JP4513701B2 - Steel plate for rapid heating and quenching and its manufacturing method - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a steel plate that is used after being formed into various shapes by press working or the like and then quenched and used, and more particularly to a steel plate that is excellent in press formability and rapid heat quenchability and a method for manufacturing the same.

  The need for ensuring passenger safety is increasing year by year, such as the release of data on car body destruction in the event of a car crash, which controls the sales of cars. Legal regulations tend to be stricter. The car body has a so-called crushable body structure that is partially deformed and absorbs shocks in the event of a collision, while the passenger compartment is designed to have a high-strength structure to protect passengers in the car. . In other words, the vehicle body has a part that requires high strength. In such a part, a molded product having a large plate thickness is used for a necessary part, a reinforcing material is attached, or a high strength steel plate is formed.

  However, increasing the thickness leads to an increase in weight, which contradicts the demand for improved fuel consumption or energy saving. For reinforcement, parts for spot welding of rib material and shock absorption are incorporated, but this also increases the weight of the vehicle body. In addition, high-strength steel sheets are difficult to press complexly due to poor press formability, and the residual stress due to processing increases as the strength increases, increasing the risk of delayed fracture. There is a limit to increasing the strength of steel sheets.

  For reinforcement of this body structure, the thickness of the steel sheet molded part applied to a specific part of the vehicle body that needs to be strengthened is not increased, or high strength steel sheet is not used, and the desired part is locally heated at high frequency. Patent Document 1 and Patent Document 2 disclose inventions of methods for quenching and improving strength. If the steel is hardened and strengthened after press forming, problems such as difficulty in forming a complicated shape and large residual stress after forming as in the case of forming a high-strength steel sheet can be eliminated. However, although the above publication shows a method of induction hardening, there is no specific description about the type of steel sheet to be applied.

  Hardened steel sheets have good press formability before quenching, high quenchability, and high strength after quenching as well as high strength by low-temperature, short-time heat treatment. Required. This is because not only the improvement of productivity but also the suppression of scale formation and the prevention of damage to the plating film when using a plated steel sheet. In the case of performing rapid heating and quenching using high-frequency heating or laser heating, it is difficult to lower the heating temperature because carbides remain and the transformation point increases.

  Patent Documents 3 and 4 disclose techniques for improving hardenability and local ductility by increasing the spheroidization rate of carbides and reducing the average carbide particle size. Since the carbide particle size is small, it seems that the strength is increased by heating at a low temperature for a short time. However, these techniques require a plurality of annealing times for a long time, so that the productivity is remarkably deteriorated.

Patent Document 5 discloses a technique for stabilizing hardenability by adding a large amount of Mn and reducing the average particle size of iron carbide. This method improves the hardenability of the steel with Mn and increases the strength even by low-temperature heating. However, since iron carbide is stabilized, it is necessary to heat for a long time during the quenching process.
JP-A-6-116630 Japanese Patent Laid-Open No. 10-17933 Japanese Patent Laid-Open No. 11-80884 Japanese Patent Laid-Open No. 11-140544 JP 2003-268489 A

  The present invention has been made to solve the above-mentioned problems of the prior art, and the problem is that the steel sheet is excellent in press formability before quenching and rapid heat quenchability, specifically low temperature quenchability, that is, heating. An object of the present invention is to provide a steel sheet having a martensite structure after quenching even at a low temperature and having excellent properties for increasing strength, and a method for producing the steel sheet.

  In order to solve the above-mentioned problems, the present inventors conducted the following experiments in order to clarify the relationship between the microstructure of the steel plate before quenching and the heating temperature of the steel plate in the quenching treatment and the hardness of the steel plate after quenching. Went. In addition, in this specification, all content of a steel component is displayed by the mass%.

  The test steel was, in mass%, C: 0.10%, Si: 0.04%, Mn: 1.0%, P: 0.011%, S: 0.007%, sol. It had a chemical composition consisting of Al: 0.021%, N: 0.002%, the balance Fe and impurities.

  A steel plate having such a chemical composition is heated to 1240 ° C., then hot-rolled at a temperature range of 850 ° C. or more, wound up at 550 ° C., and the resulting hot-rolled steel plate is pickled and 45 to 90%. Was cold rolled at a cold pressure ratio of. The obtained steel sheet was heated from 650 ° C. to 900 ° C. with a continuous annealing simulator and held for 60 seconds, then cooled to 450 ° C. at 60 ° C./s, held for 240 s, and then cooled to room temperature.

  The obtained annealed plate was subjected to microstructure observation and rapid heating test. In the rapid heating test, the annealed plate was heated to a temperature of 650 to 950 ° C. at a heating rate of 100 ° C./s with an electric heating device, and immediately after reaching the target temperature, water quenching was performed. Tissue observation was performed.

  The microstructure was observed by a cross section along the rolling direction. After corroding with nital, a three-field microscope image was taken at a magnification of 500 times, and the area ratio of the band-like tissue was measured by image processing. Here, the second phase connected in a band shape over the average ferrite grain size in the rolling direction was regarded as a band-like structure.

The low-temperature hardenability of the steel sheet was evaluated at the heating temperature for obtaining the highest hardness, and the case where the highest hardness was obtained at a heating temperature of _Ae3 point + 30 ° C. or less was considered good. In addition, _Ae3 point is _{calculated |} required from a following formula.

Here, A _e3 point is a point of equilibrium of the _three points A, at equilibrium is the temperature at which the austenite single phase if high temperatures than the temperature, maximum hardness becomes entirely martensite by subsequent rapid cooling Is obtained. However, in practice, the temperature at which the single phase becomes austenite shifts to a higher temperature side than the equilibrium point depending on the heating rate, and this tendency becomes particularly remarkable in the case of rapid heating and quenching. _It is necessary to heat to a temperature considerably higher than _e3 point. Therefore, it is preferable that the minimum heating temperature at which the maximum hardness can be obtained is closer to the _Ae3 point, which is excellent in rapid hardenability.

A _e3 (° C) = 910-203 [C%] ^1/2 +45 [Si%]-30 [Mn%]-20 [Cu%]-15 [Ni%]-11 [Cr%] + 32 [Mo %] + 104 [V%] + 400 [Ti%] + 460 [Al%] + 13 [W%] + 120 [As%]-700 [P%]
As a result of these preliminary tests, the following results (A) to (D) were obtained to complete the present invention.

  (A) FIGS. 1 and 2 are graphs showing the relationship between the hardness after quenching and the heating temperature of the steel sheet. The hardness after quenching increases with the heating temperature of the steel sheet, and the maximum quenching hardness according to the C content is obtained at the critical temperature (Tc).

(B) FIG. 3 is a graph showing the relationship between Tc and the area ratio of the band-like tissue. Tc varies depending on the microstructure of the steel sheet before quenching, and the smaller the area ratio (V _B ) of the band-like structure, the lower Tc. If V _B is made 6.0% or less, Tc becomes _Ae3 point + 30 ° C. It becomes the following.
V _B was defined as the area ratio of the second phase connected in a band shape in the rolling direction over the entire ferrite structure over twice the average ferrite grain size.

(C) Figure 4 is a graph showing the relationship between _{V B} and cold rolling reduction ratio (CR), the soaking temperature (TA). In the figure, “x” indicates a case where V _B > 6.0%, and “◯” indicates a case where V _B ≦ 6.0%. V _B has a correlation with CR and TA, and in order to reduce V _B , it is necessary to perform annealing within the temperature range indicated by the following formula (1) that is equal to or higher than _{Ac 1} point and corresponding to the cold pressure ratio. There is. The reason for this is not clear, but may be due to the following reasons.

A _c1 <(TA) <A _c1 + (A _c3 −A _c1 ) (CR−10) / 100 (1)
1. The second phase is formed after cooling in the austenitic part during annealing.
2. The higher the CR, the more austenite nucleation sites.
3. As TA increases, austenite is linked in a band shape.

(D) Therefore, by increasing the cold pressure rate and annealing at a low temperature, the generation of the band-like second phase is suppressed, and the heating temperature of the steel sheet in the quenching process can be lowered.
The present invention has been completed based on the above findings. The gist of the present invention is as follows.

(1) By mass%, C: 0.06% to 0.25%, Si: 0.3% or less, Mn: 0.5% to 1.5%, P: 0.03% or less, S : 0.02% or less, sol. A structure containing Al: 0.1% or less and N: 0.005% or less, having a chemical composition comprising the balance Fe and impurities, and comprising a ferrite phase as a main phase and a second phase of 27 area% or less And the second phase is composed of one or more of pearlite, bainite, martensite and cementite, and the second phase connected in a band shape in the rolling direction over two times the average ferrite grain size. A continuously annealed steel sheet for rapid heating and quenching, characterized in that the area ratio in the whole is 6.0% or less.

(2) The above-mentioned chemical composition (1), wherein the chemical composition further comprises one or two selected from the group consisting of B: 0.003% or less and Ti: 0.03% or less in terms of mass%. ) Continuously annealed steel sheet for rapid heating and quenching.

(3) The chemical composition is selected from the group consisting of% by mass, Cr: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, and Ni: 1.0% or less. The continuous annealing steel plate for rapid heating and quenching according to the above (1) or (2), characterized by containing one or more of the above.

(4) A steel ingot or steel slab having the chemical composition according to any one of the above (1) to (3) is hot-rolled and cold-rolled at a cold pressure ratio (CR) of 55% or more. By annealing at a soaking temperature (TA) satisfying 1), the ferrite phase is the main phase and the second phase is included, and the area ratio of the band-like structure consisting of the second phase is 6.0% or less. A method for producing a continuously annealed steel sheet for rapid heating and quenching, characterized in that:

A _c1 <(TA) <A _c1 + (A _c3 −A _c1 ) (CR−10) / 100 (1)

According to the present invention, there are provided a steel plate having excellent press formability and capable of obtaining the maximum hardness of quenching even when the heating temperature at the time of rapid heating and quenching is A _e3 + 30 ° C. or less, and a method for producing the same.

The microstructure of the present invention, the chemical composition of the steel components, and the reasons for limiting the production conditions are described in detail.
(1) Steel microstructure The steel sheet according to the present invention has a ferrite phase as a main phase and a structure including a second phase, and the area ratio of the band-like structure in the entire structure is 6.0% or less. This is because by suppressing the formation of a band-like structure, the heating temperature required to obtain the maximum hardness by rapid heating and quenching can be reduced to A _e3 + 30 ° C. or lower.

  Examples of the second phase include pearlite, bainite, martensite, cementite, and the like. In order to improve the formability before quenching, pearlite or cementite is desirable. Here, the “main phase” is a phase having the maximum volume fraction, and the second phase may contain two or more phases or structures such as pearlite and cementite. In order to improve the press formability before quenching, the area ratio of the entire second phase is preferably 10% or less.

(2) Chemical composition of steel C: C is an important element that determines the strength of steel after quenching. If the C content is too low, sufficient strength cannot be obtained after quenching. On the other hand, if the C content is too high, not only the toughness and delayed fracture resistance after quenching deteriorate, but also the ductility before quenching decreases and the press formability is impaired. Therefore, the C content is determined to be 0.06% or more and 0.25% or less. A preferable content is more than 0.09% and less than 0.12%.

  Si: Si is contained as an impurity, but has an effect of enhancing the hardenability of the steel, and therefore can be contained for the purpose of the effect of the effect. However, the Si content is set to 0.3% or less because excessive conversion causes deterioration of chemical conversion properties and plating properties. Preferably it is 0.1% or less. In order to surely obtain the effect by the above action, the Si content is preferably 0.01% or more.

  Mn: Since Mn is an important element for securing the hardenability of steel, 0.5% or more is contained. On the other hand, if it is contained excessively, the press formability before quenching is deteriorated, and the carbide is stabilized and tends to remain undissolved during heating during the quenching treatment, and the low temperature quenchability may be impaired. Therefore, the Mn content is set to 0.5% or more and 1.5% or less. Preferably, it is 1.0% or more and 1.3% or less.

  P: P is an inevitable impurity and deteriorates the formability before quenching and the toughness after quenching. Therefore, the smaller the content, the better. The P content is set to 0.03% or less. Preferably it is 0.015% or less.

  S: S is also an inevitable impurity, and deteriorates the formability before quenching and the toughness after quenching. Therefore, the smaller the content, the better. The S content is set to 0.02% or less. Preferably it is 0.01% or less.

  Al: Al is contained as an impurity, but can also be added for the purpose of deoxidation in the steelmaking process. However, if the content is too large, the production cost increases. Therefore, the Al content is determined to be 0.1% or less. Preferably it is 0.03% or less. In addition, in order to acquire the said deoxidation effect reliably, it is preferable that Al content shall be 0.005% or more.

N: N is an unavoidable impurity, and an increase in the N content deteriorates the formability before quenching. Therefore, the N content is set to 0.01% or less. Preferably it is 0.005% or less.
In the present invention, the following elements may be further contained in addition to the above elements.

  B: Since B has the effect | action which improves hardenability, it can be contained for the purpose of the effect by the said effect | action. However, since an excessive content may deteriorate the formability before quenching, the B content is preferably 0.003% or less. The effect is almost constant regardless of the B content, but 0.0003% or more is preferable for obtaining the effect more reliably.

  Ti: Ti has the effect of improving the hardenability and has the effect of improving the formability by precipitating and fixing solid solution N in the steel, so that it can be contained for the purpose of the effect of the action. However, if excessively contained, the amount of TiC precipitated increases and the strength after quenching may decrease, so the Ti content is preferably 0.03% or less. In order to reliably obtain the above effects, it is preferable to contain 0.003% or more. Moreover, since Ti has the effect | action which suppresses precipitation of BN and, as a result, there exists an effect which improves the hardenability by B, it is preferable to contain Ti and B together.

Cr, Mo, W, Ni
Since Cr, Mo, W, and Ni have the effect | action which improves hardenability, they can be contained for the purpose of the effect by the said effect | action. However, when it contains excessively, the moldability before quenching may be deteriorated. Therefore, the content of each element is preferably 1.0% or less. More preferably, it is 0.60% or less. In order to surely obtain the above effect, the content of any element is preferably 0.02% or more, and more preferably 0.06% or more.

(3) Manufacturing conditions The steel having the above chemical composition is melted by a conventional method and then rolled into a steel ingot by continuous casting or into pieces after the casting. From the viewpoint of productivity, it is preferable to use a continuous casting method.

  In the case of continuous casting, when the casting speed is 2.0 m / min or more, center segregation or V-shaped segregation of Mn is likely to occur. However, since these segregations become the formation base points of the band-like structure, they should be suppressed. The casting speed is preferably less than 2.0 m / min. On the other hand, when the casting speed is less than 1.2 m / min, the cleanliness of the slab surface portion deteriorates and the productivity also decreases. Therefore, the casting speed is preferably set to 1.2 m / min or more.

  A steel ingot or steel slab produced by continuous casting or the like is hot-rolled by a conventional method. The hot rolling conditions are not particularly limited, but it is advantageous to uniformly generate carbides in the hot rolling process. Therefore, in order to suppress the formation of a band-like structure, it is preferable to start rolling at 1000 to 1300 ° C. and perform finish rolling at 850 ° C. or higher. The coiling temperature is preferably higher in order to improve the formability after cold rolling and annealing, but if it is too high, the yield decreases due to scale formation. .

  Cold rolling is performed according to a conventional method after descaling the hot-rolled steel sheet obtained by the hot rolling by pickling or the like. In order to obtain good press formability by recrystallization annealing, the cold pressure ratio is 55% or more.

  The cold-rolled steel sheet is subjected to a treatment such as degreasing according to a conventional method as necessary, and is recrystallized and annealed. The soaking temperature (TA) at this time satisfies the following formula (1). This is for preventing the formation of a band-like structure and improving the low-temperature hardenability of the steel sheet. CR is a cold pressure rate (unit:%). For example, it is held for 5 s or more in the temperature range of 650 ° C. to 900 ° C.

A _c1 <(TA) <A _c1 + (A _c3 −A _c1 ) (CR−10) / 100 (1)
Although there are no particular restrictions other than the soaking temperature, in order to improve productivity, it is preferable that annealing is performed in a continuous annealing line, and the soaking time is preferably less than 2 minutes, and a band-like structure is formed during cooling after annealing. In order to prevent this, the average cooling rate from 650 ° C. to 450 ° C. after soaking is preferably 60 ° C./s or more.

  The steel plate produced according to the method of the present invention can be electroplated or used as a coated steel plate as a base material. Further, the steel sheet after cold rolling may be annealed in a heating furnace equipped in a hot dipping apparatus, hot dipped, and made into a plated steel sheet.

  The steel plate according to the present invention thus obtained is further divided into small pieces as necessary, and given a predetermined shape by press molding or the like, then subjected to rapid heating and quenching, and further through an annealing treatment if necessary, Final product. The “rapid heating quenching” at this time specifically indicates induction quenching, laser heating quenching, electric heating quenching, and the like, but generally indicates quenching treatment under conditions of induction quenching.

  The vehicle body part that performs such rapid heating and quenching may be any part that can reduce the weight of the vehicle body by increasing the strength, for example, the rain of a pillar, a door beam, a roof, or a bumper. There are forces. The quenching at this time may be performed in the form of local quenching on the steel sheet before forming, may be performed in the form of whole or local quenching on the parts before assembly, or may be performed in the form of local quenching after assembly. .

  According to the present invention, rapid heating hardenability, that is, low-temperature quenching is possible, and there is a great practical advantage. Although the mechanism for realizing such an effect is not clear, it is presumed as follows.

The heating temperature at which quenching is possible is determined by the austenite transformation completion temperature during heating. Austenite transformation is completed by heating temperature austenite nuclei produced from carbides at the stage of reaching the _point A, austenite grows with increasing heating temperature. The formation of the band-like structure means that austenite nucleation sites are unevenly distributed, and the higher the area ratio of the band-like structure, the more the completion of the austenite transformation is delayed. Therefore, if the formation of a band-like structure is suppressed as in the present invention, austenite nucleation during heating is caused to be uniformly dispersed and the austenite transformation is completed quickly, so that the quenchable temperature is also lowered.

Examples of the present invention will be described below.
Example 1
Steels having the compositions shown in Table 1 were melted using a laboratory vacuum melting furnace. As steel composition, C: 0.1%, Mn: 1.0%, Si: 0.04%, P: 0.012%, S: 0.005%, Al: 0.02%, N: 0 In order to see the effect of the component amount with 0.002% as the target reference composition, the other components were changed to be constant for each of C, Mn, B, Ti, Cr, Mo, W, and Ni.

  These ingots are forged into slabs for hot rolling with a thickness of 20 mm, soaked at 1200 ° C., then hot rolled to a thickness of 4 mm, and cooling conditions assuming a hot strip mill by forced air cooling or water spray The steel plate obtained immediately was put into a furnace maintained at this temperature and cooled under the conditions assuming the coil after winding. These hot-rolled steel sheets were cold-rolled after surface descaling and finished to a thickness of 1 mm, and were annealed by simulating continuous annealing at an annealing temperature of 750 ° C. using a continuous annealing simulator.

  A test piece having a thickness of 1 mm, a width of 100 mm, and a length of 490 mm was taken from each steel plate, and subjected to a quenching process under a water cooling condition after holding at a heating temperature of 950 ° C. for 2 seconds with an induction hardening apparatus having an output of 200 kW and a frequency of 10 kHz. It was. Then, it shape | molded to the JIS5 tension test piece and used for the tension test.

In the present invention, “press formability” is evaluated by tensile strength, and when it is 500 MPa or less, “excellent press formability” is defined. In addition, “rapid heat quenchability” is evaluated at the lowest heating temperature at which the highest hardness can be obtained in the rapid heat quench test, and when it is A _e3 + 30 ° C. or less, “ "

  Table 2 shows the tensile test results before and after quenching.

  Steel type no. From the results of 1-4, if the C content is low, sufficient strength cannot be obtained after quenching, and if it is too high, the toughness deteriorates. However, if within the scope of the present invention, sufficient formability is obtained before processing. It can be seen that the strength and toughness after quenching are satisfied.

  Steel type no. From the results of 5 to 8, as the Mn content is increased, the elongation is lowered and the workability is deteriorated in the state of the material before quenching, but if it is within the range of the present invention, the elongation before the processing is sufficiently large, after quenching. It can be seen that the strength of is also high.

Steel type no. From the results of 9 to 14, it can be seen that the addition of B, Ti, Cr, Mo, W, and Ni does not adversely affect the strength before quenching, the strength after quenching, and the toughness.
(Example 2)
Steel type No. in Table 1 A steel having a composition of 2 or 3 is continuously cast at a casting speed of 1.5 m / min to form a slab, then hot-rolled in a temperature range of 850 ° C. or higher, cooled and wound at 600 ° C. A rolled sheet was obtained. The obtained hot-rolled sheet was ground to a thickness of 2 to 12 mm and cold-rolled to prepare a steel sheet having a thickness of 1.2 mm with different rolling reduction.

The prepared steel sheet was annealed with a continuous annealing simulator under the conditions shown in Table 3, cooled to room temperature, a cross-sectional microstructure photograph was taken, and the area ratio V _B of the band-like carbide was measured using image analysis software. The steel sheet was measured V _B, rapid heating to various temperatures, after quenching, was cross-sectional hardness measurement. Band-like carbides ratio V _B and the cold rolling reduction ratio, the relationship between the soaking temperature shown in Table 3.

The steel plates manufactured within the scope of the present invention (trial numbers 5, 6, 9, 10, 11, 13, 14, 15, 21, 25, 26, 29, 30) are band-like carbide area ratios (pearlite ratio). V _B ) was 6.0% or less.

These steel sheets were rapidly heated and rapidly cooled to various temperatures, and the hardness after quenching was measured. Table 4 shows the hardness after quenching at V _B and each heating temperature. _Steel plates with V _B of 6.0% or less can obtain the maximum hardness by heating at A _e3 + 30 ° C, but steel plates with 6.0% or more did not reach the maximum hardness by heating at A _e3 + 30 ° C. .

Effect of heating temperature on the hardness after _quenching: is a graph showing the (V B 1.8%). Effect of heating temperature on the hardness after _quenching: is a graph showing the (V B 7.9%). A band-like carbide ratio V _B, is a graph showing the relationship between the required heating temperature Tc for obtaining the maximum hardness after quenching. Band-like carbides ratio V _B is below 6.0%, which is a graph showing the range of the annealing temperature and cold rolling reduction ratio.

Claims (4)

In mass%, C: 0.06% to 0.25%, Si: 0.3% or less, Mn: 0.5% to 1.5%, P: 0.03% or less, S: 0.0. 02% or less, sol. A structure containing Al: 0.1% or less and N: 0.005% or less, having a chemical composition comprising the balance Fe and impurities, and comprising a ferrite phase as a main phase and a second phase of 27 area% or less And the second phase is composed of one or more of pearlite, bainite, martensite and cementite, and the second phase connected in a band shape in the rolling direction over two times the average ferrite grain size. A continuously annealed steel sheet for rapid heating and quenching, characterized in that the area ratio in the whole is 6.0% or less. 2. The chemical composition according to claim 1, wherein the chemical composition further comprises one or two selected from the group consisting of B: 0.003% or less and Ti: 0.03% or less in mass%. Continuously annealed steel sheet for rapid heating and quenching. The chemical composition is, in mass%, further selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, and Ni: 1.0% or less The continuously annealed steel sheet for rapid heating and quenching according to claim 1 or 2, comprising two or more kinds. A steel ingot or steel slab having the chemical composition according to any one of claims 1 to 3 is hot-rolled and cold-rolled at a cold pressure ratio (CR) of 55% or more to satisfy the following formula (1). By annealing at a thermal temperature (TA), it has a structure including a ferrite phase as a main phase and a second phase, and the area ratio of the band-shaped structure composed of the second phase is 6.0% or less. The manufacturing method of the continuous annealing steel plate for rapid heating quenching.
A _c1 <(TA) <A _c1 + (A _c3 −A _c1 ) (CR−10) / 100 (1)

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