JPS586936A - Production of hot-rolled high-tensile steel plate for working - Google Patents

Abstract

PURPOSE:To produce a hot-rolled high-tensile steel plate of superior workability at a low cost by hot rolling carbon steel of relatively low contents of C as a blank material then applying heat treatment of specific conditions thereto thereby providing fine ferrite-pearlite structure. CONSTITUTION:The carbon steel ingot contg. 0.12-0.25% C, <0.5% Si, <1.2% Mn, <0.015% S, 0.005-0.10% sol.Al, or further contg. >=1 kind among <0.10% P, <0.0300% N and <0.010% Ca is hot-rolled to a steel plate and the final finishing temp. thereof is set in an Ar3 point- (Ar3 point + 50 deg.C). Thereafter, it is first cooled quickly at 20 deg.C/sec cooling rates from said temp. region down to an Ar3 point -Ar1 point temp. region and in this state it is held for 1-30sec or air-cooled. Finally, it is again cooled quickly at >=20 deg.C/sec cooling rates down to a 550 deg.C-Ms point temp. region, after which it is coiled. The structure of the steel plate turns to fine ferrite-pearlite structure, and the hot-rolled high tensile steel plate of superior workability is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a hot-rolled high-strength steel sheet that is inexpensive and has excellent workability.

In recent years, from the perspective of saving energy and resources, the automobile industry and other industries have been working to reduce the weight of materials, making them cheaper and more efficient.
Demand for high-strength hot-rolled steel sheets with excellent bonito workability is rapidly increasing.

Conventionally, this type of non-heat-treated hot-rolled steel sheet with a tensile strength of 50 kg/7 or more is mainly known as those strengthened by adding alloying elements such as Ti, Nb, and V. Such steel sheets require expensive additive elements, which increases costs, and they also lack versatility in terms of workability.1. It was not possible to meet a wide range of requests.

In addition, it has recently been proposed to produce low-yield-ratio hot-rolled high-strength steel sheets by cryogenic winding after hot-rolling, but the hot-rolled high-strength steel sheets obtained in this way have poor workability. Although it is rich in
At present, it requires the addition of a large amount of alloying elements such as Mn, and lacks versatility from the viewpoint of cost, so that it is not widely used.

From the above-mentioned viewpoint, the present inventors have conducted repeated research in order to provide a hot-rolled high-strength steel sheet that is inexpensive and has excellent workability without making any particular changes to the current hot-rolling process. As a result, by using carbon steel with a high C content as a raw material, after hot rolling, we performed special strong cooling that included short-time heat retention or air cooling treatment to sufficiently precipitate ferrite.
A steel sheet with fine ferrite and finely dispersed pearlite structure is obtained, and has a high yield point and tensile strength due to the refinement of ferrite grains, solid solution strengthening, and fine dispersion of pearlite grains. With Shin 11.

In addition, the carbon steel has good local elongation, high strength, and excellent workability. They came to the knowledge that if they were to use a certain material, they could obtain something with even better properties.

Therefore, this invention was made based on the above-mentioned knowledge, and C: 0.12 to 0.25 inches (hereinafter referred to as % by weight);
3i: 0.5% or less, Mn: 1.2% or less, S: O, 0.015% or less, sol, M: 0.005 to 0.10%, or further, p: 0.10 % or less, N: 0.0300% or less, Ca: 0.10% or less, containing one or more of the following, and having a composition consisting of Fe and unavoidable impurities: residue, and hot rolling the steel. Rolling A
After finishing in the temperature range from r3 point to (Ar3 point + 50℃), the temperature range is from A rg point to A r at a cooling rate of 20℃/min or more.
Rapidly cool to a temperature range of 1 point, hold in this state for 1 to 30 seconds or cool in air, and then rapidly cool again to a temperature range of 550°C to Ms point at a cooling rate of 20°C/cap or more. By winding the steel sheet from the steel sheet, we obtained a hot-rolled high-strength steel sheet that has a mixed structure of fine ferrite and pearlite, has excellent workability, and has a tensile strength of 50 to 9/- or more. It has characteristics.

Next, in the method for manufacturing a hot-rolled high-strength steel sheet for processing according to the present invention, C, Si, Mn, and S, which are components of the raw material steel.

soL, A1. The reason why the composition ranges of P, N, and Ca, as well as the rolling conditions and cooling conditions are limited as described above will be explained.

(a) C The C component has the effect of strengthening me, p, 5oky/
In order to obtain a high-strength steel plate having a tensile strength of more than mf, a content of 0.12% or more is required.
If the content exceeds 25%, the spot weldability, which is essential for high-strength steel sheets for automobiles, will deteriorate, so the content was limited to 0.12 to 0.25%.

(b) The 5i Si component has the effect of strengthening steel through solid solution hardening, and is a preferable element for obtaining high-strength steel with excellent ductility, but if its content exceeds 0.50%, Since the surface quality of the steel plate deteriorates due to the formation of so-called island scales, the content was limited to 0.5% or less.

(c), Mn The Mn component has the effect of improving the hardenability of steel and also improving local ductility by dispersing pearlite grains, but when its content exceeds 1.2 inches, the low-temperature transformed structure deteriorates. If the two-phase mixture is mixed, the processability will deteriorate and the cost will increase, so the content was limited to 1.2 inches or less.

(d) S The S component combines with Mn to produce MnS and deteriorates workability, so it is desirable to have as little as possible, but if the content is 0.015% or less, desired workability can be achieved. The content was limited to 0.0154 because it could ensure the following.

(e) soAAg is effective as a deoxidizing agent, but if its content is less than 0.005%, the deoxidizing effect cannot be expected; on the other hand, if its content exceeds 0.10%, Even if it is included, the deoxidizing effect will be saturated and no further effect can be expected, so the content was limited to 0.005 to 0.10.

(f) P The P component has the effect of solid solution strengthening of steel at low cost, and is actively added when it is particularly necessary to improve the strength, but the content is If it exceeds 0.10%, embrittlement of grain boundaries will occur, so the content was limited to 0.10% or less.

(g) N Like P, the N component also has the effect of solid solution strengthening of steel at a low cost, but if it is contained in excess of 0.0300%, it will cause blisters during casting and will reduce workability. The content was limited to 0.0300% or less.

(h) Ca The Ca component has the effect of adjusting the shape of inclusions and improving processability. However, its content is 0.
If the content exceeds 0.010%, the amount of inclusions in the steel increases, and conversely, the workability deteriorates, so the content was limited to 0.010%.

(1) Rolling conditions Hot rolling may be carried out after heating in a normal slab heating furnace, or the blooming rolled material may be directly rolled as it is, and there are no particular restrictions on the rolling start temperature. However, the final finishing temperature must be in the range of Ar3 points to (Ar3 points + 50°C).

This is because when the final finishing temperature is lower than the Ar3 point, rolling is involved in the ferrite region, and pro-eutectoid ferrite is processed, resulting in a significantly high yield point and deterioration of workability. On the other hand, the final finishing temperature was set to Ar3+5
If the temperature is higher than 0°C, the austenite grains will become coarse, and even if the temperature is rapidly cooled thereafter, a fine ferrite structure will not be obtained, and at the same time, ferrite transformation will be suppressed and bainite transformation will be promoted, which is desirable. tissue cannot be obtained.

(j) Cooling conditions After hot rolling is completed, the finishing temperature is Ar3 point ~ (Ar
The reason for first cooling from the temperature range of point 3 + 50 °C to the temperature range of point Ar3 to point Ar1 at a cooling rate of 20 °C/Sec or more is because if the cooling rate is less than 20 °C/sec, it becomes slow cooling. This is because it is not possible to suppress the crystal grain growth of pro-eutectoid ferrite, and in order to obtain fine polygonal ferrite, it is necessary to cool at a cooling rate of 20°C/approx. When monotonically cooled to a predetermined winding temperature at a cooling rate of Hold the area for 1 to 30 seconds, or
A process such as air cooling is required to sufficiently generate the desired fine polygonal ferrite. A
The time to maintain the temperature range from the rB point to the Ar1 point or to air cool this temperature range is 1 to 30 seconds, but if the time is less than 1 second, the precipitation of ferrite will be insufficient; On the other hand 3
If the time exceeds 0 seconds, ferrite transformation occurs sufficiently, and A
The effect of air cooling or holding in the r3 to ArH range, as well as the decrease in workability and strength, and the longer time required for sheet threading, so the time is limited to 1 to 30 seconds.

Next, the cooling rate from the temperature range from ArB point to Ar1 point to the coiling temperature of 550°C to Ms point was set at 20°C/min or more in order to finely disperse the pearlite. This is because fine dispersion of pearlite improves the desired workability, but the cooling rate is 20°C/see.
If it is less than that, the desired effect cannot be obtained.

In addition, the reason why the coiling temperature was set at 550°C to the Ms point was because if the temperature exceeds 550°C, QN will precipitate and strengthening by solid solution N cannot be expected. This is because if it is removed, the steel becomes so-called DuatPhase steel, and its yield point becomes too low, limiting its uses. Furthermore, by setting the winding temperature to 550°C or lower, embrittlement due to grain boundary segregation of P can be prevented, and a large amount of P can be added.
Advantageously, the temperature is below 6°C.

Next, the present invention will be explained by comparing examples and comparative examples.

Example First, each steel having the composition shown in Table 1 was melted in a converter, then continuously cast into a slab, which was then soaked at 1200°C and made into the same steel as shown in Table 1. The coil was hot-rolled to a thickness of 3.0 mm under the same rolling conditions and cooling conditions. In this way, steel plates 6 to 9 manufactured by the method of the present invention (hereinafter referred to as the steel plates of the present invention) and steel plates 1 to 5 manufactured by the comparative method (hereinafter referred to as comparative steel plates) were manufactured, respectively. In addition, comparative steel plates 1 to 5 are different from rolling conditions during hot rolling, cooling conditions, and compositional components of steel.
2″ conditions (indicated by * in Table 1),
It was manufactured outside the scope of the method of the present invention.

The results of measuring the mechanical properties of each of the steel plates thus obtained are also shown in Table 1.

In Table 1, "first cooling rate" refers to the cooling rate from the rolling end temperature to the temperature range from Ar3 point to Ar1 point, and "holding temperature" refers to the holding temperature in the temperature range from Ar3 point to Ar1 point. (air cooling), "holding time" is A, holding time in the temperature range from r3 point to Ar1 point, "second cooling rate" is from Ar3 point to A
The cooling rate from the temperature range of point r1 to the coiling temperature is shown.

From the results shown in Table 1, it is clear that the steel sheets 6 to 9 of the present invention have both high strength and excellent workability without containing expensive additive elements, for example, unlike comparative steel sheets 1 and 3. It is clear that the steel sheet has good properties as a high-strength steel sheet for automobiles, etc.

As mentioned above, according to the present invention, it is possible to produce hot-rolled high-strength steel sheets with excellent workability at a low cost without making any particular changes to the current hot rolling process and without using expensive materials. This brings about industrially useful effects such as ease of production.

Applicant: Sumitomo Metal Industries, Ltd. Agent Tomi 1) Kazuo

Claims (2)

[Claims] (1) C: 0.12~0+25chi, Si: 0.5
% or less, Mn: 1.2% or less, 3: 0,015- or less. soL, lJ: 0.005 to 0.10%, Fe and unavoidable impurities: remainder, (more than weight %) steel is hot rolled,
After completing the hot rolling in the temperature range of Ar3 point to (A13 point + 50°C), Ar is heated at a cooling rate of 20°C/sec or more.
Rapid cooling to a temperature range of 3 points to Ar1 point, and in this state 1 to 3
After holding for 0 seconds or cooling in air, return to 20℃
1. A method for producing a hot-rolled high-strength steel sheet for processing, characterized by forming a fine ferrite-pearlite structure by rapidly cooling it to a temperature range of 550° C. to Ms point at a cooling rate of /sec or more and then winding it. (2) C: 0.12-0.25chi, Si: 0.5
% or less, Mn: 1.2% or less, S: 0.015 or less, soL, AQ: 0.005 to 0.10%, and P: 0.10 or less, N: 0.030OL16 Hereinafter, hot rolling steel having a composition containing one or more of the following: Ca: O, O'LO% or less, Fe and unavoidable impurities: the remainder (more than weight%),
After completing the hot rolling in the temperature range of Ar3 point to (A13 point + 50°C), Ar is heated at a cooling rate of 20°C/sec or more.
Rapid cooling to the temperature range of point B to point Ar1, and in this state 1 to 3
After holding for 0 seconds or cooling in air, return to 20℃
/(8) A method for producing a hot-rolled high-strength steel sheet for processing, characterized by forming a fine ferrite-pearlite structure by rapidly cooling it to a temperature range of 550°C to Ms point at a cooling rate of at least 550°C and then coiling it. .