JPS637331A - Production of non-heattreated high-tensile steel plate - Google Patents

Abstract

PURPOSE:To produce a non-heattreated high-tensile hot rolled steel plate at a low cost by hot rolling a specifically composed steel consisting of C, Si, Mn, Nb and Fe at the temp. in excess of the Ar3 point, then cooling the steel under specific conditions. CONSTITUTION:The steel contg. 0.02-0.20wt% C, <=2.0% Si, 0.5-2.0% Mn, 0.010-0.100% Nb, and the balance consisting of Fe and unavoidable impurities is hot rolled at the temp. in excess of the Ar3 point. The hot rolled plate is thereafter quickly cooled down to the Ar3 point at >=10 deg.C/sec cooling rate. The hot rolled plate is cooled or held at <10 deg.C/s cooling rate for the time (t)sec or above and >=40sec to be given by the equation; t=eXp[a1+ a2X(lnNb)<2>+a3XlnNb] (where, a, a2; constant) to finely disperse the deposit of Nb into the formed ferrite particles. The hot rolled steel plate is thereafter quickly cooled down to <=450 deg.C at >=10 deg.C/s cooling rate to prevent the formation of the coarse grains of the deposit. The non-heattreated high-tensile hot rolled steel plate contg. Nb at a low ratio is thus obtd. at a low cost.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is based on steel made of steel having a specific chemical composition.
The present invention relates to a method for producing a non-tempered high tensile strength hot rolled steel sheet.

(Prior art) As a prior art related to the present invention, for example, Japanese Patent Laid-Open No. 58
As proposed in Publication No.-130221, C:
0.12 to 0.25%, Si: 0.5% or less, Mn:
1.2% or less, S: 0.015% or less, so
nAl: 0.005 to 0.10%, and contains one or more of Nb: 0.04% or less, V: 0.07% or less, Fe and unavoidable impurities: the remainder (by weight After hot rolling a steel having a composition consisting of
550℃~M at a cooling rate of 20'C/s or more, then winding. There is a method for producing a hot-rolled high-strength steel sheet for processing, which is characterized by forming a fine ferrite-pearlite structure by removing the steel.

(Problems to be Solved by the Invention) This conventional technology has no relation to Nbi in ``the time for air cooling or testing after quenching after hot rolling,'' which is the most important point in exhibiting the precipitation hardening of Nb. Only 2 to 30 seconds is specified. That is, N
The manufacturing method does not use cooling or testing time to maximize precipitation hardening (b).

(Means for Solving the Problems) The present invention solves these problems of the prior art, and in order to maximize the precipitation hardening of Nb, "air cooling or testing after quenching after hot rolling" Based on the recent findings of the present inventors, the time period for this is specified as a function of the amount of Nb, and C0.02 to 0.20%.

Si2.0% or less, Mn0.5-2.0%, Nb0
．． After hot rolling a steel containing 0.010 to 0.100% (weight %) with the remainder consisting of iron and unavoidable impurities at a temperature above the Ar3 point, it is quickly rolled to a temperature below the Ar3 point at 10°C.
/s or more, then cooled at a cooling rate of less than 10°C/s for a period of time (t seconds) given by the following formula or more and 40 seconds or less, or tested, and then 10°C/s.
The gist of the present invention is a method for manufacturing a non-thermal high tensile strength hot rolled steel sheet, which is characterized by cooling to 450° C. or lower at the above cooling rate.

tllexp (at +az X (j! n(χN
1)))" +a, In order to obtain high tensile strength by utilizing precipitation hardening of Nb, researchers focused on ``dispersing fine Nb precipitates within the ferrite grains generated by transformation'', and this resulted in high supercooling transformation. This is based on the fact that it was discovered that this can be achieved by specifying the cooling or verification time until obtaining a fine precipitation state as a function of Nbi.

Next, the reason for limiting the chemical components according to the present invention will be described.

C is required to be at least 0.02% in order to obtain high tensile strength, and the higher the amount, the higher the tensile strength, but it is limited to 0.20% or less because of the weld cracking property.

St is a necessary element to obtain high tension, but 2
．． If it exceeds 0%, weldability and toughness will be impaired, so an upper limit was set.

Mn is required at 0.5% or more to obtain high tensile strength, but 2
．． If it exceeds 0%, ferrite transformation slows down, making it difficult to utilize Nb precipitation hardening.

Nb needs to be present in an amount of 0.010% or more in order to exhibit precipitation hardening, but if it exceeds 0.1%, the effect is saturated.

Next, the reason for limiting the rolling and cooling conditions will be described.

The reason why hot rolling is carried out at more than 3 Ar points is that rolling at less than 3 Ar points is not preferable because precipitates dynamically precipitate and become coarse during rolling, which deteriorates the efficiency of precipitation hardening.

Next, the reason for cooling to below Ar3 point at a cooling rate of 10°C/s or more is because precipitation proceeds in a highly supercooled state, and in order to obtain fine precipitates, a cooling rate of 10°C/s or more is required. This is because it is necessary.

Next, the reason for cooling or testing at a cooling rate of less than 10'C/s is to finely precipitate Nb precipitates within the ferrite grains during this time.
It was found that slow cooling of less than 40 seconds is required, and that the time required is longer than the time given by equation 11. However, if the time is longer than 40 seconds, the precipitates become coarse and become ineffective. The reason for cooling again to 450°C or less at a cooling rate of 10°C/s or more is to cool as quickly as possible to 450°C or less, which is the temperature range in which the precipitates do not become coarse. A cooling rate higher than that is required.

t=exp (a, +a, X (j! n(ZNb
))”+a,,X An(ZNb) )・・・・・・(
1) (Example) Table 1 shows examples of the present invention. Regarding the minimum value of retention time, al=2.2655. in formula (1). a Z ~
0.3938. It was calculated using the value a x = 1.5221. All of them were heated in a heating furnace at a temperature of 1,200°C or higher and hot-rolled.
~■ are comparative examples.

Comparing (2) and (2), the strength is higher when the finishing temperature is Ar3 or higher.

Comparing (2) and (2), it can be seen that the strength is increased by rapidly cooling to an Ar point of 3 or lower and maintaining the temperature at -30°C.

The comparison between ■ and ■ shows the effect of the second cooling rate, and the comparison between ■ and ■ shows the effect of the first cooling rate.

In addition, the effect of the cooling stop temperature can be seen by comparing ■ and ■.

From the above, the results of the present invention are higher in strength,
Conversely, in order to obtain the same strength, it is possible to use a smaller amount of Nb.

(Effect of the invention) As can be seen from the above, according to the present invention, less Nb
With i, high strength can be obtained, cost can be reduced, and it has great industrial effects.

Claims (1)

[Claims] C0.02 to 0.20% by weight, Si 2.0% or less, M
Ar
After hot rolling at a temperature exceeding _3 point, immediately cool to below Ar_3 point at a cooling rate of 10 ° C / s or more, and then for a period of time (t seconds) or more and 40 seconds or less given by the following formula 1
A method for producing a non-thermal high tensile strength hot rolled steel sheet, which comprises cooling or testing at a cooling rate of less than 0°C/s, and then cooling to 450°C or less at a cooling rate of 10°C/s or more. t=exp[a_1+a_2×(ln(%Nb))^2
+a_3×ln(%Nb)] where a_1, a_2, a
_3 is a constant.