JP2626849B2 - Manufacturing method of high strength hot rolled steel sheet with excellent fatigue properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet suitable for undercar parts for automobiles and the like which require high strength and high fatigue characteristics.

[0002]

2. Description of the Related Art In recent years, in the automobile industry, the use of high-strength thin steel sheets has been increasing in order to ensure occupant safety and improve fuel efficiency by reducing weight. In particular, in the case of undercarriage parts and reinforcement parts of automobiles, the merit of applying this high-strength thin steel sheet is very large.

[0003] However, this high-strength thin steel sheet is usually used with a thinner thickness than the steel sheet, and in particular, the undercarriage parts are constantly subjected to repeated stress, so that the deterioration of the fatigue properties of the steel sheet is a problem. Therefore, there is a strong demand for a high-strength steel sheet having excellent fatigue properties.

[0004] Conventionally, as a technique for improving the fatigue characteristics of a steel sheet, Japanese Patent Application Laid-Open No. 63-282240 discloses C: 0.03.
~ 0.1%, Si≤1.5%, P≤0.15%, Ti≤0.0
In the thickness direction of steel containing 5% and B: 0.0002 to 0.001%, a technique in which the structure is different in the thickness direction has been proposed, but it is difficult to control the structure to achieve the structure. There's a problem. Further, JP-A-60-174850 and JP-A-62-010239 disclose a high-strength thin steel sheet excellent in the fatigue characteristics of a spot welded part.
0.3%, Mn <2.0%, P <0.12%, Si <0.05
% Or Si: 0.7-1.0% steel has been proposed, but these techniques focus on improving the fatigue properties of spot welds and are intended to improve the fatigue properties of the steel sheet itself. Rather, it is believed that the metallurgical behavior of the welds and non-welds is significantly different.

[0005]

Generally, in order to obtain a high-strength steel sheet, a method of strengthening by solid solution strengthening by adding Mn or the like, a method of strengthening by precipitation of cementite by adding C, Nb, Ti, etc. And strengthening them by precipitation strengthening. However, this method is expensive because a large amount of Mn must be added, and the other method deteriorates workability as described above. The most suitable strengthening method is the following method.

In the above method, elements such as Nb and Ti have a sufficient strengthening effect even when they are added in a small amount. In addition, in order to obtain good fatigue properties, instead of controlling the structure as described above, P,
It is said that composite addition of Cu is also effective (for example,
“Materials and Processes”, Vol. 3, No. 6 (1990) “Papers for the 120th (Autumn) Lecture Meeting”, p. The reason is considered as follows.

That is, steel has a mixed structure of a ferrite phase and a cementite phase, but since the ferrite phase is much softer than the cementite phase, strain is concentrated on the ferrite portion when a load is repeatedly applied, and fatigue fracture proceeds. Go. However, when P and Cu are added in combination, the ferrite portion is strengthened and the fatigue characteristics are improved.

This method is easy to manufacture because it is not necessary to control the structure of the steel. Further, the combined addition of P and Cu also improves the corrosion resistance, and thus is suitable for a steel plate for automobile undercarriage parts used in a severe corrosive environment. That is, Nb,
It is particularly effective to increase the strength at low cost using Ti and to add high fatigue properties by adding P and Cu in combination to obtain a steel sheet having high strength and good fatigue properties.

However, in order to strengthen the steel by using Nb and Ti, it is necessary to maintain a high temperature and uniformly diffuse these elements into the steel. At this time, Cu causes a eutectic reaction with the steel scale, elutes at the grain boundaries of the steel, and becomes a surface flaw after rolling. These surface flaws remain even after pickling, which not only impairs the appearance of the manufactured underbody parts, but also serves as a starting point for fatigue failure. For this reason, it is practically impossible to use this steel as a steel plate for underbody.

In order to suppress surface flaws caused by Cu, it is effective to add approximately the same amount of Ni as Cu. However, Ni is expensive, and it is desirable to avoid using Ni as much as possible. Japanese Patent Application Laid-Open No. Sho 56-68513 states that by specifying the slab heating furnace material furnace time and the heating furnace extraction temperature with Cu-added steel, surface flaws due to Cu can be suppressed. However, in steel sheets in which P and Cu are added in combination, P further lowers the eutectic temperature of Cu and steel scale, so that surface flaws are more likely to occur than in steel sheets in which only Cu is added. Therefore, P and C
In the case of the composite steel sheet with u added, it is necessary to take measures against surface flaws more than the steel sheet with only Cu added.

It is particularly effective to increase the strength at low cost by using Nb and Ti and to add high fatigue properties by adding P and Cu in combination to obtain a steel sheet having high strength and good fatigue properties. In the case of manufacturing using the technique described above, there is a problem that the surface properties of the steel sheet are deteriorated unless expensive Ni is added.

The present invention solves the above-mentioned problems of the prior art and provides high strength by adding Nb, Ti and the like and imparting high fatigue properties by adding P and Cu in combination without deteriorating the surface properties. It is an object of the present invention to provide a method for manufacturing a hot-rolled steel sheet that enables both of them.

[0013]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, have regulated the heating temperature and the hot rolling start temperature under appropriate adjustment of chemical components. Has been found possible.

That is, in the present invention, C: 0.01-0.
20%, Si: 2.5% or less, Mn: 0.5 to 2.5
%, P: 0.03 to 0.20% and Cu: 0.05 to
0.5%, further contains one or two of Nb and Ti in a total amount of 0.01 to 0.05%, and contains Cu (%) +
Steel having a composition satisfying the relationship of 2P (%) <0.6%
After maintaining the temperature at 0 ° C. or higher, the temperature of the steel surface was increased to 1050
A method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics, characterized in that hot rolling is started at a temperature of not more than ° C.

Hereinafter, the present invention will be described in more detail.

[Action]

First, the reasons for limiting the chemical components of steel in the present invention will be described.

C: C is effective as a strengthening element, but when added excessively, the workability deteriorates. When C is too small, a large amount of alloying element must be added instead for strengthening. Because of the problem, the C content is 0.01 to 0.2.
The range is 0%.

Si: If a large amount of Si is added, rolling becomes difficult and slab cracks occur.
5% or less.

Mn: If Mn is added in a large amount, the yield of addition to steel decreases and the steel becomes expensive. On the other hand, if it is too small, sufficient steel strength cannot be obtained.
The range is 2.5%.

Nb, Ti: Nb and Ti are elements that strengthen steel by forming carbides or hardening the structure of the steel itself. The effect on the strength of the steel when both are added is almost the same. However, if the amount is too small, there is no effect as a strengthening element, and if the amount is too large, the effect is saturated. Therefore, the addition of a certain amount or more has little meaning. For this reason,
One or two of Nb and Ti are specified in a range of 0.01 to 0.05% in total.

P, Cu: The combined addition of P and Cu has the effect of improving the fatigue strength of steel as described above. But,
If the addition amount of P and Cu is too small, there is no effect.
P must be 0.03% or more, and Cu must be 0.05% or more. On the other hand, if P is too large, the secondary workability of the steel is deteriorated, so the upper limit is 0.20%. Further, the addition of a large amount of Cu deteriorates the surface quality of the steel sheet, and the addition of more than 0.5% of Cu does not improve the surface quality even under the production conditions of the present invention. Is a condition. Therefore, the P content is 0.03 to 0.20%, and the Cu content is 0.05 to 0.25%.
It is specified in the range of 0.5%.

FIG. 4 shows the effects of P and C on the surface properties of steel.
As shown by the influence of u, the relationship Cu (%) + 2P (%) <0.6% must be satisfied in order to improve the surface properties under the production conditions of the present invention.

Next, the manufacturing conditions of the present invention will be described.
Steel having the above chemical composition is melted and cast by a conventional method,
Although it is subjected to hot rolling, it is necessary to regulate the slab heating temperature and the hot rolling start temperature to the following conditions.

First, the slab heating temperature must be 1150 ° C. or higher in order to uniformly diffuse Nb and Ti. This includes the case where the steel ingot after casting with HCR or the like is rolled as it is without ferrite transformation. FIG.
Shows the effect of the slab heating temperature on the tensile strength of steel having a composition within the range of the present invention. As can be seen from the figure, the effect (increase in strength) of Ti or Nb as a strengthening element cannot be expected unless heating is performed at a temperature of 1150 ° C. or higher.
At 0 ° C. or higher, the surface properties of the steel sheet deteriorate.

However, as shown in FIG. 2, after the same steel was heated at 1200.degree. C. and the surface properties when rolling was started at various temperatures, even if the heating temperature was 1200.degree. If it is 1050 ° C. or lower, the surface properties are good. In this case, a sufficient increase in strength is obtained by the addition of Ti and Nb. Therefore, in the present invention, the slab heating temperature is controlled to 1150 ° C. or higher for increasing the strength, and the rolling start temperature is controlled to 1050 ° C. or lower for improving the properties of the steel sheet surface.

In the hot rolling, cooling may be performed forcibly by a disk scaler or the like. The slab heating temperature is the temperature of the slab surface, and there is no problem even if the inside of the slab is 1050 ° C. or more. Furthermore, when combined addition of Ni to the present invention, it is possible to relax the conditions such as the rolling start temperature.

Next, examples of the present invention will be described.

[0028]

EXAMPLES Steel having the chemical composition shown in Table 1 was melted by a conventional method.
It was cast and hot rolled under the conditions shown in Table 2 . Table 2 also shows the results of examining the mechanical properties, surface properties, and secondary workability of the obtained rolled steel sheet. The fatigue strength was measured by JI
It was determined by a plane bending fatigue test based on SZ2247. The surface properties were evaluated by ○ (no occurrence) and × (occurrence) depending on the presence or absence of surface flaws, and the secondary workability was evaluated by reworking the cup molded product at -60 ° C.

FIG. 1 shows the relationship between the slab heating temperature and the tensile strength, FIG. 2 shows the relationship between the rolling start temperature and the tensile strength, and FIG. 3 shows the relationship between the tensile strength and the fatigue strength in order to show the effect of the addition of the P and Cu composites. Are shown separately.

Table 2 shows that all of the examples of the present invention have high strength and excellent fatigue strength under good surface properties. FIG. 3 shows the relationship between the tensile strength and the fatigue strength of a steel having a chemical component within the range of the present invention and a steel having a chemical component outside the range of the present invention in order to show the effect of the addition of P and Cu composites. As shown, if the tensile strength is the same, the P and Cu added steels have clearly higher fatigue strength. Thus, when all the conditions of the present invention are satisfied
(Example of the present invention) provides a steel sheet having high fatigue strength and excellent surface properties while maintaining the effect of strengthening by adding Nb or the like, and such a steel sheet does not satisfy any one of the conditions of the present invention. And can not be obtained.

[0031]

As described above, according to the present invention,
A steel sheet having high strength, excellent fatigue properties, and excellent surface properties can be obtained. Further, even if plating is performed on the steel plate, the material does not change at all. In particular, it is suitable as a steel plate for underbody parts and reinforcing parts of automobiles. Furthermore, it can be used for civil engineering construction because of its high fatigue properties and corrosion resistance.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between slab heating temperature and tensile strength.

FIG. 2 is a diagram showing a relationship between a rolling start temperature and a tensile strength.

FIG. 3 is a diagram showing the relationship between tensile strength and fatigue strength of P, Cu-added steel and non-added steel.

FIG. 4 is a diagram showing the influence of P and Cu on the surface properties of steel.

Claims (1)

(57) [Claims] C: 0.01% by weight (hereinafter the same)
０．２0.20%, Si: 2.5% or less, Mn: 0.5〜
2.5%, P: 0.03 to 0.20% and Cu: 0.0
5 to 0.5%, further contains one or two of Nb and Ti in a total amount of 0.01 to 0.05%, and contains Cu
(%) + 2P (%) <0.6% After a steel having a composition satisfying the relationship of 0.6% is maintained at a temperature of 1150 ° C. or more, hot rolling is started by setting the temperature of the steel surface to 1050 ° C. or less. A method for producing high-strength hot-rolled steel sheets with excellent fatigue properties.