FIELD OF THE INVENTION
00001. The present invention relates to a method of heat treatment cycle, more particularly
a simulation technique of different heat treatment cycle of high strength low alloy steel (HSLA)
which can achieve good ballistic strength alongwith adequate requirement of toughness and
hardness together.
BACKGROUND OF THE INVENTION
00002. The High Strength Low Alloy (HSLA) steel is one of the important engineering
material used widely in several applications such as structural material, automobile components,
aerospace, defence and etc. In one such applications, HSLA steel is hot pressed to the required
shape and then subjected to heat treatment to obtain high very strength and toughness properties
to achieve the ballistic strength.
00003. Initially, the steel plate is heated to the austenitizing temperature. Then,
immediately the plate is moved to pressing operation. During the steel handling and movement,
temperature drops just before pressing and further drops after pressing operation. Then, the plate
is moved for water quenching. The overall time taken from furnace to quench tank will be 5-10
minutes. The plate is then subjected to tempering cycle. Finally, the plate is tested for hardness
and toughness properties. To achieve the ballistic strength requirement, both hardness (in the range
of 290-341 HV) and toughness (59 Joules at -40°C / Charpy U notch test) properties in the
specified range shall be mandatory. Due to the changes in process parameters, poor ballistic
strength was observed repeatedly. Generally, when the hardness property increases, toughness
property deteriorate and when toughness increases, hardness tends to reach out of range. So,
appropriate balance among toughness and hardness properties is a big challenge in steel processing technology.
PRIOR ARTS
00004. US Patent titled “High strength, tough alloy steel” (Patent no. US 4170497)
discusses about a high strength, tough alloy steel is formed by heating the steel to a temperature in
the austenite range (1000°-1100°C) to form a homogeneous austenite phase and then cooling the
10 steel to form a microstructure of martensite separated by continuous thin boundary films of
stabilized retained austenite. The steel includes 0.2-0.35 % carbon, 3-4.5% chromium, and at least
one other substitutional alloying element, preferably manganese or nickel. The austenite film is
stable to subsequent heat treatment as by tempering (below 300°C) and reforms to a stable film
after austenite grain refinement.
15
00005. Patent titled “High-strength low-alloy steels” (No. US RE28878) discloses about
fully killed high-strength low-alloy steels consisting essentially of .12% to .20% carbon, 1.10% to
1.65% manganese, .05% to .20% vanadium, .005% to .025% nitrogen, .04% maximum
phosphorus, .025% maximum sulfur, .60% maximum silicon and balance iron are characterized in
20 a hot-rolled finished condition by yield strengths in excess of 80,000 psi, ultimate tensile strengths
in excess of 95,000 psi., ductilities as measured by percent elongation (2 inches) in excess of 18%
and good toughness. The steels are hot-rolled finished in the temperature range below Ac1 and
cooled at a rate within specific the range.
25 00006. US Patent 6488790, Method of making a high-strength low-alloy hot rolled steel.
A high-strength low-alloy steel is made by hot rolling a steel slab of a specified composition. The
hot rolling step is carried out at an austenitic hot roll finishing temperature. The hot rolled steel is
coiled at a temperature ranging below Ac1. The steel is characterized by a yield strength of at least
110 ksi. The steel may be further characterized by a ferrite-bainite microstructure.
30
00007. US Patent 4812182, Air-cooling low-carbon bainitic steel, Steel articles have a
granular bainite microstructure on continuous air cooling, from hot working temperature, of a low
cost, tough, high strength, low alloy (HSLA) steel having a composition which, in broadest scope,
scope, consists essentially, by weight percent, of 0.08 to 0.25% carbon, 0.30 to 1.5% silicon, 2.0
35 to 3.2% manganese and 0.005 to 0.005% boron. The steel optionally may contain, by weight
percent: up to 0.10 calcium and up to.0.20%, preferably less than 0.01%, sulfur, or up to 0.10%
lead; up to 0.10%, especially 0.04 to 0.10%, vanadium, and up to 1.5% chromium. The steel is
substantially free of molybdenum, tungsten and other alloying elements, except for residual
amounts.
10 00008. But there is always a need to provide a method for optimizing the heat treatment
for high strength low alloy (HSLA) steel for having toughness and hardness properties together.
00009. The present invention meets the long felt need to provide a method, which discloses
the modified heat treatment simulation in lab-scale specimens with different heat treatment cycle
15 using thermo-mechanical simulator. Based on the simulation, optimised heat treatment cycle will
be arrived, with requirement of good combination of toughness and hardness. The ballistic
property will be good only when both hardness and toughness requirement are obtained together.
OBJECTS OF THE INVENTION
20
000010. It is therefore an object of the present invention is to provide a heat treatment cycle
for high strength low alloy steel to receive adequate toughness and hardness properties together.
000011. Another object of the present invention is to provide a simulation technique for
25 different treatment cycles for high strength low alloy steel for having a good ballistic strength.
000012. Yet another object of the present invention is to provide a simulation technique for
different treatment cycles, which is simple yet rapid.
30 000013. Further object of the present invention is to provide a simulation technique for
different treatment cycles, which is economic and environment friendly.
SUMMARY OF THE INVENTION
000014. One or more drawbacks of conventional systems for a simulation of different heat
35 treatment cycle of high strength low alloy steel are overcome, and additional advantages are
provided through the method as claimed in the present disclosure. Additional features and
advantages are realized through the technicalities of the present disclosure. Other embodiments
and aspects of the disclosure are described in details herein and are considered to be part of the
claimed disclosure.
000015. Quenched and tempered low alloy steel formed at hot pressing process and
10 followed by heat treatment to achieve good strength and toughness. Main requirement in the
product qualification is ballistic test. Sometimes, poor ballistic strength was observed due to the
changes in the process parameters such as cooling rate, soaking temperature and time.
000016. This invention aims to simulate different heat treatment cycles, to meet the
15 requirement of good ballistic strength with requirement of toughness and hardness property. The
specimen is subjected to heat treatment cycle of 930°C for 1 hour. This is followed by cooling
cycle to 520°C corresponding to the pressing process conducted in the actual situation. Then, the
sample is quenched in water to reach room temperature. Then the material is tempering treatment
at 660°C for 1 hour. After the HT, microstructure and mechanical properties are evaluated.
20
000017. The conventional heat treatment cycle is modified and additional tempering cycle
at 450°C was introduced to hot formed product using thermo mechanical simulator. Based on the
results, optimised heat treatment cycle was arrived. The ballistic property will be good only when
both hardness and toughness requirement are obtained together.
25
000018. Various objects, features, aspects, and advantages of the inventive subject matter
will become more apparent from the following detailed description of preferred embodiments.
30 000019. It is to be understood that the aspects and embodiments of the disclosure described
above may be used in any combination with each other. Several of the aspects and embodiments
may be combined to form a further embodiment of the disclosure.
35 000020. The foregoing summary is illustrative only and is not intended to be in any way
limiting. In addition to the illustrative aspects, embodiments, and features described above, further
aspects, embodiments, and features will become apparent by reference to the drawings and the
following detailed description.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
000021. The illustrated embodiments of the subject matter will be best understood by
reference to the drawings, wherein like parts are designated by like numerals throughout. The
following description is intended only by way of example, and simply illustrates certain selected
10 embodiments of devices, systems, and processes that are consistent with the subject matter herein,
wherein:
000022. Fig. 1 illustrates the determination of critical temperature (Ac1 and Ac3)
15 000023. Fig. 2 illustrates the CCT diagram of high strength low alloy (HSLA) steel.
000024. Fig. 3 illustrates the microstructures of HSLA steel at different cooling rate.
000025. Fig. 4 illustrates the heat treatment thermal cycle of HT Cycle
000026. Fig. 5 illustrates the heat treatment thermal cycle of HT Cycle 2
20
000027. Fig. 6 illustrates the heat treatment thermal cycle of HT Cycle 3
000028. Fig. 7 illustrates the heat treatment thermal cycle of HT Cycle 4
25 000029. Fig. 8 illustrates the dilation plot of HT Cycle 4
000030. The figure depict embodiments of the disclosure for purposes of illustration only.
One skilled in the art will readily recognize from the following description that alternative
30 embodiments of the methods illustrated herein may be employed without departing from the
principles of the disclosure described herein.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
35 000031. While the embodiments of the disclosure are subject to various modifications and
alternative forms, specific embodiment thereof have been shown by way of the figures and will be
described below. It should be understood, however, that it is not intended to limit the disclosure to
the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications,
equivalents and alternative falling within the scope of the disclosure.
40
000032. It is to be noted that a person skilled in the art would be motivated from the present
disclosure to arrive at a process for simulation of different heat treatment cycle of high strength
6
45
5 low alloy steel. Such a method for evaluating the same may vary based on combination of one
low alloy steel. Such a method for evaluating the same may vary based on combination of one or
more ingredients. However, such modifications should be construed within the scope of the
disclosure. Accordingly, the drawings illustrate only those specific details that are pertinent to
understand the embodiments of the present disclosure, so as not to obscure the disclosure with
details that will be clear to those of ordinary skill in the art having benefit of the description herein.
10
000033. As used in the description herein and throughout the claims that follow, the
meaning of “a”, “an”, and “the” includes plural reference unless the context clearly dictates
otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on”
15 unless the context clearly dictates otherwise.
000034. The terms “comprises”, “comprising”, or any other variations thereof used in the
disclosure, are intended to cover a non-exclusive inclusion, such that a method, alloy, toughness,
20 hardness, simulation, austenitizing that comprises a list of components does not include only those
components but may include other components not expressly listed or inherent to such method, or
assembly. In other words, one or more elements in a system or device proceeded by
“comprises…..a “does not, without more constraints, preclude the existence of other elements or
additional elements in the system, apparatus or device.
25
000035. The present subject matter relates to a process for simulation of different heat
treatment cycles during hot pressing of high strength low alloy steel (HSLA) for attaining good
ballistic strength alongwith adequate toughness and hardness property.
30
000036. The process uses a dilatometry based heat treatment simulation experiments by
using thermo mechanical simulator to observe the phase transformation occurrence.
35 000037. The process comprises the steps of:
i) heat treatment of steel plates at a temperature of 930°C with sufficient soaking
period i.e. for 1 hour;
ii) simulating at a temperature of 660° in the thermo mechanical simulator;
40 iii) further austenitizing at a temperature of 930°C alongwith subsequent tempering at
660°C;
7
5 iv) cooling to room temperature in different cooling rate depending on different phases;
characterized in that an additional tempering treatment below the critical
temperature (AC1) at a temperature of 450°C and another subsequent stress relieving heat
treatment cycle at a temperature of 660°C was incorporated to have good ballistic property of the
10 steel sheets.
000038. AC1 and AC3 temperature of high strength low alloy steel (HSLA) grade denote
the starting and completion of austenite phase which are to be 753°C and 810°C respectively.
15 000039. The cooling rate are utilized for this process are 100, 50, 20, 10, 5, 2, 1 and 0.5°C/s.
000040. Table 4 provides variation of cooling time and cooling rate in different cycles.
Table 4.
HT
CYCLE
Cooling time and cooling rate from 930°C 1
st HT 2
nd HT
HT
CYCLE
1
930-700: 10
mins CR:
0.38°C/s
700-560:
15 s
CR:25°C/
s
560-520:
5 mins
CR:
0.13°C/s
520-60:
130 s
CR:3.5°C/s 660°C / 0.5
hr
Nil
HT
CYCLE
2
930-700:
10 mins
CR: 0.38°C/s
700-560:
15 s
CR:
25°C/s
560-520:
5 mins
CR:
0.13°C/s
520-60:
130 s
CR:3.5°C/s
930°C / 0.5
hr 660°C /
0.5 hr
HT
CYCLE
3
930-700:
10 mins
CR: 0.38°C/s
700-560:
15 s
CR:
25°C/s
560-520:
5 mins
CR:
0.13°C/s
520-60:
130 s
CR:3.5°C/s 450/1 hr,
Quenching;
660°C /1
hr
HT
CYCLE
4
930-700:
10 mins
CR: 0.38°C/s
700-560:
15 s
CR:
25°C/s
560-520:
5 mins
CR:
0.13°C/s
520-60:
130 s
CR:3.5°C/
s
450/1 hr
No
Quenching
660°C /1
hr
20 000041. The temperature of phase change was detected by dilatometer (Table 2) and
incorporated in the cooling curve (as illustrated in Fig 2). Microstructure and hardness (Table 3)
8
5 observed on these different cooling rate (as illustrated in Fig 3) were done to correlate the
transformation temperature. Only Martensite phase was observed in cooling rate from 100-50°C/s.
Martensite and bainite are observed together in the cooling rate of 20-5°C/s. Only bainite phase
was seen in the cooling rate range of 2-1°C/s. Ferrite and bainite phases are appearing in the
cooling rate of 0.5°C/s.
10 Table 2
Cooling
rate,
°C/s
Specimen
no
Cooling
rate, °C/s
Fs-Ff Bs Bf Ms Mf
100 353 100 - - - 367 174
50 354 50 - - - 386 167
20 355 20 - 425 389 388 189
10 348 10 - 506 364 363 201
5 349 5 - 567 359 358 205
2 356 2 - 555 231 - -
1 351 1 - 528 227 - -
0.5 352 0.5 761-643 566 344 - -
Table 3
Cooling
rate, °C/s 100 50 20 10 5 2 1 0.5
Specimen
No 353 354 355 348 349 356 351 352
Reading 1 569 557 561 543 442 434 430 338
Reading 2 566 591 575 496 540 406 378 297
Reading 3 532 575 597 508 424 361 434 274
Avg HV10 556 574 578 516 469 400 414 303
000042. The additional tempering treatment occurs at 450°C with or without cooling to
15 room temperature and the subsequent stress relieving heat treatment/ second tempering process at
660°C for the required soaking period (as illustrated in Fig 6 &7). The additional heat treatment
was believed to modify the retained austenite phase present in lath boundaries of martensite and
bainite phases (as illustrated in Fig 8). The dilation measurement of steel on heat treatment cycle
which was used for correlating the microstructural phases with mechanical properties. Table 5
20
9
5 gives about the mechanical strength to withstand the ballistic test will be good only when both
hardness and toughness requirement are obtained together.
Table 5
HT Cycle Toughness, Joules @ -40°C Hardness HV10
HT Cycle 1 122, 121 Avg: 121 364, 361
HT Cycle 2 90, 119 Avg: 105 299, 302
HT Cycle 3 133,138 Avg: 136 340, 316
HT Cycle 4 127, 131 Avg : 129 334, 314
000043. The plot of dilation versus temperature is generated for different cooling rate to
10 observe the phase changes the micrograph and cab be illustrated in continuous cooling
transformation (CCT) diagram (as illustrated in Fig 2).
000044. The product qualification can only be checked by ballistic test. Sometimes, poor
ballistic strength was observed due to the changes in the process parameters such as cooling rate,
15 soaking temperature and time.
000045. This ballistic property will be good only when both hardness and toughness
requirement are obtained together. The present invention aims to simulate different heat treatment
20 cycles, to meet the requirement of both toughness and hardness property.
000046. Table 1 illustrates about the value of nominal thickness, yield stress, tensile stress,
elongation % and charpy notch.
Table 1
Nominal
thickness, mm
Yield stress,
MPa
Tensile stress,
MPa
Elongation, % Charpy 3 mm U
Notch (-40°C)
45 883 Min 981 Min 13 59
25
000047. Each of the appended claims defines a separate invention, which for infringement
purposes is recognized as including equivalents to the various elements or limitations specified in
the claims. Depending on the context, all references below to the “invention” may in some cases
refer to certain specific embodiments only. In other cases, it will be recognized that references to
30 the “invention” will refer to subject matter recited in one or more, but not necessarily all, of the
claims.
10
5 000048. Groupings of alternative elements or embodiments of the invention disclosed herein
are not to be construed as limitations. Each group member can be referred to and claimed
individually or in any combination with other members of the group or other elements found
herein. One or more members of a group can be included in, or deleted from, a group for reasons
of convenience and/or patentability. When any such inclusion or deletion occurs, the specification
10 is herein deemed to contain the group as modified thus fulfilling the written description of all
groups used in the appended claims.
Equivalents:
15
000049. With respect to the use of substantially any plural and/or singular terms herein,
those having skill in the art can translate from the plural to the singular and/or from the singular to
the plural as is appropriate to the context and/or application. The various singular/plural
permutations may be expressly set forth herein for sake of clarity.
20
000050. It will be understood by those within the art that, in general, terms used herein, and
especially in the appended claims (e.g., bodies of the appended claims) are generally intended as
“open” terms (e.g., the term “including” should be interpreted as “including but not limited to”,
25 the term “having” should be interpreted as “having at least”, the term “includes” should be
interpreted as “includes but is not limited to”, etc.). It will be further understood by those within
the art that if a specific number of an introduced claim recitation is intended, such an intent will
be explicitly recited in the claim, and in the absence of such recitation no such intent is present.
For example, as an aid to understanding, the following appended claims may contain usage of the
30 introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the
use of such phrases should not be construed to imply that the introduction of a claim recitation by
the indefinite articles “a” or “an” limits any particular claim containing such introduced claim
recitation to inventions containing only one such recitation, even when the same claim includes
the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an”
35 (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the
same holds true for the use of definite articles used to introduce claim recitations. In addition, eve
it a specific number of an introduced claim recitation is explicitly recited, those skilled in the art
will recognize that such recitation should typically be interpreted to mean at least the recited
40 11
5 number (e.g., the bare recitation of “two recitations”, without other modifiers, typically means at
least two recitations, or two or more recitations).
000051. The terminology used herein is for the purpose of describing particular
10 embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated
that several of the above-disclosed and other features and functions, or alternatives thereof, may
be combined into other systems or applications. Various presently unforeseen or unanticipated
alternatives, modifications, variations, or improvements therein may subsequently be made by
those skilled in the art without departing from the scope of the present disclosure as encompassed
15 by the following claims.
000052. While various aspects and embodiments have been disclosed herein, other aspects
and embodiments will be apparent to those skilled in the art. The various aspects and embodiments
20 disclosed herein are for purposes of illustration and are not intended to be limiting, with the true
scope and spirit being indicated by the following claims.
25
30
12
35
5 We claim:
1. A process for simulation of heat treatment cycle of high strength low alloy (HSLA) steel
for attaining good ballistic strength comprises the steps of:
i) heat treatment of steel plates with sufficient soaking period;
10 ii) simulating of the plates in the thermo mechanical simulator;
iii) further austenitizing at 930°C alongwith subsequent tempering at 660°C; and,
iv) cooling to room temperature in different cooling rate depending on various phases;
characterized in that an additional tempering treatment below the critical
15 temperature (AC1) at a temperature of 450°C and another subsequent stress relieving heat
treatment cycle at a temperature of 660°C was incorporated to have good ballistic property of the
steel sheets.
2. The process for simulation of heat treatment cycle of high strength low alloy steel as
20 claimed in claim 1, wherein the initial heat treatment occurs at temperature of 930°C for a duration
of 1 hour.
3. The process for simulation of heat treatment cycle of high strength low alloy steel as
claimed in claim 1, wherein the simulation or tempering occurs at a temperature of 660°C for 1
25 hour.
4. The process of simulation of heat treatment cycle of high strength low alloy steel as
claimed in claim 1, wherein the different cooling rates are 100, 50, 20, 10. 5. 2. 1 and 0.5°C/s.
30 5. The process for simulation of heat treatment cycle of high strength low alloy steel as
claimed in claim 1, wherein the cooling rate of only martensite phase was 100 - 50°C/s.
6. The process for simulation of heat treatment cycle of high strength low alloy steel as
claimed in claim 1, wherein the cooling rate of martensite and bainite is 20 - 5°C/s.
35
13
5 7. The process for simulation of heat treatment cycle of high strength low alloy steel as
claimed in claim 1, wherein the cooling rate of only bainite phase is 2 - 1°C/s,
8. The process for simulation of heat treatment cycle of high strength low alloy steel as
claimed in claim 1, wherein the cooling rate of ferrite and bainite phase is 0.5°C/s.