JPH11279685A - Steel sheet excellent in impact absorptivity after nitriding treatment, high strength press-formed article excellent in impact absorptivity, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet most suitable for producing a high strength press-formed article excellent in impact absorptivity with high dimensional accuracy at a low cost, and to produce a high strength press formed part excellent in impact absorptivity. SOLUTION: This steel sheet excellent in impact absorptivity after nitriding treatment has a composition consisting of, by weight, <=0.003% C, <=0.05% Si, 0.1-1% Mn, <=0.1% p, <=0.02% S, 0.03-0.06% Sol.Al, 0.0002-0.0015% B, <=0.003% N, Ti in an amount in the range where the value of Ti* defined by Ti*%=Ti%-48/14×N%-48/12×C%-48/32×S% becomes -0.01 to 0.075%, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention is most suitable for structural members such as members in automobile parts and reinforcing members such as reinforcements, but any other mechanical structure which requires energy absorption upon impact deformation. Steel sheet, which is suitable for manufacturing high-strength press-formed products with excellent shock-absorbing ability with high dimensional accuracy and at low cost, and high-strength press-formed bodies with excellent shock-absorbing capacity And its manufacturing method.

[0002]

2. Description of the Related Art In recent years, there has been a demand for a reduction in the weight of a vehicle body in order to improve the fuel efficiency of an automobile. Also, from the viewpoint of collision safety, there is a growing need for high strength, high rigidity, and high shock absorption capacity of the body. To meet these needs, structural members such as members that require strength and reinforcement members such as reinforcement are converted from conventional soft steel plates to high-tensile steel plates with TS (tensile strength) of 340 MPa or more. Is planned.

[0003] However, the high tensile strength steel sheet has an elongation and an inferior r value as compared with the soft steel sheet, so that, of course, the formability is low and it is difficult to press-form a complex shape.

[0004] Further, as the YP (yield point) increases, the springback increases, and it is difficult to obtain good dimensional accuracy. As for the body panel, it is desired to increase the yield strength of the product from the viewpoint of ensuring the dent resistance. However, if the strength of the material is increased, the dimensional accuracy after pressing is deteriorated, and the moldability is also deteriorated, thereby causing design restrictions.

As one of means for solving this problem, there is a technique for strengthening a soft steel sheet by performing a heat treatment after forming. In this method, the steel sheet has high formability before quenching, and has a high strength by a subsequent heat treatment. Therefore, there is an advantage that a complicated shape can be pressed and high strength can be obtained.
However, there is a problem in that a large heat input is involved during quenching, so that the member is deformed by thermal distortion, and high dimensional accuracy of the member cannot be obtained.

On the other hand, as a technique for hardening steel by heat treatment, carburizing and nitriding are widely used mainly for parts requiring wear resistance such as gears. These techniques are originally surface hardening techniques for cast and forged products. Recently, there has been disclosed a technique in which a steel sheet is press-formed and used for parts requiring wear resistance, fatigue strength, and seizure resistance, such as tools, parts for mechanical structures, and automobile parts (Japanese Patent Application Laid-Open No. 9-1997). No. 25543,
JP-A-9-25544). As described in the specification, these techniques are mainly intended to improve the wear resistance of automobile drive transmission parts and the like by increasing the surface hardness, and try to increase the strength of the members themselves. For example, the technical idea is fundamentally different from the post-press quenching technique. Conventional nitrided steel has a large increase in surface hardness, but has a problem in that toughness is deteriorated due to the formation of a remarkably hardened layer on the surface layer. In addition, since the thickness of this cured layer is conventionally at most about 400 μm,
A non-nitrided layer at the center remains, and a sufficient effect on the strength of the entire press-formed member cannot be obtained.

As a steel sheet having excellent nitriding or nitrocarburizing properties,
JP-A-54-21916, JP-A-55-7604
6, JP-A-1-96330, JP-A-8-3
Various technologies are disclosed in JP-A-5013 and JP-A-9-25517.

Japanese Patent Application Laid-Open No. 6-136438 discloses a technique for hardening the inside of a plate by using precipitation strengthening of ε-Cu together with nitriding.

[0009]

However, Japanese Unexamined Patent Publication No.
Japanese Patent Application Laid-Open No. 4-21916 is intended for abrasion resistance and fatigue resistance properties. For the purpose of the present invention, the purpose is to use parts for machine structural use because the Ti concentration is too high and Cr is contained. Poor in strength and toughness. Furthermore, the prior art does not show that the Ti concentration is remarkably excellent in the strength toughness after nitriding in the present invention. JP-A-55-760
No. 46 and Japanese Unexamined Patent Publication No. Hei 9-25517 require the addition of Cr, but Cr has the effect of increasing the hardness of the hardened surface layer, but it causes embrittlement, so that nitriding for parts for mechanical structures requiring toughness is required. It must not be added to steel sheets. Further, since the added element is expensive, it is not industrially preferable. JP-A-1-96
No. 330 discloses a method of nitriding in a plate manufacturing stage, and has a different technical idea from the present invention. In addition, this prior document does not show that the strength and toughness after nitriding are particularly excellent in the very limited ranges of C and Ti shown in the claims of the present invention. The technique disclosed in Japanese Patent Application Laid-Open No. 8-35013 is not excellent in strength after nitriding because the amount of solid solution Ti that contributes to an increase in strength due to nitriding is not sufficient as compared with the present invention.

Further, Japanese Patent Application Laid-Open No. 6-136438 discloses C
The u-added steel has remarkable generation of surface defects due to eutectic melting of Cu during hot rolling, and cannot be applied to parts requiring good surface properties.

As described above, in the prior art, it was difficult to manufacture a high-strength member having a high shock absorbing ability, which is essential for reducing the weight of an automobile, with high dimensional accuracy.

An object of the present invention is to provide a steel sheet which is suitable for producing a high-strength press-formed body having excellent shock-absorbing ability with high dimensional accuracy and at low cost, a high-strength press-formed body having excellent shock-absorbing ability, and production thereof. It is to provide a method.

[0013]

In order to solve the above problems and achieve the object, the present invention uses the following means.

(1) The steel sheet of the present invention has a C:
0.003% or less, Si: 0.05% or less, Mn:
0.1-1%, P: 0.1% or less, S: 0.02%
And Sol. Al: 0.03 to 0.06% and B:
0.0002 to 0.0015%, N: 0.003% or less, and Ti as Ti ^* : -0.0.
It is a steel sheet excellent in impact absorption capacity after nitriding treatment, characterized in that it is contained in the range of 0.01 to 0.075%, the balance being Fe and unavoidable impurities.

Ti ^* % = Ti% −48 / 14 × N% −48 / 12 × C% −48 / 32 × S% (1) (2) The steel sheet of the present invention is expressed as a steel component by weight%. The steel sheet according to (1), further comprising Nb: 0.005 to 0.03%, and having excellent shock absorbing ability after the nitriding treatment.

(3) The molded article of the present invention is a press molded article made of a steel sheet having the composition described in the above (1) or (2), having a nitrided layer over the surface or central portion of the molded article. And a high-strength press-formed body excellent in impact absorbing ability, having a cross-sectional hardness Hv of 150 to 300.

(4) A method of manufacturing a molded article according to the present invention is characterized in that a steel sheet having the composition described in the above (1) or (2) is formed into a predetermined shape and then subjected to nitriding or nitrocarburizing treatment. This is a method for producing a high-strength press-formed body having excellent dimensional accuracy and shock absorbing ability.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of intensive studies to solve the above-mentioned problems, the present inventors have obtained the following findings.

C: In ultra-low carbon steel of 0.003% or less, Ti is replaced by Ti ^* % = Ti% -48 / 14 × N% -48.
Ti ^* % defined as / 12 × C% −48 / 32 × S%
Is set to -0.01 to 0.075%, the optimum strength increase due to nitriding can be obtained. Further, B is 0.00
The addition of 2 to 0.0015% is indispensable for suppressing brittle fracture after nitriding while obtaining high strength and obtaining high shock absorbing ability. On the other hand, by controlling the additive elements as in claim 1 of the present invention from the viewpoints of material properties such as press formability and manufacturability, a steel sheet having excellent post-nitridation shock absorption capacity is provided. In addition, by performing nitriding or nitrocarburizing treatment after forming the above steel sheet, it is excellent in formability and shape freezing property during forming, and thereafter, by performing nitriding or nitrocarburizing treatment,
It is possible to manufacture a press-formed body having a high shock absorbing ability without impairing the dimensional accuracy.

Based on the above findings, the present inventors have proposed C:
In a very low carbon steel of 0.003% or less, a solid solution Ti concentration (Ti ^* ) defined by an amount excluding nitrides, carbides, sulfides and composites thereof from a total amount of Ti added is controlled within a certain range. Further, by adding a certain amount of B, a steel sheet having excellent press formability and shock absorbing ability after nitriding treatment is found, and after the steel sheet is formed into a predetermined shape, nitriding or nitrocarburizing treatment is performed. Thus, a high-strength press-formed body having excellent shock absorbing ability and a method for producing the same were found, and the present invention was completed.

That is, the present invention is intended to produce a high-strength press-formed body having excellent shock-absorbing ability with high dimensional accuracy and low cost by limiting the steel composition and the manufacturing conditions of the press-formed body to the following ranges. It is possible to provide an optimum steel plate, a high-strength press-formed body excellent in impact absorbing ability, and a method for producing the same.

The reasons for adding the components of the present invention, the reasons for limiting the components, and the conditions for producing the press-formed body will be described below.

(1) Component composition range Ti ^* : -0.01 to 0.075%, where Ti ^* % =
Ti% -48 / 14 × N% -48 / 12 × C% -48 /
32 × S%.

Ti is one of the most important chemical components of the present invention, and the control of solid solution Ti excluding nitride, carbide, sulfide and those forming a complex thereof is included in the total amount of Ti. It is indispensable for balancing strength and toughness after nitriding. The solid solution Ti has the formula Ti ^* % = Ti% −48 / 14 × N% −48 /
12 × C% −48 / 32 × S%, which is
It is necessary to control in the range of 0.01 to 0.075%. If it is less than -0.01%, the increase in strength due to nitriding is not sufficient, and high strength cannot be obtained. On the other hand, if this exceeds 0.075%, the member becomes brittle after nitriding,
High impact absorption energy cannot be obtained.

C: 0.003% or less When the C concentration exceeds 0.003%, the steel sheet strength is increased due to the fine precipitation of TiC, and the formability is significantly reduced. Therefore, C must be less than 0.003%.

Si: 0.05% or less When Si exceeds 0.05%, the strength increases due to solid solution strengthening, and the formability is reduced. Therefore, the upper limit of the content is 0.05%.

Mn: 0.1-1% Mn precipitates as MnS together with solid solution S and suppresses surface flaws caused by S. If it is less than 0.1%, the effect is not sufficient. If it exceeds 1%, the strength increases due to solid solution strengthening, and the formability decreases. Therefore, its content is 0.1-1%.

P: 0.1% or less P is a solid solution strengthening element like Mn and Si.
If it exceeds 300, the moldability will be significantly deteriorated. Therefore, the upper limit of the content is 0.1%.

S: 0.02% or less Since S causes surface flaws as described above, S is 0.02% or less.
It is necessary to control the following.

Sol. Al: 0.03 to 0.06% Sol. Al is added as a deoxidizer. If it is less than 0.03%, the effect is not sufficient. If the content exceeds 0.06%, the effect is saturated and uneconomical.
6% or less.

N: 0.003% or less N is dissolved as N so as to prevent the formation of a texture suitable for improving the r value during annealing, or to generate a stretcher strain mark at the time of forming due to strain aging and to strengthen at the time of nitriding. In order to fix the contributing solid solution Ti as TiN and reduce the effect of nitriding, it is desirable to reduce as much as possible. If it exceeds 0.003%, its adverse effect becomes remarkable, so it is necessary to make it 0.003% or less.

B: 0.0002 to 0.0015% B is an important constituent element of the present invention, and B is contained in an amount of 0.0002 to 0.0015%.
By adding 0.0015%, when the material after nitriding is subjected to impact deformation, cracks are less likely to occur, and the impact absorbing ability is significantly increased. Although the detailed mechanism is unknown, it is possible that B segregates at the grain boundaries to strengthen the grain boundaries and improve the local deformability after nitriding. This effect is 0.0002%
If less than 0.0015%, the effect of strengthening the grain boundary exceeding 0.0015% is not only saturated, but also the ductility is reduced due to the increase in strength due to solid solution B. Further, the r-value also decreases, so that the moldability deteriorates remarkably.

In the present invention, in addition to the above elements, Nb may be further contained in the following range for the purpose of improving material formability.

Nb: 0.005 to 0.03% Nb has the effect of improving the r value and elongation of the material. However, less than 0.005% has no effect.
%, The effect saturates and the cost increases,
0.005 for applications that require particularly high material formability
０．０0.03% must be added.

Further, Cu is eutectic melted at the time of hot rolling and remarkably deteriorates surface properties.

By adjusting to the above component composition range, it becomes possible to obtain a steel sheet which is optimal for producing a high-strength press-formed body having excellent shock absorbing ability with high dimensional accuracy and at low cost.

The press-formed body having such characteristics can be manufactured by the following manufacturing method.

(2) Step of Manufacturing Pressed Body (Manufacturing Method) First, a steel sheet having the component composition of the above (1) is formed into a predetermined shape by press forming. Since the steel sheet is excellent in press formability and shape freezing property, it is possible to form a press formed body having a complicated shape with high dimensional accuracy. Thereafter, nitriding or nitrocarburizing is performed. Nitriding,
The effect of the soft nitriding treatment is the same regardless of whether it is gas nitriding or plasma (ion) nitriding. The nitriding conditions are the same as in the conventional case for surface hardening, and there is no problem.
It is appropriate that the nitriding time is about 1 hour to about 10 hours.

The reason why the steel sheet containing the composition (1) is subjected to the nitriding treatment or the nitrocarburizing treatment is to generate and strengthen a hardened layer (nitrided layer) having a uniform hardness distribution in the thickness direction. . If the temperature or time of the nitriding treatment is low, the effect of this nitriding is not sufficient.

The press-formed body of the present invention obtained by the above-described manufacturing method has a nitrided layer over the surface or central portion of the plate of the formed body, and has a cross-sectional hardness Hv of 150 to 30.
It is characterized by being 0. This nitrided layer is desirably formed uniformly to the center of the plate.

When the minimum value of the sectional hardness Hv is less than 150,
Insufficient nitriding effect. The reason for setting Hv to 150 is T
S: Based on a strength equivalent to that of a member using a 45 kg / mm ² material. On the other hand, if the maximum value of the cross-sectional hardness (generally, the surface hardness) after nitriding exceeds Hv300, cracking becomes noticeable during impact deformation, and high absorption energy cannot be obtained. Therefore, the surface hardness (Hv) after nitriding treatment is 300
It must be:

The press-formed body having the component (1) and subjected to nitriding or nitrocarburizing has good dimensional accuracy, high strength, and does not become brittle, so that it has high shock absorption. Has ability.

Further, a member having good dimensional accuracy, high strength and high shock absorbing ability can be manufactured by the above method.

In the present invention, the conditions for producing the steel sheet are not particularly limited. In other words, the method for smelting steel, the method for rolling during the production of a steel sheet, and the method for heat treatment may be any conditions that are normally employed.

The effects of the present invention will be proved by the following examples.

[0046]

EXAMPLE A cold-rolled steel sheet having a thickness of 1.2 mm containing various chemical components and concentrations as shown in Table 1 (A to D: steel of the present invention,
E to I: Comparative steel). Nitriding treatment is performed by using a gas nitrocarburizing method at 570 ° C. in a mixed gas of NH ₃ , N ₂ , H ₂ and CO.
× 2 hours. After nitriding, a tensile test is performed to determine YP (yield point), TS (tensile strength), and El (elongation). Compared with the tensile properties (YP, TS, El, and r values) before nitriding, the characteristic change due to nitriding is calculated. investigated.

The cross-sectional hardness distribution in the thickness direction was measured at a pitch of 100 μm by Vickers hardness (load: 10 g).
The maximum value (Hv max) and the minimum value (Hv min) were determined. The shock absorption capacity is 200m long with a square cross section of 40mm square
The average deformation load when the hat-shaped column test piece of m was deformed by impact at a deformation speed of about 10 m / sec was evaluated. Since the absorbed energy is expressed as the product of the average deformation load and the displacement,
The absorbed energy up to a certain amount of deformation is proportional to the average deformation load.

As shown in Table 2, the steels A to D of the present invention have a small springback amount because the material before nitriding has a low YP.
The elongation is 50% or more and the moldability is good. When these steels are nitrided, they have a TS (tensile strength) of 300 MPa.
The above marked increase in strength was exhibited. Also the cross-sectional hardness (Hv)
Is 150 or more and the distribution in the thickness direction is as small as 30 or less. The average deformation load is about 28k for the material in this embodiment.
N was 45 kN or more, which is 70% or more.

On the other hand, the comparative steels E and I, in which Ti ^* is lower than the range of the present invention, have a small increase in strength due to nitriding, and all have Hv min of less than 150. Therefore, the increase in the average deformation load is small and is not suitable. Further, the comparative steel H in which Ti ^* exceeds the range of the present invention has a large increase in strength, but Hv m
Since ax exceeds 300, it breaks brittlely and high absorption energy cannot be obtained. Comparative steel F without B added
Has a low absorbed energy because it is brittlely broken by impact deformation. Comparative steel G having a high B content exceeding the range of the present invention had low formability due to a decrease in elongation (El) and r value, and was not suitable for forming a member having a complicated shape.

FIG. 1 shows the steels A, B, and B of the test steels shown in Tables 1 and 2 to which B was added and Ti ^* (% by weight) was varied.
Ti ^{* of} C, D, comparative steels E, H, and comparative steel F without B
And the relationship between the average deformation load. It contains 0.0002 to 0.0015% of B within the scope of the present invention, and Ti ^* is-
Inventive steels A, B, C, in the range of 0.01 to 0.075%
D has an extremely high absorption energy by nitriding treatment,
It can be seen that it is most suitable for a member for a high-strength automobile structure and a reinforcing member having excellent shock absorbing ability such as a member and a reinforcement.

Further, it can be seen that a press-formed body excellent in impact absorption capacity and dimensional accuracy can be easily manufactured by performing a nitriding treatment after pressing using the steel sheet according to claim 1 of the present invention.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

As described above, according to the present invention,
By specifying the steel composition and the manufacturing conditions, it becomes possible to manufacture a high-strength press-formed body having excellent shock absorbing ability with high dimensional accuracy and at low cost, and an extremely useful effect in industry can be expected.

[Brief description of the drawings]

FIG. 1 shows Ti ^* (% by weight) of a steel sheet according to an example of the present invention.
FIG. 4 is a diagram showing a relationship between the average deformation load and the average deformation load.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Akihide Yoshitake 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd.

Claims (4)

[Claims] 1. The method according to claim 1, wherein C: 0.003% or less in weight%,
i: 0.05% or less; Mn: 0.1 to 1%;
0.1% or less, S: 0.02% or less, Sol. A
l: 0.03-0.06% and B: 0.0002-0.
0015%, N: 0.003% or less, and Ti as defined by the following formula (1): Ti ^* : -0.01 to 0.075
%, With the balance being Fe and unavoidable impurities, the steel sheet having excellent shock absorbing ability after nitriding treatment. Ti ^* % = Ti% −48 / 14 × N% −48 / 12 × C% −48 / 32 × S% (1) 2. The steel composition further comprises Nb:
Characterized by containing 0.005 to 0.03%,
The steel sheet excellent in impact absorbing ability after the nitriding treatment according to claim 1. 3. A press-formed body made of a steel sheet having the composition according to claim 1 or 2, having a nitrided layer over a surface layer portion or a central portion of the formed body, and having a cross-sectional hardness Hv of 150 to 300. A high-strength press-formed body having excellent shock absorbing ability. 4. A high-strength steel sheet having excellent dimensional accuracy and shock-absorbing ability, wherein a steel sheet having the composition according to claim 1 or 2 is formed into a predetermined shape and then subjected to nitriding or nitrocarburizing treatment. A method for manufacturing a press-formed body.