, if the C content becomes too high, the steel sheet for hot pressing used for hot pressing becomes hard and it becomes difficult to ensure the cold workability before hot pressing. 30% or less. Preferably it is 0.14-0.28%, More preferably, it is 0.18-0.25%.

Si: 2.0% or less (excluding 0%)
Si is an element effective for enhancing the hardenability of the steel sheet for hot pressing and ensuring the strength of the hot press-formed part stably. However, if the Si content becomes too high, it becomes difficult to soften the steel sheet for hot pressing, and the Ac3 point rises remarkably, and ferrite remains in the heating stage during hot pressing, ensuring strength. Since Si cannot be used, the Si content is set to 2.0% or less. Preferably it is 0.02-1.8%, More preferably, it is 0.3-1.5%.

Mn: 0.5 to 3.0%
Mn, like Si, is an extremely effective element for enhancing the hardenability of the steel sheet for hot pressing and for ensuring the strength of the hot press-formed part stably. In order to effectively exhibit such an action, it is necessary to contain 0.5% or more of Mn. However, if the Mn content is too high, it becomes difficult to soften the steel sheet for hot pressing, so the Mn content is 3.0% or less. Preferably it is 0.7 to 2.6%, and more preferably 1.0 to 2.4%.

Cr: 1.0% or less (excluding 0%)
Cr is an element contained in steel as an impurity, but it is also an extremely effective element for enhancing the hardenability of the steel sheet for hot pressing and ensuring the strength of the hot press-formed part stably. . However, if the Cr content becomes too high, it becomes difficult to soften the steel sheet for hot pressing, so the Cr content is 1.0% or less. Preferably it is 0.8% or less, More preferably, it is 0.5% or less. In addition, in order to acquire the effect by the said action | operation more reliably, it is preferable that Cr content shall be 0.1% or more.

Ti: 0.01-0.10%
Since Ti has a higher nitride forming ability than B and Al, it is possible to suppress the formation of BN and AlN that cause toughness deterioration of press-formed parts by sufficiently containing Ti. In order to exhibit such an action effectively, it is necessary to contain Ti 0.01% or more. However, if the Ti content is too high, Ti combines with C in the steel to form a large amount of TiC, making it difficult to soften the steel sheet for hot pressing, so the Ti content is 0.10%. The following. Preferably it is 0.07% or less, More preferably, it is 0.05% or less.

B: 0.0003 to 0.0100%
B is an element that is very effective for enhancing the hardenability of the steel sheet for hot pressing and ensuring the strength of the hot pressed steel sheet member stably. In order to exhibit such an action effectively, it is necessary to contain 0.0003% or more of B. However, if the B content becomes too high, it becomes difficult to soften the steel sheet for hot pressing, so the B content is set to 0.0100% or less. Preferably it is 0.0005 to 0.0080%, More preferably, it is 0.0008 to 0.0050% or less.

  The steel sheet of the present invention basically contains the above components, with the balance being iron and unavoidable impurities (P, S, N, Al, O, etc.).

  In addition, the steel sheet of the present invention can contain the following permissible components as long as the effects of the present invention are not impaired.

Cu: 0.50% or less (excluding 0%),
Ni: 0.50% or less (excluding 0%),
Mo: At least one of 0.50% or less (excluding 0%) These elements effectively act to enhance the hardenability of the steel sheet for hot pressing. However, even if contained excessively, the above action is saturated, and it is economically wasteful. Therefore, it is recommended that each element is contained at the upper limit value or less. In addition, in order to exhibit the said effect | action more effectively, it is still more preferable that the minimum is 0.10% with respect to each element.

V: 0.10% or less (not including 0%) and / or Nb: 0.10% or less (not including 0%)
V and Nb have the effect of forming fine carbides and making the structure fine by the pinning effect. However, even if contained excessively, the above action is saturated, and it is economically wasteful. Therefore, it is recommended that each element is contained at the upper limit value or less. In addition, in order to exhibit the said effect | action more effectively, it is further more preferable that the lower limit shall be 0.01% for each element.

  The steel plate of the present invention may be a non-plated steel plate or a plated steel plate. In the case of a plated steel sheet, the type of plating may be any of general zinc-based plating and aluminum-based plating. The plating method may be any one of hot dipping, electroplating, etc., and may be subjected to alloying heat treatment after plating, or may be subjected to multilayer plating.

  Next, the preferable manufacturing method for obtaining the said steel plate of this invention is demonstrated below.

[Preferred production method of the steel sheet of the present invention]
First, steel adjusted to the above component composition range is melted in a converter, and the melted steel is made into a slab by continuous casting.

The slab is heated and held at 1100 to 1300 ° C. for 5 to 120 minutes, and hot rolling (hot rolling) is performed under the condition that the finishing temperature becomes the temperature of the austenite region.
By making the steel structure an austenite single phase and finishing the processing at a temperature of the austenite region, by preventing the fine graining of the ferrite generated after winding, the final hot press steel sheet can be softened. realizable.

[Cooling rate after hot rolling: 10 ° C./s or more, winding temperature: 300 to 450 ° C.]
After completion of hot rolling, it is cooled at an average cooling rate of 10 ° C./s or more and wound up at 300 to 450 ° C.
Thus, by rapidly cooling to 300 to 450 ° C., a certain amount of bainite structure having a high dislocation density can be contained in the steel structure, and as a result, recrystallization is promoted in the annealing process after the subsequent cold rolling. Thus, the aspect ratio reduction (spheroidization) of cementite produced with recrystallization is realized.
In addition, Mn and Si, which are substitutional elements, are rapidly cooled to a temperature range of 300 to 450 ° C., which is difficult to move, so that during cooling after winding, Mn flows into the cementite and concentrates. Therefore, it is possible to prevent the concentration of Si from being discharged and the concentration from being lowered.

[Cold rolling ratio: 30-80%]
The hot-rolled material obtained in the hot-rolling step is subjected to cold rolling (cold rolling) after pickling, and the cold rolling rate is 30 to 80%.
By increasing the degree of cold working, recrystallization is promoted in the next annealing step, which contributes to a reduction in the aspect ratio (spheroidization) of cementite that is generated along with the recrystallization. However, if the cold rolling rate is too high, the degree of extension of the ferrite grains increases, and the strength of the steel sheet for hot pressing obtained by subsequent annealing increases, making it difficult to soften.

[Annealing temperature: 500 to 750 ° C., annealing time: 10 to 300 s]
The cold-rolled material obtained in the cold-rolling step is annealed, and the annealing temperature is 500 to 750 ° C. and the annealing time is 10 to 300 s.
By holding at 500 ° C. or higher for a certain period of time, recrystallization occurs, and the cementite aspect ratio is reduced (spheroidized). Recrystallization is more likely to occur for longer periods of time at higher temperatures, which is preferable for reducing the aspect ratio (spheroidization) of cementite. On the other hand, alloying elements tend to concentrate in cementite, and when hot-pressing steel plates for high-temperature pressing. Hardenability during high-speed and short-time heating is degraded.

  In addition, although the steel plate which the said annealing was complete | finished is good also as a steel plate for the hot press which concerns on this invention, what applied the electroplating layer and the hot dipping layer to the surface further with respect to the steel plate which the said annealing was complete | finished to this invention. It is good also as the steel plate for such a hot press. Alternatively, in order to suppress scale that may occur during the manufacture of hot-press molded parts and / or to facilitate shape processing during hot pressing, metals, organic substances or inorganic substances, or these A steel sheet for hot pressing according to the present invention may be formed by forming a coating layer made of a composite compound or a mixture on the surface.

  Using a vacuum induction furnace (VIF), a test steel containing the chemical components shown in Table 1 below was melted, cast into a 50 kg ingot, and cooled. After heating the obtained ingot to 1200 ° C., it was once hot-rolled to a plate thickness of 25 mm and used as a test material. This specimen is heated again to 1200 ° C., then hot-rolled at a finishing temperature of 900 ° C. to each plate thickness, and then rapidly cooled at a cooling rate of 20 ° C./s to simulate winding. The rapid cooling was stopped at the take-up temperature, and after putting in an atmospheric furnace heated to the take-up temperature and maintaining for 30 minutes, the furnace was cooled to obtain a hot-rolled sheet. And this hot-rolled sheet was cold-rolled to obtain a cold-rolled sheet having a thickness of 1.4 mm. This cold-rolled sheet was annealed at each annealing temperature and time using a continuous annealing line (CAL) simulator to obtain an annealed sheet. Moreover, when plating on the surface, after performing the annealing process at each annealing temperature and annealing time using a plating simulator, it was immersed in a plating bath, plated, and a hot-dip plated plate was obtained. The annealed plate and hot-dip plated plate thus prepared were used as hot press steel plate specimens. Each manufacturing condition is shown in Table 2 below.

  For each hot-press steel sheet specimen produced in this way, the area ratio of ferrite containing cementite and the inclusion in the ferrite by the measurement method described in the above section [Mode for carrying out the invention] The average aspect ratio, Si concentration, and Mn concentration of the cementite were measured. Moreover, in order to evaluate the mechanical property before hot pressing about each test material, the No. 5 test piece as described in JISZ2201 was produced, the tensile test was done according to JISZ2241, and the tensile strength was measured.

  Next, in order to evaluate the hardenability at the time of hot pressing for each of the steel sheet test materials for hot pressing, each test material is heated to 900 ° C. at a heating rate of 80 ° C./s with an electric heating device, After holding for 90 s, cooling is performed at various cooling rates, and by analyzing the steel structure of each specimen after the heat treatment, a cooling rate at which martensite can be ensured by 90% or more in area ratio is obtained, and the critical cooling rate As an index for evaluating the hardenability.

  Further, each of the steel sheets for hot pressing was heated to 900 ° C. at a heating rate of 80 ° C./s with the above-mentioned electric heating device, held for 90 s, and then immediately pressed to perform mold quenching. . In order to evaluate the mechanical properties after hot pressing of the test material obtained after hot pressing, a No. 5 test piece described in JIS Z 2201 was prepared, and a tensile test was performed according to JIS Z 2241. The tensile strength was measured.

  The measurement results are shown in Table 3 below.

  As shown in Table 3 above, Test No. Reference numerals 1, 10 to 12, and 16 to 21 are comparative steel sheets that do not satisfy at least one of the component composition and steel structure requirements defined in the present invention. Tensile strength before hot pressing, critical cooling rate, hot pressing At least one of the later tensile strengths does not meet the acceptance criteria.

  In contrast, test no. 2 to 9 and 13 to 15 are invention steel plates that satisfy all the requirements of the steel structure provision of the present invention as a result of producing them under the recommended production conditions using steel types that satisfy the range of the component composition of the present invention. The tensile strength before hot pressing, the critical cooling rate, and the tensile strength after hot pressing all satisfy the acceptance criteria, while reducing the steel plate strength before hot pressing, It was confirmed that a steel sheet for hot pressing capable of ensuring sufficient hardenability even with heating for a short time was obtained.

Claims (3)

Ingredient composition is in mass% (hereinafter the same for chemical ingredients)
C: 0.10 to 0.30%,
Si: 2.0% or less (excluding 0%)
Mn: 0.5 to 3.0%
Cr: 1.0% or less (excluding 0%),
Ti: 0.01-0.10%,
B: 0.0003 to 0.0100%,
Each of which comprises iron and inevitable impurities,
Steel structure
Made of ferrite and cementite,
The area ratio of ferrite enclosing cementite is 50% or more,
The average aspect ratio of cementite encapsulated in the ferrite is 2 or less,
When the Si concentration of cementite contained in the ferrite is Si _θ , the Mn concentration is Mn _θ , the Mn content of the whole steel sheet is Mn, and the Si content is Si, Mn _θ /Mn≦1.3, Si _θ A steel sheet for hot pressing characterized by satisfying /Si≧0.8. Ingredient composition further
Cu: 0.50% or less (excluding 0%),
Ni: 0.50% or less (excluding 0%),
The steel sheet for hot pressing according to claim 1, comprising at least one of Mo: 0.50% or less (not including 0%). Ingredient composition further
V: 0.10% or less (not including 0%) and / or Nb: 0.10% or less (not including 0%)
The steel sheet for hot pressing according to claim 1 or 2, wherein the steel sheet for hot pressing is contained.