JPS583017B2 - Manufacturing method for low yield ratio hot-rolled high-strength steel sheet with excellent cold workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a low yield ratio hot-rolled high-strength steel sheet with excellent cold workability.

Conventionally, high-strength steel plates have often been used as thick structural steel plates.

However, in recent years, there has been an increasing tendency to use hot-rolled high-strength steel sheets in order to reduce the weight of automobiles, vehicles, industrial machinery, and the like.

However, in fields where these relatively thin high-tensile steel plates are used, the steel plates are often press-formed.
Conventional high-strength steel plates with high yield points and high yield ratios have low material ductility and cannot be subjected to severe machining, tend to have poor accuracy in molded products due to springback after deformation, and have high yield points that make it difficult to use tools. There were various problems such as large wear and mold galling.

On the other hand, high-strength steel sheets with a low yield ratio and high tensile strength have a low yield point, but are highly work hardened and have a sufficient yield point or yield strength after forming, so they are suitable for these applications and are popular with customers. requested by.

The standards required here are steel plates with a tensile strength of 50 kg/mm2 or more and a yield ratio of 70% or less.

Bainite steel is a technology for manufacturing steel plates with such materials.

As is well known, this is a high-strength steel that takes advantage of the high strength of bainite, which is a low-temperature transformation product of austenite, but in order to manufacture it in a normal hot strip mill, it is necessary to use Nb, V, Ti, etc. In addition to precipitation strengthening elements, increasing the amount of Mn, Ni, Cr,
As a result of the need to add Mo etc., the component cost increases,
There were problems in using it for automobile steel sheets, etc., which require low cost.

Furthermore, it is inappropriate to use carbonitride-forming elements such as Nb, V, and Ti, which have been added to conventional high-strength steel for the purpose of increasing toughness, because they increase the yield ratio (usually 80%). .

In addition, a method has been proposed in which the hot-rolled coil is annealed above the transformation point using heat treatment equipment such as a continuous annealing line and then rapidly cooled to lower the yield ratio, but in this case, one additional step is required. This will inevitably lead to an increase in costs.

The present inventors have conducted various studies in order to overcome these drawbacks of the conventional technology and produce a low-yield-ratio hot-rolled high-strength steel sheet that is low cost, has excellent cold workability, and is also excellent in severe bending and burring workability. This led him to invent the following manufacturing method.

That is, the gist of the present invention is that C0.05-0
．． 15%, Si≦0.70%, Mn 0.80 to 2.
00%, S≦0.015%, Zr 2≦Zr/S≦10
Or rare earth element (REM) 1.3≦REM/S≦5
Ar3 containing steel with the remainder consisting of iron and unavoidable impurities
Finish rolling at a temperature of +40℃ or less, and after rolling the temperature is 16℃/
The present invention provides a method for producing a hot-rolled high-strength steel sheet with a low yield ratio and excellent cold workability, characterized by cooling to the Ar' point or lower at a rate of not less than 1 second and not more than 70° C./second.

The reasons for limiting the constituent elements of the present invention will be explained below.

First of all, regarding the ingredients, if C exceeds 0.15%, it will be difficult to process.
It is undesirable because it reduces weldability, and 0.05%
If it is less than this, the necessary strength cannot be obtained, so this range was set.

Si is useful as a deoxidizing element and a reinforcing element, so it may be added, but if it exceeds 0.70%, it impairs weldability, which is not preferable.

Mn is an essential element when applying the method of the present invention, and the amount added can be changed depending on the strength level, but if it is less than 0.80%, the structure necessary to lower the strength and yield ratio cannot be obtained; If it exceeds %, ductility and weldability will be impaired, so it was limited to this range.

Furthermore, in order to improve properties such as bendability and stretch flangeability, which are significantly deteriorated by inclusions stretched in the rolling direction, S
is limited by reducing MnS-based inclusions and by reducing Zr, R
In order to reduce the amount of EM added, the upper limit is set to 0.015%.

The range of addition of Zr and REM, which are sulfide shape controlling elements, to the amount of S varies depending on the bond strength of these elements with O, N, etc., so 2≦Zr/S≦10, 1.3≦REM/S
≦5 is appropriate.

Both of these lower limits are the amounts necessary to change MnS into a sulfide composition that does not easily undergo plastic deformation under hot conditions. This is determined by the fact that inclusions increase and workability is adversely reduced.

The steel having the above components may be melted by a normal steel manufacturing method, and the steel billets may be manufactured by either ingot-blowing rolling or continuous casting.

Next, the rolling conditions of the method of the present invention will be described.

Heating may be done by heating and rolling in a normal slab heating furnace or by directly rolling the bloomed material.
, V, and other additive elements, there is no restriction on the heating temperature from the viewpoint of carbonitride solutionization.

Further, since there is no particular restriction on the rolling start temperature, it may be the lowest temperature required from the rolling end temperature.

The rolling end temperature is limited to Ar3 or higher and Ar3+40°C or lower.

This restriction is to control the hardenability by making the austenite grains finer and to obtain a structure suitable for the purpose of the present invention.

In other words, if the finish rolling temperature exceeds Ar3+40°C, the austenite grains during the final rolling will not become sufficiently fine, and if the cooling rate after rolling is increased as it is, an intermediate structure will occur, leading to deterioration of workability. .

Further, if the Ar point is less than 3, the yield point increases, workability deteriorates, and target mechanical properties cannot be obtained, so a lower limit is set.

Also, the reason why the cooling rate after rolling is set to 16°C/second or more is as follows.
If it is less than this, ferrite-pearlite transformation occurs and the low yield ratio targeted by the present invention cannot be obtained.

On the other hand, when cooling to below the Ar' point at a cooling rate of 16°C/second or more, a structure is obtained in which pro-eutectoid ferrite and austenite, which precipitate during the cooling process after rolling, are mixed with finally transformed bainite in an appropriate ratio. A steel plate with the target level of strength and yield ratio can be obtained.

On the other hand, if the cooling rate below Ar' after rolling exceeds 70°C/sec, the required pro-eutectoid ferrite will not be generated in the present invention from the fine-grained austenite due to rolling, or the proportion thereof will be extremely small, and the remaining Most of the steel becomes an intermediate structure or a structure containing a large proportion of martensite, resulting in a significant decrease in ductility and an increase in yield ratio.

Therefore, the upper limit of the cooling rate is 70°C/sec.

Next, the effects of the present invention will be explained using examples.

Table 1 shows the chemical composition and rolling conditions of a steel plate obtained by rolling a steel plate that has undergone melting, ingot making, and blooming in a converter to a thickness of 2.3 mm in a hot strip mill.

The rolling conditions are determined by the difference between the finishing exit temperature (FT) and the Ar3 transformation point (
FT-Ar3) and the cooling rate in the transformation region, which is represented by the average cooling rate on the runout table determined by the finishing exit temperature, winding temperature, and rolling speed.

Steels A to D in this table are Al-Si killed steels, and steels E to I are Si killed steels.

Here, the Ar3 transformation point is 770°C for steels A-D and 790°C for steels E-I.

Further, the winding temperatures used to calculate the cooling rate in the transformation region are all lower than the lower limit of Ar' of 500°C.

Table 2 shows the mechanical test values of the steel plates obtained in Table 1.

Tensile test is JIS No. 5 L direction test piece, bending test is JIS No. 15
The C-direction test piece with a width of 0 mm (end face shear cut) and the hole expansion test were conducted using a punched hole test piece with a diameter of 20 mm.

The critical bending radius for the bending test was determined as the minimum bending radius at which the crack length was 10% or less of the specimen width when bent at 180°.

Steels A, B, and C have the composition and rolling conditions within the range of the present invention, and exhibit high tensile strength, low yield ratio, and excellent cold formability, as well as improved bendability and stretch flangeability. It's remarkable.

On the other hand, although the composition and rolling finishing temperature of Steel D are within the range of the present invention, the cooling rate in the transformation region is low, and the low yield ratio that is the characteristic of the present invention cannot be obtained, and the tensile strength is lower than that of the present invention steel with the same composition. You can only gain strength.

Steels E and H are also examples of the present invention, whereas steel F, which has the same composition but has a high finishing temperature during rolling, increases the yield point and deteriorates cold workability even for the same strength.

G is an example in which Nb is added to the base component of E and F steels, and the strength increases, but the yield point increases significantly, resulting in a steel plate with a high yield ratio, which is completely contrary to the purpose of the steel of the present invention. It shows.

Furthermore, I steel is an example in which the effects of the present invention were not obtained due to the high cooling rate, and it is clear that the total elongation and workability evaluation test values are decreased.

As is clear from the above, the present invention can produce steel sheets with high strength, low yield ratio, and excellent cold workability using Nb, V, and Ti.
It can be manufactured at low cost by hot rolling without using elements such as
This method is particularly suitable for hot strip mills, and is an industrially excellent method.

Claims (1)

[Claims] 1 C0.05-0.15%, Si≦0.70%, M
n0.80-2.00%, S≦0.015%, Zr 2
≦Zr/≦10 or rare earth element (REM) 1.3≦
Steel containing REM/S≦5 with the remainder consisting of iron and unavoidable impurities is finished rolling at a temperature of Ar3 + 40°C or less, and after rolling is heated with Ar at a speed of 16°C/sec to 70°C/sec.
A method for manufacturing a hot-rolled high-strength steel sheet with a low yield ratio and excellent cold workability, which is characterized by cooling to a temperature below the ' point.