JPS63282240A - High tensile strength rolled steel plate having excellent fatigue characteristics - Google Patents

Abstract

PURPOSE:To produce the titled steel plate by prepg. the hot rolled steel plate contg. specific ratios of C, Mn, Si, P, Ti, N and B and having specific structure of surface layer and central layer independently. CONSTITUTION:The hot rolled steel plate consisting of, by weight, 0.03-0.1% C, 0.5-2.0% Mn, <=1.5% Si, <=0.15% P, <=0.05% Ti as well as <=about 0.005% N in >=3.42 N (%) Ti, 0.0002-0.001% B and the balance Fe with inevitable impurities is prepd. In said steel plate, the structure in which the average grain size of ferrite grains present on the layer of the thickness from the surface to 0<h<(0.1-0.25)t where (t denotes plate thickness) is regulated to <=4mum and the sectional structure area rate is regulated to >=90% is specified. The central layer interposed therebetween has the structure in which either or both of martensite and residual austenite having 5-15% sectional structure area rate are dispersed into >=75% ferrite matrix. The hot rolled steel plate having <=0.7 yield ratio and >=0.5 ratio of fatigue strength to tensile strength is thereby obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a two-phase high-strength hot-rolled steel sheet with excellent fatigue properties.

(Prior Art) Currently, there is a strong demand for high-strength steel sheets with excellent workability, mainly for steel materials for automobiles. In order to meet this demand, a recently developed duplex steel (hereinafter referred to as DP steel) in which a highly hard transformed phase is dispersed and precipitated in a ferrite structure has been developed.
shows high strength and long ductility. For example, JP-A-56-2%24, JP-A-57-137452
(Japanese Patent Publication No. 61-50125) has been attracting attention.

However, steel materials for automobiles (e.g. wheels, etc.)
Since there are many places where repeated stress is applied, the quality of the material is often determined by fatigue strength rather than tensile strength.From this point of view, DP steel generally has a fatigue strength/tensile strength ratio of 0.
．． It almost never exceeds 5 and cannot be said to have excellent properties.

On the other hand, attempts to make the ferrite structure finer for the purpose of increasing fatigue strength can be seen in JP-A-58-123823 and JP-A-58-174544; The aim is to obtain a microstructure, and there are strict restrictions on alloying elements and rolling conditions.

In addition, steels aiming for good toughness have been developed in which the layer near the surface has a fine ferrite structure and the other center layer has a transformation phase mainly composed of bainite or martensite. balance is inferior to that of DP steel (Japanese Unexamined Patent Publication No. 59-170238).

(Problems to be Solved by the Invention) As mentioned above, conventional DP steel has a low yield ratio;
Although it has good properties such as an excellent balance of strength and toughness, there is a problem that its fatigue properties are not necessarily excellent.

The present invention was made with the object of providing a high-tensile hot-rolled steel sheet with excellent fatigue properties, which solves the problems in the prior art described above.

(Means to solve the problem) The features of the present invention are as follows.

(1) C in weight%: 0.03-0.1% Mn: 0.5-2.0% Si≦1.5 Wood P≦0.15! Ik Ti≦1.05% and Ti≧3.42X N (!I;
)B: 0.0002~0.001! The remainder consists of Fe and unavoidable impurities, and from the surface 0 < h <
(0,1 to 0.25) The average grain size of the ferrite grains present in the layer with a thickness of t is 4 μm or less, the cross-sectional structure area ratio is 9ON or more, and the structure of the central layer sandwiched between them is , has a structure in which one or both of martensite and retained austenite of 5 or more and 15% or less are dispersed in a ferritic ground with a cross-sectional structure area ratio of 75% or more, a yield ratio of 0.7 or less, and a fatigue strength A high tensile strength hot rolled steel sheet with excellent fatigue properties and a tensile strength ratio of 0.5 or more. However, t: board thickness.

(2) C at weight t: 0.03-0.1! kMn: 0.5 ~ 2.0! <Si≦1.5% P≦0.15% Ti≦1.05! k and Ti≧3.42x N (!k
) B: Contains 0.0002 to 0.001%, and further contains one or more of Ni, Cr, and Mo in a total of 0.5 or less, and one or two of Nb and V in a total of 0, The average of ferrite grains containing either or both of 0.08% or less, the balance consisting of Fe and unavoidable impurities, and existing in a layer with a thickness of 0 < h < (0,1 to 0.25) t from the surface. The grain size is 4 μm or less, the cross-sectional structure area ratio is 90% or more, and the structure of the center layer sandwiched between them is 5% or more and 15 or less in the cross-sectional structure area ratio of 75 or more. , a high-strength hot-rolled steel sheet with excellent fatigue properties, having a structure in which martensite and/or retained austenite are dispersed, a yield ratio of 0.7 or less, and a ratio of fatigue strength to tensile strength of 0.5 or more. .

The present invention will be explained in detail below.

First, the reason for limiting the composition of the steel of the present invention will be explained.

C is an element necessary to obtain the second phase. The reason why the lower limit of C content was set to 0.035 is that a two-phase structure cannot be obtained with an amount less than this, and the reason why the upper limit was set to 0.1% is that if it is added in excess of this, This is because the cross-sectional structure area ratio of the ferrite in the center layer does not reach 75% or more, making it impossible to achieve the object of the present invention.

Mn also contributes to the generation of the second phase. The lower limit of the Mn content is set at 0.5* because this is the minimum value necessary to generate martensite of 5 or more.

On the other hand, the reason why the upper limit is set to 2.0 t is that if more than this is added, it becomes difficult to generate 75% or more of ferrite in the center layer.

The addition of Si and P is preferable to obtain a two-phase structure, but the addition of S
If the i content exceeds 1.5%, problems will arise in the weldability of the steel material, so the upper limit is set at 1.5%. Also, P is 0.1
5! If added in excess of Ii, brittle fracture is likely to occur during processing of the steel material, so the upper limit is set at 0.15.

Addition of B increases the hardenability of the steel material and makes it easier to form a two-phase structure, but if the amount added exceeds 0.001%, not only does the effect of addition become saturated, but the ductility of the steel material deteriorates. Therefore, the upper limit of the amount of B added is 0.001! Totosu.

On the other hand, setting the lower limit of the amount of B added to 0.0002t is as follows:
This is because if no more than this is added, it will not contribute to improving the hardenability of the steel material.

N causes hardening of ferrite and deteriorates the ductility of steel materials. Therefore, the upper limit of its content is set at 0°005.

Ti forms precipitates with N and contributes to the purification of ferrite, but as its content increases, it also forms precipitates with C, hardening the steel and reducing ductility, so the upper limit of the amount added is set. Let be 0.05'li.

On the other hand, the lower limit of the amount of Ti added is set to Ti≧3.42X N (!Ii) in relation to the N content.
This is to remove solid solution N in the form of TiN by adding a chemical equivalent of TiN, suppress B--N bonding, and bring B into a solid solution state to contribute to improving the hardenability of the steel material.

Although the addition of Nb, V, Ni, (:r, Mo) is not an essential condition of the present invention, the addition of these elements contributes to grain refinement of the steel surface layer or improvement of hardenability. Add selectively.

However, since Nb and V form precipitates, when added in large amounts, the yield ratio of the steel material increases due to precipitation hardening. In the present invention, in order to satisfy the yield ratio <0.7,
Total addition amount of one or both of Nb and V is 0.08%
is the upper limit.

On the other hand, Ni, Cr, and Mo are elements that improve the hardenability of steel materials and are therefore advantageous for obtaining a two-phase structure. Not only this, but also the cost increases, so the upper limit of the total amount of these elements added is set at 0.5 mm.

The structure of a hot rolled steel sheet is determined from the sheet surface to the thickness h (o<h< (
0.1 to 0.25) t, t: plate thickness) The reason why the ferrite structure fraction of the layer is 90 or more and the average grain size is 4 μm or less is that the ferrite structure of the layer of thickness h from the plate surface This is because if the tissue fraction is less than this, the fatigue strength/tensile strength ratio will be less than 0.5, and the problems of the present invention cannot be solved.

In addition, the reason why the average grain size of the ferrite structure in the layer with a thickness h from the plate surface is set to be 4 μm or less is because when the average grain size exceeds this, the significant difference from the grain size of the ferrite structure in the center layer becomes smaller. This is because no significant improvement in the fatigue strength of the steel material will be seen.

On the other hand, the lower limit of the thickness h of the fine-grained ferrite layer is set to 10 of the plate thickness t.
The reason for choosing 04 is that if the thickness is less than this, the fatigue strength of the steel material will not be significantly improved.

In addition, the upper limit of the thickness h of the fine-grained ferrite layer is set to 25 of the plate thickness t.
The reason for choosing t is that if the fine-grained ferrite layer becomes thicker than this, the characteristics of low yield ratio and high ductility, which are the advantages of duplex steel, will no longer be observed.

On the other hand, the reason why the structure of the center layer of the plate is limited to a structure in which martensite and/or retained austenite with a cross-sectional texture ratio of 5 to 15% is dispersed in a ferrite base with a cross-sectional texture ratio of 75 or more is that under this condition, This is because the inventors have found that the low yield ratio and good strength-ductility balance, which are the excellent characteristics of duplex steel, cannot be obtained with other materials.

Next, preferred conditions for manufacturing the steel plate of the present invention will be explained.

Rolling temperature: When the rolling finishing temperature exceeds Ar, +100℃,
Ferrite grains generated by transformation become larger and Ar
If it is less than 3, it will become processed ferrite or coarse grains grown from it, so the rolling finishing temperature should be set as Ar3+b ('C) = 900-509 x C (wt%) +2
4x Specified in S-5-64x.

Reduction ratio: If the final reduction ratio in finish rolling is made smaller, the grain size of ferrite produced by transformation becomes larger, so the final reduction ratio in finish rolling is set to at least 10 t. The upper limit of the final rolling reduction ratio in finish rolling is set to 60% or less, at which temperature rise due to heat generation during processing does not occur significantly.

During hot rolling of steel, the friction coefficient between the rolls and the material is high and the temperature of the surface of the rolled plate is low, so immediately after rolling, the dislocation density near the plate surface is higher than that in the center layer, and If transformation occurs, there is a high possibility that fine ferrite will be produced. However, if it takes a long time to start transformation or if the cooling rate is low, the advantage of the layer near the plate surface in refining the ferrite grains is lost. Therefore, cooling of the steel material after rolling starts within 1 second, and the cooling rate at that time is 50° C./S or more.

On the other hand, in order to make the central layer of the plate thickness a two-phase structure, Ar,
Once in the temperature range of -100°C to Ar3-200°C, forced cooling is stopped, and ferrite transformation is allowed to proceed by cooling, constant temperature treatment, or at a low cooling rate, and then from the temperature range of 550°C or higher to 50°C. It is preferable to cool to 250°C or less at a cooling rate of /S or more.

(Example) Steel consisting of the components shown in Table 1 was subjected to continuous hot rolling under the rolling and cooling conditions shown in Table 2. In Table 2, the primary cooling start time means the time from the end of finish rolling until entering the cooling zone. Further, forced cooling by the cooling device is stopped between primary cooling and secondary cooling. Table 3 shows the characteristics of each rolled material. Here, YR is yield stress YS/tensile strength T
means S. Further, ER is the ratio of fatigue strength/tensile strength. Moreover, the thickness of the h layer means the thickness of a layer in which 90 tons or more of ferrite grains of 4 μm or less are present.

In the materials of rolling Nos. 12 and 13, many coarse grains of 60 μm' were observed on the outermost surface. As is clear from Table 3, the steel of the present invention has a YR of 0.7 or less and an ER of 0.5 or more, exhibiting excellent properties not found in conventional materials.

(Effects of the Invention) The steel of the present invention has a low yield ratio and an excellent strength-toughness balance, which are characteristics of conventional composite structure hot-rolled steel sheets, and also has improved fatigue strength due to the fine grain structure near the surface. It can be used in parts that require high strength and formability, as well as fatigue resistance against repeated loads, such as automobile parts discs, contributing to improved safety.
This is an invention with high industrial value.

Procedure Nefusho (own weight) June 29, 1986

Claims (2)

[Claims] (1) In weight%, C: 0.03-0.1% Mn: 0.5-2.0% Si≦1.5% P≦0.15% Ti≦0.05% and Ti≧3.42 ×N (%) B: 0
．． 0002~0.001% The remainder consists of Fe and unavoidable impurities, with 0<
The average grain size of the ferrite grains present in the layer with a thickness of h < (0.1 to 0.25) t is 4 μm or less, and the cross-sectional structure area ratio is 90% or more, and the central layer is sandwiched between them. The structure is 5% in ferrite ground with a cross-sectional structure area ratio of 75% or more.
It has a structure in which martensite and/or retained austenite are dispersed in an amount of 15% or less, a yield ratio of 0.7 or less, and a fatigue strength to tensile strength ratio of 0.5 or more. High tensile strength hot rolled steel sheet. However, t: board thickness. (2) In weight%, C: 0.03-0.1% Mn: 0.5-2.0% Si≦1.5% P≦0.15% Ti≦0.05% and Ti≧3.42 ×N (%) B: 0
．． 0002 to 0.001%, and further contains one or more of Ni, Cr, and Mo in a total of 0.5% or less, and one or two of Nb and V in a total of 0.08% or less. The average grain size of ferrite grains containing either one or both of them, the remainder being Fe and unavoidable impurities, and existing in a layer with a thickness of 0<h<(0.1-0.25)t from the surface is 4 μm Below, the cross-sectional structure area ratio is 90% or more, and the structure of the center layer sandwiched between them is martensite and One or both of the retained austenites have a dispersed structure, the yield ratio is 0.7 or less, and the ratio of fatigue strength to tensile strength is 0.
．． High tensile strength hot rolled steel sheet with excellent fatigue properties of 5 or more.