JP5821794B2 - Hardened steel, its manufacturing method, and hardened steel - Google Patents

Description

  The present invention relates to a hardened steel material suitable for machine structural parts such as body structural parts and undercarriage parts of automobiles, a manufacturing method thereof, and a steel material for hardening.

  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 plate widely used for automobiles, press formability decreases as the strength of the steel plate increases, making it 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 of 780 MPa class or higher (hereinafter also referred to as “TS”).

  In order to solve such a problem, Patent Document 1 does not use a high-strength steel sheet for a specific part of a vehicle body that requires strength, but locally heats the part to perform a quenching process. A method for improving strength is disclosed. Patent Document 2 discloses a quenching type ultra-high strength electric resistance welded steel pipe having a TS1620 MPa or more and a manufacturing method thereof. Moreover, in patent document 3, the technique regarding quenching member manufacture which is excellent in toughness with TS1.8GPa or more is disclosed.

JP-A-6-116630 JP 2001-164338 A Japanese Patent Laid-Open No. 2007-302937

  However, for members having a TS of 1.8 GPa or more, the actual situation is that a technique for further improving toughness is desired.

  A specific problem of the present invention is to provide a hardened steel material having a high toughness of which TS is 1.8 GPa or more and 2.0 GPa or less but having a toughness superior to that of the prior art, a manufacturing method thereof, and a steel material for quenching. It is.

  As a result of intensive studies to improve the toughness of a quenched member having a TS of 1.8 to 2.0 GPa, for example, after rapid heating and quenching, the present inventors have adjusted the chemical composition of the steel material and the steel structure of the quenched steel material, We have newly found that toughness is greatly improved by optimizing the heat pattern during quenching. The gist of the present invention completed based on the findings is as follows.

(1) By mass%, C: 0.25% to 0.31%, Mn: 0.5% to 1.0%, Nb: 0.01% to 0.15%, B: 0.0. 0001% to 0.01%, Cr: 0.005% to 0.05%, Si: 0.005% to 0.1%, Al: 0.005% to 1%, P: 0.00. 05% or less, S: 0.03% or less, N: 0.01% or less and satisfying the formulas (1) and (2), the balance being Fe and impurities, the former austenite average A hardened steel material having a steel structure composed of martensite having a section length of 10 μm or less and having mechanical properties 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)
6 ≦ Mn / Si ≦ 20 (2)
Here, the element symbol in a formula represents content (mass%) of each element in steel.

  (2) The chemical composition is mass% in place of part of Fe, Cu: 1% or less, V: 1% or less, Mo: 1% or less, Ni: 3% or less, and Bi: 0.02% The hardened steel material according to (1) above, which contains one or more selected from the group consisting of:

  (3) 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 hardened steel material according to (1) or (2) above, which contains one or more selected from the group consisting of:

(4 ) A quenching steel material for producing the quenched steel material according to any one of (1) to (3) above , wherein ₃ points of A c are 900 ° C. or lower.

(5) The steel for quenching described in the above (4) is heated to a temperature range of (Ac ₃ points + 40 ° C.) to (Ac ₃ points + 200 ° C.) at an average heating rate of 50 ° C./second or more, and the temperature After holding for 10 seconds or less in the region, cooling to a temperature region below the Ms point at an average cooling rate of 200 ° C./second or more , (1) to (3) above A method of manufacturing hardened steel.

  Next, the reason why the present invention is limited to each range will be described. In the following description, “%” for alloy elements represents “mass%”.

(Chemical composition)
The chemical composition of the steel for quenching and the hardened steel in the present invention is defined as follows.

C: 0.25% or more and 0.31% or less C is a very important element that enhances the hardenability of steel and mainly determines the strength after quenching. If the C content is less than 0.25%, it is difficult to ensure TS1.8 GPa or more after quenching. Therefore, the C content is 0.25% or more. Preferably it is 0.27% or more. On the other hand, if the C content exceeds 0.31%, the strength after quenching becomes too high, so that the toughness deterioration becomes significant. Therefore, the C content is 0.31% or less. Preferably it is 0.30% or less.

Mn: 0.5% or more and 1.0% or less Mn is an extremely effective element for enhancing the hardenability of the steel and stably securing the strength after quenching. However, if the Mn content is less than 0.5%, not only the effect is not sufficient, but also the adhesion of the scale generated during heating becomes high, and peeling removal after quenching becomes difficult. Therefore, the Mn content is 0.5% or more. Preferably it is 0.6% or more. On the other hand, if the Mn content exceeds 1.0%, the above effect is saturated, and toughness deterioration after quenching becomes significant. Therefore, the Mn content is 1.0% or less. Preferably it is 0.8% or less.

Nb: 0.01% or more and 0.15% or less Nb suppresses recrystallization and forms fine carbides to make austenite grains fine when steel is heated to a temperature range of Ac ₃ points or more. Therefore, it has the effect of greatly improving toughness. 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. On the other hand, when the Nb content exceeds 0.15%, the effect is saturated and not only unnecessarily increases in cost but also the toughness deterioration after quenching becomes remarkable. Therefore, the Nb content is 0.15% or less. Preferably it is 0.10% or less.

B: 0.0001% or more and 0.01% or less B is effective for enhancing the hardenability of the steel and further enhancing the effect of ensuring the stability of the strength after quenching. It is also an important element in that it segregates at the grain boundaries to increase the grain boundary strength and improve the toughness and delayed fracture resistance after quenching. Further, it has an effect of suppressing austenite grain growth during heating. However, if the B content is less than 0.0001%, the effect is not sufficient. Therefore, the B content is 0.0001% or more. Preferably it is 0.001% or more. On the other hand, when the B content exceeds 0.01%, the effect is saturated and the cost is increased. Therefore, the B content is 0.01% or less.

  The grain boundary segregation amount of B is affected by the austenite grain size during quenching. 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, it is preferable to determine the upper limit of the B content according to the austenite grain size when subjected to quenching, that is, the prior austenite grain size in the quenched member, because the effect of B segregation can be obtained efficiently. Specifically, it is preferable that the following formula (3) is satisfied.

B content (ppm) ≦ exp (4.57−0.571 × ln (r)) (3)
Here, r is the average section length (μm) of the prior austenite grains.

  In order to satisfy the above formula (3), the relationship between the chemical composition and the austenite grain size at the time of quenching is obtained empirically, and the chemical composition and quenching conditions may be adjusted.

Cr: 0.005% or more and 0.05% or less Cr is an extremely effective element for enhancing the hardenability of the steel and stably securing the strength after quenching. However, if the Cr content is less than 0.005%, the effect is not sufficient. Therefore, the Cr content is 0.005% or more. On the other hand, when the Cr content exceeds 0.05%, the effect is not only saturated, but during heating, dissolution of carbides mainly composed of cementite tends to be delayed, and toughness deterioration after quenching becomes remarkable. . Therefore, the Cr content is 0.05% or less. Preferably it is 0.02% or less.

Si: 0.005% or more and 0.1% or less Si is an effective element for enhancing the hardenability of the steel and securing the strength after quenching stably. However, if the Si content is less than 0.005%, the effect is not sufficient. Therefore, the Si content is 0.005% or more. On the other hand, when the Si content exceeds 0.1%, the effect is not only saturated, but also the adhesion of the scale generated during heating becomes high, so that it becomes difficult to remove after quenching. Preferably it is 0.08% or less.

Al: 0.005% or more and 1% or less Al is an effective element for enhancing the hardenability of the steel and stably securing the strength after quenching. However, if the Al content is less than 0.005%, the effect is not sufficient. Therefore, the Al content is 0.005% or more. Preferably it is 0.01% or more. On the other hand, when the Al content exceeds 1%, the effect is not only saturated, but also the cost is unnecessarily increased. Therefore, the Al content is 1% or less. Preferably it is 0.8% or less.

P: 0.05% or less, S: 0.03% or less, N: 0.01% or less These elements are elements that are generally contained as impurities, but increase the hardenability of the steel, and after quenching It is an element that is effective in ensuring strength stability. Accordingly, one or more of these elements may be positively included. However, even if it contains more than the said upper limit, the effect is small and causes a cost increase unnecessarily. For this reason, content of each alloy element shall be the said range.

Ti: 3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5
Ti has the effect of greatly improving the toughness after quenching because it suppresses recrystallization and forms fine carbides to make austenite grains fine when the steel is heated to Ac ₃ points or more. When the Ti content is less than (3.42N + 0.001)%, it is difficult to sufficiently obtain the above effect. 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)%, the effect is saturated and the cost is unnecessarily increased. Therefore, the Ti content is set to (3.42N + 0.5)% or less. Preferably, it is (3.42N + 0.08) or less.

6 ≦ Mn / Si ≦ 20
Mn / Si, which is a value obtained by dividing the Mn content by the Si content, is a parameter that greatly affects the scale adhesion. That is, when Mn / Si is less than 6 or more than 20, adhesion of the scale generated during heating becomes high, and peeling removal after quenching becomes difficult. Therefore, Mn / Si is within the above range.

One or more selected from the group consisting of Cu: 1% or less, V: 1% or less, Mo: 1% or less, Ni: 3% or less, and Bi: 0.02% or less. It is an element that improves the hardenability of the steel and is effective in ensuring the stability of the strength after quenching. Ni further has the effect of increasing the cleavage fracture strength and greatly improving the toughness after quenching. Bi further has the effect of making the structure uniform and further increasing the toughness after quenching. Therefore, you may contain 1 type, or 2 or more types of these elements.

  However, even if Cu, V, Mo, and Ni are contained in the upper limit value or more, the effect is small, and the cost is unnecessarily increased. Moreover, about Bi, when the content exceeds 0.02%, hot workability will deteriorate and hot rolling will become difficult. Therefore, the content of each alloy element is within the above range. The Ni content is preferably 1% or less, and the Bi content is preferably 0.015% or less. In order to obtain the above effect more reliably, Cu: 0.005% or more, V: 0.005% or more, Mo: 0.005% or more, Ni: 0.01% or more, and Bi: 0.001% It is preferable to satisfy at least one of the above. The content of Ni is more preferably 0.1% or more, and the content of Bi is more preferably 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. It is an element that contributes to inclusion control at the time, especially the fine dispersion of inclusions, and has the effect of increasing toughness after quenching. 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.

(Steel structure of hardened steel)
The steel structure of a hardened steel material shall consist of martensite whose prior austenite average section length is 10 μm or less.

  When the prior austenite average section length exceeds 10 μm, it is difficult to ensure good toughness under high strength of TS1.8 GPa or more. Accordingly, the steel structure of the hardened steel material has an old austenite average section length of 10 μm or less.

  Moreover, when the steel structure of the quenched steel material is mainly composed of a phase or structure other than martensite, it is difficult to ensure a high strength of TS1.8 GPa or more. Accordingly, the steel structure of the quenched steel material is assumed to be martensite.

  In addition, although the steel structure of hardened steel materials consists of martensite, it includes phases or structures other than martensite which are inevitably mixed in production. For example, several percent of retained austenite or carbide may be contained.

(Strength of hardened steel)
The strength of the hardened steel material is 1.8 GPa or more and 2.0 GPa or less in TS.

  If it is less than TS1.8 GPa, toughness does not become a problem so much, and good toughness can be ensured without depending on the present invention. Therefore, in the present invention, it is assumed that the strength of the hardened steel material is 1.8 GPa or more in TS.

  If it exceeds TS 2.0 GPa, the toughness is significantly lowered as the strength is increased, and it becomes difficult to ensure good toughness. Therefore, the strength of the hardened steel material is 2.0 GPa or less in TS.

(Hardening steel)
The steel material for quenching is a steel material before quenching, has the above chemical composition, and preferably has an Ac ₃ point of 900 ° C. or lower.

At the time of quenching, in order to make the steel for quenching into an austenite single phase, it is necessary to heat to a temperature range of at least Ac ₃ points or more. Therefore, from the viewpoint of heating cost and productivity, the lower the Ac ₃ point is, the more preferable, specifically, 900 ° C. or less is preferable. More preferably, it is 850 degrees C or less.

  The steel material for quenching includes, in addition to a steel plate, a steel pipe, a shaped steel, and the like, a member processed into a desired shape before quenching.

(Method of manufacturing hardened steel)
The quenching steel material heats the quenching steel material to a temperature range of (Ac ₃ points + 40 ° C.) to (Ac ₃ points + 200 ° C.) at an average heating rate of 50 ° C./second or more, and in the temperature range for 10 seconds. After holding below, it is preferable to manufacture by cooling to a temperature range below the Ms point at an average cooling rate of 200 ° C./second or more.

  By setting the average heating rate to 50 ° C./second or more, austenite grain growth in the heating process during quenching can be suppressed, and after quenching, the prior austenite average section length can be easily set to 10 μm or less. Therefore, the average heating rate is preferably 50 ° C./second or more. More preferably, it is 100 ° C./second or more. The higher the average heating rate is, the faster the austenite grain growth is suppressed.

By setting the heating temperature to (Ac ₃ points + 40 ° C.) or higher, the structure during quenching can be surely set to an austenite single phase, and the structure and the concentration distribution of various elements can be made uniform. The desired strength can be obtained stably and high toughness can be ensured. Therefore, the heating temperature is preferably (Ac ₃ points + 40 ° C.) or higher. More preferably (Ac ₃ points + 80 ° C.) or more.

By setting the heating temperature to (Ac ₃ points + 200 ° C.) or less, austenite grain growth in the heating process during quenching is suppressed, and after quenching, the prior austenite average section length is easily set to 10 μm or less. . Therefore, the heating temperature is preferably (Ac ₃ points + 200 ° C.) or less. More preferably, it is (Ac ₃ points + 180 ° C.) or less.

  After the heating, a structure composed of martensite is obtained by cooling to a temperature range below the Ms point at a cooling rate equal to or higher than the upper critical cooling rate, but the quenching steel plate is 200 ° C / ° C by water cooling or oil cooling. It is preferable to cool to a temperature range below the Ms point with an average cooling rate of at least 2 seconds.

  As a suitable heating method of the present invention, any method may be adopted as long as rapid heating and rapid cooling are achieved. For example, an induction heating quenching method or an electric heating quenching method can be used.

  Moreover, the hardened steel material and the hardened steel material of the present invention can be provided with a plating film on the surface for the purpose of imparting corrosion resistance or the like. Examples of the plating film include Zn-based plating and Al-based plating.

  Moreover, when applying the hardened steel material which concerns on this invention to components for motor vehicles etc., the heat processing by paint baking is given to a hardened member. At this time, there is a case where a slight decrease in strength is observed, but a large change that does not fall within the scope of the present invention does not occur.

  Moreover, since the steel for quenching of the present invention is heated to the austenite temperature range during quenching, the mechanical properties at room temperature before heating are not so important, and the steel structure before heating is not particularly specified.

Examples of the present invention will be described below.
A steel plate (plate thickness: 1.4 mm) having the chemical composition shown in Table 1 was used as a test material. The manufacturing method of the plate as the test material is as follows. That is, the slab melted in the 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 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 steel sheet was stripped by pickling and then cold-rolled to a thickness of 1.4 mm.

  A test piece having a size of 1.4 t × 15 w × 200 L was cut and collected from the steel plate described above, energized and heated under the heating conditions shown in Table 2, and then immediately quenched with water. Various test pieces were collected from the quenched part, and subjected to cross-sectional microstructure observation, old austenite average section length measurement by cutting method, tensile test (JIS No. 13 B test piece), Charpy impact test, and scale peelability investigation.

  In the peelability evaluation of the scale, the remaining state of the scale after water cooling was visually confirmed, and the case where it was peeled off was indicated as “good”, and otherwise, it was indicated as “failed”.

Moreover, Ac ₃ points | pieces of each steel type were calculated | required by measurement of the thermal expansion change of a test piece at the time of the above-mentioned heating.
For the Charpy impact test, after grinding the 1.4 mm thick steel plate after quenching to 1.2 mm thickness, four sheets are stacked and screwed, then a V-notch test piece is prepared and used for the Charpy impact test. did.

The toughness evaluation, the impact value at 0 ℃ is the ○ as passed if it exceeds 40 J / cm ^2, if it exceeds 50 J / cm ^2, as a very excellent toughness, in the table was expressed as ◎. When it did not satisfy the above criteria, it was written as “x”.

  As a general rule, the evaluation of other items was discontinued or not conducted for any item that failed even one of the evaluation items.

  Test No. which is an example of the present invention. 1 to 10 indicate that TS is 1.8 to 2.0 GPa and toughness is also good. On the other hand, test No. which is a comparative example. No. 11 is low in strength and steel type No. 12 is too strong. In addition, Test No. In 13. Since the amount of Mn is high, the toughness is low. In No. 14, the Nb content is high, so the toughness is low. In addition, Test No. In No. 15, Nb is not contained, so the toughness is low. In No. 16, since Ti and B are not contained, toughness is low. Test No. In No. 17, the toughness is low due to the high Cr content. Test No. In No. 18, the amount of Si is high and the scale peelability is poor. Test No. In 19 and 20, since Mn / Si is out of the scope of the present invention, the scale peelability is poor. Test No. In No. 21, the amount of Mn is low and the scale peelability is poor.

Claims (5)

In mass%, C: 0.25% to 0.31%, Mn: 0.5% to 1.0%, Nb: 0.01% to 0.15%, B: 0.0001% or more 0.01% or less, Cr: 0.005% to 0.05%, Si: 0.005% to 0.1%, Al: 0.005% to 1%, P: 0.05% or less , S: 0.03% or less, N: 0.01% or less, satisfying the formulas (1) and (2), the balance being Fe and impurities, the former austenite average intercept length Is a hardened steel material having a steel structure composed of martensite of 10 μm or less and 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)
6 ≦ Mn / Si ≦ 20 (2)
Here, the element symbol in a formula represents content (mass%) of each element in steel.   The chemical composition is, in place of a part of Fe, in mass%, Cu: 1% or less, V: 1% or less, Mo: 1% or less, Ni: 3% or less, and Bi: 0.02% or less. The hardened steel material according to claim 1, comprising one or more selected from the group.   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 hardened steel material according to claim 1 or 2, comprising one or more selected from the group. The steel for hardening for manufacturing the hardened steel according to any one of claims 1 to 3, wherein _three points of A c are 900 ° C or lower. The steel for quenching according to claim 4 is heated to a temperature range of (Ac ₃ points + 40 ° C.) to (Ac ₃ points + 200 ° C.) at an average heating rate of 50 ° C./second or more, and 10 seconds in the temperature range. The method for producing a hardened steel material according to any one of claims 1 to 3, wherein after cooling, the steel sheet is cooled to a temperature range below the Ms point at an average cooling rate of 200 ° C / second or more.