JPS5841326B2 - Method for producing high-strength hot-rolled steel sheets for processing - Google Patents

Description

[Detailed description of the invention] This invention uses steel with similar composition to general hot-rolled steel sheets, does not require the addition of special elements (such as Ti, Nb, V, etc.) By improving the hot rolling process,
Tensile strength: 55-75 kg/mA1 elongation 126% or more,
The present invention relates to a method for manufacturing a high-strength hot-rolled steel sheet with excellent workability and a yield ratio of 65% or less.

In recent years, there has been a trend in the automobile industry to reduce the weight of automobile bodies as one of the measures to improve the fuel efficiency of automobiles, and there is a demand for thinner steel sheets and high-strength hot-rolled steel sheets with excellent workability from the viewpoint of safety. It's increasing.

Conventionally, in order to obtain high-strength hot-rolled steel sheets with excellent workability, Ti
, Nb 2 , V, and other special elements have been used to improve strength through solid solution hardening and precipitation hardening through carbonitride formation.

However, these elements are expensive, leading to increased costs, and resource constraints are expected in the future.

In view of the current situation, this invention is a new method for producing high-strength hot-rolled steel sheets at low cost that can guarantee the above-mentioned material properties using steels with similar composition to general-use 51-Mn steel. The main feature is that a steel plate with a specified composition is hot-rolled in the ferrite-austenite dual-phase region, then left to cool for a certain period of time, then cooled and rolled at a low temperature to produce ferrite-marten. The aim is to create a steel sheet with a composite structure of site and retained austenite.

This invention will be explained in detail below. The basic component is C
: 0.04-0.15%, Mn: 0.5-2.0%,
Si: 0.5 to 2.0%, S: limited to 0.005% or less.

C and Mn are essential elements for securing the necessary strength and obtaining a composite structure, C: less than 0.04%, Mn: 0.04%.
If it is less than 5%, it is difficult to obtain a composite structure, and if it is C0.15% or more and Mn2.0% or more, the ductility deteriorates significantly and weldability is also impaired.
．． 5 to 2.0%.

When Si is added in an amount of 0.5% or more, it reduces the solid solution (C) from the ferrite grains, purifies the ferrite grains, and promotes the concentration of C element in the untransformed austenite grains more significantly. It is recognized that it has the function of making it easier to obtain a suitable composite structure, and improves the strength-ductility balance of the steel plate.

FIG. 1 shows the relationship between tensile strength and ductility of 51-Mn steel containing 0.8% and 1.1% Si and CrMn steel containing no Si according to the present invention.

The amount of Si decreases in the order of (4)>(B)>(C) in the figure.
The larger the amount, the better the balance of tensile strength and ductility.

In addition, increasing Si increases the A3 transformation temperature, expands the finishing temperature range during hot rolling, and eases hot rolling work conditions, so it is an extremely effective element in manufacturing.

However, if Si exceeds 2.0%, it will increase hot rolling scale flaws and deteriorate the paintability of the product.
．． Set to 0%.

S is set to 0.005% or less in order to reduce the formation of MnS-based inclusions and improve hole expandability.

Elements such as Ca and REM are effective in making inclusions into fine spherules and improving hole expandability, so they are preferably added and their desirable contents are 0.005% or less and 0.0% or less, respectively.
It is less than 5%.

The hot rolling finishing temperature is below Ar3 point, preferably ferrite + austenite dual phase region temperature up to Ar3 point - 150°C, for example, 900 to 750°C, and rolling is completed.

As a result, a fine-grained mixed structure containing ferrite grains into which deformation bands have been introduced and untransformed austenite grains is formed.

This fine-grained mixed structure is then rapidly cooled and rolled at a low temperature.
This is advantageous in stably producing a homogeneous composite structure of ferrite grains, martensite grains, and retained austenite.

The most important feature of this invention is that a water-free time of 3 to 7 seconds is required from the end of finish rolling to the start of water injection cooling.

Figure 2 shows Si containing 0.8% Si according to the present invention.
- Time and elongation (El) until the start of water injection cooling of Mn-based steel,
It shows the relationship with yield ratio (YR).

EJ' and YR when water injection cooling started within a short time of 1 second after finish rolling and after a relatively long time of 10 seconds, and YR when water injection cooling started after 5 seconds. Both are inferior in comparison.

As a result of many experiments, it has been found that a predetermined elongation of 26% or more and yield ratio of 65% or less can be obtained in 3 to 7 seconds until the start of water injection cooling.

In other words, there is an appropriate value for the time until water injection cooling starts,
If it is shorter or longer than this, the material will deteriorate.

The reason for this is not necessarily clear, but if the time until the start of water injection cooling is short, the recovery of the ferrite grains after introducing the deformation zone is insufficient and many dislocations are built in, resulting in a decrease in El.
■ Increase.

In the case of a long time, ferrite and untransformed austenite grains grow coarsely and rapidly cool, and martensite after transformation becomes grain-sized and coarsely distributed, resulting in a decrease in EJ'.
It is also presumed that this is due to the fact that the water injection start temperature shifts to a lower temperature side and the solid solution of C in the ferrite grains increases, causing age hardening due to recuperation after winding and increasing YR.

Average cooling rate after starting water injection cooling (FT-CT %r i
me is at least the cooling rate necessary for untransformed austenite to transform into martensite, that is, at least 30°C/sec, and the winding temperature is at most 150'C to prevent tempering of the formed martensite, and winding is performed at this temperature. Make a coil.

Example Examples according to the invention are shown in Table 1.

Inventive Examples 1 to 11 are obtained by hot rolling steel having predetermined components according to the present invention.

Comparative Examples 12 to 19 are steels with the same composition as Invention Examples 2 to 6 that were rolled under hot rolling conditions different from the conditions of the present invention, and Comparative Example 20 was steel with a different composition from this invention. Hot rolling was performed according to the processing conditions of the invention.

In other words, invention example (1) is a coil that is made of steel having a predetermined composition, has a finishing temperature A r of 3 points or less (actually 750°C), is held for 5 seconds until the start of water injection cooling, is cooled at 40°C/SeC, and is wound at 1000C. , Invention example (2) is steel having predetermined components,
Finishing temperature Ar 3 points or less (actual result: 840°C), holding for 3 seconds until the start of water injection cooling, cooling at 35°C/sec, coil winding at 150°C, invention example (3) is a steel with a predetermined composition, finishing Temperature A r 3 points or less (actual soo'c), held for 5 seconds until water injection cooling starts, cooled at 40℃/Sec, 130
The coil wound in 'C, Invention example (4), is made of steel with a predetermined composition, finishing temperature Ar 3 points or less (actual result: 840°C), held for 5 seconds until water injection cooling starts, cooled at 60°C/sec, 1
Coil wound at 50°C, invention example (5) is steel with a specified composition, finish temperature ArJ or lower (actual performance: 830°C), held for 5 seconds until water injection cooling starts, cooled at 80°C/Sec, 1
Inventive example (6), a coil wound at 30°C, is made of steel with a predetermined composition, and the finishing temperature A r is 3 points or less (actual result: 850°
C), the coil was held for 7 seconds until the start of water injection cooling, cooled at 50°C/sec, and wound at 140°C; invention example (7) was steel with a predetermined composition, and the finishing temperature was Ar3. A or less (Achievement 86
0°C), held for 5 seconds until water injection cooling starts, 45°C/Se
Coil cooled at C and wound at 130'C, invention example (8)
is a steel having a predetermined composition, and the finishing temperature A r 3. Below 50°F (actual 850'C), held for 5 seconds until water injection cooling starts, 4
Inventive example (9), which is a coil cooled at 5°C/sec and wound at 120°C, is made of steel having a predetermined composition and finished at a finishing temperature of Ar3. a
Below, (actual 880℃), held for 5 seconds until water injection cooling started, cooled at 50℃/SeC, coil wound at 130℃,
Invention example (10) is a steel having predetermined components, and the finishing temperature is A.
r3 points or less (actual 860℃), until water injection cooling starts, 5
The coil was held for seconds, cooled at 35°C/SeC, and wound at 140°C. Inventive example (11) is steel with a predetermined composition, finishing temperature A r 3 points or less (actual performance: 900°C), water injection cooling started. , hold for 5 seconds, cool at 50°C/sec, 140
The coil was wound at ℃, but in each case the tensile strength was 55 to 75 k
g/my? Material values of t, elongation of 26% or more, and yield ratio of 65% or less were obtained.

In comparative examples (12) and (13), the time until the start of water injection was 1
Since the time is as short as ~2 seconds, elongation decreases, yield value increases, and predetermined material quality values are not obtained.

Comparative example (14)? In case (15), since the time until the start of water injection is as long as 8 to 10 seconds, the elongation decreases, the yield value increases, and a predetermined material quality value is not obtained.

In Comparative Example (16), the cooling rate was as slow as 20° C./sec, so the elongation decreased, the yield value increased, and the desired material quality values were not obtained.

In Comparative Example (17), the winding temperature was as high as 300° C., so the elongation decreased, the yield value increased, and the desired material properties were not obtained.

In Comparative Example (18), the finishing temperature was very low at 730°C, so the elongation was low and the yield ratio was high, making it impossible to obtain the desired material properties.

In Comparative Example (19), the finishing temperature was as high as 920° C., so the elongation decreased, the yield ratio increased, and the predetermined material quality values were not obtained.

Since Comparative Example (20) does not contain Si, the elongation decreases and the predetermined material quality value is not obtained.

From the above results, according to the present invention, the tensile strength is 55 to 7.
5 kg/va? High-strength hot-rolled steel sheets with excellent workability that have material values of TS elongation of 26% or more and yield ratio of 65% or less can be produced at low cost using current equipment without the addition of special elements, resulting in industrial benefits. is big.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between TS and El of steels with different Si contents, and FIG. 2 is a diagram showing the relationship between El and YR and the holding time until the start of water injection cooling.

Claims (1)

[Claims] IC: 0.04~0.15%, Mn: 0.5~
2.0%, Si: 0.5-2.0%, S: 0.005%
Hereinafter, when hot finish rolling a steel plate of steel whose balance is composed of Fe and unavoidable elements, the hot rolling is finished in the ferrite austenite biphase region with an Ar point of 3 or less, and 3 to 7 days after finishing the hot rolling.
A method for manufacturing a high-tensile hot-rolled steel sheet for processing, characterized in that water injection cooling is started after seconds have elapsed, and the process is rapidly cooled to 150° C. or lower at an average cooling rate of 30° C./sec or higher and then rolled up.