JP5521818B2 - Steel material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel material excellent in toughness and ductility, a manufacturing method thereof, and a quenching steel plate suitable as a material for the steel material used for machine structural parts such as automobile body structural parts and underbody parts. About.

  In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel materials and reduce the weight used. In steel plates widely used in automobiles, press formability decreases with increasing steel plate strength, making it difficult to manufacture complex shapes. Specifically, there are problems that the ductility is reduced and fracture occurs at a high degree of processing, and that the spring back and wall warp are 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 of 780 MPa class or higher (hereinafter also simply referred to as “TS”).

  On the other hand, as disclosed in Patent Document 1, in a method called hot pressing in which a heated steel plate is press-formed, press forming is performed in a state where the steel plate is soft and highly ductile at a high temperature. The shape can be formed with high dimensional accuracy. Furthermore, by strengthening the steel sheet in the austenite region and quenching (quenching) in the mold, the steel material can be strengthened by martensitic transformation at the same time.

  In Patent Document 2, after forming into a predetermined shape at room temperature in advance, heating to an austenite region and quenching in a mold ensure the formability of the steel sheet and increase the strength of the steel material after forming. A pre-press quench method is disclosed.

Such a hot press method and a pre-press quench method are excellent forming methods that can simultaneously ensure the formability of the steel sheet during forming and the strengthening of the steel material after forming.
By the way, the steel structure of the steel material thus obtained is a martensite single-phase structure generally considered to have poor toughness.

  Therefore, Patent Document 3 discloses to improve the toughness of the steel material by controlling the cooling rate during hot pressing, and Patent Document 4 discloses to improve the toughness of the steel material mainly by reducing the prior austenite grain size. It has been proposed that

British Patent Publication No. 1490535 Japanese Patent Laid-Open No. 10-96031 JP 2004-353026 A JP 2006-152427 A

  The present invention provides a steel material having an extremely high tensile strength of 2.0 GPa or more and further having good toughness and ductility, and a method for producing the same, in order to meet the recent demand for higher strength. It is another object of the present invention to provide a quenching steel plate suitable as a material for the steel material.

In order to obtain a steel material having excellent toughness and ductility while having an extremely high tensile strength of 2.0 GPa or more, the present inventor has intensively studied as follows.
That is, Mn is an element that is particularly effective in enhancing the hardenability of the steel and ensuring the strength of the steel material after quenching stably, so that the strength after quenching is about 1.5 GPa. Conventionally, it has been frequently used as a main component of steel sheets for quenching treatment such as hot pressing.

  However, when the strength after quenching becomes 2.0 GPa or more, deterioration of toughness due to segregation of Mn itself becomes obvious, and Mn which promotes grain boundary segregation such as P causing grain boundary embrittlement. It has been found that toughness deterioration due to action is also manifested, and the two together may lead to significant toughness deterioration.

  Therefore, when the strength after quenching is set to 2.0 GPa or more, in order to ensure good toughness, the upper limit of the Mn content is set as compared with the conventional technique in which the strength after quenching is about 1.5 GPa. Must be strictly limited.

Therefore, after strictly limiting the upper limit of the Mn content, a method for setting the strength after quenching to 2.0 GPa or more and further providing good toughness was studied.
As a result, it has the same effect as Mn, and further contains Ni having the effect of greatly improving the toughness by increasing the cleavage fracture strength, and also has the same effect as Mn, and further at the grain boundary. The idea was to add B, which has the effect of greatly improving toughness by segregating and increasing the grain boundary strength, to increase the strength after quenching to 2.0 GPa or more and to provide better toughness.

However, when the hot-pressed steel material was actually made on the basis of the above idea and the toughness was confirmed, it was found that the effect of B could be obtained or not.
Therefore, when further investigation was made, the action of Mn that promotes grain boundary segregation also affects the grain boundary segregation of B. When the upper limit of the Mn content is strictly limited, the grain boundary area where B segregates. It has been found that the size of has a great influence on the toughness of steel.

  That is, in general, it is said that the smaller the prior austenite particle size is, the more preferable it is to improve the toughness. However, the smaller the prior austenite particle size, the larger the grain interfacial area where B segregates. When the upper limit is strictly limited, there is a possibility that the amount of B segregating at the grain boundary is insufficient as compared with the grain boundary area. In such a case, the effect of B cannot be obtained sufficiently. It turned out.

  Therefore, when strictly limiting the upper limit of the Mn content as in the present invention, it is inappropriate to aim to simply reduce the prior austenite grain size in accordance with the idea of improving toughness in the prior art, It is necessary to have a suitable prior austenite grain size. And by doing in this way, favorable ductility can be provided.

  The present invention has been made on the basis of the above new findings, and the gist thereof is as follows.

(1) By mass%, C: 0.33% to 0.40%, Ni: 2.0% to 5.0%, Mn: 0.01% to less than 0.5%, B: 0.00. 0001% or more and 0.01% or less, Al: 0.01% or more and 3% or less, P: 0.05% or less, S: 0.03% or less, N: 0.01% or less, and the following formula Ti is contained within a range satisfying (1), and Cr: 0.5% or less, Si: 0.5% or less, Cu: 1% or less, V: 1% or less, Nb: 1% or less, Mo: Martens containing one or more selected from the group consisting of 1% or less and Co: 3% or less, the balance having a chemical composition consisting of Fe and impurities, and an average austenite grain size of 5 μm or more It has a steel structure consisting of sites, and has mechanical properties such that tensile strength is 2.0 GPa or more and total elongation is 5% or more. Steel material and wherein the door.
3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5 (1)
Here, N and Ti indicate the contents (unit: mass%) of N and Ti in the chemical composition, respectively.

(2) A steel material having the chemical composition described in (1) above is held in a temperature range of Ac ₃ points or higher for 5 minutes or more, and then subjected to a quenching process for cooling to the Mf point at a cooling rate higher than the upper critical cooling rate. A method for producing a steel material characterized by the above.

  According to the present invention, a steel material having a tensile strength of 2.0 GPa or more and further having good toughness and ductility is provided. Moreover, the method of manufacturing the steel material stably and the steel plate for hardening process suitable as a raw material of the steel material are also provided.

It is explanatory drawing of the shape of the test piece for measuring Ac ₃ points | pieces and an upper critical cooling rate.

  Hereinafter, a steel material, a manufacturing method thereof, and a steel sheet for quenching according to the present invention will be described. In the following description, “%” defining the chemical composition is “% by mass” unless otherwise specified.

1. Chemical composition of steel materials and steel sheets for quenching treatment (1) C: 0.33% or more and 0.40% or less C is an important factor that enhances the hardenability of steel and mainly determines the strength of the steel material after quenching. Element. If the C content is less than 0.33%, it is difficult to secure a TS of 2.0 GPa or more with the strength of the steel material after quenching. Therefore, the C content is 0.33% or more. On the other hand, if the C content exceeds 0.40%, the strength of the steel material after quenching becomes too high, and the toughness may be significantly deteriorated. Therefore, the C content is set to 0.40% or less. Preferably it is 0.38% or less.

(2) Ni: 2.0% or more and 5.0% or less Ni is an element that is particularly effective for enhancing the hardenability of steel and stably securing the strength of the steel material after quenching. Furthermore, it is an important element that greatly improves the toughness of the steel material by increasing the cleavage fracture strength. If the Ni content is less than 2.0%, it is difficult to obtain the above effect. Therefore, the Ni content is 2.0% or more. Preferably it is 3.0% or more. On the other hand, even if the Ni content exceeds 5.0%, the above effect is saturated and disadvantageous in cost. Therefore, the Ni content is 5.0% or less. Preferably it is 4.0% or less.

(3) Mn: 0.01% or more and less than 0.5% Mn is an element that is particularly effective for enhancing the hardenability of steel and stably securing the strength of the steel material after quenching. If the Mn content is less than 0.01%, it is difficult to obtain the above effects. Therefore, the Mn content is 0.01% or more. Preferably it is 0.1% or more. On the other hand, when the Mn content is 0.5% or more, when the strength of the steel material after quenching is set to 2.0 GPa or more, deterioration of toughness due to segregation of Mn itself becomes obvious, and grain boundaries Degradation of toughness due to the action of Mn, which promotes grain boundary segregation such as P, which causes embrittlement, is also manifested, and together they may lead to significant toughness degradation. Therefore, the Mn content is less than 0.5%. Preferably it is 0.2% or less.

(4) B: 0.0001% or more and 0.01% or less B is an element that is particularly effective for enhancing the hardenability of the steel and stably securing the strength of the steel material after quenching. Furthermore, it segregates at the grain boundaries to increase the grain boundary strength and is an important element that greatly improves the toughness of the steel after quenching by suppressing excessive grain growth of austenite in the heating process during quenching. . If the B content is less than 0.0001%, it is difficult to obtain the above effect. Therefore, the B content is 0.0001% or more. Preferably it is 0.0010% or more. On the other hand, if the B content exceeds 0.01%, the above effects are saturated and disadvantageous in cost. Therefore, the B content is 0.01% or less. Preferably it is 0.0030% or less.

(5) Ti: Range satisfying 3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5 Ti is wasted as BN by combining with N in steel in preference to B Is suppressed, and the effect of B is improved. When the Ti content is less than (3.42N + 0.001)% (“N” in 3.42N means N content (unit: mass%), the same applies hereinafter), it is difficult to obtain the above effect. is there. 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, when the Ti content exceeds (3.42N + 0.5)%, a large amount of Ti-based precipitates are formed in the steel, and the toughness of the steel sheet for quenching treatment and the steel material after quenching is remarkably deteriorated. There is a case. Therefore, the Ti content is set to (3.42N + 0.5)% or less. Preferably, it is (3.42N + 0.08)% or less.

(6) Al: 0.01% or more and 3% or less Al is an element that is effective in enhancing the hardenability of the steel and stably securing the strength of the steel material after quenching. Furthermore, by raising the Ms point of steel, it has the effect of promoting automatic tempering in quenching and improving the toughness of the steel material after quenching. If the Al content is less than 0.01%, it is difficult to obtain the above effects. Therefore, the Al content is 0.01% or more. Preferably it is 0.03% or more. On the other hand, if the Al content exceeds 3%, the above effect is saturated and disadvantageous in cost. Therefore, the Al content is 3% or less. Preferably it is 1% or less.

(7) P: 0.05% or less, S: 0.03% or less, N: 0.01% or less P, S and N are elements that are generally contained as impurities, but the hardenability of steel. It is also an element that is effective in enhancing and stably securing the strength of the steel material after quenching. Therefore, you may make it contain actively. However, if the content of each element exceeds the above upper limit, the toughness of the steel may be significantly deteriorated. Therefore, the contents of P, S and N are within the above range. In order to obtain the above effect more reliably, the P content is preferably 0.002% or more, the S content is preferably 0.002% or more, and the N content is 0.002%. The above is preferable.

(8) Cr: 0.5% or less, Si: 0.5% or less, Cu: 1% or less, V: 1% or less, Nb: 1% or less, Mo: 1% or less, and Co: 3% or less One or more selected from the group These elements are effective in enhancing the hardenability of the steel and ensuring the strength of the steel material after quenching stably. Furthermore, Co has the effect of enhancing the toughness of the steel material after quenching by increasing the Ms point of the steel to promote automatic tempering during quenching. Accordingly, one or more of these elements are contained. However, if the content of each element exceeds the upper limit, the above effect is saturated and disadvantageous in cost. Therefore, the content of these elements is within the above range. In order to obtain the above effect more reliably, the Cr content is preferably 0.005% or more, the Si content is preferably 0.005% or more, and the Cu content is 0.001 or more. V content is preferably 0.001 or more, Nb content is preferably 0.001 or more, Mo content is preferably 0.001 or more, Co content The amount is preferably 0.01% or more. The Co content is more preferably 1% or more.

2. Steel structure of steel material The steel structure of a steel material shall consist of martensite whose prior austenite average particle diameter is 5 μm or more.

  If the steel structure contains a phase or structure other than martensite, it may be difficult to secure a tensile strength of 2.0 GPa or more, or the toughness may be significantly deteriorated. Accordingly, the steel structure shall be composed of martensite, except for other phases and structures that are inevitably mixed. Other phases and structures that are inevitably mixed include residual austenite that remains untransformed and carbide that precipitates during automatic tempering. If the sum of these volume ratios is 5% or less, there is no actual harm. The “martensite” in the present invention includes automatic tempered martensite.

  As for the prior austenite grain size, generally, the finer the prior austenite grain size, the better the toughness of the steel material. The

  However, since the grain interfacial area where B segregates increases as the prior austenite grain size becomes smaller, when the upper limit of the Mn content is strictly limited as in the present invention, the grain is larger than the grain interfacial area. There is a possibility that the amount of B segregating at the boundary may be insufficient. In such a case, the effect of B, which is segregated at the grain boundary and improves the toughness by increasing the grain boundary strength, cannot be sufficiently obtained. . Further, the ductility of the steel material may be significantly reduced. For this reason, it is inappropriate to simply aim for refinement of the prior austenite grain size, and an appropriate grain size is required.

  Therefore, in order to obtain a steel material having good toughness and ductility while having a tensile strength of 2.0 GPa or more, in the present invention, a steel structure composed of martensite having a prior austenite average particle diameter of 5 μm or more; To do. The prior austenite average particle size is preferably 10 μm or more. The upper limit of the prior austenite average particle size is not particularly specified, but if the prior austenite particle size becomes excessively coarse, the effect of toughness deterioration due to coarsening of the prior austenite particle size becomes significant, making it difficult to ensure toughness. There is a case. Therefore, the prior austenite average particle size is preferably 20 μm or less.

3. Mechanical properties of steel materials As for the mechanical properties of steel materials, the tensile strength is 2.0 GPa or more and the total elongation is 5% or more.

In order to meet the needs for higher strength in recent years, the tensile strength of steel is set to 2.0 GPa or more, and the total elongation is set to 5% or more in order to provide good toughness.
In addition, as an index of toughness, it is preferable that a Charpy impact value at −40 ° C. in a 1/4 size Charpy test is 30 J / cm ² or more.

  Moreover, as a representative example of the steel material, a door guard bar or a bumper reinforcement, which is a reinforcing part for automobiles, can be exemplified. These are preferably produced by hot pressing.

4). Steel Material Manufacturing Method The steel material manufacturing method includes quenching treatment in which the steel material having the above chemical composition is held in a temperature range of Ac ₃ points or higher for 5 minutes or more and then cooled to the Mf point at a cooling rate of the upper critical cooling rate or higher. It is preferable to perform the treatment.

Ac is maintained in a temperature range of ₃ points or more by converting the steel structure into an austenite single phase structure and then performing quenching treatment to cool to the Mf point at a cooling rate higher than the upper critical cooling rate. This is to secure a TS of 2.0 GPa or more as a steel structure. The holding temperature is preferably (Ac ₃ + 100 ° C.) or higher. The upper limit of the holding temperature does not need to be specified in particular, but it is preferable that the prior austenite average particle size is 20 μm or less as described above.

Further, holding at a temperature range of ₃ points or more for Ac for 5 minutes or more promotes grain growth after forming an austenite single-phase structure so that the average austenite grain size becomes 5 μm or more, and then Mf at a cooling rate higher than the upper critical cooling rate. This is to ensure good toughness and ductility by setting the prior austenite average particle diameter of the steel material obtained by performing the quenching treatment to a point to 5 μm or more. The holding time is preferably 10 minutes or longer. The upper limit of the holding time is not particularly required, but as described above, it is preferable that the prior austenite average particle diameter is 20 μm or less.

The steel material to be held for 5 minutes or more in a temperature range of Ac ₃ points or more may be a flat steel plate, or may be a steel material previously formed into a predetermined shape.
Further, the steel material may be molded after being held in a temperature range of Ac ₃ points or more for 5 minutes or more and before performing a quenching treatment for cooling to the Mf point at a cooling rate equal to or higher than the upper critical cooling rate. . In addition, the steel material may be formed at the same time as the quenching process for cooling to the Mf point at a cooling rate equal to or higher than the upper critical cooling rate. Since the steel is soft and rich in workability, it is hot or warm, so that it is possible to obtain a steel with high dimensional accuracy, which is preferable.

  The form of the molding is not particularly limited, and may be appropriately selected depending on the shape of the target steel material. For example, bending molding, drawing molding, bulging molding, hole expansion molding, and flange molding are exemplified.

  Moreover, the means for performing the molding is not particularly limited, and may be appropriately selected according to the target molding mode. For example, press molding may be performed using a press die, or roll molding may be performed using a roll.

  According to the hot press method in which press molding is performed using a press mold, the quenching process of cooling to the Mf point at a cooling rate higher than the upper critical cooling rate can be performed by the press mold, and the molding process and quenching are performed. Since it becomes possible to integrate a processing process, it is preferable.

  After the quenching process, a shot blasting process may be performed for the purpose of removing the scale. This shot blasting has the effect of introducing a compressive stress into the surface of the steel material, and therefore has the advantage that delayed fracture is suppressed and the fatigue strength is improved.

5. Steel plate for quenching treatment The steel plate for quenching treatment of the present invention has the above chemical composition, and the upper critical cooling rate is 60 ° C./s or less.

The definition of the chemical composition is as described above.
If the upper critical cooling rate exceeds 60 ° C./s, it may be difficult to obtain a steel structure composed of martensite depending on the mode of quenching. Accordingly, the upper critical cooling rate is set to 60 ° C./s or less. Preferably it is 30 degrees C / s or less. For example, in a hot press, the steel material is usually cooled by a steel mold. In this case, the cooling rate in the cooling is usually 60 ° C./s or more.

In addition, it is not necessary to prescribe | regulate especially the steel structure and mechanical characteristic of the steel plate for hardening processing. Ac is maintained in a temperature range of ₃ points or more during the quenching process, so that the steel structure that the quenching steel sheet has at room temperature, that is, the stage before the quenching process is canceled, and quenching is performed. This is because the mechanical properties of the steel sheet for processing at normal temperature do not affect the formability immediately before or during the quenching process. Accordingly, the steel sheet for quenching treatment may be any one of a hot-rolled steel sheet and a cold-rolled steel sheet (full hard material, annealed material) and a plated steel sheet based on any of them, and the manufacturing method thereof is particularly There is no limitation. When the steel sheet for quenching treatment is a plated steel sheet, the plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electro-Zn—Ni alloy plating. Examples of the hot dip plating layer include hot dip galvanizing, alloyed hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, etc. The

However, in the case of forming before holding in the temperature range of Ac ₃ point or higher in the quenching process, it is desirable that the steel plate is as soft as possible and has good formability. For example, the tensile strength is 590 MPa or less. It is preferable to have a mechanical property of

Examples of the present invention will be described below.
A slab having a chemical composition shown in Table 1 melted in a laboratory was heated at 1250 ° C. for 30 minutes, and then hot-rolled at 900 ° C. or higher to obtain a steel plate having a thickness of 6 mm. After hot rolling, water-spray cooling to 600 ° C., charging into 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 . About the hot-rolled steel sheet thus obtained, the scale was removed by surface grinding, and the sheet thickness was 2.7 mm by cold rolling. A test piece having a thickness of 2.7 mm, a width of 110 mm, and a length of 240 mm was taken from the cold-rolled steel sheet obtained in this manner, and the results are shown in Table 2 in a heating furnace (gas furnace) in which the air-fuel ratio was set to 1.1. It heated on the conditions shown, took out from the heating furnace, and performed the hot press immediately after that using the flat steel metal mold | die. Note that time is held, it refers to the time after charged into the furnace, since it reaches the Ac ₃ point to removal from the furnace. The cooling rate was also measured by attaching a thermocouple to the steel plate.

The test material thus obtained was subjected to a prior austenite particle size measurement by a cutting method and a tensile test (JIS No. 5 test piece).
Further, a 1/4 size (plate thickness: 2.5 mm) V-notch Charpy test piece was prepared and subjected to a Charpy impact test. The toughness was evaluated as acceptable when the impact value at −40 ° C. was 30 J / cm ² or more.

The _three Ac points and the upper critical cooling rate of each steel type were measured by the following method.
A cylindrical test piece (FIG. 1) having a diameter of 3.0 mm and a length of 10 mm is 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. In addition, measurement of thermal expansion change during cooling, measurement of Vickers hardness (load 49N, number of measurements: 3) of the obtained specimen and structural observation were performed, and the upper critical cooling rate was estimated from the results.

In Tables 1 and 2, the numerical values indicating the chemical composition, production conditions, steel structure characteristics and mechanical properties are underlined, indicating that they are outside the scope of the present invention.
Example No. which is an example of the present invention. 1 to 6 have a tensile strength of 2.0 GPa or more and have good performance. On the other hand, Example No. which is a comparative example. In 7-11, since the chemical composition did not satisfy the scope of the present invention, any performance was unsatisfactory.

Claims (2)

In mass%, C: 0.33% to 0.40%, Ni: 2.0% to 5.0%, Mn: 0.01% to less than 0.5%, B: 0.0001% or more 0.01% or less, Al: 0.01% or more and 3% or less, P: 0.05% or less, S: 0.03% or less, N: 0.01% or less, and further, the following formula (1) In addition, Ti is contained within a range that satisfies the following conditions: Cr: 0.5% or less, Si: 0.5% or less, Cu: 1% or less, V: 1% or less, Nb: 1% or less, Mo: 1% or less And Co: consisting of martensite containing one or more selected from the group consisting of 3% or less, the remainder having a chemical composition consisting of Fe and impurities, and the prior austenite average particle size of 5 μm or more It has a steel structure, has a tensile strength of 2.0 GPa or more, and a total elongation of 5% or more. Steel and butterflies.
3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5 (1)
Here, N and Ti indicate the contents (unit: mass%) of N and Ti in the chemical composition, respectively. A steel material having the chemical composition according to claim 1 is held in a temperature range of Ac ₃ points or higher for 5 minutes or more, and then subjected to a quenching process for cooling to a Mf point at a cooling rate equal to or higher than the upper critical cooling rate. Steel manufacturing method.

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