JP5835622B2 - Hot-pressed steel plate member, manufacturing method thereof, and hot-press steel plate - Google Patents

Description

  The present invention relates to a hot-pressed steel sheet member, a manufacturing method thereof, and a hot-press steel sheet. More specifically, the present invention is suitable for machine structural parts such as automobile body structural parts, undercarriage parts, etc., and has high strength heat with little strength variation, excellent toughness, and excellent delayed fracture resistance. The present invention relates to a hot-pressed steel plate member, a manufacturing method thereof, and a hot-press steel plate.

  In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel used for the vehicle body and reduce the weight used. In a thin steel sheet widely used for automobiles, press formability decreases with increasing strength, and it becomes difficult to manufacture a complicated shape. Specifically, there arises a problem that the ductility is lowered and the fracture occurs at a site where the degree of processing is high, or the spring back and the wall warp become large and the dimensional accuracy is deteriorated. Therefore, it is not easy to produce a part by press molding using a steel plate having high strength, particularly a tensile strength (hereinafter also referred to as “TS”) of 780 MPa class or higher.

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

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

  Thus, since hot pressing is an excellent forming method, its application has expanded in recent years, and research on hot pressing has been further advanced.

  By the way, the microstructure of a member obtained by hot pressing is conventionally considered to be a martensite single-phase structure and its strength does not vary much.

  However, as described in Non-Patent Document 1, it has been clarified by recent research that a great difference occurs in strength depending on the part even in the same member. In other words, this suggests that there is a risk of significant strength reduction depending on hot pressing conditions.

British Patent No. 1490535 Japanese Patent Laid-Open No. 10-96031

Iron and Steel, Japan Iron and Steel Association, 2010, Vol. 96, No. 6, pages 378-385

  In order to cope with such a problem, a method of simply preventing a decrease in strength by simply adding a large amount of a hardenability improving element or the like to the steel sheet can be considered. However, addition of a large amount of elements such as hardenability improving elements is not practical because it generally leads to a decrease in toughness and delayed fracture resistance.

  The present invention has been made in view of the above-described prior art, and provides a hot-pressed steel sheet member having little strength variation, excellent toughness and delayed fracture resistance, a manufacturing method thereof, and a hot-press steel sheet. That is.

  The inventors of the present invention have made extensive studies on hot-pressed steel sheet members to reduce strength variation and to ensure excellent toughness and delayed fracture resistance. As a result, the present inventors have newly found a method for satisfying the above characteristics by using a steel material having an appropriate steel plate component and performing hot pressing under appropriate conditions. The gist of the present invention is as follows.

(1) By mass%, C: 0.26% to 0.35%, Mn: 1.5 1 % to 2.0%, Nb: 0.01% to 1.0%, B: 0 0.0001% to 0.01%, Cr: 0.5% or less, P: 0.05% or less, S: 0.03% or less, Si: 0.5% or less, Cu: 1% Hereinafter, it contains one or more selected from the group consisting of V: 1% or less, Mo: 1% or less, Al: 1% or less, and N: 0.01% or less. ) Containing Ti that satisfies the formula, the balance having a chemical composition consisting of Fe and impurities,
Having a steel structure having a prior austenite average particle size of 10 μm or less,
A hot-pressed steel sheet member having a mechanical property of a tensile strength of 1.8 GPa or more and 2.0 GPa or less.

3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5 (1)
Here, Ti and N in Formula (1) show content (unit: mass%) of each element in steel.

  (2) The hot-pressed steel sheet member according to (1), wherein the chemical composition contains Ni: 3% or less in mass% instead of part of Fe.

  (3) The hot composition according to (1) or (2), wherein the chemical composition contains, by mass%, Bi: 0.02% or less instead of part of Fe Pressed steel plate member.

  (4) The chemical composition is mass% in place of part of Fe, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% The hot-pressed steel sheet member according to any one of items (1) to (3), comprising one or more selected from the group consisting of:

(5) the steel material Ac ₃ point or more and heated to (Ac ₃ point + 100 ° C.) below the temperature range, the time that the steel is exposed to air in the completion of the heating to the start of the hot press 1 0 seconds the hot pressing applied as within, the upper critical cooling rate cooling rate higher than room temperature, characterized in that cooling, (1) hot pressing steel member according to any of paragraphs to item (4) Manufacturing method.

  (6) A steel sheet having the chemical composition according to any one of items (1) to (4), and hot between any one of items (1) to (4) A steel plate for hot pressing, which is used for a material of a pressed steel plate member.

  According to the present invention, there are provided a hot-pressed steel sheet member having little strength variation, excellent toughness and delayed fracture resistance, a manufacturing method thereof, and a hot-press steel sheet.

FIG. 1 is an explanatory view showing the situation of a four-point bending delayed fracture test. FIG. 2 is an explanatory diagram showing a hot press test procedure. FIG. 3 is an explanatory view showing a cylindrical test piece.

1. Chemical Composition The reason why the chemical composition of the hot-pressed steel plate member (hereinafter also simply referred to as “steel plate member”) and the hot-pressed steel plate according to the present invention is defined as described above will be described. In the following description, “%” representing the content of each alloy element means mass% unless otherwise specified.

(C: 0.26% to 0.35%)
C is a very important element that enhances the hardenability of the steel sheet and mainly determines the strength after hot pressing. In particular, in order to ensure TS1.8 GPa or more in strength after hot pressing, the C content needs to be at least 0.26%. Therefore, the C content is 0.26% or more. Preferably it is 0.28% or more. On the other hand, if the C content exceeds 0.35%, the strength after hot pressing becomes too high, and the toughness deterioration becomes remarkable. Therefore, the C content is 0.35% or less. Preferably it is 0.33% or less.

(Mn: 1.5% to 2.0%)
Mn is an element that is very effective for enhancing the hardenability of the steel sheet and stably securing the strength after hot pressing. However, if the Mn content is less than 1.5%, the effect is not sufficient. Therefore, the Mn content is 1.5% or more. On the other hand, when the Mn content exceeds 2.0%, the effect is saturated and the toughness and delayed fracture resistance after hot pressing are greatly deteriorated. Therefore, the Mn content is 2.0% or less. Preferably it is 1.7% or less.

(Nb: 0.01% to 1.0%)
Nb has the effect of greatly improving the toughness after hot pressing because it suppresses recrystallization and forms fine carbides to make austenite grains fine when the steel sheet is heated to Ac ₃ points or higher. . However, if the Nb content is less than 0.01%, it is difficult to obtain the effect by the above action. Therefore, the Nb content is 0.01% or more. Preferably it is 0.02% or more, More preferably, it is 0.04% or more. On the other hand, when the Nb content exceeds 1.0%, the effect by the above action is saturated, and the cost is unnecessarily increased. Therefore, the Nb content is 1.0% or less. Preferably it is 0.15% or less, More preferably, it is 0.1% or less.

(B: 0.0001% to 0.01%)
B is an element effective for enhancing the hardenability of steel and further enhancing the effect of ensuring the strength stability after hot pressing. It is also an important element in that it segregates at grain boundaries to increase grain boundary strength and improve toughness and delayed fracture resistance. Furthermore, it has the effect | action which improves toughness by suppressing the grain growth of the austenite in the heating process at the time of using for a hot press. If the B content is less than 0.0001%, it is difficult to obtain the effect by the above action. Therefore, the B content is 0.0001% or more. Preferably, it is 0.001% or more.

  On the other hand, if the B content exceeds 0.01%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the B content is 0.01% or less.

  The grain boundary segregation amount of B is affected by the austenite grain size when subjected to hot pressing. That is, the smaller the austenite grain size, the more B segregation sites increase, so that more B can segregate. On the other hand, as the austenite grain size increases, the segregation sites of B decrease, so that the segregable amount of B decreases. Therefore, the upper limit of the B content is determined according to the austenite grain size when subjected to hot pressing, that is, the prior austenite grain size in the hot-pressed steel sheet member, so that the effect of B segregation can be obtained efficiently. It is preferable because it is possible. Specifically, it is preferable that the following formula (2) is satisfied.

B content (ppm) ≦ exp (4.57−0.571 × ln (r)) (2)
Here, r is the average section length (μm) of the prior austenite grains.

  In order to satisfy the above formula (2), the relationship between the chemical composition and the austenite grain size when subjected to hot pressing should be determined empirically, and the chemical composition and hot pressing conditions should be adjusted. .

(Cr: 0.5% or less, P: 0.05% or less, S: 0.03% or less, Si: 0.5% or less, Cu: 1% or less, V: 1% or less, Mo: 1% or less , Al: 1% or less and N: 0.01% or less selected from the group consisting of 0.01% or less)
These elements are elements that are effective in enhancing the hardenability of the steel and ensuring stable strength after hot pressing. Therefore, you may contain 1 type, or 2 or more types of these elements. However, since the effect is small even if it contains more than the said upper limit, and it causes a cost increase unnecessarily, content of each alloy element shall be the said range. In order to obtain the above effect more reliably, Cr: 0.01% or more, P: 0.0001% or more, S: 0.0001% or more, Si: 0.01% or more, Cu: 0.01% It is preferable to satisfy at least one of V: 0.01% or more, Mo: 0.01% or more, Al: 0.01% or more, and N: 0.0001% or more.

(3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5)
Ti has the effect | action which improves the effect by B by fixing N in steel. When the Ti content is less than (3.42N + 0.001)%, it is difficult to obtain the effect by the above action. Therefore, the Ti content is set to (3.42N + 0.001)% or more. Preferably, it is (3.42N + 0.02)% or more. On the other hand, if the Ti content exceeds (3.42N + 0.5)%, a large amount of Ti-based precipitates are generated, and the toughness is deteriorated. Therefore, the Ti content is set to (3.42N + 0.5) or less. Preferably, it is (3.42N + 0.08)% or less.

(Ni: 3% or less)
Ni is a very effective element in order to enhance the hardenability of the steel sheet and to stably secure the strength after hot pressing. Furthermore, it has the effect of increasing the cleavage fracture strength and greatly improving toughness. Furthermore, it has the effect of improving delayed fracture resistance. Therefore, Ni may be included. However, when the Ni content exceeds 3%, the effect is saturated and the cost is increased. Therefore, the Ni content is 3% or less. Preferably it is 1.0% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable that Ni content shall be 0.01% or more, and it is more preferable to set it as 0.1% or more.

(Bi: 0.02% or less)
Bi is an extremely effective element for enhancing the hardenability of the steel sheet and stably securing the strength after hot pressing. Moreover, it is also an element which has the effect | action which makes a structure | tissue uniform and raises the toughness after a hot press further. Furthermore, it has the effect of suppressing hydrogen penetration into the steel sheet and improving delayed fracture resistance. Therefore, Bi may be included. However, if the Bi content exceeds 0.02%, hot workability deteriorates and hot rolling becomes difficult. Therefore, the Bi content is 0.02% or less. Preferably it is 0.015% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable that Bi content shall be 0.001% or more, and it is more preferable to set it as 0.002% or more.

(One or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less)
These elements contribute to inclusion control during steelmaking, in particular to fine dispersion of inclusions, and have an effect of increasing toughness after hot pressing. Therefore, you may contain 1 type, or 2 or more types of these elements. However, when the content exceeds 0.01%, deterioration of the surface properties may become obvious. Therefore, the content of each element is as described above. In addition, in order to acquire the effect by the said action | operation more reliably, it is preferable that content of at least 1 of these elements shall be 0.0003% or more.

  Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

2. Steel structure In a hot-pressed steel sheet member, if the prior austenite average particle size exceeds 10 μm, it is difficult to ensure good toughness. Therefore, the prior austenite average particle size is 10 μm or less. Preferably it is 8 micrometers or less. From the viewpoint of ensuring good toughness, the prior austenite average particle diameter is preferably as fine as possible. Accordingly, the lower limit of the prior austenite average particle size need not be specified, but is usually 3 μm or more.

  As will be described later, the steel sheet for hot pressing is heated to the austenite region during hot pressing and is often press-formed in that temperature region. It is not very important and it is not necessary to specify the steel structure. Therefore, any of hot-rolled steel sheets, cold-rolled steel sheets (full hard material and annealed material), and plated steel sheets may be used as the hot-press steel sheet, and the manufacturing method is not particularly limited. For example, examples of the plated steel sheet include an aluminum-based plated steel sheet and a zinc-based plated steel sheet.

3. Mechanical properties When the tensile strength is less than 1.8 GPa, it is not possible to meet the recent needs for higher strength, and toughness and delayed fracture resistance do not become a significant problem. Therefore, the tensile strength of the hot pressed steel sheet member is 1.8 GPa or more.

  On the other hand, if the tensile strength exceeds 2.0 GPa, it is difficult to ensure good toughness and delayed fracture resistance. Therefore, the tensile strength of the hot pressed steel sheet member is 2.0 GPa or less.

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

When the heating temperature of the steel material to be subjected to hot pressing is less than Ac ₃ points, since a part of the steel structure is not austenitic, high strengthening by quenching cannot be achieved for the part, and the tensile strength of the hot pressed steel plate member It becomes difficult to make the thickness 1.8 GPa or more. Therefore, the heating temperature of the steel material used for hot pressing is set to Ac ₃ points or higher.

On the other hand, when the heating temperature of the steel material to be subjected to hot pressing exceeds (Ac ₃ points + 100 ° C.), the grain growth of austenite proceeds excessively, and the old austenite average particle size of the steel structure of the hot pressed steel sheet member is 10 μm. It becomes difficult to make the following. Therefore, the heating temperature of the steel material used for hot pressing is set to (Ac ₃ points + 100 ° C.) or less.

The holding time for holding the steel material to be subjected to hot pressing in a temperature range of Ac ₃ points or more (Ac ₃ points + 100 ° C.) or less is not particularly defined, but it is 1 minute or more from the viewpoint of surely promoting the austenitization of the steel materials. In view of suppressing the grain growth of austenite, it is preferably 10 minutes or less.

  The heating method of the steel material to be subjected to hot pressing is not particularly required, and the steel material is heated at several degrees C / second to several hundred degrees C / second by furnace heating, high-frequency heating, electric heating, or the like.

  The heated steel material is then subjected to hot pressing, but the steel material is usually exposed to air cooling while the steel material is conveyed from the completion of heating to the start of hot pressing. For this reason, if the conveying time is too long, ferrite transformation occurs, and it becomes difficult to set the tensile strength of the hot-pressed steel sheet member to 1.8 GPa or more. The cooling rate in the air cooling is about several tens of degrees centigrade / second. In order to prevent the ferrite transformation from occurring at such a cooling rate, the steel material having the above chemical composition can be heated from the completion of heating to the hot. What is necessary is just to make time to a press start less than 15 second. Therefore, the hot pressing is performed by setting the time during which the steel is exposed to air cooling from the completion of the heating to the start of the hot pressing for less than 15 seconds.

  Cooling of the steel material in the hot press is important in order to set the hot pressed steel plate member to a structure mainly composed of martensite and the tensile strength of the hot pressed steel plate member to 1.8 GPa or more. Therefore, the cooling rate of the steel material in the hot press is set to the upper critical cooling rate or more and cooled to room temperature.

  The steel structure thus obtained is a structure in which lath martensite and autotemper martensite are singly or mixed, but inevitably several percent of retained austenite, bainite, or the like may be mixed.

Examples of the present invention will be described below.
A slab melted in a laboratory having the chemical composition shown in Table 1 was heated at 1250 ° C. for 30 minutes and hot-rolled at 900 ° C. or higher to obtain a steel plate having a thickness of 4 mm. After hot rolling, after water spray cooling to 600 ° C., charging in a furnace, holding at 600 ° C. for 30 minutes, and then gradually cooling to room temperature at 20 ° C./hour to simulate a hot rolling winding process. . The obtained hot rolled sheet was made to have a thickness of 2.6 mm by cold rolling after removing the scale by pickling.

The steel plate thus obtained was heated in a gas furnace set to an air-fuel ratio of 1.1 under the conditions shown in Table 2, and then taken out from the heating furnace and transported (air-cooled) and then flat steel plate Hot pressing was performed using a mold. In the hot press, a thermocouple was attached to the steel sheet, and the cooling rate was also measured. In addition, the holding time in Table 2 is the time from the time when it reaches Ac ₃ point to the time when it reaches the heating temperature until it is removed from the heating furnace.

  The obtained hot pressed steel sheet member was subjected to old austenite particle size measurement (average slice length measurement) by a cutting method, tensile test (JIS No. 5 test piece), Charpy impact test, and 4-point bending delayed fracture test.

In the Charpy impact test, 1 / 4-size (plate thickness: 2.5 mm) V-notch Charpy test pieces were prepared and evaluated. The toughness evaluation, if the impact value at -40 ℃ is 30 J / cm ² or more 50 J / cm ² or less as acceptable, in the table is represented as ○, if it exceeds 50 J / cm ² is very In the table, it was indicated as “◎” as being excellent in toughness.

  In the 4-point bending delayed fracture test, a 1.2 t × 7.0 w × 68 L (mm) test piece was prepared, and as shown in FIG. 1, various strains (stresses) were applied to the test piece by a 4-point bending jig. And then immersed in hydrochloric acid (30 ° C., pH = 1) for 100 hr, and the amount of limit strain at which cracking occurs was investigated. In the delayed fracture resistance evaluation, when the critical strain amount is 0.6% or more and 1.0% or less, it is determined to be acceptable. In the table, it was indicated as “◎” as being excellent in delayed fracture property.

  In addition, a test piece having a length of 200 mm and a width of 50 mm was taken from the cold-rolled steel sheet, heated under the conditions shown in Table 2, and then subjected to a hot press test that was sandwiched between the split molds shown in FIG. At this time, the clearance width W was 70 mm, and the upper and lower clearances CL were each 0.2 mm. The holding at the bottom dead center was performed for 60 seconds with a pressing force of 49 kN. About the test piece obtained by this hot press test, the cross-sectional Vickers hardness (load 9.8 N) of the position shown in FIG. 2 is measured, and the minimum Vickers hardness in the clearance portion and the portion where the mold is in direct contact are measured. The difference from the average value of Vickers hardness is defined as △ Hv, and △ Hv is 20 or more and 50 or less, and is indicated as ◯ in the table, and when it is less than 20, there is very little hardness variation. In the table, it was written as ◎.

In addition, Ac ₃ point and upper critical cooling rate of each steel type were measured by the following method. A cylindrical test piece having a diameter of 3.0 mm and a length of 10 mm shown in FIG. 3 was cut out from the hot-rolled steel sheet and heated to 900 ° C. at a temperature increase rate of 10 ° C./second in an inert gas atmosphere. After holding for a minute, it was cooled to room temperature at various cooling rates. The Ac ₃ points were measured by measuring the change in thermal expansion during heating at that time. Moreover, the measurement of the thermal expansion change during cooling, the Vickers hardness measurement (load 49N, number of measurements: 3) of the obtained test piece, and the structure observation were performed, and the upper critical cooling rate was estimated from those results.

  As a general rule, any item that failed even one of the evaluation items was canceled or not evaluated.

The results are shown in Table 2.
Example No. which is an example of the present invention. From 1 to 9, it is clear that the hardness variation is small and the toughness and delayed fracture resistance are excellent.

  On the other hand, Example No. which is a comparative example. Nos. 10 and 11 do not satisfy the range of the present invention in terms of tensile strength.

  Example No. No. 12 has a small Mn content and is out of the scope of the present invention, so the hardness variation is large.

  Example No. Since No. 13 has a high Mn content and is out of the scope of the present invention, delayed fracture resistance is inferior.

  Example No. No. 14, because the Nb content deviates from the scope of the present invention, the prior austenite grain size becomes coarse and the toughness is inferior.

  Example No. No. 15 is inferior in toughness because the Ti content and B content deviate from the scope of the present invention.

  Furthermore, Example No. In No. 16, the air cooling time is too long, so the tensile strength does not satisfy the scope of the present invention.

Claims (6)

In mass%, C: 0.26% to 0.35%, Mn: 1.5 1 % to 2.0%, Nb: 0.01% to 1.0%, B: 0.0001% In addition to containing 0.01% or less, Cr: 0.5% or less, P: 0.05% or less, S: 0.03% or less, Si: 0.5% or less, Cu: 1% or less, V 1% or less, Mo: 1% or less, Al: 1% or less, and N: 0.01% or less, or one or more selected from the group consisting of 0.01% or less, and further, the following formula (1): Containing satisfactory Ti, the balance having a chemical composition consisting of Fe and impurities,
Having a steel structure having a prior austenite average particle size of 10 μm or less,
A hot-pressed steel sheet member having a mechanical property of a tensile strength of 1.8 GPa or more and 2.0 GPa or less.
3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5 (1)
Here, Ti and N in Formula (1) show content (unit: mass%) of each element in steel.   The hot-pressed steel sheet member according to claim 1, wherein the chemical composition contains Ni: 3% or less in mass% instead of part of Fe.   The hot-pressed steel sheet member according to claim 1 or 2, wherein the chemical composition contains Bi: 0.02% or less in mass% instead of part of Fe.   The chemical composition is, in place of a part of Fe, in mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less. The hot-pressed steel sheet member according to any one of claims 1 to 3, comprising one or more selected from the group. The steel material is heated to Ac ₃ point or more (Ac ₃ point + 100 ° C.) below the temperature range, the heat time the steel to the completion of the heating to the start of the hot pressing is exposed to air as within 1 0 seconds The method for producing a hot-pressed steel sheet member according to any one of claims 1 to 4, wherein the hot-pressed steel sheet member is subjected to hot pressing and cooled to room temperature at a cooling rate equal to or higher than the upper critical cooling rate.   It is a steel plate which has the chemical composition of any one of Claim 1 to Claim 4, Comprising: As a raw material of the hot press steel plate member of any one of Claim 1 to Claim 4 A steel sheet for hot pressing, which is used for a use.