JP4264306B2 - Precipitation strengthened high strength steel - Google Patents

Description

【００２７】
【表２】

【００２８】
表２に示したとおり、発明例はいずれも、Cuクラスターの粒径が５nm以下と小さく、かつ体積率が５％以上と高いため、優れた析出強化効果が得られている。また、降伏比（ＹＲ）も85％以下と小さいことから、成形加工性も良好であることが分かる。
これに対し、比較例である鋼種ＣおよびＤは、Cu量が少ないため、Cuクラスターの体積率が小さく、また体積率を上昇させた場合にはCuクラスターの粒径が大きいため、析出強化効果が小さく、その結果得られる引張強度も低い。
【００２９】
実施例２
表１中の鋼種ＡおよびＣについて、時効温度を変更した場合における引張強度の変化について調査した。
得られた結果を表３に示す。
【００３０】
【表３】

【００３１】
同表に示したとおり、発明例である鋼種Ａでは、100 ℃以上の時効温度において強度上昇量が大きいが、比較例の試料Ｃでは 800℃以上の時効温度において強度上昇が見られるものの、その値は 500 MPa未満と小さかった。
【００３２】
【発明の効果】
かくして、本発明に従い、時効処理後に微細なCuクラスターを適量析出させることにより、析出強化効果が大きく、かつ成形加工性に優れた析出強化型高強度鋼を安定して得ることができる。
【図面の簡単な説明】
【図１】 Cuクラスターの体積率と引張強度との関係を示した図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a precipitation-strengthened high-strength steel, and particularly intends to improve the forming processability advantageously.
[0002]
[Prior art]
In recent years, due to increasing interest in environmental issues, demand for material development for energy conservation has increased, and in the fields of automotive steel sheets and building structural members, for example, there has been an increasing demand for the development of high-strength steel that is essential for weight reduction of materials ing.
[0003]
Examples of means for increasing the strength of steel include precipitation strengthening, solid solution strengthening, structure strengthening, and fine grain strengthening.
Among these, solid solution strengthening is achieved by dissolving elements such as P, Si and Mn in the structure, and structure strengthening is achieved by dispersing structures such as ferrite, bainite, martensite, and pearlite, thereby strengthening the grain refinement. Is intended to increase the strength by refining the crystal grains of the structure.
[0004]
Precipitation strengthening is intended to increase the strength by dispersing precipitates. In this case, control of the size and form of the precipitates is important for increasing the strength. In general, the smaller the precipitate size and the higher the precipitate volume ratio (volume ratio), the greater the strength increase effect.
[0005]
As an example of improving the material properties by controlling the size of the precipitates, high strength for machining with improved fatigue properties by controlling the precipitation size (particle size) of pure Cu particles to 2nm or less in a composite steel sheet Cold-rolled steel sheets have been proposed (for example, Patent Document 1).
However, this technique not only precipitates pure Cu particles of 2 nm or less in order to increase the tensile strength of the steel, but also ensures the ductility, so that the cooling rate after hot rolling, the coiling temperature, after cold rolling annealing. It is required to strictly control the cooling rate and the cooling stop temperature.
In addition, with this technology, the yield strength increases as the tensile strength of the steel increases, so that the steel shape cannot be formed into a desired shape, especially when the steel shape is a plate material, due to the performance of the spring back and press machine during forming. There was a problem.
[0006]
[Patent Document 1]
JP-A-11-279690 (Claims)
[0007]
[Problems to be solved by the invention]
The present invention advantageously solves the above-mentioned problems, and does not impair ductility, and is easy to process with relatively low strength during molding, and after molding, the strength can be advantageously improved by an aging phenomenon. The purpose is to propose precipitation strengthened high strength steel.
[0008]
[Means for Solving the Problems]
Now, as a result of intensive research to achieve the above object, the inventors have made use of fine clusters that have the same BCC (Body Centered Cubic) structure as the parent phase of steel. For example, it has been found that ductility is not impaired, and it is easy to process with relatively low strength during molding, while high strength is achieved by aging after molding.
The present invention is based on the above findings.
[0009]
That is, the gist configuration of the present invention is as follows.
1. In mass%
C: 0.002 to 1.0 %,
Si : 0.005 to 1.0 %,
Mn: 0.1 ~ 2.0%,
Cu: 1.5 ~ 4.0%,
P: 0.01 to 0.1 %,
Al : 0.1 % or less and
S: 0.010 % or less
The remainder is composed of Fe and inevitable impurities, and after aging treatment, a Cu cluster having a particle size of 5 nm or less and having a BCC structure is 5% or more and 20 % or less by volume ratio with respect to the total Cu volume ( excluding 20%) containing to precipitation strengthening type high-strength steel, characterized in Rukoto.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the research results that led to the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.002%, Si: 0.01%, Mn: 0.1%, P: 0.01%, S: 0.005%, Al: 0.04%, N: 0.002%, and Cu: 0 to 2% of the steel slab whose components were adjusted After hot rolling at a temperature of 1200 ° C and a rolling finish temperature of 900 ° C, winding at room temperature, pickling, cold rolling from 3.5 mm to 1.0 mm, and then annealing at 800 ° C for 20 minutes .
Then, after performing aging treatment for 20 minutes at the temperature of 300-650 degreeC, the JIS No. 5 tensile test piece was created and tensile strength TS (Tensile Strength) was measured.
[0013]
In addition, a sample for a transmission electron microscope was collected from each of the obtained samples at a thickness of 1/4, and the size (grain size) of a Cu cluster having a particle size of 5 nm or less by an EDX (Energy Dispersive X-rayspectrometer). (Diameter) was measured and the volume ratio was calculated.
In addition, the volume ratio of Cu cluster shall be shown by the ratio with respect to the volume of all Cu. Here, the volume of all Cu is a value obtained by dividing the total Cu mass in steel by the specific gravity of Cu.
The obtained results are shown in FIG.
[0014]
As shown in the figure, after aging treatment, a good increase in strength is achieved when Cu clusters with a BCC structure with a diameter of 5 nm or less are present in a volume ratio of 5% or more with respect to the total Cu volume. There was found.
[0015]
In the present invention, among the Cu clusters having a BCC structure, the Cu clusters having a particle size of 5 nm or less are particularly targeted. It is difficult to produce a large number of precipitates exceeding 5 nm. This is because it does not effectively act on precipitation strengthening.
[0016]
Next, the reason why the preferred component composition of steel is limited to the above range in the present invention will be described.
C: 0.002 to 1.0%
The smaller the C, the better the ductility, but it is disadvantageous in terms of cost to reduce the content to less than 0.002%. On the other hand, if it contains more than 1.0%, the workability and weldability deteriorate significantly. The amount was limited to a range of 0.002 to 1.0%.
[0017]
Si: 0.005 to 1.0%
Si, as a solid solution strengthening element, contributes to an increase in strength without reducing ductility, so it is contained in an amount of 0.005% or more, but the effect is saturated even if it is contained in excess of 1.0%. In order to adversely affect the properties, the Si content was limited to a range of 0.005 to 1.0%.
[0018]
Mn: 0.1 to 2.0%
Mn is a useful element that not only increases the strength of the steel but also prevents hot brittleness due to S, and is actively added in the range of 0.1% or more. However, if it exceeds 2.0%, workability deteriorates. Therefore, the amount of Mn was limited to the range of 0.1 to 2.0%.
[0019]
Cu: 1.5 ~4.0%
Cu is the most important element in the present invention.
In the present invention, as described above, the ductility of steel (ferrite) as a parent phase is caused by precipitating 5% or more by volume ratio of Cu clusters having a particle size of 5 nm or less with respect to the total Cu volume. It is intended to increase the strength without lowering. If the Cu amount is less than 1.5 %, the absolute amount of the Cu cluster described above is insufficient. On the other hand, if it exceeds 4.0%, the effect reaches saturation, so the Cu amount is limited to a range of 1.5 to 4.0%.
The Cu cluster was observed with a transmission electron microscope (TEM), the particle size was measured with EDX, and the volume ratio was calculated. As a result, it was found that when the BCC structure Cu cluster having a particle size of 5 nm or less is 5% or more by volume, it is easy to process and the strength increase is large due to the aging phenomenon after molding.
[0020]
P: 0.01 to 0.1%
P is an element effective in increasing the strength without decreasing the ductility. However, if the content is less than 0.01%, the effect is poor. On the other hand, if added over 0.1%, the workability and weldability are reduced. Since P has an adverse effect, P is limited to the range of 0.01 to 0.1%.
[0021]
Al: 0.1% or less
Al needs to be added for deoxidation, but if it is added too much, non-metallic inclusions increase and the elongation deteriorates, so the content was limited to 0.1% or less.
[0022]
S: 0.010% or less If too much S causes a drop in ductility or cracking during hot rolling, it is desirable to reduce it as much as possible, but 0.010% or less is acceptable.
[0023]
The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
0.01 to 2.0% of one or more selected from Mo, Nb, Ti, V, Cr and Ni in total
All of these elements are elements that contribute effectively to improvement in strength. However, if the content is less than 0.01%, the effect of addition is poor. On the other hand, if the content exceeds 2.0%, the effect is saturated. Therefore, these contents are contained in the range of 0.01 to 2.0% in either case of single addition or compound addition. It is preferable to make it .
[0024]
Moreover, in this invention, it is not necessary to manage manufacturing conditions strictly like the said patent document 1, and hot rolling, winding, cold rolling, etc. should just be performed according to a conventional method. However, as expected in the present invention, in order to precipitate 5% or more of Cu clusters having a BCC structure with a diameter of 5 nm or less, these rolling performed after hot rolling or cold rolling, or as necessary It is necessary to perform heat treatment (aging treatment) at a temperature of 100 to 600 ° C. after the subsequent annealing treatment.
[0025]
【Example】
Example 1
Steel having the composition shown in Table 1 is melted in a converter, continuously cast, reheated at a heating temperature of 1200 ° C, hot rolled at a rolling finish temperature of 850 to 930 ° C, and then 550 ° C or less. After rolling at a temperature of 50 ° C., pickling, forming a 0.7 to 1.3 mm cold rolled sheet by cold rolling at a rolling reduction of 60 to 80%, annealing at 800 ° C. for 20 minutes, and then 300 to 650 An aging treatment was performed at a temperature of 20 ° C. for 20 minutes.
The steel sheet thus obtained was processed into a JIS No. 5 tensile test piece described in JIS Z 2201, and a tensile test described in JISZ 2241 was performed.
Further, a sample for a transmission electron microscope was taken from the position of the thickness of the steel sheet after the aging treatment, and the size (particle diameter) of the Cu cluster having the BCC structure was measured by EDX, and the volume ratio was calculated.
The obtained results are shown in Table 2.
[0026]
[Table 1]

[0027]
[Table 2]

[0028]
As shown in Table 2, all of the inventive examples have an excellent precipitation strengthening effect because the particle size of the Cu cluster is as small as 5 nm or less and the volume ratio is as high as 5% or more. Moreover, since the yield ratio (YR) is as small as 85% or less, it can be seen that the moldability is also good.
On the other hand, steel types C and D, which are comparative examples, have a small amount of Cu, so the volume ratio of Cu clusters is small, and when the volume ratio is increased, the grain size of Cu clusters is large, so the precipitation strengthening effect And the resulting tensile strength is low.
[0029]
Example 2
For steel types A and C in Table 1, the change in tensile strength when the aging temperature was changed was investigated.
The obtained results are shown in Table 3.
[0030]
[Table 3]

[0031]
As shown in the table, the steel type A, which is an example of the invention, has a large increase in strength at an aging temperature of 100 ° C. or higher. The value was small, less than 500 MPa.
[0032]
【The invention's effect】
Thus, according to the present invention, by precipitation of an appropriate amount of fine Cu clusters after the aging treatment, a precipitation strengthening type high strength steel having a large precipitation strengthening effect and excellent formability can be obtained stably.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the volume fraction of Cu clusters and tensile strength.

Claims (1)

In mass%
C: 0.002 to 1.0 %,
Si : 0.005 to 1.0 %,
Mn: 0.1 ~ 2.0%,
Cu: 1.5 ~ 4.0%,
P: 0.01 to 0.1 %,
Al : 0.1 % or less and
S: 0.010 % or less
The remainder is composed of Fe and inevitable impurities, and after aging treatment, a Cu cluster having a particle size of 5 nm or less and having a BCC structure is 5% or more and 20 % or less by volume ratio with respect to the total Cu volume ( excluding 20%) containing to precipitation strengthening type high-strength steel, characterized in Rukoto.

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