JPS5827328B2 - Manufacturing method of low yield ratio hot-rolled high-strength steel plate for cold working - Google Patents

Description

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

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 pressed bottom-shaped.
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 have been various problems, such as the wear of the parts, which tends to cause dog-shaped 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 required level here is a tensile strength of 50Kf/yd or more,
This is a steel plate with 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, Nb, V, Ti, etc. In addition to precipitation-strengthening elements, increasing the amount of Mn, Ni, and C to improve hardenability
r, M.

As a result, the cost of the components increases, which poses a problem for use in automobile steel sheets, etc., which require low cost.

In addition, Nb and V have been added to conventional high-strength steel for the purpose of making it tougher.

The use of carbonitride-forming elements such as Ti is unsuitable because it increases the yield ratio (usually ≧80%).

In addition, a method has been proposed in which the hot-rolled coil is annealed above the Ac1 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 overcome these difficulties of the prior art and have previously invented and applied for a method for producing high ductility, low yield ratio, hot rolled high tensile strength thin steel sheets with a thickness of 4 mm or less; There are problems in terms of processability when applying it to low yield ratio hot rolled high tensile strength steel plates for processing, so efforts were made to manufacture steel plates with a thickness of more than 4 mm, and low yield low yield ratio hot rolled high tensile strength steel plates for cold working were developed at low cost. In order to manufacture this, we conducted various studies and came up with the following manufacturing method.

That is, the gist of the present invention is that C0,05~
0.15%.

Ar3
Rolling is finished at a temperature below 40°C above Ar3, and Ar
From above 3 points, quench with appropriate cooling means such as water cooling,
The method of manufacturing a hot-rolled high-strength steel plate with a low yield ratio for cold working with a thickness of more than 4 mm, which is then rolled at a temperature of 300°C or less, is also suitable for more severe bending, burring, and other applications. As a method to improve workability so as to withstand CO, 05 to 0.15%, Si≦0.
70%, Mn 0.80-2,00%, S≦0.
015%, containing Zr of 2≦Zr/S≦10 or rare earth elements (REM) of 1.3≦REM/S≦5, with the remainder consisting of iron and unavoidable impurities.
+ Finish rolling at a temperature of 40℃ or less, and after rolling A
A method for producing a low-yield-ratio hot-rolled high-strength steel plate for cold working with a thickness of more than 4 peaks, which is characterized by rapidly cooling from above the rg point using an appropriate cooling means such as water cooling, and then rolling it up at a temperature of 300°C or less. It is in.

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, as a manufacturing method for steel sheets that improves properties such as bendability and stretch flangeability, which are subject to significant deterioration due to inclusions stretched in the rolling direction, limiting S is to reduce MnS-based inclusions and reduce Zr and REM. The upper limit is set to 0 to reduce the amount added.
．． 015%.

The range of addition of Zr and REM, which are sulfide shape controlling elements, to Si is within 0.5% of these elements. Since it differs depending on the bonding strength with N etc., 2≦Z r/S≦10.1.3≦REM/
S≦5 is appropriate.

These lower limits are the amount necessary to change MnS into a sulfide composition that does not easily undergo plastic deformation under hot conditions, and the upper limit is the amount at which the sulfide shape improvement effect of each element is saturated, and further additions are required to change the sulfide composition to a sulfide composition that does not easily undergo hot plastic deformation. 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 slag heating furnace or by directly rolling the bloomed material.
, V and the like, so there is no restriction on the heating temperature from the viewpoint of carbonitride solution.

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 above Ar3 point and below Ar3 +40°C.

Here, the term "Ar higher than 3 points" refers to a temperature of Ar3+1'C or higher.

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.

That is, if the finish rolling temperature exceeds Ar3+40°C, the austenite grains during the final rolling will not become sufficiently fine, and if the austenite grains are then rapidly cooled after rolling, a coarse intermediate structure will be generated, leading to deterioration of workability.

Further, setting the finish rolling temperature to above the Ar3 point is necessary in order to start the rapid cooling after rolling from above the Ar3 point.

Regarding cooling after completion of rolling, it is preferable to start water cooling using a normal run-out table or the like from above the Ar3 point.

The reason for this is that when the component steel of the present invention is rolled under the above conditions, the austenite grains become finer and the hardenability decreases.
This is because even if it is rapidly cooled from above r 3, it will not be baked.

In other words, it is assumed that after the start of rapid cooling, pro-eutectoid ferrite is uniformly precipitated in a high temperature range, and retained austenite, which is roughly concentrated in C, is transformed into martensite and/or bainite in a lower temperature range, thereby obtaining the desired composite structure steel. Become.

Next, the reason why the winding temperature is set to 300° C. or less is to obtain the above-mentioned low-temperature transformed phase; if it is higher than this, the final transformed structure becomes pearlite, and the low yield ratio targeted by the present invention cannot be obtained.

If it is below 300℃, a structure is obtained in which pro-eutectoid ferrite, which precipitates during the cooling process after rolling, and martensite and/or bainite, which are the final transformation of austenite, are mixed in an appropriate ratio, and the target level of strength and strength is obtained. A steel plate having a yield ratio is obtained.

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

Table 1 shows the chemical composition and rolling conditions of steel plates obtained by rolling steel slabs that have undergone melting, ingot making, and blooming in a converter to a plate thickness of 4.5 mm in a hot strip mill. The chemical composition and rolling conditions of a steel plate obtained by continuously casting a steel plate after melting in a furnace and rolling it to a thickness of 6 mm using a hot strip mill are summarized as Steel G.
~■ Shown.

The rolling conditions are the difference between the finishing exit temperature (FT) and the Ar 3 transformation point (FT-Ar3), the quenching start temperature, and the winding temperature.

In this table, steels A to C, G to H are inventive steels, D-F, I
is a comparison steel.

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

JI8 No. 5 direction test piece for tensile test, 15 for bending test
The test was carried out using a C-direction test piece (with the end surface sheared as it was) with a width of 0 mπ.

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

Steels A, B, C, G, and H have the composition and rolling conditions within the scope of the present invention, and exhibit high tensile strength, low yield ratio, and excellent cold-opening formability. Excellent bendability due to addition of Zr and REM.

On the other hand, although the composition and rolling finishing temperature of Steel D are within the range of the present invention, due to the high winding temperature, 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 get so much.

Even though the E steel has the same composition as the steel of the present invention, the rolling finishing temperature is A.
Since the r point is below 3, the yield point increases and cold workability deteriorates even for the same strength.

Steel F has the same composition and tensile strength as Steel C, but
Since the finishing temperature is higher than the range of the present invention, the yield ratio increases,
It is clear that the ductility is also reduced.

Steel (1) is also a steel with the same composition as Steel G, but is an example in which the finishing temperature is higher than the range of the steel of the present invention, so the ductility is particularly significantly reduced.

As is clear from the above, the present invention is a method that can inexpensively produce steel sheets as hot-rolled, which have high strength and a low yield ratio, and are particularly suitable for cold working. This is an excellent method.

Claims (1)

[Claims] I C0.05-0.15%, Si≦Q, 70%
, MnO, OS ~ 2.00%, the remainder is iron and unavoidable impurities.
Finish rolling at a temperature of 0 °C or less, and after rolling, Ar3
A method for producing a low-yield-ratio hot-rolled high-strength steel plate for cold working with a thickness of more than 4 mm, characterized by rapidly cooling the steel plate from above a point and rolling it at a temperature of 300° C. or less. 2 C0.05~0.15%, Si20.70%,
Mn0.80-2.00%, S≦0.015%, Zr
2≦Zr/S≦10 or rare earth element (REM) 1
,3≦REI! 14/S≦5, with the remainder consisting of iron and unavoidable impurities.
Finish rolling at a temperature of 40℃ or less, and after rolling, Ar3
A method for producing a low-yield-ratio hot-rolled high-strength steel plate for cold working with a thickness of more than 4 mm, characterized by rapidly cooling the steel plate from above a point and rolling it at a temperature of 300° C. or less.